microstructure evaluation and wear-resistant properties of
TRANSCRIPT
KTH Industrial Engineering and Management
Microstructure Evaluation and Wear-Resistant
Properties of Ti-alloyed Hypereutectic High
Chromium Cast Iron
Qiang Liu
Doctoral Dissertation
Stockholm 2013
Division of Applied Process Metallurgy
Department of Materials Science and Engineering
Royal Institute of Technology
SE-100 44 Stockholm
Sweden
Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i
Stockholm, framlägges för offentlig granskning för avläggande av Teknologie
Doktorsexamen, fredagen den 27 september, kl. 13.00 i sal B1, Materialvetenskap,
Brinellvägen 23, Kungliga Tekniska Högskolan, Stockholm
ISBN 978-91-7501-842-3
Qiang Liu Microstructure Evaluation and Wear-Resistant Properties of Ti-alloyed
Hypereutectic High Chromium Cast Iron
Division of Applied Process Metallurgy
Department of Materials Science and Engineering
School of Industrial Engineering and Management
Royal Institute of Technology
SE-100 44 Stockholm
Sweden
ISBN 978-91-7501-842-3
© Qiang Liu (刘强), July 2013
Tryck: Universitetsservice US AB, Stockholm, Sweden
To my beloved parents
献给我的父亲母亲
Abstract High chromium cast iron (HCCI) is considered as one of the most useful wear
resistance materials and their usage are widely spread in industry. The mechanical
properties of HCCI mainly depend on type, size, number, morphology of hard
carbides and the matrix structure (γ or α). The hypereutectic HCCI with large volume
fractions of hard carbides is preferred to apply in wear applications. However, the
coarser and larger primary M7C3 carbides will be precipitated during the solidification
of the hypereutectic alloy and these will have a negative influence on the wear
resistance.
In this thesis, the Ti-alloyed hypereutectic HCCI with a main composition of
Fe-17mass%Cr-4mass%C is studied based on the experimental results and calculation
results. The type, size distribution, composition and morphology of hard carbides and
martensite units are discussed quantitatively. For a as-cast condition, a 11.2μm border
size is suggested to classify the primary M7C3 carbides and eutectic M7C3 carbides.
Thereafter, the change of the solidification structure and especially the refinement of
carbides (M7C3 and TiC) size by changing the cooling rates and Ti addition is
determined and discussed. Furthermore, the mechanical properties of hypereutectic
HCCI related to the solidification structure are discussed.
Mechanical properties of HCCI can normally be improved by a heat treatment process.
The size distribution and the volume fraction of carbides (M7C3 and TiC) as well as
the matrix structure (martensite) were examined by means of scanning electron
microscopy (SEM), in-situ observation by using Confocal Laser Scanning Microscope
(CLSM), Transmission electron microscopy (TEM) and electron backscattered
diffraction (EBSD). Especially for the matrix structure and secondary M7C3 carbides,
EBSD and CLSM are useful tools to classify the fcc (γ) and bcc (α) phases and to
study the dynamic behavior of secondary M7C3 carbides. In conclusion, low holding
temperatures close to the eutectic temperature and long holding times are the best heat
treatment strategies in order to improve wear resistance and hardness of Ti-alloyed
hypereutectic HCCI.
Finally, the maximum carbides size is estimated by using statistics of extreme values
(SEV) method in order to complete the size distribution results. Meanwhile, the
characteristic of different carbides types will be summarized and classified based on
the shape factor.
Keywords: High Chromium Cast Iron, cooling rate, Ti addition, M7C3, TiC, carbides
size distributions, volume fraction, heat treatment, microstructure, mechanical
properties, wear resistance, statistics of extreme values (SEV), maximum carbides
size.
Acknowledgements
I would like to take this opportunity to thank the people who have contributed and
supplied the help to this work.
All of first, I would like to express my sincere gratitude to two great supervisors
Professor Keiji Nakajima and Professor Pär Jönsson. Your endless support,
enthusiasm, knowledge and scientific discussion are very helpful for me during my
study and growth at KTH, Sweden. Your guidance makes benefit greatly on my
research life.
I would like to thank Professor Qiang Wang, associate Professor Hongwei Zhang,
associate Professor Kai Wang and Mr. Xiangkui Zhou from Northeastern University,
China. Many thanks for yours simulation support and experiment help.
I would like to say many thanks to Professor Hiroyuki Shibata at Tohoku University,
Japan. Thank you very much for taking care of me all the time when I was in Japan.
Your knowledge and experiences are really helpful to my work. I also want to say
thanks to assistant Professor Junichi Takahashi (Tohoku University) for helpful
discussion and nice suggestions on the statistical analysis. Thanks also to Akifumi
Harada for the help of SEM measurement and teaching me many Japanese words at
Tohoku University.
I also appreciate the help and support from Dr. Peter Hedström, KTH. Thank you very
much for your experiment support, very nice discussion and many good suggestions.
Thanks also to Ms. Wenli Long, who help me a lot on the technical problem at KTH
lab. The same thanks to associate Prof. Lyubov Belova (KTH) for the help with TEM
sample preparation by using Focused Ion Beam (FIB) technique. Mr. Oskar Karlsson
from Swerea KIMAB is acknowledged, who has been very helpful on EBSD
measurements.
Many thanks for the help from the colleagues at the Department of Material Science
and Engineering (MSE), KTH. I have a lot of fun from you during sharing Chinese
food, ice-cream time, basketball time and innebandy time. Special thanks to Erik
Roos for teaching me Swedish and your friendly help and care, especially when I was
in the hospital. Special appreciations are also to my Chinese friends: Rongzhen, Jun,
Zhili, Zhi, Yanyan, Ni, Xiaolei, Mei, Mu, Ying, Changji, Xiaobin, Bu, many thanks
for your kindly help and happy moments during living in Stockholm.
All colleagues in the division of TPM are thanked for your help and friendship, many
thanks to Dr. Mikael Ersson for introducing the innebandy and Mjöd to me. It is
very nice experiences for me to feel the Swedish culture.
I would like to acknowledge to China Scholarship Council for the financial support.
Finally, I wish to thank my family: my sister, Yuanyuan, thank you very much for
taking care of mother and father when I was abroad, I owe you a lot; Mama and Baba,
thank you very much for your endless love and support.
Qiang Liu, Stockholm, July 2013.
Preface
This is a systematic study from melting and solidification to heat treatment of a wear
resistant high chromium cast iron. The results on Ti-alloyed hypereutectic high
chromium cast iron can be widely be applied to improvements of wear resistant parts
in mineral industry plants, power plants, steel making plants, and others. Moreover,
for the development of new superior wear resistant materials. I wish you can get some
`hint` when you read this thesis, which is intend to contribute with new knowledge in
the production of wear parts.
The present thesis is based on the following supplements:
Supplement I:
“Effect of Cooling Rate and Ti Addition on the Microstructure and Mechanical
Properties in As-Cast Condition of Hypereutectic High Chromium Cast Irons”
Qiang Liu, Hongwei Zhang, Qiang Wang, Xiangkui Zhou, Pär G. Jönsson and Keiji
Nakajima
ISIJ International, Vol. 52(2012), No. 12, pp. 2210-2219.
Supplement Ⅱ:
“Solidification and Phase Transformations of Ti-added Hypereutectic High
Chromium Cast Irons”
Qiang Liu, Hongwei Zhang, Qiang Wang, Xiangkui Zhou, Pär G Jönsson and Keiji
Nakajima
ICRF 2012, 1st International Conference on Ingot Casting, Rolling and Forging,
05.06.2012, Aachen. Düsseldorf: Stahl-Zentrum 2012, ICRF-31.pdf (CD-ROM).
Supplement Ⅲ:
“Effect of Heat Treatment on Microstructure and Mechanical Properties of
Ti-alloyed Hypereutectic High Chromium Cast Iron”
Qiang Liu, Peter Hedström, Hongwei Zhang, Qiang Wang, Pär G. Jönsson and Keiji
Nakajima
ISIJ International, Vol. 52(2012), No. 12, pp. 2288-2294.
Supplement Ⅳ:
“Dynamic Precipitation Behavior of Secondary M7C3 Carbides in Ti-alloyed
High Chromium Cast Iron”
Qiang Liu, Hiroyuki Shibata, Peter Hedström, Pär Göran Jönsson and Keiji Nakajima
ISIJ International, Vol. 53(2013), No. 7, pp. 1239-1246.
Supplement Ⅴ:
“Estimation of the Maximum Carbide Size in a Hypereutectic High Chromium Cast
Iron Alloyed with Titanium”
Qiang Liu, Pär Göran Jönsson and Keiji Nakajima
Accepted by ISIJ International for publication, Vol. 53(2013), No. 12.
The contributions by the author to the different supplements of the dissertation:
Supplement I: The literature survey, a major part of the experimental work, and a
major part of the writing.
Supplement Ⅱ: The literature survey, a major part of the experimental work, and a
major part of the writing.
Supplement Ⅲ: The literature survey, a major part of the experimental work, and a
major part of the writing.
Supplement Ⅳ: The literature survey, a major part of the experimental work, and a
major part of the writing.
Supplement Ⅴ: The literature survey, a major part of the experimental work, and a
major part of the writing.
Parts of the work have been presented at the following conference:
“Estimation of the Maximum Carbide Size in a Hypereutectic High Chromium Cast
Iron Alloyed with Titanium”
Qiang Liu, Pär Göran Jönsson and Keiji Nakajima.
3rd
International Symposium on Cutting Edge of Computer Simulation of
Solidification, Casting and Refining (CSSCR2013), May 20-23, 2013, Stockholm,
Sweden and Helsinki, Finland.
Contents
Chapter 1 Introduction .............................................................................................................. 1
1.1 Background ................................................................................................................................... 1
1.2 The purpose and overview of thesis .............................................................................................. 2
Chapter 2 Methodology .............................................................................................................. 5
2.1 Experiment methods ...................................................................................................................... 5
2.1.1 Melting and solidification process .......................................................................................... 5
2.1.2 Heat treatment process ........................................................................................................... 8
2.1.3 Microstructure observations and image analysis .................................................................. 10
2.1.4 Mechanical properties measurement .................................................................................... 14
2.2 Theoretical works ........................................................................................................................ 14
Chapter 3 Results and Discussion ........................................................................................... 17
3.1 Phase diagram and solidification path of Ti-alloyed hypereutectic HCCI .................................. 17
3.1.1 Phase diagram ....................................................................................................................... 17
3.1.2 Solidification path ................................................................................................................ 19
3.2 Microstructure of Ti-alloyed hypereutectic HCCI ...................................................................... 20
3.3 Effect of cooling rate on as-cast structure ................................................................................... 21
3.4 Effect of Ti addition on as-cast structure..................................................................................... 23
3.5 Effect of Ti addition and cooling rate on the volume fraction in as-cast condition of the
hypereutectic HCCI ........................................................................................................................... 27
3.6 Classification of M7C3 carbides for as-cast condition ................................................................. 29
3.7 Effect of cooling rate and Ti addition on the mechanical properties in as-cast condition
of the hypereutectic HCCI ................................................................................................................. 33
3.8 Evolution of matrix after heat treatment...................................................................................... 35
3.9 Evolution of carbides after heat treatment ................................................................................... 39
3.9.1 Evolution of primary and eutectic M7C3 carbides after heat treatment ................................ 39
3.9.2 Evolution of TiC carbides after heat treatment..................................................................... 42
3.9.3 Evolution of secondary carbides during heat treatment ....................................................... 44
3.10 Mechanical properties improved by heat treatment .................................................................. 52
3.11 Estimation of the maximum carbide size by using SEV method .............................................. 54
3.11.1 Estimation of maximum primary M7C3 carbides size ........................................................ 54
3.11.2 Estimation of maximum TiC carbides size ......................................................................... 57
3.11.3 Estimation of maximum secondary M7C3 carbides size ..................................................... 58
Chapter 4 Concluding Discussion ........................................................................................... 63
Chapter 5 Conclusions and Future Work ............................................................................... 69
References ................................................................................................................................ 73
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Chapter 1
Introduction
1.1 Background
The Fe-Cr-C system alloys with high carbon have widely been applied to wear
resistant parts in steel making plants, power plants, mineral industry plants, and others
[1, 2]. In all industrial fields, however, the improvement of the abrasion
wear-resistance is strongly required because of its short service life.
In the first half of the 1980s, High Chromium Cast Iron (HCCI) with a composition
range of 2.0-4.3%C [2] and 12-30%Cr [3] were started to be applied to roll materials
for hot rolling. The wear performance level of hypereutectic HCCI is mainly
controlled by a high volume fraction (30 to 60%) of M7C3 type carbides (1200~1600
HV) [4], M23C6 type carbides (1600-2520HV) [5], MC type carbides (2400-4000HV)
[5], etc. These carbides will precipitate during the solidification and heat treatment,
which lead to an improvement of the final material properties.
Compared to the hypoeutectic HCCI, the hardness of hypereutectic HCCI is obviously
improved via a higher carbon and chromium content. This is due to that carbonization
inside the hypereutectic HCCI highly increased. Moreover, these higher additions will
also cause an increase of the volume fraction of carbides. However, the coarser and
larger primary carbides will be precipitated during the solidification of the
hypereutectic HCCI. These coarser primary carbides have a negative effect on the
improvement of the hardness and wear resistance of all alloys. Therefore, many
investigations have been executed with the aim of refining the solidification structure
and improving the properties of hypereutectic HCCI. Here, additions of alloy
-2-
elements, such as Ti, Nb, V, Zr etc. [6-13], have been used to form the NaCl type
carbides. Those elements, which are strong carbide forming elements, will react with
carbon and form NaCl type carbides, such as TiC, NbC, VC etc. Also, these can act as
heterogeneous nuclei for the precipitation of M7C3 type carbides during the cooling
and solidification of the melt. [6-8] This, in turn, results in significant refinements of
the final carbides size.
Furthermore, for many applications, a higher wear resistance is required to improve
the service life of the wear components. In comparison to the as-cast alloy, the
heat-treated alloy with higher wear resistance is prior to service. The reason is that the
matrix changes from an austenitic structure (fcc, γ) into a martensitic structure (bct, α’)
and new carbide precipitates (M7C3 [3, 14] or M23C6 [15-17] form within the matrix.
Therefore, many studies [4, 14-15, 18-22] focusing on improving the mechanical
properties of cast iron, by alternative heat treatment processes, have been reported.
Most of the previous studies are focused on identifying the new carbides by using
transmission electron microscopy (TEM). [14, 16-20, 22-26] They have furthermore
studied how the new carbides influence the final properties.
At present, few reports discuss how the carbide size distribution influences their final
solidification structure and properties. However, they only show the effects of the
average diameter and volume fraction of the total amount of carbides on the material
properties. [6-9, 11] In addition, for heat treated HCCI, only a few studies
quantitatively discuss how the size distribution of carbides and martensite units affect
the final properties of hypereutectic HCCI. However, we must point out that the size,
number, volume fraction and distribution of these carbides and martensite units will
also affect the final properties of the hypereutectic HCCIs significantly. Hence, a
quantitative study on the carbides size distribution and how these carbides influence
on mechanical properties is desired.
1.2 The purpose and overview of thesis
In this thesis, we quantitatively study the size distribution, composition and
-3-
morphology of carbides and martensite units for as-cast condition and heat treatment
condition of Ti-alloyed hypereutectic HCCI with main composition of
Fe-17mass%Cr-4mass%C based on following five supplements. An outline of this
thesis is given and organized as follows:
Fig. 1.1. The scope and focus of different supplements in the present thesis.
As shown in Fig. 1.1, the content of this thesis is mainly divided into three parts to
study the relationship between the microstructure and the final mechanical properties
of a hypereutectic HCCI alloyed with titanium. The first one is the as-cast part, which
is related to the melting and solidification of hypereutectic HCCI alloys during the
casting process. It corresponds to Supplement I [27] and Supplement Ⅱ [28]. The
solidification path, the microstructure refined by different cooling rates and titanium
additions, and the mechanical properties of hypereutectic HCCI were studied both
experimentally and theoretically.
The second part of the thesis is focus on heat treatment, which is related to the matrix
phase transformation and the secondary carbides precipitation during the heat
treatment process. It corresponds to Supplement Ⅲ [29] and Supplement Ⅳ [30].
-4-
The relationship between carbide precipitation, martensite units and mechanical
properties are discussed based on the volume fraction and the size distribution of
carbide precipitates (M7C3 carbides and TiC carbides) as well as the martensite units.
A possible mechanism is finally proposed to explain the improvement of mechanical
properties through the heat treatment process. In addition, the best heat treatment
strategy is suggested.
In the final part of thesis, which is related to Supplement Ⅴ [31], more accurate
carbides size distribution results were obtained by supplementing the maximum
carbides size information to the normal size distribution. Moreover, the combined
method between a size distribution analysis and a maximum analysis is proposed.
Then, the relationships between the carbides size distribution including the maximum
carbides size and the mechanical properties can be fully explained by synthetically
consider evaluation of carbides and matrix.
-5-
Chapter 2
Methodology
In the present thesis, the solidification process, the carbides precipitation behavior, the
phase transformations as well as these phenomena influence on the mechanical
properties of a Ti-alloyed hypereutectic HCCI were studied both experimentally and
theoretically. However, the experimental studies were taken as the major research
tools in this thesis. The detail information about experiment methods and theoretical
methods will be introduced as following sections.
2.1 Experiment methods
2.1.1 Melting and solidification process
Based on the requirements during actual production, melts were prepared in a
laboratory furnace. high-carbon ferro-chromium (Fe-8mass%C-60mass%Cr),
ferro-molybdenum (Fe-0.05mass%C-58.4mass%Mo), pure iron, pig iron
(Fe-3.8mass%C-0.34mass%Mn-1.8mass%Si), ferro-titanium(Fe-30mass%Ti),
ferro-manganese(Fe-75.9mass%Mn), and pure nickel were melted in a 8 kg capacity
graphite crucible. This was done using a medium frequency induction furnace (45Kw,
7k Hz) and using an air atmosphere. The alloy compositions are shown in Table 2.1.
Pure iron, pig iron and high carbon ferro-chromium were first melted. Then,
ferro-manganese and ferro-molybdenum were added. Finally, pure nickel and
ferro-titanium were added into this mother alloy.
The holding time over the TiC precipitation temperature was kept as a value of
-6-
Table 2.1 Chemical compositions of HCCI used in the experiments. (mass.%)
C Cr Ti Mn Mo Ni Si N S O P Fe
Sand
4.01 16.2 - 2.04 1.01 1.24 0.775 0.020 0.018 0.0086 0.026 Bal.
3.79 17.0 1.53 2.08 1.05 1.09 0.416 0.016 0.017 0.014 0.025 Bal.
Graphite
4.9 16.4 - 1.59 0.75 1.09 0.845 0.019 0.022 0.0046 0.025 Bal.
4.03 15.6 0.74 1.72 0.93 1.08 0.992 0.014 0.020 0.0047 0.023 Bal.
4.01 17.4 1.48 1.91 0.89 1.11 0.951 0.015 0.017 0.011 0.024 Bal.
3.91 17.5 3.36 1.95 1.01 1.08 0.966 0.025 0.025 0.031 0.022 Bal.
Metal
3.92 14.7 - 1.950 0.95 1.05 0.529 0.015 0.020 0.0037 0.024 Bal.
3.90 16.8 1.06 1.95 0.99 1.15 0.708 0.014 0.014 0.0094 0.024 Bal.
3min±1min, in order to prevent the agglomeration of TiC carbide precipitation.
Finally, the molten hypereutectic HCCI were poured into the mold at a temperature
value of 1450±10°C.
Fig. 2.1. Schematic diagram of melting process of hypereutectic HCCI with 1.5mass% Ti addition and
casting in the graphite mold.
Composition
s Mold
-7-
The following three mold types were used: sand mold (silica sand), graphite mold,
metal mold (ASTM: 1045). A schematic diagram of the melting process is shown in
Fig. 2.1.
The dimension of the ingot is 120mm (L) ×84mm (W) ×55mm (H). A schematic view
of the ingot and the thermocouple (Type-B) position is shown in Fig. 2.2.
Fig. 2.2. Schematic view of ingot and cutting position of sample for microstructure observation.
0 100 200 300 400 500 600 7001000
1100
1200
1300
1400
1500
Sand mold Graphite mold Metal mold
25oC/min 80
oC/min 110
oC/min
Time, (s)
Tem
pera
ture
, (o
C)
Sand mold
Metal mold
Graphite mold
Fig. 2.3.Typical cooling curves measured in three kinds of molds for hypereutectic HCCI with
1.5mass% Ti addition.
-8-
In addition, the cooling curves obtained from the three different molds are shown in
Fig. 2.3. From this information the following different cooling rates were obtained:
25oC/min, 80
oC/min and 110
oC/min within 10% errors for the sand mold, graphite
mold and metal mold, respectively.
2.1.2 Heat treatment process
In this thesis, the heat treatment process was carried out in two ways: one using a
conventional heat treatment, which is mainly focused on an evaluation of the matrix
and carbides after heat treatment. Another way is by using the in-situ observations,
which mainly focused on studying the dynamic precipitation behavior of secondary
carbides during the whole heat treatment process from heating and holding to cooling.
2.1.2.1 Heat treatment for matrix and carbides evaluation
The specimens used for the heat treatments were selected from the ingots, which were
casted in a graphite mold with a 1.5mass% titanium addition. Samples of the
dimensions 10mm (L) ×10mm (W) ×10mm (H) were cut and then heat-treated at a
furnace temperature of 900oC, 1000
oC and 1050
oC for 2hr and 6hr respectively. The
0
200
400
600
800
1000
1200
200000
10 oCmin10
oCmin
906oChr
1008oChr
1058oChr
air cooling
1005oChr
903oChr
T
em
pe
ratu
re,
(oC
)
Time, (s)
1057oChr
air cooling
10000 200000 10000
Fig. 2.4. Histories of sample surface temperatures.
-9-
samples were air cooled from the elevated temperature to room temperature. The
surface temperature of the sample during heat treatment was measured and calibrated
by using a R-type thermocouple. The temperature histories of sample surface are
shown in Fig. 2.4. It was found that the difference between the furnace temperature
and the actual temperature of the sample surface was less than 10 oC.
2.1.2.2 In-situ observation for secondary carbides evaluation
The specimens, which were used for in-situ observations, were selected from the
ingots that were casted in the graphite mold for HCCI grades with 1.5mass% titanium
additions. It is the same as the above heat treatment process. The cast ingots were cut
into small pieces (3mm (L) × 2mm (W) × 2mm (H)). Each specimen was carefully
polished following the preparation procedure as is described in Table 2.2.
Table 2.2 Sample preparation for in-situ observation and EBSD measurement
Step
Disk name and type No.
(Company: BUEHLER)
Abrasive Size and
suspension
Load(N)
Speed(rpm) and
Direction
Time
(minutes)
Step 1
ApexHercules® S rigid
No.41-27400-412-001
9μm
diamond suspension
30
150
(Comply)
5
Step 2
MicroFloc
No.40-8322
3μm
diamond suspension
30
150
(Comply)
5
Step 3
ChemoMet®
No.40-7922
0.05μm
colloidal silica
30
150
(Contrary)
2
The polished specimens were set into a high purity alumina crucible (Φ5.5mm O.D. ×
Φ4.5mm I.D. ×5mm height) and study in a Confocal Laser Scanning Microscope
(CLSM) equipped with an infrared heating furnace. The wavelength of 1.5mW He-Ne
laser, which is used in CLSM setup, is 632.8nm. A detailed description of the CLSM
setup can be found in previous works. [32, 33] The specimens were heated to 890°C
-10-
and 1026°C using a 10°C/min heating rate and thereafter they were held for 2 and 6
hours, respectively. The atmosphere in the CLSM was pure Ar; moreover, a titanium
foil was wound around the upper part of the crucible to avoid oxidation of the
specimen surface during the experiments. Once the desired temperature and holding
time was reached, the specimen was cooled at a cooling rate of 90°C/min. The furnace
temperature was measured by a PtRh30%–PtRh6% thermocouple (type B), which was
inserted at the bottom of the crucible. In this case, the actual specimen temperature
was calibrated by using a Pt–PtRh13% thermocouple (type R) through welding of a R
type thermocouple wire on the surface of the specimen. It was found that the
temperature difference between the specimen surface and the furnace was 13°C and
31°C at 890°C and 1026°C, respectively. The temperature history of the specimen
surface and furnace is illustrated in Fig. 2.5.
0
100
200
300
400
500
600
700
800
900
1000
1100
(Specimen:890oC)
(Specimen:1026oC)
100500
10oC/min
Difference:31oC Furnace
Specimen surface
Difference:13oC
Tem
pera
ture
, (o
C)
Time, (min)
90oC/min
150 100500 150
90oC/min
10oC/min
Fig. 2.5. Temperature history of specimen surface and furnace during the heat treatment process.
2.1.3 Microstructure observations and image analysis
2.1.3.1 Carbide analysis by LOM and SEM
The as-cast samples were cut at a position close to the thermocouple measurement
position, as shown in Fig. 2.2. Thereafter, the as-cast samples were polished and
etched by using a solution of 5g FeCl3+10ml HCl+100ml ethanol. Then, the size
-11-
distribution of carbides (primary M7C3, eutectic M7C3, secondary M7C3 and TiC) was
measured on photographs obtained by using a light optical microscope (LOM) (Leica
DMR) at a ×10 magnification. This is done both for primary M7C3 carbides and
eutectic M7C3 carbides, where one pixel represented 0.3356μm. Back scattered
electron (BSE) setting was used in the scanning electron microscopy (SEM) (Hitachi:
S-3700N) together with a ×600 magnifications when studding TiC carbides, where
one pixel represented 0.1656μm. Moreover, the BSE mode was used in SEM
(FE-SEM, HITACHI, S-4800 type and FEG, JEOL JSM-7401F) along with a ×10000
magnification to study secondary M7C3 carbides, where one pixel represented
0.0093μm. The composition of carbides including Fe, Cr, Ti, Mo, Mn and other
elements were determined with the Energy Dispersive X-ray spectrometer (EDX)
method. The number of carbides per unit area, NA, was calculated using Equation
(2-1).
NA (i) = n (i) /Sobs (2-1)
where n(i) is the counted number of carbides for each selected step (i) of the size
distribution. Moreover, Sobs is the total area which was observed on the photographs,
which were taken by LOM (Sobs=0.3152mm2 for primary M7C3 carbides and eutectic
M7C3 carbides) and SEM (Sobs=0.0314 mm2 for TiC carbides and Sobs=107.78μm
2 for
secondary M7C3 carbides) respectively. The size of carbides, d, and the volume
fraction of carbides were calculated as the equivalent diameter of a circle by using an
image analyzer, which was the commercial software WinROOF. The size
distributions for carbides and martensite unit were plotted using a logarithmic scale.
The value of size d(j), for the j-th step, ∆(j), of a log-normal distribution can be
determined as follows:
d(j) or D(j) = 10∆(j) (2-2)
where ∆(j) is the width of the j-th step for log-normal distributions (∆(j)= ∆(j-1)+0.15).
This range is from -0.15 to 2.4 for primary M7C3 carbides and eutectic M7C3 carbides
(d), from -1.05 to 2.1 for TiC carbides (d) and from -2.25 to 2.4 for secondary M7C3
-12-
carbides and martensite unit (D). In this case, the mean value for the j-th step of the
size distribution, d(j) or D(j) varies from 0.85 to 214.50μm, from 0.11 to 107.51μm,
and from 0.0068 to 214.5038μm respectively.
2.1.3.2 Carbide and matrix analysis by EBSD
An electron backscattered diffraction (EBSD) analysis was carried out to identify the
phases in the specimens. [34, 36] Austenite (fcc, γ), martensite (bct, α’) and carbides
(M23C6, M7C3 and TiC) were included in the analysis. The flatness and surface quality
of specimens is crucial for EBSD analysis [35] in order to properly index phases such
as e.g. martensite. [35] In this study, the specimens used for EBSD analysis were
polished; first by manual polishing using SiC abrasive papers (P240-P320) together
with water, secondly with 9μm and 3μm diamond suspensions. Finally, specimens
were polished with a 0.05μm colloidal SiO2 by automatic polishing. The detailed
preparation procedure for the hypereutectic HCCI samples is shown in Table 2.2.
The EBSD measurements were performed using a LEO 1530 FEG (Field Emission
Gun)-SEM equipped with a HKL system and the HKL Channel 5 software. The
specimens were tilted 70o towards the detector and the microscope was operated at an
acceleration voltage of 15kV, in high current mode, and using an aperture of 120μm.
Two different step sizes of 0.1μm and 0.03μm were used. The area scanned with a
0.1μm step size was 200μm×150μm and the area scanned with a step size 0.03μm was
10μm×10μm. The reason for using two step sizes was the rather broad size
distribution. The size and volume fraction of carbide and martensite units were
determined by using image analysis made on EBSD maps.
The EBSD measurements were further complemented by energy dispersive X-ray
spectroscopy (EDS) mapping using the INCA software. The EDS analysis was
conducted using a 20kV acceleration voltage and a 30μm aperture during operation in
the high current mode. The EDS analysis enabled a more sound identification of the
phases since both crystal symmetry (EBSD) as well as chemical composition (EDS)
could be considered for the observed microstructural features.
-13-
The number of carbide precipitates and martensite units per unit area (NA) was
calculated by Equation (2-1). In this case, the parameter Sobs is the total area of the
micrographs, which was 0.0314 mm2
for the SEM determinations. For EBSD the
following values were used: Sobs=0.0001mm2 (0.03μm step size) and Sobs=0.03mm
2
(0.1μm step size). The size of martensite units (D) was calculated as the equivalent
diameter of a circle, which has the same area as the measured martensite unit.
2.1.3.3 Secondary carbide identification by TEM
Thin-foil specimens (~40nm) after heat treatment, which were prepared by focused
ion beam (FIB)-SEM (FEI, Nova 600) technique, were used to identify the type of
secondary carbides. Transmission electron microscopy was conducted using a TEM,
JEOL JEM-2100F, at an operating voltage of 200 kV. Moreover, the secondary
carbides type was evaluated as a M7C3 type in the Ti-alloyed hypereutectic HCCI
grades.
2.1.3.4 Estimation of the maximum carbides size by SEV method
In present thesis, the statistics of extreme values (SEV) method was used to predict
the maximum carbides size in a Fe-17mass% Cr-4mass% C hypereutectic HCCI. The
analysis procedure of the SEV was performed according to Murakami s method
[37-40] and following the ASTM E2283-03 standard. [41] The as-cast samples with
different titanium additions, casting in the different molds and heat treated at different
conditions were used to determine the maximum carbides size. The size of a unit area
for primary M7C3 carbides and TiC carbides is 0.505mm2 (LOM) and 0.017mm
2
(SEM), respectively. This corresponds to a ×5 magnification micrograph (one pixel
represented 0.555μm) in a LOM and a ×400 magnification micrograph (one pixel
represented 0.124μm) in a SEM, respectively. In addition, the size of a unit area for
secondary M7C3 carbides is 107.8μm2
at a magnification of ×10000 and 420.3μm2
at
magnification of ×5000. The maximum carbide size was measured in each unit area.
Totally, 40 unit areas were examined for the carbide extreme value analysis according
to the suggestions in the Murakami’s method for the inclusion. [42] The total
observed area for primary M7C3 carbides TiC carbides and secondary M7C3 carbides
-14-
was 20.20mm2, 0.692mm
2, and 0.106mm
2, respectively. The maximum size of
carbides, dAmax, was calculated as the equivalent diameter of a circle by using the
WinROOF image analyzer, which is commercial software.
2.1.4 Mechanical properties measurement
The mechanical properties: bulk hardness and wear loss of the hypereutectic HCCI
were investigated in the as-cast condition and in the heat treatment condition. The
bulk hardness was measured by using a Rockwell type hardness tester, HR-150A with
the Rockwell C scale (HRC). Overall, eight measurements were done for each sample.
The wear resistance was evaluated by using a single platform rotary abraser (model:
ML-100 type, Zhangjiakou Taihua Machine Factory). The wear tests were performed
at a platform speed of 60rpm under a load of 100N. The size of the wear test samples
was 4.1mm (φ) × l0mm (H). The tests were performed on a 80 mesh Al2O3 abrasive
paper at room temperature. The total sliding distance and the testing time of the
samples on the abrasive paper were 9m and 25s, respectively. The mass loss of the
specimens due to abrasion was calculated by measuring the weight of the specimens
before and after tests with a 0.001mg accuracy. Three measurements were carried out
per sample.
2.2 Theoretical works
In this thesis, the phase diagram and volume fraction of carbides in HCCI were
predicted numerically by coupling a micro-segregation model with thermodynamic
equilibrium calculations at solid/liquid interfaces.
The Scheil model, which is based on an assumption of no diffusion in solids, indicates
that the new phases always precipitate from the residual liquid in a solidification
sequence. Furthermore, the already formed solids remain frozen.
The Partial Equilibrium (PE) model distinguishes the diffusivities of substitutionals as
negligible and interstitials as complete in solids. In the present HCCI system, only C
behaves as an interstitial. Then the compositions and the amounts of the phases are
-15-
adjusted based on equalization of the chemical potentials of the interstitials,following
the equations below:
1SL
C Cµ µ (2-3)
The mass conservation of the interstitial component C before and after partial
equilibrium:
1 1
1
1
( ) ( )0
1 ( ) 1 ( )
S SL LSLC C C C
SL
C C
w w w wf f
w w
(2-4)
The unchanged u-fraction of substitutional components such as Fe element before and
after partial equilibrium in each phase:
1 ( )( )
1
LL LCFe FeL
C
ww w
w
(2-5)
1
1 1
1
1 ( )( )
1
SS SCFe FeS
C
ww w
w
(2-6)
The mass conservation of the substitutional components before and after partial
equilibrium:
1( )
1 ( )
LL LC
L
C
wf f
w
(2-7)
1
1 1
1
SS S
S
1( )
1 ( )
C
C
wf f
w
(2-8)
The chemical potential of C in a phase is determined through phase equilibrium
calculations, based on data of the phase composition, temperature and pressure. In the
above equations,L
Cw , ( )L
Cw are the mass compositions of carbon in the liquid before
and after partial equilibrium, 1S
Cw , 1( )
S
Cw are the corresponding averaged mass
composition of C in solid 1S .
1 1 1 1* ( ) /S S S Sk k
C C
k k
w w f f (2-9)
-16-
where 1* S
Cw is the newly formed composition in S1 at the k th step. The
parametersLf , 1Sf ( 1S
kk
f ) are the mass fractions of the liquid and the solid phases
1S at k th step, respectively.
The partial equilibrium among several phases (i.e. liquid and solid phase j =1, m) is
regarded as equivalent to several partial equilibriums between two phases (e.g. liquid
and one of the solids,1S ). Therefore, the mass conservation of the interstitial
component among all phases follows equation (2-10) instead of equation (2-4),
( ) ( )0
1 ( ) 1 ( )
j j
j
j
S SL LSLC C C C
SLj
C C
w w w wf f
w w
(2-10)
The calculations are based on the ideas of Chen and Sundman. [43, 44] They are
performed in Thermo-Calc/TQ interface [45] by a coupling of thermodynamic
equilibrium calculations with the alloy database TCFE6. [46] A temperature step
of -1oC was used in the calculations. The calculations by the PE and Scheil models
were stopped when the liquid fraction in the mass was less than 10-4
.
-17-
Chapter 3
Results and Discussion
The results in the present thesis include both experimental studies and theoretical
studies. These will be shown in the following sequence in this chapter: firstly, the
phase diagram and solidification path of Ti-alloyed hypereutectic HCCI will be
presented, it corresponds to section 3.1; secondly, the solidification (as-cast) structure
evolution and its influence on the mechanical properties of as-cast Ti-alloyed
hypereutectic HCCI will be concerned, it corresponds from section 3.2 to section 3.7;
then, the relationship between heat treatment structure evolution and final mechanical
properties was discussed from section 3.8 to section 3.10; finally, in the section 3.11,
the maximum carbides size will be discussed in order to compete the size distribution
results. Meanwhile, the characteristic of different carbides types will be summarized
and classified based on the shape factor.
3.1 Phase diagram and solidification path of Ti-alloyed hypereutectic
HCCI
3.1.1 Phase diagram
The phase diagram of Ti-alloyed hypereutectic HCCI, which is based on the sample
composition given in Table 2.1, is created by thermodynamic equilibrium calculations
by using the Thermo-Calc©
software. [45, 46] The phase diagrams of the
hypereutectic HCCI with different Ti content are shown in Fig. 3.1. It was found that
the solidification was initiated with a formation of primary M7C3 carbides, which
began to precipitate at 1337°C, Thereafter, the eutectic reaction L γ+M7C3 took
-18-
Fig. 3.1. Phase diagram of the hypereutectic HCCI with (a): 0mass%, (b): 0.75mass%, (c):1.5mass%
and (d): 3.0mass% Ti addition by using Thermo-Calc calculation. A zone: LIQUID; B zone: LIQUID+
TiC (FCC_A1#2); C zone: LIQUID+M7C3; D zone: LIQUID+ γ (FCC_A1)+TiC (FCC_A1#2)+M7C3;
E zone: γ (FCC_A1)+M7C3.
place during the cooling from 1256°C to 1215°C. This is valid for the case without Ti
additions, as illustrated in Fig. 3.1(a). On the other hand, in the case with a Ti addition,
the precipitation sequence is changed as shown in Fig. 3.1(b), 3.1(c) and 3.1(d). In the
case of a 1.5mass% Ti addition, FCC_A1#2 (corresponds to the NaCl type carbides
such as TiC carbide) first precipitates at 1549°C. Then, the primary M7C3 carbides
precipitate at 1317°C. Finally, the eutectic reaction L γ+M7C3 occur from 1258°C.
Moreover, the precipitation temperature of the TiC carbide increases from 1459°C to
-19-
1734°C with an increased Ti content from 0.75mass% to 3.0mass%.
3.1.2 Solidification path
The predicted solidification path and composition evolution in the residual liquid of
hypereutectic HCCI are shown in Fig. 3.2 and Fig. 3.3.
1000 1100 1200 1300 1400 1500 16000.0
0.2
0.4
0.6
0.8
1.0
Primary M7C
3
M3C
Eutectic M7C
3
MC
FCC
Vo
lum
e f
rac
tio
n o
f p
ha
se
s,
-
Temperature, (oC)
C : 4wt%
Cr: 20wt%
Ti: 1.5wt%
PE
LIQ
Fig. 3.2. Predicted solidification path for normal composition of HCCI by PE prediction.
The solid phases precipitate in the order of MC→M7C3→FCC→M3C, as shown in
Fig. 3.2. MC carbide precipitates firstly and acts as the nuclei of the M7C3 carbide
[27]. Afterwards it forms the eutectic FCC phase and eutectic M7C3 carbide, which act
as the matrix of HCCI. Also, a M3C carbide appears at the final stage of solidification.
Its amount is so small that its effect is omitted here. Thus, the carbides including MC
and M7C3 account for approximately 33% in volume fraction of the hypereutectic
HCCI. The composition evolution in the residual liquid is closely related to the phase
-20-
0.0 0.2 0.4 0.6 0.8 1.0
0
5
10
15
20
25
Primary
Co
mp
osit
ion
in
liq
uid
, (w
t%)
Volume fraction of solid, (-)
T
C
Cr
Ni
Mo
Mn
Si
Ti
precipitation
M7C
3
precipitationMC
Eutectic
1000
1100
1200
1300
1400
1500
1600
Tem
pera
ture
, ( o
C)
C : 4wt%
Cr: 20wt%
Ti: 1.5wt%
Fig. 3.3. Variation of liquid composition during the solidification sequence for normal composition of
HCCI by PE prediction.
precipitation. As shown in Fig. 3.3, the composition of Ti in the liquid firstly
decreases largely corresponding to the first steep decrease in temperature curve. It
indicates the formation of a MC (i.e. TiC) carbide. Secondly, the Cr composition in
the residual liquid decreases greatly, relating to the second steep decrease in the
temperature curve. It is due to the formation of primary M7C3 carbide. Afterwards, the
Cr composition decreases smoothly. It indicates the formation of a FCC phase and
eutectic M7C3 carbides.
3.2 Microstructure of Ti-alloyed hypereutectic HCCI
Fig. 3.4 shows the microstructure of Ti-alloyed hypereutectic HCCI as the conditions
of as-cast and heat treatment. The solidification (as-cast) microstructure consists of
primary M7C3 carbides with a large hexagonal columnar structure, the eutectic M7C3
carbides with an irregular shape, TiC carbides with a cubic structure and a
-21-
Fig. 3.4. Microstructure of Ti-alloyed hypereutectic HCCI, (a) LOM and (b) (c) (d) SEM for carbides,
(e) and (f) EBSD for matrix
matrix containing mainly of an austenite phase (γ), as shown in Fig. 3.4(a)-(c) and (e).
After heat treatment, the secondary carbides will precipitate from austenite matrix, as
shown in Fig. 3.4(d). Meanwhile, the matrix phase will change from austenite (γ) into
martensite (α´) after heat treatment, as shown in Fig. 3.4(f).
3.3 Effect of cooling rate on as-cast structure
As mentioned in chapter 2, three kinds of cooling rates, which were examined in the
three kinds of mold, were obtained. The idea for refining the microstructure by
increasing the cooling rates is based on: that the nucleation probability of primary
M7C3 carbides increases due to the increase of the precipitation undercooling degree.
It is apparent from the photographs in Fig. 3.5(a) that in the case without Ti addition,
the size of primary M7C3 carbides decreases as the cooling rate increase. It is also
apparent from the comparison between Fig. 3.5(a) and 3.5(b) that the size of primary
M7C3 carbides in the case of a 1.5mass% Ti addition is significantly smaller than for
those in the case without a Ti addition. This is due to a precipitation of TiC carbides in
the melt. The size distribution of M7C3 carbides for different conditions was shown in
-22-
Fig. 3.5. Optical microscope observations for M7C3 carbide under different cooling rate,
(a) Ti=0mass%, (b) Ti=1.5mass%
Fig. 3.6. As shown in Fig. 3.6(a) representing a case without Ti additions, the
maximum size of primary M7C3 carbides (>11.2μm) decreases from 151.9 to 53.9μm
with an increased cooling rate (from a sand mold to a metal mold). In addition, the
number of eutectic M7C3 carbides, which in general has a size less than 11.2μm,
increases from 0.81×103 mm
-2 to 4.80×10
3 mm
-2 with an increased cooling rate.
10-1
100
101
102
103
10-1
100
101
102
103
104
4.80103
mm-2
0.81103
mm-2
53.9 um
151.9 um
Eutectic M7C
3
Ti=0%
Sand mold
Graphite mold
Metal mold
N
A, (m
m-2)
d, (m)
Primary M7C
3
Peak value
(a)
-23-
10-1
100
101
102
103
10-1
100
101
102
103
104
(b)
Peak value
53.9 um
Eutectic M7C
3
Ti=1.5%
Sand mold
Graphite mold
Metal mold
N
A,
(mm
-2)
d, (m)
Primary M7C
3
2.49103
mm-2
1.29103
mm-2
Fig. 3.6. Effect of cooling rate on M7C3 carbide size distribution of the hypereutectic HCCI with (a):
without Ti addition; (b): 1.5mass% Ti addition by using optical microscope photos.
Moreover, as shown in Fig 3.6(b) representing a case with a 1.5mass% Ti addition,
the maximum size of primary M7C3 carbides (>11.2μm) is not changed to a large
degree. However, the number of eutectic M7C3 carbides (<11.2μm) increases from
1.29×103 mm
-2 to 2.49×10
3 mm
-2 with an increased cooling rate, when a 1.5mass% Ti
addition is used.
If Fig. 3.6(a) with Fig. 3.6(b) are compared, it can be seen that the size of primary
M7C3 carbides in the case of a 1.5mass%Ti addition is significantly smaller than that
for a case without a Ti addition. Furthermore, that the maximum sizes are 53.9μm and
151.9μm respectively. Making the story short, the size of primary M7C3 carbides of
the hypereutectic HCCI can be refined both by Ti additions and by increasing the
cooling rate.
3.4 Effect of Ti addition on as-cast structure
As mentioned in chapter 1, strong carbide forming elements, such as Ti, Nb, V, Zr etc.,
will react with carbon to form NaCl structure (MC type) carbides. Those MC type
carbides can act as heterogeneous nuclei for the precipitation of M7C3 type
-24-
Fig. 3.7. SEM and EDX analysis for TiC carbide with 1.5mass% Ti addition in the graphite mold.
(Point A and C: Ti(C,N); Point B and D: (Ti, Mo)C)
carbides during the cooling and solidification of the melt. Therefore, based on this
idea, Ti was selected to study the influence on the solidification structure of HCCI. As
shown in Fig. 3.7, it is confirmed that the TiC carbides can act as a nuclei of M7C3
carbides. More specifically, it was found that TiC carbides can not only act as nuclei
Fig.3.8. Light optical microscope observations for M7C3 carbide precipitate found in the graphite mold
(a). Ti=0%, (b). Ti=0.75%, (c). Ti=1.5%, (d). Ti=3.0%.
-25-
of M7C3 carbides, but they also contain a Ti(C, N) core. This is illustrated by the black
point A, C in Fig. 3.7. Furthermore, EDX results show that the main composition
particle in B and D is Ti, Mo and C, it should be (Ti, Mo)C. However, the chemical
composition of the black points A and C shows that Ti, C and N are dominant, and
that the TiC carbides should be Ti(C, N).
Fig. 3.8 shows the micrographs solidification structure for different Ti additions. The
microstructure of M7C3 carbides is refined significantly with increasing Ti contents.
As shown in Fig. 3.9, the size distributions of M7C3 carbides also resemble a
log-normal distribution.
10-1
100
101
102
103
10-1
100
101
102
103
104
27.0 um
151.9 umEutectic M
7C
3
Graphite mold
Ti=0%
Ti=0.75%
Ti=1.5%
Ti=3.0%
N
A, (m
m-2)
d, (m)
Primary M7C
3
2.84103
mm-2
2.00103
mm-2
Peak value
Fig. 3.9. Effect of Ti addition on M7C3 carbides size distribution of the hypereutectic HCCI in the
graphite mold obtained by using optical microscope photos.
It can be seen that the maximum size of the primary M7C3 carbides decreases from
151.9 to 27.0μm with an increased Ti addition in the graphite mold. However, the size
and number of eutectic M7C3 precipitates do not change to a large degree (from
2.00×103 mm
-2 to 2.84×10
3 mm
-2) with an increased Ti content.
The back scattered electron (BSE) observations for TiC carbides are shown in Fig.
3.10. TiC carbides are mostly located inside or at the boundary of M7C3 carbides, as
shown in Fig. 3.10. Moreover, TiC carbides starts to grow and agglomerate when the
-26-
Fig. 3.10. SEM observation for TiC carbide, (a): Ti=0.75mass%, (b): Ti=1.5mass%, (c): Ti=3.0mass%.
10-1
100
101
102
103
10-1
100
101
102
103
104
Graphite mold
Ti=0.75%
Ti=1.5%
Ti=3.0%
N
A,
(mm
-2)
d, (m)
Fig. 3.11. Effect of Ti addition on TiC carbide size distribution of the hypereutectic HCCI in the
graphite obtained by using SEM photos
Ti content is 1.5mass%, as shown in Fig. 3.10(b). It is also apparent from the
photographs in Fig. 3.10(c) that larger and coarser clusters of TiC carbides exist. The
size distribution of TiC carbides in the graphite mold for different Ti additions is
-27-
shown in Fig. 3.11. These carbides generally have a size less than 10μm. Moreover, it
can be seen that both the size and number of TiC carbides increase with an increased
Ti addition.
10-1
100
101
102
103
10-1
100
101
102
103
104
Ti=1.5%
Sand mold
Graphite mold
Metal mold
N
A,
(mm
-2)
d, (m)
Fig. 3.12. Effect of cooling rate on TiC carbide size distribution of the hypereutectic HCCIs with
1.5mass% Ti addition obtained by using SEM photos
Also, Fig. 3.12 shows the size distribution of TiC carbides with 1.5mass% Ti addition
in a sand mold, a graphite mold and a metal mold. The average size of TiC carbides
with a 1.5mass% Ti addition does not practically change with an increased cooling
rate. Also, the number of TiC carbides does not change due to the increased cooling
rate. Consequently, it is clearly concluded that the precipitation of TiC carbides was
not seemingly affected by the cooling rate. Thus, it is confirmed that the cooling rate
will not affect the precipitation behavior of TiC carbides.
3.5 Effect of Ti addition and cooling rate on the volume fraction in
as-cast condition of the hypereutectic HCCI
The experiment results and the calculation results of the volume fractions of M7C3
carbides and TiC carbides are shown in Fig. 3.13.
-28-
Fig.3.13. Calculation results and experiment results on volume fraction of M7C3 carbides and TiC
carbide
It can be seen that the volume fraction of M7C3 carbides decreases with an increased
cooling rate. The condition in the low cooling rate seems to be more close to the
condition in the equilibrium state. However, the 5-10 vol.% of M7C3 carbides did not
fully precipitate in the high cooling rate conditions. Moreover, the volume fraction of
M7C3 carbides decreases with a Ti addition due to the reaction between Ti and C.
Furthermore, the volume fraction of Ti carbides increases with increased Ti additions
in the graphite mold. However, the volume fraction of TiC carbides does not change
although the cooling rate changed. If the experimental results are compared to the
-29-
calculation results, it can be seen that the tendency is the same. Moreover, that the
experimental results are closer to the PE calculation results. The agreement between
experiment and calculation results is within 5%, which is encouraging. This is due to
that the PE calculation considers the carbon back diffusion and consequently the
results will be closer to the real experimental situation.
3.6 Classification of M7C3 carbides for as-cast condition
In this study, it is suggested that the M7C3 type carbides in the HCCI can be classified
into primary M7C3 carbides and eutectic M7C3 carbides depending on the time of
precipitation. The primary carbides precipitate in the melt before solidification (that is
during cooling of the melt until the solidification temperature is reached). The eutectic
M7C3 carbides precipitate during the eutectic reaction. The precipitation possibility of
M7C3 carbides before and during solidification is estimated for the sample
composition given in Table 2.1. This was done based on PE model calculations. The
volume fraction of precipitated primary M7C3 carbides, eutectic M7C3 carbides and
1000 1100 1200 1300 1400 1500 1600 1700 18000.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
M7C
3:
Ti=0%
Ti=0.74%
Ti=1.48%
Ti=3.36%
TiC:
Ti=0.74%
Ti=1.48%
Ti=3.36%
Vo
lum
e F
racti
on
, (%
)
Temperature, (C)
Eutectic M7C
3
Primary M7C
3
TiC
Fig. 3.14. Volume fraction of M7C3 carbide and TiC carbide of the hypereutectic HCCI with different Ti
addition obtained by using PE model calculation for Fe-3.98mass%C-16.8 mass%Cr-1.86
mass%Mn-0.943 mass%Mo-1.09 mass%Ni-0.97mass% Si. "∙": transformation points of M7C3 carbide
-30-
TiC carbides are plotted against the temperature in Fig. 3.14. It can be seen that the
precipitation temperature of primary M7C3 carbides decreases with an increased Ti
content. However, the precipitation temperature of TiC carbides increases slightly
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50
5
10
15
20
25
30
35
40
(a) border size: 7.9m
Total M7
C3
PE Scheil Experiment
Total M7C
3:
Primary M7C
3:
Eutectic M7C
3:
Vo
lum
e F
rac
tio
n,
(%)
Ti, (mass%)
4.9mass%C
4.9mass%C
4.0mass%C
4.9mass%C
4.0mass%C Eutectic M7
C3
Primary M7
C3
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50
5
10
15
20
25
30
35
40
PE Scheil Experiment
Total M7C3:
Primary M7C3:
Eutectic M7C3:
Vo
lum
e F
rac
tio
n,
(%)
Ti, (mass%)
4.9mass%C
(b) border size: 11.2m
4.9mass%C
4.0mass%C
4.9mass%C
4.0mass%C
Total M7
C3
Eutectic M7
C3
Primary M7
C3
Fig. 3.15. PE model prediction and experimental results of total, primary and eutectic M7C3 carbides in
the hypereutectic HCCI with different Ti additions. M7C3 carbide’s border size: (a) d=7.9μm, (b)
d=11.2μm
-31-
with an increased Ti content.
Fig. 3.15 shows PE model prediction and experiment results of total, primary and
eutectic M7C3 carbides in the hypereutectic HCCI for different Ti additions. Here, (a)
d=7.9μm and (b) d=11.2μm were adopted as the M7C3 carbide’s border size, d.
Comparing Fig. 3.15(a) with 3.15(b), it can be seen that experimental results with a
11.2μm border size are closer to the calculation results from the viewpoint of the
primary M7C3carbides volume fraction (see ○ mark). Regarding eutectic M7C3
carbides (see ∆ mark), the experiment data are 5% lower than the calculation curve.
This might be due to the difficulty of deciding the threshold from the gray images of
the optical microscope photos. Moreover, the experimental results are close to the
Partial Equilibrium (PE) predictions. The difference between the experimental results
and the calculation results is lower than 5%. This can be explained by that the carbon
back diffusion is taken into consideration in the PE calculation. Therefore, the results
will be closer to experiment results. It is also found that the C content has a large
influence on the total volume fraction of M7C3 carbides. More specifically, the total
volume fraction increases with an increased C content, as shown in Fig. 3.15.
-32-
0 2 4 6 8 10 30 40 50 60 70 80 90 1000
1
2
3
4
30
40
50
60
70
Co
mp
os
itio
n o
f c
arb
ide
, (a
t%)
(b):
Graphite mold
d, (um)
Fe
Cr
Ti
Mo
Mn
Primary M7C
3Eutectic M7C
3
0 2 4 6 8 10 30 40 50 60 70 80 90 1000
1
2
3
4
30
40
50
60
70
Co
mp
os
itio
n o
f c
arb
ide
, (a
t%)
(c):
Sand mold
Fe
Cr
Ti
Mo
Mn
d, (um)
Primary M7C
3Eutectic M
7C
3
Fig. 3.16. Relationship between composition and size of primary M7C3 carbides and eutectic M7C3
carbides of the hypereutectic HCCI with 1.5mass% Ti addition under different cooling rate: (a) Metal
mold, (b) Graphite mold and Sand mold.
In summary, Fig. 3.15 shows that the total volume fraction of M7C3 carbides
decreases as the Ti content increases; the volume fraction of primary M7C3 carbides
decreases with an increased Ti content. However, the volume fraction of eutectic
M7C3 carbides increases with an increased Ti content. The contents of Fe, Cr, Ti, Mo,
-33-
Mn in primary M7C3 carbides and eutectic M7C3 carbides are plotted against the
carbide sizes in Fig. 3.16 for the metal mold, graphite mold and sand mold for a
1.5mass% Ti addition. In general, it is seem that the content of Cr and Ti increases
with an increased M7C3 carbide size, d. However, the content of Fe and Mo decreases
with an increased M7C3 carbide size; Moreover, the content of Mn does not change
with an increased M7C3 carbide size for the sand, graphite and metal molds. It is
concluded that the primary M7C3 carbides are dominant at higher Cr contents, and that
the eutectic M7C3 carbides have lower Cr contents.
In summary, based on the analysis of size distribution and chemical composition of
M7C3 carbides, the M7C3 carbides were roughly classified into primary M7C3 carbides,
which in general have size larger than 11.2μm. Moreover, into eutectic M7C3 carbides
which in general have a size smaller than 11.2μm.
3.7 Effect of cooling rate and Ti addition on the mechanical properties
in as-cast condition of the hypereutectic HCCI
In this section, the effect of cooling rate and Ti addition on the mechanical properties
of the hypereutectic HCCI is discussed, but only for as-cast conditions. The bulk
hardness of the hypereutectic HCCI is determined by the hardness of carbides, such as
TiC and M7C3 [21] and the matrix. The matrix of the hypereutectic HCCI will
obviously not change by a Ti addition. This is due to that it will mainly exist as a
single austenite phase (γ-phase) together with small amounts of martensite (α´-phase)
under as-cast conditions. Therefore, the bulk hardness of the hypereutectic HCCI is
mainly affected by the volume fraction of carbides. Fig. 3.17 shows the bulk hardness
of the hypereutectic HCCI with different Ti additions for different cooling rates. The
tendency is that the bulk hardness increases slightly with increased cooling rates and
titanium contents. As shown in Fig. 3.13, when Ti is added, the volume fraction of
TiC carbides increase and the volume fraction of M7C3 carbides decrease. TiC
carbides might make a bigger contribution to the bulk hardness than M7C3 carbides,
because the hardness of TiC carbides is in the range of 2000-3200HV. [4] However,
-34-
Fig 3.17. Mechanical properties of the hypereutectic HCCI with different Ti additions and different
cooling rates.
the bulk hardness of the hypereutectic HCCI in the graphite mold shows a big scatter
because the TiC carbides are not uniformly distributed. The wear loss of the
hypereutectic HCCI is determined by the amount of matrix and M7C3 carbides,
although the hardness of TiC carbides is larger than that of the M7C3 carbides and the
matrix. However, the presence of a small amount of TiC carbides will not contribute
to the wear loss. It can be seen that there is a tendency of an increased wear loss with
an increased Ti content in the graphite mold experiments, as shown in Fig. 3.17. This
is due to the decrease of the volume fraction of M7C3 carbides. It seems that the
austenite phase (γ-phase) which surrounds the agglomerated TiC might be easily
peeled off during the wear application process. This is leads to an increased Ti content.
In summary, the hardness and wear loss of the hypereutectic HCCI do not change
significantly with increased Ti contents and cooling rates under as-cast conditions.
As a summary for as-cast results, the mainly focus point on the carbides (M7C3 and
TiC) evolution is quantitative discussed by changing the cooling rates and titanium
contents. Base on this work, the heat treatment process can be performed in order to
improve the mechanical properties of the hypereutectic HCCI.
-35-
3.8 Evolution of matrix after heat treatment
From this section, we will set our focus point on how the matrix and the carbides
(M7C3 and TiC) evolution and their influence on the mechanical properties though
heat treatment process. As mentioned in the chapter 2, the specimens, which were
casted in a graphite mold with a 1.5mass% titanium addition, were used for heat
treatments. The reason for the casting process and selected chemical composition
(Table 2.1) is explained in detail in supplement Ⅲ. Firstly, the matrix evolution after
heat treatment will be introduced. The matrix of hypereutectic HCCI includes both
austenite and martensite before and after heat treatment, as shown in Fig. 3.18 and
Fig. 3.19. However, the austenite is dominating in the as-cast condition and the
martensite dominates after heat treatment. EBSD phase maps with 0.1μm and 0.03μm
step sizes are shown in Fig. 3.18 and Fig. 3.19, respectively. It is clear that the matrix
essentially consists of austenite with a small amount of martensite in the as-cast
condition. The inverse pol figure (IPF) map in Fig. 3.19 shows the orientation of the
martensite units, and it is clear that the martensite unit size differs between the
different heat treatment conditions.
Fig. 3.18. EBSD phase colored maps using 0.1μm step size. (Color in phase maps: Blue: M7C3; TiC:
Yellow; Green: Austenite; Red: Martensite; Greyscale: band contrast)
-36-
Fig. 3.19. EBSD phase colored and inverse pole figure (IPF for martensite) colored maps using
0.03μm step size. (Color in phase maps: Blue: M7C3; TiC: Yellow; Green: Austenite; Red: Martensite;
Greyscale: band contrast)
The size distribution of martensite units in as-cast condition and after the different
heat treatments is shown in Fig. 3.20. It shows the results when combining data from
the 0.03μm and 0.1μm step sizes. These are also plotted with a logarithmic scale.
From Fig. 3.20, it is seen that large martensite units above 4μm is present under the
conditions of high temperatures or long holding time: 900oC×6hr, 1000
oC×6hr and
1050oC×6hr. Compared to the higher temperatures, the size of the martensite unit
-37-
10-2
10-1
100
101
102
103
100
101
102
103
104
105
106
107
NA
, (m
m-2
)
D, (m)
Martensite:
As cast
900oC2hr
1000oC2hr
1050oC2hr
900oC6hr
1000oC6hr
1050oC6hr
0.1m step0.03m step
Fig.3.20. Size distribution of martensite unit.
decreased at lower heat treatment temperatures. The volume fraction of martensite is
shown in Fig. 3.21. The EBSD maps, which were used for calculating the volume
fraction of martensite includes the non-indexed regions. The volume fraction of
martensite calculated by using the 0.1μm and the 0.03μm step size EBSD maps give
0 2 4 6 80
10
20
30
40
50
60
70
Minimum
Martensite
0.1m step:
As cast
900oC
1000oC
1050oC
Maximum
Vo
lum
e F
racti
on
, (%
)
Time, (hr)
0.03m step:
As cast
900oC
1000oC
1050oC
Fig. 3.21. Volume fraction of martensite
-38-
quite different results, as shown in Fig. 3.21. Thus, from the viewpoint of size
distribution and volume fraction of martensite, the formation behavior of martensite is
summarized as follows:
(1) At longer holding times during heat treatment, such as 6hr, the volume fraction of
martensite increases, compared to shorter holding time.
(2) At long holding times, such as 6hr, a lower holding temperature results in a more
refined martensite.
(3) Thus, from the viewpoint of the transformation promotion of martensite (volume
fraction) and the refinement of martensite unit (size distribution), longer holding time
such as 6hr and lower holding temperatures such as 900oC are recommended.
Fig. 3.22. SEM observations before heat treatment (a, c and e) and after heat treatment (b, d and f) at
the same position.
-39-
3.9 Evolution of carbides after heat treatment
Fig. 3.22 shows SEM observations from the same position before and after heat
treatment. The size of primary M7C3 carbides and eutectic M7C3 carbides increases at
higher temperatures and longer holding times, as shown in Fig. 3.22(c)-(f). The
primary and eutectic M7C3 carbides grow and finally merge with each other, as
marked by A and A in Fig. 3.22 (c) and (d). This is also indicated by the length of
mark B and B in Fig. 3.22 (e) and (f), which shows that the distance decreases from
9.0μm to 5.3μm. Compared to the microstructure before heat treatment, it can be seen
from Fig. 3.22(b) that the secondary carbides precipitate from the matrix during heat
treatment. The following section 3.9.3 will discuss the secondary carbides in detail
after heat treatment.
3.9.1 Evolution of primary and eutectic M7C3 carbides after heat
treatment
The carbides mainly include primary M7C3 carbides and eutectic M7C3 carbides both
before and after heat treatment, as shown in Fig. 3.18 and Fig. 3.22. It is also noticed
Fig.3.23. SEM observations of M7C3 and TiC after different heat treatments.
-40-
based on SEM and EBSD observations that fine secondary carbides precipitate in the
matrix after heat treatment, as shown in Fig. 3.4(d) and Fig. 3.23. A detail information
regarding secondary carbides will be described in section 3.9.3. The size distribution
of M7C3 carbides from SEM and EBSD are shown in Fig. 3.24(a) and (b). From Fig.
3.24(a) and (b), it is possible to clearly recognize the precipitation of secondary
carbides (<1μm) after heat treatment. In order to understand the precipitation behavior
of M7C3 carbides, besides the size distribution, the total carbide volume fraction has
to be taken into consideration.
10-1
100
101
102
103
100
101
102
103
104
105
M7C
3:
As cast
900oC2hr
1000oC2hr
1050oC2hr
900oC6hr
1000oC6hr
1050oC6hr
Eutectic M7C
3
NA
, (m
m-2
)
d, (m)
Secondary M7C
3Primary M
7C
3 (a)
10-1
100
101
102
103
100
101
102
103
104
105
M7C
3:
As cast
900oC2hr
1000oC2hr
1050oC2hr
900oC6hr
1000oC6hr
1050oC6hr
Eutectic M7C
3
NA
,(m
m-2
)
d, (m)
Secondary M7C
3Primary M
7C
3(b)
Fig. 3.24. M7C3 carbide size distribution in the hypereutectic HCCI by using (a): SEM images, (b):
EBSD maps
-41-
The total volume fraction of M7C3 carbides (primary, eutectic and secondary)
obtained from SEM images and EBSD maps using image analysis is shown in Fig.
3.25.
Fig. 3.25. Volume fraction of M7C3 and TiC carbides
From the size distribution in Fig. 3.24 and the total carbide volume fraction in Fig.
3.25, the precipitation behavior of M7C3 carbides is summarized as follows:
(1) For lower holding temperatures and shorter holding times during the heat
treatment, such as 900oC×2hr, the number of fine secondary M7C3 carbides (less than
1μm) and eutectic M7C3 carbides (less than 11.2μm) increases. On the contrary, at
higher holding temperatures and longer holding times of heat treatment, such as
1050oC×6hr, the size of coarse primary M7C3 carbides larger than 11.2μm increases.
(2) This behavior is due to the following reasons. The precipitation of secondary
M7C3 occurs actively at 900oC close to the eutectic temperature and finishes
completely within 2hr. The coarsening of all sizes of M7C3 carbides occur actively at
higher holding temperatures and longer holding times during heat treatment.
-42-
(3) Thus, from the viewpoint of the precipitation of M7C3 (volume fraction) and the
growth control/refinement of M7C3 (size distribution), a lower holding temperature
such as 900oC is preferred. This is regardless of the holding time during heat
treatment.
(4) According to the reports of Wiengmoon, et al. [16, 22, 23] and Pearce, [47] in cast
irons with higher Cr contents, such as 30mass% Cr, the secondary carbides were of
M7C3 type or M23C6 type. These precipitate during conventional heat treatment, as
shown in Table 3.1, however, only the M7C3 type carbides were examined based on
the EBSD results in the present work.
Table 3.1 Secondary carbide type under different C and Cr content
Type C (mass%) Cr (mass%) References
M7C3 type
2.8 18 S K. Hann, et al
2.4 30 J.T.H. Pearce,
M23C6 type
2 26 E. Pagounis, et al
2.4 30 A. Wiengmoon, et al
1.09 10.48 Y. Wang, et al
3.9.2 Evolution of TiC carbides after heat treatment
The TiC carbides with a high precipitation temperature are uniformly distributed
within the M7C3 carbides and the matrix, as shown in Fig. 3.18 and Fig. 3.23.
The size distributions of TiC in as-cast condition and after various heat treatments are
shown in Fig. 3.26(a) and (b). Based on the volume fraction in Fig. 3.25 and the size
distribution in Fig. 3.26, the precipitation behavior of TiC can be summarized as
follows:
(1) At longer holding times of heat treatment, such as 6hr, the number of all sizes of
TiC carbides increases.
-43-
10-1
100
101
102
103
100
101
102
103
104
105
TiC:
As cast
900oC2hr
1000oC2hr
1050oC2hr
900oC6hr
1000oC6hr
1050oC6hr
N
A, (m
m-2
)
d, (m)
(a)
10-1
100
101
102
103
100
101
102
103
104
105
(b)
NA
, (m
m-2
)
d, (m)
TiC:
As cast
900oC2hr
1000oC2hr
1050oC2hr
900oC6hr
1000oC6hr
1050oC6hr
Fig. 3.26. TiC size distribution of hypereutectic HCCI by using (a): SEM images and (b): EBSD maps
(2) This behavior is caused due to the following reasons. The precipitation of TiC
occurs mainly in the melt and finishes completely at higher temperatures above
1100oC. Moreover, the coarsening of all sizes of TiC particles occurs actively under
longer holding times during heat treatment.
(3) Thus, from the viewpoint of the precipitation of TiC (volume fraction, size and
number), longer holding time such as 6hr is preferred, regardless of the holding
temperature level, as the heat treatment condition.
-44-
3.9.3 Evolution of secondary carbides during heat treatment
3.9.3.1 Identification of secondary carbides
Fig. 3.27 shows a TEM micrograph, the selected area diffraction pattern (SADP) and
the EDS results for a secondary carbide in the condition of 890oC×6hr. It shows that
the secondary carbides have a size of ~250nm in diameter. The SADP in Fig. 3.27(b)
form the particle, marked in Fig. 3.27(a), is indexed to be a M7C3 carbide type in the
Fe-4 mass%C-17 mass%Cr-1.5 mass% Ti hypereutectic HCCI alloy. It is also confirm
the EBSD identification regarding the secondary carbides. In addition, the EDS result
from the secondary M7C3 carbide also shows that the composition of this particle is
rich in Cr, Fe and C, as is shown in Fig. 3.27(c). It is different from the matrix
(Martensite, α´), which is only Fe rich, as is shown in Fig. 3.27(d).
Fig. 3.27. TEM micrograph of secondary carbide in a condition of (890°C×6h): (a) a bright–field TEM
micrograph and (b) a selected area diffraction pattern (SADP) of secondary M7C3 carbide and (c) an
EDS analysis of a secondary M7C3 carbide and (d) an EDS analysis of matrix .
-45-
3.9.3.2 In-situ observations of secondary M7C3 carbide precipitation
The images were extracted from a series of CLSM video sequences at a rate of 30
frames per second during the whole heat treatment. Fig. 3.28 illustrates the in-situ
observations of the microstructure evolution in the Ti-alloyed HCCI heated to 890°C
Fig. 3.28. In-situ observations of secondary carbide precipitations during the heat treatment process by
using a CLSM to study sample for an 890°C×2hr condition.
-46-
and held there for 2hr with a heating rate 10°C/min. The corresponding temperature
and time during the heat treatment are also displayed in the images. It was found that
the ferrite (bcc, α) start to form from the matrix at approximately 575°C during the
heating process, as is shown in Fig. 3.28(b). The ferrite (bcc, α) phase should be
stable at low and intermediate temperatures as determined from the phase diagram for
a Fe-17 mass% Cr- 4 mass% C- 1.5 mass% Ti alloy, which was calculated in a
supplement Ⅲ. It is the retained austenite (fcc, γ) that starts to transforms into ferrite
(bcc, α) at 575°C. The ferrite will disappear again when approaching the holding
temperature where austenite (fcc, γ) is the stable phase. The temperature where the
ferrite austenite transformation takes place is approximate 703°C, which is
estimated by the phase diagram of this alloy. In addition, the volume of the ferrite
increased with an increased temperature, as shown in Fig. 3.28(c). Moreover, at
666°C, it seems that some precipitates start to appear from surface of primary M7C3
carbides, as shown in Fig. 3.28(c). These precipitates increased with an increased
temperature. However, the precipitates inside the samples were polished and studied
carefully by using SEM after the heat treatment in Fig. 3.23 and none of precipitates
were found on the surface of primary M7C3 carbides. Hence, this indicates that the
observed feature on the primary M7C3 carbides is a surface phenomenon. Furthermore,
at 840°C, the precipitates start to appear from the matrix as shown in Fig. 3.28(d) and
the matrix phase is austenite (fcc, γ) according to the phase diagram. From Fig.
3.28(e)-(h), it can be seen that the number of precipitates increased and the sizes
became larger with an increased time and temperature during the holding and cooling
process. The increasing number of precipitates during the heat treatment is illustrated
in Fig. 3.29. The total area for the counted precipitate number is 4856.0μm2 (73.8μm
(L) × 65.8μm (W)). It was found that the precipitation starts at approximately 840°C
and 847°C for the 890°C×2hr and 1026°C×2hr conditions, respectively. Moreover, the
number of precipitates increased sharply at these temperatures. It was also found that
the precipitate number per unit area did not change too much when the temperature
reached the holding temperatures. Comparing the two different temperatures, the
-47-
0 20 40 60 80 100 120 140 160 180 200 220 240 260
104
0.8
1.6
2.4
3.2
4.0
4.8
890C2hr
52C1026C1026C
847C840C
876C
Time, (min)
NA
, (m
m-2)
HoldingHeating
55C
700C
890C
890C890C
849C
890C890C
Cooling
6.4
0
5.6 1026C2hr
1026C1026C1026C
Fig. 3.29. Change of number of secondary carbides with time and temperature during heat treatment
for samples with 890°C×2hr and 1026°C×2hr condition based on in-situ observation results.
number of secondary carbides is higher for 890°C than for 1026°C.
Fig. 3.30 shows the evolution of the size distribution of secondary carbides during
heat treatment with holding temperatures of 890°C and 1026°C. In this section, only
the secondary carbides, which are larger than 1μm, are discussed because of a limited
resolution of the CLSM. The smaller secondary carbides (<1μm) will be discussed in
detail in section 3.9.3.3. From Fig. 3.30(a), it was found that the number of secondary
carbides increased with an increased heating temperature. Moreover, the number of
carbides did not change to a large extent during the holding and cooling stage.
However, the number of secondary carbides decreased with increased holding and
cooling times. It can also be seen that the size of the secondary carbides increases
from the heating stage to the cooling stage. Compared to the 890°C×2hr condition, the
same tendency was obtained for a 1026°C×2hr condition, as seen in Fig. 3.30(b).
However, it was found that the number of secondary carbides is lower for a
1026°C×2hr condition compared to a 890°C×2hr condition.
-48-
10-2
10-1
100
101
101
102
103
104
105
106
Resolution limited zone
890C2hr:
Cooling
Holding
d, (m)
NA, (m
m-2)
79min 840C
80min 849C
85min 890C
205min 890C
220min 55CCooling:
Holding:
Heating:
Heating
(a)
10-2
10-1
100
101
101
102
103
104
105
106
d, (m)
NA, (m
m-2)
80min 847C
85min 1026C
205min 1026C
220min 52CCooling:
Holding:
Heating:
Cooling
Holding
Heating
1026C2hr: (b)
Resolution limited zone
Fig. 3.30. Size distribution of secondary carbides determined in-situ observation by using CLSM for
the following conditions: (a) 890°C×2hr and (b) 1026°C×2hr.
Fig. 3.31 shows the evolution of volume fraction of secondary carbides, which was
obtained from the in-situ observation. The volume fractions of secondary carbides
increase gradually with an increased heat treatment time. In addition, it was found that
the volume fraction of secondary carbides at lower temperatures is a little bit higher
-49-
0 20 40 60 80 100 120 140 160 180 200 220 240 2600
5
10
15
20
25
30
35
40
1026C1026C
1026C
890C2hr
840C
55C
700C890C890C890C
849C
890C890C
CoolingHolding
Vo
lum
e F
rac
tio
n,
(%)
Time, (min)
Heating
52C1026C1026C
1026C2hr
Fig. 3.31. Volume fraction of secondary carbides as a function of time determined from in-situ
observation by using CLSM.
0 2 4 6 8 100
5
10
15
20
25
30
35
40
Vo
lum
e f
racti
on
, (%
)
Time, (hr)
M23
C6
M7C
3
M23
C6
M23
C6
M7C
3
M7C
3
2.91 26.96 SEM
Wiengmoon,
et al[24]
Carbides
type
dataCLSM
References Content Method Temperature
C(%) Cr(%) 900oC 1050
oC
Present work 4 17 SEM, CLSM
2.3 30 SEM
Wiengmoon,
et al[16] [18]
Wiengmoon, 2.72 26.6 SEM
et al[25]
3 20
Fig. 3.32. Comparison of the volume fraction of secondary carbides between data from present work
and data from Wiengmoon, et al [16, 18, 24, 25].
than that at higher temperatures. However, the volume fraction will not change to a
large extent (5%±2%) when the holding time and holding temperature change after
holding for 2hr, as shown in Fig. 3.32.
-50-
As a confirmation of the results in the present study, data from Wiengmoon, et al. [16,
22, 24, 25,] were used as a comparison. The same tendency between the present
results and Wiengmoon, et al. [16, 22] results can be found. However, compared to
the present study, the volume fraction of secondary carbide is much higher in
Wiengmoon, et al. studies. [16, 22] One possible reason for this difference is that the
Cr content is higher in the Wiengmoon, et al. study. [16, 22] Furthermore, the volume
fraction values (~5%) in this study agree with the Wiengmoon, et al. results [23, 25].
3.9.3.3 Size distribution of secondary carbides
Fig. 3.33 shows the secondary carbides for different heat treatment conditions
observed in SEM. It can be seen that the number of secondary carbides at lower
Fig. 3.33. SEM observations of secondary M7C3 carbides for the following heat treatment conditions:
(a) 890°C×2hr, (b) 1026°C×2hr, (c) 890°C×6hr and (d) 1026°C×6hr.
temperatures such as 890°C (Fig. 3.33(a) and (c)) is higher than that at higher
temperatures such as 1026°C (Fig. 3.33 (b) and (d)). In addition, the size of secondary
carbides seems to increase with an increased holding temperature and time.
-51-
10-2
10-1
100
101
101
102
103
104
105
106
890C2hr
1026C2hr
890C6hr
1026C6hr
SEM CLSM
d, (m)
NA, (m
m-2)
CLSMSEM
Present work(a)
10-2
10-1
100
101
101
102
103
104
105
106
900C 1025CRef. :2.3% C-30% Cr
d, (m)
NA, (m
m-2)
Wiengmoon, et al [22]
SEM
950C 1025C
Wiengmoon, et al [16]
(b) References
Fig. 3.34. Comparison of the size distribution of secondary M7C3 carbides. (a) data from present work
and (b) data analyzed based on SEM images from Wiengmoon, et al. [16, 22].
Fig. 3.34(a) shows the combination size distribution of secondary carbides results
from the SEM study after heat treatment and in-situ observation by using CLSM in
this study. It was found that the number of secondary M7C3 carbides per unit area
increases with a decreased temperature (as 890°C×2hr). Meanwhile, compared to a
lower holding temperature and a shorter holding time (as 890°C×2hr), the size of
-52-
secondary M7C3 carbides increases with increased holding times and holding
temperatures (as 1026°C×6hr). Furthermore, SEM micrographs from the literatures
[16, 22] were studied by using an image analyzer and the size distribution results are
shown in Fig. 3.34(b). The total area, Sobs, which were taken based on the SEM
images from literatures, [16, 22] were 181.9μm2 and 81.2μm
2 respectively. Compared
to the data in Fig. 3.34(a), it can be seen that these previous results (order of size and
number) agree well with the results of the present study. The number of secondary
carbides is high at a lower temperature (as 900°C). Moreover, the size of secondary
carbides is large at higher temperature (as 1025°C).
In a short summary of secondary M7C3 carbides, the results from this section show
that a lower temperature and a shorter holding time, as 890°C×2hr, is preferred to
precipitate secondary M7C3 carbides.
3.10 Mechanical properties improved by heat treatment
Previous studies on the effect of a matrix structure on the wear resistance suggest that
a martensitic matrix offers a better wear resistance than an austenitic matrix. [48, 49]
It is also reported [1, 2] that two competing effects will influence the final properties.
These effects are (a) a high carbon martensite phase, which forms at higher
temperatures tends to increase the hardness, and (b) the increasing retained austenite
content at higher temperatures, tends to reduce the hardness. Fig. 3.35 gives the
correlation between hardness and wear resistance in as-cast condition and after heat
treatment. It is seen that both the hardness and the wear resistance is improved
markedly after heat treatment. In addition, the wear resistance increases with an
increased holding time from 2hr to 6hr. However, both the hardness and wear
resistance decrease with an increased holding temperature from 900°C to 1050°C. In
-53-
48 50 52 54 56 58 60 62 64 660.30
0.32
0.34
0.36
0.38
0.40
0.42
0.44
0.46
0.48
0.50
0.52
As cast
900C,1000C,10502hr
900C,1000C,10506hr
We
ar
los
s (
g/m
in)
Hardness (HRC)
900C6hr
1000C6hr
1050C6hr 900C2hr
1050C2hr
1000C2hr
As cast
Fig. 3.35. Mechanical properties of the hypereutectic HCCI.
the as-cast condition, the matrix almost consists solely of austenite. Clearly the
hardness of austenite is lower than that of martensite. By considering the results of
volume fraction and size distribution of M7C3, TiC and martensite units in sections
3.8 and 3.9 on the observed properties in Fig. 3.35, the mechanism of improved
mechanical properties was deduced as follows:
(1). Longer holding time leads to the transformation promotion of martensite (volume
fraction increase) as well as the refinement of martensite units (size distribution).
Consequently, longer holding times will increase the wear resistance, due to the
altered matrix toughness and the decreased hardness, which is due to the coarsening
of M7C3.
(2). Meanwhile, longer holding time leads to the growth promotion of TiC (volume
fractions) and the increase number of TiC carbides. Therefore, longer holding times
will also increase the wear resistance from the view point of the precipitation of TiC
carbides (volume fraction, size and number).
-54-
(3). In addition, a lower holding temperature leads to precipitation promotion of M7C3
(volume fraction) and the growth control/ refinement of martensite unit (size
distribution). Consequently, lower holding temperatures will generate an increased
wear resistance and hardness due to the fine dispersions of M7C3 carbides.
As a summary, the relationship between carbide precipitation, martensite units and
mechanical properties are discussed base on the volume fraction and the size
distribution of carbide precipitates (M7C3 and TiC) as well as martensite units.
Possible mechanism is proposed to explain the improvement of mechanical properties
through the heat treatment process. And the best heat treatment strategy with a lower
holding temperature close to eutectic temperature and a longer holding time, such as
900oC×6hr, is the most suitable heat treatment conditions for improving the wear
resistance properties of Ti-alloyed hypereutectic HCCI.
3.11 Estimation of the maximum carbide size by using SEV method
In this section, in order to obtain complete size distribution results, the maximum
carbide size in a hypereutectic HCCI produced with different cooling conditions,
titanium additions and heat treatment conditions was estimated by using the statistics
of extreme values (SEV) method. The experiments conditions and prediction results
for all carbides are summarized as table in detail in supplement Ⅴ.
3.11.1 Estimation of maximum primary M7C3 carbides size
Fig. 3.36 and Fig. 3.37 are plots of the size (dAmax) versus the reduced variate (yi)
from six groups samples. These extreme value distribution (EVD) figures confirm
that a linear relationship holds between the size of the maximum carbides and the
reduced variate. The reduced variate of each size data, yi, was calculated by using the
following equation:
ln ln( )1
i
iy
n
(1≤i≤n) (3-1)
n is the number of a unit volume or a unit area. It was found that the sand mold had
-55-
0 50 100 150 200 250 300-2
-1
0
1
2
3
4
5
6
Primary M7C
3 carbides:
SEV method
Ti=0%
Re
du
ce
d v
ari
ate
, y
i in
Eq
. (3
-1)
Sand mold
Graphite mold
Metal mold
dAmax, (m)
X fit
x high
x low
Fig. 3.36. Comparisons of the EVD for primary M7C3 carbides under different casting molds without
titanium addition.
0 50 100 150 200 250 300-2
-1
0
1
2
3
4
5
6
Primary M7C
3 carbides:
SEV method
Ti=0%
Ti=0.75%
Ti=1.5%
Ti=3.0%
Re
du
ce
d v
ari
ate
, y
i in
Eq
. (3
-1)
dAmax, (m)
Graphite mold
X fit
x low
x high
Fig. 3.37. Comparisons of the EVD for primary M7C3 carbides with different titanium additions in the
graphite mold
the slowest cooling rate, as expected. This means that the hypereutectic HCCI alloyed
has the largest primary M7C3 carbides and the smallest slope of the EVD regression
-56-
lines among the studied molds. The latter is especially true for carbide sizes larger
than 150μm, as seen in Fig.3.36. In addition, for the smaller size carbides, such as for
1.5mass% and 3.0mass% titanium additions have a similar slope of the EVD
regression lines, as seen in Fig. 3.37. It s likely this is due to the different growing
mechanisms. In present study, the shape factor, Circularity, was calculated as follows
in order to classify the type of carbides:
2
4 ACircularity
p
(3-2)
where A is the area of the shape and p is the perimeter. Fig. 3.38 shows a typical size
distribution of primary M7C3 carbides circularity versus the dAmax value for different
molds and titanium additions.
Fig. 3.38. Typical size distribution of primary M7C3 carbides circularity versus the dAmax value for
different molds and titanium additions
It was found that the circularity value of primary M7C3 carbides was in the range of
-57-
0.2-0.7. Moreover, that the circularity value is decreased slightly with an increased
cooling rate. However, the circularity values are not change too much in case of
different titanium additions.
In summary, the circularity value of maximum primary M7C3 carbides, which have
the typical hexagonal shape, is approximately 0.43, as shown in Fig. 3.38.
3.11.2 Estimation of maximum TiC carbides size
Fig. 3.39 gives an extreme value distribution (EVD) for experiments using the
graphite mold and different titanium contents. It confirms that a linear relationship
holds between the size of the maximum TiC carbide and the reduced variate.
Moreover, it was found that the tendency plots for a 3.0% titanium condition was
suddenly changed in the region of a reduced variate value of yi >2.5 and for a dAmax
0 10 20 30 40-2
-1
0
1
2
3
4
5
6
SEV method
Graphite mold
TiC carbides:
Ti=0.75%
Ti=1.5%
Ti=3.0%
Re
du
ce
d v
ari
ate
, y
i in
Eq
. (3
-1)
dAmax, m)
x fit
X low
X high
Fig. 3.39. Comparisons of EVD for TiC carbides with different titanium additions for graphite Mold
value >15μm, as shown in Fig. 3.39. It was believed that is due to the different shapes
and sizes of TiC carbides. It can be seen in Fig. 3.40 that the circularity value of TiC
carbides decrease from 0.45 to 0.16 as the titanium content is increased from 0.75% to
3.0%. In addition, the difference circularity value of TiC carbides is due to the shape
-58-
change of TiC carbides. Fig. 3.40 also shows that the typical single TiC carbides have
a cubic shape, which circularity value is about 0.45 for the case with a 0.75% titanium
addition. However, the single TiC carbides seem to be adhered to each other and start
to form cluster for the experiments with a 1.5% titanium addition. The circularity
value is around 0.36. Furthermore, many TiC carbides aggregate with each other and
form big cluster for the experiments with an addition of 3.0% titanium. Then, the
circularity value is reduced to 0.16.
Fig. 3.40. Typical size distribution of TiC carbides circularity versus the dAmax value for different
titanium additions.
3.11.3 Estimation of maximum secondary M7C3 carbides size
Fig. 3.41 shows the extreme value distribution (EVD) for different heat treatment
conditions. It confirm that a linear relationship holds between the size of the
maximum secondary M7C3 carbide and the reduce variate. In general, it was found
that the slope of the EVD regression lines is not changed by changing the heat
-59-
0 1 2 3 4 5 6-2
-1
0
1
2
3
4
5
6
Ti=1.5%
SEV method
Secondary carbides:
Graphite mold
890oC2hr
1026oC2hr
890oC6hr
1026oC6hr
Re
du
ce
d v
ari
ate
, yi
in E
q.
(3-1
)
dAmax, (m)
X fit
X low
X high
Fig. 3.41. Extreme value distribution (EVD) of secondary M7C3 carbides for different heat
treatment conditions.
Fig. 3.42. Typical size distribution of secondary M7C3 carbides circularity versus the dAmax values for
different heat treatment conditions.
treatment conditions, as shown in Fig. 3.41. However, in the region of a reduced
variate value yi>1.5 and for a dAmax>2μm, it was found that the tendency plots were
-60-
0 20 40 60 80 100-2
-1
0
1
2
3
4
5
6
SEV method Graphite mold
Ti=1.5%
R
ed
uc
ed
va
ria
te, y
i in
Eq
. (3
-1)
dAmax, m)
Secondary M7C
3 carbides
TiC carbides
Primary M7C
3 carbides
X fit
X low
X high
Fig. 3.43. Comparisons of the EVD of primary M7C3 carbides, TiC carbides and secondary M7C3
carbides for graphite mold with 1.5% titanium addition.
a little bit changed for the 1026oC×2hr/6hr conditions. In addition, the shape of the
secondary M7C3 carbide is rod-like and irregular and the mean circularity value is in
range of 0.17-0.35, as shown in Fig. 3.42.
A summary of the extreme value distribution (EVD) among primary M7C3 carbides,
TiC carbides and secondary M7C3 carbides for a case with a 1.5% titanium addition
and casting in a graphite mold is shown in Fig. 3.43. The slope of the regression line
for these three carbide types is different. The reasons for this discrepancy in the EVD
value depend on their characteristic, such as the shape factor. It was found that the
typical morphology of primary M7C3 carbides, TiC carbides and secondary M7C3
carbides is of a hexagonal shape, a cubic shape and an irregular shape, respectively.
Moreover, the corresponding circularity value for three carbide types is quite different.
Compared to the circularity value of TiC carbides and secondary M7C3 carbides, the
circularity value of primary M7C3 carbides is larger (about 0.43) than the one for TiC
carbides and secondary M7C3 carbides. A comparison of the estimated and observed
-61-
0 50 100 150 200 250 3000
50
100
150
200
250
300
Ti=1.5%moldGraphite
Ti=0%
Graphite mold
Metal mold
1/1
Primary M7C
3 carbides
Es
tim
ate
d m
ax
imu
m s
ize
, (
m)
Observed maximum size, (m)
Ti=0%Sand mold
Ti=0%
Graphite moldTi=0.75%
Graphite
Ti=1.5%
Ti=3.0%
mold
TiC carbides
Secondary M7C
3 carbides
Fig. 3.44. Comparisons of the estimated and observed maximum size of primary M7C3 carbides, TiC
carbides and secondary M7C3 carbides. Note: The error bar is standard error.
maximum size of primary M7C3 carbides, TiC carbides and secondary M7C3 carbides
for the different experimental conditions is shown in Fig. 3.44. It can be seen that the
estimated maximum size of primary M7C3 carbides are larger than the observed ones.
This is especially true for the larger primary M7C3 carbides, such as in the sand and
graphite mold experiments without titanium additions. Moreover, the estimated
maximum size shows a large error bar (18.9μm in sand mold). However, the smaller
carbides, such as primary M7C3 carbides in the graphite mold with 1.5% and 3.0% Ti
additions, TiC carbides and secondary M7C3 carbides show a smaller error. The errors
are 0.92μm-2.13μm for TiC carbides and 0.17μm-0.5μm for the secondary M7C3
carbide, respectively. The predicted smaller carbides with small errors result in agree
well with the observed one, as shown in Fig. 3.44.
-62-
In summary, the complete size distribution results can be obtained base on the
maximum carbides size analysis. In addition, the characteristic of the different
maximum carbide types are summarized and classified based on the shape factor.
Then, based on a combination of a size distribution analysis and a maximum size
analysis, the relationship between the carbide size distribution including the
maximum carbides size and the mechanical properties can be discussed.
-63-
Chapter 4
Concluding Discussion
This work has had the aim to develop superior abrasion resistant castings. Thus, a
systematic research has been carried out to study the solidification and heat treatment
of hypereutectic High Chromium Cast Iron (HCCI) containing mainly
Fe-17mass%Cr-4mass%C. The work, which is presented in Supplement I, started
with the aim to refine the carbides size and improve the mechanical properties of
hypereutectic HCCI alloyed with Ti. In Supplement I, the primary M7C3 carbides size
was successfully refined with an increased cooling rate and titanium addition. The
mechanical properties, such as hardness, were improved with an increased cooling
rate and titanium addition. However, the mechanical properties, such as the wear
resistance, were found to become worse when too much titanium was added to the
as-cast condition.
The prediction of the fraction of a solidification structure and a phase diagram of
hypereutectic HCCI (Fe-17mass%Cr-4mass%C multicomponent alloys) was also
done in Supplement I. This was performed by using a multiphase micro-segregation
model (Gulliver-Scheil and Partial Equilibrium), which was coupled with the
CALPHAD method and using the Thermo-Calc software database. Furthermore, in
Supplement Ⅱ , the solidification path and effect of C, Cr, Ti content on the
compositions and amounts of Fe-17mass%Cr-4mass%C multicomponent alloys were
studied both experimentally and theoretically. Here, the focus was to understand the
solidification mechanism of hypereutectic HCCI more deeply.
-64-
Finally, an as-cast sample with a 1.5mass% titanium addition and being casted in a
graphite mold was selected as a good candidate for the following heat treatment
studies.
Supplement Ⅲ focus on different heat treatment processes that were carried out on
as-cast hypereutectic HCCI to study how improved mechanical properties could be
obtained. The relationship between carbide precipitation, martensite units and
mechanical properties were discussed after heat treatment. This was done based on
the volume fraction and the size distribution of carbide precipitates (M7C3 and TiC) as
well as the martensite units. The best heat treatment strategy, which is 900oC×6hr, is
suggested in Supplement Ⅲ. However, the main focus of Supplement Ⅲ is set on
the martensite units. The results show that the fine martensite units make a big
contribution for wear resistance properties of hypereutectic HCCI. However, in
Supplement Ⅲ, it did not go into a lot of detail about the behavior of the secondary
carbides, which precipitated from austenite matrix during the heat treatment process.
Consequently, in Supplement Ⅳ , in-situ observation on secondary carbide was
performed by using a Confocal Laser Scanning Microscope (CLSM) in order to
understand the dynamic precipitation behavior of secondary carbides. The type of
secondary carbide is identified as a M7C3 type by using transmission electron
microscopy (TEM). It was found that the precipitation of secondary M7C3 carbides
started during the heating process (~840°C). Furthermore, the precipitation of
secondary carbides will almost be finished within 2hr after reaching the holding
temperature. This is due to that the volume fraction of secondary carbides appeared to
be unaffected by the holding time and the holding temperature after a holding time of
2hr.
Finally, the focus in Supplement Ⅴ was to get more exactly size distribution results
to fully explain the relationship between the size distribution and the mechanical
properties. More specifically, the maximum carbide size, such as primary M7C3
carbides, TiC carbides and secondary M7C3 carbides in Fe-17mass% Cr-4mass% C
hypereutectic HCCI alloyed with titanium, was studied by using the statistics of
-65-
extreme values (SEV) methods. Moreover, the combined method between a size
distribution analysis and a maximum analysis was proposed in Supplement Ⅴ. A
more accurate carbides size distribution result was obtained by supplementing the
maximum carbides size information to normal size distribution, as shown in Fig. 4.1.
Fig. 4.1. Combination of size distribution analysis and maximum size analysis of primary M7C3
carbides, TiC carbides and secondary M7C3 carbides.
-66-
Then, based on a combination of a size distribution analysis (Supplement I, Ⅲ,Ⅳ)
and a maximum size analysis (Supplement Ⅴ), the relationship between the carbides
size distribution including the maximum carbides size and the mechanical properties
could be discussed.
Fig. 4.1(a) shows that the size of the primary and eutectic M7C3 carbides decreased
and the number of primary and eutectic M7C3 carbide increased with an increased
cooling rate ranging from a sand mold to a metal mold. As a result, the hardness of the
hypereutectic HCCI in an as-cast condition increased with a decreased primary and
eutectic M7C3 carbide size, as shown in Fig. 4.1 and Fig. 4.2.
48 50 52 54 56 58 60 62 64 660.30
0.32
0.34
0.36
0.38
0.40
0.42
0.44
0.46
0.48
0.50
0.52
Primary M7C
3:
Sand mold
max. 259.1m
TiC: max. 26.7mTi=3.0%
Ti Hardness Wear lossMold
Hardness, HRC
Sand : 0%
Graphite: 0%
0.75%
1.5%
3.0%
Metal: 0%
W
ea
r lo
ss
, g
/min
Primary M7C
3:
Graphite mold
max. 196.6m
Primary M7C
3:
Metal mold
max. 132.9m
TiC: max. 19.8mTi=0%-1.5%
Fig. 4.2. Mechanical properties of the hypereutectic HCCI in as-cast conditions.
Moreover, this tendency was also confirmed by the predicted maximum primary
M7C3 carbide sizes for different molds. In short, the fine primary and eutectic will
improve the hardness of hypereutectic HCCI. In addition, compared to 0%-1.5%
titanium additions, it was found that the predicted maximum TiC carbide size, which
was found for a 3.0% titanium addition, is 26.7μm, as shown in Fig. 4.1(b). Moreover,
higher titanium additions of 3.0% were found to be worse to improve the wear
resistance of hypereutectic HCCI in as-cast conditions, as shown in Fig. 4.2. The
reason might be that the agglomerated TiC carbides for 3.0% titanium additions are
easily peeled off during the wear application. As discussed in chapter 3, the wear
-67-
48 50 52 54 56 58 60 62 64 660.30
0.32
0.34
0.36
0.38
0.40
0.42
0.44
0.46
0.48
0.50
0.52
Graphite mold
Ti=1.5%
max. 3.79m
Secondary M7C
3:
As-cast
900C,10502hr
900C,10506hr
W
ear
loss
, g/m
in
Hardness, HRC
900C6hr
1050C6hr900C2hr
1050C2hrAs-cast
max. 2.58m
Secondary M7C
3:
max. 2.16m
Secondary M7C
3:
max. 3.15mSecondary M
7C
3:
Fig. 4.3. Mechanical properties of the hypereutectic HCCI with different heat treatment conditions.
resistance properties of hypereutectic HCCI are mainly affected by size and number
of martensite units after heat treatment. More specifically, the presence of fine
martensite units for a 900oC×2hr/6hr conditions make a big contribution to improve
the wear resistance properties of hypereutectic HCCI, as shown in Fig. 4.3. However,
the secondary M7C3 carbide, which precipitated from the matrix during the heat
treatment process, might also have an influence on the mechanical properties of a
hypereutectic HCCI. This is particularly true for the hardness of hypereutectic HCCI.
As shown in Fig. 4.1(c), it was found that the size of the secondary M7C3 carbide
increased as the heat treatment conditions changed from 900oC×2hr/6hr to
1050oC×2hr/6hr. This result led to a reduced hardness of hypereutectic HCCI, as
shown in Fig. 4.3.
-68-
-69-
Chapter 5
Conclusions and Future Work
5.1 Conclusions
This thesis focuses on refining the microstructure and improving the mechanical
properties of Ti-alloyed hypereutectic high chromium cast iron (HCCI). The size
distribution, composition and morphology of these carbides and martensite units in
the Ti-alloyed hypereutectic HCCI with main composition of Fe-17mass%
Cr-4mass% C were quantitatively studied by using both experimental methods and
theoretical methods. The most important conclusions reached in this study can be
summarized as follows:
General conclusions of supplement I:
1). The microstructures of as-cast Ti-alloyed hypereutectic HCCI were refined by an
increased cooling rates and titanium addition;
2). The M7C3 carbides were roughly classified into “primary M7C3 carbides” and
“eutectic M7C3 carbides” with a 11.2μm border size based on size distribution results
and volume fraction results; The TiC carbides with high formation temperatures can
not only act as nuclei of M7C3 carbides, but they also contain a core, Ti(C, N). It was
also found that TiC carbides precipitation was not obviously affected by the cooling
rate;
3). The hardness of the hypereutectic HCCI increased slight with an increased the
cooling rats and titanium addition. The wear resistance were found become worse
-70-
when too much titanium addition. However, the mechanical properties (hardness and
wear resistance) do not change significantly under the as-cast conditions.
General conclusions of supplement Ⅱ
1). The precipitation sequence of MC and M7C3 carbides is greatly affected by the
nominal contents of C, Cr and Ti elements in the HCCI. The composition of Cr in the
M7C3 phase increases with both the Cr and Ti contents, but decreases with the C
content. Both the increase of C and Cr lead to an increased volume fraction of M7C3
carbide. However, they have no influence on MC carbide;
2). The PE predictions shows that a potential exists to predict the solidification path of
HCCI at cooling rates between 0.4°C
/s to 1.5
°C
/s. This conclusion is based on the
agreement between the predictions and the experimental thermal analysis data from
the ingot.
General conclusions of supplement Ⅲ:
1). From the viewpoint of the precipitation promotion of M7C3 (volume fraction) and
the growth control/ refinement of M7C3, the growth promotion of TiC (volume
fractions) and the transformation promotion of martensite (volume fraction) and the
refinement of martensite unit (size distribution), the best heat treatment process for
improving the wear resistance properties is suggested as 900oC×6hr.
General conclusions of supplement Ⅳ
1). The secondary carbides were identified as a M7C3 type found in Fe-4 mass%C-17
mass%Cr-1.5 mass% Ti hypereutectic HCCI alloys.
(2). Precipitation of secondary M7C3 carbides started during the heating process
(~840°C) and the number of these secondary M7C3 carbides increased sharply at
temperatures higher than 840°C. Moreover, the precipitation of secondary M7C3
-71-
carbides will almost finish within 2hr after reaching the holding temperature since the
volume fraction of secondary carbides appeared unaffected by the holding time and
holding temperature after holding for 2hr. In summary, a lower temperature and a
shorter holding time, such as 890°C×2hr, are preferred to precipitate secondary M7C3
carbides.
General conclusions of supplement Ⅴ
1). The shape factor (circularity value) of maximum primary M7C3 carbides, TiC
carbides and secondary M7C3 carbides was found to be different. It leads to different
slope of the extreme value distribution (EVD) regression lines for these carbides. The
large sized carbides such as primary M7C3 carbides have a lower slope than the
smaller carbides such as TiC carbides and secondary M7C3 carbides.
2). The combined method between a size distribution analysis and a maximum
analysis is proposed in this study. Moreover, a more accurate carbides size
distribution result was obtained by supplementing the maximum carbides size
information to normal size distribution.
3). The combination of a size distribution analysis and a maximum analysis results
showed that the primary M7C3 carbides and secondary M7C3 carbides in as-cast
condition and heat treatment condition are related to the hardness of hypereutectic
HCCI. More specifically, it will decrease with an increased maximum size of primary
M7C3 carbides and secondary M7C3 carbides. Moreover, it is confirm the wear
resistance property will become worse due to increasing the size of TiC carbides
(titanium addition) from viewpoint of maximum size analysis.
5.2 Suggestions for future work
In the present thesis, Ti as a carbide promoter was selected to refine the
microstructure of hypereutectic HCCI. Actually, other strong carbides forming
-72-
elements, such as Zr, Hf, V, Nb and B can also be used to refine the microstructure of
hypereutectic HCCI according to the wear applications. Moreover, the complex
addition of those elements in hypereutectic HCCI may also improve the
microstructure due to the nucleation theory.
The three-dimensional 3D X-ray computed tomography technic or electric extraction
experiment for hypereutectic HCCI might be used to reconstruct or extract 3D
carbides information. Thus, it will be possible to get more clearly (3D) experimental
results on the carbides size distribution. In addition, statistical analysis methods, such
as Schwartz-Saltykov (ss) method, is also very important to estimate the 3
dimensional (3D) carbides size distribution base on 2 dimensional (2D) carbides size
distribution by using Schwartz-Saltykov (ss) method. Furthermore, these results may
thereafter be used to verify the prediction and calculation.
In order to obtain very accurate carbides size distribution, not only maximum carbides
size, the small size and volume fraction, which is omitted due to the resolution of
characterization equipment, also should be estimated by statistical methods.
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