A R C HI V E S

o f

F O UND R Y E NG I NE E R I N G

Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

ISSN (1897-3310)

Volume 11 Issue 4/2011

157 – 162

29/4

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1 , I s s u e 4 / 2 0 1 1 , 1 5 7 - 1 6 2 157

Evaluation of cast GX40CrNiSiNb24-24 steel

after-service conditions

R. Zapała*, B . Kalandyk, J. Głownia Department of Cast Alloys and Composites Engineering, Faculty of Foundry Engineering,

AGH University of Science and Technology, Reymonta 23 Str., 30-059 Krakow, Poland

*Corresponding author. E-mail address: zapala@agh.edu.pl

Received 15.07.2011 accepted in revised form 27.07.2011

Abstract In this study, an attempt was made to assess changes in the microstructure of centrifugally cast tubes from GX40CrNiSiNb24-24 steel after

a long-term operation under the conditions of a catalytic conversion of methane with steam. Changes in phase morphology occurring as a

result of high temperature were identified.

Keywords: Metallography, Heat resistance Cr-Ni cast steel, Microstructure, High temperature degradation, Carbides

1. Introduction

Hydrogen, which is one of the sources of so-called "clean

energy", has become an alternative to the drastically diminishing

fossil fuel resources. Due to the fact that the product of hydrogen burning is water, this element is an ideal fuel used in internal

combustion engines or fuel cells. Currently, hydrogen is produced

mainly from fossil fuels and used in the processes of chemical

synthesis (e.g. the synthesis of ammonia, aldehydes, ketones), in

metallurgy and aerospace (rocket fuel) [1]. Hydrogen is produced mainly from natural gas in the process of steam reforming at

temperatures of up to 950ºC under the pressure of 2÷4MPa. This

process is carried out in a reformer, whose basic element is the

furnace composed of the centrifugally cast chrome-nickel steel

tubes. Placed vertically in the furnace, the tubes are filled with nickel catalyst (ceramic rings coated with a nickel-based alloy).

The working tubes are exposed to corrosive environments (air,

water vapour, carbon and sulphur) for approximately 100000

hours [2÷4].

Tubes operating in installations for the reforming and pyrolysis are made, among others, from the cast GX40CrNiSi25-

20 steel [5÷8]. This material has good resistance to gas corrosion

(appropriately high content of chromium) and shows reasonable resistance to stress, even at temperatures down to 1050 º C.

In as-cast state, the structure of cast GX40CrNiSi25-20 alloy

is composed of an austenitic matrix with large precipitates of

carbides, located mainly at the grain boundaries. To improve the

performance properties of the alloy, nickel content is increased and an addition of niobium is introduced, the latter one being a

strongly carbide-forming element. Niobium forms carbide

eutectic at grain boundaries; scarce carbide particles are also

present inside the grains. In as-cast niobium-inoculated alloys,

one can find fine precipitates of niobium carbides (NbC), adhering to the carbides of M 23C6 type. Carbides of M 23C6 type

contain in their composition besides chromium also certain

amounts of Fe [9].

2. Methods investigation

The material for studies were samples of the cast

GX40CrNiSiNb24-24 steel taken from a tube operating in installations for the catalytic conversion of methane with steam.

Samples were cut out from an area distance by about 1.5 m from

the tube inlet operating at about 800°C for 110000 hours.

158 A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1 , I s s u e 4 / 2 0 1 1 , 1 5 7 - 1 6 2

Metallographic studies were carried out on surfaces perpendicular

to the tube axis using a Nikon light microscope and a Hitachi S-

3500L scanning microscope, and also a NORAN attachment for

analysis of the chemical composition. Samples were etched with a

Mi 15Fe reagent. The chemical composition of the examined cast steel is shown

in Table 1.

Table 1.

Chemical composition of the examined cast steel

Material

Chemical composition, wt%

C Si Mn Cr Ni Nb

GX40CrNiSiNb

24-241

0,3

0,5

1,0

2,5

max

2,0

23,0

25,0

23,0

25,0

0,8

1,8

MB2 0,5 1,3 1,2 24,2 23,1 1,3

1 according to PN-EN10295 2 examined material

To get more information on the phase composition of the

examined material, an X-ray phase analysis was carried out.

Diffraction studies were conducted on a Siemens Kristalloflex 4H

diffractometer. The radiation of copper anode tube of λ = 1.54 Å was used. Diffraction lines were recorded at a voltage of 35kV

and 20mA current intensity in the 2θ angle range.

The aim of the conducted studies was to obtain structural

characteristics of material used for the tubes centrifugally cast

from heat- resistant steel GX40CrNiSiNb24-24.

3. Results and discussion

Figure 1 shows structure of the examined casting. On the tube

section one can distinguish the three major structural zones, viz.

of chill grains, columnar and equiaxial. The zone of frozen grains

in the examined material has been partly destroyed by the effect of the corrosive environment (Fig. 1.1).

Fig. 1. Structure of casting made from the GX40CrNiSiNb24-24

steel

To evaluate the morphology of the dendritic structures, the

secondary dendrite arm spacing (DAS) was determined.

Measurements of the DAS parameter were taken only for the

well-developed secondary arms located near the tube outer

surface and middle wall part. It has been assumed that the width of the secondary dendrite arms is approximately equal to spacing

between the precipitates found in the interdendritic spaces. The

DAS parameter for both tube wall areas was similar and

amounted to about 30 μm.

Figure 2÷4 shows after-service microstructure of the cast austenitic GX40CrNiSiNb24-24 steel as seen under a light

microscope.

The metallographic analysis showed that the examined

material is characterised by a dendritic structure with numerous

precipitates of complex morphology present in both dendrites and interdendritic spaces.

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1 , I s s u e 4 / 2 0 1 1 , 1 5 8 - 1 6 2 159

Fig. 2. After-service microstructure of GX40CrNiSiNb24-24 cast

steel - outer surface of tube, optical microscope

Fig. 3. After-service microstructure of GX40CrNiSiNb24-24 cast

steel - middle part of tube, optical microscope

Fig. 4. After-service microstructure of GX40CrNiSiNb24-24 cast

steel - inner surface of tube, optical microscope

160 A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1 , I s s u e 4 / 2 0 1 1 , 1 5 7 - 1 6 2

For SEM examinations, areas located in middle part of the

tube wall were chosen (Fig. 5÷7). The precipitates located in the

interdendritic spaces, and visible on SEM images show clear

enrichment in Nb, Ni and Si (Fig. 5, spectrum 2). It confirms the

fact reported in the literature concerning the examined material that at high temperatures the precipitates of NbC are unstable, and

after long-term operation at temperatures around 900°C undergo

transformation forming an intermetallic Ni16Nb6Si7 type phase

enriched in Si (G phase) [11]. The analysis of the chemical

composition of these precipitates, located in different areas of the central zone of the tube wall, showed significant differences in the

content of Nb (30÷45%wt), Si (6÷10%wt) i Ni (30÷40%wt). It

can be concluded that the observed differences in the composition

of these phases may be caused by varying degrees of

advancement of the niobium carbide NbC transformation into the G phase [1].

The precipitates visible on SEM images as dark areas,

showing mainly the enrichment in chromium, and less frequently

in chromium and iron, are chromium carbides of Cr23C6 and

(Cr,Fe)23C6 type (Fig. 5, spectrum 1).

Fig. 5. After-service microstructure on the cross-section of a tube

cast from GX40CrNiSiNb24-24 steel, including the results of

chemical analysis, SEM-EDS

Fig. 6. Microstructure on the cross-section of a tube cast from

GX40CrNiSiNb24-24 steel with surface distribution of elements,

SEM-EDS

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1 , I s s u e 4 / 2 0 1 1 , 1 5 8 - 1 6 2 161

In dendrites, the lamellar precipitates of an average size of

about 5μm prevail. Typically they are arranged in respect of each

other at an angle of about 60 and 120 degrees. SEM examinations

showed enrichment of this phase in Cr compared to the alloy

matrix (Fig. 7). Based on the literature data it can be concluded that these are probably the precipitates of σ phase, because

operation of the examined material under the conditions of steam

reforming in a temperature range of 850 ÷ 920ºC may contribute

to the formation of acicular precipitates of this phase [1, 10].

Fig. 7. Microstructure on the cross-section of a tube cast from GX40CrNiSiNb24-24 steel with linear chemical analysis, SEM-EDS

162 A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1 , I s s u e 4 / 2 0 1 1 , 1 5 7 - 1 6 2

Fig. 8. The diffractogram of GX40CrNiSiNb24-24 cast steel

Figure 8 shows an example of diffraction pattern obtained

on the examined material. The analysis of diffraction patterns

revealed the presence of the following phases: γ,

(Cr, Fe)23C6, NbC, Ni16Nb6Si7, Nb3Ni2Si.

4. Conclusions

On the entire cross-section of the test tube, two types

of precipitates were found: acicular (inside the dendrites)

and complex of globular shapes (mainly in the

interdendritic spaces).

Complex precipitates show enrichment in Nb, Ni and Si

(bright areas) and in Cr and Fe (dark areas).

The phase enriched in Nb, Ni and Si is probably the G

phase which is a product of the transformation of niobium carbide, while phases enriched in Cr and Fe are Cr23C6

and/or (Cr,Fe)23C6.

The acicular precipitates enriched in Cr are probably the

precipitates of σ phase. The X-ray phase analysis revealed the presence of the

following phases: γ, (Cr, Fe)23C6, NbC, Ni16Nb6Si7, Nb3Ni2Si.

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