creep-resistant austenitic cast steel

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

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

ISSN (1897-3310)Volume 8

Issue 2/2008

115 120

25/2

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 8 , I s s u e 2 / 2 0 0 8 , 1 1 5 - 1 2 0 115

Creep-resistant austenitic cast steel

B. Piekarski*, J. Kubicki

Szczecin University of Technology, Al. Piastw 17, 70-310 Szczecin *Corresponding author. E-mail adress: [email protected]

Received 08.02.2008; accepted in revised form 17.03.2008

Abstract The study reviews the reference literature and gives some results of own investigations concerning changes of phase composition that

take place in creep-resistant austenitic cast steel of two generations due to an ageing treatment. The cast steel of the first generation contains in its chemical composition only the alloying additives like nickel, chromium and silicon. Compared with the first group, the chemical composition of the second generation cast steel has been enriched with additions of niobium and/or titanium. Keywords: Austenitic cast steel; Phase composition; Precipitation process.

1. Introduction

Among different types of austenitic cast steel, for operation at high temperatures the cast steel containing nickel from 18 to 38%, chromium from 15 to 26%, silicon up to 2.5% and carbon from 0,15 to 0,6% (% max.) is used most frequently. This chemical composition guarantees obtaining a stable austenitic structure at both high and ambient temperatures. Increasing the content of iron in Fe-Ni-Cr alloys definitely deteriorates their heat- and creep-resistance, though on the other hand it is the main cost-effective factor in this group of materials.

The oldest standard on the chemical composition of creep-resistant steel castings is the standard developed by ACI (American Casting Institute). The cast steel designation system introduced by this standard is still used in worldwide technology, while the proposed alloy grades serve as a reference and example in the development of various national standards. In Poland, since 2004, the European Standard PN-EN 10295 Creep-resistant steel castings has been used. Compared with the former Polish Standard PN-90/H-83159 Heat- and creep-resistant cast steel, it covers a wider range of alloys and chemical compositions, fully satisfying the current demand of different industry sectors.

At present, three generations of the creep-resistant austenitic cast steels are used [1]:

- 1st generation. Common type cast steel. Its main alloying elements are nickel, chromium and silicon. It started to be used worldwide in 50-ties and 60-ties of the past century.

- 2nd generation. Common type cast steel, additionally enriched with niobium and titanium. Introduced to industry in 70-ties of the past century.

- 3rd generation. The cast steel stabilised with niobium and/or titanium and, additionally, inoculated with rare earth metals. Used in industry since 80-ties of the past century.

All other modifications introduced successively to the chemical composition of creep-resistant austenitic cast steels aimed at a simultaneous improvement of the following three parameters: creep resistance, operating temperature and resistance to the effect of aggressive gas environments [15]. To achieve this purpose, numerous elements, like Nb, Ti, Zr, V, W, Co, Mo were introduced to the Cr-Ni and Ni-Cr cast steels, as single additions or in combination, and in amounts from tenth to several percent. Due to this, it was possible to obtain a combined effect of the solution hardening of matrix (austenite) and precipitation hardening of both matrix and grain boundaries [1, 2].

Conferring long life to the elements of machines and equipment operating at high temperatures is a complex design task requiring careful examination of numerous factors that determine the life of parts during performance. These factors fall into the three basic categories [6]:

1. Material-related factors determined by the behaviour of material and the process of its manufacture. When the material is put in service, it undergoes some changes caused by its attempt to obtain a state closer to the state of thermo-dynamic equilibrium than the state possessed initially, considered optimum for the utilisation properties. These, very undesirable, changes can be counteracted mainly through proper choice of the chemical and phase constituents of the material.

2. Environmental factors resulting from the operating conditions. They include mechanical loads, corrosive effect of the environment, thermal shocks, friction, etc. These processes may co-act with each other and with the above-mentioned spontaneous transformations, speeding up or slowing down their course.

3. Design and engineering factors related with casting design and operation, including correct layout and manufacture of casting, strict observance of recommendations regarding proper performance conditions as well as diagnosis of failures, correct maintainance and repair procedures, modifi-cations introduced to the cast material and casting design as a result of experience acquired during its performance. The effect of the above mentioned factors, usually acting jointly, is responsible for the following types of material degradation: creep, thermal fatigue or thermal-mechanical fatigue, corrosion and erosion [16].

In this study, using data given in the reference literature and the results of own investigations, the effect of material-related factors on the structural stability of cast steels of the first and second generation was analysed and examined.

2. The structural stability of cast steel

In as-cast state, the matrix of austenitic cast steels is composed of a supersaturated solid solution of austenite. The structure of these alloys usually comprises also a carbide-austenite eutectic, distributed mainly on the ternary grain boundaries, and in the case of alloys containing below 0.2%C single carbides present in the grain boundaries [7, 8]. The number and size of the carbide particles depend, first of all, on the cooling rate and carbon content in alloy [810].

In the first generation cast steel, the precipitating primary carbides are chromium carbides of the M7C3 and M23C6 type (symbol M in the stoichiometric formula of carbides corresponds to chromium atoms mainly, but also to the atoms of iron, nickel, manganese, etc. [7, 10]). The content of chromium carbides of the M7C3 type in the as-cast structure of steel increases (compared with the M23C6 carbide content) along with an increasing carbon content (Fig. 1) [8, 10]. In the second generation cast steel containing carbide-forming elements (mainly Nb and/or Ti), the structure of alloy undergoes some important changes in respect of the structure described above. In as-cast state, the eutectic is formed of M7C3 and/or M23C6 carbides (depending on the Cr/C ratio) and (depending on the type of the stabilisers used) of simple carbides of the TiC and/or NbC type [1, 7, 13].

Time, hrs

JM

3 /

(412

)7C

JM

C(4

40)

236

400200 600 10008000

5

15

10

0

20

- 0.21 - 0.31- 0.39

C, wt-%

Fig. 1. Effect of 36Ni-18Cr cast steel ageing time on the relative

volume content of carbides [8]

Compared to the first generation cast steel with the same carbon content, the content of carbide eutectic increases as well as the massiveness of carbides present in this eutectic [14]. Joint additions of niobium and titanium introduced to cast steel give rise to the formation of straight and complex simple carbides. In each of these cases, the simple carbides are characterised by different morphologies [15, 16].

The process of annealing changes in a significant way the phase composition and morphology of the precipitates. In the first generation cast steel, the secondary chromium carbides of the M23C6 type start precipitating (Fig. 1). Their content in structure is function of the annealing time and temperature and of the alloy chemical composition carbon and chromium content mainly. An increase in the content of carbide phase is accompanied by a respective change in the chemical composition of the matrix, i.e. a high-rate chromium impoverishment Fig. 2. An example of the steel structure as-cast and after annealing is shown in Figure 3.

2

4

6

8

Car

bide

s fra

ctio

n, w

t-%

0

Cont

ent o

f Cr,

wt-%

20

80

90

10

15

85

75

matrix

carbids

a

b

400200 600 10008000Time, hrs

C, wt-%- 0.21 - 0.31- 0.39

Fig. 2. Effect of 36Ni-18Cr cast steel annealing time on:

a) chromium content in carbides and matrix, b) the content of carbide phase [8]

The shape and size of the precipitating secondary carbides

definitely affect the alloy creep behaviour and plastic properties. In this respect, more beneficial are fine globular particles instead of large precipitates [6]. The structure still contains chromium


carbides of the M7C3 and M23C6 type; the content of the former ones drops to zero after a longer time of annealing (Fig. 1, 4), mainly due to the M7C3 + M M23C6 transformation [7, 13].

10 m

a

b

Fig. 3. Microstructure of 0.4C-17Cr-34Ni steel as-cast (a) and

after annealing at 900oC/ 100 h (b) [8]

Time, hrs100 10001 10

700

600

800

900

1000

M CM

MC

7 3

23 6

+

Tem

pera

ture

, oC M C23 6

M C7 3

Fig. 4. Effect of time and temperature on the transformation of

M7C3 carbides into M23C6 carbides in 0.4C-25Cr-20Ni cast steel [7]

The M7C3 M23C6 transformation is proceeding between the

M7C3 carbides and austenite, and it is possible to have both transformations in-situ together with an independent nucleation of M23C6 carbides and simultaneous dissolution of M7C3 carbides in matrix. In low-carbon cast steel, the procesess of precipitation are proceeding mainly along the grain boundaries, while in medium- and high-carbon cast steels they also take place inside the austenite grains [810, 17]. The structure of the aged cast steel may also comprise chromium nitrocarbides of the M2(C,N) type [7]. Long-term annealing at temperatures below 950oC [7] makes carbides of both M23C6 and M7C3 type undergo further transformation into M6C carbides (Fig. 5). The M6C carbides may contain even up to 40% nickel and 10% silicon [7]. Contrary to M7C3 M23C6 transformation, the M7C3 (M23C6) M6C transformation originates not on the carbide/ matrix interface but most probably in the crystal lattice of chromium carbides [7, 18].

coagulation M C6

M C6

M C M M C23 6 6 + M CM

M C

7 3

6

+ 10010 1000

Time, hrs

Tem

pera

ture

, oC

600650

700

750800

850900

9501000

Fig. 5. Time and temperature effect on the formation of M6C carbides in 0.4C-55Ni-25Cr cast steel [7]

The precipitates rich in nickel and silicon, present in the

microstructure of first generation steel casting withdrawn from use, are shown in Figure 6, while the results of local micro-analysis of the chemical composition are compiled in Table 1.

1

2

10 m

Fig. 6. Microstructure of casting made from 36Ni-17C steel

operating for about 5000 hours in a heat treatment furnace [8]

Table 1. Microanalysis (wt.%.) of the precipitates shown in Figure 6

The place of analysis Si Fe Cr Ni 1 0.2 17.5 75.6 6.5 2 8.8 4.6 48.1 37.4

matrix 1.6 46.2 13.8 38.4

In cast steels of the first and second generation, annealing can give rise to a combined precipitation of M23C6 carbides and phase . However, the probability of the precipitation of this phase is rather small, because in cast alloys carbon content is usually much higher than it is in alloys assigned for plastic working. Carbon binds chromium into carbides, and as an immediate result of this effect a drop in the content of this element in matrix below a level indispensable for the precipitation of phase is observed [20]. The cast steel containing above 0.05% carbon is after solutioning characterised by an austenitic structure with M23C6 carbides. During annealing at temperatures below 1000oC, from the austenite after solutioning are precipitating carbides of the M23C6 type, while phase is nucleating only when carbon content in the austenite drops to a critical level (for 25Cr-20Ni steel this level is 0.006%) [20]. So, obtaining a low carbon level in the matrix of creep-resistant high-carbon cast steel is practically impossible, especially when the cast steel matrix is expected to have at the same time a high chromium concentration. Under given conditions of annealing and above the predetermined critical


carbon content, when the state of equilibrium between carbides and austenite has been reached, austenite continues behaving like a carbon-saturated solid solution [20]. Besides carbon content, the precipitation of phase also depends on the chemical composition of alloy [2] and its occurrence is usually preceded by a long-term annealing (Fig. 7). A typical arrangement of the needle-like precipitates of phase in the cast steel matrix is shown in Figure 8.

10000 100000100 1000

700

600

800

900

1000

Tem

pera

ture

, oC

Time, hrs

Fig. 7. Time and temperature effect on the precipitation of phase

in 0.4C-25Cr-20Ni cast steel [7]

Fig. 8. Precipitates of phase in the structure of pipe

centrifugally cast from 24Cr-24Ni Nb steel after 95000 hours of operation [19]

The strong carbide-forming elements, like niobium and/or

titanium are usually addded to cast steel to form carbides much more stable than the chromium carbides and to obtain due to this an improvement of numerous physical and mechanical properties. For low-carbon steels, the knowledge in this field has a well-grounded fundamentals [6, 11, 21, 22].

The phase composition of the second generation cast steel is more complex than that of the cast steel of the first generation, mainly because its structure contains the co-existing complex and simple carbides of both primary and secondary origin (Fig. 9).

In the structure of low-carbon cast steel during performance are nucleating only carbides of the M23C6 and MC type. Carbides of the MC type may contain chromium, the amount of which usually decreases with the elapsing time of ageing [24, 25]. After a long- term annealing, even two strong, carbide-forming elements added jointly to steel are not capable of either stabilising or improving the resistance to the growth of MC precipitates this effect, by the way, is also obtained when only one of the two elements is present. In cast steel containing niobium and titanium, both these elements are found in the precipitates of MC carbides [23, 24]. An addition of niobium and titanium to 25Cr-20Ni cast steel produces structure with more uniform distribution of the secondary precipitates and less continuous network of eutectic

carbides than it is possible to obtain with the sole addition of niobium [25].

a

b

10 m

Fig. 9. Microstructure of 30Ni-18Cr cast steel after the process of annealing at 900oC/ 300h [23]: a) 0.03Nb and 0.03Ti, b) 0.08Ti

and 1.75Nb

Investigations of the effect of titanium and niobium on the structure of 0.4C-25Cr-20Ni cast steel indicate that niobium does promote the precipitates refinement at a temperature of 950oC but with temperature this role is taken over by titanium (Fig. 10). The morphology and distribution of the secondary carbides in matrix depend on a total titanium/ niobium-carbon ratio (Fig. 11). The content of the secondary simple carbides in the structure of cast steel at a given temperature and time can be used for calculation of the true operating temperature of the individual parts of casting. An example of the practical application of quantitative metallography is given in [15].

without of Nb or Ti1,79%Nb0,76%Ti0,15%Ti+0,6%Nb

900 1000 1050950 1100

0,25

0,30

0,35

0,40

0,45

Temperature, Co

Size

of c

arbi

ds,

m

Fig. 10. Effect of stabilisers and temperature on the size

of secondary carbides in 0.4C-25Cr-20Ni cast steel after 1000 hour annealing [13]

In stabilised cast steels also the primary simple carbides are

not stable. Their decomposition is said to follow the reaction of MC + M M6C [7]. The formation of M6C carbides is usually accompanied by a drop in the content of other carbides present in the cast steel structure with simultaneous increase in the total content of carbide phases (Fig. 12).


(Ti+Nb)/C

0,2

0,1

0,3

0,4

0,5

1

2

4

5

3

0,5 1,00

l, m

,

2

1

Fig. 11. Changes in microstructure of 0.4C-25Cr-20Ni cast steel

caused by joint addition of niobium and titanium after the process of annealing at 1050oC/ 1000 hours [13]:

1 mean carbide dimensions, 2 mean carbide spacing

connectingM CNbCM C

23 6

6

Time, hrs

Cont

ent o

f car

bids

, wt-%

Fig. 12. Kinetics of M6C carbide precipitation in 25Cr-20Ni

(2.1Si-1.56Nb) cast steel annealed at a temperature of 800oC [14]

However, the authors of some studies [2628] doubt it very much that the precipitating nickel- and silicon-rich phases can be the M6C type carbides. According to these authors it is much more probable that the precipitating phases are phase G of a symbol of M6Ni16Si7. An optimum temperature for the formation of phase G in 25Cr-20Ni cast steel is 750800oC [26]. The presence of this phase in the form of precipitates around the particles of NbC carbides lying on the grain boundaries was observed as early as after two hours of annealing (Fig. 13). The process of the formation of phase G in austenitic cast steel promotes an increase of the silicon content in alloy. In 35Ni-20Cr cast steel, the beginning of the precipitation of phase G is shifting together with the silicon content towards shorter times of annealing and higher temperatures, with an upper value of temperature exceeding 980oC (Fig. 13). Annealing at a temperature equal to or higher than 980oC does not result even after a very long time of annealing (up to 2400 hours) in MC G transformation [15, 29]. The results of the studies described in [2931] also point out to a significant role of silicon in the process of phase G formation. No traces of phase G were observed in the structure of samples made from the 25Cr-20Ni cast steel stabilised with niobium and titanium and containing 0.7%Si, exposed to the effect of creep for 2647 hours at a temperature of 8001000oC.

Within an area of the formation of phase G, the presence of chromium carbide precipitates has also been observed (Fig. 14),

and this may mean that they are a by-product of the MC G transformation. The results of the chemical analyses in micro-regions [23] prove the presence of a nickel- and silicon-rich phase on the MC carbide/matrix interface.

1 52 510 20 100

700

900

800

1000

1100

0

M C G NbC23 6+ +

1.84%Si

2.62%Si

1.63%Si

M C NbC23 6+

Time, hrs

Tem

pera

ture

, oC

Fig. 13. The curves of phase transformation in a time-temperature

system for 0.4C-35Ni- 20Cr-1.97Nb cast steel [29]

M23C6

G

10 m

M23C6

G

Fig. 14. Examples of the presence of phase G in stabilised 30Ni-18Cr cast steel; annealing at 900oC/ 300 hours [16]: a) 1 % Ti, b)

1.67% Nb, 0.05% Ti.

However, opinions of researchers vary as to the occurrence in austenitic cast steel of M6C carbide or phase G.

Over the past decade, some remarks appeared in literature on the presence in the structure of the niobium-stabilised and aged 36Ni-18Cr cast steel of phase rich in silicon and of a low carbon content, called silicide and having the structure of M6C carbide [32, 33]. From the results of studies presented in [34] it follows that the structure of this phase extends over a wide range of chemical compositions, e.g. A3BB3C, A2B3B SiC, where carbon can be replaced with atoms of silicon, and A are atoms of V, Cr, Mn, Fe, Co or Ni, while B are atoms of Nb, Ta, Mo or W.


3. Conclusions

As follows from the above presented analysis of the reference literature, an addition of niobium and/or titanium introduced to the creep-resistant austenitic cast steel cannot guarantee obtaining a fully stable structure in this alloy within the range of operating temperatures, i.e. 600900oC.

It can be generally stated that in cast steels with an addition of the stabilising elements it is possible to have the transformation of MC-type carbides into phase G during casting operation at high temperatures. The authors of this study did not find in the available literature a single proof of an unfavorable effect of the said phase on the utilisation properties of the creep-resistant austenitic cast steel.

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creep-resistant austenitic cast steel

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