Effect of Alloying Elements on Variation of Micro-Hardness during Heat Treatment of Hypoeutectic High Chromium Cast Iron

Transcription

1 Materials Transactions, Vol. 49, No. 10 (2008) pp to 2330 #2008 The Japan Institute of Metals Effect of Alloying Elements on Variation of Micro-Hardness during Heat Treatment of Hypoeutectic High Chromium Cast Iron Sudsakorn Inthidech 1 and Yasuhiro Matsubara 2 1 Faculty of Engineering, Mahasarakam University, Mahasarakam 44150, Thailand 2 Department of Materials Science and Metallurgical Engineering, Kurume National College of Technology, Kurume , Japan Hypoeutectic cast irons containing 16 mass% and 26 mass% Cr with single additions of Ni, Cu, Mo and V as well as without alloy addition were prepared to investigate variations of micro-hardness of matrix during heat treatment. In the as-hardened state, Ni and Cu decreased the micro-hardness but Mo increased it slightly. By contrast, V increased the micro-hardness in 16 mass% Cr but reduced it in 26 mass% Cr cast irons. The volume fraction of retained austenite (V ) was positively correlated with alloy content except for V addition and it was high at elevated austenitizing temperatures. Tempered micro-hardness curves showed secondary hardening and the degree of secondary hardening (H D ) was greater in alloyed specimens comparing with alloy-free specimen. The H D was closely related to V in as-hardened state, and the more the V, the greater the H D. The maximum tempered micro-hardness (H MTmax ) was obtained in the specimen tempered at 698 to 873 K depending on the kind and the amount of alloying element where the V was less than 20%. The H MTmax values of Mo and V containing specimens increased with the V in the as-hardened state. The highest value of H MTmax was obtained in those samples containing 3 mass% Mo in both series of the cast irons. The mechanism of secondary hardening in Mo and V containing cast irons was mainly by both the precipitation of special secondary carbide and the transformation of destabilized as-hardened retained austenite into martensite providing the high microhardness. [doi: /matertrans.mra ] (Received March 3, 2008; Accepted July 2, 2008; Published September 25, 2008) Keywords: hypoeutectic high chromium cast iron, alloying element, heat treatment, micro-hardness, volume fraction of retained austenite 1. Introduction High alloy cast irons containing mass% Cr have been employed as abrasion wear resistant materials for a long time. It is well known that 15 mass% to 16 mass% Cr cast irons have been preferably used for rolling mill rolls in steel plants, while cast irons with 25 mass% to 28 mass% Cr are present as rollers and tables of pulverizing mills in mining and cement industries because of their high abrasion wear resistance and suitable toughness. High chromium cast irons of hypoeutectic composition are preferred as they avoid the formation of primary carbides that would reduce toughness. In these hypoeutectic cast irons, the as-cast microstructure consists of the primary austenite () dendrites and a eutectic structure of austenite () +M 7 C 3 carbides. 1 5) The structure of eutectic carbide varies depending on the kind and the amount of carbide forming elements and carbon content. The eutectic structure shows a colony morphology with fine carbides in the central region and coarse carbides at the boundary regions. The size of colony is controlled by the eutectic freezing range and cooling rate. 4) On the other hand, the matrix structure can be changed by the controlled cooling rate of the casting in the mold. In the high Cr cast iron, austenite exists in either of as-cast and as-hardened states. The presence of austenite is valuable because of its high toughness and can increase the hardness by the formation of strain-induced martensite. 2,6) However, large amounts of retained austenite are considered to deteriorate the spalling resistance of the cast irons. 2) It has been reported that the martensitic matrix with some retained austenite showed a higher wear resistance to spalling than the fully austenitic matrix. 2,3) Heat treatment of high chromium cast irons has been conducted in order to get the optimum combination between the quantity of austenite and martensite for the high abrasion wear resistance and other mechanical properties. In conclusion, heat treatment is of great importance in improving the wear resistance as well as mechanical properties of high chromium cast irons. It is well known that the hardness of eutectic carbide rises with increasing level of Cr. However, it was reported somewhere that the macro-hardness does not always correspond to the Cr content in specimens with the same eutectic ratio. 7) Since the eutectic M 7 C 3 carbides in cast irons with 10 to 26 mass% Cr do not change in the configuration during destabilization heat treatment, 8) the micro-hardness of the matrix greatly affects the macro-hardness. The micro-hardness is closely related to the amount of retained austenite and martensite which is determined by the kind and amount of alloying elements and the heat treatment condition. In spite of many studies on the heat treatment of high chromium cast iron, 2,3,6 14) systematic research on the effects of third elements on micro-hardness of matrix during heat treatment have not been reported. In this program, the effects of third alloying elements added singly to 16 mass% Cr and 26 mass% Cr cast irons with hypoeutectic composition were determined. Alloying elements are Ni or Cu which is usually added to improve the hardenability and Mo or V which promotes the precipitation hardening. The investigations were focused on the variation of micro-hardness and volume fraction of retained austenite (V ) associated with heat treatment condition. Finally, the mechanism of the secondary hardening was clarified. 2. Experimental Procedure 2.1 Preparation of test specimens Hypoeutectic high chromium cast irons containing 16 mass% and 26 mass% Cr with a separate addition of Ni or Cu up to 2 mass% and Mo or V up to 3 mass% as well as without alloy addition were produced using a high frequency

2 Effect of Alloying Elements on Variation of Micro-Hardness during Heat Treatment of Hypoeutectic High Chromium Cast Iron 2323 Table 1 Chemical composition of test specimens. Number Element (mass%) C Cr Si Mn Ni Cu Mo V No No No No No No No No No No No No No No No No No No No No No No induction furnace. The charge materials were melted down and superheated to 1853 K. The melt was poured from 1773 to 1793 K into preheated CO 2 mold with a cavity size of 25 mm in diameter and 65 mm length. After pouring, the melt was covered with dry exothermic powder to prevent the riser from fast cooling. The specimens were sectioned by a wire-cut machine to obtain disk-shaped test pieces of 7 mm thickness. The chemical compositions of the test specimens are shown in Table Heat treatment Specimens were annealed at 1173 K for 10.8 ks and then cooled in a furnace. The annealed specimens were hardened by fan air cooling (FAC) with an approximate cooling rate of 12 K/s at austenitizing temperatures of 1273 K and 1323 K for 5.4 ks. The hardened specimens were tempered at 50 K intervals from 573 K to 873 K for 7.2 ks and cooled in air. 2.3 Measurement of hardness and retained austenite The micro-hardness measurements were carried out using a Micro-Vickers hardness tester with applied load of 1 N (0.1 kgf). The volume fraction of retained austenite (V ) was obtained by an X-ray diffraction method using a special goniometer with automatic rotating and swinging sample stage. The diffraction peaks chosen for calculation were (200) and (220) planes for ferrite () or martensite and (220) and (311) planes for austenite (). 15) 3. Experimental Results and Discussions 3.1 As-hardened microstructure of test specimen In order to comprehend the behavior of the matrix structure in the as-hardened state, the sample microstructures were observed by SEM focusing on the carbides within the matrix. As an example, the microstructures of 16 mass% Cr cast irons hardened from 1323 K are shown in Fig. 1 and the constituent phases in the microstructure are shown by labels representatively on the photograph of specimen with 1 mass% Mo. The matrix structure consists of comparably large amounts of fine precipitated carbides, certain martensite and retained austenite. G. Laid II and Powell 14) reported that the secondary carbides precipitated in as-hardened state of high chromium cast iron are normally the M 7 C 3 and/or M 23 C 6 types. In the case of 26 mass% Cr cast irons, the matrix was also dominated by secondary carbides, martensite and some retained austenite. Some pearlite appear in the matrix of the specimen with V. This may occur because V reduces C content in the matrix and resultantly decreases the hardenability of the cast iron. 16,17) The secondary carbides are less in the amount and seem to be of smaller size than those in 16 mass% Cr cast irons. 3.2 Effect of alloying elements on variation of microhardness and volume fraction of retained austenite (V ) As-hardened state The primary purpose of the destabilization treatment is to produce a martensitic structure from an austenitic matrix supersaturated with C and Cr in the as-cast state. Generally, the final micro-hardness depends greatly on the hardness of martensite itself and V, the greater the amount of martensite and the lower V, the higher is the micro-hardness. The micro-hardness and V were measured in all the ashardened specimens and the relationships between alloying elements and micro-hardness are illustrated in Fig. 2. Over all, when compared with alloy-free specimens, the microhardness decreased with increasing Ni and Cu contents and increased gradually with increasing Mo content. The V content increases the micro-hardness in 16 mass% Cr but unexpectedly reduces it in 26 mass% Cr cast irons. With respect to the effect of austenitizing temperature, the micro-hardness is substantially low in 16 mass% Cr cast irons and high in 26 mass% Cr cast irons when the austenitizing temperature is elevated. The highest micro-hardness is obtained in the Mo-bearing specimen. In 16 mass% Cr cast irons, it is clear that the largest growth of micro-hardness is obtained in the specimens with Mo, followed by those with V, Cu and Ni, respectively. The increase of micro-hardness in Ni and Cu bearing specimens is determined by the proportion of austenite and martensite in the matrix. Ni has a stronger effect on stabilizing austenite than does Cu. Mo delays the pearlite transformation greatly but the decreasing rate of Ms temperature is rather small. The micro-hardness of Mo specimens rises due to an increase in martensite and the precipitation of special molybdenum carbides. 8,18) V promotes the precipitation of vanadium carbide with high hardness during destabilization and increases the amount of martensite by decreasing the carbon content in austenite. 8,16,17) In the case of 26 mass% Cr cast irons, the decreasing rate is different from 16 mass% Cr cast irons. The largest increasing rate is obtained in cast irons with Mo followed by Cu, Ni and V containing cast irons,

3 2324 S. Inthidech and Y. Matsubara Content Alloy Alloy-free 1 mass% 2 mass% Ni Cu Mo V Fig. 1 As-hardened microstructures of 16 mass% Cr cast irons with and without a third alloying element. Hardening: 1323 K 5.4 ks. respectively. The effects of Ni, Cu and Mo on the increasing rate are as mentioned above. As for V-bearing specimens, it can be explained that since most of austenite transforms into pearlite and small amount of martensite during cooling, the hardness is lowered greatly. Effects of Ni, Cu, Mo and V on the V are shown respectively in Fig. 3. The V increases gradually with increasing alloy content except for V. The V values are much greater in 16 mass% Cr cast irons than those in 26 mass% Cr cast irons and they are high when the austenitizing temperature increases. The growth of V by Ni addition is the greatest followed by the order of Cu, Mo and V. As the austenitizing temperature increases, the increasing rate of V in Ni, Cu and Mo bearing cast irons is positive but that of V-bearing cast irons is negative. It has been reported that V has the strongest effect in lowering the rate of Ms temperature followed by Ni, Cu and Mo. 8) When it is considered that the lowered rate of Ms temperature is connected to the increasing rate of V, Ni and Cu correspond well to the report. Ni and Cu increase the V greatly. This is because both alloying elements dissolve all in austenite and stabilize it. Mo and V are strong carbide forming elements and they prefer to dissolve in their eutectic carbides during solidification. 18,19) The rest of Mo and V, not consumed in carbide

4 Effect of Alloying Elements on Variation of Micro-Hardness during Heat Treatment of Hypoeutectic High Chromium Cast Iron 2325 (a) Ni (b) Cu (c) Mo (d) V Fig. 2 Effects of alloying elements on micro-hardness in as-hardened cast irons with 16 mass% Cr and 26 mass% Cr. (a) for Ni, (b) for Cu, (c) for Mo and (d) for V. (a) Ni (b) Cu (c) Mo (d) V Fig. 3 Effects of alloying elements on volume fraction of retained austenite (V ) in as-hardened cast irons with 16 mass% Cr and 26 mass% Cr. (a) for Ni, (b) for Cu, (c) for Mo and (d) for V. formation, dissolves in austenite, and it participates in the solid state transformation and affects Ms temperature. The amount of dissolution of an element in austenite is determined by the partition coefficient (Kx ). It was reported that the partition coefficient in austenite of Mo (K Mo ) is greater than that of V (K V ), and the dissolution of Mo and V increases with increasing Cr content. 20) Even if both Mo and V are strong carbide formers, the ability of V is much larger than that of Mo. Therefore, V can form its carbide with even less dissolution in austenite. Consequently, simultaneously C concentration in austenite reduces greatly. This causes the rise of Ms temperature and subsequently, the V decreases.

5 2326 S. Inthidech and Y. Matsubara Alloy-free Ni Mo 1 mass% 1 mass% 2 mass% 2 mass% Fig. 4 Relationships between micro-hardness, volume fraction of retained austenite (V ) and tempering temperature in 16 mass% Cr cast irons with Ni, Mo and without third alloying element Tempered state The relationships between micro-hardness, V and tempering temperature of 16 mass% Cr and 26 mass% Cr cast irons for alloy-free and Ni or Mo addition are shown in Fig. 4 and Fig. 5, respectively. In each test specimen, the tempered micro-hardness curve first increases to a maximum point and then decreases with increasing tempering temperature. That is to say, it shows secondary hardening. The tempered microhardness shows more or less a secondary hardening due to both precipitation of secondary carbides and martensite transformed from destabilized austenite. The degree of secondary hardening (H D ), which is defined as the difference in hardness between the maximum tempered microhardness (H MTmax ) and the hardness at which the secondary hardening begins, is greater in the case of 1323 K austenitization. The H D is much greater in alloyed cast irons compared with that in the alloy-free cast iron. The H MTmax is high in the cast iron hardened from 1323 K austenitization. The H MTmax is obtained when the cast iron is tempered from 698 K to 823 K. This effect depends on the kind and the amount of alloying element. It is clear from these results that the higher austenitizing temperature, the more of both the H D and H MTmax. The tempering temperature to obtain the H MTmax shifts to the high temperature side when the austenitizing temperature is elevated. This is because the higher tempering temperature is needed to decompose the large amount of austenite in the as-hardened state into martensite under the same holding time. The V in the as-hardened state begins to decline gradually when the tempering temperature rises to 673 K and then they decrease abruptly. The tempering temperature at which the V disappears ranges from 773 to 873 K. The V values at the H MTmax are less than 20%. The relationship between the micro-hardness and V in the tempered state is shown in Fig. 6(a) and Fig. 6(b) for 1273 and 1323 K austenitizations, respectively. The micro-hardness decreases after the maximum value which is about 5% V in 1273 K and 10% to 20% V in 1323 K austenitizations. For 1273 K austenitization, there is no difference in the extent of decrease of micro-hardness between chromium content of

6 Effect of Alloying Elements on Variation of Micro-Hardness during Heat Treatment of Hypoeutectic High Chromium Cast Iron 2327 Ni Alloy-free Mo 1 mass% 1 mass% 2 mass% 2 mass% Fig. 5 Relationships between micro-hardness, volume fraction of retained austenite (V ) and tempering temperature in 26 mass% Cr cast irons with Ni, Mo and without third alloying element. (a) Austenitized at 1273 K (b) Austenitized at 1323 K Fig. 6 Relationships between micro-hardness and V of tempered cast irons with third alloying elements. (a): austenitized at 1273 K, (b): austenitized at 1323 K for 5.4 ks. the cast irons. In 1323 K austenitization, the decreasing tendency is separated into two groups, 16 mass% Cr and 26 mass% Cr cast irons, but the extents of decline are almost the same. At the same V value, the micro-hardness of 16 mass% Cr cast irons is higher than that of 26 mass% Cr cast irons. The specimens distributed with exceptionally high hardness contain Mo. This may be because some molybdenum carbides can precipitate secondarily in the matrix.

7 2328 S. Inthidech and Y. Matsubara (a) 16 mass% Cr cast iron (b) 26 mass% Cr cast iron Fig. 7 Relationships between maximum tempered micro-hardness (H MTmax ) and volume fraction of retained austenite (V ) in the as-hardened state. (a) 16 mass% Cr and (b) 26 mass% Cr cast irons. (a) Ni (b) Cu (c) Mo (d) V Fig. 8 Effects of alloying elements on maximum tempered micro-hardness (H MTmax ) in 16 mass% Cr and 26 mass% Cr cast irons. (a) for Ni, (b) for Cu, (c) for Mo and (d) for V. The H MTmax, as connected to V in as-hardened state, is shown in Fig. 7(a) for 16 mass% Cr and Fig. 7(b) for 26 mass% Cr cast irons. In 16 mass% Cr cast irons, when the V increases, the H MTmax rises a very little in the cast irons with Ni and Cu and it increases gradually in the cast irons with Mo and V. In 26 mass% Cr cast irons, the H MTmax of the cast irons with Cu, Mo and V rises remarkably with an increase in the V. On the other hand, the H MTmax does not change much in the cast iron with Ni. This means that in the range of less than 10% V, the micro-harness is not only effected by the transformation of austenite into martensite but also the precipitation of secondary carbides from the carbide reaction during tempering. In the specimens with Mo, the H MTmax increases with the V in both series of the cast irons. The reason is due to an increase in the amount of the precipitated special molybdenum carbides and martensite transformed from destabilized austenite as the V in the ashardened state increases. The H MTmax of the V-bearing cast iron increases in the small range of V less than 5%. This suggests that although a small mount of dissolution in austenite occurs, V can promote its secondary carbides precipitation during tempering and causes a greater increase of micro-hardness. The effects of alloying elements on the H MTmax are shown in Fig. 8. Ni and Cu do not show a significant effect on the H MTmax. However, Ni seems to decrease the H MTmax slightly in 26 mass% Cr cast irons. Mo increases the H MTmax gradually in both series of the cast irons. V increases the H MTmax a little in 16 mass% Cr cast irons but reduces it in 26 mass% Cr cast irons. At the same alloy content, the H MTmax value of each specimen does not make much difference by increasing the austenitizing temperature. When

8 Effect of Alloying Elements on Variation of Micro-Hardness during Heat Treatment of Hypoeutectic High Chromium Cast Iron 2329 the degree of increase in the H MTmax is considered, Mo shows the highest degree, and followed by V, Cu and Ni in 16 mass% Cr cast irons. In 26 mass% Cr cast irons, on the other hand, the degree decreases by the order of Mo, Cu, Ni and V, respectively. The difference in the order of increase between Ni and V shows obvious results in this investigation. It can be explained by that V decreases the hardenability and it also promotes the pearlite transformation. Therefore, the increasing degree of the cast iron with V is low in 26 mass% Cr cast iron. The highest values of H MTmax, 721 HV0.1 in 16 mass% Cr and 730 HV0.1 in 26 mass% Cr cast irons with 3 mass% Mo, were obtained Mechanism of secondary hardening The experimental results show that retained austenite is closely related to the micro-hardness in both the as-hardened state and tempered states and apart from the behavior of secondary hardening. Secondary hardening in tempering is considered to occur by the following mechanism; (a) precipitation of secondary carbides from martensite, (b) precipitation of secondary carbides from retained austenite in the as-hardened state and (c) transformation of retained austenite destabilized by tempering into martensite. In this study, it is believed that the increasing degree of the microhardness is greater in the mechanism (a) and (c). This reason is explained as follows; In the case of high chromium cast iron containing strong carbide former, so-called carbide reaction takes place at the high tempering temperature over 773 K, 21) at which the alloying element can defuse by itself to form its special carbide, say, Mo 2 C or VC with extremely high hardness compared with M 2 C[(FeMo) 2 C] or MC[(FeV)C] type formed in the reaction (b). It is quite right that martensite transformed from the decomposed austenite in the post-cooling after tempering has high hardness and contributes to an increase in the secondary hardening (c). The effect of V on the H D is shown in Fig. 9. It is clear that the H D is closely related to the V in the as-hardened state and it increases in proportion to the V. It has been clarified that the V is not only determined by Cr and C contents but also the kind and amount of alloying elements and austenitizing temperature. It can be concluded from Fig. 9 that V in the as-hardened state contributed more to the secondary hardening. The relationship between the H D and H MTmax values was obtained using 16 mass% Cr cast irons containing much more V in the as-hardened state and it is illustrated in Fig. 10. Even if the H D increases, the H MTmax values in alloy-free, Ni or Cu bearing cast irons do not make much difference. The H MTmax values of Mo and V bearing cast irons increase slightly with an increase in the H D and they are in higher position than those in alloy-free, Ni or Cu containing cast irons. Since Ni and Cu do not form their special carbides, the tempering behavior of Ni and Cu cast irons must be the same as an alloy-free specimen, that is, the precipitation of chromium carbides of M 7 C 3 or M 23 C 8,14,18) 6 could contribute to the secondary hardening. Even if the specimens with more V in the as-hardened state have large H D, the H MTmax values did not show dominant difference. This is because the hardness at which the secondary hardening occurs is lower when the H D increases. Fig. 9 Effects of volume fraction of retained austenite (V ) in the ashardened state on the degree of secondary hardening (H D ) in cast irons with third alloying elements. Fig. 10 Relationship between maximum tempered micro-hardness (H MTmax ) and the degree of the secondary hardening (H D ) in 16 mass% Cr cast irons. In the case of cast irons containing Mo, higher H MTmax values are obtained. It is considered that the micro-hardness increases due to the precipitation of special carbides which are of much higher hardness than chromium carbides by tempering or carbide reaction at high temperature. The martensite transformed from destabilized austenite also raises the micro-hardness. Therefore, the increase in the H MTmax value of cast irons with Mo must be due to the precipitation of special molybdenum carbides. For cast irons with V, it was reported that V preferred to dissolve in eutectic carbides, and the hardness of eutectic carbides was increased. 8,19) As V dissolves into carbides preferentially, the dissolution of V in austenite is decreased and certainly the V content in martensite is reduced. As a result, the amount of special vanadium carbide precipitating from martensite by carbide reaction is decreased. Even if the H D is increased by V addition, the hardness of the martensite transformed from austenite is low because the C content is reduced by

9 2330 S. Inthidech and Y. Matsubara increasing V content. According to Fig. 10, the H MTmax shows the similar behavior to that in Fig. 7(a). These reasons are mostly same as the discussions described above. Here, it can be concluded that the alloying element showing the highest effect on the H MTmax is Mo followed by V. In order to improve the matrix hardness after tempering, the high V in the as-hardened state is necessary to increase the secondary hardening, say, by employing a high austenitizing temperature. In the case of Ni and Cu, they do not show a dominant effect on the H MTmax. Such alloying elements should be added to improve the hardenability. It is presumed that the secondary hardening of 26 mass% Cr cast irons can be caused by the similar mechanism to the case of 16 mass% Cr cast irons even if the V value is less. 4. Conclusions The effect of alloying element on the variation of microhardness of hypoeutectic high chromium cast irons containing 16 mass% Cr and 26 mass% Cr with single addition of Ni, Cu, Mo and V was systematically investigated. After annealing at 1173 K, the cast irons were hardened by fan air cooling at 1273 and 1323 K austenitizations for 5.4 ks. The hardened specimens were tempered at 50 K intervals from 573 to 873 K for 7.2 ks and cooled in air. The micro-hardness of matrix and volume fraction of retained austenite (V ) were measured. The effects of heat treatment condition and alloying elements on the matrix hardness as well as the V in matrix were clarified. The following conclusions have been drawn from the experimental results. (1) The micro-hardness decreased gradually with an increase in Ni and Cu content, but it increased slightly with Mo content in both the 16 mass% and 26 mass% Cr cast irons. V did not show any significant effect on the micro-hardness in 16 mass% Cr cast irons but it reduced the micro-hardness greatly in 26 mass% Cr cast irons. When the austenitizing temperature increased, the micro-hardness decreased in 16 mass% Cr cast irons but increased in 26 mass% Cr cast irons. (2) The V rose with an increase in the amount of alloying element except for V. At the same alloy content, the V was greater in 16 mass% Cr cast irons than that in 26 mass% Cr cast irons. An increase in austenitizing temperature led to an increase in the V overall. (3) The curve of tempered micro-hardness showed a secondary hardening due to the precipitation of special carbides formed by carbide forming elements of Mo and V and the transformation of destabilized austenite into martensite. The degree of secondary hardening was greater in the cast irons hardened from higher austenitizing temperature, 1323 K. The peak of the microhardness which was expressed as H MTmax (maximum tempered micro-hardness) was mostly obtained when the cast iron was tempered at 748 to 823 K. (4) The H MTmax value was high in the case of 1323 K austenitization and it appeared on the higher temperature side as the austenitizing temperature increased. The H MTmax value was obtained when the V was less than 20% and it was slightly increased with an increase of V in the as-hardened state. (5) Mo increased the H MTmax slightly in both 16 mass% and 26 mass% Cr cast irons. Ni and Cu did not show a major effect on the H MTmax. V increased the H MTmax a little in 16 mass% Cr cast irons but reduced it greatly in 26 mass% Cr cast irons. The highest value of H MTmax was obtained in the cast irons containing 3 mass% Mo. (6) Secondary hardening occurred in this study mainly by the following mechanism, (a) precipitation of secondary carbide from martensite at high temperature. 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