Consideration on the Intergranular Tempered Martensite Embrittlement
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Consideration on the Intergranular Tempered Martensite Embrittlement H. KWON and C. H. KIM A loss in toughness occurs in the tempering temperature range of 250 ~ to 450 ~ although the strength decreases upon tempering of martensitic steels. This loss in toughness has been called tempered martensite embrittlement (TME). According to the fracture mode, the TME can be classified into two types: transgranular and intergranular TME. The transgranular TME has been generally observed in high purity steels. Ithas been reported that this embrittlement is associated with the formation of interlath carbides following the decomposition of retained austenite as first detected by detailed transmission electron microscopy.~ On the other hand, the intergranular TME has been long observed in commercial purity steels. Recently, however, this embrittlement was also detected in some high purity steels. 5'6'7 H. KWON, formerly with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, is now with the Department of Metallurgical Engineering, Kook-Min University, Seoul, Korea. C.H. KIM is Professor, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Seoul, Korea. Manuscript submitted April 19, 1983. METALLURGICALTRANSACTIONS A
It has been pointed out that the combined action of impurities and carbides at the grain boundaries is necessary to the intergranular TME. 5-1~ Most impurity segregation for this kind of the embrittlement occurs during austenitization. H Thus, this segregation alone in austenite phase is not sufficient to cause a sudden drop in toughness after tempering near the TME temperature. Upon tempering near the TME temperature, plate-like carbides form at the grain boundaries. These grain boundary carbides are apparently essential for the intergranular TME. They can act as slip barriers and help initiate the intergranular cracks at the already impurity-weakened grain boundaries. This leads to a drop in toughness. TME has been usually characterized by a trough in roomtemperature impact energy as a function of tempering temperature, reflecting an increase in the ductile-brittle transition temperature. However, since the occurrence of the embrittlement trough is dependent on the test temperature, it is not always desirable that the test temperature be fixed at room temperature for characterization of TME. Thus, TME can be characterized by a trough in impact energy as a function of tempering temperature, for a given test temperature. Recently, the influence of the test temperature for the TME phenomenon was systematically studied. 4'7'8 Particularly, for the intergranular TME, the TME phenomenon was observed only when the fracture test was performed below a critical test temperature. 78 Briant and Banerji8 reported that the fracture energy was inversely proportional to the amount of intergranular fracture in phophorous doped Ni-Cr steels. However, they did not consider the correlation between the impact energy and the amount of
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