Tempered martensite embrittlement in Fe-Mo-C and Fe-W-C steel
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I.
INTRODUCTION
A L T H O U G H the strength decreases upon tempering of martensitic steels, a loss in toughness occurs in the tempering temperature range of 250 ~ to 450 ~ This loss in toughness has been referred to as tempered martensite embrittlement (TME).I-~ TME has been usually characterized by a trough in impact energy, as a function of tempering temperature. Recently, the development of analytical transmission electron microscopy has made it possible to characterize the influence of the retained austenite on the TME phenomenon. McMahon and Thomas 6 showed that in Fe-Cr-C steels TME was correlated with the thermal decomposition of interlath retained austenite, followed by the formation of interlath cementite during the tempering process. Furthermore, Clark and Thomas 7 suggested that the absence of resolvable amounts of interlath retained austenite resulted in the absence of the TME phenomenon in Fe-Mo-C steels. However, Goolsby 8 reported the appearance of a clear TME trough associated with interlath cementite precipitates in Fe-Mo-C steel even though he could not detect any interlath retained austenite at the time he performed his experiment. King et al 9 regarded the coarsening of interlath cementite as the basic cause of TME in Fe-C steel. Moreover, it was suggested by Bhadeshia and Edmonds 3 that TME was controlled by the interlath (or intralath) cementite coarsening rather than by the destabilization of the interlath retained austenite in Fe-Mo-C steels and Fe-V-C steel. In general, the impact toughness is influenced by the change of test temperature, and is represented by the ductilebrittle transition curve. The TME phenomenon causes a substantial increase in the ductile-brittle transition temperature. It has been reported that the TME trough with the tempering temperature was affected by change of test temperature) 99-H Those reports were not consistent and not definitely explained, however. It is the purpose of this research to determine (1) if the retained austenite is present in Fe-Mo-C and Fe-W-C steel, H. KWON and C.H. KIM are Graduate Student and Professor, respectively, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology,P.O. Box 150 Cheongryang, Seoul, Korea. Manuscript submittedJuly 27, 1982. METALLURGICALTRANSACTIONSA
(2) how the interlath cementite resulting from the decomposition of the interlath retained austenite is correlated with TME, and (3) how the TME phenomenon is influenced by the change of test temperature.
II.
EXPERIMENTAL PROCEDURE
Approximately 20 kg experimental alloy ingots 100 x 100 • 250 mm were prepared from high-purity base materials by means of vacuum induction melting. These ingots were then hot forged to form bars 15 x 15 mm in cross section. The alloy compositions are listed in Table I. Three millimeter thick, 25 mm gage length tensile specimens and standard size Charpy V-notch impact specimens were made from these alloys. All of the test specimens were austenitized at 1200 ~ for one hour in a flowing argon atmos
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