The Tensile Properties of an Fe-4 Pct Mo-0.2 Pct C Martensite Tempered under Applied Stress
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I.
INTRODUCTION
IT is
well known that mechanical properties of alloys are affected when alloys are stressed or deformed at elevated temperature where solute atoms can move easily. Sometimes, a Portevin-Lechatelier effect,l-7 stressorientation dependency of precipitates, 8'9'1~or embrittlement phenomena "-~6 are observed. In this investigation, Fe-4 pct Mo-0.2 pct C martensite, which can be remarkably precipitation hardened by Mo2C17-21during tempering, was employed. When this martensite was tested by tension at 600 ~ to 700 ~ typical premature failure was observed. This temperature range is higher than that of so-called temper brittleness in martensite. 22-27The temper brittleness is considered to be caused by segregation of impurities (especially, phosphorus) to prior austenite grain boundaries while it is reported that Mo is effective to prevent temper brittleness 22'2s-3~ (especially, 0.25-0.5 pct Mo). And there are many theories for the mechanism of segregation on grain boundary. 32-37 So, to make clear the interrelation between this premature failure and those phenomena (temper brittleness and secondary hardening of Mo2C), the quenched Fe-Mo-C specimens were tempered under applied stress and tensile-tested at room temperature, and the effect of tempering temperature, tempering time, and applied stress on this phenomenon were studied. In addition, the effects of the direction of applied stress and of the solution treatment temperature on the embrittlement phenOmenon were investigated. SHOJI FUJIWARA, Research Assistant, and IMAO TAMURA, Professor, are with the Department of Metal Science and Technology, Kyoto University, Kyoto 606, Japan. TAKATOSHI OGAWA, formerly with Kyoto University, is now with the Japan Steel Works, Ltd., Muroran, Hokkaido, Japan. YOSHIFUMI OHMURA, formerly with Kyoto University, is now with Sanyo Special Steel Company, Ltd., Himeji, Hyogo, Japan. This paper is based on a presentation made at the "Peter G. Winchell Symposium on Tempering of Steel" held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees. METALLURGICALTRANSACTIONS A
II.
EXPERIMENTAL PROCEDURES
A. Material Samples were prepared by vacuum induction melting. The chemical compositions are shown in Table I. Four ingots were prepared, and mainly No. 1 ingot was used. No. 3 ingot having 0.034 pct P was used for studying the effects of solution treatment. By using specimens of the rest of the ingots, no significant difference was observed in the results of this investigation. After hot-forging, they were hot-rolled and cold-rolled to sheet 0.3 mm thick or colddrawn to wire 3 mm in diameter. From the sheet, tensile test pieces shown in Figure l(a) were machined. The wire was cut off in 100 mm length, and both ends of specimens were deformed to 6 mm in diameter by heading. The centers of specimens were electrolytically polished for the gauge section (Figure l(c)). B. Heat Treatment Most of the specimens were a
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