The evaluation of tempered martensite embrittlement in 4130 steel by instrumented charpy V-notch testing
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INTRODUCTION
P L A I N carbon and low alloy martensitic steels tempered in the range 250 ~ to 450 ~ are susceptible to an embrittlement characterized by a trough in room temperature notch toughness v s tempering temperature. 1-14 The embrittlement is commonly referred to as tempered martensite embrittlement (TME). In general, the level of toughness, the severity of embrittlement as governed by the depth of the trough, the temperature range of embrittlement, and the failure mechanism depend on complex interactions between steel composition, heat treating conditions, test temperature, and testing method. Both intergranular 5'11'12and transgranular 5-8 fracture have been observed to accompany TME. The combination Of impurity segregation, primarily P, to austenite grain boundaries during austenitizing and formation of cementite at prior austenite grain boundaries on tempering both appear to be necessary for the intergranular form of TME. 5'12 The transgranular fracture associated with TME may be interlath, i . e . , between the parallel martensite laths, 6'7'13 or it may be translath, i . e . , across the martensitic laths. 5'8'14 Cementite formed during tempering is believed to provide an easy fracture path for interlath fracture 6 or crack initiation sites for translath fracture. 8 Both interlath 6'15 and grain boundary ~6 cementite is formed by the decomposition of retained austenite on tempering in the TME range. Horn and Ritchie 7 have proposed that mechanical instability of the interlath austenite also contributes to TME. Bhadeshia and Edmonds 14 have suggested that in an Fe-C-Mo steel the E ZIA-EBRAHIMI, Postdoctoral Research Associate, and G. KRAUSS, AMAX Foundation Professor, are both with the Department of Metallurgical Engineering, Colorado School of Mines, Golden, CO 80401. 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
TME is controlled by relatively coarser intralath cementite rather than by the interlath cementite resulting from the decomposition of small amounts (less than 2 pct) of retained austenite present between the martensite laths. As noted earlier the method and conditions of toughness testing significantly affect the ability to detect TME. Charpy impact energy v s tempering temperature of hardenable steels always show a minimum and/or plateau associated with specimens tempered between 250 ~ and 450 ~ However, plane-strain fracture toughness data sometimes are in direct contradiction to Charpy data. For example, Kula and Anctil, 4 working with 4130 steel, have shown that no minimum could be found in room temperature Kic as a function of tempering temperature, although CVN impact energy clearly showed TME. Similarly, Walker and M a y 17 have not found a minimum in room temperature K~c values in a Ni-Cr-Mo-V steel. Materkowski and Kra
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