The effects of tempering reactions on temper embrittlement of alloy steels
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INTRODUCTION
M O L Y B D E N U M is the oldest palliative for the problem of temper embrittlement of steel.~ The mechanism by which molybdenum operates in this regard remained obscure until recently. For some time, based on Russian work, 2 it was thought that molybdenum inhibited the diffusion of phosphorus, which is the most common embrittling element. Later, this was disproved, 3 but it was found that molybdenum, among other elements, had a strong effect in lowering the solubility of phosphorus in iron, 3'4 that molybdenum also retarded temper embrittlement by tin and antimony, 5 and that the molybdenum effect seemed to disappear after very long aging times at elevated temperatures. 5 Subsequently, in a study of kinetics of temper embrittlement in a NiCrMo steel doped with phosphorus, it was found that molybdenum greatly retarded the initial embrittling rate, but that at longer aging times large amounts of temper embrittlement o c c u r r e d , 6 as shown in Figure 1. This was interpreted as due to a molybdenum-phosphorus scavenging reaction at short aging times followed later by precipitation of molybdenum in carbides, resulting in the ultimate release of phosphorus which was then free to segregate to grain boundaries. This kind of reaction could account for the very slow embrittlement kinetics observed by Gould in NiCrMoV steels, 7 as shown in Figure 2. More recent work on 2.25 Cr 1 Mo steels 8 and 12 Cr steel has confirmed the previous explanation of the molybdenum effect. The issue of temper embrittlement of the CrMoV high pressure rotor steel has been raised in the United States in the past several years, particularly as a result of the CrMo steel disc failures at Hinckley Point in England in 1969 '0 and the CrMoV steel rotor failure at Gallatin, Tennessee, in 1974." The bainitic CrMoV rotor steel has always been acknowledged to have low fracture toughness because of its Z. QU is Staff Scientist with The Institute of Metal Research, Academia Sinica, Shenyang, Peoples' Republic of China. C.J. McMAHON, Jr. is Professor, Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104. 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.
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