The effects of double austenitization on the mechanical properties of a 0.34C containing low-alloy Ni-Cr-Mo-V steel
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I.
INTRODUCTION
A combination of high strength and high toughness has been the goal pursued during the development of high-strength steels. To achieve this purpose, several heat-treatment methods have been advanced, including high-temperature austenitization (HAT), tl,21 rapid cyclic austenitization,]3j double austenitization (DA), 14,5,61 and duplex treatment (DT). tTl The advantage of DA treatment was proposed by Rao and Thomas t4,5] and Sarikaya e t a / . t61 These authors studied the experimental 2000 MPa ultimate tensile strength Fe-4Cr-0.3C steels tempered at 200 ~ to 265 ~ In order to eliminate undesirable alloy carbide morphologies from the microstructure, it was necessary to austenitize the Fe-4Cr-0.3C steel at a high temperature (1100 ~ for 1 hour). However, a very high austenitizing temperature led to an increase in grain size. It was shown that improvements in toughness could be achieved by DA treatment, viz., austenitizing at a high temperature of 1100 ~ plus a low temperature of 870 ~ (both for 1 hour). This treatment has combined the benefits of achieving a homogeneous austenite phase free from alloy carbides that has small austenite grains. Some other research on the tentative application of DA has been attempted. In the work of Kar et al., tsl the 52,100 ball bearing steel was double austenitized at 1150 ~ + 900 ~ and martempered at 255 ~ before quenching to room temperature. An A533B pressure vessel steel, studied by Bowen and Knott, t9j was double austenitized at 1250 ~ + 900 ~ but the steel was not tempered. It should be mentioned that all the foregoing steels reported were tempered below 300 ~ By exercising, a processing method may usually influence several metallurgical factors simultaneously, and E. CHANG, Professor, and C.Y. CHANG, Graduate Student, are with the Department of Materials Science and Engineering, National Cheng-Kung University, Tainan, Taiwan, Republic of China. C.D. LIU, Superintendent, is with the Taiwan Machinery and Manufacturing Corp., Kaohsiung, Taiwan, Republic of China. Manuscript submitted April 11, 1991. METALLURGICALAND MATERIALS TRANSACTIONS A
these factors will in turn determine the impact or fracture toughness of high-strength steels. Thus, the subject was intensively debated in the literature. It was proposed that prior-austenite grain size, tl~ pack size, t9'lll retained austenite, tS,6,s,12-141 twinning, tl3,151 size, shape, and spacing of inclusion, t16,171 carbide size, tls'lg,2~ interparticle spacing of carbide, tlS'211 duplex martensite-bainite structure, 18'22"231 s u b s t r u c t u r e , t24] matrix flow stress, t12'251 segregation either during austenitizationtlSj or during tempering, [18'26'271 and a combination of impurity segregation and carbide formation (tempered martensite embrittlement)tES-3u are possible important factors determining the toughness of tempered martensite. The beneficial effects of DA treatment on the martensitic high-strength steels tempered to 200 ~ to 265 ~ were attributed to several causes, such as grain refining, increase
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