Fracture behavior in medium-carbon martensitic Si- and Ni-steels
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INTRODUCTION
IN the
case of brittle fracture of martensitic steels, the susceptibility to intergranular fracture has been considered mainly from the point of the presence of impurities and/or carbides at the grain boundaries. ~-9 However, the weakness of grain boundaries means that the grain boundaries are relatively brittle compared with the matrix. Thus, the level of matrix toughness can play an important role in controlling brittle fracture behavior. In other words, in the condition of similar grain boundary features, if the resistance to transgranular cleavage fracture is large (i.e., the matrix toughness has a high value), the occurence of intergranular fracture may be relatively easy as a brittle fracture mode, but otherwise (i.e., the matrix toughness has a low value), the transgranular cleavage fracture may occur with relative ease. In martensitic steels, the matrix toughness has been correlated mainly with the microstructural features. ~0-22Recently, Kwon and Kim 8'23 reported that the occurrence of tempered martensite embrittlement can be influenced by the variation of matrix toughness with test temperature. And Zia-Ebrahimi and Krauss 24 considered the important role of both carbides and the matrix (by its effect on strain hardening) on the room temperature toughness and fracture in the mechanism of tempered martensite embrittlement. On the other hand, the effect of alloying additions on intrinsic matrix toughness, which is dependent upon the inherent dislocation motion, has been nearly neglected. However, since the intrinsic matrix toughness becomes a basic factor in controlling the microstructure-sensitivity for transgranular cleavage fracture, whether either intergranular or transgranular cleavage fracture occurs may be also influenced by the intrinsic matrix toughness. It is known that Ni is a powerful alloying element which improves intrinsic matrix toughness and Si is a potent alloying element which lowers toughness in ferritic steels. 25'26'27 In martensitic steels, however, there are few reports indiHOON KWON is Assistant Professor, Department of Metallurgical Engineering, Kookmin University, 861-1, Jeongneung-Dong, Seongbuk-Ku, Seoul 132, Korea. CHONG HEE KIM is Professor, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Cheongryang, P.O. Box 131, Seoul 131, Korea. Manuscript submitted March 28, 1985. METALLURGICALTRANSACTIONS A
cating such effects of alloying elements on the intrinsic matrix toughness. 28'29 Even these reports have been also restricted to low-carbon (C
o 350~ 550~ (a)
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-20O -100 0 100 TEST TEMPERATURE, OC Fig. 1--Variation of Charpy impact energy with test temperature in the 350 ~ and 550 ~ tempered conditions for the Si-steel. 1174--VOLUME 17A, JULY 1986
(b) Fig. 2--Scanning electron micrographs of fracture surfaces showing the intergranular fracture at room temperature in the Si-steel: (a) 350 ~ tempered condition and (b) 550 ~ tempered condition. METALLURGICALTRANSACTIONS A
(a)
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