Mechanical properties of 0.40 pct C-Ni-Cr-Mo high strength steel having a mixed structure of martensite and bainite
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INTRODUCTION
IT has generally been recognized that the microstructure resulting from phase transformation plays a predominant role in determining mechanical properties of heat-treated high strength low alloy steels. The microstructure is classified primarily into single phase transformation products such as martensite or bainite and the mixed structures of martensite and nonmartensitic decomposition products. So far, considerable effort has been directed toward revealing the relationship between the microstructural features of a single phase martensite or bainite and the mechanical properties. However, very little study has been systematically made to clarify the controlling factor that affects mechanical properties of the mixed structure of martensite and nonmartensitic decomposition products, in spite of the facts that such mixed structures have frequently been encountered in commercial practice and that improvement in mechanical properties has often been associated with such mixed structures. 1-~ Thus, we believe that better comprehension of mixed structures will provide new suggestions for modification of commercial heat treatment techniques and/or lead to new ideas for developing better mechanical properties of high strength low alloy steels. Bailey5 has found that mechanical properties of leaner alloy steels such as AISI 1340 and 5140 were significantly impaired when 30 vol pct nonmartensitic decomposition products appeared and that the impairment of the mechanical properties became greater as the reaction temperature of YOSHIYUKI TOMITA and KUNIO OKABAYASHI are with the Department of Metallurgical Engineering, College of Engineering, University of Osaka Prefecture, 4-804 Mozu-Umemachi, Sakai, Osaka 591, Japan. Manuscript submitted July 15, 1983. METALLURGICAL TRANSACTIONS A
the nonmartensitic decomposition products increased. A higher alloy steel such as AISI 2340, however, which appeared to have the least carbon enrichment of austenite, was affected least by the presence of 30 vol pct nonmartensitic decomposition products. From these results, he suggested that the main controlling factor governing the impairment was a higher carbon martensite which was formed as a result of carbon enrichment of austenite. Hehemann e t a l . ~ have revealed the influence of bainite on mechanical properties of AISI 4340 steel at high strength levels. They have demonstrated that as the temperature of formation of the bainite is lowered, damage to mechanical behavior becomes less severe until at reaction temperatures near the Ms. reduction in area and toughness of mixed structure is equivalent or slightly superior to that of fully martensitic structure. The detrimental effect of high temperature bainite on mechanical behavior is associated primarily with the strength differential between bainite and martensite: However, based on our recent investigations6'7,8 of the microstructure and mechanical properties of high and ultrahigh strength low alloy steels having a comparatively low strength second phase, it appears likely t
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