The bainite transformation in chemically heterogeneous 300M high-strength steel
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I.
INTRODUCTION
THE
medium-carbon steel alloyed with several substitutional solutes, 300M, has a typical chemical composition of Fe-0.45C-1.7Si-0.7Mn-l.9Ni-0.8Cr0.4Mo-0.1V (weight percent). The different alloying elements have many purposes, ~J-4~ but they all serve to enhance hardenability to such an extent that a 10-cmdiameter bar can be air-cooled to a uniform martensitic microstructure. Molybdenum also helps to limit impuritycontrolled temper embrittlement effects, and nickel has an intrinsic beneficial influence on the toughness of the martensite, m Vanadium serves to restrict austenite grain growth during the austenitizing heat treatment, usually carried out at a temperature not high enough to completely dissolve particles of vanadium carbides. The composition is, in fact, based on the popular 4340 steel, which acquires its optimum strength and toughness in the quenched and tempered martensitic condition. The 4340 steel unfortunately suffers from tempered martensite embrittlement, in which the toughness goes through a minimum at a tempering temperature of ~-300 ~ At low tempering temperatures, the cementite particles are too fine to be detrimental. Although at high tempering temperatures they are relatively coarser, the yield strength is, by then, rather low so that the material can accommodate coarser particles. It is at the intermediate temperatures, where the curve of toughness v s tempering temperature shows a minimum, that there is an unfavorable combination of cementite particle size and yield strength. The 300M steel was designed to avoid this problem by increasing the silicon concentration to a level high enough to retard the coarsening of cementite and, hence, to shift the toughness minimum to a higher temS.A. KHAN, Research Student, and H.K.D.H. BHADESHIA, Lecturer, are with the Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, United Kingdom. This paper is based on a presentation made in the symposium "International Conference on Bainite" presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee. METALLURGICAL TRANSACTIONS A
pering temperature. It can, therefore, be tempered at a relatively low temperature and, hence, avoid the large loss of strength associated with tempering at elevated temperatures.[l,2.3] Both 4340 and 300M steel are best utilized in the quenched and tempered condition. When attempts are made to use 4340 steel in a'bainitic condition, the toughness obtained is not impressive because of the coarse cementite particles associated with upper bainite in this medium-carbon steel. On the other hand, when 300M steel is isothermally transformed to upper bainite, cementite does not readily precipitate, and the microstructure then contains only bainitic ferrite and residual austenite at the isothermal transformation temperature.* *Throughout this paper, the term "residual austen
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