Deformation, fracture, and mechanical properties of low-temperature-tempered martensite in SAE 43xx steels
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I. INTRODUCTION For applications that require high strength, hardenable, low-alloy, medium-carbon steels are austenitized, quenched to martensite, and tempered at low temperatures between 150 7C and 200 7C. The martensitic microstructure consists of lath-shaped martensitic crystals arranged parallel to one another in groups termed packets, and each parent austenite grain is divided into several packets.[1,2] Tempering at low temperatures, between 100 7C and 200 7C, according to the early work of Cohen and his co-workers, is commonly referred to as the first stage of tempering.[3,4] In this stage of tempering, the carbon supersaturation of the as-quenched (AQ) martensite is relieved by the precipitation of fine transition carbides. These carbides have been identified as epsilon carbide with a hexagonal structure[5] or eta carbide with an orthohombic structure,[6] and have been shown by transmission electron microscopy to form in linear clusters of particles 2 to 5 nm in size.[6,7] Although there is considerable literature information available concerning the hardenability, hardness, and mechanical properties for hardenable low-alloy medium carbon steels,[8,9] relatively few investigations have coupled the deformation and fracture characteristics of ultra-highstrength, low-temperature-tempered (LTT) martensitic steels to mechanical properties, especially when tempering is performed below 200 7C. The relationship of strength to fracture is especially important in the ultra-high-strength LTT martensitic steels that have low toughness and, depending on carbon content and heat-treating conditions are susceptible to various forms of embrittlement.
MARIO SAEGLITZ, Research Engineer, is with the Deutsche Bahn, AG, 14774 Brandenburg-Kirchmoeser, Germany. GEORGE KRAUSS, John Henry Moore Professor, is with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. Manuscript submitted March 13, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
Recent articles that have explored the deformation and fracture of LTT martensitic steels emphasize that high strengths are obtained by high rates of strain hardening and correlate with increasing densities of transition carbides with increasing steel carbon content.[10,11,12] In steels containing up to 0.5 mass pct C, the fracture of the LTT martensite is ductile and occurs by microvoid nucleation, growth, and coalescence at inclusions and carbide particles retained after austenitizing. The transition carbides and the substructure of the martensite play no direct role in fracture but determine the magnitude of the ultimate tensile strength and the amount of postuniform elongation or necking required to generate the stresses that cause microvoid nucleation. The LTT martensitic steels containing more than 0.5 mass pct C fracture by brittle intergranular mechanisms associated with phosphorous segregation and carbide formation at austenite grain boundaries during austenitizing and quenching.[12,13] Although this grain boundary embrittlement is simi
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