The effects of grain-refining precipitates on the development of toughness in 4340 steel
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I.
INTRODUCTION
THE resistance of high-strength steels to transgranular fracture has been related to a variety of factors such as austenite grain size,[1,2,3] martensite morphology,[4,5,6] temper carbide morphology,[7–10] the presence of nonmartensitic transformation products,[11,12] and the content and dispersion of interlath retained austenite,[4–6,11] nonmetallic inclusions,[13–15] and other large second-phase particles in the microstructure.[16–20] Iron-based carbides[2–5,21] and other small particles[22,23] retained through heat treatment also have been implicated as features with the potential to degrade the toughness of tempered martensite. Various investigators[1,4,22,24] have noted the effects of small second-phase particles on the fracture resistance of high-strength steels, particularly as regards the increases in fracture toughness that result from the dissolution of the particles with increases in austenitization temperature. Spitzig,[25] on the other hand, speculated that ε carbide could be responsible for the nucleation of fine-scale microvoids in lightly tempered, 0.45 wt pct C alloy steels containing appreciable amounts of aluminum (0.023 to 0.027 wt pct) and nitrogen (70 to 90 ppm). Krauss[26,27] and Saeglitz and Krauss[28] more recently have suggested that the strain hardening associated with interactions between dislocations and temper carbides only provides a necessary condition for tensile separation when the flow stress is raised to the ductile fracture strength in lightly tempered structures. A sufficient condition for secondary microvoid initiation in the strain hardened matrix is derived from the presence of comparatively larger second-phase particles in the microstructure, although the particles postulated to limit ductility were never identified as fractographic features in these investigations. M.J. LEAP, Research Specialist, and J.C. WINGERT, Research Analyst, are with the Materials Science Department, The Timken Company, Canton, OH 44706-0930. Manuscript submitted February 23, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
With the exception of work by Garrison,[29] Gore et al.,[30] Garrison and Handerhan,[31] Yano et al.,[32–34] and Chang et al.,[35] very little effort has directly focused on the degradation in toughness that results from the presence of smaller second-phase precipitates in high-strength steels. However, establishing a direct link between the content and dispersion of these features and the toughness of tempered martensite provides a fundamentally necessary basis for understanding the mechanisms governing the resistance to brittle and ductile fracture in a wide variety of commercial air-melt steels. Grain-refining precipitates such as aluminum nitride and microalloy carbonitrides have been more recently identified as microstructural features that can significantly degrade the fracture resistance of grain-refined, high-strength steels,[36–39] and a process has been developed[40] to optimize the impact toughness of these steels via the refinement of grain-refining
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