Stress-state effects on the stress-induced martensitic transformation of carburized 4320 steels
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INTRODUCTION
AFTER carburization, bearing steels are hardened by austenitizing and oil quenching. At room temperature, the microstructure of carburized steels processed in this way consists of both martensite and metastable austenite phases. Part of the parent austenite phase is retained, because the driving force for transformation from austenite to martensite during quenching is not sufficient to complete the transformation due to internal stress on the austenite phase imposed by the martensitic regions. Under a favorable applied stress or with a further decrease in temperature, the retained austenite transforms to martensite with a corresponding increase in volume of about 4 pct.[1,2] The dimensional changes resulting from this volumetric expansion can exceed tight tolerances in bearings and other applications,[3] influencing fatigue and fracture behavior. Thus, it is important to understand the mechanism behind these changes with respect to applied stress conditions. The effect of stress on martensitic transformation was first extensively studied by Kulin et al.[4] Since this work, numerous studies have considered the effect of stress state on martensitic transformation of ferrous alloys.[2–9] Within a specific range above Ms (martensite start temperature), a stress applied to an austenitic or a martensitic-austenitic composite sample induces the onset of transformation. In this regime, transformation occurs in a stress-assisted manner on the same sites which are responsible for transformation taking place without any applied stress.[6,10] If the new nucleation sites are formed by plastic deformation, the mechanism is called strain-induced transformation.[10,11] The I. KARAMAN and M. BALZER, Graduate Research Assistants, and HUSEYIN SEHITOGLU, Professor and Associate Head, are with the Mechanical and Industrial Engineering Department, University of Illinois at Urbana-Champaign, Urbana, IL 61801. H.J. MAIER, formerly Visiting Researcher, Mechanical and Industrial Engineering Department, University of Illinois, is Senior Engineer with the Institut fu¨r Werkstofftechnik, Universita¨t-GH Siegen, 57068 Siegen, Germany. Manuscript submitted July 3, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
initial yield behavior shows whether the mechanism is stress-induced or strain-induced, according to the change in the yield strength with temperature. Recent studies show that stress-induced transformation is the dominant mechanism for the initial yield behavior of carburized 4320 steels.[2,3] However, with further deformation, the distinction between stress-induced and strain-induced transformation and deformation due to a slip mechanism becomes less obvious. The kinetics of stress-induced transformation is generally related to the ambient temperature, Ms temperature, carbon content, amount of retained austenite, austenite and martensite strengths, and hydrostatic and effective stress.[10,12,13] Although a large number of researchers have studied the effects of the uniaxial stress state on the martensitic transformatio
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