Fatigue strength of TRIP steels
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Fatigue Strength of TRIP Steels G. B. OLSON, R. CHAIT, M. AZRIN, AND R. A. GAGNE Studies l~ of the fatigue behavior of metastable austenitic steels have shown interesting differences between the behavior of high strength TRIP steels 7 and that of lower-strength metastable austenites, while identifyinga marked contrast between the influence of the deformation-induced martensitic transformation under strain-control vs stress-control conditions. Fatigue crack propagation (FCP) studies (controlled AK) have indicated that the deformation-induced transformation retards crack propagation in the lower strength austenites, particularly at low AK, 1and also exerts a beneficial influence in high strength TRIP steels, though to a much lesser extent3 In smooth bar fatigue tests on lower strength austenites, the transformation was found to reduce fatigue life under conditions of controlled plastic strain amplitude? Under controlled total strain amplitude, the transformation was found to be detrimental to low cycle fatigue life, but it was indicated that a small amount of transformation may be beneficial at high cycles? Similarly, the low cycle fatigue properties of high-strength TRIP steels were found to be degraded by the deformation-induced transformation under controlled total strain amplitude conditions? Under stress-control, however, the fatigue life of the lower strength metastable austenites is found to be greatly enhanced by the transformation; for smooth bar tests with a stress ratio of R = 0, fatigue limits in excess of the yield strength have been reported. 6 The above trends are summarized in Table I, with the lengths of the arrows used to give some qualitative indication of the changes. This study was undertaken to extend the stress-control fatigue tests to the high-strength TRIP steels and determine whether the beneficial effect of the deformation-induced transformation persists to the high strength levels. In addition, fatigue data generated
under stress-control conditions may provide a more useful design criterion for many applications. In a previous study, 8,9both air and vacuum melts were prepared of a TRIP steel of nominal composition: Fe-9 Cr-8 Ni-4 Mo-2 Mn-2 Si-0.3 C. Solution-treated austenitic billets were strengthened by warm-extrusion to reductions of area of 40, 60, or 80 pct in the temperature range 400 to 850 ~ (480 to 730 K). Melt compositions and processing details are given in Refs. 8 and 9. Preliminary room-temperature tensile tests revealed that the as-extruded material was too stable with respect to martensitic transformation during testing, resulting in lower than expected values of uniform elongation. A tempering treatment designed to alter the austenite matrix composition through carbide precipitation was found to restore the correct austenite stability for optimum room-temperature tensile properties. A 1 h temper at 1100 ~ (870 K) produced a markedly increased uniform elongation and a higher ultimate tensile strength. The overall tensile properties of the extruded and tempered material were
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