Low cycle fatigue of a high strength metastable austenitic steel

Abstract

The low cycle fatigue (LCF) behavior of a high strength, metastable austenitic steel called TRIP steel has been studied. High strain LCF experiments on cylindrical, well-polished specimens under diametral strain control were carried out. To study the effect of a mixed austenite-martensite matrix, LCF tests were also done on the TRIP steel after inducing significant amounts of martensite in the austenite matrix by means of a very high unidirectional prestrain. To establish the role played by the martensite transformation, tests were also run above the M_D. The amount of martensite induced was magnetically measured by means of a “permeameter” built specifically for this purpose. It was found that the LCF life of the TRIP steel, both at room temperature (in the presence of martensitic transformation) and at 200°C (in the absence of the transformation), was related to the plastic strain range, ε_PR, by the Manson-Coffin law. Either cyclic hardening or softening occurred at room temperature, depending primarily upon the plastic strain range used in cycling. Hardening was observed below 3 pct plastic strain range. For LCF tests at 200°C, cyclic softening was observed in all cases. The hardening and softening behavior has been found to depend on the martensitic transformation taking place in these steels during cycling. However, the LCF life correlated best to the percent reduction in area, independent of the extent of the martensite transformation.