Internal and effective stresses analysis during cyclic softening of F82H martensitic stainless steel.

A ARMAS; I ALVAREZ; M AVALOS; C PETERSEN; R SCHMIT

Paris, Francia

Conferencia; International Conference on Temperature-Fatigue Interaction, 9th International Spring Meeting; 2002

European Structure Integrity Series

Low cycle fatigue tests were performed on samples of F82H mod. stainless steel with two different initial microstructures: “ferritic-pearlitic” and “tempered martensite”. Samples of both microstructures show, after the first tensile ramp, an exponential cyclic softening that continues up to failure. Whereas samples with “tempered martensite” structure display the most striking softening behaviour. The observed differences and the evolution in flow stress at different temperatures were analysed by studying internal and effective stresses obtained from the hysteresis loops after the method originally proposed by Cottrell. From this analysis during cycling of the steel F82H mod. at 723K it can be concluded that the strong softening obtained in samples with “tempered martensite”-structures are produced by a decrease in internal stresses. A different behaviour of internal and effective stresses was revealed during the softening of samples with “femtic-pearlitic”-structure. The internal stress remains almost constant from the beginning of the test up to the failure of the specimen but the effective stress behaves like the applied stress. These differences in behaviour correlated with observations from electron microscopy are discussed in the paper.