Metastability and fatigue behavior of austenitic stainless steels

This study presents the results of a detailed investigation of metastability and susceptibility to deformation induced α’-martensite formation of several austenitic steels (AISI 304, AISI 321, AISI 348 and two batches from AISI 347) in the solution-annealed state. Besides conventional characterization of metastability by calculating stacking-fault energy and threshold temperature (designated as MS and Md30), the present work introduced a new method for determining susceptibility to α’-martensite formation. The method was based on dynamically applied local plastic deformation and non-destructive micro-magnetic measurement of α’-martensite content. The parameter Iξ was established, which correlated very well with the grade of α’-martensite formation during cyclic loading. The cyclic deformation and phase transformation behavior of cyclically loaded specimens from different metastable austenitic steels were investigated in total-strain and stress controlled fatigue tests with load ratio R = -1 at ambient temperature. The influence of the strain rate on the cyclic deformation and phase transformation behavior was also examined. During the fatigue tests, besides stress-strain hysteresis and temperature measurement, in situ micro-magnetic measurements were performed. Using the compressive measured data, the influence of plastic induced self-heating of the specimen and the strain rate on α’-martensite formation was analyzed.