Abstract
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.
Highlights
Fatigue is a process of mechanical failure resulting from the application of repeated cyclic loading with stresses mostly below stress limit determinates in monotonic tensile tests [1]
Metastabilty of metastable austenitic steels is mainly influenced by the chemical composition [4,5,6,7]
Several empirically determined equations for calculating stacking-fault energy (SFE) and MS and Md30 temperature thresholds have been developed to estimate the metastabilty of austenite
Summary
Fatigue is a process of mechanical failure resulting from the application of repeated cyclic loading with stresses mostly below stress limit determinates in monotonic tensile tests [1]. To enhance the monotonic and especially cyclic strength of metallic materials, microstructural modification is possible due to chemical alloying and heat treatment. The second class comprises austenitic steels, which were discovered to obtain a passivity (and, “stainlessness”). Since the chromium contents of typical austenitic stainless steels exceed 16 wt%, their equilibrium microstructure at room temperature would be fully ferritic, if no other austenite-stabilizing alloying elements were added to the material. The elements most often used to obtain an austenitic microstructure are nickel, manganese, carbon and nitrogen [3]. Changes in chemical composition influence the passivity of austenitic stainless steel and significantly affect the metastability of the austenitic microstructure [4,5,6,7]
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