Abstract
The phase transformation from paramagnetic austenite to ferromagnetic α′-martensite significantly affects not only the mechanical but also the magnetic properties of austenitic stainless steel. The volume fraction of α′-martensite, often determined by magnetic methods, can be used as an indicator of (fatigue) damage. This study investigates the changes in α′-martensite morphology and microstructure, as well as the magnetic properties caused by different fatigue conditions that nevertheless resulted in the same volume fraction of ferromagnetic α′ phase. To achieve an equal α′ volume fraction, a magnetic force balance was developed and implemented in situ in fatigue tests. After the target magnetization was reached, the tests were stopped, and the specimens were analyzed with a vibrating sample magnetometer; scanning electron microscopy, including EBSD; and magnetic force imaging. The results show that α′-martensite morphology changes characteristically with strain amplitude. High strain resulted in coarse martensite, whereas the transition to lower strain amplitude added a significant portion of fine-grained α′. Moreover, a good correlation between fatigue strain amplitude, grain size, and coercivity was found, with an R-squared value above 0.9.
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