Strain-Induced Martensite Characteristic of AISI 316 Austenitic Stainless Steel Subjected to Low-Cycle Fatigue Through Magnetic and Ultrasonic Nondestructive Evaluation

Strain-induced martensite transformation characteristics of AISI 316 austenitic stainless steel subjected to low-cycle fatigue were investigated by magnetic and ultrasonic nondestructive evaluations. A low-cycle fatigue test was conducted through total strain amplitude control ranging from 0.5 % to 1.7 % and up to failure. The micrographs observed by an optical microscope and transmission electron microscope depict the variation of microstructural features, such as deformation slip, dislocation, dislocation substructure, strain-induced martensite, etc. The dislocation density increased with the strain amplitude. Most of all, strain-induced martensite increased monotonously with strain amplitude. The first cyclic hardening was discussed in terms of the dislocation multiplication and dislocation density. Then the second cyclic hardening was determined to be due to the strain-induced alpha'-martensite nucleation during cyclic fatigue. The nucleation and growth of strain-induced martensite evolving from plastic deformation gives rise to its magnetic properties and ultrasonic nonlinearity and is dependent on the strain amplitude.