Strengthening via the Formation of Strain-Induced Martensite and the Effects of Laser Marking on the Microstructure of Austenitic Stainless Steel

The deformation behavior and microstructure characteristics of 304L stainless steel during strip rolling and bar extrusion at different strains and temperatures, from room to liquid-nitrogen temperature, were investigated with Vickers hardness, light microscopy, and electron-backscatter-diffraction. The relative volume fractions of transformed martensite at different stages of the deformation process were assessed using Ferritescope MP-30. It was found that during rolling and extrusion the relative volume fraction of martensite increases with increasing strain and decreasing temperature. According to the enhancement of the mechanical and magnetic properties after isothermal treatment at 673 K (400 °C), it is assumed that both, ε-martensite and α′-martensite, are present in the deformation microstructure, indicating the simultaneous stress-induced transformation and strain-induced transformation of austenite. The effects of the laser surface treatment and the local appearance of a non-magnetic phase due to the α′ → γ transformation after the laser surface treatment were also investigated.