Role of metastable austenite in the fatigue resistance of 304L stainless steel produced by laser-based powder bed fusion

The fatigue crack growth (FCG) behavior and fatigue strength of 304L stainless steel (SS) manufactured by the laser powder bed fusion (LB-PBF) process were investigated. Effect of build orientation, microstructure, and temperature--considering that the alloy undergoes temperature-dependent stress-induced martensitic transformation (SIMT)--were determined. FCG rates were found to be broadly independent of the build orientation and microstructure, although it is reduced in shorter builds due to the presence of higher compressive residual stress in them. Microstructural investigations of the fatigue crack reveal that SIMT occurs at the crack tip of alloys tested at room temperature, whereas the same was negligible at 150 °C. SIMT induces dilatation and shear, which enhances crack closure and retards FCG rates at RT compared to that at 150 °C. Using the microstructural observations of the transformed zone, an estimate of the crack closure due to SIMT is provided. Finally, failure envelopes or the Kitagawa-Takahashi diagram was prepared for different temperatures to facilitate a damage tolerant design approach.