Effect of laser shock peening as a pretreatment on ion nitriding of precipitation hardened stainless steel

Laser shock peening (LSP) is emerging as an advanced technology to improve the fatigue life of engineering products. Also, this can improve wear and corrosion resistance as add-on benefits to the properties of the materials. In the present investigation, a systematic understanding of the hybrid surface treatment that combines LSP with ion nitriding, also known as plasma nitriding (PN) of precipitated hardened (PH) stainless steel was conducted to improve the effective case hardening layer. The effect of input parameters such as laser spot diameter, the number of superimposed layers, and overlap rate was investigated on the performance characteristics, which include surface morphology, microhardness, and the change in microstructure after the proposed treatment. The surface characterization results reveal that an 80% laser overlap rate modified the surface roughness of PN samples owing to higher surface roughness values induced by ablation during the LSP treatment. The microhardness and optical microscopy (OM) results revealed an increase in the effective hardening layer thickness of about 115% for a 40% overlap rate owing to the absence of blind spots. In addition, XRD analysis revealed the presence of Fe4N and CrN compounds on the surface of the PN samples. Scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) elemental mapping reveals the presence of a nitrogen-rich phase in the effective hardened layer and increased effective hardened layer thickness owing to grain refinement of LSP pre-processing.