Effect of laser peening on surface integrity of AISI 304 stainless steel

• Main Author: Chua, Jia Jun
• Other Authors: Upadrasta Ramamurty
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/157908
• Institution: Nanyang Technological University

AISI 304 stainless steel (SS304) is widely used across multiple industries due to its strong corrosion resistance and good low-temperature strength. However, surface strengthening of SS304 is quite challenging as it does not undergo phase transformation during traditional heat treatment. This is due to its high nickel content, stabilising the austenite phase even at room temperature. In this project, laser shock peening (LSP) is utilised to strengthen the SS304 surface. LSP is a mechanical surface enhancement method that employs high-intensity shock waves to introduce compressive residual stresses and enhance the material's mechanical properties. Generally, an ablative layer is used during LSP to protect the metal surface. The effect of the ablative layer on hardness, microstructure, surface topography and residual stress of SS304 steel was investigated in this study. The results exhibit that both LSP with an ablative layer and without an ablative layer induce similar surface hardening. In both cases, higher pulse energy (11J) generated higher hardness than lower pulse energy (7 J). Similarly, surface roughness for LSP without an ablative layer was found to be larger than for LSP with an ablative layer. This is due to the formation of surface oxides from the thermal effects of the laser. Cross-section microstructure observation also revealed the presence of a thermally damaged layer in the case of LSP without an ablative layer. For LSP with an ablative layer, the depth of compressive stresses was greater than 1 mm at higher pulse energy (11 J) than ~0.8 mm at lower pulse energy (7 J). However, peening at lower pulse energy (7 J) produced a higher magnitude of compressive stresses at the surface. This is attributed to the strong horizontal recoil shock wave along the surface during high energy peening, which reduces the compressive stress magnitude at the surface. Experiments for LSP without an ablative layer show a similar residual stress profile despite using different pulse energies. The reason for this behaviour is not known from the current analysis.