Study on the effect of multiple laser shock peening on residual stress and microstructural changes in modified 9Cr-1Mo (P91) steel

This paper presents an experimental study on the effect of laser shock peening (ISP) parameters on resultant surface residual stress and microstructural evolution in P91 steel. P91 steel samples were subjected to single and multiple LSP treatments. Single LSP treatment was performed with laser power density (LPD) in the range of 3.9-5.5 GW cm(-2) while multiple LSP treatment was carried out a fixed LPD of 3.9 GW cm(-2). The surface of laser shock peened samples was investigated with respect to residual stress, microstructure and micro-hardness through X-ray diffraction (XRD), scanning and transmission electron microscopy and micro-hardness measurements. Single LSP treatment produced significant increase in the magnitude of compressive residual surface stress (-570 to -610 MPa) with respect to unpeened sample with compressive surface stress of around +/- 85 MPa. However with increasing the LPD beyond 3.9 GW cm(-2), the compressive residual surface stress has marginally increased. On the other hand, in case of multiple LSP treated samples, the magnitude of compressive residual surface stress has been observed at increased depth of 975 mu m (double LSP) and similar to 1200 mu m (triple LSP) compared to single LSP impact (similar to 750 mu m). Further, microstructural analysis carried out using electron microscopy of laser peened samples demonstrated that in the near surface region, martensite lath structure under gone severe refinement with respect to the unpeened sample having average lath width similar to 1.2 mu m. It has also been noticed that grain refinement is found be more prominent in the case of multiple LSP treated samples. The grain refinement in LSP treated samples was also confirmed by XRD peak broadening which progressively increases with an increase in laser impacts. Electron microscopic characterization revealed that in triple LSP treated P91 samples, well-defined polygonal subgrains of average size similar to 115 +/- 10 nm have been observed at near surface (20 mu m depth from top).