Enhancement of oxidation resistance of modified P91 grade ferritic-martensitic steel by surface modification using laser shock peening

In this paper, effect of laser shock peening (LSP) on high temperature oxidation behavior of P91 steel has been studied. P91 samples were subjected to single and triple LSP treatments at a constant laser power density of 3.9 GW.cm(-2). Single and triple LSP treatment led to the formation of ultrafine subgrains of sizes similar to 300 +/- 30 and similar to 105 +/- 10 nm respectively at near surface comparing to unpeened P91 sample (similar to 1200 +/- 120 nm). Further, unpeened and LSP treated samples were oxidized at temperatures of 600, 650 and 700 degrees C for time 5 to 200 h in air. Compare to unpeened sample, single and triple peened samples exhibited two and five fold reduction in weight gain after 200 h of oxidation at highest WA temperature (700 degrees C). Improvement of oxidation resistance in LSP treated samples is attributed to the formation of continuous layer of protective oxides such as (Fe,Cr)(2)O-3, Cr2O3 and MnCr2O4 due to enhanced diffusivity of Cr and Mn from bulk during early stage of oxidation. The enhanced diffusivity Cr and Mn in LSP treated samples have been further supported by reduction in apparent activation energy of oxidation, which is evaluated by modelling the time-temperature dependence of weight gain data in purview of kinetics theory.