Microstructure and microhardness of pearlitic steel after laser shock processing and annealing

Abstract

The microstructure evolution of the high carbon pearlitic steel after laser shock processing (LSP) with different laser pulse energy and high temperature annealing was investigated. After LSP, the cementite lamella were bent, fractured and broken into granules. Fragmentation and dissolution of the cementite lamella were enhanced by increasing the laser pulse energy. Results show that the ferrite lattice parameter increased due to carbon atom dissolution in the ferrite matrix, and the corresponding ferrite X-ray diffraction peaks shifted significantly towards the smaller diffraction angles. After annealing at 650°C for 30 min, an ultrafine duplex microstructure (ferrite+cementite) was formed on the surface. After LSP with a high energy, equiaxed ferrite grains were refined to 400 nm and the cementite lamella were fully spheroidised with the particle diameter of ∼150 nm. The corresponding grain size of ferrite and cementite under low pulse energy was 500 and 300 nm respectively. After annealing, the ferrite peaks significantly shifted towards the higher diffraction angles, and the ferrite lattice parameter decreased. The microhardness initially increases after LSP and then slightly decreases after subsequent annealing but remained higher than without LSP.