Effect of laser shock peening and hot isostatic pressing on the microstructure of MoNiCr nickel based alloy

Bricín David - Centrum výzkumu Řež s.r.o. (Czech Republic)

This study aimed to evaluate the change in the microstructure of MoNiCr nickel-based alloy, because of specimen surface modification by laser shock peening (LSP) followed by heat treatment using the hot isostatic pressing technology (HIP). MoNiCr nickel-based alloy is a material that is considered a potential material for producing parts for molten salt reactors that operate at elevated temperatures above 700 °C in a corrosive environment of fluoride or chlorine salts. The initial dendritic structure of this alloy is stable and it’s hard to recrystallize it. Thus, it’s necessary to use suitable forming operations to get a recrystallized austenitic grain structure. One of these possible technologies is laser shock peening which can generate high-pressure residual stresses into material surface and structure rearrangement. HIP was used to recrystallize the surface area affected by LSP. The experimental specimens consisted of plates with a nominal thickness of 5 mm and a diameter of 200 mm. In the center of these samples, surface treatment was performed in a square grid with dimensions of 100×100 mm using LSP. Several variants of LSP treatment were done which differ in laser pulse energy density. HIP was done in an argon atmosphere, holding a temperature of 1150 °C and pressure of 120 MPa. Change in specimens’ surface topography was evaluated by optical profilometer Keyence and Scanning electron microscope Tescan Mira 3. Change in microstructure close to the treated surface was evaluated by optical microscope CarlZeiss Observer Z1m and scanning electron microscope Tescan Mira 3. Metallography analysis was supplemented by HV0.1 microhardness analysis using semiautomatic hardness tester DuraScan. Results of the experiment show that LSP caused plastic deformation of the surface which was associated with change in the surface roughness and microhardness. The HIP process also caused recrystallization of the grains in the surface modified by LSP in some cases.  The following research will focus on the corrosion fatigue tests.

Keywords: LSP, HIP, MoNiCr, laser shock peening, hot isostatic pressing