Metallurgical characterization of laser hardened, mechanically textured 2.5 Ni-Cr-Mo low alloy steel and optimization using RSM

Abstract

Surface modification of 2.5 Nickel-Chromium-Molybdenum low alloy steel is carried through laser transformation hardening technique under different laser parameters. A 4 kW high power diode laser with a flat top-hat beam of dimension 10x1mm is employed in this study. In this investigation, substrate surface was roughened with abrasive paper to minimize the specular reflection and to increase the absorption rate of laser beam. Microstructure analysis, corrosion analysis, residual stress distribution and the percentage of austenite retained in the hardened region were done. Response Surface Methodology (RSM), a statistical method used for analyzing input variables (Laser power (LP), scan speed (SS) and focal plane distance (FPP)) that influences the hardening process and response optimization, is employed in this study. Characterization analysis of specimen reveals the presence of lath type martensite microstructure in the hardened region with nearly 2.5 times increase in hardness (700 ± 10HV0.5) is achieved with a combination of 1000 W laser power, 240 mm/min scanning speed and 290 mm laser head distance (optimal solution) obtained from the RSM. By performing residual stress analysis, compressive type residual stress at the laser hardened region's cross-section is confirmed. With the optimum laser parameters, a peak residual stress value of −211 MPa is achieved. Retained austenite percentage analysis and corrosion analysis were carried out at the hardened region of RSM's optimal solution. It is observed that 5.76% of retained austenite is obtained in the hardened region using laser hardening. In corrosion, the hardened track obtained using the optimal solution showed a significant improvement in corrosion resistance than that of the base metal due to microstructural homogenization.