Influence of laser polishing on surface roughness and microstructural properties of the remelted surface boundary layer of tool steel H11

Abstract

This work investigates the influence of multi-step laser polishing on microstructural properties of the remelted surface layer of tool steel H11. Four different sets of process parameters were selected for laser polishing of initially annealed samples made from tool steel H11. In a sequential, multi-step process using continuous and pulsed laser radiation (Nd:YAG) a significant reduction of surface roughness was achieved.. The remelted layers were analyzed using roughness measurements, white light interferometry, X-ray diffractometry, electron backscatter diffraction, glow discharge emission spectroscopy, and nanoindentation hardness measurements. Laser polishing leads to a grain refinement and a significant increase in hardness. A minimal surface roughness of Ra = 50 nm was achieved in an Argon process atmosphere with an additional 6 vol% CO2. In particular the carbon concentration was significantly reduced within the remelted layer. The lower carbon concentration is correlated with a decreased maximal surface hardness down to 366 HV. High residual tensile stresses of up to 926 MPa can be introduced by laser polishing. Overall, high temperature gradients and, in particular, decarburization due to carbon diffusion processes were identified to be the major driving force for significant changes in surface roughness and microstructural properties.

Keywords

• Laser polishing
• AISI H11
• Surface roughness
• Decarburization
• Micro hardness
• Martensite formation