Surface quality, microstructure, and mechanical properties of the SKD 61 tool steel with prior heat treatment affected by single- and double-pass continuous wave laser polishing

Abstract

This study is made to investigate the effects of applying double-pass continuous wave laser polishing on the parameters of areal average surface roughness, two mechanical properties, and microstructure of SKD 61 tool steel. These parameters are partly compared to those in the single-pass specimens. The specimens before conducting double-pass laser polishing are prepared using the conditions of the specimen with the smallest surface roughness in the single-pass specimens, with the aim of examining the possibility of further reducing surface roughness. The smallest surface roughness value of the double-pass specimens has a value slightly larger than that of the single-pass specimens. An appropriate shallow melting for a substantial reduction in the grinding marks of the as-received specimen is used to prepare the single-pass and double-pass specimens with the smallest surface roughness. The surface roughness value is controlled by the contents and intensities of α-ferrite and γ-austenite in the sample, thus the composite grain size. In the single-pass specimens, the austenite lattices appear when the deposited energy is higher than the critical value, laying between 60.75 and 66.81 J/mm2. In the double-pass specimens, the austenite lattices disappear when deposited energy is higher than the critical value between 81.00 and 91.13 J/mm2. The growth of γ-austenite can increase the composite grain size. These two kinds of polishing show the same characteristic, which is that the growth of γ-austenite can increase the composite grain size. The intensity of the γ-austenite and the composite grain size are two of the governing factors with regard to the surface roughness of the specimen. Either a decrease in deposited energy in the double-pass specimens or an increase in deposited energy in the single-pass specimen is favorable for an increase in surface roughness in the non-zero intensity region of austenite.