Effects of Laser Energy Density On Carbide Dissolution, Element Distribution and Microstructure Evolution of AISI P20 Steel During Laser Surface Quenching

Laser surface quenching (LSQ) was performed on AISI P20 mould and hot-working die steel with an objective to improve surface characteristics. The steel was treated under three different process parameter conditions. The microstructure, element distribution and residual stresses were investigated through SEM, EDS and XRD analyses. The effect of laser energy density on carbide dissolution/ablation, microstructure evolution were thoroughly investigated. The dissolution/ablation of carbides significantly affected the formation of martensite and retained austenite, and the distribution of elements and phase in the microstructure. The results of the study and analyses of treated surface revealed that the LSQ treatment significantly improved the microstructure, eliminated the pores or other defects. Furthermore, the degree of carbide dissolution/ablation was closely related to the laser energy density. Comparing to Cr7C3, Cr3C2 was more difficult to dissolve at lower laser energy density. Thus, those incompletely dissolved Cr3C2 would hinder the growth of austenite and reduce the carbon content in austenite and lead to the formation of low-carbon martensite. The highest laser energy density (150 J/mm2), was able to produce finer microstructure and significantly reduced the inhomogeneity in distribution of Cr between the poor and the rich Cr areas.

Effect of prior cold ring rolling on carbide dissolution during the austenitizing process of an M50 bearing steel

The influence of prior cold ring rolling (PCRR) on carbide dissolution during the austenitizing process of an M50 bearing steel was investigated by combining microstructural observations with kinetics analysis. The microstructural results show that the PCRR leads to a reduction in the volume fraction and the mean diameter of the undissolved carbides. The matrix of the as-quenched specimens after PCRR is enriched with more carbon and alloying elements than those without PCRR, which further confirms that the dissolution behavior of the alloy carbides is enhanced by PCRR during the austenitizing process. The kinetics of the transformation from ferrite to austenite (α → γ) without and with PCRR are determined by differential scanning calorimetry (DSC) upon heating. The austenite onset (Ac1) and end (Ac3) temperatures both decrease with as the thickness reduction from the PCRR increases. The activation energy for the α → γ transformation is calculated and shows a significant decline when the PCRR process is applied. In addition, the PCRR process results in a slight increase in the hardness regardless of the austenitizing temperature.