Surface Modification Of H13 Steel By Laser Cladding

H13(4Cr5MoSiV1), the mostly widely used hot work die steel, which has high thermal strength, wear resistance, impact-toughness and excellent thermal fatigue resistance, is widely used to manufacture various forging dies, hot extrusion dies and casting dies for magnesium and aluminum alloys. Because hot work dies suffer from various physical and chemical actions, such as wear, thermal fatigue, erosion, stress corrosion and surface thermal welding in service, their service life are not long. It is well known that failure of mould starts from the surface. To improve the surface microstructure and mechanical properties is an effective way to extend its service life. Laser cladding is used to improve surface properties of H13 steel, especially its surface hardness, wear resistance, corrosion resistance and heat resistance by making use of high radiation, high brightness, sound directivity and monochromaticity of laser beam, and so its service life is extended. In practical, large area cladding should be done, but the diameter of the incident laser spot is usually too small to meet the requirement. Over lapping between contiguous laser scanning zones is the simplest solution. As a novel materials system beyond traditional metallurgical design, the high entropy alloys (HEAs) consist of multiple principal elements in equiatomic or close-to-equiatomic ratios. In HEAs, simple solid solutions with FCC or BCC structure tend to form with the absence of intermetallic compounds and elaborate phases during solidification.In this paper, the H13 steel was modified using high entropy alloys and Ni60 alloy without and with the addition of Yttria by laser cladding techniques. A Nd-YAG pulsed solid laser device wras used in the study and the laser parameters were optimized. The cross-sectional microstructure, phase assemblages, micro-hardness, abrasive resistance and corrosion resistance of the laser cladding layer were characterized by using optical microscopy, scanning electronic microscopy with EDS, X-ray diffractometer, micro-hardness tester, high temperature and high speed abrasion tester and Electrochemical workstationThe multipass fabrication parameters for the laser cladding of H13 steel were obtained: current 135 A, scanning rate 200 mm/min, defocusing amount 45 mm and Overlapping ratio 45%. The high entropy alloy coating with good microstructure and property was prepared on the H13 steel. The coating is compact with a few cracks and well bonded with the substrate.Two kinds of coating materials’cladding layers both own the typical microstructure of laser rapid solicitation. The laser cladding layer is a hybrid structure composed with planar, columnar and dendritic crystals; planar crystal exists at the interface between the cladding layer and substrate; the bottom and middle side of the cladding layer are mainly composed of columnar and dendritic grains which grow perpendicularly to the bonding face, in the form of "band" structure. The surface of cladding layer is dendritic grains which are in disordered direction. The HEAs laser cladding layer on the H13 steel is mainly composed of solid solution with BCC and FCC structure. The hardness of the cladding layer is enhanced mainly through the refining strengthening and solid solution strengthening mechanism. And the Ni60 laser cladding layer has (Ni, Fe) solid solution for the matrix, which contains Cr2sC6, CrB and other precipitates. Hard precipitates make the hardness of the cladding layer increase. The average hardness of two kinds of coating materials’cladding layer were 486HV and 576HV respectively, which are 2.5 times and 2.9 times that of the H13 steel substrate,respectively. The results of high-temperature abrasion test demonstrated that the abrasive resistance of two kinds of cladding layers after 4000r(16min) of test. The weight loss of the HEAs cladding layer and Ni60 cladding layer were 43% and 40% of that of the H13 substrate. The abrasion mechanisms of the H13 steel substrate is adhesive wear, while the main abrasion mechanism of cladding layer is abrasive wear. The formation of hard particles and solid solutions are the main reason for the enhancement of the wear resistance of the two kinds of cladding layers.The anodic polarization curve of the laser cladding layer shows that corrosion potential of the laser cladding layer is markedly increased compared with the H13 steel substrate. The superior corrosion resistance of laser cladding relates to their fine microstructure and large amounts of nickel, chromium, silicon, and other anti-corrosion elements. The addition of Y2O3 refines the grain. It changes the typical morphology of rapidly solidified laser coating into very small dendrites and equiaxed grains. Microstructure becomes more uniform and defects in the coating such as cracks and pores are inhibited. Although the hardness decreased slightly and the composition has no changes, the dense microstructure, hard cladding uniform precipitates refined, relative increase of the toughness parts; they make an increase in high-temperature wear-resistance of laser cladding layers. The addition of Y2O3 has little effect on the corrosion resistance of laser cladding layer.