Microstructures And Properties Of Hard Materials Coating On Die Steel Surface By Vacuum Powder Sintering

Recently developed surface treatment and composite technology by vacuum powder sintering have been widely used for preparation of coating from hard materials on steel surface. It was coated with prepared mixed powder on surface of steel, sintered in vacuum furnace. The major excellences exist in high thickness, high bonding strength and low quality cost in coating.In this dissertation, microstructures and properties of the preparated coating from ternary boride based cermet and cemented carbide by vacuum powder sintering on substrate of 45 steel,H13 steel and W18Cr4V steel were studied. The effect of different sintering temperatures on microstructures of the coating and the interface morphology by SEM,EDS,EMPS,XRD and hardness tester was analysed. The mechanism of their formation was studied. Using specially designed tensile test method the bonding strength between coating and substrate was precisely measured. Based on hardness(HRA) measurement of the coating and microhardness(HV) distribution on cross-section of the sample the ductility-strength characteristic of the coating was analysed. Wear resistance of coated sample was compared with that of H13 steel and W18Cr4V steel and wear mechanism of the coating was analysed. Thermal microhardness(HV) at temperatures of 300℃,500℃and 700℃on the surface of coated H13 steel sample was measured .The difference of the thermal strength between coated and uncoated sample was compared. Thermal fatigue test of coated H13 steel sample was conducted by Uddeholm method. The morphologies of thermal fatigue crack on coated and uncoated samples, their distribution on cross section and surface microhardness(HV) variation before and after thermal cycle were analysed. An approach to mechanism of the thermal fatigue formation of the coating was performed. From above mentioned experimental results the fallowing can be concluded:The coating from ternary boride cermet has microstructure of Mo₂FeB₂+α-Fe at pertinent sintering temperatures ranging from 1220℃to 1240℃. While the coating from cemented carbide has microstructure of WC+α-Fe at pertinent temperature ranging from 1280℃to 1300℃. The bonding strength on the metallurgically bonded interface between coating and substrate exceeded 400MPa.HRA and HV values on the coated exceeded 80 and 1000 respectively. The microhardness distribution on cross section at both sides of the interface changed smoothly. The wear resistance for the coating was higher than for H13 steel and W18Cr4V steel. Wear mechanism could be expounded by the comprehensive wear both micro-cutting and plastic deformation.The coated H13 steel from ternary boride cermet with HV value of about 1000 at temperature of 700℃exhibited very high thermal strength. The coated H13 steel from ternary boride cermet exhibited higher thermal fatigue resistance than uncoated H13 steel while the coated H13 steel from cemented carbide possessed less thermal fatigue resistance than uncoated H13 steel. The higher thermal fatigue resistance of coated H13 from ternary boride cermet was attributed to great number of fine hard phase Mo₂FeB₂ distributed in martensite matrix and great increase of boundary so as to increase resistance again crack propagation while the lower thermal fatigue resistance of coated H13 steel from cemented carbide appeared due to coarse and polygonal WC grains which were susceptible to stress concentration so as to originate and propagate crack easily.