Study On Microstructure And Properties Of Cobalt-based Alloy Powder Plasma-arc Surfacing Layers

The valve sealing surface is often easily damaged when valves suffer from being squeezed, wear, corrosion or high temperature oxidation conditions in the process of use, which results in the failure of valves. This causes serious losses. The cobalt-based alloy is an ideal high-temperature alloy to partially strengthen valve sealing surface for its excellent performance such as heat resistance, abrasive wear resistance, impact resistance, corrosion resistance and so on. The plasma-arc surfacing is an energy-saving, high-efficient and low dilution rate of surface strengthening technology. It is widely used to repair or improve the surface properties of parts. In this paper, PTA process with optimized technical parameters was conducted on the 304 stainless steel substrate using Stellite 6, Stellite F and Tribaloy 400 (hereafter called T-400) cobalt-based alloys. The influence of element composition on microstructures and properties of different cobalt-based alloy surfacing layers was analyzed. Meanwhile, the wear types of surfacing layers were also studied at normal temperature and 600℃.The study shows that both welding current and power feeding rate affect the dilution rate and surface quality of cobalt-based alloy surfacing layer. The optimal parameters are:32mm/min welding speed,0.2L/min plasma gas flow rate,35g/min power feeding rate,99A transferred arc current. Under the optimum conditions, surfacing layer won good shape, defects-free and low dilution rate.Microstructure and morphology of cobalt-based alloy surfacing layers were investigated. The microstructure of Stellite 6 alloy surfacing layer is changed distinctly. There are different crystallization morphologies in different zones:planar crystal in the fusion zone, cellular crystal near interface, columnar dendrite crystal with a certain direction within the central of surfacing layer and small isometric crystal near surface. The main phases of Stellite 6 alloy surfacing layer are y-Co, M23C6. M7C3, CoCx. M is Cr, W, Fe etc. Stellite F alloy surfacing layer has similar distribution of microstructure, and its main phases are y-Co. N23C6(M is Cr, W, Fe),Cr7C3, Cr2Ni3, M17W3. The microstructure of T-400 alloy surfacing layer has no apparent crystalline region, mainly has few columnar crystals, fine dendrite and isometric crystal. Its main phases are y-Co and intermetallic compounds Co3Mo2Si, CoMoSi, Co2M03, Co7Mo6 etc.The hardness and electrochemical corrosion test show that the hardness of Stellite 6, Stellite F and T-400 cobalt-based alloy surfacing layers is higher than 304 stainless steel with 2 times,1.8 times and 2.8 times as many as that of base metal respectively. T-400 alloy surfacing layer has the highest average micro-hardness with 5OOHVo.5. By comparing with the base metal, the corrosion resistance of three surfacing layers in 3.5wt% NaCl solution is better too. Their corrosion current density is 2.47μA/cm2,4.01μA/cm2,1.91μA/cm2 with 0.293 times,0.476 times and 0.227 times as few as that of base metal respectively. T-400 alloy surfacing layer has the best corrosion resistance in 3.5wt% NaCl solution.The wear tests of three alloys surfacing layers and base metal were carried out at normal temperature and 600 P<0. The results indicate that:(1) At normal temperature, the wear loss of Stellite 6, Stellite F, T-400 alloy surfacing layers is lower than 304 stainless steel with 0.27 times,0.37 times and 0.67times as few as that of base metal respectively. Stellite 6 alloy surfacing layer has the highest relative wear resistance because of the oxide film formed on the surface to reduce the wear loss. The second is Stellite F. T-400 surfacing layer has the lowest relative wear resistance for the existence of a large number of hard brittle inter-metallic compounds. Its plasticity is poor, and produces the "furrows" when worn, then broken off. Stellite 6 and Stellite F alloy surfacing layers have the same wear types with adhesive wear, oxidation wear and abrasive wear. The main wear types of T-400 alloy surfacing layer are abrasive wear with slightly oxidation wear.(2) At 600℃, The average friction coefficient of Stellite 6, Stellite F, T-400 alloy surfacing layers is 1.25,0.38,1.43 respectively. T-400 alloy surfacing layer has the smallest wear scar width, followed by Stellite 6, Stellite F. The main wear types of three cobalt-based alloys surfacing layers are different during high temperature wear. The wear of Stellite 6 alloy surfacing is caused by abrasive wear and fatigue wear, accompanied by oxidative wear; The main wear types of Stellite F alloy surfacing layer are oxidation wear and adhesive wear; The wear of T-400 alloy surfacing layer is mainly caused by oxidation wear with slightly abrasive wear, adhesive wear.