The Study On The Resistances To Corrosion Of Fe-based Powder Metallurgy Valve Seats

The effects of manufacturing and treatment processes on the microstructure and mechanical properties of V561,4080A, AR20 and V581 iron-based powder metallurgy valve seats were analyzed by optical microscope, X-ray diffraction, SEM, EDS and other instruments. The influences and mechanisms of manufacturing and treatment processes and chemical compositions on the corrosion resistance of valve seats were researched by high temperature oxidation test, high temperature sulfuration test and electrochemical corrosion test. The results showed increasing the sintering temperature and adopting the copper infiltration process are able to enhance density, compactness, strength and hardness of valve seats. The enhancement of compactness improves the resistances to oxidation, sulfuration and electrochemical corrosion. Cryogenic treatment +tempering can also enhance the strength of valve seats, but reduce density, compactness, hardness and the reduction of compactness results in the decrease of resistance to corrosion. Oxide films of these valve seats mainly contain oxides of Fe and Cu, and some loose acicular structures that composed of Fe2O3 appear on the oxide film surfaces of V561 valve seats of different manufacturing and treatment processes; sulfide films mainly contain sulfides of Fe, Cu and Co. Sulfide films of valve seats without copper infiltration are mainly loose strip structures that composed of CuFe2S3 and FeS, and spherical particles that composed of CusFeS4 appear on the sulfide film surfaces of samples with copper infiltration. The densities of valve seats of three materials are in order of AR20>V581>4080A, the mechanical properties are in order of V581>4080A>AR20, but their densities are similar. Their resistances to oxidation are in order of V581>4080A>AR20, and their oxide films mainly contain oxides of Fe and Cu; their resistances to sulfuration are in order of AR20>4080A>V581, and their sulfide films mainly contain sulfides of Fe, Cu and Co; their resistances to electrochemical corrosion in H2SO4, HNO3 and NaCl solution are in order of V581>AR20>4080A, but in NaOH solution, AR20>V581>4080A.