Research On Microstructure And Properties Of Cast Iron Piston Ring Duplex Treated With Coating And Multicomponent Thermochemicial Treatment

Piston ring is one of the most important parts of engine, and it plays a vital role in the working process. Better wear and corrosion resistance is required for the severe work condition of the piston ring. Only the piston ring undergone surface treatment can be employed in the engine. At present, the main heat treatments of piston ring are nitriding and chroming. However, chroming is harmful to the environment. The excellent wear resistance after nitriding depends on the thickness of compound layer. However, the compound layer formed in the nitriding process is thin. It is necessary to enhance the thickness of the compound layer. The purpose of this paper is developing a new process, which can enhance the thickness of the compound layer with excellent wear resistance and corrosion resistance. The samples used in this paper are common piston rings, which are made of nodular cast iron.The piston rings were treated with three heat treatments:nitriding, multi-component thermochemical treatment and duplex heat treatment. Microstructures were observed by optical microscopy and laser scanning confocal microscopy; the types of phase were characterized by XRD; salt pray corrosion test and electrochemical polarization curve test were applied to compare their corrosion resistance; their hardness and wear resistance were also measured.The results show that compound layers have formed on the surface of piston rings after three heat treatments. The compound layer of duplex heat treatment is the thickest, being up to27μm. It is nine times that of nitriding. The compound layer of multi-component thermochemical treatment is12μm, which is three times thicker than that of nitriding. The hardness of the compound layer formed in nitriding process, multi-component thermochemical treatment and duplex heat treatment is905HV0.05,875HV0.05and780HV0.05respectively. The wear resistance of the compound layers listed in descend order is accordance with the results of hardness test. The results of salt spray corrosion test show that the corrosion resistance of the compound layer formed after nitriding is6times that of received sample, while that of multi-component thermochemical treatment is8times and duplex heat treatment is64times. This was also approved by the electrochemical test.In conclusion, a thicker compound layer with excellent wear resistance and corrosion resistance is forming in the duplex heat treatment process.