The Failure Analysis On The Valve Seat And Valve Guide Of Gasoline Engine

In this dissertation, the mechanism of thermal fatigue crack initiation and propagation, the composition and structure of sulfide corrosion film and the mechanism of sulfide corrosion process of the V531, V571 material valve seat and the Ycl.8,1528 material valve guide were studied with the metallurgical microscope, X-ray diffraction, SEM, EDS and etc. The physical and mechanical properties of the part of failure valve seat and valve guide were measured, and the failure causes were analyzed. The results show that the crack initiation and growth rate of V531 is greater than V571’s, and the crack initiation and growth rate of 1528 is greater than Ycl.8’s. The main crack initiates at pores and depression which at the edge of sample and expands between pores. The expansion path of crack in pearlite depends the orientation relationship between the pearlite lamellae and tensile stress, but in the hard phase, expansion path of crack is almost straight line. S in MoS2 reacts with Fe to FeS, FeS distribute in the matrix, promoting crack initiation and expansion. The crack initiation and propagation rate decrease with the temperature decreasing, and the oxidative corrosion and propagation of thermal fatigue crack promote each other. The sulfide film of four material samples all contain CuFe2S3, FeS, CuS, CuFeS2 and there are NiS and CoS2 in the sulfide film of V571. At the beginning of sulfide corrosion, Fe, Cu, Ni ions migrate rapidly, react with S ions to generate sulfide. With the constant of sulfide corrosion, the sulfides of Cr obstacle the migration of Cu, Ni, Co ions enhancing of limited, Sulfide corrosion degree deepening. The V571’s corrosion resistance is superior to the V531’s, and the Yc1.8’s corrosion resistance is superior to the 1528’s. The corrosion resistance of the same material in the HNO3 solution is superior to the corrosion resistance of that in the H2SO4 solution. The agglomeration of hard particles will lead to the uneven distribution of hardness. The higher porosity is the main cause of the low hardness and cutting difficult, Cu remaining on the surface will cause the mutual adhesion between the products. Excessively low carbon content is the reason of serious wear failure, while excessively high carbon content causing a low crushing strength. The weight of copper will have a direct impact to the effect of copper infiltration. Excessively low carbon content of the material powder, raising in the powder handing process and the production process parameters such as dew point temperature and protective atmosphere flow may result to the excessively low carbon content of the product. Finally, the corresponding improvement measures were put forward for each failure mode and cause.