Preparation And Basic Researches On Exhausting Valve And Valve Seat Used For Heavy Duty Engines

A novel PM alloy used for valve seat of heavy duty engine has been prepared. Series theoretical problems such as ambient densification of multi-powders, hot densification of green compacts and heat treatment of the sintered alloy have been studied on this PM valve seat alloy. At the same time, a novel nickel-base alloy for valve of heavy duty engines has been prepared. Series problems such as plasticity of the valve alloy at elevated temperatures, inertia friction of the alloy with steel 4Cr10Si2Mo, cladding of stellite 6 powders on the valve alloy have been studied. At last, oxidization-resistance and wear-resistance of the both alloys have also researched. Property and microstructure evolution in preparing of the valve/valve seat materials and properties of the two materials at an operating mode of heavy duty engines have been studied using hardness measurement, tensile test at room temperature and at elevated temperatures, X-ray diffraction(XRD) ,micro-strain analysis, optical microscopy(OM), electronic microstructure analysis(SEM,TEM), atomic force microscopy(AFM) , finite element method(FEM) and thermal-mechanics calculation etc. Important conclusions can be summarized as follows:1. By researches on ambient compaction of multi-powders, a new compaction equation has been suggested asEffect of various factors on cracks in compaction of multi-powder has been discussed. Results show that the aggregation of Cr-Fe and Mo-Fe powder is the main reason for the formation of micro-cracks, the inhomogeneity of density in vertical direction and the misalign of dies can lead to crack formation and propagation. FEM simulation has been used to study evolution of strain, stress and density in unidirectional compaction of multi-powder. Simulation results show that density of top layer in green compacts is higher than that of bottom layer. A new friction-assisted compaction has been suggested. And FEM simulation has also been used to study this new compaction. Simulation results show that density of green compacts made in friction-assisted method is much more homogenized than that in unidirectional method.2. Shrinkage of the alloy in sintering has been discussed. It is discovered when the density of green compacts get to a threshold value, density of the sintered alloy will not increase with increasing of compact density. By theoretical deduction, a equation for calculation of threshold density has been brought out as:v.ρ²=C₁ρ²+C₂·m²+C₃mρEffects of sintering density on physical and mechanical property have been studied. It reveals that it is an effective way of increasing sintering density to promote properties of the alloy. A equation for calculation of conductivity coefficient at elevated temperatures has been suggested as: k_t=[1+α·(T-273)]·(1-ε)/(1+4.46ε²)·k_s.Thermal simulator has been used to study the effects of such parameters as holding time and sintering temperature on microstructure and mechanical properties of the alloy. It is discovered that a suitable sintering process is sintering at 1240℃and holding 30 min.3. Continuous cooling transformation (CCT) diagram and isothermal temperature transformation (TTT) diagram of the PM valve seat alloy has been studied on a Gleeble1500 simulator. Results show that there are obvious phase transformation regions of pearlite and bainite on CCT and TTT diagrams. Carbides precipitate at a slow cooling rate and M_s of the alloy is 296℃. Effects of such parameters as quenching temperature, tempering temperature and tempering holding time on microstructure and mechanical properties have also been discussed. It is found that a suitable heat treatment of the alloy is quenching after holding 30 min. at 1100℃and tempering at 750℃for 2 hours. Thus, hardness of the alloy is 42 HRC, tensile strength of the alloy is 446MPa, transverse rupture of the alloy is 996MPa and impact toughness of the alloy is 3.2J/cm². A thermal dynamics calculation program has been used to calculation phases of the alloy at 750℃and the calculation results has been compared with experimental data. It is discovered that part of the original carbides of the alloy formed in sintering is stable after being tempered at 750℃for 2 hours, which leads to a good mechanical property of the alloy.4. Based on a discussion of oxidization resistance of the alloy from 250℃to 650℃, it is found the oxidization resistance of the alloy is excellent at temperatures lower than 450℃. Oxides formed at 250℃is "needle-like". Oxides formed at 550℃is "piece-like" and that formed 650℃is "flower-like". Chemical analysis shows that chrome is gathering at the boundary between the layer of oxides and matrix. Grain boundary is an important entrance for ambient oxygen from surface of samples to inner matrix.5. Deformation behavior of a nickel-base alloy Ni76Cr19AlTi has been studied on a thermal simulator. It is found that suitable deforming temperature of the alloy is 1050±30℃and the swain rate can be higher than 1S^-1. Based on the Zerrilli-Armstrong constitutive equation used for f.c.c, metals, an improved constitutive equation for the studied nickel-base alloy has been suggested. Microstructure and mechanical property of the alloy after die forging has also been investigated. A finite element code has been used to investigate the die forging process. Simulation results can reasonably explain the microstructure evolution and mechanical property of the alloy after die forging.6. Friction welding has been used to join Ni76Cr19AlTi nickel-base alloy with 4Cr10Si2Mo steel. Strength of the welds is 1120MPa. Microstructure analysis shows that the-ultra-high dislocation density in nickel-base alloy and martensite transformation in 4Cr10Si2Mo steel makes a high strength of the welds. Plasma cladding of stellite 6 on Ni76Cr19AlTi-made valve has been researched. Researching results show that there are no obvious imperfects in the interface between stellite 6 coatings and the matrix.7. Based on a discussion of the oxidization resistance of Ni76Cr19AlTi nickel-base alloy from 600℃to 800℃, it is found the oxidization resistance of the alloy is excellent at these temperatures. The oxidization of the alloy depends on the diffusion rate of chrome and Ti at the temperatures.8. Sliding wear-resistance of the friction pair consisting of PM valve seat alloy and Ni76Cr19AlTi nickel-base alloy has been studied. It is found that wear loss of the PM alloy is minor if the counterpart nickel-base alloy has been coated with stellite 6 cobalt-base alloy, however if the counterpart nickel-base alloy is uncoated, then wear of the PM alloy is severe. In the condition of the friction pair consisting of PM valve seat alloy and the coated Ni76Cr19AlTi nickel-base alloy, effects of load and wearing time on wear mechanism of the PM alloy have been investigated. It is discovered that furrowing is a major wearing of the PM alloy if the loading is low. If the loading is heavy and the wearing time is short, wear loss is mediate. If he loading is heavy and the wearing time is long, PM alloy is worn severely. An engine simulator has been used to analyze the impact wear of the PM valve seat and the coated nickel-base alloy valve. Results show that if the ambient temperature is low, wear of both the PM valve seat and the coated nickel-base alloy valve is minor, if the ambient temperature is high, for example, higher than 700℃and the number of impacts is small, minor adhesion is the major wear mechanism. If the ambient temperature is high, for example, higher than 700℃and the number of impacts is big, severe adhesion is the major wear mechanism. At last, a comment on wear-resistance of the valve seat alloy by the sink quantity of the valve. It is found that valve sink quantity of the studied pair of PM valve seat/coated nickel-base valve is 0.065mm, much smaller than that of the pair of high-chrome valve seat/4Cr14Ni14W2Mo valve.