Research On High-temperature Tribological Performance Of Ni30 Valve-ST30 Valve Seat Of High-speed And Heavy-duty Diesel Engine

The valve-seat contact pair is the main tribological system of the diesel engine.During operation,this contact pair is subjected to huge alternating impact load and high-temperature and high-speed gas erosion,and the working condition is extremely bad.Friction wear is the main failure mode of valve-seat contact pair,which leads to the decrease of sealing characteristics of combustion chamber,thus reducing the performance and service life of the diesel engine.The Fe-Ni based superalloy valve material and Fe based P/M seat material has been widely used in the diesel engine due to good wear resistance.According to the practical application conditions of these two materials,this paper focuses on the high-temperature mechanics,fatigue and friction and wear performance of this pair,which lays a technical foundation for improving the reliability of valve-seat contact pair of high-speed heavy-duty diesel engine and the wide application of new materials.(1)Two crucial heat treatments,ageing process and solution-ageing process,were applied to Ni30 valve material respectively,and the high-temperature mechanical properties at room temperature(25 ℃),500 ℃～750 ℃ were investigated.The results show that the Ni30 valve material still had high strength at high temperature,and the sample treated by solution-aging maintained good high-temperature stability above 600 ℃.The fracture morphology was dominated by dimples and micro-pores at medium and low temperatures.High temperature led to the decrease of grain boundary strength and slippage,which promoted the fatigue crack propagation.Thus,the fracture morphology was mainly inter-granular fracture and crack.Meanwhile,high temperature oxidation and creep aggravated the tearing and crack propagation,and greatly reduced the material strength.The aging treated samples showed brittle fracture mechanism above 600 ℃,while the solution-ageing treated samples still displayed plastic fracture mechanism at 600 ℃,showing good high-temperature stability.(2)The high-temperature rotary bending fatigue properties of aging treated Ni30 samples and solution-aging treated Ni30 samples at 500 ℃～750 ℃ were studied.The fatigue strength of aging treated samples was higher than that of solution-aging treated samples at500 ℃～600 ℃,but the fatigue strength decreased sharply at 650 ℃～750 ℃.At650 ℃～750 ℃,the fatigue strength of solution-aging treated samples was higher.Fatigue cracks initiated from the surface,and obvious oxidation and creep voids occurred at high temperatures.Solution-aging treatment enhanced the creep resistance and slowed down the growth rate of fatigue crack.(3)The wear properties of Ni30 valve-ST30 seat material contact pair under different temperatures and loads were studied and the wear mechanism was clarified.With the increase of temperatures,the wear rate of Ni30 valve materials and ST30 seat materials decreased first and then increased.The wear rates of Ni30 valve materials and ST30 seat materials reached the lowest at 600 ℃ and 500 ℃,respectively.The Ni30 valve-ST30 seat material contact pair was mainly abrasive wear at 25 ℃,and with the increase of temperature,it changed into a mixed wear mechanism composed of abrasive wear,oxidation wear and adhesive wear at500 ℃,oxidative wear at 600 ℃ and fatigue spalling at 700 ℃.Under different loads,the wear rate of Ni30 valve materials and ST30 seat materials increased gradually,and the wear mechanism was mainly adhesive wear.Under the condition of high temperatures and heavy load,the friction and wear performance of solution-aging treated Ni30 valve-ST30 seat contact pair was better.(4)The bench impact durability of Ni30 valve-ST30 seat contact pair under high temperature,high load and low cycle conditions and high temperature,medium load and high cycle conditions were studied on the bench simulation tester.The results show that under two working conditions,the total wear amount of solution-aging treated Ni30 valve-ST30 seat contact pair was decreased by 16.31% and 22.54%,respectively.It can be seen that solution-aging treatment can improve the impact performance of valve materials.At high temperature,high load and low cycle condition,the wear mechanism of contact pair was adhesive wear and fatigue spalling.At high temperature,medium load and high cycle condition,the wear mechanism was adhesive wear,oxidation wear and fatigue spalling.