| Engine valve is the key component of engine valve train,it works with the seat insert to seal the engine cylinder and it controls the intake of fresh air and the discharge of burnt gas.The valve’s operating stability and durability would directly influence the power output,fuel economy,durability,exhaust emission and service life of the internal combustion engine.The operating conditions of valve and seat insert are significantly severe,including high temperature,high stress and corrosive atmosphere,so the wear failure problems are prone to occur.With the improvement of engine performance,especially the application of supercharging and strengthening technology,leading to the speed of engine,the explosion pressure of combustion chamber,the operating temperature of valve is getting higher and higher,and the work conditions of valve and seat insert are becoming further severe.Although the materials with better performance have been developed for solid exhaust valve,solid valve still cannot meet the requirements of improving the high performance index of the engines.Due to the two prominent advantages of valve operating temperature reduction and valve weight reduction,sodium filled valve has become one of the main methods to improve engine performance,which has received more and more attention.Based on the urgent demand of high-performance valve for an engine and the analysis of advantages and disadvantages of different valve preperation methods,this dissertation proposed a new method,method of upsetting and drilling and friction welding,to manufacture the hollow head and sodium filled valve（HHSV）.Furthermore,the friction and wear performance and rotation bending fatigue performance of 42Cr9Si2 and 23-8N valve materials were investigated.The strengthening effect of ultrasonic surface rolling（USR）technology for the surface enhancement of 23-8N exhaust valve material was investigated.A novel bench simulation tester for valve and seat insert component has been developed,and the tester was used to conduct the durability test of the HHSV components.There are important theoretical significance and practical value of this dissertation.The research of preparation method and the key manufacturing process of novel sodium filled valve was carried out.By analyzing the valve manufacturing process,a method of upsetting and drilling and friction welding was proposed to manufactue HHSV.Based on the comparison of advantages and disadvantages of different HHSV manufacturing methods,the whole manufacturing processes of the method of upsetting and drilling and friction welding have been determined.The key manufacturing process of 42Cr9Si2 and 23-8N HHSV,friction welding sealing of valve hollow head,was studied.The weld quality under different processing parameters was discussed,and the range of optimized processing parameters was obtained.Two types of HHSVs with small batch have been prepared.Compared to the solid valve,the weight of HHSV was reduced by 16.14%17.44%.The friction and wear performance of 42Cr9Si2 and 23-8N valve materials were studied,and the related wear mechanisms of the two materials at high temperatures were clarified.A pin-on-disc friction and wear tester was used to investigate the friction and wear behaviors of42Cr9Si2 and 23-8N valve materials at different temperatures.It was found that friction and wear behaviors of 42Cr9Si2 and 23-8N materials were significantly changed at the critical temperature of 550℃ and 600℃,respectively.When the test temperature is lower than the critical temperature,both of the friction coefficient and wear loss of valve material were at relatively high levels.The predominant wear mechanism of 42Cr9Si2 material was classfied as a mixture abrasive wear,oxidation wear and adhesive wear.The wear mechanism of 23-8N materials was classified as a mixture of abrasive wear and adhesive wear,accompanied by a certain extent of oxidative wear.When the test temperature reached the critical temperature,the protective oxide films were formed on the worn surface,and the friction coefficient and wear loss of valve material were significantly reduced.The predominat wear mechanism of42Cr9Si2 material was characterized as mild oxidative wear,accompanied by adhesive wear.And the predominant wear mechanism of 23-8N material was characterized as oxidative wear,accompanied by adhesive wear.The rotating bending fatigue performance of 42Cr9Si2 and 23-8N valve materials were investigated,and the fracture mechanisms of the two materials were classified.The S-N curve was expressed by a three-parameters power function,and the S-N curves functions of two valve materials at different temperatures were determined by the method of correlation coefficient optimization.The fatigue strength of the martensite 42Cr9Si2 material at three temperatures ranging from 25℃ to 650℃ was determined.It was found that the fracture patterns were classfied as surface fracture modes.The typical fracture surface can be divided into three regions:crack initiation region,crack propagation region and fracture region.The fatigue strength of the 23-8N valve material at seven temperatures ranging from 25℃ to800℃ was determined.It is found that the predominant fracture patterns were also classified as surface fracture mode and the typical fracture can be divided into three regions.At 300℃,there were cracks initiated on the sub-surface of a specimen due to the presence of inclusion,and the fracture moded was classfied as sub-surface fracture mode.The research on USR strengthening technology of 23-8N exhaust valve materials was carried out.The surface strengthening mechanism of the valve material was discussed,and the influence of strengthening technology on the wear resistance and fatigue performance of materials was analyzed.The USR technology was used to the surface strengthening of the23-8N exhaust valve material,leading to a severe plastic deformation modified layer with a certain thickness was generated on the material surface.Compared to the untreated materials,the grains of modified layer materials were significantly refined,forming plate-like nanoscale grains with a thickness of 50150 nm.The dislocation within the grains in modified layer was increased,and the Kernel Average Misorientation between the grains was increased.After USR treatment,the surface roughness Ra was decreased from 0.84μm to 0.21μm,the produced maximum residual compressive stress was-908 MPa,the maximum microhardness was increased to 430.6 HV0.2,and the depth of hardening material was 800μm.In addition,both of the tensile strength and fatigue strength of USR treated 23-8N valve material at 25℃ and 650℃ were significantly increased.Furthermore,the wear resistance of the USR treated material can be improved under the conditions of fretting wear and sliding wear test.The construction of bench simulation tester and the durability experimental research of HHSV components were conducted.A novel bench simulation tetser that can simulate the actual operating temperature and explosion pressure of the engine valve has been developed.Then,the durability test was conducted,and the durability of HHSV components was evaluated via 10 million times of impact cycles.The design and manufacturing parameters of HHSV were optimized as follows:23-8N material was selected for valve head,suffered with nitriding treatment,42Cr9Si2 material was selected for valve guide,suffered with nitriding treatment,as well as quenching treatment for head face of valve guide,valve seating face was processed by plasma surfacing with Stellite F alloy.Consequently,valve head durability and wear resistance of valve seating face could meet the requirements.In the high-temperature dry friction durability test conditions,the predominant wear mechanism of valve and seat insert contact pair was classified as a interaction of adhesive wear,oxidative wear and surface fatigue wear.The temperature distributions of the solid exhaust valve and the hollow guide sodium filled exhaust valve of a gasoline engine were investigated.Compared to the solid exhaust valve of an gasoline engine,the overall maximum temperature of the hollow guide sodium filled exhaust valve was decreased from 745℃ to 590℃,with a decrease by 20.80%,the cooling effect of sodium valve was then verified. |
