Study On Microstructure And Properties Of Low Nickel Austenitic Gas Valve Alloy For Internal Combustion Engine

Internal combustion engines are widely used in various fields such as transportation in modern society,including various land machinery vehicles,ship hosts and some civil aviation fields.The improvement of working efficiency of internal combustion engine is generally realized by increasing the working temperature of combustion chamber,so as to improve the fuel utilization.The improvement of working temperature puts forward more stringent requirements for the high-temperature performance of internal combustion engine materials.Austenitic gas valve alloy has excellent mechanical properties,structural stability and good economic benefits.It is widely used in the field of gas valve steel.In this thesis,the solid solution and aging heat treatment system of a low nickel austenitic gas valve alloy21-4NWNb,the microstructure stability after long-term heat exposure at 750℃ and high temperature oxidation resistance were studied,and the internal relationship between microstructure characteristics and macro mechanical properties was studied.21-4NWNb alloy has fine grains as forged,and there are a large number of insoluble M₂₃C₆carbides and Nb（C,N）phases.At five solution temperatures of 1000℃,1050℃,1100℃,1150℃and 1200℃for 30min respectively,when the solution temperature reaches 1150℃,the grains of the alloy are obviously coarsened,the grain size grade reaches grade 9,the insoluble particles in the crystal are reduced,and the hardness of the alloy is greatly reduced.It shows that the solid solution effect is better when holding at1150℃～1200℃for 30min.The aging test was carried out on the alloy treated by solid solution at 1150℃,the grain size did not change significantly after the test.The hardness is the highest after 750℃×4h test,reaching 323.3hv.The strengthening precipitate of the alloy during aging is WN phase by transmission electron microscope.WN precipitates and Fe matrix maintain（110）_WN//（220）_γPositional relationship.In the practical application of 21-4n NWNb alloy,the grain size grade is required to be grade 8 or finer,and the error is within grade 3.The grain size of the alloy after solid solution at 1050℃reaches grade 12.In order to study the changes of microstructure and properties of the alloy in practical application,the alloy after heat treatment at1050×30min,750×4h was subjected to long-term insulation test at 750℃.The grain size of the alloy did not change significantly during heat preservation.When holding for 500h,needle like particles were formed near the M₂₃C₆carbide in the crystalσmutually.With the increase of holding time,the number ofσphases increases gradually,and the number and size of WN precipitated phase particles in the alloy also increase gradually.The transmission analysis of the samples insulated for 500h and 2000h shows that there are a large number of uniform WN precipitated phase particles in the matrix,WN precipitates and Fe matrix maintain（110）_WN//（220）_γPosition direction.When holding for 0～200h,the hardness and strength of the alloy show an upward trend.The hardness reaches the peak value of 324.1HV at 200h,and the strength reaches the peak value of 971.3MPa at 200h.After holding for 200h,the hardness and strength began to decline slightly at the same time,with the lowest hardness of 305.6HV and the lowest strength of 896.3MPa.The highest elongation of the alloy is 22.2%,which shows a downward trend,and the lowest is 15.6%.When 21-4NWNb alloy was oxidized at 750℃,its weight gradually increased with the extension of oxidation time.At the initial stage of oxidation from 0h to 200h,the oxidation weight gain of the alloy increases rapidly and the oxidation rate is relatively low.When the oxidation time is extended to 500～1000h,the oxidation weight gain of the alloy decreases compared with the early stage of oxidation,and the oxidation rate also slows down relatively.When the oxidation test is carried out to 1500h～2000h in the later stage of oxidation,the oxidation weight gain of the alloy is very slow,and the oxidation rate is greatly reduced to 0.0035mg/（cm2·h）.The oxidation rating of the alloy is complete oxidation resistance.The outer oxide film of the alloy gradually grows completely with the extension of holding time,and the oxide film is composed of uniform and dense granular oxide.EDS and XRD showed that the outer oxide film was the mixed oxide of Fe₂O₃and Mn₂O₃,and the inner layer was Cr₂O₃oxide.When the oxidation time reaches 1500h,the alloy reaches the level of complete oxidation resistance.