| D6A steel is a kind of medium-carbon low-alloy steel with ultrahigh strength good hardenability,which is developed from D6AC.D6AC can be obained by modulating and tempering of D6A.It has a lot of advantages,such as high strength,good stress corrosion resistance,low cost and reliable application.Currently,it is used in aerospace and military applications.The conventional heat treatment process of D6A alloy steel is quenching and low-temperature tempering.Although the tempered martensitic steel has high strength,however,poor plastic toughness limits the scope of application of the material.The studies have shown that the comprehensive mechanical properties of bainite/martensite composite obtained by the banite isothermal treatment process are better than that of tempered martensite in medium and low carbon low alloy steels.Therefore,good plasticity of D6A alloy steel can be obtained by the bainitic isothermal treatment process.In this paper,the D6A alloy steel with good mechanical properties was prepared by bainitic isothermal treatment.The influence of different isothermal treatment processes on the microstructure and mechanical properties of the experimental steel was analyzed by means of OM,SEM,XRD,EBSD,hardness test and tensile test.The product of strength and elongation of D6A steel is as high as 19835MPa·%.The main research contents and results are as follows:(1)The phase transition point of experimental steel was measured and the results show that Ac1,Ac3,Ms and Mf are 742?,782?,305? and 185?,respectively.The CCT curves were drawn according to the expansion curves at different cooling rates.The martensite region was determined to be 185??305?,the bainite region was presumed be ranged from 185? to 438?.The micro structure is mainly bainite with a small amount of ferrite at the cooling rates of 0.1?/s and 0.5?/s,but only martensite exists when the cooling rate is more than 10?/s.(2)Study the effects of different austenitizing annealing temperature,isothermal temperature and isothermal time on micro structure and mechanical properties of experimental steel,which is treated with isothermal quenching-bainite process.The results show that the microstructure is made of lower bainitic ferrite,martensite and carbide.As the annealing temperature increases from 820? to 880?,the tensile strength increases from 15 03 MPa to 1817MPa,the elongation increases to the maximum of 12.4%at 860?.As the isothermal temperature increased from 225? to 300?,the tensile strength decreased from 1762MPa to 1576MPa,and the grain size reachs the minimum value of 770nm at 275?.As the isothermal time was extended from 30min to 180min,the tensile strength and hardness reachs the maximum of 1612MPa and 585HV,respectively at 120min.After isothermal process,the tensile strength is 1612MPa,elongation is 12.4%,resulting in the highest product of streng and elongation up to 19835MPa·%?(3)With the isothermal temperature increasing from 275? to 350?,the tensile strength gradually decreased from 1825MPa to 1488MPa,the elongation increased from 8.2%to 11.5%.After salt-quenching-bainite isothermal process,the tensile strength is 1685MPa,companied with an elongation of 11.2%and consequently a product of strength and ductility of 18872MPa·%.(4)By comparing the mechanical properties,the optimum parameters of isothermal treatment are determined as follows:After annealing at 860? for 10min,the steel was cooled to 275? in salt bath and incubated for 120min,the experimental steel has the best comprehensive mechanical properties in this process. |
PDF Full Text Request