| Marine engineering equipment has long been subjected to various loads and seawater erosion,and the maintenance cost is high.In order to reduce safety hazards and extend the service life of marine engineering equipment,this requires marine engineering steels to have better strength,plastic toughness,seawater corrosion resistance and good welding performance.At present,China’s research on steel for offshore engineering equipment is still lower than the international level,and some key structural steels still rely on imports,so it is extremely urgent to develop marine steel with excellent performance.In this paper,1MnCrMoNi alloy steel for offshore engineering is subjected to different processes of quenching+tempering and austempering,and through metallographic microscope,scanning electron microscope（SEM）,X-ray diffraction（XRD）,hardness tester,universal stretching.Testing equipment,impact testing machine,electrochemical workstation and simulated seawater immersion test and other analytical test methods to study austenitizing temperature and time,quenching cooling method,tempering temperature and austempering temperature and time on the microstructure and properties of alloy steel Influencing the law and mechanism,and optimizing the heat treatment process with the best comprehensive performance.The results show that the microstructure of as-cast 1MnCrMoNi alloy steel is pearlite with hardness of HRC25.7.The tensile strength,yield strength,elongation rate and area shrinkage are 783MPa,523MPa,7.9%and 29.8%,respectively.The impact toughness was94.5J/cm2.The austenitizing temperature and austenitizing time have a significant effect on the number,morphology and size of the lath martensite structure in the alloy steel.When the austenitizing temperature is in the range of 815915℃,with the increase of the austenitizing temperature,the content of the lath martensite and the plate spacing is increased,the lath martensite is coarsened,and the hardness of the alloy steel increased first and then decreased.When the austenitizing time is 10 to 50min,the austenitization proceeds more and more with the prolongation of the holding time,and the microstructure is accompanied by coarsening.The optimal quenching process optimized by analysis is:austenitizing at 835℃for 30min oil quenching.The martensite structure obtained under this process is uniform and fine,and the hardness of the alloy steel is HRC42.4.The alloy steel under the above optimal quenching process is tempered,and the tempering temperature ranges from 200 to 600℃.It is found that with the increase of tempering temperature,the microstructure changes into tempering martensite,tempering Tritzite and tempering Soxhlet respectively.The hardness rises first and then decreases,secondary hardening occurs at 200500℃,the tensile strength decreases gradually,and the impact toughness decreases and then increases.The tempering brittleness is obvious in the temperature range of 300500℃After tempering at 600℃,the tensile strength,yield strength,elongation and reduction of area of the alloy steel are 784MPa,669MPa,13.1%and 42.2%,respectively,and the impact toughness is 147J/cm2,which has the best comprehensive mechanical properties.The 1MnCrMoNi alloy steel after austenitizing at 835℃for 30min was subjected to isothermal quenching treatment in the range of 300 to 450℃.With the increase of austempering temperature,the amount of martensite decreases and the amount of bainite increases,and a small amount of pearlescence appears under the isothermal quenching process at 450℃.The tensile strength of alloy steel decreases first and then increases,the elongation first increases and then decreases,the shrinkage rate of the section shows“M”trend,the impact toughness first rises and then decreases;when the austempering time is1080min,with the extension of the isothermal quenching time,the bainitic content is increased.the tensile strength,elongation and reduction of the section first increase and then decrease.The impact toughness of the alloy steel increases with the extension of the isothermal quenching time.The optimal austempering process optimized by analysis is:austenitizing at 835℃for 30min+400℃for austempering for 60min.The structure obtained under this process is uniform and fine,the content of bainite in the structure is large,the content of hard phase martensite is low,and there is a small amount of retained austenite which is favorable for performance,and the tensile strength,yield strength,elongation and section shrinkage of 1mncrmoni alloy steel are 1050MPa,745MPa,11.5%and 49.0%,respectively,impact toughness is 132J/cm2,hardness is HRC32.9,and has the best comprehensive mechanical properties.Electrochemical test results show that the electrochemical corrosion performance of as-cast,400℃austempering and 600℃tempering 1MnCrMoNi alloy steels is similar,and the corrosion resistance of 600℃tempering is better than 400℃austempered alloy steel.The results of simulated seawater immersion corrosion show that the corrosion rate of the as-cast alloy steel and the alloy steel after the two optimal processes are reduced within 13weeks.When the immersion time is within 13 months,the corrosion rate decreases slowly and gradually becomes stable.The corrosion products increase and thicken with the immersion time.The corrosion resistance of the alloy steel after tempering at 600℃is slightly better.The corrosion products of the inner rust layer of the three-state alloy steel are mainly composed of three phases of fibrite（γ-FeOOH）,goethite（α-FeOOH）and chromium oxide（CrOOH）,and the outer rust layer corrosion product has Fe（OH）3,goethite（α-FeOOH）,tetragonal fibrite（b-FeOOH）,fibrite（γ-FeOOH）,Fe2O3/Fe3O4 and FeOCl.Although the hardness of 1MnCrMoNi alloy steel after isothermal quenching is lower than that of conventional quenching+tempering heat treatment,the tensile strength and plasticity are better,the impact toughness and corrosion resistance are similar,therefore,the comprehensive properties of the 1MnCrMoNi alloy steel treated by isothermal quenching are better. |
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