Effects Of Heat Treatment On The Structure And Properties Of 4Cr2Ni4Mo Alloy Steel

In recent years,China has made great progress in the marine engineering and shipbuilding industry.Steel for marine engineering must have the characteristics of high strength,high plastic and toughness,welding and corrosion resistance in order to cope with the special marine environment.Domestically developed marine platform steel plates have been widely used.However,the high-strength and large-thickness materials used in key parts still depend on imports.So it is urgent to increase the research and development of high-end marine steel.In this paper,the self-designed4Cr2Ni4Mo alloy steel is subjected to stepped quenching and austempering treatment.The effect of stepped quenching temperature and time,and austempering temperature and time on the microstructure,mechanical properties and corrosion resistance of4Cr2Ni4Mo alloy steel has been specifically investigated through analytical test methods such as metallographic microscope,hardness tester,scanning electron microscope?SEM?,X-ray diffractometer?XRD?,universal tensile testing machine,impact testing machine,electrochemical workstation and simulated seawater immersion test.The heat treatment process with the best comprehensive performance has been optimized.The results show that the as-cast microstructure of 4Cr2Ni4Mo alloy steel is needle-shaped martensite,whose hardness is HRC53.After quenching and tempering treatment,the microstructure of the alloy steel is coarse lath martensite.The tensile strength,the yield strength,the elongation and the reduction of area are 1830 MPa,1480 MPa,6.3%and 9%,respectively.The hardness is HRC45.The impact toughness is 27 J/cm².After the steppeded quenching treatment,the microstructure of the alloy steel is mainly lath-like and needle-shaped martensite.In the range of 380-540?,with the increase of the quenching temperature,the needle-shaped martensite firstly decreases and then increases.The hardness and the tensile strength is in the"M"type trend.The elongation and the reduction of area firstly shrink,then rise and then fall.The impact toughness firstly rises and then tends to flatten.When stepped quenching at 380?for3h,some austenite undergoes granular bainite transformation,resulting in a decrease in strength,hardness and impact toughness.In the range of 1-4h,with the prolongation of the quenching time,the needle-shaped martensite is reduced.The tensile strength and elongation,the reduction of area firstly increase and then decrease.The hardness and impact toughness increase slowly.When stepped quenching at460?for 4h,part of the lower bainite appeared in the structure,and the tensile strength decreased significantly.Based on the comprehensive analysis,it is determined that the suitable quenching process of 4Cr2Ni4Mo alloy steel is austenitizing at 860?for 30min and then stepped quenching at 460?for 3h.After treated by the optimized process,the tensile strength,the yield strength,the elongation and the reduction of area of the 4Cr2Ni4Mo alloy steel are 1950MPa,1500MPa,7%and 16%.The hardness is HRC52,and the impact toughness is13.3J/cm².The microstructure of 4Cr2Ni4Mo alloy steel after austempering is mainly bainite.In the range of 280-360?,the microstructure tranforms to lower bainite,upper bainite and granular bainite as the austempering temperature rises.The tensile strength,the yield strength and the impact toughness firstly increased,then decreased and then increased.The elongation decreased gradually,and the hardness and the reduction of area firstly decreased and then increased.When the austempering temperature is 280?,the structure is martensite,and the strength and the toughness are lower.After austempering at 300?,the microstructure is B_L/M multiphase structure with good comprehensive mechanical properties.When the austempering temperature rises to 340?and 360?,the microstructure is upper bainite and granular bainite,respectively.The strength and the ductility of the alloy steel are lower.In the range of 4-24h,with the extension of austempering time,the content of bainite in the structure increased and gradually coarsened.The strength of the alloy steel firstly decreased,then increased and then decreased.The elongation and the reduction of area firstly increased and then decreased.There is a slight upward trend,and the impact toughness drops sharply after austempering for 24h.When the austempering time is 16h,the obtained B_L/M multiphase structure has good mechanical properties.The optimum austempering process is austenitizing at 860?for 30 min and then austempering at 300?for 16 h.The tensile strength,the yield strength,the elongation and the reduction of area are 1560 MPa,1300 MPa,6.3%and15%,respectively.The hardness is HRC45.The impact toughness is 27 J/cm².The electrochemical test results show that the corrosion tendency of the alloy steel after stepped quenching is the smallest.The results of immersion experiments show that the corrosion rate of 4Cr2Ni4Mo alloy steel decreases rapidly and gradually becomes stable with the prolongation of immersion time.The alloy steel treated by stepped quenching has the lowest corrosion rate and the best corrosion resistance.The main components of rust layer in corrosion products are made up of Fe₃O₄,?-FeOOH and?-FeOOH.