| In recent years,with the development of petrochemical and coal chemical industries in the direction of secondary deep processing and environmental friendliness,technological innovation marked by "hydrogenation technology and its application" has risen to a national strategy.The need for equipment to be large-scale,long life and clean green is more urgent.Traditional hydrogen Cr-Mo steel has been unable to meet the increasingly demanding performance requirements."Large thickness,high cleanliness,homogenization,high strength and toughness" has become the development direction of a new generation of hydrogen Cr-Mo steel.12Cr2Mo1R steel and 12Cr2Mo1VR steel are low-carbon hydrogen steels mainly composed of chromium and molybdenum,which have good resistance to temper brittleness,high temperature creep resistance and hydrogen corrosion resistance.In order to meet its performance requirements,it is necessary to effectively control phase transformation and the precipitation behavior of the experimental steels during heat-treatment.In this paper,12Cr2Mo1R steel and 12Cr2Mo1VR steel were taken as the research objects based on the national key research and development project "New Generation PWR Nuclear Island Steel and Large Single and Extra Heavy Plate for Hydrogen Chemical Industry".The heat treatment process,microstructure and the mechanical properties of the experimental steels were studied through heat treatment experiments.The main research work and results of this paper are as follows:(1)Thermo-Calc thermodynamic calculation software was used to calculate the thermodynamic equilibrium of 12Cr2Mo1R steel and 12Cr2Mo1VR steel.It was concluded that with the increase of temperature,the carbide transformation of Cr23C6+Mo6C?Cr23C6 in 12Cr2MolR experimental steel mainly occurs at 600? and the carbide transformation of Cr23C6+Mo6C(520?)?Cr23C6+ VC+(Mo,V)2C(790?)?VC mainly occurs in 12Cr2Mo1VR experimental steel.(2)The austenite continuous cooling transformation of 12Cr2Mo1R steel and 12Cr2Mo1 VR steel was studied by using Formaster-? thermal expansion phase transformation instrument.The experimental results show that Aci=766?,A,3=860? for 12Cr2Mo1R steel,Aci=769?,Ac3=880? for 12Cr2Mo 1 VR steel.The cooling rate of 12Cr2Mo 1R experimental steel for obtaining bainite structure is 0.3?50C/s,while that of 12Cr2Mo1VR experimental steel is 0.1?5?/s.(3)The austenite grain growth of 12Cr2Mo1R and 12Cr2Mo1VR experimental steels at high temperature was studied by means of heat preservation and quenching experiments.The results show that the austenite grain size of 12Cr2Mo1R experimental steel and 12Cr2Mo1VR experimental steel increases with the increase of heating temperature and the extension of holding time in the temperature range of 850-1050?.The austenite grains were refine and the austenite grain coarsening rate were inhibited by the additi'on of V in the experimental steel in the range of 850-950?.(4)The effects of tempering temperature on the microstructure and mechanical properties of 12Cr2Mo1R and 12Cr2Mo1VR experimental steels were studied by heat treatment of hot rolled steel sheets in box resistance furnace.The results show that all of the microstructure of 12Cr2Mo1R and 12Cr2Mo1VR steel with different tempering temperatures is mixed structure compose of granular bainite and lath bainite.With the increase of tempering temperature,the strength and hardness of 12Cr2Mo1R and 12Cr2Mo1VR experimental steels decreases gradually,while the impact toughness increases gradually.The carbides containing Cr with size range of 15-130nm,found as precipitated phase in 12Cr2Mo1R experimental steel mainly distributed inside the grain.For 12Cr2Mo1VR experimental steel,due to the addition of V,the number of precipitated phases increases,the size is refined and the distribution is dispersive.In 12Cr2Mo1VR experimental steels,the carbides containing Cr precipitated at grain boundaries,ranging in size from 100 to 260 nm,are conducive to grain boundary hardening and durability improvement;Mo and V composite carbides with sizes of 20 to 60 nm and VC with sizes of 5 to 15 nm were precipitated in the grain.The type,size and distribution of these carbides are beneficial to the improvement of strength and toughness.In order to obtain the best matching of strength and toughness,the tempering temperature for 12Cr2Mo1R steel ranges from 680?to 700? and that of 12Cr2Mol VR steel ranges from 700? to 720?. |
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