| With the modification,upgrading and updating of pressure vessel under hydrogen atmosphere,the demand of 12Cr2MolR steel for pressure vessel under critical hydrogen atmosphere,which are used widely in the petroleum and chemical industry,as well as energy fields,is constantly rising.Due to its critical service environment,high requirements of properties,12Cr2MolR steel were not able to be produced stably in domestic and had been depended on import for a long time.Based on fully knew its application environment,chemical composition system,microstructure and technical requirements,and production performances of both domestic and foreign steel plants.The basic production approach,key technical points and common problems in service period are summarized.Then combined with the existing condition,developed the production process,and finally the 12Cr2MolR plate with thickness of 12 mm was successfully produced in the laboratory.Microstructure and processing properties were studied.The main contents are as follows.(1)Expansion experiment was carried out to measure the static CCT curve of 12Cr2MolR steel.Through analyzing the austenite transformation law under different cooling rates,the critical cooling rate to obtain desired bainite for 12CrMolR steel was 0.5?2 0C/s.(2)Thermal simulation experiment was conducted to investigate the influences of controlled rolling and controlled cooling process parameters(finish rolling temperature,cooling rate and finish cooling temperature)on microstructure of 12Cr2MolR steel.The results indicate that granular bainite and lath bainite transformation are two main intermediate transformation types,and the transformation start temperature of granular bainite is higher than lath bainite.With the decreasing of finish rolling temperature,increasing of cooling rate,or decreasing of finish cooling temperature,on the one hand,the main microstructure from granular bainite to lath bainite accompanied by smaller grain size;on the other hand,M/A islands,distributed in the matrix,volume fraction decreases,from bigger irregular blocky shape to smaller needle,granular shape.Through reasonable control of rolling and cooling process parameters,as well as the morphology,size,quantity and volume fraction of M/A islands,strength and toughness of the steel can achieve a good match.(3)Effect of different heat treatment parameters on microstructures and mechanical properties of 12Cr2MolR steel were investigated.The results show that with the increasing of normalizing temperature or holding time,the island microstructures,distributed in the matrix of granular bainite,became more evenly with the bigger size.With the increasing of tempering temperature or holding time,the number of carbides,precipitated in the tempered bainite,became more and more with the relative bigger size,as well as more evenly distributed.Meanwhile,the strength decreases,while ductility and toughness increase gradually.The final heat treatment parameters were as follows:normalizing temperature was 930 ? with 30 min holding time,tempering temperature was 730 ? with 50 min holding time.(4)The variation reasons of mechanical properties after 12Cr2MolR steel tempering were analyzed.By analyzing microstructures at different tempering temperatures,it can be deducted that the variations of mechanical properties were mainly attributed to the comprehensive impacts among dislocation recovered soften within bainite matrix,second phase of precipitation strengthening,carbon and alloy elements exsolution mechanism on the 12Cr2MolR steel during tempering process.(5)Minimum simulated post weld heat treatment,maximum simulated post weld heat treatment and step cooling treatment were conducted to inspect the processing properties of 12Cr2MolR steel.The results show that tempered bainite can be obtained with good temper resistance after Min.PWHT,Max.PWHT and Min.PWHT+S.C,as well as strength and toughness maintained at a high level.After three different heat treatments,impact energies under serial temperatures were measured,ductile-brittle transition temperature curves were drawn and ductile-brittle transition temperature were evaluated,respectively.Finally,an indicator that can be used to evaluate the level of sensitivity to temper embrittlement was calculated,vTr54+2.5?vTr54=-107.375 ?,which was far less than the demand of 10 ?.Therefore,12Cr2MolR steel has a good temper embrittlement resistance can be concluded.(6)High temperature tensile properties of 12Cr2MolR steel under different temperature were measured.The results show that the steel has high strength and heat-resistance from 200 to 600 ?.With the increasing of temperature,tensile and yield strength only decrease by a small range,but obviously from 500 to 600 ?,while elongation ratio and percentage reduction of area both increase.Finally,the research results show that through appropriate composition design,high level of smelting,reasonable rolling and cooling processes,and suitable heat treatment process,12Cr2MolR steel can have a good comprehensive mechanical properties and meet the technical requirements at the same time. |
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