| The nuclear power is becoming more and more concerned because it is safety,high efficiency,economy and environmental.Nuclear reactor pressure vessel(RPV)is an important component of the nuclear power plant and it has long been serviced in harsh environments such as high temperature,high pressure and neutron irradiation.In addition,the nuclear RPV is often subjected to the cyclic thermal stress from start-up and shut-down processes,emergency stop and temperature fluctuations frequent during its service.Therefore,the ASME code regards the fatigue damage suffered by nuclear RPV during service as a safety assessment standard.SA508-? steel,as candidate material for RPV,make up for the problem of poor hardenability and insufficient low-temperature toughness of the current material SA508-? steel.The heavy forgings SA508-? steel diameter is ?2000 mm,and the wall thickness is 700 mm far exceeding the size of the previous RPV materials.The increase of the wall thickness will bring the wall thickness effect of the new generation RPV materials during the quenching and tempering heat treatment:The surface forms often martensite microstructure due to the high cooling rate and the core forms granular bainite microstructure due to the low cooling rate.The different microstructures will cause different mechanical and fatigue properties.The M/A islands in the granular bainite will decompose into bainite ferrite and carbides during the quenching and tempering heat treatment,which is one of the essential heat treatment methods for nuclear RPV materials before application.The evolution of granular bainite microstructures has a decisive influence on the mechanical and fatigue properties of the materials.The forging of nuclear RPV belongs to large forging,and mixed crystal is the main defect after forging.Therefore,the forging process of casting SA508-? steel is studied by hot compression test in order to obtain uniform microstructures and prepare for the subsequent fatigue tests.The main investigation contents and conclusions are as follows:(1)The forging process of SA508-? steel was studied by thermal simulation tests.The hot compression tests of SA508-? steel were carried out at a temperature 950-1250?,a strain rate 0.001-1 s-1,and a true strain of 0.7,respectively.The hot deformation activation energy of SA508-? steel was calculated to be 328.73 kJ mol-1 based on the results of the hot compression tests.Meanwhile,the constitutive equation and dynamic recrystallization equation of SA508-? steel were established according to the stress-strain curve.And then the processing map of SA508-? steel was drawn according to the ture stress-ture strain data.Finally,the optimal forging process of SA508-? steel was obtained by the analysis of hot processing map and microstructure:The temperature was 1050-1175?,and the strain rate was 0.01-0.1s-1.The grains in the optimal forging process zone were homogeneous and fine,the grain size was 18-62 ?m.(2)The mechanical and fatigue properties of SA508-? steel with martensite microstructure and granular bainite microstructure were studied.The mechanical tests results showed that the ultimate tensile strength and impact toughness of SA508-? steel with martensite microstructure were 830 MPa and 158 J,respectively,and those of SA508-? steel with granular bainitte were 811 MPa and 115 J,respectively.SA508-? steel with martensite microstructure had higher strength and impact toughness than SA508-? steel with granular bainite microstructure.The impact tests results showed that the martensite material belonged to typical dimple fracture,while the granular bainite material belonged to brittle fracture.The fatigue tests results showed that the fatigue life of SA508-?steel with martensite was 2717 cycles,and that of SA508-? steel with granular bainite was 1545 cycles under the strain amplitude of 10.45%.The fatigue life of the martensite material was greater than that of the granular bainite.Martensite material had fewer crack initiation points,narrower fatigue striations separation,and more high-angle grain boundaries(HAGBs)volume fractions than granular bainite material.The fewer crack initiation points meant fewer fatigue cracks,the narrower fatigue striations separation meant slower crack propagation rate,and the more HAGBs volume fraction could more effectively hinder fatigue crack propagation,resulting in the higher fatigue life of SA508-? steel.(3)The effect of tempering temperature on microstructure evolution of granular bainite SA508-? steel and the effect of microstructure evolution on its mechanical properties and fatigue properties were studied.Granular bainite specimens were quenching and tempering heat treatment at 595?,620?,630?,640? and 675? for 15 h,respectively.It was found that the M/A islands decomposed when the tempering temperature was 595?.More strengthening phase M/A islands decomposed when the tempering temperature increased to 630?,resulting in the increase of softening phase bainite ferrite matrix,the stress concentration around M/A was released,the crack initiation nucleation points decreased,the ultimate tensile strength of material decreased form 781 MPa to 738 MPa,the toughness increased from 112 J to 126 J,and the fatigue life increased from 2145 cycles to 2853 cycles under the strain amplitude of 10.45%.When the tempering temperature increased to 675?,the tempering temperature exceeded AC1 temperature and reached the intercritical tempering temperature.The microstructure obtained was ferrite,lath martensite and a small amount of undissolved carbides after intercritical tempering at 675?.The martensite structure provided strength,and the ultimate tensile strength of material increased to 863 MPa.The fatigue crack initiation occured in the ferrite phase and at the phase boundary between ferrite and martensite,the number of crack initiation points increased,the toughness of material decreased to 57 J,and the fatigue life decreased to 1509 cycles.(4)The effect of tempering time on the decomposition of M/A islands in granular bainite and the effect of M/A islands decomposition and decomposition products on its mechanical properties and fatigue properties were studied.Granular bainite specimens were quenching and tempering heat treatment at 630? for 30 min,2 h,5 h,15 h,30 h and 45 h,respectively.The results showed that the decomposition of M/A islands had already occurred after tempering for 30 min,and found that the decomposition of M/A islands at grain boundaries occurred preferentially.The ultimate tensile strength of the material was 906 MPa,the impact toughness was 75 J,and the fatigue life was 1257 cycles under the strain amplitude of 10.45%.More strengthening phase M/A islands decomposed into bainite ferrite matrix and fine carbides with the increase of tempering holding time,the stress concentration was released,the crack initiation nucleation points decreased,and the crack propagation rate decreased,resulting in the increase of toughness and fatigue life of the material.The increase of softening phase bainite ferrite matrix caused the decrease of ultimate tensile strength.The M/A islands had completely decomposed into M3C carbides and bainitic ferrite matrix when tempered for 45 h,the crack initiation nucleation points were the least,the crack propagation rate was the slowest,the ultimate tensile strength of material decreased to 675 MPa,the toughness increased to 156 J,and the fatigue life increased to 2205 cycles,the fatigue properties was the best. |
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