Study On Heat Treatment,microstructure And Properties Of SA508 Gr.3 Steel Heavy Forging For Nuclear Power Plant

SA508 Gr.3 steel is widely used in the manufacture of heavy forgings for pressurized water reactor nuclear power plant,especially for vessels used in the nuclear island.As the power of single reactor has been continuously improved,the size and weight of a vessel increased accordingly.In this case,thick-walled heavy forgings will be the trend in the future.For instance,the maximum wall thickness of the forgings used in nuclear island vessels of CAP1400 and Hualong One is even more than 500 mm.These forgings show a significant size effect during the quenching and tempering processes.The slow cooling rate in center leads to low and uneven low-temperature toughness in heavy forgings which affects the quality of the product severely.Therefore,it is urgent to improve the low-temperature toughness and homogeneity in SA508 Gr.3 steel heavy forgings in nuclear industry.This article has studied the transformation of undercooled austenite and evolution of tempered microstructure in SA508 Gr.3 steel systematically.The effects of intercritical heat treatment and pre-tempering processes on microstructure and mechanical properties of the core in 520 mm thick-wall heavy forgings with an actual cooling rate of 7.5?/min was studied.On the above basis,a new heat treatment process of "quenching+intercritical heat treatment+two-step tempering" was designed.Compared with quenching and tempering process,effects of the new heat treatment process on microstructure and property were investigated.It also revealed the sensitivity of different quenching cooling rates on heat treatment process.At the same time,the thermal aging behaviors of different heat-treatment steels were investigated.On the above basis,a new process has been explored to improve the low-temperature toughness and homogeneity of thick-walled heavy forgings.The transformation of undercooled austenite and evolution of tempered microstructure in SA508 Gr.3 steel were investigated.The results indicated that the undercooled austenite mainly transformed into bainite during the hot manufacturing process and it was common to transform incompletely for bainite in SA508 Gr.3 steel.The uneven enrichment of carbon in the undercooled austenite,resulting in the stability of the austenite increased so that the bainitie transformation was incomplete to form residual austenite or martensite-austenite(M-A)constituents.The amount of bainitie transformation was related to the isothermal temperature.After isothermal treatment at 500?,the amount of bainitie transformation was only 40%,while that was up to 80%at 400?.During isothermal tempering,the M-A constituents decomposed and the carbide precipitated in the bainite only at high temperatures,suggesting that SA508 Gr.3 steel has high tempering stability.The heat treatment process of "quenching+intercritical heat treatment+tempering" of the steel was investigated.The results suggested that the fraction of M-A constituents and ferrite transformed from undercooled austenite increased gradually,as the temperature of intercritical heat treatment decreased from 800?to 750?.There were still M-A constituents after tempering,resulting in the low impact toughness.The low-temperature toughness of specimens was even lower than that of traditional process of "quenching+tempering".When the temperature of intercritical heat treatment was at 750?,more coarse-stringer and massive M-A constituents,and massive ferrite formed as the cooling rate decreased from water cooling(1500?/min)to 4?/min.The impact toughness was significantly reduced after tempering indicating that this process was sensitive to the cooling rate in SA508 Gr.3 steel.This manifested that it was difficult to obtain a good homogenization after intercritical heat treatment in practical heavy forgings.The heat treatment process of "quenching+intercritical heat treatment+two-step tempering" was studied.Compared with quenching,the fraction of M-A constituents increased after intercritical heat treatment at 750?.When pre-tempering at 300?500 ?,M-A constituents exhibited different decomposition characteristics.Pre-tempering at 400 ? can promote the decomposition of M-A constituents and the precipitation of fine carbides which not only reduced the number of crack initiation cites but also hindered crack propagation to obtain good low-temperature toughness.Compared with the specimens with "quenching+tempering" or "quenching+intercritical heat treatment+tempering",the low-temperature toughness of specimens with the new process has improved by 64%and 88%,respectively.In addition,the new process can improve the impact toughness of the core in thick-walled heavy forgings and achieved the goal of homogenization.At the same time,the new process has widened the temperature range of intercritical heat treatment from 20? to above 50?.The thermal aging behaviors of steels with "quenching+intercritical heat treatment+tempering" and "quenching+intercritical heat treatment+two-step tempering" were studied.It was suggested that M-A constituents decomposed to form fine carbides at the initial stage of thermal aging at 450? so that the impact toughness increased.After 1 000 hours of thermal aging,the M-A constituents were completely decomposed.The carbides and blocks coarsened,and the fraction of large-angle grain boundaries decreased.These changes weaken the hindering effect on the crack propagation leading to the decrease of the impact toughness.