| Nuclear power is a clean,efficient and safe energy source and increasingly favored by countries around the world At present,the nuclear power reactor type in China is AP1000.The main steel used is SA73 8 Gr.B It has high strength,good low-temperature-impact toughness,and is widely used in nuclear power plant containment and structural modules.Based on the national key R&D project,the phase transformation,precipitation behavior,strengthening and toughening mechanism of SA738 Gr.B steel plate for nuclear reactor containment during heat treatment were studied.The main research work and results are as follows:(1)The thermodynamic calculation of SA738 Gr.B steel was carried out by JMatPro.The results show that there are four kinds of carbide precipitation in SA738 Gr.B steel below 900?,among which MC carbide formation temperature is the highest,but the precipitation amount is the least;M3C and M23C6 carbide can be mutually transformed near 610??660?,which is the main precipitation in SA738 Gr.B steel;The size of M3C is stable,while the size of M23C6 increses significantly with the increasing of tempering time.(2)The temperature of AC1,AC3 and Ms of SA738 Gr.B nuclear power steel are 684?,824? and 396?,respectively;the critical cooling rate for banite is 2?/s and for martensite is 10?/s,and the microhardness of steel increases with the increase of cooling rate;the original austenite grain size and the average size of martensite packet and block microstructure of steel show a trend of gradual coarsening with the increase of austenitizing temperature,and the original austenite grain size increases with the increase of austenitizing temperature The growth range of austenitizing temperature is smaller than that of packet and block;the strength of steel tempered with different austenitizing temperature has little correlation with the microstructure size of martensite;the impact toughness at-45? has a linear relationship with the original austenite grain,the packet and block size of martensite,Block is the effective grain size of low temperature toughness(3)The strength and toughness of SA738 Gr.B steel matches well after tempering at 630 ??650? for 1h,The structure composed of lath ferrite+polygonal ferrite+M3C carbide;when tempered at 670?,the strength and impact toughness of steel reduced compared with that tempered at 630??650?.A small number of twin martensites formed along the grain boundary,and the amount of carbide precipitation reduced;when the tempering temperature is increased,the amount of carbide precipitation is reduced.At 690? and 710?,the tensile strength of the steel increased,the yield strength continued to decrease,the impact toughness sharply decreased,and a large number of twin martensites were precipitated in the structure,accounting for 18.6%and 34.9%respectively;M3C and MC carbides were precipitated in SA738 Gr.B steel.The microstructure analysis shows that the main reason for the increase of strength and the decrease of toughness is the increase of volume fraction of twin martensite(4)After tempering at 650? for 2h?5h,the tensile strength of SA738 Gr.B steel is stable,while the yield strength and toughness reduced sharply.The structure composed of lath ferrite+polygonal ferrite+carbide.With the increasing of tempering time,M3C changes into M23C6 alloy carbide with the growing size,which affects the toughness of steel.When the tempering time is 8h?10h,the tensile strength of the steel increase,while the yield strength is stable and the toughness reduced sharply.The microstructure consist of polygonal ferrite and twin martensite.(5)On the basis of the heat treatment experiment in the laboratory,the industrial trial production of SA738 Gr.B ultra wide 4600mm,ultra thick 100mm steel plate for the containment of nuclear reactor has been completed,and the quenched and tempered performance meets the standard requirements. |
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