Investigation On Technology Of Improving Cryogenic Toughness Of High Strength Stainless Steel

As is well known,stabilizing retained austenite and forming reverse transformation austenite is one of the main means to improve the ultra-low temperature toughness of high strength steel.However,the effect of retained /reverse transformation austenite in high strength stainless steel on toughness is complex.The retained austenite after solution treatment of high strength stainless steel significantly improves the ultra-low temperature toughness,but the retained austenite is very sensitive to the contents of alloy elements such as Cr,Ni and Mo.The window of Cr,Ni and other alloy elements corresponding to about 20-30%austenite is very narrow,and the effect of reverse transformation austenite formed by overaging on ultra-low temperature toughness of high strength stainless steel is not clear.therefore,this paper studies a new thermal stabilization process for different types of high strength stainless steel to increase the content of retained austenite after solution treatment,which provides a new way to improve ultra-low temperature impact toughness.At the same time,a new solid solution treatment process to increase the content of retained austenite and reduce the stability of reverse transformation austenite is studied.The results are as follows.The effects of retained austenite obtained by quenching and tempering of07Cr16Ni6 high-strength stainless steel and reversed austenite obtained by overaging of 00Cr11Ni11 Mo Ti maraging stainless steel on the notch tensile strength and impact toughness at cryogenic temperature have been studied.The test results of mechanical properties have indicated that although the strength and toughness are similar at room temperature,the notch tensile strength and impact toughness of07Cr16Ni6 steel are significantly higher than 00Cr11Ni11 Mo Ti steel at cryogenic temperature.Nearly 90% retained austenite of 07Cr16Ni6 steel are strain-induced transformation during the formation and propagation of cracks,and a large amount of energy is absorbed to improve cryogenic temperature toughness.The reversed austenite formed by the over-aging of 00Cr11Ni11 Mo Ti steel is difficult for straininduced martensite transformation to occur because of its high stability,so the improvement of cryogenic temperature toughness is limited.The effect of a new thermal stabilization treatment technology below the aging temperature that is added before the cold treatment on the microstructure and cryogenic toughness of maraging stainless steel has been studied.The results have shown that the heat stabilization treatment at temperature in a range from 200℃ to450℃ after solution treatment obviously suppresses the further transformation of austenite to martensite during the subsequent cold treatment at-73℃,eventually,more austenite is retained.Although the tensile strength after aging at 480℃ is reduced by 7.0-11.5%,the U-notch impact energy at liquid nitrogen temperature is increased by more than 2.7 times.The effect of thermal stabilization treatment at400℃ is more obvious,and the corresponding liquid nitrogen temperature U-notch impact energy is increased by 3.7 times,indicating that the thermal stabilization treatment technology improves the cryogenic toughness significantly.The effect of repeated low temperature solution treatment of 12Cr-10Ni-Mo-Ti maraging stainless steel after solution treatment at 1000 ℃ on the room temperature strength and cryogenic temperature toughness of 12Cr-10Ni-Mo-Ti maraging stainless steel has been investigated.The results have shown that Cr-Ni-Mo-Ti maraging stainless steel is treated at 1000 ℃ followed by low temperature solution treatment to form austenite by α ’-γ shear reverse transformation,which not only inherits the grain morphology and size of austenite,but also increases the martensitic transformation resistance and significantly reduces the formation temperature of reverse transformation austenite due to the high density of defects in austenite,resulting in more austenite remaining.Even 16.4% austenite is retained after double solid solution treatment at 750 ℃.15% reverse transformation austenite is formed after peak aging at 460 ℃.More than 30% retained/reverse transformation austenite is beneficial to improve ultra-low temperature toughness.Therefore,the impact energy of liquid nitrogen temperature V notch reaches the highest value of more than 90 J,and the tensile and yield strength is only slightly lower than that of solid solution treatment at 1000 ℃.