Key Technologies And Mechanisms About Improving Cryogenic Toughness Of 9Ni Steel

To improve cryogenic toughness of 9Ni steel,the influence of different processes on the microstructure and properties of 9Ni steel were studied.Meanwhile,the microscopic toughening mechanism of steel was discussed.First,the transformation behavior of 9Ni steel in the process of continuous cooling from the austenite was systematically investigated by Gleeble-3500 thermal simulation machine,based on the linear expansion and combined with the microstructure analysis,the corresponding Continuous Cooling Transformation(CCT) diagram was constructed;Secondly,the effects of different processes on cryogenic toughness of 9Ni steel were studied.Finally,the microscopic toughening mechanism of 9Ni steel were analyzed by using OM,SEM,TEM, EBSD and internal friction experiments.The phase transformation laws of 9Ni steel during continuous cooling process were explored,the results show that: the cooling speed <5 ℃/s,the room temperature microstructures exists a certain amount of ferrite whose ratio and size decrease with cooling rate increasing,the martensitic transformation temperature decrease;cooling speed is 5 ℃ /s~10 ℃ /s,the room temperature microstructures consisted of some bainite and lath martensite;cooling rate is up to 10 ℃/s,it will get all of martensite.The influence of different cooling rate on microstructure and properties of 9Ni steel were investigated.The results show that: the cooling rate has a great influence on the cryogenic toughness of 9Ni steel,the maximum difference of the impact toughness value at-196 ℃ is 75 J,and the impact toughness value of 9Ni steel can reach 165 J by QT process.The mainly reason that is:the size of the retained austenite is coarse and unstable under the slow cooling conditions.The effect of lamellarizing heat treatment on the microstructure and properties of9 Ni steel was studied.The results show that: the microstructure of 9Ni steel are martensite matrix and some retained austenite and ferrite after RLT or QLT processes,meanwhile,the retained austenite is distributed evenly in the matrix.The cryogenic toughness of 9Ni steel after double phase region heat treatment was higher than QTprocess.The cryogenic toughness of 9Ni steel reached 188 J by RLT process,and it was stable.The evolution of microstructure and toughening mechanism of different processes were investigated.The results show that:the retained austenite after QT process with granular distribution in grain boundaries,and showed the K-S orientation relationship with the matrix;the content of retained austenite is more,the alloying element of C,Ni is higher in the austenite,and the austenite size is fine and homogeneous,the toughness of 9Ni steel is better;the impact energy of the steel increase with the average misorientation angle increasing.After intercritical heat treatment,the size of ferrite is relatively larger,the toughness of 9Ni steel is better;the existence of ferrite,refinement of the original austenite grain,and tempering lath martensite microstructure refinement,the impact energy of the steel increased.