The grain size and precipitate amount which are affected by heat treatment have significant impact on the properties of high nitrogen austenitic stainless steel. In this study, Cr18Mn18 high nitrogen steel sheet is employed to investigate the effects of precipitate on austenitic grain size. It can be seen that the lamella precipitates which are rich in nitrogen and chromium nucleate in the austenite grain boundary and grow inward into grain when aged at 800 ℃ through electron probe micro-analyzer. The transmission electron microscopy results demonstrate that the precipitate is Cr2N and its morphology are detected as ellipsoid-like with major axis of 100-300 nm and minor axis of 50-100 nm roughly. The experiment show that coarsen of the austenite grain is quite critical at 1000-1100 ℃. However, the samples which pre-precipitated at 800 ℃ for 240 min to obtain the most nitride precipitate exhibits much smaller grain size than the as-rolled samples after solid solution treated at 1000, 1050 and 1100 ℃ for 240 min. The results show that the nitride precipitates in the grain boundary can effectively pin the austenite grain boundary and inhibit the grain growth.
Li Jingyuan, Feng Shufei, Fang Fei, Chen Yulai University of Science and Technology Beijing, Beijing 100083,China
Aging precipitation and solid solution heat treatment were carried out on three steels which have chromium content of 18%,manganese content of 12%,15%,18%,and nitrogen content of 0.43%,0.53%,0.67%,respectively,The mechanisms of precipitation and solid solution of high nitrogen austenitic stainless steel were studied using the scanning electron microscopy,transmission electron microscopy,electron probe micro analysis and mechanical testing.The results show that,Cr2N is the primary precipitate in the tested stainless steels instead of Cr23C6.Cr2N nucleates at austenitic grain boundaries and grows towards inner grains with a lamellar morphology.By means of pre-precipitation of Cr2N at 800 °C,the microstructure of the steels at solid solution state can be refined,thus improving the strength and plasticity.After the proposed treatment,the tensile strength,the proof strength and the elongation of the tested steel reach 881 MPa,542 MPa and 54%,respectively.
Composition segregation and microstructure nonuniform of H13 steel have been obviously improved though adding nitrogen element. The microstructure and property of H13 steel with nitrogen contents of 0.021% and 0.085% are studied under various quenching and tempering states. Dual quenching that is quenching between 1000-1010 ℃ twice is employed in order to increase the tensile strength and hardness of coupons. The temperature and time are explored for improving the toughness under optimal quenching treatment. For extrusion die steel, the tensile strength can reach a satisfactory level and impact toughness are met the requirements of NADCA at the temperature.
Chen Yulai, Liu Biao, Zhao Peng, Tang Di, Li Jingyuan University of Science and Technology Beijing, Beijing 100083,China
