Microstructure And Property Analysis On New Type Nitriding Steel Micro-alloyed With Niobium

For a long time, 38CrMoAl steel is the unique dedicated nitriding steel in Chinese standard. There are lots of deficiencies exist in 38CrMoAl steel:strength and heat resistance can not be meet the high demand of the substrate; low hardenability; relatively, the heat treatment temperature is too high. Since the stability of aluminum-containing ferritic is very high and the aluminum is difficult to dissolve in austenite, the holding time is longer than that of alloy steel for about 0.5 times, all of those make 38CrMoAl steel can not meet the demands of high-standard parts. While there are 9 kinds of nitriding steels in Euro standard, new high-performance dedicated nitriding steels urgently need to be developed in China.While contrast the characters of composition of nitriding steels developed by domestic and foreign scholars, a new fine grain nitriding steel was developed in this paper. The content of Si and Mn were increased to enhance the hardenability and heat resistance; Cr and Mo are the main alloy elements, V and Nb were added as micro-alloying elements in order to improve the mechanical properties and nitriding performance; the content of S, P and other harmful elements were controled. The phase transformation performance, mechanical properties, hot deformation property and nitriding performance of new steel were studied. Simulated by thermodynamic software-Jmatpro, OM, SEM, TEM, EDS, 3DAP and some other micro-analytical techniques were used to investigate the microstructures and micro-mechanism of new nitriding steel BTHJ. Main research results are listed as following:1. Granular bainite were obtained when cooling rate between 0.1and 0.3℃/s, and the critical cooling rate of mantensite is 3℃/ s, while cooling rate was under 0.1℃/s, the microstructures of BTHJ is bainite and pearlite. The bainite transformation activation enegy of BTHJ is 54.5KJ/mol caculated by JMA formula. The bainite transformation showed more obvious character of incomplete transformation while isothermal transformation, the bainite transformation temperature is 425℃-Ms. Isothermal transformation kinetics of pearlite can be expressed by Johnson-Mehl formula.2. The hardenibility of BTHJ is excellent, it can be hardend between 880℃and 960℃and the harden radius is more than 100mm while austenite temperatue between 900℃and 960℃. The grain size of BTHJ is over 9 grade while quenched at 880℃～960℃. Compared to 38CrMoAl steel, the hardenibility of BTHJ is improved significantly.3. The mechanical properties of BTHJ is better than that of 38CrMoAl for its excellent strength, ductility and toughness. The microstructure of BTHJ after quenching and tempering is quite fine, and fine carbides were dispersed in matrix investigated by OM and SEM, a small amount of undissolved MC carbide which contained Nb were found, too. The TEM results showed that the width of martensite lath of BTHJ is about 150nm and various forms of carbides which contain Cr, Mn and V dispersed in matrix, but the carbides contained Mo is rare, which suggests that most of Mo dissolved in matrix, thereby enhanced the tempring resistance. The 3DAP results showed that the most of alloy element dissolved in matrix after quenching, after tempering, M2C type fine carbides which contain Cr, Mn, V were precipitated, while most of Mo and Nb were still dissolved in matrix which played a role in the effect of solid solution strengthening and enhancing heat resistance.4. After holding at 550℃for 60 hours, the hardness of BTHJ steel droped from 41.4HRC to 37.6HRC, exhibited excellent thermal stability. OM and SEM investigation showed that the matrix recrystallized and the quantity of carbides increased significantly. The carbides extraction results suggested that the type of carbides did not changed before and after thermal stability test, the types of carbides were M23C6, M7C and M3C, the TEM results showed that of decreasing of hardness is mainly caused by the coarsening of carbides and the precipitation of Mo. 5. The obvious dynamic recrystallization were occurenced when the BTHJ steel deformed at 1100℃,0.01 s^-1, with the decreasement of deformation temprature and increasement of deformation rate, the deformation resistance increased and the dynamic recrystallization turned to be not obvious. The hot deformatin formula of BTHJ as following: (ε|&) = 2. 42e⁵⁴ [sinh(0.0065σ)]^7.26exp(-591570/RT); The harden rate was used to determine the critical deformation condition, the critical strain increased with the increasement of lnZ, the relation between critical strain and lnZ can be express by the following function:ε_c = 0.01366lnZ-0.53.6. After nitriding, the highest surface microhardness was over 950HV, the results of plasma nitriding test showed that BTHJ steel can obtained deeper nitriding case than 38CrMoAl in short period of time, while gas nitriding showed that after long time nitriding, the depth of BTHJ and 38CrMoAl almost similar, but due to the fine grain, the nitriding case of BTHJ did not exist nervatied microstructure, and the hardness gradient of BTHJ was much more gentle, which can improve the stability of residual stress, thereby improve the fatigue life. Totally, the nitriding performance of BTHJ is much better.