淬火温度对Nb微合金化齿轮钢18CrNiMo7-6组织演变及力学性能的影响

通过Thermo-calc热力学计算软件、扫描电镜、光学显微镜、冲击试验及拉伸试验等,研究了淬火温度对Nb微合金化齿轮钢18CrNiMo7-6组织及力学性能的影响。结果表明:随着淬火温度的升高,Nb微合金化齿轮奥氏体平均晶粒尺寸增加,但保持在20 μm 以下,晶界稳定性较高;根据Thermo-calc热力学计算结果可知,主要存在的碳氮化物为Cr₇C₃、Cr₂₃C₆、NbC以及AlN,其中Cr₇C₃、Cr₂₃C₆固溶温度较低,分别为730 ℃和749 ℃,NbC、AlN固溶温度较高,分别为1180 ℃和1070 ℃,NbC和AlN为主要钉扎晶界、细化晶粒的碳氮化物;NbC中存在少量的N元素,在一定温度下,NbC有向Nb(C,N)转变的趋势。随着淬火温度的升高,屈服强度呈降低趋势,抗拉强度在860 ℃出现平台,冲击性能先升高后降低。含Nb齿轮钢18CrNiMo7-6具有较宽的工艺设计窗口,最佳热处理工艺为860 ℃淬火+180 ℃低温回火,此时抗拉强度为1455 MPa,屈服强度为1229 MPa,冲击吸收能量为100 J,硬度约为44 HRC。

Effect of quenching temperature on microstructure evolution and mechanical properties of Nb microalloyed gear steel 18CrNiMo7-6

Effect of quenching temperature on microstructure and mechanical properties of the Nb microalloyed gear steel 18 CrNiMo7-6 was studied by means of Thermo-calc thermodynamic calculation software,scanning electron microscope,optical microscope,impact test and tensile test.The results show that with the increase of quenching temperature,the average austenite grain size of the Nb microalloyed gear steel increases,but remains below 20μm,and the grain boundary stability is high.According to Thermo-calc thermodynamic calculation results,the main precipitates are Cr₇C₃,Cr₂₃C₆,NbC and AlN,among which the solution temperatures of Cr₇C₃ and Cr₂₃C₆ are relatively low(730℃and 749℃,respectively),while the solution temperatures of NbC and AlN are relatively high(1180℃and 1070℃,respectively).NbC and AlN are the carbonitrides that mainly pin grain boundaries and refine grains.There is a small amount of N element in NbC precipitates,and NbC tends to change to Nb(C,N)at a certain temperature.The yield strength decreases with the increase of quenching temperature,the tensile strength plateaus at 860℃,and the impact absorbed energy increases first and then decreases.The Nb microalloyed gear steel 18 CrNiMo7-6 has a wide process design window.The optimal heat treatment process is quenching at 860℃and tempering at 180℃,after which the tensile strength is 1455 MPa,the yield strength is 1229 MPa,the impact absorbed energy is 100 J,and the hardness is about 44 HRC.