18CrNiMo7-6钢齿轮组织性能异常分析

某18CrNiMo7-6钢生产的齿轮因为组织性能异常,在铣齿过程中齿根位置出现难加工的问题。采用光学显微镜、场发射扫描电镜以及EPMA电子探针分析18CrNiMo7-6钢齿轮组织性能异常的原因。结果表明,该齿轮出现难加工的原因是齿根位置出现大量贝氏体导致硬度过高。EPMA电子探针验证了齿根位置贝氏体的出现与元素偏析有关。JMatPro计算结果表明,C与合金元素含量增加使贝氏体转变的温度范围扩大,贝氏体转变的临界速率降低,使钢能够在较低的冷却速率下产生贝氏体组织,合金元素中Cr、Mn对贝氏体转变的影响效果最为显著。当C与合金元素Mn、Cr的含量增加50%左右时,贝氏体转变的临界冷速由0.1 ℃/s 降低为0.02 ℃/s,0.1 ℃/s冷速下贝氏体转变温度范围扩大到49 ℃。

Analysis of abnormal microstructure and properties of 18CrNiMo7-6 steel gear

18CrNiMo7-6 steel gear was difficult to machine in the root of the gear during the milling process because of its abnormal microstructure and properties. The reasons for abnormal micro structure and properties of 18CrNiMo7-6 steel gear were analyzed by metallographic microscope (OM), field emission scanning electron microscope (FESEM) and EPMA electron probe. The results show that the difficult-machining of the gear can be attributed to the formation of bainite in the root of the gear that results in excessive hardness. The EPMA electron probe verify that the appearance of bainite at tooth root is related to chemical composition segregation. The JMatPro calculation results show that the increase of C and alloying elements expand the temperature range of bainite transformation and reduces the critical rate of that. The bainite structure is formed at a low cooling rate. The influence of Cr and Mn elements on bainite transformation is the most significant factor. When the content of C, Cr and Mn is increased by about 50%, the critical cooling rate of bainite transformation decrease from 0.1 ℃/s to 0.02 ℃/s, and the bainite transformation temperature range at 0.1 ℃/s is expanded to 49 ℃.