Research Organization Temperature Carburizing Steel Gears And Fatigue Performance

High temperature carburization is a new technology for gear production, which has advantages such as energy-saving, high productivity and low pollution. The grain coarsening is very much concerned in high temperature carburization since the carburizing temperature is as high as1000℃. The commonly used gear steels, in which A1N precipitates act as grain refiners, are easy to have the grain coarsening problem when carburized at high temperature. Due to the high stability of Nb/Ti precipitates at high temperatures, Nb/Ti microalloying is frequently used in gear steels for high temperature carburization. In the paper, four kinds of gear steels, i.e.20CrMn,20CrMnTi (0.054%Ti),20CrMnNb (0.077%Nb) and20CrMnTiNb (0.048%Nb+0.038%Ti) are prepared to study the effect of Nb/Ti microalloying on the austenite grain growth behavior. With the rotating bending fatigue tests, the fatigue properties and fatigue failure mechanism of the four gear steels carburized at1000℃are studied and compared with those carburized at930℃.The results of austenite grain growth behavior of the experimental steels indicated that the mean distance of precipitates in the steel20CrMnTiNb with0.048%Nb and0.038%Ti was the smallest and grain growth trend was the lowest due to the strongest pinning effect of the precipitates. When austenitized at1000℃for10h, the20CrMnTiNb steel was fine-grained with the average austenite grain size of14μm. When austenitized at930～1000℃, the steel20CrMn was not easy to get grain coarsening problem in comparison to the steels20CrMnTi and20CrMnNb because of a large amount of the fine and dispersed AlN precipitates. While austenitized at1000～1200℃, the grain growth tendency of the steel20CrMn was much higher than those of the steels20CrMnTi and20CrMnNb due to full dissolving of AlN precipitates to lose the pinning effect. Moreover, under the different austenitizing conditions, the average precipitate distance of the steel20CrMnNb with0.077%Nb was smaller than that of the steel20CrMnTi with0.054%Ti. As a result, the grain growth tendency of the steel20CrMnNb was lower than that of the steel20CrMnTi.The results of microstructure and mechanical properties of the experimental steels after carburization showed that after1000℃carburization, the pinning effect of NbC and TiN precipitates in the steel20CrMnTiNb was the strongest to result in the finest microstructure, the highest tensile strength (1610MPa) and excellent impact toughness (Charpy impact energy KU2at the room temperature:93J; KU2at-40℃:73J). The impact toughness of the steel20CrMnNb (Charpy impact energy KU2at the room temperature:103J; KU2at-40℃:74J) was a little bit higher than that of the20CrMnTiNb steel since its tensile strength (1560MPa) was lower. The mechanical properties of the steels20CrMn and20CrMnTi were lower than those of the20CrMnTiNb steel. Compared with930℃carburization, all the experimental steels decreased to varying degrees in comprehensive mechanical properties due to the coarsened microstructure.The results of the rotating bending fatigue tests of the experimental steels showed that the fatigue properties after carburization depended on the grain size in the carburized case as well as the oxgen content in the steels. After1000℃high temperature carburization, with the finest austenite grain in the carburized case and the most tortuous grain boundaries in the steel20CrMnTiNb, the fatigue cracks originated from carburized layer propagated hardly. Besides, the oxygen content (10ppm) in the steel20CrMnTiNb was lowest to dramatically decrease the probability of crack initiation from inclusions. Consequently, the fatigue strength of the steel20CrMnTiNb was the highest (1050MPa). The steel20CrMnTi was the worst in fatigue properties (840MPa) due to the largest grains in hardened case and the highest oxygen content (19ppm) which might produce large inclusions to remarkably increase the probability of crack initiation from inclusions. Furthermore, the effective carburized case depth (0.7mm) and retained austenite content (6.9%) in the carburized layer in the20CrMnTi steel were too low compared with other experimental steels. The fatigue properties of the steels20CrMn and20CrMnNb were between those of the20CrMnTiNb steel and the20CrMnTi steel. After930℃carburization, the fatigue cracks of the specimens mainly originated from surface matrix due to high retained austenite content (exceeding30%) to decrease the fatigue strengths compared with those after1000℃high temperature carburization.