Carburizing Gear Steel Deformation And Fatigue Performance

High heat treatment distortion and poor fatigue properties are two problems commonly occurred in the production and application of transmission gears, both of which are dependent on the gear materials. This work is aimed to clarify the effect of gear materials on heat treatment distortion and fatigue properties through studies on common gear steels including 20Cr2Ni4A,20CrMoH, and 22CrMoH, and newly developed microalloyed gear steels such as 20CrMoNbH.By using Navy-C ring specimens, heat treatment distortion for the 20Cr2Ni4A heavy duty gear steel has been studies and compared with that of the new gear steel 15Cr2MnNiMoV (steel C2) after carburizing, followed by air cooling, oil cooling, and water cooling, respectively. Results show that the heat treatment distortion of the new gear steel C2 is lower than that of the 20Cr2Ni4A steel. Heat treatment distortion of the test steels has been found to increase linearly with the increase of the core hardness of the specimens.Heat-treatment distortion of the Nb-B microalloyed case hardening steels has also been investigated by means of Navy-C ring specimens. Results show that the case hardening steels which were microalloyed with Nb and B have fine prior austenite grain sizes in the carburized case as a result of the pinning effect of fine Nb(C,N) particles on austenite grain coarsening during carburization. Heat-treatment distortion of 22CrMoH and 20CrMoH steels has been increased by the addition of boron, which can remarkably improve hardenability and ideal diameter (DI value) of hardening; however it has been decreased by the addition of niobium, which can evidently refine the prior austenite grain sizes. As a result, Nb-B microalloying in case hardening steels can increase hardenability while it does not cause significant increase in heat treatment distortion.By using the rotating bending fatigue test method, the fatigue properties of the 20Cr2Ni4A steel and C2 steel have been studied with carburized specimens. Results show that the new gear steel C2 has a bending fatigue limit of 925 MPa, evidently higher than that (865 MPa) of the 20Cr2Ni4A steel. The higher fatigue limit C2 is largely due to its higher surface hardness and a deeper carburized case depth. In addition, a more uniform carburized case depth of the C2 steel after grinding due to its lower heat treatment distortion may contribute to its higher fatigue limit to some extent.Rotating bending fatigue test results of carburized specimens show that the fatigue limit of the 20CrMoNbH steel is 1085 MPa, while that of the 20CrMoH steel is only 995 MPa. Observations of fracture surfaces of fatigue specimens showed that the fatigue crack originated in the carburized layer, and then propagated along the prior austenite grain boundaries, indicating that the grain refinement of the carburized layer could improve the fatigue strength through preventing propagation of fatigue crack.