Carburized Gear Steel Microstructure And Mechanical Properties

For heavy duty gear steels, high contact fatigue strength and bending fatigue strength are needed.20Cr2Ni4A is a commonly used heavy duty gear steel and a new heavy duty gear steel18Cr2Ni2MoVNb has been developed in recent years to overcome the low fatigue performance of the20Cr2Ni4A steel. In this study, the fatigue properties of the steels18Cr2Ni2MoVNb and20Cr2Ni4A have been tested by the way of rotating bending fatigue tests after carburization, and their contact fatigue properties were tested by the same way. The effects of cryogenic treatment on fatigue performance of the18Cr2Ni2MoVNb steel has also been investigated by the way of rotating bending fatigue tests.The austenite grain growth kinetics test results show that after Nb and V microalloyed in18Cr2Ni2MoVNb, its austenite grains are well refined, because Nb-containing and V-containing particles existing at the grain boundary can suppress grain growth. When austenitized at the temperature range of880-940℃for10h, the average austenite grain size of the18Cr2Ni2MoVNb steel is lower than that of20Cr2Ni4A steel and austenite grain growth tendency of the18Cr2Ni2MoVNb steel is significantly lower than that of the20Cr2Ni4A steel. The grain size in the carburized case of the18Cr2Ni2MoVNb steel is also much finer than that of the20Cr2Ni4A steel.The mechanical properties of experimental steels have been tested, the results show that they have high strength and good ductility:the tensile strength is more than1300MPa, and the room-temperature Charpy impact energy for U-notch specimens is over100J. Compared with the20Cr2Ni4A steel, the18Cr2Ni2MoVNb steel has a slightly lower strength due to its lower carbon content, and it has higher impact toughness due to its fine grain size.Rotating bending fatigue tests of the experimental steels show that when the case depth is1.5mm, the rotating bending fatigue limit of the18Cr2Ni2MoVNb steel is1025MPa, which is much higher than that of865MPa of the20Cr2Ni4A steel. The observations of the fatigue fracture surfaces show that the fatigue crack propagates along the austenite grain boundaries in the carburized case. The much finer grain size in the18Cr2Ni2MoVNb steel can hinder the fatigue crack propagation more effectively, resulting in higher rotating bending fatigue properties.Contact fatigue tests of the18Cr2Ni2MoVNb steel show that when the case depth is1.5mm, the contact fatigue limit of the18Cr2Ni2MoVNb steel is3725MPa, which is higher than that of3575MPa of the20Cr2Ni4A steel. Observations of the fracture surfaces of the rolling contact fatigue specimens show that the18Cr2Ni2MoVNb steel has a lower oxygen content and oxide inclusion quantities and size than those in the20Cr2Ni4A steel, which can hinher rolling fatigue crack initiation and result in a higher contact fatigue life.After cryogenic treatment at-80℃for the18Cr2Ni2MoVNb steel, the retained austenite content in the carburized surface has decreased from16%to6%, and its surface hardness and compressive residual stress have increased by50HV and30MPa, respectively. The rotating bending fatigue limit of the18Cr2Ni2MoVNb steel has a decrease of60MPa after cryogenic treatment, presumably due to the decrease of the retain austenite content.