Effect Of Materials And Heat Treatment On The Bending Fatigue Properties Of Heavy-Load Bulldozer Gears

Usually,20 CrMnTi steel was chosen as the manufacture of bulldozer.However,at the suitcase of large module gear(m?8),the low hardenability can't meet the needs of technology.The purposes of the thesis are focused on the study of the bending fatigue behaviors of heat treated heavy load bulldozer gears with different materials as well as the effect of heat treating process in order to provide the bases for the application of gears.Fatigue machine,X ray diffractometer(XRD),optical microscope,scanning electron microscope(SEM),and EDS(Energy Dispersive Spectrometer)had been taken to measure and characterize the bending fatigue behaviors,hardness,and microstructure of three kind of carburizing steels 20 CrMnTi,20CrMnMo and 22CrNi2 MoNb as well as the ionic nitride 42 CrMo steel.The bending fatigue behaviors of 20 CrMnTi steel gear with different carburizing depth were studied in order to provide the effect of heat treating process.The experimental results denote that the carburizing steels 22CrNi2 MoNb and 20 CrMnTi have the fine grains and microstructure,and the reasons are mainly due to the existence of micro-alloy elements Nb and Ti in steels in which case can pin the grain boundary during the long time carburizing process.The fine grains and microstructure of 22CrNi2 MoNb and 20 CrMnTi steels leads to higher mechanical properties.The coarser microstructure of 20 CrMnMo steel after carburizing leads to lower property.42 CrMo steel is of the compound with the depth about 100?m as well as the diffusion transition layer with the depth about 300?m.The bending fatigue behaviors show that 22CrNi2 MoNb steel has the highest fatigue life about 14 times that of 20 CrMnTi steel,the 20 CrMnTi steel is secondary,and the 20 CrMnMo steel has the lowest fatigue life in three carburizing steels under the condition of 120 KN load.Generally speaking,the bending fatigue behaviors of carburizing gears are better than that of 42 CrMo steel gear after ionic nitride.The lower bending fatigue behavior of 42 CrMo steel gear after ionic nitride is mainly due to the shallow ionic nitride layer.The experimental results also denote that the deep the carburized layer,the better the bending fatigue life.The fatigue fracture morphology consists of three zones,i.e.the crack source,the stable propagation and instantaneous fracture zones.The fatigue crack sources are mainly come from the micro-voids and micro-crack nearby the surface as well as the machining defects at the surface of gear.22CrNi2 MoNb steel can be used as the manufacture of gear with higher request for the bending fatigue life,while 20 CrMnTi steel is suited for the common request for the bending fatigue life.