Overload Fatigue Performance Of Carburized Gear Steel

Generally, high rotating-bending and rolling-contacting fatigue strengths are required for heavy-load gear steel because heavy-load gears must bear heavy cyclic loading and friction. Recently, some important ways to improve fatigue properties of heavy-load gear steels are developed, such as surface strengthening and micro-alloying which can improve surface hardness and refine grain size. The steel 17CrNiMo6 is a heavy-load gear steel widely used in Germany, which has good properties of high strength, toughness and fatigue performance. The steel was renamed as 17Cr2Ni2Mo according to Chinese National Standard for Steels. To further improve its properties, we are developing a new heavy-load gear steel 17Cr2Ni2MoVNb by the addition of V and Nb micro-alloying elements into 17Cr2Ni2Mo. This work focuses on phase transformation, hot ductility and fatigue strength of the steels 17Cr2Ni2Mo and 17Cr2Ni2MoVNb through thermal dilatometer test, thermal simulator test and rotating-bending fatigue test. Meanwhile, it also focuses on the effect of case depth on fatigue strength of the steel 17Cr2Ni2MoVNb.(1) Results of thermal dilatometer test show that the zone of pearlite separated from that of bainite, forming an excessive steady zone of undercooling austenite at 550℃on the continuous cooling transformation diagrams (CCT diagram) of the steels 17Cr2Ni2Mo and 17Cr2Ni2MoVNb. The temperatures of the points Ac3 and Ms of the steel 17Cr2Ni2MoVNb are 805℃and 450℃and those of the steel 17Cr2Ni2Mo are 800℃and 430℃, respectively. As the cooling rate increase, microstructures of ferrite+pearlite, bainite+ferrite+perlite, bainite, bainite+martensite and martensite of the steels appear in turn and the hardness is increased in order.(2) Results of high temperature tensile test show that the tensile strength of the V (0.1%) and Nb (0.036%) micro-alloyed 17Cr2Ni2MoVNb steel is higher than that of the 17Cr2Ni2Mo steel, since micro-alloying elements can result in grain refinement strengthening and solution strengthening. Meanwhile, V and Nb are not detrimental to the ductility since the steel has low [N] (0.0057%) content. Hot ductility of the 17Cr2Ni2Mo steel containing higher [N] (0.0130%) is below that of the 17Cr2Ni2MoVNb steel in the test temperature range of 600-900℃and 1050-1200℃. The differences in [N] content and in temperature for phases transformation result in the distinction for precipitation of AlN andα-ferrite between 17Cr2Ni2MoVNb and 17Cr2Ni2Mo, which in turn influence the hot ductility.(3) Results of rotating-bending fatigue test show that the fatigue strength of carburized steel of 17Cr2Ni2MoVNb and 17Cr2Ni2Mo are 935MPa and 945MPa, respectively. The fatigue failure is mostly initiated by surface inclusion and the critical size of inclusion is not evidently different. The results on fatigue properties of the steel 17Cr2Ni2MoVNb with different case depths show that fatigue strength of the steel has the trend of increasing along with increasing of carburized case depth (relative case depth) on the premise of improving of surface hardness. In addition, surface hardness is the importance factor which affects fatigue strength of carburized steels.