Micro-alloying Heavy Gear Steel Fatigue Performance

High bending fatigue strength and contact fatigue strength are needed for heavy duty gear steels.Microalloying technology is one of important approaches to improve fatigue properties of heavy duty gears.In this study,experimental steels G1(0.06Nb+0.13V), G2(0.04Nb)and G3(0.08Nb)have been prepared by adding Nb and V to heavy duty gear steel 17CrNiMo6 and the fatigue properties and fatigue fracture mechanisms of the experimental steels after carburization have been investigated by means of rotating bending fatigue tests and contact fatigue tests.The results of austenite grain growth kinetics show that austenite grains are refined in Nb microalloyed heavy duty gear steels because Nb(C,N)particles are effective for grain boundary pinning and can suppress grain growth.With the increase of Nb content(≦0.08%) in the steels,grain growth activation energy increases due to more Nb(C,N)particles.The more the Nb content is,the more obviously the grains are refined especially in high austenitizing temperature or for long holding time.Microstructure and mechanical properties tests indicate that experimental steels still keep high strength,toughness and hardenability after microalloying.Microalloyed steels possess higher toughness and finer carburized case and core microstructures.The maximum hardness in the carburized case of rotating bending fatigue specimens increases with the increase of Nb content in the steels 17CrNiMo6,G2 and G3.The maximum hardness of carburized case near the surface of G1 microalloyed by Nb and V is higher than that of 17CrNiMo6.Rotating bending fatigue properties of experimental steels have been improved after Nb microalloying.The more the Nb content,the higher the rotating bending fatigue limit in the following order:17CrNiMo6,G2 and G3.Nb can successfully be used to refine grain size because Nb(C,N)particles are effective for grain boundary pinning.Grain boundary area enlarges because of grain size refining,which can hinder fatigue crack propagation,thus it markedly enhance rotating bending fatigue properties of experimental steels.Compared to 17CrNiMo6,the rotating bending fatigue limit of G1 increases by 8.5%,but lower than G2. Adding Nb and V together can improve the rotating bending fatigue limit of experimental steels,but inferior to adding Nb only.The results of contact fatigue tests show that the contact fatigue rating life of 17CrNiMo6 is equivalent to that of the Nb/V microalloyed steel G1.Under the action of alternative stress,there is stress concentration phenomenon at brittle oxide inclusions from which the fatigue crack initiates,leading to the formation of flaking.Oxide inclusions have great influence on contact fatigue properties.Oxygen content and oxide inclusion quantities in G1 are obviously higher than those in 17CrNiMo6,resulting in the decrease of contact fatigue life.Contact fatigue properties of experimental steels may be improved by reducing oxygen content and controlling oxide inclusions quantities.