Carburizing Process And Fatigue Properties Of 18CrNiMo7-6 Gear Steel

The surface strengthening process of carburizing heat treatment introduce carbon atoms into the surface of the material to obtain a high-strength surface and a hightoughness core,which can greatly improve the fatigue performance of the gear and increase the service life.In this thesis,the carburizing time,carburizing temperature,environmental carbon potential and other parameters in the heat treatment process were studied with the help of DEFORM software,and four heat treatment processes with effective case depth of 0mm,0.3mm,and 0.8mm were designed.And verified the accuracy of the simulation results by preparing samples.Then performed a rotating bending fatigue test on the heat-treated samples to observe the fatigue properties of the different effective hardened layer samples,and performed a fracture analysis on the samples after fatigue fracture to infer the fracture mechanism.In addition,the evolution of retained austenite,hardness,and residual stress were also measured.The main work and gains are as follows:(1)The heat treatment process simulation and process design were carried out by using DEFORM,and the mathematical model and finite element model used in heat treatment simulation were analyzed.Then the carburizing heat treatment parameters were studied.The results show that the carburizing temperature has the greatest influence on the effective hardened layer depth,followed by the carburizing time,and the environmental carbon potential has the least influence on the effective hardened layer.(2)With the effective case depth deepens,the fatigue performance of the sample increases.In the test,the fatigue performance of 0.8mm effective case depth is the best,and its fatigue limit is 1108 MPa,which is much higher than that of the matrix680 MPa.And the relative hardening depth corresponding to the best fatigue performance is 0.107.(3)The gradient structure and residual compressive stress introduced by carburizing heat treatment have a significant improvement on the fatigue properties of the material.The carburizing heat treatment process can inhibit the initiation and change the type of fatigue cracks of materials.For the un-carburized samples,the fatigue source is mainly the discontinuous tool marks produced by machining,and the fatigue sources of the carburized samples are mostly inclusions distributed outside the effective hardened layer.(4)For the samples with inclusions as the main fatigue cracks,the cracks initiate at the stress concentration of the interface between the inclusions and the matrix material,and form a "fish-eye" morphology after a long period of slow growth behavior.When the cracks expand to the effective hardened layer the cracks begin to rapidly expand toward the core due to the high strength and high performance of the hardened layer that can hinder the propagation of cracks.When the crack propagation is large enough,the hardened layer is not enough to hinder the propagation of the crack,and the instantaneous fracture occurs after the crack propagates rapidly in the carburized layer for circumferentially,then the morphology of the instantaneous fracture zone located opposite the fatigue source is formed.(5)After the carburizing heat treatment,the austenite present on the surface of the sample will be gradually consumed during the fatigue test.The evolution of the residual stress in the fatigue specimen process is the result of the competition between the austenite transformation and the relaxation effect.When the austenite transformation is dominant,the residual compressive stress increases,and vice versa.