Predictions of distortion and hardness after heat treatment by numerical simulation are very useful for determining the optimum condition and for controlling the distortion of machinery parts. In this article, the combination method of orthogonal experiment and numerical simulation is used to optimize the standard heat treatment condition by reducing the distortion after carburizing quenching. A helical gear made of carburizing steel 20MnCrS5 is simulated using three-dimensional coupled analysis that is based on thermo-mechanical theory. The good agreement between the experiment and simulation is verified by the comparison between the experimental data and the simulated data. Firstly, the influencing factors of distortion after carburizing quenching are investigated and discussed. Four influence factors of heat treatment are selected as follows: carburizing time, the cooling time before 860°C, the holding time at 860°C, and the gear orientation during quenching. Next, selection of the optimal case is then determined by comparing the distortions. Finally, a new optimization method of minimum distortion after carburizing quenching is provided.
Distortion and fatigue are both important criteria for evaluating carburizing and quenching process. An optimized process was proposed to reduce distortion and improve fatigue strength simultaneously. Mild steel 20MnCrS5 were heat treated using standard condition and optimized condition respectively. The microstructure, hardness, residual stress, domain size, fatigue performance and crack growth rate with different conditions were studied. Due to carburization, the near surface of the materials have different microstructures with different carbon concentration. The carburized layer, subsurface layer and central layer were selected to prepare the fatigue specimens and to be evaluated. The strengthening effect was verified by comparing the fatigue limit and the crack growth rate. The strengthening mechanism was analyzed by comparing microstructure, retained austenite, residual stress and domain size. The results show that with the optimized condition the fatigue performance at different layers are improved while achieving higher surface hardness. The joint action of domain refinement, more compressive residual stress and less retained austenite results in the strengthening.
Water-jet cavitation peening (WCP) is used to increase the hardness and induce the formation of a compressive residual stress layer on the surface through the shock wave pressure, which is produced by the collapse of small bubbles of a cavitation jet at the surface of materials. In this article, a pressure sensitive paper is applied to determine the pressure field distribution of WCP. WCP is carried out on the bearing steel 100Cr6 with different microstructures, which are heat-treated differently. The hardness, residual stress distribution, and microstructure of bearing steels at the same observation points are compared, respectively, before and after WCP. The research indicates that both the hardness and residual stress are increased with the peening time. It also proves that this method has different strengthening limits for materials with different microstructures because of the different mechanisms of WCP.
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