Effects of High Temperature Tempering on Microstructure and Hardness Distribution on 18Cr2Ni4WA Alloy Steel Heavy-duty Gears: Numerical Simulation and Experiments

Abstract

In the present work, simulations of gas carburizing heat treatment of heavy-duty gears on a coal-cutting machine fabricated by a 18Cr2Ni4WA alloy steel were carried out by using a finite element method software. The physical property database of the 18Cr2Ni4WA alloy steel was constructed in the software. The effects of the high temperature tempering process on the microstructure and hardness distributions were simulated and studied. The hardness distribution results obtained by simulation are in good agreement with the experimental results, which verifies the accuracy of the simulation. The evolution of the volume fractions of the austenite, martensite, and bainite microstructure after high temperature tempering was simulated, while the change of the retained austenite varying with the number of high temperature tempering was studied. The results show that with the increase of the number of high-temperature tempering, the content of retained austenite in the gear decreases, and it could reduce the surface carbon concentration. The hardness variation, calculated by the Maynier equation through secondary development, is shown to be governed by the joint influence of carbon concentration and microstructure distribution. The hardness distribution from the surface layer to the core is 62HRC-32HRC after the heat treatment process with twice high tempering, which meets the needs of the practical application of the gears.

Microstructural Investigation and Impact Strength of Sinter-Hardened PM Steels: Influence of Ni Content and Tempering Temperature

“…The present study focused on the 160-300 • C range to explore the possibility of applying high-temperature tempering to PM steels intended for demanding wear resistance applications (e.g., gears) [11,43,44]. Such a range is associated with the first and second tempering stages [45].…”

“…Temperatures above 300 • C were not considered, in order to avoid the third tempering stage, consisting of the decomposition of martensite into ferrite and globular carbides [45]. This kind of microstructure is not suitable for wear-demanding applications [43,44]. At the same time, lowering the tempering temperatures results in residual stresses, jeopardizing the material's properties [20].…”