Medium frequency induction annealing equipment easily solves crack defects in 20CrMnTiH steel gears

The gear workpiece material is 20CrMnTiH steel. The heat treatment process is carburizing at 930°C, furnace cooling to 830-840°C and oil quenching, and the depth of the carburized layer is 1.2-1.6mm. During use, it was found that cracks appeared at the root of the splines after the gear was quenched, with a depth of about 20mm.

The inspection found that the gear hardenability was too high and it was completely hardened after carburizing and quenching. The core structure was martensite + bainite, its specific volume was large, and the surface of the workpiece was in a state of tensile stress. Due to the high carburizing heating temperature, the structure of the carburized layer is coarse martensite and about 40% (volume fraction) retained austenite.

Analysis shows that when the carbon content of the workpiece is above 1.10% (mass fraction), the grains grow rapidly as the temperature rises. The carbon content of the gear surface is about 1.2%-1.4% (mass fraction). When the carburizing temperature is high, the austenite grains grow, making the martensite needles thicker and the gear performance becoming brittle. At the same time, because the gear is completely quenched, when austenite transforms into martensite, the volume expands, and the surface of the workpiece is subject to tensile stress. The residual austenite content on the tooth surface is high, so the compressive stress is small, so the gear eventually forms a surface tensile stress state. . During quenching, the workpiece generates thermal stress. The combined action of tissue stress and thermal stress causes the gear to cause stress concentration at the root of the spline shaft. When the tensile stress is greater than the fracture strength of the workpiece, the gear forms quenching cracks and cracks under the action of alternating stress. The expansion causes the gear to break.

The high hardenability of the gear and the coarse martensite are the main causes of crack failure in the workpiece. Therefore, selecting steel with suitable hardenability and adjusting and improving the heat treatment process are the main means. The adjusted heat treatment process is carried out using medium frequency induction annealing equipment so that the workpiece can obtain a suitable fine needle or hidden needle martensitic structure, enhance the strength and toughness of the workpiece, and prevent cracks and fracture defects in the workpiece.

After heat treatment using medium-frequency induction annealing equipment, the cracks in the gear disappeared, and the workpiece qualification rate reached more than 95%, meeting the technical and production requirements. Even better, this heat treatment process is suitable for large-scale production and can greatly improve workers' production efficiency.