Process robustness in reduced-time vacuum carburizing of automotive gear steels

Abstract This study evaluated the influence of vacuum carburizing parameters on the microstructure, microhardness profile, and case hardening depth (CHD550) of TL 4227 and TL 4521 steels used in highly loaded gear components. Experiments were performed on industrial gear components using an ALD ModulTherm system at 960, 980, and 1,000 °C, followed by gas quenching and tempering. The work focused on industrial validation of VW TL gear steels and shortened carburizing cycles based on previous temperature-focused results. Metallographic examination and microhardness profiling were performed to establish correlations between process parameters and case properties. CHD550 increased from approximately 0.65 mm to 0.9 mm with increasing carburizing temperature, while surface hardness remained around 740 HV1. A reduction of the total carburizing cycle time by approximately 15–20% resulted in a limited decrease in CHD550 (9–12%), while surface hardness, martensitic microstructural homogeneity, and surface integrity were preserved. The carburizing regime at 980 °C provided the most balanced combination of case depth, hardness, and structural homogeneity. Moderate adjustment of carburizing temperature and cycle duration shortened the process while maintaining compliance with VW 50019:2011-05 under the studied industrial conditions and component geometries.