High-temperature surface decarburization in 8Cr4Mo4V high alloy steel by metal-carbon coupling diffusion

The high-temperature decarburization behavior of 8Cr4Mo4V high alloy steel was investigated between 700–1100 °C for aircraft bearings application to control this surface degradation for extending the service life of high-performance components. Microstructural analysis indicated that the formation of multi-layered oxide scales is attributed to the sequential outward diffusion and oxidation of alloying elements from the steel matrix. Internal decarburization was particularly pronounced in the temperature range of 700–800 °C, where the martensitic structure underwent transformation into bainite, during which significant diffusion of Cr and carbon occurred. Notably, the outward diffusion of Cr, V and Mo induces lattice distortion and creates vacancy-assisted pathways for rapid carbon transport. TEM characterization revealed that the non-decarburized region is primarily composed of iron (BCC) with a minor presence of carbide phases, whereas the decarburized region exhibits a polycrystalline structure, showing a disordered atomic arrangement. This interfacial decarburization was explained by metal-carbon coupling diffusion, culminating in a sharp decline in surface hardness. The atomic-scale insights offered a precise guideline for developing surface protection strategies during heat treatment.