Influence of Aluminum on the Retained Austenite Content and Stresses in the Surface Layer of Steel after a Hybrid Low-Pressure Carburizing Process with Metallization

The article presents a unique method of modifying the surface layer properties of low-alloy steels. This method consists of the simultaneous saturation of the surface layer with aluminum and carbon. A hybrid heat treatment process is used, which integrates diffusion metallization with low-pressure carburizing (LPC). An aluminum coating is applied to the surface of the steel by physical vapour deposition or slurry method to saturate the surface layer with aluminum. The steel is then annealed to create a diffusion layer and subjected to LPC. Gas quenching in nitrogen and low-temperature tempering are performed after carburization. This surface treatment tenfold reduces the amount of retained austenite in the carburized layer. This is particularly critical for carburized steels that include austenite-stabilizing elements, such as nickel and manganese. This causes an increase in surface hardness. It also changes the state of residual stress compared to ordinary LPC to one that is more favorable from the point of view of operation-compressive stresses are generated in the technological surface layer. A new method of thermochemical treatment of steel, the hybrid process, a combination of aluminizing with a low-pressure carburizing process, is studied. This treatment reduces the amount of retained austenite in carburized steels to about 5%vol. The result is a higher surface hardness after the process and a better stress state in the carburized layer, strongly influencing the funcional properties.image (c) 2024 WILEY-VCH GmbH