Cold-Deformed Passive Alloy Nitrogen Hardening

The patent applies preliminary action by performing high-temperature treatment (austenitization) before the low-temperature surface hardening process. This preliminary high-temperature step dissolves any existing carbides and nitrides, eliminates sensitization, and creates a uniform austenitic structure. Only after this preparatory treatment is the low-temperature nitriding or carburizing performed, ensuring that the surface hardening does not trigger carbide/nitride formation because the material is already in a stable austenitic state with dissolved alloying elements.

The patent converts the harmful effect of cold deformation (which creates dislocations and makes the material susceptible to sensitization) into a beneficial outcome. The high-temperature austenitization treatment transforms the cold-deformed microstructure by dissolving precipitates and creating a homogeneous austenitic structure. The subsequent rapid cooling and low-temperature surface hardening then produce expanded austenite or martensite with superior properties, effectively converting the previously harmful cold work into a benefit by enabling a more effective heat treatment response.

The patent segments the heat treatment process into distinct stages: a relatively brief high-temperature austenitization step followed by rapid cooling and then a low-temperature surface hardening step. This segmentation allows the high-temperature treatment to be limited to just what is necessary for austenite formation and dissolution of carbides, rather than maintaining high temperature for the entire process. The rapid cooling then locks in the austenitic structure, and the final low-temperature step completes the surface hardening without risk of sensitization.

The patent applies parameter changes by utilizing high temperature to fundamentally alter the state of the passive oxide layer and the material's ability to dissolve nitrogen and carbon. At high temperature during austenitization, the oxide layer becomes permeable and the material can absorb nitrogen and carbon without requiring removal of the passive layer. The subsequent rapid cooling and low-temperature treatment then complete the surface hardening while maintaining corrosion resistance, as the passive layer reforms or is preserved in the low-temperature regime where it provides protection.