Simulation of the ferritic nitriding process

Abstract

The Lehrer diagram of pure iron is widely used to select the nitriding process of alloy steels. In the present work, the Lehrer diagram of AISI 4140 was determined for the first time by computational thermodynamics. It is demonstrated that the Lehrer diagram of AISI 4140 differs from that of pure iron significantly. Nitriding experiments for AISI 4140 were also carried out to verify the predictions from computational thermodynamics. The scanning electron microscopy results show the existence of two phases in the compound layer and the transmission electron microscopy results at the interface between the compound layer and the diffusion zone verified the coexistence of γ′-Fe₄N and ϵ-Fe_2–3(C,N) phases. These experimental results agree well to the customised Lehrer diagram of AISI 4140 constructed from computational thermodynamics.

Keywords:

• Nitriding process
• Lehrer diagram
• CALPHAD
• AISI 4140

The support of the Center for Heat Treating Excellence (CHTE) at Worcester Polytechnic Institute and the member companies is gratefully acknowledged. Special thanks to Bodycote Worcester for their support through facilities and experimental work and to Thermo-Calc for assistance and continuing support.