Nitriding of Steels

Nitriding is among the most versatile thermochemical surface engineering treatments of steels and is widely applied to improve the performance of steel components with respect to fatigue, wear and corrosion. The deliberate introduction of nitrogen to steels dates back to the early 1900s, where it was discovered that steels applied in the ammonia synthesis suffered from embrittlement due to the ingress of nitrogen, which is an undesired corrosion phenomenon. This gave the inspiration for the deliberate and controlled introduction of nitrogen as an alloying element. The strengthening mechanisms on which nitriding relies, are solid solution strengthening by interstitially dissolved elements, precipitation strengthening by alloying element nitrides, and the formation of interstitial surface compounds (iron-based nitrides). There exists a wide range of commercial processes to introduce nitrogen into steels, including many variants of gas, plasma and salt-bath based processes. This article will not elucidate the various processing routes, but rather focus on the mechanisms by which nitrogen in steels contributes to the improvement of their performance. As a thermodynamics reference for the dissolution of nitrogen in steels, equilibrium reactions with nitrogen-providing gas molecules are given. This article is subdivided in nitriding treatments for low-alloyed steels and for stainless steels.