Over the years, this case-hardening process has been known by many names, like Tufftride® or Tennifer, Melonite® QPQ, and ARCOR ®.
Basically, it is a molten salt nitriding process that operates at low temperatures, typically ranging between 1,000 °F and 1,100 °F, depending on material and product geometry. The case produced consists of two parts, an epsilon iron nitride compound layer, and a diffusion zone. The iron nitride layer is typically less than 0.001”. Unlike gas nitriding, this process works well with less expensive, low alloy steels.
This nitrided case is extremely hard, has a low coefficient of friction, with good corrosion resistance and increased fatigue strength properties. When carried one step further, re-introducing the part into an oxidizing salt bath, an oxide will develop in the surface of the compound layer, and a handsome jet-black finish can be obtained.