Salt bath ferritic nitrocarburizing
Salt bath ferritic nitrocarburizing is also known as liquid ferritic nitrocarburizing or liquid nitrocarburizing[8] and is also known by the trademarked names Tufftride[2] and Tenifer.[9]
The most simple form of this process is encompassed by the trademarked Melonite process, also known as Meli 1. It is most commonly used on steels, sintered irons, and cast irons to lower friction and improve wear and corrosion resistance.[10][11]
The process uses a salt bath of alkali cyanate. This is contained in a steel pot that has an aeration system. The cyanate thermally reacts with the surface of the workpiece to form alkali carbonate. The bath is then treated to convert the carbonate back to a cyanate. The surface formed from the reaction has a compound layer and a diffusion layer. The compound layer consists of iron, nitrogen, and oxygen, is abrasion resistant, and stable at elevated temperatures. The diffusion layer contains nitrides and carbides. The surface hardness ranges from 800 to 1500 HV depending on the steel grade. This also inversely affects the depth of the case; i.e. a high carbon steel will form a hard, but shallow case.[10]
A similar process is the trademarked Nu-Tride process, also known incorrectly as the Kolene process (which is actually the company's name), which includes a preheat and an intermediate quench cycle. The intermediate quench is an oxidizing salt bath at 400 °C (752 °F). This quench is held for 5 to 20 minutes before final quenching to room temperature. This is done to minimize distortion and to destroy any lingering cyanates or cyanides left on the workpiece.[12]
Other trademarked processes are Sursulf and Tenoplus. Sursulf has a sulfur compound in the salt bath to create surface sulfides which creates porosity in the workpiece surface. This porosity is used to contain lubrication. Tenoplus is a two-stage high-temperature process. The first stage occurs at 625 °C (1,157 °F), while the second stage occurs at 580 °C (1,076 °F).[13]
