Liquid nitriding is a subcritical surface enhancement process with one of the longest track records of success of any case hardening technology.
PVD coatings involve the deposition of thin (2-10 microns; 0.0001" – 0.0004") films on the surface of tools and components.
Liquid Nitriding (LN) is a common term for a diffusion process that is actually liquid nitrocarburizing; a thermo-chemical reaction whereby nitrogen, primarily, and some carbon are diffused into the surface of iron-based materials. The nitrogen combines with the iron to form an iron-nitride compound layer that provides improved surface properties; e.g. resistance to wear, friction, corrosion, and fatigue.
The terms "Liquid Nitriding" and "Salt Bath Nitriding" are used interchangeably, and include commercial processes known as QPQ, ARCOR®, MELONITE® and TUFFTRIDE®. All are actually nitrocarburizing processes that use molten salts as a source of nitrogen (+ some carbon). Different trade names are used due to differences in the chemistry of the nitriding chemicals and the secondary processing steps.
Nitriding provides only nitrogen to the surface of the work piece, and is normally accomplished in gas or plasma atmospheres, using much longer cycles to achieve deep diffusion depth. Nitrocarburizing supplies both nitrogen and some carbon; can be performed in either liquid (salt bath) or gas atmospheres; and uses much shorter time cycles to produce comparatively shallow diffusion depth.
MELONITE®, TUFFTRIDE® and ARCOR® are all liquid nitrocarburizing processes, and trademarks of the HEF Group (TS USA and HEF USA are US subsidiaries). MELONITE® and TUFTRIDE® are identical processes – the former term is prevalent in North America, and the latter term in the rest of the world. However, ARCOR® represents a family of LN processes, including ARCOR® C, V, N, DT, and others, which were developed to address specific applications and/or materials. These ARCOR® treatments provide a more robust and consistent compound layer; are operationally easier to control and environmentally friendlier than other liquid nitriding treatments.
"QPQ" stands for "Quench-Polish-Quench", which describes a sequence of secondary steps following the liquid nitriding step. These steps entail the sequence: (1) OXIDATION: 2-3 microns of the surface layer is transformed to an iron oxide. This is done by immersing the parts in specially formulated 'salts' between 400°C - 425°C (750°F -800°F); (2) POLISHING: to improve surface finish and (3) RE-OXIDATION: to recover the oxide layer thickness that may have been lost during the polishing step. QPQ is prescribed when a smooth surface finish and maximum corrosion protection are required.
Nitriding, or, nitrocarburizing, can be accomplished using four different media: 1) Liquid; 2) Gas; 3) Plasma; and 4) Fluidized Bed. All methods are intended to accomplish similar - though not identical – results. However, liquid is considered the benchmark for uniformity, consistency, and flexibility. Liquid Nitriding also provides the best combination of wear and corrosion protection and the shortest processing times.
Liquid Nitriding is not a coating or plating: it is a diffusion process that modifies/transforms the surface of the treated component. This modified surface layer is well integrated with the bulk material – hence it is not susceptible to flaking or peeling.
Liquid Nitriding is often referred to as a heat treatment; but, this is technically incorrect. True heat treating processes operate at higher temperatures, and transform the crystal structure of the steel to produce the desired mechanical properties. Liquid Nitriding achieves results by means of a chemical reaction (at temperatures lower than conventional heat treatment) that leads to the formation of a hard nitride compound on the surface of the component.
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