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It is well known that due to process characteristics - like best reproducibility on a high-quality level - nitrocarburizing in ionic liquids provides excellent resistance to wear, pitting, galling, seizure and surface-fatigue resistance to the treated parts. However, the corrosion protection is still moderate. This problem can be solved by post heat treatment in oxidizing salt melts, producing a very thin but compact oxide layer on the surface of the nitride layer. Combined with polishing and impregnation, the oxidized parts can have smooth, attractive black surfaces, allowing dramatic improvements in corrosion resistance up to 1,000 hours in salt-spray tests without losing the previously mentioned benefits.
This article discusses the application of Controlled Liquid Ionic Nitrocarburizing (CLIN) processes like TENIFER® and ARCOR® to replace galvanic coatings like chrome, nickel and zinc plating due to excellent corrosion resistance and wear properties. It also highlights economical and environmental advantages of their usage. Due to easy handling, complex plant equipment is not required. The process times are rather short and allow flexible work without building up bigger buffer capacities for the workload.
Introduction
CLIN is a family name of modern and environmentally friendly processes for nitrocarburizing and oxidation of steel and cast iron. Diffusing nitrogen and carbon results in a so-called compound layer, which possesses a nonmetallic character. The outstanding advantage of this edge zone in relation to other coatings is the firm compound diffused on the base material and not applied on the surface. Therefore, they exhibit a very good adhesion, and crack sensitivity is clearly reduced. Depending upon material used, these layers possess hardnesses from 800-1500 Vickers. The compound layer is supported by the underlying diffusion layer. CLIN-treated parts offer eminent protection against wear, seizure, galling, pitting and fatigue.
Process Characteristics
Basically, all kinds of ferrous material - tool steels, mild steels, valve steels, austenitic steels, cast iron or sintered materials - can be nitrocaburized in salt melts without any special preliminary pre-treatment. The process sequence is not complicated. After a short pre-cleaning and preheating in air to 350-400°C (662-752°F), the parts are nitrocarburized in the salt melt, generally for 60-120 minutes. Treating temperature is usually 570-590°C (1058-1094°F). In special cases, lower (480°C) or higher temperatures (630°C) are possible. Water, air, nitrogen, vacuum or an oxidizing...