What is Nitriding?
Nitriding is still one of the most common surface treatment processes world-wide and was developed in the early 1900's. Basically elemental nitrogen is diffused into a steel surface to provide a hard, nitrogen saturated structure which forms layers metal nitrides on top and into the steel. The resulting layer structure is a surface compound zone or white layer (WL), the diffusion zone (DZ) and a transition zone before reaching the base material. Each of these layers provide particular properties in an application. For example the WL provides wear and corrosion resistance, whereas the DZ supplies wear resistance and support for surface layers. The thickness and depth of these layers also have a major influence on the performance. Furthermore brittle layers and/or shallow diffusion zones also give performance problems.
Conventional nitriding processes do not allow for the tailor-made formation of the surface layers and this is where our nitriding is different. In some cases a thick WL and deep DZ is required, whereas in other cases different sies of layer provide the correct performance. Our WL is also ductile rather than brittle and with the correct steel selection and heat treatment it treatment ensures the correct thickness WL and DZ is formed.
Black Nitriding, QPQ
When sufficient white layer is formed it can be given an additional high temperature black oxide layer which helps improve both corrosion and wear resistance. This is commonly known as Black Nitriding. QPQ stands for Quench-Polish-Quench whereby the parts are given pre-, intermediate and final polish process to create a smooth, black surface which the benefits of improved corrosion and wear resistance vs. black nitriding. Our Diffox coating is a combination of 1. controlled WL and DZ. 2. a pre-polished surface and 3. a blackening cycle giving the best combination of all properties.
Sub-Critical Nitriding for Stainless Steel
It is well known that conventional high temperature nitriding negatively influences the corrosion resistance of stainless steel. With Sub-Critical Nitriding (SCN), the nitriding temperature is maintained below critical levels whilst nitrogen is diffused into the surface. This results a hard (~72 HRc) layer, 20 – 30 µm deep, which does not negatively affect the corrosion resistance and importantly, which gives a surface highly resistant to galling even on soft stainless steels (304/316/ etc.). We have performed SCN on several stainless' including austenitics, martensitics and duplex stainless steels with excellent success in applications ranging from injection moulding, pump components, ball valves and machinery components.
Contact us regarding optimizing the nitriding treatment for your applications.