What is Nitriding?
Nitriding was developed in the early 1900’s and is still one of the most common surface treatment processes world-wide. The process involves diffusing elemental nitrogen into a steel surface to provide a hard, nitrogen saturated structure which forms two distinct metal nitride layers:
a. surface compound zone or white layer (WL), and
b. the diffusion zone (DZ) underneath the WL
Each of these layers provide particular properties in an application. For example the WL provides wear and corrosion resistance, whereas the DZ supplies fatigue resistance, wear resistance and support for the surface WL. The thickness, depth and composition of these layers also have a major influence on the performance of a part.
Conventional nitriding processes do not allow for the tailor-made formation of the surface layers and this is where our specialized nitriding is different. In some cases a thick WL and deep DZ is required, whereas in other cases different thicknesses of layers 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. The WL is also low in porosity which is essential for certain applications.
Due to nitriding being a diffusion and metallurgical reaction-based process, the hardness of the nitrided layer depends on the level of alloying in the steel. Mild or low alloy steels do not show a great response where alloyed steels can achieve high surface hardness. “Nitriding steels” such as 1.8850 are designed to create very hard surfaces and can achieve over 70 HRc (eqv).
Mould and Tool Steel Nitriding
For injection moulds and other tooling, the formation of a thicker WL is not desired as this increases surface roughness and may spall under high shear. Our mould nitriding process minimizes the formation of a thin WL to provide a smoother surface and improved polishability. Dies can be used straight from the process but the mirror finish can also easily be restored.
Diffox vs 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 compared to just 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.
Nitriding for Stainless Steel
Nitriding of stainless steels require a special process. Find out more about it here.