Learn how gas nitriding increases the hardness of tool steel

What is gas nitriding?

Gas nitriding is a common method of creating a hard surface on steel. It works by diffusing nitrogen into the steel from a nitrogen-rich gas. The gas is usually ammonia (NH3), which is why this method is also referred to as ammonia nitriding.

Gas nitriding is a mature process which has been around for almost a century. It’s relatively easy to control the process parameters guiding the thickness and composition of the nitriding layer.

The main benefits

Steel that is gas nitrided can reach a surface hardness of more than 67 HRC for excellent wear resistance of the steel. It also gives the surface anti-galling properties, which is important in applications with poor lubrication.

The treated metal experiences a minimum of distortion and deformation and only minor changes in dimensions. Dimensional changes should be accounted for pre-nitriding to reach the desired measurements post-nitriding. Thanks to the low temperatures in the process the metal will not be subject to tempering which could make the steel softer.

The nitrided surface improves fatigue life and other fatigue-related properties such as notch sensitivity. An additional benefit is that gas nitrided steel generally becomes more resistant to corrosion.

How gas nitriding works

Gas nitriding works at a relatively low temperature compared to other case-hardening techniques
No quenching is needed
Lower temperatures minimize the risk for distortion of the steel parts
Precise control of the chemical potential of nitrogen in the nitriding atmosphere by controlling gas flow rate of nitrogen and oxygen.
The steel is nitrided on all surfaces that are exposed to the gas, which is usually an advantage
Large batch sizes possible, only limited by the size of the furnace and the gas flow
The technique allows for precise control of the nitriding results
Relatively low equipment cost, especially compared with plasma nitriding

Things to consider when gas nitriding

The result depends to a large extent on the steels surface – oil, cutting fluids or other contaminants will disturb the process
Surfaces sometimes need to be activated when treating steels with a high chromium content
Adequate protection is needed; ammonia is harmful when inhaled at a high concentration

Applications

Steel components that are surface treated with gas nitriding can withstand higher surface pressure and wear. Typical applications include crankshafts, camshafts, cam followers, valve parts, gears, screws, die-casting tools, forging dies, extrusion dies, firearm components, injectors and plastic-mold tools.

Different types of surface hardening

Laser hardening

Laser hardening of tool steel can give high surface hardness that penetrates relatively deep into the material, typically up to 2 mm. Laser hardened surfaces become very smooth, which reduces friction and the risk for material sticking to the surface. Laser hardening is also called laser peening.

Induction hardening

Induction hardening is a common process for the surface hardening of steel. The steel components are heated by means of an alternating magnetic field to a high temperature followed by immediate quenching. By quenching the heated part in water, oil, or a polymer-based liquid, the surface layer is altered to form a martensitic structure which is harder than the base metal.

Surface etching

Surface etching of metal is called chemical etching or photo etching. It is a subtractive metal machining process which uses chemistry to alter the surface geometry of a steel piece, for example when producing steel molds for making car interior panels.

Shot peening

Shot peening is a cold process which compresses the surface of the steel by striking it with round metallic, glass, or ceramic particles with a high enough force to create plastic deformation. Its main application is to avoid the propagation of microcracks in a surface.

Gas nitriding of Toolox^® 44

By nitriding Toolox^® you can reach a surface hardness of 58‑65 HRC for highly wear resistant applications with a long service life. By coating the nitrided surface through PVD it’s possible to achieve even greater hardness or resistance to corrosion. The high strength of Toolox^® minimizes the risk of cracking and chipping of the surface layer.

A tool steel well suited for surface treatment

All kinds of surface treatments work better on a homogenous steel with high surface quality and consistent properties. Toolox® engineering and tool steel from SSAB meet the strictest demands on suitability for gas nitriding and other surface treatment techniques.