Gas nitriding of tool steel

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.

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.

• 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

• 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

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.

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.

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.