The difference between hard nitriding and soft nitriding

The term “soft nitriding” does not imply that the resulting hardness is lower than that of so-called “hard nitriding”; rather, it conveys simplicity, convenience, and low cost.

Hard nitriding
Nitriding is a type of chemical heat treatment, also known as case nitriding. It involves placing the workpiece in a furnace atmosphere containing nitrogen (including ammonia gas), selecting appropriate temperature, gas flow rate, and ammonia concentration ratio, and maintaining these conditions for a specified period of time. This allows nitrogen atoms to diffuse into the surface of the workpiece, forming a highly hardened surface nitrided layer that imparts superior wear resistance, fatigue resistance, and other enhanced properties.
The temperature for nitriding treatment typically ranges from 480 to 540°C (this range ensures both the required tempering hardness in the core of the workpiece and the desired hardness of the nitrided layer). The duration of the treatment varies depending on the required depth of the nitrided layer (usually between 0.1 and 0.65 mm), generally ranging from 15 to 70 hours, or even longer.
This conventional nitriding is also known as hard nitriding, in contrast to soft nitriding.
Soft Nitriding
The technical term for soft nitriding is “nitrocarburizing.” The elements diffused into the steel surface are primarily nitrogen, with carbon added as a secondary component. Essentially, soft nitriding is a low-temperature nitrocarburizing process dominated by nitrogen diffusion. Compared to hard nitriding, the resulting diffusion layer has lower hardness and less brittleness, which is why it’s referred to as “soft nitriding.”
The soft nitriding methods are broadly categorized into gas soft nitriding and liquid soft nitriding. Currently, gas soft nitriding is the most widely used method in domestic production. Gas soft nitriding involves low-temperature co-diffusion of carbon and nitrogen in an atmosphere containing active carbon and nitrogen atoms; the typical temperature range is 560–570°C, and the nitriding time usually lasts 2–3 hours.
Currently, a problem with gas soft nitriding is that the thickness of the iron-nitride compound layer on the surface is relatively thin (0.01–0.02 mm), and the hardness gradient within the nitrided layer is steep. Therefore, this process is not suitable for applications under heavy-load conditions.
The term “soft nitriding” does not imply that the resulting hardness is lower than that achieved by so-called “hard nitriding”; rather, it conveys simplicity, convenience, and lower costs.
Hard nitriding is applied to workpieces subjected to high loads and requiring relatively high resistance to contact fatigue, with emphasis placed on the depth of the diffusion layer.
The advantage of soft nitriding is its fast diffusion rate; it’s typically used for workpieces with low loads and where a shallow case depth is required.