Maximum surface hardness is achieved by Nitriding.

Nitriding is a surface hardening process in which nitrogen is diffused into the surface of a metal to form a hard nitride layer. It is a low temperature, case-hardening process that is commonly used to enhance the wear resistance, fatigue resistance, and corrosion resistance of steels.

The process of Nitriding involves the following steps:

1. Cleaning the surface of the metal to be nitrided to remove any impurities or contaminants.
2. Placing the cleaned metal in a sealed chamber that is filled with a nitrogen-rich atmosphere.
3. Heating the chamber to a temperature between 495°C and 565°C (923°F and 1050°F) for a period of several hours.
4. During this time, nitrogen atoms diffuse into the surface of the metal, forming a hard nitride layer.

Advantages of Nitriding:

1. Nitriding produces a very hard surface layer, which results in increased wear resistance and fatigue resistance.
2. The process is relatively low temperature, which means that the core of the metal remains relatively unaffected.
3. Nitriding is a cost-effective way to improve the properties of steels, without the need for expensive alloys or coatings.
4. The process can be applied to a wide range of steels, including low-alloy steels, stainless steels, and tool steels.

Disadvantages of Nitriding:

1. Nitriding is a time-consuming process, which can take several hours or even days to complete.
2. The process requires specialized equipment and skilled operators, which can make it expensive.
3. The depth of the nitrided layer is limited, typically to a few micrometers.
4. The process may not be suitable for all applications, such as those that require a thick, wear-resistant coating.