What is Nitriding Heat Treatment Process?
Nitriding is a heat treatment process used to enhance the surface properties of certain metals, particularly steels. It involves the diffusion of nitrogen into the surface of the material, forming a hard and wear-resistant layer. The process is typically carried out at temperatures between 500°C and 600°C (932°F and 1112°F) in an atmosphere of ammonia gas.
During nitriding, the nitrogen atoms diffuse into the metal lattice, forming nitrides. This results in an increased surface hardness, improved wear resistance, and enhanced fatigue strength of the material. The depth of the nitrided layer can be controlled by adjusting the duration of the treatment.
Nitriding Working Principle
Nitriding, also known as nitridation, is a process that involves introducing nitrogen into workpieces to create nitrides within them. This results in the formation of a hard compound layer composed of iron nitrides on the surface of the workpiece.
Below this layer is the diffusion zone, where nitrogen becomes embedded in the metal matrix of the workpiece, thereby enhancing its fatigue strength. Nitriding steels are particularly effective in achieving high values of fatigue strength through this process.
To carry out nitriding, temperatures ranging from 500°C to 580°C are required, and the duration of treatment can vary from one to one hundred hours. The thickness of the compound layer and the development of porous zones depend on the temperature and duration of the treatment.
The nitriding hardness depth (NHD) is determined by referencing the limit hardness, which is 50 HV higher than the core hardness of the workpiece.
Types of Nitriding Heat Treatment
The three most commonly used types of nitriding heat treatment processes are
- salt bath nitriding,
- gas nitriding, and
- plasma nitriding.
Salt bath nitriding
In salt bath nitriding, the workpieces are partially dipped into a bath of molten salts containing nitrogen. This process allows for controlled diffusion of nitrogen into the surface of the material, resulting in increased hardness and wear resistance.
Gas nitriding
Gas nitriding involves exposing the workpieces to an atmosphere of ammonia gas at elevated temperatures. The ammonia dissociates into nitrogen and hydrogen, and the nitrogen diffuses into the surface of the material. This process is commonly used for steels and other ferrous alloys.
Plasma nitriding
Plasma nitriding utilizes a low-pressure plasma environment to introduce nitrogen into the surface of the workpieces. The plasma is created by applying a high voltage to a gas, typically a mixture of nitrogen and hydrogen. This process offers precise control over the nitriding parameters and is often used for complex-shaped components.
All three processes can also be used for partial nitriding, where only specific areas of the workpiece are treated. In salt bath nitriding, the workpiece is partially dipped into the bath, while in plasma nitriding, a protective compound or mechanical means can be used to limit the nitriding to specific regions.
Advantages of Nitriding heat treatment
- Increased surface hardness
- Improved wear resistance
- Enhanced fatigue strength
- Reduced friction and improved lubricity
- Retention of core toughness
- Resistance to corrosion and oxidation
Disadvantages of Nitriding heat treatment
- Limited depth of hardened layer
- Potential distortion of the component
- Requirement for precise temperature control
- Longer processing time compared to other heat treatments
- Limited applicability to certain materials and geometries