Carbonitriding
Carbonitriding is a kind of metal surface modification technology that is mainly used to improve toughness surface properties, wear resistance and fracture surface properties of steel.
Definition
Carbonitriding is a Hardening technique for ferrous metals that is similar to carburizing, but it incorpo rates nitrogen in addition to carbon in the surface layer of the material. It is done at relatively low temperatures compared to other hardening pro cesses. Carbonitriding process is usually intended for low carbon steels (0.15 % to 0.25 %).
Background
Carbonitriding was developed in the early 1950s to improve the toughness properties of steel. In carbonitriding, steel is heated to a temperature of 870 to 910 °C in an atmosphere of cyanide-coated, ammonia-saturated water vapor, which diffuses into the steel, forming a nitride case of mainly carbon and nitrogen. Carbon can diffuse during the process, increasing its concentration in the steel’s case layer. By controlling the process variables, the case grades can be changed and can also affect the resulting case hardness.
Principle
Carbonitriding is a variant of carburizing, in which carbon and nitrogen are diffusely incorporated into the surface of the material. During the carbonitriding process, the steel parts are heated in an atmosphere of ammonia-saturated, cyanic-coated water vapor. The process temperature is usually 870 to 910 °C. The ammonia converts to hydrogen cyanide, which then diffuses into the surface of the steel. The carbon and nitrogen in the atmosphere react to form various unstable compounds, which are then diffused into the steel.
During carbonitriding, it is important to keep the parts in an inert atmosphere, as this will prevent oxidation and oxidation-related issues. Additionally, it is important to ensure that the atmosphere is maintained within a certain temperature and atmosphere pressure range.
Process Steps
1. Cleaning: The steel parts are first carefully cleaned with a liquid detergent to remove any oil, dirt, or contaminants from the surface.
2. Carburizing Compound Application: A carburizing compound (typically cyanide) is then applied to the surface of the steel parts.
3. Heating: The steel is then placed into a furnace and heated to a temperature of 870 to 910 °C. During the entire process, the atmosphere should be kept in a totally inert state (free from oxygen and other reactive substances).
4. Cooling: Once the desired temperature is reached, the parts are removed from the furnace and allowed to cool slowly in an inert atmosphere.
5. Post Treatment: After cooling, the parts are subjected to post-treatment processes, such as annealing, quenching, or tempering, to obtain the desired properties.
Benefits
Carbonitriding offers several benefits, including:
• Increased fatigue strength
• Improved wear and scuff resistance
• Reduction of material distortion
• Enhanced corrosion resistance
• Improved surface hardness
Applications
Carbonitriding is primarily used in the manufacturing of ferrous components such as gears and other machine parts. It is also used in the automotive, aerospace and military industries for various components, such as transmissions, valves and bushings.
Conclusion
Carbonitriding is an effective way to improve the surface properties, wear resistance and fracture surface properties of steel components. It offers numerous benefits, including increased fatigue strength, improved wear and scuff resistance and enhanced corrosion resistance, making it a versatile process for many industries.
