Carbonitration

Carbonitriding is a type of chemical-thermal treatment for surface hardening of steel and cast iron parts, which consists of simultaneously saturating their surface with nitrogen and carbon. Carbonitriding technology is most often used instead of traditional nitriding. Carbonitriding can also be recommended as an alternative to the processes of carburizing, nitrocarburizing, surface hardening with high-frequency current, and galvanic chromium plating.

The name «carbonitration» was established by the developer of the process. It means simultaneous saturation of the surface of steel products mainly with nitrogen and to a lesser extent with carbon. Unlike nitrocarburizing, carbonitration is carried out at significantly lower temperatures.

The carbonitriding process is carried out in a salt melt, and the main diffusing elements are nitrogen and carbon. In gas nitriding, such an element is nitrogen, formed during the dissociation of ammonia. This significantly affects the structure of the diffusion layer. In gas nitriding, nitride phases are formed, and in carbonitriding, carbonitride phases.

The surface hardness and layer thickness for each specific material are practically the same, but due to the structural features of the surface zone, the carbonitrided layer has a wear resistance that is 2 times higher. The fatigue strength and contact endurance characteristics are equivalent.

The duration of holding of parts during processing by the carbonitriding method is 5 or more times shorter than during gas nitriding (with the same requirements for the thickness of the hardened layer). The heating time of parts is significantly reduced, and cooling can be carried out immediately after unloading the parts from the bath in air, water or oil. Sequence and approximate duration of operations:

To carry out the carbonitriding process, a shaft electric furnace-bath with a crucible made of titanium alloy is used, preventing contamination of the salt melt and significantly improving the quality of the hardened layer. Before carbonitriding, parts are necessarily preheated to 300-350 ° C in a separate furnace. To clean the parts from adhering salt, rinsing baths with hot or cold water are used.

For the fusing of the bath, salts synthesized from ammonium carbon compounds (melamine, melon, dicyandiamide) are used.

• Carbonitriding is a finishing process, so the parts must arrive in a final mechanically processed condition. For the most critical parts (with precise fit dimensions and a high roughness class), it is recommended to leave an allowance of 0.01-0.03 mm (determined individually in each specific case), for polishing (honing) to obtain the specified roughness and dimensional accuracy. Other parts can be used without additional mechanical processing. It is not recommended to remove the carbonitrided layer by grinding.
• To ensure sufficient strength and toughness of the core, the parts in the blank state must be subjected to hardening with high tempering (thermal improvement) — for alloy steels, normalization — for carbon steels.
• To avoid warping (deformation) during carbonitriding, complex-shaped parts, such as shafts with a large number of steps, large-diameter rings with deep grooves, thin-walled bushings, etc., after rough machining, it is recommended to subject them to tempering to stabilize the dimensions. After tempering, the parts undergo final machining (grinding) and are sent for carbonitriding.