A thermochemical heat treatment process primarily involving carburization with simultaneous nitrogen infiltration. To a certain extent, it overcomes the limitations of nitriding—where the layer is very hard but shallow—and carburizing—where the hardened depth is substantial but the surface hardness is comparatively lower. The most widely applied methods are gas carbonitriding and salt bath carbonitriding. The gas carbonitriding medium is a mixture of carburizing and nitriding agents, such as kerosene or ethanol with ammonia injection; endothermic or exothermic atmosphere gas supplemented with high-carbon-potential enrichment gas and nitrogen; or continuous drip-feeding of triethanolamine or alcohol containing dissolved urea.
After carbonitriding, quenching, and tempering, the microstructure consists of nitrogen-bearing martensite, carbonitrides, and retained austenite. A carbonitrided layer approximately 0.6–1.0 mm deep exhibits strength and wear resistance comparable to a carburized layer 1.0–1.5 mm deep. To minimize distortion, medium-load gears and similar components can use carbonitriding below 870 °C as a substitute for carburizing at 930 °C.
The basic process is as follows:
Pre-treatment
Degreasing and cleaning to remove surface oil, dust, and oxides.
Pickling and passivation to remove surface rust, forming a dense oxide film that enhances infiltration efficiency.
Preheating the workpiece to the temperature required for carburization to promote atomic diffusion.
Carburizing and Nitriding
Carburizing and nitriding constitute the core of the carbonitriding process, with the objective of driving carbon and nitrogen atoms into the workpiece surface.
Carburizing: Infiltrating carbon atoms into the workpiece surface at elevated temperature to form a high-hardness, wear-resistant carbide layer.
Nitriding: Infiltrating nitrogen atoms into the workpiece surface at elevated temperature, yielding higher surface hardness, wear resistance, and corrosion resistance.
Post-treatment
Enhancing workpiece performance and stability.
Quenching: Rapidly cooling the carburized or nitrided workpiece to increase the hardness and strength of the diffusion layer.
Tempering: Heating the quenched workpiece to a specific temperature followed by slow cooling to relieve quenching stresses, improve toughness, and enhance machinability.
Grinding and polishing: Removing excess material from the diffusion layer surface to improve surface finish and dimensional accuracy.
Precautions
Temperature, time, and atmosphere during carburizing and nitriding must be strictly controlled to ensure the required case depth and hardness are achieved.
Workpiece surface roughness and geometry affect infiltration effectiveness and must be taken into account.
Some distortion may occur in the workpiece; finish machining is required to meet dimensional accuracy requirements.
author: Wangbo