Carbon and nitrogen elements are infiltrated into the workpiece surface at the same time. The process is called carbonitriding. The treatment temperature of this process is lower than the ordinary carburizing temperature, which is about 700 ℃ ~ 960 ℃. The most widely used temperature is between 700 ℃ and 860 ℃. The medium of nitriding is an active medium, which contains carbon and nitrogen elements.

It is precisely because this process is mainly carburized, so the nitrided cementite or nitride is the main structure in the carburizing layer, the surface hardness of the workpiece is improved due to the existence of these hardening phases, and the amount of active nitrogen atoms infiltrated determines the structure and hardness of the carburized layer.

In the process of carbonitriding, the nitrogen atoms entering the surface of the sample change the austenite transformation temperature and decrease its value. The reason is that both carbon and nitrogen can expand the austenite region, and the solubility of carbon and nitrogen increases with the decrease of austenite transformation temperature, so the content of carbon and nitrogen in the layer increases.

However, the decrease of transformation temperature in the cooling process will also lead to the incomplete transformation of austenite, and there will be a certain amount of retained austenite in the layer. Although these retained austenite can improve the fatigue resistance and toughness of the workpiece, it will reduce the hardness of the carburized layer.