Surface treatment

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Chemical heat treatment refers to the process in which a steel part is placed in a chemical medium containing one or several infiltrating elements, and these elements are caused to diffuse from the surface toward the interior through heating, holding at a certain temperature, and cooling. The purpose of chemical heat treatment is to improve properties such as surface hardness, wear resistance, heat resistance, corrosion resistance, and fatigue strength of metals by altering their chemical composition and microstructure at the surface. By infiltrating different elements into the metal surface, various properties can be achieved; therefore, the chemical heat treatment of metals is often named after the elements that are infiltrated, such as carburizing, nitriding, carbonitriding, boriding, aluminizing, and chromizing. (1) Carburizing: It is a heat treatment process in which a steel component is heated in a carburizing medium, usually at temperatures between 900°C and 950°C, and held at that temperature to allow carbon atoms to penetrate into its surface. This increases the carbon content in the surface layer and creates a certain carbon concentration gradient. It is the most widely used chemical heat treatment method in the machinery manufacturing industry. Depending on the carburizing agent used, carburizing methods can be divided into solid carburizing, liquid carburizing, and gas carburizing. Steels used for carburizing are mostly low-carbon steels or low-carbon alloy steels with a carbon content of 0.15%~0.30%. Carburizing is mainly used for parts that require very high surface hardness, as well as sufficient strength and toughness in their core, such as gears and piston pins. To achieve the desired properties, workpieces are usually subjected to quenching plus low-temperature tempering after carburizing. (2) Nitriding: It is a chemical heat treatment process in which nitrogen atoms are introduced into the surface layer of a workpiece at a certain temperature in a specific medium, thereby increasing the nitrogen concentration in that surface layer. Common methods include liquid nitriding, gas nitriding, and ion nitriding. Traditional gas nitriding involves placing the workpiece in a sealed container, introducing flowing ammonia gas and heating it; after maintaining this temperature for a period of time, the ammonia gas decomposes to produce ammonia atoms, which then continuously adsorb, diffuse, and penetrate into the surface layer of the workpiece. The nitriding temperature is very low, usually ranging from 500°C to 580°C, and no quenching is required after nitriding; as a result, the deformation of the workpiece is minimal. This process is often used for gears made of high-speed tool steel, grinding machine spindles, as well as those components that require precise dimensions and resistance to corrosion and wear. However, this process has a long cycle time, and certain steels (such as lead-copper alloys) become highly brittle on the surface after nitriding. (3) Carbonitriding: It is a chemical heat treatment process in which carbon and nitrogen atoms are simultaneously diffused into the surface layer of steel, in order to increase the hardness and wear resistance of the metal surface layer. The temperature range for carbonitriding is quite wide; nitridation can be carried out at temperatures between 550°C and 950°C. In daily practice, carburitriding is often divided into three categories based on temperature: high-temperature carburitriding (900°C–950°C), medium-temperature carburitriding (800°C–870°C), and low-temperature carburitriding (500°C–600°C). Different carburizing temperatures result in different carbon and nitrogen concentrations in the carburized layer. At high temperatures, carburization is the dominant process, with very little nitrogen infiltration ； At low temperatures, nitrogen diffusion is the dominant process, with very little carbon diffusion, which is commonly referred to as soft nitridation. It is most widely used only when the amounts of carbon and nitrogen diffusion are both appropriate at moderate temperatures. Therefore, carbonitriding usually refers to medium-temperature carbonitriding. Compared with single carburizing, carbonitriding is faster, has a shorter production cycle, results in less deformation and cracking of the workpiece, and enables the workpiece to achieve higher hardness, wear resistance, fatigue strength, and resistance to interlocking.