Several Methods Of Surface Hardening in Steel Machining

Surface hardening processes are important for components in industries such as machinery, electronics, medical devices, aerospace, and oil and gas. Depending on the application, some parts require higher surface hardness or strength. And some parts that need to deal with complex working stress not only need wear-resistant surface, but also need core strength and toughness to withstand impact stress.
Two common methods are generally used to obtain these different properties.
1. Change the chemical composition of the surface before and after conditioning. Processes used include carburizing, nitriding, cyaniding and carbonitriding.
2. The heating and quenching process only hardens the surface layer. The most common methods used for surface hardening are flame hardening and induction hardening.
Carburizing process
By heating the part in a carbonaceous medium, carbon diffuses to a controlled depth across the surface of the part. The resulting depth of carburization (commonly called carburization depth) depends on the carbon potential of the medium used and the time and temperature of the carburization process.
The steels most suitable for carburizing to increase toughness are those with sufficiently low carbon content (usually less than 0.3%). Carburizing temperatures range from 1550-1750°F (843-954°C), with temperature and time adjusted for varying surface depths.
The choice of steel, hardenability and type of hardening depends on section size, required core hardness and service requirements.
The three most commonly used carburizing techniques

liquid carburizing
Heat the steel with molten barium or sodium cyanide. In addition to carbon, the case also absorbs nitrogen, making its surface hard.
Carburizing package
This is done by sealing solid carbon material and steel in a closed container, then heating it.
gas carburizing
This process heats steel in a gas with a specific carbon content. It allows you to tightly control the carbon content. Using either method, the part can be quenched after the carburizing cycle without reheating, or it can be air cooled and then reheated to austenitizing temperature before quenching.
The depth of the cabinet can be adjusted according to the load conditions in use. However, maintenance features often only require hardfacing of selected areas of the part. Cover the non-encapsulated areas with a layer of copper plating or a commercial paste so that the carbon penetrates only the exposed areas. Another method is to carburize the entire part and machine it before hardening. Removes the shell from the selected area.
heat treatment of steel
Nitriding process
The process is performed by heating the steel part in ammonia and cracked ammonia at temperatures between 900°F and 1150°F. The formation of nitrides allows the formation of a thin but hard shell. For this process to be successful, the steel needs elements with strong nitriding capabilities. These elements include special non-standard steel grades containing chromium molybdenum and aluminum. In addition, the main advantage of this process is that it can be quenched, tempered and machined before nitriding. This is because only a slight deformation occurs during nitriding.
Cyanide process
The process entails heating the part in a bath of molten sodium cyanide to a temperature just above the transition range, followed by rapid cooling to achieve thinner hardness.
Carbonitriding
This process is similar to cyanidation. The difference is that the absorption of carbon and nitrogen is accomplished by heating the part in a gaseous atmosphere containing hydrocarbons and ammonia. For parts to be quenched, use temperatures of 1425-1625°F (774-885°C), or 12000-1450°F (649-788°C) if liquid quenching is not required. Increase. lower temperature.
flame hardened
In this process, steel parts are heated rapidly by direct exposure to an open flame of hot gases. The surface is heated to a temperature above the transformation range, followed by some cooling to fully harden. Our flame hardened steels have a carbon content of 0.30-0.60%. Spray the quenching agent on the surface slightly away from the heating flame. Immediate tempering is required and can be done in a conventional furnace or by the flame tempering process, depending on part size and cost.
induction hardening
The process is somewhat similar to flame hardening, with the main difference being that a high frequency electric current surrounds the steel part being hardened. Therefore, the heating depth depends on the frequency, the thermal conductivity of the surface and the degree to which the surface is heated. Spray water at appropriate intervals to quench the part. Therefore, after reaching a certain temperature, the parts can be immersed in an oil bath for oil quenching.