Which of the following heat treatment processes is usually applied to castings ?

ANS:D - Carburising

Carburizing is a heat treatment process used to increase the carbon content at the surface of a low-carbon steel component, typically to improve its hardness and wear resistance. During carburizing, the steel is heated in a carbon-rich atmosphere at elevated temperatures, typically between 850°C to 950°C (1562°F to 1742°F), for an extended period of time. This allows carbon atoms from the atmosphere to diffuse into the surface layers of the steel. The process is typically carried out in a controlled environment, such as a sealed furnace containing a carbonaceous material like charcoal or a gas rich in carbon, such as methane or propane. The carbon atoms diffuse into the surface layers of the steel and form iron carbides, increasing the carbon content and creating a hardened surface layer. Carburizing is commonly used in the manufacture of gears, camshafts, bearings, and other components where increased surface hardness and wear resistance are desired. After carburizing, the component may undergo additional heat treatments such as quenching and tempering to achieve the desired combination of hardness and toughness throughout the part.