Answer:
Surface hardening is a process used to increase the hardness of the surface layer of a metal component while maintaining a tough and ductile core. It is commonly used to improve the wear resistance and fatigue strength of materials. There are several surface hardening processes, and each has its own characteristics and requirements.

Nitriding:
Nitriding is a surface hardening process in which nitrogen is diffused into the surface of a metal component to form a hard nitride layer. This process is typically performed at temperatures below the lower critical temperature of the material, which means that it does not require quenching. Nitriding is commonly used for low-alloy steels, stainless steels, and cast irons. The nitride layer formed during nitriding enhances the surface hardness, wear resistance, and corrosion resistance of the material.

Induction hardening:
Induction hardening is a surface hardening process that uses electromagnetic induction to heat the surface of a metal component. The heated surface is then rapidly quenched to achieve the desired hardness. Induction hardening is commonly used for steels and cast irons, and it can provide a high hardness and wear resistance in a localized area of the component.

Flame hardening:
Flame hardening is a surface hardening process that involves heating the surface of a metal component using a high-temperature flame and then quenching it to achieve the desired hardness. The flame can be produced by burning a fuel gas such as acetylene or propane. Flame hardening is commonly used for large and irregularly shaped components, and it can provide a high hardness and wear resistance in specific areas of the component.

Case carburizing:
Case carburizing, also known as carburizing or carburization, is a surface hardening process that involves introducing carbon into the surface layer of a low-carbon steel component. The component is heated in a carbon-rich atmosphere, usually containing hydrocarbons, at elevated temperatures. After the desired carbon concentration is achieved, the component is quenched to form a hardened case. Case carburizing is commonly used for gears, shafts, and other components that require high wear resistance.

In summary, out of the given options, nitriding is the only surface hardening process that does not require quenching. Nitriding is performed at temperatures below the lower critical temperature of the material, allowing for the diffusion of nitrogen without the need for rapid cooling.