Quenching is a process in which a heated metal is rapidly cooled in a liquid medium, usually water or oil, to achieve the desired hardness. However, there are certain hardening processes in which quenching is not necessary, such as case carburizing, flame hardening, and nitriding. These processes can achieve the desired hardness without the need for quenching.

a) Case Carburizing:
Case carburizing is a surface hardening process in which the carbon content of the outer layer of a low-carbon steel is increased by diffusing carbon into the surface. This is achieved by exposing the steel to a carbon-rich atmosphere at high temperatures for an extended period. The carbon atoms diffuse into the surface of the steel, forming a high-carbon layer, while the core remains relatively unchanged.

In case carburizing, the carbon-rich atmosphere provides the carbon necessary for the hardening process. The high temperatures during the process allow the carbon to diffuse into the steel's surface, creating a hardened layer. Since the carbon is already present during the heating process, there is no need for quenching to achieve the desired hardness.

b) Flame Hardening:
Flame hardening is a surface hardening process that involves heating the surface of a metal using a high-temperature flame and then rapidly cooling it. In this process, a focused flame, such as an oxy-acetylene flame, is directed onto the surface of the metal, raising its temperature to the austenitizing range. The heated surface is then quenched by the ambient air or by a water spray, achieving the desired hardness.

In flame hardening, the rapid cooling achieved by quenching is sufficient to achieve the desired hardness. The high temperature attained by the surface during the heating process transforms the structure of the metal, and the subsequent quenching locks in the hardened structure. Therefore, additional quenching is not necessary.

c) Nitriding:
Nitriding is a surface hardening process in which nitrogen is diffused into the surface of a metal to form a hard nitride layer. This is typically done by exposing the metal to a nitrogen-rich atmosphere at elevated temperatures for an extended period. The nitrogen atoms diffuse into the metal's surface, reacting with the base material to form a hardened layer.

In nitriding, the nitrogen-rich atmosphere provides the necessary nitrogen for the hardening process. The high temperatures during the process allow the nitrogen to diffuse into the metal's surface, creating a hardened layer. Since the nitrogen is already present during the heating process, there is no need for quenching to achieve the desired hardness.

d) Any of the above processes:
The correct answer is option 'D' because all of the above processes (case carburizing, flame hardening, and nitriding) can achieve the desired hardness without the need for quenching. Each process utilizes different mechanisms to achieve hardening, such as diffusion of carbon or nitrogen, or transformation of the metal's structure. The specific mechanism used determines the necessity of quenching.

For flame hardnening water or oil quenching Is done...so answer is wrong...should be nitriding.