Quenching is a heat treatment process in which steel is heated above its critical temperature, held for a certain period, and then cooled at a rate higher than the critical cooling rate to obtain a non-equilibrium microstructure dominated by martensite (bainite or retained single-phase austenite may also be obtained as required). Quenching is the most widely used heat treatment process for steel.

There are four fundamental heat treatment processes for ferrous metals: annealing, normalizing, quenching and tempering. Annealing: The workpiece is heated to a suitable temperature, held for a duration determined by material and part size, and then cooled slowly (the slowest cooling rate). Its purpose is to bring the internal metal microstructure to or near an equilibrium state, achieving favorable processing and service properties, or preparing the microstructure for subsequent quenching.

Normalizing: The workpiece is heated to an appropriate temperature and then cooled in still air. Its effect is similar to annealing, yet it yields a finer microstructure. It is commonly used to improve the machinability of materials, and sometimes serves as the final heat treatment for parts with less stringent requirements.

Tempering: To reduce the brittleness of quenched steel parts, the quenched workpiece is held for an extended time at a suitable temperature above ambient temperature and below 710 °C, followed by cooling. This process is defined as tempering.

Quenching refers to the heat treatment where an austenitized workpiece is cooled in a controlled manner to form martensitic or bainitic structures. Common variants include water quenching, oil quenching, air quenching, etc.

Annealing, normalizing, quenching and tempering are known as the “four major heat treatment processes” for bulk heat treatment. Quenching and tempering are closely associated and frequently used in combination.