Learn About Quenching in Metalworking

How This Method Hardens Steel

Quenching metal
••• Arthur Siegel - Library of Congress CALL NUMBER: LC-USW36-812 [P&P]/Wikipedia Commons/Public Domain

Quenching is a rapid way of bringing a metal back to room temperature after heat treatment (such as annealing, normalizing or stress relieving) to prevent the cooling process from dramatically changing the metal's microstructure. Quenching results in the hardening of steel at the same temperature that full annealing does.

How Is Quenching Executed?

Special purpose polymers, forced air convection, fresh water, salt water and oil can all be used to carry out the quenching process. Water is an effective medium when the goal is to have the steel to reach maximum hardness. However, using water can lead metal to crack or become distorted. If extreme hardness isn't necessary, then other media such as mineral oil, whale oil or cottonseed oil may be used in the quenching process instead.

The Impact of the Quench Rate

With a slower quench rate, the thermodynamic forces have more of an opportunity to change the microstructure. Sometimes, this outcome is preferred, which is why different media are used to perform quenching. Oil, for example, has a quenching rate that's much lower than water. Quenching in a liquid medium requires stirring the liquid around the piece of metal to reduce steam from the surface. Pockets of steam counter the quench by air cooling until they evaporate. 

Why Is Quenching Performed?

Often used to harden steels, water quenching from a temperature above the austenitic temperature will result in carbon getting trapped inside the austenitic lath. This leads to the hard and brittle martensitic stage. Austenite refers to iron alloys with a gamma-iron base, and martensite is a hard type of steel crystalline structure. Quenched steel martensite is very brittle and stressed. As a result, quenched steel typically undergoes the tempering process. 

Typically, steel will be subsequently tempered in oil, salt, lead baths or in furnaces with air circulated by fans to restore some of the ductility (ability to withstand tensile stress) and toughness lost by conversion to martensite. After the metal is tempered, it is cooled quickly, slowly or not at all, depending on the circumstances, particularly whether the metal in question is vulnerable to post-temper brittleness.

In addition to the martensite and austenite temperatures, heat treatment of metal involves the ferrite, pearlite, cementite and bainite temperatures. The delta ferrite transformation occurs when the iron is heated to a high-temperature form of iron. According to The Welding Institute in Great Britain, it forms "on cooling low carbon concentrations in iron-carbon alloys from the liquid state before transforming to austenite."

Pearlite is created during the slow cooling process of iron alloys. Bainite comes in two forms: upper and lower bainite. It is produced at slower cooling rates slower than martensite formation but at a faster cooling rate than ferrite and pearlite form.

Quenching prevents steel from breaking down from austenite into ferrite and cementite. The goal is for the steel to reach the martensitic phase.