How Quenching Hardens Steel in Metalworking
Quenching is a rapid way of bringing a metal back to room temperature after heat treatment to prevent the cooling process from dramatically changing the metal's microstructure. Metalworkers do this by placing the hot metal into a liquid or sometimes forced air. The choice of liquid or the forced air is referred to as the medium.
How Quenching Is Executed
Common media for quenching include special purpose polymers, forced air convection, fresh water, salt water, and oil. Water is an effective medium when the goal is to have the steel to reach maximum hardness. However, using water can lead to metal cracking or becoming distorted.
If extreme hardness isn't necessary, mineral oil, whale oil, or cottonseed oil may be used in the quenching process instead. The process of quenching can look dramatic to those not familiar with it. As the metalworkers transfer the hot metal to the chosen medium, steam rises from the metal in great volume.
The Impact of the Quench Rate
Slower quench rates give thermodynamic forces a greater opportunity to change the microstructure, and this often can be a bad thing if that change in the microstructure weakens the metal. 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 can counter the quenching process, so it is necessary to avoid them.
Why Quenching Is 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 a tempering process. This involves reheating the metal to a temperature below a critical point, then allowing it to cool in the air.
Typically, steel will be subsequently tempered in oil, salt, lead baths, or 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 cooling rates slower than martensite formation but at a faster cooling rate than ferrite and pearlite.
Quenching prevents steel from breaking down from austenite into ferrite and cementite. The goal is for the steel to reach the martensitic phase.
Each medium available for the quenching process has its own benefits and drawbacks, and it is up to the metalworkers to decide what is best based on the specific job. These are some of the options:
- Caustics: These involve water, different concentrations of salt water, and soda. These are the fastest ways to cool metals during the quenching process. Aside from possibly warping the metal, safety precautions also must be taken when using caustic sodas, as they can be harmful to the skin or eyes.
- Oils: This tends to be a most popular method because some oils still can cool metals rapidly but without the same risk as water or other caustics. Oils do come with risks, though, because they are flammable. Therefore, it's important for metalworkers to know the limits of the oils they are working with in terms of temperatures and load weights in order to avoid fires.
- Gases: While forced air is common, nitrogen is another popular option. Gases often are used for finished metals, such as tools. Adjusting the pressure and exposure to the gases can control the rate of cooling.