The so-called metal quenching is a method of rapidly reducing the metal back to room temperature after heat treatment (e.g., annealing) in order to prevent the significant change in the microstructure of the metal during the cooling process. Quenching results in an increase in the degree of hardening of the steel at the same temperature at which it is fully annealed.
How do you perform quenching?
Special polymers, forced air convection, fresh water, brine, and oil can all be used for quenching. When the goal is to maximize the hardness of the stainless steel tube, water is an effective medium. However, using water can cause the metal to crack or deform. If extremely high hardness is not required, other media such as mineral oil, whale oil or cottonseed oil can be used during the quenching process.
Effect of quenching rate
The slower quenching and more thermodynamic changes in the stainless steel tube are opportunities for changing the microstructure. Sometimes this result is preferred, which is why different media are used for quenching. For example, the quenching speed of oil is much lower than that of water. Quenching in a liquid medium requires stirring the liquid around the metal to reduce surface vapors. The steam pocket is quenched by air cooling until it evaporates.
Typically used to harden stainless steel tubes, water quenching from austenite temperatures will cause carbon trapped inside the austenitic slats. This leads to a martensitic hardening and brittle stage. Austenite refers to an iron alloy having a γ-iron base, and martensite is a rigid steel crystal structure. Quenched steel martensite is very fragile and stressed. Quenched steel is usually subjected to tempering.
Normally, metals such as stainless steel pipes will be tempered in the oil, salt, lead bath, or air circulated by the fan in the furnace to restore some ductility (ability to withstand tensile stress) and loss of toughness by conversion to martensite . After the metal is tempered, depending on the situation, in particular whether the metal is susceptible to brittleness after tempering, it is cooled rapidly, slowly or not at all.
In addition to martensite and austenite temperatures, the heat treatment of metals involves ferrite, pearlite, cementite, and bainite temperatures. When iron is heated to the high temperature iron form, delta ferrite transformation occurs. According to the Welding Institute in the United Kingdom, it formed "low carbon concentrations from liquid cooled iron-carbon alloys prior to transformation to austenite."
Pearlite is easily produced during the slow cooling of iron alloys. Bainite has two forms: upper and lower bainite. It is produced at a slower cooling rate than martensite formation, but at a faster cooling rate than ferrite and pearlite forms. Quenching prevents the steel from decomposing austenite into ferrite and cementite, with the goal of making the steel martensitic.
Source: China Steel Tubes Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)