On 2018-08-08 14:48:54
How is quenching performed? Understand quenching in metal working
Quenching is a rapid method of bringing metals back to room temperature after heat treatment to prevent significant changes in the microstructure of metals during the cooling process. Metalworkers do this by putting hot metal into a liquid or sometimes forced air. The choice of liquid or forced air is called a medium.
How is quenching performed?
Commonly used quenchants include specialty polymers, forced air convection, fresh water, salt water, and oil. Water is an effective medium when the goal is to make the steel strong. However, the use of water can cause metal to crack or deform. If extreme hardness is not required, mineral oil, whale oil, or cottonseed oil may be used in the quenching process.
The quenching process may seem dramatic to those unfamiliar with it. As the metalworker transfers the hot metal to the chosen medium, steam rises in great quantities from the metal.
The effect of quenching rate
The slower quenching rate provides a greater opportunity for thermodynamic forces to change the microstructure, which is usually a bad thing if the microstructure changes weaken the metal. Sometimes this result is preferred, which is why different media are used for quenching. For example, the quenching rate of oil is much lower than that of water. Quenching in a liquid medium involves stirring the liquid around the sheet metal to reduce steam from the surface. Bubbles will hinder the quenching process, so it is necessary to avoid the formation of bubbles.
Usually used to harden steel, water quenching at temperatures above austenite causes carbon to be trapped in austenite slates. This leads to a hard and brittle martensitic phase. Austenite is an alloy of iron with a metal-iron base, while martensite is a rigid steel crystalline structure. Martensite brittleness and stress of hardened steel are high. As a result, hardened steel usually undergoes a tempering process. This involves reheating the metal to a temperature below the critical point and then cooling it in the air.
Typically, the steel is then tempered in an oil, salt, lead bath, or furnace, where the air is circulated through a fan to restore some of the ductility (the ability to withstand tensile stress) and the toughness lost through conversion to martensite. After tempering, metals are cooled rapidly, slowly, or not at all, depending on the situation, especially whether the metal in question is susceptible to brittleness after tempering.
In addition to martensite and austenite temperatures, the heat treatment of metals also includes ferrite, pearlite, cementite and bainite temperatures. Ferritic transformation occurs when iron is heated to a high temperature.
Pearlite is produced during the slow cooling of ferroalloys. There are two forms of bainite: upper bainite and lower bainite. It cools slower than martensite formation, but faster than ferrite and pearlite.
Quenching prevents steel from decommissioning from austenite to ferrite and cementite.