3/18/2021 0 Comments Cast Metal Cooling
Pearlite is created during the slow cooling process of iron alloys.Ryan Wojes wrote about commodities and metals for The Balance and worked as a metallurgist for more than 13 years.Quenching is a rapid way of bringing metal back to room temperature after heat treatment to prevent the cooling process from dramatically changing the metals 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. Common media for quenching include special-purpose polymers, forced air convection, freshwater, saltwater, 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 isnt 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. 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 thats 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. Often used to harden steels, water quenching from a temperature above the austenitic temperature will result in carbon getting trapped inside the austenitic lath. ![]() 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. 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.
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