By using the diagram, a heat treat cycle can be developed that will provide the desired grain structure and properties required. The diagram is a function of temperature and time, showing the grain structure that will be formed based on how quickly the material is cooled or quenched.
The slower the cooling process, the more austenitic grain structure will remain, providing a soft material with good ductility but lower strength. A very fast cool produces a total martensite grain structure, making a product high in strength but not ductile. The tempering process is an essential stage in heat treatment, especially in very fast cooling, as it brings back ductility. Before we can start the quenching process we need to heat the steel to a high heat.
Heating to this temperature causes a grain structure called austenite to form. An austenitic grain structure produces a very soft metal. After the metal is heated, we need to rapidly cool the steel. This has to do with the crystalline structure of the steel and the differences between martensitic and austenitic stainless steels.
The tempering process will lower the hardness while increasing ductility. It generally involves heating the metal again for a set period of time and then allowing it to cool in still air rather than quenching it again.
Depending on the required properties, the temperature and other variables of both the quenching and tempering processes can be fine tuned for the desired outcome. While the tempering of steel has been developed over thousands of years, aluminum is a much newer metal and the quenching techniques are more recent.
This may be why people are more likely to associate quenching with stainless steel, even though it is certainly possible to quench aluminum if desired. As with stainless steel, there are a number of possible liquids that can be used with aluminum, with water being the most common. Water is the most widely used choice for a number of reasons, starting with the fact that water is readily available and usually the cheapest option.
Another advantage is that water offers the rapid quenching speeds that are required for certain properties in various alloys. Water also allows for flexibility by altering its temperature as needed.
Water can be used to quench aluminum with a number of techniques, including cold water quenching, hot water quenching, and water spray. Other quenching materials include still air, air blasting, glycols, polymers, liquid nitrogen, oils, brine solutions, and more.
To fine tune the quenching process and draw out very specific and targeted properties, you can do some combination of the above. The major drawback to cold water quenching is that as with stainless steel, it can lead to undesirable warping or distortion in the aluminum. At Clinton Aluminum, we pride ourselves on our ability to help our clients through every step of the material procurement process, ensuring that not only do they get the right product fitted to their needs, but that they maximize their investment and get exactly what they need in a timely and friendly manner.
Contact us today to learn more about which quenching options make the most sense for your application. Clinton Aluminum All rights reserved. Alloys may be air cooled, or cooled by quenching in oil, water, or another liquid, depending upon the amount of alloying elements in the material. Hardened materials are usually tempered or stress relieved to improve their dimensional stability and toughness. Steel parts often require a heat treatment to obtain improved mechanical properties, such as increasing increase hardness or strength.
The hardening process consists of heating the components above the critical normalizing temperature, holding at this temperature for one hour per inch of thickness cooling at a rate fast enough to allow the material to transform to a much harder, stronger structure, and then tempering. Steel is essentially an alloy of iron and carbon; other steel alloys have other metal elements in solution.
Heating the material above the critical temperature causes carbon and the other elements to go into solid solution. Quenching "freezes" the microstructure, inducing stresses.
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. 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.
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