The Basics of the Heat Treating
Heat treating is a metallurgical process used to change the physical or chemical properties of a metal, allowing it to change in terms of ductility, hardness, strength, toughness and elasticity. This is done through a process of heating, cooling and reheating.
One of the goals in producing heat treat equipment is to create a tool that will be both hard enough to stand up under service conditions and tough enough not to crack, either in its manufacture or in use. Five factors contribute to making successful heat treat equipment including good design, steel of the proper grade, correct heat treatment, proper grinding and proper use of the tool.
Complex heat treating cycles are often used to optimize an alloy’s mechanical properties. Controlled heating of the metal can repair some of the defects appearing in the metal.
First, the metal is heated to an extremely high temperature, referred to as the critical temperature. This extreme heat will cause internal changes to the structure of the metal. The metal should remain at this temperature for a specific amount of time to ensure that the properties of the metal are altered consistently throughout. This step is also known as annealing, and produces a refined microstructure in the metal.
If the metal is then left to cool naturally, the metal will return to normal. In heat treatment, however, a process called quenching is used to control the rate at which the metal cools. Quenching involves the use of liquids, such as oil, water or brine, which each have a natural thermal conductivity, or gases to cool the metal faster than it would naturally, allowing the metal to retain some of the changes introduced during the initial heating phase.
Finally, the metal is heated again, this time to a temperature below the critical temperature. This second round of heat, referred to as tempering, is used to correct some of the issues that can be caused by the previous steps. The initial heating and quenching processes result in metal that is stressed, making it brittle and hard. Tempering relieves the stresses, increasing the toughness and ductility of the metal while retaining sufficient hardness and strength. Reheating the metal is an important step of the heat treatment process that should be performed soon after quenching.
When tempering steel, colors appear on the surface of the metal, which can be used to determine the thickness of the iron oxide forming on the surface, which is a reliable indicator of the temperature of the steel.
A common misconception is that heat treating is a difficult process, but with the proper equipment, any size company can do heat treating in-house. Purchasing your own heat treat furnace enables you to do your own heat treating, saving time and reducing the costs of your project.
“A Knights Furnace heat treat furnace with automatic temperature control is ideal for this precision work.” (Bill Knight)