Heat treatment is the process of heating metal without letting it reach its molten, or melting, stage, and then cooling the metal in a controlled way to select desired mechanical properties. All heat treatments involve heating and cooling metals, but there are three main differences in process: the heating temperatures, the cooling rates, and the quenching types that are used to land on the properties you want.
Stages of Heat Treatment
There are three stages of heat treatment:
? Heat the metal slowly to ensure that the metal maintains a uniform temperature
? Soak, or hold, the metal at a specific temperature for an allotted period of time
? Cool the metal to room temperature
The Heating Stage
During the heating stage, the foremost aim is to make sure that the metal heats uniformly. You get even heating by heating slowly. If you heat the metal unevenly, one section may expand faster than another, resulting in a distorted or cracked section of the metal. You choose the heating rate according to the following factors:
? The heat conductivity of the metal. Metals with high heat conductivity heat faster than those with low conductivity.
? The condition of the metal. Tools and parts that have been hardened, or stressed, previously should be heated slower than tools and parts that haven’t.
? The size and cross-section of the metal. Larger parts or parts with uneven cross sections need to be heated more slowly than small parts to allow the inside temperature to be close to the surface temperature. Otherwise, there’s a risk of cracking or excessive warping.
The Soaking Stage
The purpose of the soaking stage is to keep the metal at the appropriate temperature until the desired internal structure takes shape. The “soaking period” is how long you keep the metal at the appropriate temperature. To determine the correct length of time, you will need the chemical analysis and mass of the metal. For uneven cross-sections, you can determine the soaking period using the largest section.
Generally, you shouldn’t bring the temperature of the metal from room temperature to the soaking temperature in one step. Rather, you’ll need to heat the metal slowly to just below the temperature where the structure will change, and then hold it until the temperature is consistent throughout the metal. After this step of “preheating”, you more quickly heat the temperature to the final temperature that you’ll need. Parts with more complex designs may require layers of preheating to prevent warping.
The Cooling Stage
In the cooling stage, you’ll want to cool metal back to room temperature, but there are different ways to do this depending on the type of metal. It may need a cooling medium, a gas, liquid, solid, or combination thereof. The rate of cooling depends on the metal itself and the medium for cooling. It follows that the choices you make in cooling are important factors in the desired properties of the metal.
Quenching is when you rapidly cool metal in air, oil, water, brine, or another medium. Usually quenching is associated with hardening because most metals that are hardened are cooled rapidly with quenching, but it is not always true that quenching or otherwise rapid cooling results in hardening. Water quenching, for example, is used to anneal copper, and other metals are hardened with slow cooling.
Not all metals should be quenched – quenching can crack or warp some metals. Generally, brine or water can rapidly cool metal, while oil mixtures are better for a slower cooling. The general guidelines are that you can use water to harden carbon steels, oil to harden alloy steels, and water to quench nonferrous metals. However, as with all treatments, the rate and medium of cooling you choose must fit the metal.
ABOUT KEVA
Keva Casting is a professional casting company that focuses on Construction, Machinery, Automobile, Hardware, Pipeline and many other industries.
We provide top quality products with the best service based on our complete production chain including our own mold design, advanced production techniques, and full management of the supply chain. Keva Casting has two factories located in Zhucheng, China's Shandong province.
We use "shell molding", which is a new technique that has been developed in recent years.This new technique has simplified production procedures and reduced air pollution compared with traditional sand casting and investment casting. Shell making and core making are faster to ensure production in enormous quantities. The dimensions and surface finishing are satisfactory for all kinds of industrial requirements. Besides supplying our own formwork & scaffolding systems, our casting products also supply OEM services in various industries.