The Metal Heat Treatment Process Explained
The metal heat treatment process is the process of heating and cooling metals. Through the different types of metal heat treatment, you can change the properties of both ferrous and non-ferrous metals, adapting your materials to your needs. Knowing when to use heat treatment and what specific technique to use is a significant step in the manufacturing process, which is why we’ve created a guide to heat-treating metal.
Why heat treat?
The metal heat treatment process allows you to change the properties of metals. Different types of metal heat treatment allow you to choose between hardness and softness, which has a major effect on the strength of the metal and its malleability. In general, the idea of heat treatment in the forging industry is to arrive at a balance between surface hardness and core toughness. Some processes can reduce stresses created by earlier cold working techniques, and others can adapt the chemical processes of the metal. You can ensure your metals have the desired properties for your work by using one or a combination of these techniques.
Heat treating: the basics
Heat treatment boils down to heating a metal, retaining the elevated temperature for the correct time, and then cooling the metal back to room temperature. When the metal is heated, its mechanical properties change, as the temperatures change the material’s microstructure. There are three main variables: 1) the heating temperature, 2) the rate of cooling, and 3) the type of quenching used.
The first step in the metal heat treatment process is heating. The metal must heat evenly. If different parts of the piece are allowed to reach different temperatures, the metal may crack or distort. The metal must be heated slowly, with the heating rate chosen according to a set of factors: heat conductivity, condition of the metal, and the piece’s size, geometry and cross-section. Heating metal changes its mechanical properties, altering the grain structure from a solid solution to an alloy or mechanical mixture, or vice versa. The temperature you aim for depends upon the desired result you’re pursuing. Once you have reached the appropriate temperature, you can move on to the next stage.
Holding, or soaking, the metal at the appropriate temperature is the second step of heat treatment. The duration you hold a metal depends upon what properties you are seeking to achieve. For instance, if you only need to harden the surface of a metal, the metal will need to soak for less time. Once your metal has reached the target grain structure, the next step is to bring the metal back to room temperature.
Cooling the metal is the final step of the heat treatment method. There are two basic approaches: rapid and slow cooling. Several types of metal heat treatment use rapid cooling, often achieved by cooling metal in a quenching medium such as water or brine. Hardening is achieved by rapid cooling,
A high cooling rate is not appropriate for all metals, some of which can crack or warp under high pressure. Different quenching mediums can slow cooling rates, such as oil or air, used for hardening alloy steels. When precise slow cooling is needed, one possibility is to place the metal in a gradually cooling furnace, allowing for a steady descent to lower temperatures.
Types of metal heat treatment
Annealing is one of the most common types of heat treatment. In this metal heat treatment process, the metal is heated beyond the upper critical temperature and then subjected to slow cooling. This process reduces the tensile strength and hardness of the metal, softening it to allow for easier forming and cold working. Annealing is also stress relieving, removing deformations created by previous processes.
Another metal heat treatment process is normalising. In this process, you generally heat metal above its typical upper critical temperature, and then cool the piece in air at ambient temperature. This approach creates a consistent grain size and composition, increasing the metal’s tensile strength, compared to that of annealing.
The most popular type of metal heat treatment is hardening. Unsurprisingly, this increases the hardness of a metal. By heating a metal piece to a specific temperature and then using water or brine for rapid cooling, the hardening process increases the strength and brittleness of a metal.
This is why it is important to get the right balance between surface hardness and core toughness, in order to mitigate the brittleness that can arise from rapid cooling. There are multiple different hardening processes. Case hardening is when only the surface of a metal is hardened, a common technique for shafts and other structures at high risk of material wear. Other types include differential, flame and induction hardening.
Stress relieving is another important heat treating method, typically applied to metal pieces previously subjected to other processes. Internal stresses are relieved by heating a metal to just below its lower critical temperature, and then slowly and uniformly cooling the piece. Another way to reduce internal stress is by tempering a metal. Tempering reduces excess hardness created by the hardening process and requires a much lower holding temperature.
At Greg Sewell Forgings we are partnered with companies that offer a wide range of forging heat treatment services. Their capabilities range across different heat treatment techniques and can be combined with our custom metal fabrication services to suit all your needs. Our commitment to high-quality and cost-effective service, and our experience working across multiple industries, means we always produce exceptional metalwork.