There is a wide range of heat treatment processes available in the industry, but why do we heat treat metals? We will cover what are heat treatments, what their objectives are, how they’re achieved. Also, we will discuss the properties of steel when cooled at different rates.
What Is The Heat Treatment Of Metals?
Heat treatment is a thermal process (with many sub-processes) used to manipulate metals physical properties. In some cases, the chemical properties will be affected as well. Metals are heated to extreme temperatures and cooled at different rates to manipulate the properties of the metal depending on the goal.
However, some processes do not involve heating the metal to extreme temperatures. There are many heat treatment processes used in the industry that you will be aware of: precipitation strengthening, solid solution strengthening, case hardening, annealing etc.
Why Do We Heat Treat Metals?
What are the objectives of heat treating metals? As discussed above, we want to manipulate the properties of the metal, but why? Depending on the environment or system the metal is in, it will require different properties. For example, structural steel will need to be of high strength to sustain its high loading. Additionally, knives require steels with a high hardness to allow them to remain sharp.
As the range of environments metals are present in is vast, many properties need modifying. As mentioned above, we need strengthened steel and hardened steel for specific applications. Wear resistance is another property commonly manipulated. Increasing the wear resistance of metals will increase the life of some components. A great example of this is the materials used in bearings.
Ductility is a property that we may want to manipulate if the environment will represent a varying load and some deformation in the metal is required, e.g. in steel cables. Below is a table of objectives for heat treatments and the process which helps us achieve them.
Why We Use Certain Heat Treatment Processes
| Objective | Heat Treatment |
| Increase Strength | Quenching, Precipitation Hardening |
| Increase Wear Resistance | Case Hardening |
| Increase Ductility | Tempering, Annealing |
| Increase Toughness | Tempering, Recrystallisation Annealing |
| Remove Internal Stress | Stress Relief Annealing |
Heat treatment processes work off the principle of manipulating grain sizes which is one of the most effective ways to determine the behaviour of the metal.
Cooling Rate of Steel During Heat Treatment
Steel is the most common material used in engineering. A method for altering the properties of steel, as mentioned earlier, is by controlling the cooling rate. Below are a diagram and a table showing the roles of different cooling rates for steel when heated to an extreme temperature, approximately 950°C. At this temperature, steel forms the phase Austenite which has a high carbon content.
| Phase | Hardness (Hv) | Tensile Strength (MPa) | Ductility (%) | Toughness Charpy (J) |
| Pearlite | 300 | 950 | 15 | 40 |
| Bainite | 400 | 1200 | 16 | 20 |
| Martensite | 500 | 1800 | 6 | <10 |
From the table and diagram above, we can see that as we increase the cooling rate, both the hardness and tensile strength of the material increases. On the other hand, the properties ductility and impact toughness decrease. These changes are a result of the increased level of carbon in the microstructure. When heated to the Austenitic phase, carbon readily diffuses into the microstructure. The rapid cooling from quenching does not allow the carbon to diffuse out of the structure, locking it in. The result is steel containing a high level of carbon.
Summary
Manipulating just the cooling aspect of metal can result in very different properties of the metal. Matching the cooling rate with alternative heating temperatures leads to an even larger variance in metal properties. The process allows metals properties to be manipulated extensively depending on their environment and their role. I hope you now understand why we heat treat metals and the different objectives behind the methods.
Related Posts: What Are The 3 Stages In Precipitation Hardening?, What Is Solid Solution Strengthening?, What Is The Annealing Heat Treatment Process?
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