The ultimate guide to Heat Treating
Our guide to heat treating will explain all you need to know and more. Heat treating is a pre and post-manufacturing process which is used to change a number of properties of metals and their alloys. The primary use of heat treating is to make the metal suitable for a particular application.
Properties effected by the process
The process effects the metal to change various properties such as:
- Toughness
- Strength
- Machinability
- Hardness
- Ductility
- Elasticity
- Formability
Heat treating can also affect the physical and mechanical properties of metal in order to change the use of it to alter future manufacturing carried out on it at a future date.
What changes in the metal
In order to discuss heat treating it is important to understand the structure and phases of metals and alloys.
- Grain Structure – The arrangement of atoms in a metal.
- Grain Size – The size of the individual crystals of metal. Large grain size is generally associated with low strength, hardness, and ductility.
Steel is composed of crystals which have defined structures determined by the arrangement of its atoms. Within Iron (the base element of steel) are 2 common crystal structures:
Body-centered-cubic (BCC) and Face-centered-cubic (FCC). In the example of an FCC structure, it is able to absorb higher quantities of Carbon than in a BCC structure. This is because of an increase in the interstitial sites where carbon can sit between the iron atoms, in other words, there are gaps for the carbon.
During the process to turn the metal (iron) into the alloy (steel), carbon is introduced. This addition of carbon interrupts the geometry of the crystal structures and increases strength, therefore the change in the crystal structure is critical to successful heat treating.
Iron-Carbon Phases
Steel exists in various phases; Ferrite, Austenite, and Cementite. The Ferrite is the low carbon version, cementite is the high carbon steel and austenite sits between and occurs only above 1333°F.
Ferrite at room temperature has a BCC structure which will only allow it to absorb low amounts of carbon. The unabsorbed carbon separates away from the BCC structure to form carbides which join together to form an extremely hard crystal structure called cementite.
However, when the ferrite is heated up to high temperatures above 1333°F the structure changes from BCC to FCC, therefore allowing more carbon to be absorbed, this is called austenite. The austenite phase allows the cementite to dissolve into austenite if this is allowed to cool slowly the carbon will separate out of the ferrite and the structure will change back from FCC to BCC. The small pockets of cementite will reform within the ferrite and the steel will revert to having the same properties it did prior to being heated.
If however the steel is cooled rapidly or quenched in oil, water, etc, the carbon contained within will not have time to exit the cubic structure and will be trapped within it. This creates something called martensite, the microstructure which produces the most sought after of mechanical properties in steel fasteners.
Types of Heat Treating
Heat treating can affect a number of different aspects of the metal including strength, hardness, toughness, machinability, formability, ductility, and elasticity. It can also affect the physical and mechanical properties of metal to change the use of the metal or alter future work on the metal.
Due to the large variety of metal and alloys as well as service requirements, there is a large variety of heat treating processes. This guide to heat treating would not be complete without the actual types of heat treating:
Hardening
In this process, the metal is heated to a temperature which causes the elements in the metal to switch to being a solution. Prior to these defects in the crystal lattice structure of the metal are the main source of plasticity. The heat-treating process addresses these deficiencies in the steel by making the metal into a reliable solution with fine particles to strengthen the metal. Once this phase is achieved the solution is rapidly quenched to trap the particles in the solution.
Anealing
In the annealing process, various metals such as aluminum, steel, copper, brass or silver are heated to a set temperature and held there for a certain amount of time so that a transformation occurs, it is then air-cooled.
The process is used to increase the ductility of the metal and to decrease the hardness so that the metal can be worked more easily. Most of the metals can be cooled either quickly or slowly but the ferrous metals such as steel must be cooled gradually. Annealing is also used to make the metal more stable, allowing harder metals to be machined without the risk of cracking.
Case Hardening
This process allows the surface of the metal to be hardened while keeping the interior soft. Low carbon iron and steel has to have carbon-infused into the surface. Case hardening is used as a final step after the part has been machined. The process itself used high heat in combination with various elements and chemicals to achieve the hardened layer.
Typical hardening can make metals more brittle, that is when case hardening comes into its own when an application requires a flexible inner layer with a durable outer layer.
Tempering
Tempering is a process by which heat treating is used to increase resilience in iron-based alloys such as steel. The natural properties of iron-based metals make them very hard but also brittle, in fact, they may be too brittle for the most common uses. By tempering the metal, it’s hardness, ductility and strength can be changed to make it easier to machine.
The metal is heated to a temperature just below the critical point as this will reduce the brittleness of the metal whilst still maintaining its hardness. A higher temperature is used to make a more ductile metal.
Yet another option is the purchase of metal which has already been hardened or to harden the material prior to machining. Though this makes the part more difficult to machine, it minimizes the risk of the final part changing in size and away from agreed tolerances. This also eliminates the need to further finish the part through grinding.
Normalizing
This is a type of annealing process for steel which heats it to 150-200°F higher than in typical annealing. The temperature is held for long enough to allow the transformation to occur. Any steel treated using this method has to be air-cooled. The heat-treating creates small austenitic grains whilst the air cooling produces more refined ferritic grains. The normalization process improves the strength, machinability, and durability of the steel.
