Heat Treatment Process – Explanation
The mechanical properties of a metal or an alloy can be enhanced or molded to our desired requirements by the application of a manufacturing process called heat treatment. Heat treatment is employed during different stages of the manufacturing process to acquire desired characteristics. It can be used to make the specimen mechanically strong or hard. If performed under a controlled atmosphere, it can make the specimen more ductile and malleable. Sometimes, metalworking like welding or forging can cause the specimen to alter its properties, consequently, heat treatment can be used to restore the properties of the specimen to pre-work conditions.
An important consideration while altering the mechanical properties of a metal or an alloy is the fact that a compromise should be made between different preferred properties. Generally, enhancing one property results in the loss of another desirable attribute. For example, making the specimen stronger or harder can result in loss of ductility and as a result, the material will become more brittle. Similarly, stress removal will make the material more ductile yet less strong.
Heat Treatment Process – Working
Metals and their alloys have a crystalline structure, composed of small units called a crystal lattice. These lattices grow to form crystal grains. The orientation and size of these grains are critical to the mechanical and physical properties of the material. Smaller grain sizes produce more resilient, more strong, and tougher specimen while larger grain sizes will result in a ductile material with lower tensile strength. Therefore, controlling the environment of the manufacturing process can enable us to manipulate the growth of grain and consequently, the properties of our material. This manipulation can be done through a heat treatment process which involves heating the specimen of a metal or an alloy to extreme temperatures, sometimes as high as 2400F but usually below its melting point. This heated material is then kept at this high temperature for some time and allowed to cool. Thus, carefully monitoring and controlling the heat treatment process can result in a material with desirable characteristics.
Heat Treatment Process – Stages
To obtain the required properties, the heat treatment process must be carefully carried out. The parameters governing the success of the heat treatment process are the three stages involved in the process.
The first stage in the heat treatment process is heating the material to a specific high temperature. The heating process is important as it results in a change in the microstructure of the material and as a result, a change in properties occurs. Heating must be done slowly and gradually to remove any temperature gradient across the material specimen. If performed rapidly, thermal shocks can appear in the lattice, making it brittle and susceptible to failure. Heating requirements depend upon several factors and vary from specimen to specimen. Some of the factors influencing heating temperatures are:
- The conductivity of material: the higher the conductivity of the material, the more rapidly will the heat transfer occur.
- The existing condition of the material: the specimens that are pre-worked like welded or forged will have enhanced thermal stresses and therefore need slower heating.
- The geometry of specimen: irregular cross-sectioned specimen requires slower cooling than a small or regular geometry specimen.
The next stage in the heat treatment process is the soaking stage. This involves keeping the heated specimen at a high temperature for a certain period. The soaking stage is crucial to the final mechanical properties of the material since it is during this stage the internal crystal lattice and grain take their final shape. The microstructure of crystalline metal or alloy grows depending upon the time the material is allowed to soak.
Soaking times are also dependent upon the geometry of the material being subjected to the heat-treatment process. Materials with larger cross-sections require a longer soaking period to allow heat diffusion throughout their crystal lattice. On the contrary, materials with smaller areas and regular geometries do not need a long time for soaking.
The next and final stage in heat treatment of a metal or an alloy is bringing the heated material back to normal room temperature. This step is also crucial to the final properties of the material and should be carried carefully. Cooling rates determine the hardness, strength, and ductility of the final product. If performed rapidly, a harder and stronger material is achieved but ductility is compromised. However, slower cooling rates can provide a ductile material, but the strength is at lower levels.
Another important consideration is the cooling medium. Cooling can be achieved through the introduction of cooling mediums like oil, water, or brine, or it can be performed by the action of air currents. The selection of a cooling medium is also dependent upon the type of material being subjected to the heat-treatment process, in addition to the desired final properties. An inappropriate cooling medium can result in warping and cracking of the material. Therefore, extensive research should be carried out before selecting the coolant for your material.
Heat Treatment Process – Techniques
Controlling the abovementioned stages can result in different properties and thus to obtain a material with desirable properties, there are various techniques of heat treatment which are mentioned below:
Annealing is the heat treatment process that is usually carried out at temperatures above the crystallization temperature, followed by cooling at slow rates usually by air. This method is generally preferred when ductility of a metal or an alloy is desired, and its hardness is to be lowered. This prepares a metal or its alloy for cold working. Further, the stress and dislocation generated by cold working can be removed by the heat treatment using the annealing technique.
Annealing, when performed, first breaks the existing crystal lattice and grain structure by heating beyond the crystal’s crystallization temperature and then allows the fine-grained structure to grow gradually. Thus, removing any dislocations and enhancing its ductility.
- Case Hardening
Case Hardening is another technique of heat treatment process employed to enhance the hardness of the metal or alloy. This technique, in contrast to other heat treatment processes, hardens the outer surface of the material only while maintaining the mechanical properties of the core unchanged. This technique is cost-effective and sometimes is preferred over other heat treatment methods as it provides a hardened exterior with the ductility of the core intact.
Case hardening is done by heating to high temperatures, followed by soaking but the cooling is done rapidly. This quenching technique does not allow the inner crystal structures to change their microstructure by abrupt cooling. Rapid cooling can be achieved by using quenching in a coolant medium like water or oil.
Sometimes, after the material has been hardened by quenching and rapid cooling, another heat treatment technique is used to reduce the hardness of the material to enhance its toughness. The hardening process can result in brittleness of the material that is sometimes an unwanted characteristic and can hinder the utility of the material. Therefore, tempering is used to lower the hardness and brittleness to enhance the ductility of the specimen.
Tempering is done usually by reheating the quenched material to temperatures usually below critical temperatures. This heated material is then allowed to air-cooled. As a result, hardness is reduced a little, allowing the material to be more ductile and less brittle.
- Through Hardening
Through hardening is similar to case hardening since it improves the hardness of the material. However, in contrast to the case hardening, this heat treatment process hardens the material throughout the specimen and not just the exterior.
Heat Treatment Process – Specifications
Usually, while performing the heat treatment process, the processing technique is not enough to understand the product completely. Certain specifications are needed to properly comprehend the final properties desired. These specifications are:
- Case Hardening:
Generally, while performing case hardening heat treatment process, there are two types of specifications i.e., effective case depth and total case depth.
For thinner cases after heat treatment, effective case depth specifications are used. They mention the depth of carbon diffusion from the surface. For thicker cases, total case depth is specified. This measures the distance from the exterior into the case and expresses the hardness level. These hardness levels are usually expressed on Rockwell scale B (HRB).
- Through hardening
Through hardening is usually specified by the hardness levels. These levels are usually expressed as tolerance range since achieving a uniform level of hardness is difficult to change in geometry. It makes use of the Rockwell hardness scale C (HRC) to enlist the hardness levels.
Annealing is also specified by the Rockwell hardness scale C (HRC).