Types of Heat Treatment of Steel and What is Heat Treatment?
Firstly, the question that we need to ask is “What is Heat Treatment?” Some people don’t know heat treatment, it’s not something that is related to medical.
So, what is heat treatment?
Heat treatment is a process where the heating and the cooling of metals, by using specific methods that we can require any desired properties whether how hard or how soft the metal should be.
There are considered “ferrous and non-ferrous metals” that underwent heat treatment before going for the end product.
Up until now, there’s a lot of different methods that have been developed. Even up until this day, metallurgists are still working to improve the cost-efficiency of these processes.
They have developed the term called “cycles” to produce various grades that require a different rate of heating, holding and cooling of the metal.
Whether it is a heat treatment of steel, aluminium, copper and etc, they will go under heat treatment to make the metal alloys harden or should I say “better” to serve your applications.
Why Heat Treatment?
There are few reasons when considering heat treatment and it is a necessary process to produce different kinds of metal.
Some procedures made the metal soft and others can even make their hardness increased!
Other than that, these types can also affect the electrical and heat conductivity of these materials.
Heat treatment relieves the stresses that are induced earlier during cold working processes.
Some develop desirable chemical properties to metals.
So, in choosing the right method of heat treatment really rely on what type of metal and the required properties to reach the goal.
Sometimes, a metal part needs to go to many heat treatment process such as superalloys that are used in the aircraft manufacturing industry.
They need to undergo up to 6 different heat treating procedure for it to perfect for the application.
Here are 5 Types of Heat Treatment:
- Case Hardening
Hardening is a way of heat treatment where we need to harden the metal.
The metal is heated with a temperature where the elements in the metal become a solution.
The crystal lattice structure of the metal is its the primary source to “give”. By doing heat treatment, those deficiencies can turn into a reliable solution with fine particles that can strengthen the metal.
So, heat treating can utilise those deficiencies by bringing the metal into a reliable solution with really fine particles that its job is to strengthen the metal.
After the metal is heated thoroughly with the right temperature, it will become a solid solution.
Then, will be quickly quenched to trap the particles in the solution.
Overall, hardening made the metal alloy stronger by having those impurity particles.
2. Case Hardening
The case hardening can be referred literally by the name itself. It covers the case. Which means, the outer layer of the metal is hardened while the inner layer will stay soft.
For the metals with low carbon content like iron and steel, we have to infuse with additional carbon.
Normally, case hardening will be used as the final step after the piece have been machined.
A combination with other elements and chemicals need to use high heat to produce the hardened part for the outer layer.
This is because hardening can make the metals more brittle. This method, case hardening is useful for applications that require flexible metal and also being a durable wear layer.
The heat treatment of steel really depends on what shape or type of method you choose to serve the purpose of your application.
This part of the process where the heat treatment method requires metal to be heated in a specific temperature and held at the temperature for some time to allow transformation to occur.
Then, after that, the metal needs to be air-cooled.
The metals that can undergo this heat treatment are aluminium, copper, steel, silver and brass. The reason for this process is to increase the metal’s ductility and decrease its hardness to make it easy to work with.
Copper, silver and brass can be easily quickly cooled where ferrous metals like steel always need to be cooled gradually.
This is to allow annealing to occur or it won’t be ductile enough in the process.
The process of Annealing is used before the metal is machined to improve its stability. Creating harder material that cannot be cracked or damaged.
It’s a process for the steel where it’s heated at 150 – 200°F which is higher than the process of annealing.
The process is longer than annealing as they are held at a critical temperature to make the transformation happen. For steel that used this method must be air-cooled.
Normalizing causes smaller austenitic grains, while air cooling process more refined ferritic grains.
It improved the machinability, ductility and strength of the steel.
To remove columnar grains and dendritic segregation, you must use the standardization process during the casting.
Remember that metal failure has resulted from uncontrolled stress, so normalizing steel before any kind of hardening can help to fulfil the goal of the project.
Increasing the resilience of iron-based alloy is one of the methods for tempering. Iron-based alloys like steel are very hard and often too brittle to be used for any purposes.
The reason we do tempering is that we can change the hardness, ductility and the strength of the metal.
The metal under the tempering process will be heated with a temperature below the critical point because lower temperatures reduce brittleness but still can maintain the hardness.
To increase the plasticity of the metal with less hardness and strength, you will need a higher temperature to achieve the goal.
If you want to skip the tempering part, it’s better to purchase a hardened metal material. Even though it’s going to be hard to machine, but it eliminates the risk for the part sizes changing, unlike a post-machining heat treatment process.
Now, knowing that you’ve known the different method of heat treatment. There are also phases where each phase where a metal going through a different phase when the metal either to be hardened or making it soft and brittle.
Diagrams of Phases
Heat treatment will undergo these diagrams of phases that show metal in different temperatures and different chemical compositions.
This diagram shows one of the phases of metal of its behaviour towards heat treatment. It’s important to know each metal types have a different set of phases when undergoing heat treatment.
As you can see in the x-axis shows the carbon content while the y-axis shows the temperature.
Took into account that 2.14% of carbon is the limit when steel becomes cast iron.
There are various regions where metal exists in different microstates. These regions are as follows: A1, A2, A3 and Acm.
Along with these phases, the carbon content value can pass through.
A1: The upper limit of the cementite/ferrite phase.
A2: the limit where iron loses its magnetism. This is where “Curie Temperature” occurs where metal loses it’s magnetism.
A3: The interface that separates the Austenite + Ferrite phase from the y (Gamma austenite phase)
Acm: The interface that separates y Austenite from Austenite + Cementite field.
Since you know the types of heat treatments mentioned above. There are other commons heat treatments method that you should know about.
Most Common Heat Treatment Methods:
They bring certain qualities and serve a different purpose to reach the goal for the applications.
Heat Treatment methods:
— Stress relieving
What Types of Metals That Are Suitable for Heat Treating?
If you think that most metal can, then you might be wrong. Not all metals are suitable but the most common ones do.
And I know you know the names, coming from an industry background.
The majority of ferrous metals that can be treated are; alloys of copper, magnesium, nickel, brass, titanium and aluminium.
So, there’s around 80% of the metals that can undergo heat treatment. Ferrous metals that can be heat treated include cast iron, stainless steel and as well as other grades of steel.
Processes like hardening, annealing, normalising, stress relieving, case hardening, nitriding, and tempering are generally done on ferrous metals.
Aluminium is suitable for annealing, heat treating and ageing. Heat treatment for aluminium is a precision process. It must be controlled carefully during each stage of the process.
Lastly, not every material is suitable for heat treatment. There are certain methods for each metal to follow their specific steps of heat treatment. Every material should be identified as a separate material as they are only suitable for the specific method to achieve the desired result.
Using the phase diagrams above may help in terms of planning and notice the effect of each method.