What is Heat Treatment?
Heat treatment is the process of heating and cooling metals, using specific predetermined methods to obtain desired properties. Both ferrous, as well as non-ferrous metals, undergo heat treatment before putting them to use.
Over time, a lot of different methods have been developed. Even today, metallurgists are constantly working to improve the outcomes and cost-efficiency of these processes. For that, they develop new schedules or cycles to produce a variety of grades. Each schedule refers to a different rate of heating, holding, and cooling the metal.
These methods, when followed meticulously, can produce metals of different standards with remarkably specific physical and chemical properties.
The Benefits of Heat Treatment
There are various reasons for carrying out heat treatment. Some procedures make the metal soft, while others increase hardness. They may also affect the electrical and heat conductivity of these materials.
Some heat treatment methods relieve stresses induced in earlier cold working processes. Others develop desirable chemical properties to metals. Choosing the perfect method really comes down to the type of metal and the required properties.
In some cases, a metal part may go through several heat treatment procedures. For instance, some superalloys used in the aircraft manufacturing industry may undergo up to six different heat-treating steps to optimize them for the application.
Heat Treatment Process Steps
In simple terms, heat treatment is the process of heating the metal, holding it at that temperature, and then cooling it back. During the process, the metal part will undergo changes in its mechanical properties. This is because the high temperature alters the microstructure of the metal. And microstructure plays an important role in the mechanical properties of a material.
The final outcome depends on many different factors. These include the time of heating, time of keeping the metal part at a certain temperature, rate of cooling, surrounding conditions, etc. The parameters depend on the heat treatment method, type of metal, and part size.
Over the course of this process, the metal’s properties will change. Among those properties are electrical resistance, magnetism, hardness, toughness, ductility, brittleness, and corrosion resistance.
As we already discussed, the microstructure of alloys will change during heat treatment. Heating is carried out in line with a prescribed thermal profile.
An alloy may exist in one of three different states when heated. It may either be a mechanical mixture, a solid solution, or a combination of both.
A mechanical mixture is analogous to a concrete mixture where cement binds sand and gravel together. Sand and gravel are still visible as separate particles. With metal alloys, the mechanical mixture is held together by the base metal.
On the other hand, in a solid solution, all the components are mixed homogenously. This means that they cannot be identified individually even under a microscope.
Every state brings along different qualities. It is possible to change the state through heating according to the phase diagram. The cooling, though, determines the final outcome. It is possible for the alloy to end up in one of the three states, depending solely on the method.
During the holding or soaking stage, the metal is kept at the achieved temperature. The duration of that depends on the requirements.
For example, case hardening only requires structural changes to the surface of the metal in order to increase surface hardness. At the same time, other methods need uniform properties. In this case, the holding period is longer.
The soaking time also depends on the material type and part size. Larger parts need more time when uniform properties are the objective. It just takes longer for the core of a large part to reach the required temperature.
After the soaking stage is complete, the metal must be cooled in a prescribed manner. At this stage, too, structural changes occur. A solid solution on cooling may stay the same, become a mechanical mixture completely or partially, depending on various factors.
Different media such as brine, water, oil or forced air control the rate of cooling. The sequence of cooling media named above is in decreasing order of effective rate of cooling. Brine absorbs heat fastest, while air is the slowest.
It is also possible to use furnaces in the cooling process. The controlled environment allows for high precision when slow cooling is necessary.
Heat Treatment Methods
There are quite a few heat treatment techniques to choose from. Every one of them brings along certain qualities.
The most common heat treatment methods include:
- Stress relieving
In annealing, the metal is heated beyond the upper critical temperature and then cooled at a slow rate.
Annealing is carried out to soften the metal. It makes the metal more suitable for cold working and forming. It also enhances the metal’s machinability, ductility, and toughness.
Annealing is also useful in relieving stresses in the part caused due to prior cold working processes. The plastic deformations present are removed during recrystallization when the metal temperature crosses the upper critical temperature.
Metals may undergo a plethora of annealing techniques such as recrystallization annealing, full annealing, partial annealing, and final annealing.
In this process, the metal is heated to a temperature that is 40° C above its upper critical temperature.
This temperature is higher than the one used for hardening or annealing. After holding it at this temperature for a designated period of time, it is cooled in air. Normalizing creates a uniform grain size and composition throughout the part.
Normalized steels are harder and stronger than annealed steel. In fact, in its normalized form, steel is tougher than in any other condition. This is why parts that require impact strength or need to support massive external loads will almost always be normalized.
The most common heat treatment process of all, hardening is used to increase the hardness of a metal. In some cases, only the surface may be hardened.
A workpiece is hardened by heating it to the specified temperature, then cooling it rapidly by submerging it into a cooling medium. Oil, brine, or water may be used. The resulting part will have increased hardness and strength, but the brittleness increases too simultaneously.
Case hardening is a type of hardening process in which only the outer layer of the workpiece is hardened. The process used is the same but as a thin outer layer is subjected to the process, the resultant workpiece has a hard outer layer but a softer core.
This is common for shafts. A hard outer layer protects it from material wear. When mounting a bearing to a shaft, it may otherwise damage the surface and dislocate some particles that then accelerate the wearing process. A hardened surface provides protection from that and the core still has the necessary properties to handle fatigue stresses.
Ageing or precipitation hardening is a heat treatment method mostly used to increase the yield strength of malleable metals. The process produces uniformly dispersed particles within a metal’s grain structure which bring about changes in properties.
Precipitation hardening usually comes after another heat treatment process that reaches higher temperatures. Ageing, however, only elevates the temperature to medium levels and brings it down quickly again.
Some materials may age naturally (at room temperature) while others only age artificially, i.e. at elevated temperatures. For naturally ageing materials, it may be convenient to store them at lower temperatures.
5. Stress Relieving
Stress-relieving is especially common for boiler parts, air bottles, accumulators, etc. This method takes the metal to a temperature just below its lower critical border. The cooling process is slow and therefore uniform.
This is done to relieve stresses that have built-in up in the parts due to earlier processes such as forming, machining, rolling, or straightening.
Tempering is the process of reducing excess hardness, and therefore brittleness, induced during the hardening process. Internal stresses are also relieved. Undergoing this process can make a metal suitable for many applications that need such properties.
The temperatures are usually much lower than hardening temperatures. The higher the temperature used, the softer the final workpiece becomes. The rate of cooling does not affect the metal structure during tempering and usually, the metal cools in still air.
In this heat treatment process, the metal is heated in the presence of another material that releases carbon on decomposition.
The released carbon is absorbed into the surface of the metal. The carbon content of the surface increases, making it harder than the inner core.
What Metals Are Suitable for Heat Treating?
Although ferrous metals account for the majority of heat-treated materials, alloys of copper, magnesium, aluminum, nickel, brass, and titanium may also be heat treated.
About 80% of heat-treated metals are different grades of steel. Ferrous metals that can be heat treated include cast iron, stainless steel, and various grades of tool steel.
Processes like hardening, annealing, normalizing, stress relieving, case hardening, nitriding, and tempering are generally done on ferrous metals.
Copper and copper alloys are subjected to heat treatment methods such as annealing, ageing and quenching.
Aluminum is suitable for heat treatment methods such as annealing, solution heat treating, natural and artificial ageing. Heat treatment for aluminum is a precision process. Process scope must be established and it should be controlled carefully at each stage for the desired characteristics.
Evidently, not all materials are suitable for the forms of heat treatment. Similarly, a single material will not necessarily benefit from each method. Therefore, every material should be studied separately to achieve the desired result. Using the phase diagrams and available information about the effect the aforementioned methods have is the starting point.
What is heat treatment?
Heat treatment is the process of heating metal without letting it reach its molten, or melting, stage, and then cooling the metal in a controlled way to select desired mechanical properties. Heat treatment is used to either make the metal stronger or more malleable, more resistant to abrasion, or more ductile.
What is heat treatment examples?
For example, aluminum alloy automotive castings are heat-treated to improve hardness and strength; brass and bronze items are heat-treated to increase strength and prevent cracking; titanium alloy structures are heat-treated to improve strength at high temperatures.
What are the two types of heat treatment?
Following are the different types of heat treatment processes:
- Induction Hardening.
- Flame Hardening.
Why do we do heat treatment?
There are many advantages of heat treatment, including It can change a material’s physical (mechanical) properties and it aids in other manufacturing steps. It relieves stress, making the part easier to machine or weld. Increases strength, making the material ductile or more flexible.
How many types of heat treatment are there?
In this post, we’ll cover the four basic types of heat treatment steel undergoes today: annealing, normalizing, hardening, and tempering.
What are the 3 stages of heat treatment process?
Stages of Heat Treatment
- The Heating Stage.
- The Soaking Stage.
- The Cooling Stage.
What are the five basic heat treatment process?
Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing, and quenching.
How does heat treatment work?
In simple terms, heat treatment is the process of heating the metal, holding it at that temperature, and then cooling it back. During the process, the metal part will undergo changes in its mechanical properties. This is because the high temperature alters the microstructure of the metal.
What is quenching and tempering?
Quenching and tempering are processes that strengthen materials like steel and other iron-based alloys. These processes strengthen the alloys by heating the material while simultaneously cooling in water, oil, forced air, or gases such as nitrogen.
What is steel and alloy of?
Fundamentally, steel is an alloy of iron with low amounts of carbon. There are thousands of different types of steel that are created to suit different kinds of applications. These broadly fall into 4 types – carbon steel, tool steel, stainless steel, and alloy steel.
What is annealing in heat treatment?
annealing, treatment of a metal or alloy by heating to a predetermined temperature, holding for a certain time, and then cooling to room temperature to improve ductility and reduce brittleness.
What are the advantages of tempering?
Tempering helps to relieve stress making the metal easier to weld or machine. Increases strength while making the material more flexible and ductile. Increases hardness and introduces wear-resistant properties to the surface or through the entire metal.
What are types of annealing?
Some of the Different Types of Annealing Process:
- Complete Annealing. With this method, steel parts are heated until they’re roughly 30°C hotter than their critical transformative temperature.
- Isothermal Annealing.
- Spherical annealing.
- Recrystallization Annealing.
- Diffusion Annealing.
Do you normalize before heat treating?
Normalizing Process Overview. Normalizing heat treatment helps to remove impurities and improve ductility and toughness. During the normalizing process, the material is heated to between 750-980 °C (1320-1796 °F).
What is the main purpose of annealing?
The main advantages of annealing are in how the process improves the workability of material, increasing toughness, reducing hardness, and increasing the ductility and machinability of a metal.
What is soaking time in heat treatment?
Soak time is the amount of time the steel is held at the desired temperature, which is in this case 1500 degrees Fahrenheit. When the soak time is complete, very quickly but carefully take the sample out with tongs.
What is the purpose of hardening?
Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain.
Does heating metal weaken it?
This simple act, if heated to an exact temperature range, can create a more pure, hard metal. It’s often used to create steel that is stronger than annealing the metal, but also creates a less ductile product. So, heat can indeed make metal weaker.
What is the difference between tempering and hardening?
Hardening or quenching is the process of increasing the hardness of a metal. Tempering is the process of heating a substance to a temperature below its critical range, holding, and then cooling.
What happens if you over temper steel?
With high enough temperature and long enough time, the steel will be softer than if you didn’t quench it at all and just let it slowly cool down. So depending on what your goal is you can absolutely over-temper a blade. It will make the blade softer but also less brittle.
Why tempering process is mandatory after quenching?
It is mandatory to temper the steel after it has been hardened. This is simply because a new phase has been created, which is the martensite. Remember that it is necessary to progress into the austenite phase before martensite can be created.
Why tempering is required after quenching?
After being quenched, the metal is in a very hard state, but it’s brittle. The steel is tempered to reduce some of the hardness and increase ductility. It’s heated for a set period of time at a temperature that falls between 400° F and 1,105° F.
What is brass an alloy of?
brass, an alloy of copper and zinc, of historical and enduring importance because of its hardness and workability.
What is the hardening process?
The hardening process consists of heating the components above the critical (normalizing) temperature, holding at this temperature for one hour per inch of thickness cooling at a rate fast enough to allow the material to transform to a much harder, stronger structure, and then tempering.
Why induction hardening is required?
Induction hardening is a heat treatment process carried out to enhance the mechanical properties in a localized area of a ferrous component. The resultant hardened area improves the wear and fatigue resistance along with strength characteristics.
What materials can be flame hardened?
Flame hardening is performed on parts made of mild steel, alloy steels, medium carbon steels, and cast iron. As its name suggests, flame hardening uses direct heat from oxy-gas flames.
How do you do temper process?
Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air.