In this article we are going to learn what is heat treatment process and also about the various different types of heat treatment processes in detail.
What is Heat Treatment Process ?
Heat treatment refers to a combination of heating and cooling of a metal or alloy in the solid state for the purpose of obtaining desired properties. Changes in properties result from micro structural changes in the material produced by heat treatment operations.
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.
Basics of Heat Treatment
Although iron and steel constitute the vast majority of heat treated materials, alloys of aluminum, copper, magnesium, nickel, and titanium may also be heat treated.
Heat treating processes require three basic steps. Or we can say Stage of heat treatment are :
- Heating to a specified temperature
- Holding at that temperature for the appropriate amount of time
- Cooling according to prescribed methods
Temperatures may range as high as 2400°F and time at temperature may vary from a few seconds to as many as 60 hours or more.
In the furnace, some materials are cooled slowly, while others must be quenched. Treatment at -120°F or lower is required for some cryogenic processes. Water, brine, oils, polymer solutions, molten salts, molten metals, and gases are some examples of quenching media. Each has its own set of characteristics that make it ideal for specific tasks. 90 percent of the parts, on the other hand, are quenched in water, oil, gas, or polymers.
A. Heating Stage
During the heating stage, the primary goal is to ensure that the metal heats uniformly. Slowly heating ensures even heating. If you heat the metal unevenly, one section may expand faster than another, resulting in a distorted or cracked section of metal. You select the heating rate based on the following factors:
1. The metal’s heat conductivity: Metals with a high heat conductivity heat up more quickly than those with a low conductivity.
2. The metal’s condition: Tools and parts that have previously been hardened or stressed should be heated at a slower rate than tools and parts that have not.
3. The metal’s size and cross-section: To allow the inside temperature to be close to the surface temperature, larger parts or parts with uneven cross sections must be heated more slowly than small parts. Otherwise, you risk cracking or excessive warping.
B. The Soaking Stage
The soaking stage’s purpose is to keep the metal at the proper temperature until the desired internal structure takes shape. The “soaking period” refers to how long the metal is kept at the appropriate temperature. You will need the chemical analysis and mass of the metal to determine the correct length of time. The soaking period for uneven cross-sections can be determined by using the largest section.
In general, you should not bring the metal’s temperature from room temperature to soaking temperature in a single step. Rather, slowly heat the metal to just below the temperature at which the structure will change, and then hold it there until the temperature is consistent throughout the metal. After this “preheating” step, you can quickly heat the temperature to the final temperature that you’ll require. To prevent warping, parts with more complex designs may require multiple layers of preheating.
C. Cooling Stage
During the cooling stage, you’ll want to return the metal to room temperature, but there are different ways to do this depending on the type of metal. It may require a cooling medium, such as a gas, liquid, solid, or a combination of these. The rate of cooling is determined by the metal and the medium used for cooling. As a result, the cooling options you choose have a significant impact on the desired properties of the metal.
Quenching is the rapid cooling of metal in air, oil, water, brine, or another medium. Most metals that are hardened are cooled rapidly with quenching, so quenching is usually associated with hardening; however, quenching or other rapid cooling does not always result in hardening. Copper, for example, is annealed using water quenching, and other metals are hardened using slow cooling.
Purpose of Heat Treatment
However, these serve one or more of the following purposes :
- Improve machinability
- Relieve internal stresses.
- Improve mechanical properties such as ductility, strength, hardness, toughness, etc.
- Change the grain size.
- Increase resistance to heat and corrosion.
- Modify electrical and magnetic properties.
- Change the chemical composition.
- Remove gases
Advantages of Heat Treatment
After heat treatment, materials are :
- More durable product.
- Steel becomes tougher, stronger.
- Easier to weld.
- Becomes more flexible.
- Increases its wear-resistance.
- Increase in overall lifetime of the part.
Heat treatment is done for a variety of reasons. Some procedures soften the metal, while others harden it. They may also have an impact on the electrical and thermal conductivity of these materials.
Some heat treatment methods relieve stresses caused by previous cold working processes. Others add desirable chemical properties to metals. Choosing the best method is ultimately determined by the type of metal and the required properties.
A metal part may be subjected to multiple heat treatment procedures in some cases. Some super alloys used in the aircraft manufacturing industry, for example, may go through up to six different heat treating steps to be optimized for the application.
Types of Heat Treatment Process
The aforesaid purposes of heat treatment may be served by one or more of the following processes of heat treatment :
5. Case Hardening :
6. Surface Hardening
a.) Induction hardening
b.) Flame hardening
7. Diffusion Coating
Annealing is one of the most important widely used operations in the heat treatment of steel.
Annealing is a heat treatment in metallurgy and materials science that changes the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It entails heating a material above its recrystallization temperature, holding that temperature for an appropriate amount of time, and then cooling.
The purpose of annealing is to obtain one or more of the following effects :
- Soften the steel.
- Improve machinability.
- Increase or restore ductility and toughness.
- Relieve internal stresses.
- Reduce or eliminate strucural inhomogeneity.
- Refine grain size
- Prepare steel for subsequent heat treatment.
Normalising is a heat treatment process used to make a metal more ductile and tough after it has been hardened thermally or mechanically. Normalising is the process of heating a material to a high temperature and then allowing it to cool back down to room temperature by exposing it to room temperature air after it has been heated. This heating and slow cooling changes the microstructure of the metal, lowering its hardness and increasing its ductility.
When steel is cold-structure is distorted, and the metal may be brittle and unreliable. The internal structure of hot-worked forged part may also be distorted owing to being worked at a very low temperature. It can also be seen that a variable finishing temperature will result in a variable structure for forgings of the same carbon content. Likewise, if a casting is poured at a somewhat indefinite temperature and cools at different rates in different parts, it may be unreliable. Normalizing is, therefore, used particularly for the following
The purpose of Normalising is to obtain one or more of the following effects :
- To eliminate coarse-grained structure.
- To remove internal stresses that may have been caused by working
- To improve the mechanical properties of the steel
In addition to all these purposes, it may be used to increase the strength of medium carbon steels to a certain extent (in comparison with annealed steels), to improve the machinability of low carbon steels, to improve the structure of welds, etc.
The operation of hardening is applied to all tools and some important machine parts intended for especially heavy duty service as well as to all machine parts made of alloy steel.
The purposes of hardening with subsequent tempering are :
1. To develop high hardness to resist wear and to enable it to cut other metals.
2. To improve strength, elasticity, ductility, and toughness.
The process consists of:
1. Heating the steel to a temperature above critical point.
2. Holding at this temperature for a considerable period.
3. Quenching (rapid changing) in water, oil or molten salt bath.
Tempering, in metallurgy, the process of improving the properties of a metal, particularly steel, by heating it to a high temperature, but below the melting point, and then cooling it, usually in air. The process toughens by reducing brittleness and internal stresses.
When a piece of steel is taken out of the quenching medium, as already stated, it is hard, brittle and will have severe unequally distributed internal stresses besides other unfavorable characteristics. In general, tempering restores ductility and reduces hardness and results in some decrease in hardness. The primary objects of tempering are, therefore, as follows:
- To stabilize the structure of the metal.
- To reduce internal stresses produced during previous heating.
- To reduce some of the hardness produced during hardening and to increase the ductility of the metal.
- To give the metal eight strucural condition combined with toughness and shock resistance.
The tempering treatment requires :
- Reheating the steel after hardening to temperatures below Ac1 point ( psk line in fig 6.6
- Holding bit for a considerable time.
- Slow cooling. It is desirable that the temperature of the steel shall be maintained for not less than 4 to 5 minute for each millimeter of the section.
5. Case Hardening
The oldest known method of producing a hard surface on steel is case hardening or carburising. The steel used for this purpose is usually a low carbon steel of about 0.15 per cent carbon, which does not respond appreciably to heat treatment. In course of the process, the outer layer is converted into a high carbon steel with a carbon content ranging from 0.9 to 1.2 per cent carbon. If it receives proper heat treatment, it will have are extremely hard surface on the outside and a soft ductile core.
Cyaniding is a process of producing hard surface on low carbon or medium carbon а steels by immersing the steel in a molten salt bath containing cyanide maintained at 800°C to 900°C and then quenching the steel in water or oil. The hardness produced by this treatment is due to the presence of compounds of nitrogen as well as of carbon in the surface layer.
Nitriding is a process of producing hard surface layer on alloy steels only, Nitriding consists essentially of heating the steel in an atmosphere of ammonia gas at temperature of 500°C to 650°C without further heat treatment. The ammonia is dissociated and the nascent nitrogen combines with elements in the steel to form nitrides. These nitrides give extreme hardness to the surface. A hard surface layer usually from 0.2 to 0.4 mm in depth is produced in 50 hours.
Nitriding is the last operation after shaping and heat treatment process. Thus after forging, the sequence of operations is : (a) oil hardening at 850°C to 900°C, (b) tempering at 600°C to 650°C, (c) rough machining, (d) stabilizing (to remove internal stresses) at 525°C to 550°C, (e) final machining and ultimately, (f) nitriding.
Nitriding is used on many automotive, airplane, and diesel engine wearing parts, as well as on numerous miscellaneous parts such as pump shafts, gauges, drawing dies, gears, clutches, and mandrels. Its use is limited by the expense necessary for the treatment and the comparatively thin case obtained.
6. Surface Hardening
A. Induction hardening
Induction heating has proved satisfactory for many surface hardening operations as required on the bearing areas of crankshafts, camshafts, axle shafts and similar wearing surfaces. It differs from ordinary case-hardening practice in that the analysis of the surface steel is not changed, the hardening being accomplished by an extremely rapid heating and quenching of the wearing surface which has no effect on the interior core. The hardness obtained in induction hardening is the same as that obtained in conventional treatment and depends on the carbon content.
B. Flame hardening
The process of hardening steel by heating it with the flame of an oxyacetylene torch is known as flame hardening which, like the induction hardening process, is based on rapid heating and quenching of the surface by water. The flame is directed to the desired part without heating the remainder of the work efficiently to affect it. The advantages in favour of its application are as follows:
- Because it heats quickly, flame heating is convenient when hardness is required only for a limited depth, the remainder retaining its original toughness and ductility.
- Flame heating makes it possible and practical to harden a part or all of a piece of work that is too large or too inconvenient to place in a furnace.
- The amount of time required for heating is less with flame heating than with a furnace
7. Diffusion Coating
Diffusion coating, or metallic cementation, is the process of impregnating the surface of steel with aluminium, chromium, silicon, boron, beryllium and other elements.
Diffusion coating is accomplished by heating and holding steel parts in direct contact with one of the above elements which may be in the solid, liquid or gaseous state. This process imparts a number of valuable properties to steel, among which are high heat, corrosion and wear resistance. In many cases, steel subjected to diffusion coating may be used as a substitute for a high alloy steel.
Each metal alloy has its own phase diagram. As previously stated, heat treatment is performed in accordance with these diagrams. They depict the structural changes that occur at different temperatures and chemical compositions.
Let’s use the iron-carbon phase diagram as an example because it’s the most well-known and widely taught at universities.
The iron-carbon phase diagram is a useful tool for learning about the heat treatment behavior of various carbon steels. The x-axis represents the alloy’s carbon content, while the y-axis represents the temperature.
Note that the limit at which steel becomes cast iron is 2.14 percent carbon.
The diagram depicts various regions where the metal can be found in various microstates such as austenite, cementite, and pearlite. These areas are denoted by the boundaries A1, A2, A3, and Acm. When the temperature or carbon content value passes through these interfaces, phase changes occur.
- A1: The upper limit of the cementite/ferrite phase.
- A2: The limit where iron loses its magnetism. The temperature at which a metal loses its magnetism is also called Curie temperature.
- A3: The interface that separates Austenite + Ferrite phase from the γ (Gamma) austenite phase.
- Acm: The interface that separates γ Austenite from the Austenite + Cementite field.
The phase diagram is an important tool for determining whether or not heat treatment will be beneficial. Each structure contributes different qualities to the final product, and the heat treatment is chosen accordingly.
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.
- Metal parts put into furnace
- Jet engine parts going into a furnace
- 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 requirement.
- 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.
What Metals Are Suitable for Heat Treating?
Although ferrous metals account for the majority of heat treated materials, copper, magnesium, aluminum, nickel, brass, and titanium alloys may also be heat treated.
Approximately 80% of heat treated metals are different grades of steel. Cast iron, stainless steel, and various grades of tool steel are all ferrous metals that can be heat treated.
Ferrous metals are commonly subjected to processes such as hardening, annealing, normalizing, stress relieving, case hardening, nitriding, and tempering.
Heat treatment methods such as annealing, ageing, and quenching are used on copper and copper alloys.
Aluminium is suitable for annealing, solution heat treating, natural and artificial ageing, and other heat treatment methods. Aluminium heat treatment is a precise process. The scope of the process must be established, and it must be carefully controlled at each stage to ensure that the desired characteristics are achieved.
Evidently, not all materials are suitable for the various types of heat treatment. Similarly, a single material may not benefit from all methods. As a result, each material should be studied separately in order to achieve the desired result. The starting point is to use phase diagrams and available information about the effects of the aforementioned methods.
Heat Treatment of Steels
Steel heat treatment is the heating and cooling of metals to change their physical and mechanical properties without causing them to change shape. Heat treatment is a method for strengthening materials, but it can also be used to change mechanical properties such as formability, machining, and so on.
The most common application is metallurgical, but metal heat treatment can also be used in the production of glass, aluminum, steel, and a variety of other materials.
We have tried to cover all the aspects of Heat treatment starting from what is heat treatment process, then the types of heat treatment processes including the methods such as Annealing, Case Hardening, Tempering, Normalising and many more. We also discussed advantages and disadvantages of heat treatment process.
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