This article is about the heat treatment process which used in manufacturing industries for changing some properties of the material. Heat treatment processes involve high heating of metal at some temperature and sudden cooling it using a quenching medium. In this article you will learn heat treatment processes and their classification. we will also see the Purpose of heat treatment processes, why they are carried out.

Introduction to heat treatment :

Steel and other alloys have a large number of applications in engineering practice under varying conditions, requiring different properties in them. At one place they may be subjected to bending while at the other to twisting. They may be required to withstand various types of stresses and as a tool, materials to have hardness, especially red hardness, combined with toughness along with anon-brittle cutting edge. They may be required to bear static or dynamic loads, revolve at extremely high speeds, operate in highly corrosive media, carry an extremely hard skin with a tough core, subjected to fatigue and creep, etc. Such varying conditions of their applications require these materials to possess specific properties of the required order to successfully serve under these conditions. But, a material may lack in some or all of these properties either fully or partially. These deficiencies are fulfilled through the process of heat treatment. Generally all steels can be heat treated as per need. Aluminum is the only non-ferrous metal which can be effectively heat treated.

The process of heat treatment involves heating of solid metals to specified (recrystallization)temperatures holding them at that temperature and then cooling them at suitable rates to enable the metals to acquire the desired properties to the required extents. All this takes place because of the changes in size, form, nature, and the distribution of different constituents in the microstructure of these metals. All heat treatment processes, therefore, comprise the following three stages of components:

Stages of heat treatment Process :

1. Heating the metal to a predefined temperature.

2. Holding it at that temperature for sufficient time so that the structure of the metal becomes uniform throughout.

3. Cooling the metal at a predetermined rate in a suitable media to force the metal to acquire a desired internal structure and thus, obtain the desired properties to the required extent. All this takes place because of the changes in size, form, nature, and the distribution of different constituents in the microstructure of these metals.

Read More about heat treatment process : 3 Steps Of Heat Treatment Process | Basic Of Heat Treatment

 Purpose of Heat Treatment

Metals and alloys are heat treated to achieve one or more of the following objectives:

1. To relieve internal stresses set up during other operations like casting, welding, hot and cold working, etc.

2. To improve mechanical properties like hardness, toughness, strength, ductility, etc.

3. To improve machinability

4. To change the internal structure to improve their resistance to heat, wear, and corrosion.

5. To effect a change in their grain size.

6. To soften them to make suitable for operations like cold rolling and wire drawing.

7. To improve their electrical and magnetic properties.

8. To make their structure homogenous to remove coring and segregation.

9. To drive out trapped gases.

Classification of Heat Treatment Processes

Various heat treatment processes can be classified as follows:

1. Annealing.

2. Normalizing.

3. Hardening.

4. Tempering.

5. Case hardening.

6. Surface hardening.

7. Diffusion coating.

Annealing

Annealing is indeed one of the most important heat treatment processes. The internal structure of the metal gets stabilized through this process. This heat treatment is given to the metal to achieve one on more of the following objectives:

1. To refine the grains and provide a homogenous structure.

2. To relieve internal stresses set up during earlier operations.

3. To soften the metal and, thus, improve its machinability.

4. To effect changes in some mechanical, electrical, and magnetic properties.

5. To prepare steel for further treatment or processing.

6. To drive out gases trapped during casting.

7. To produce the desired macrostructure.

Different type of annealing processes can be classified as follows:

1. Full annealing – The main objectives of this type of annealing are to soften the metal, relieve its stresses, and refine its grain structure. It is also known as high-temperature annealing. In this process complete phase recrystallization takes place and, therefore, all imperfections of the previous structure are wiped out. This involves heating of steel to a temperature about 30 degrees to 50 degrees above the higher critical point for hypereutectoid steels, and by the same amount above the lower critical point for hypereutectoid steels, holding it at that temperature for sufficient time to allow the internal changes to take place and then cooling slowly.

2. Process annealing –The purpose of process annealing is to remove the ill effects of cold working and often the metal so that its ductility is restored and it can be again plastically deformed or put to service without any danger of its failure due to fracture. It is also known as a slow temperature annealing or sub-critical annealing or commercial annealing. The process is extremely useful for mild steels and low carbon steels and is cheaper and quicker than full annealing. Also, less scale is produced during this process. The main output of this process is increased ductility and plasticity, improved shock resistance, reduced hardness, improved machinability, and removal of internal stresses. During cold working operations like cold-rolling, wire drawing, a metal gets severely strain-hardened.

3. Spheroidise annealing – The main purpose of spheroidise annealing is to produce a structure of steel that consists of globules or well-dispersed spheroids of cementite in the ferrite matrix. Following are the main methods through which the above objective can be obtained:

1. High carbon steels: Heating the steel to a temperature slightly above the lower critical point (say between 730 deg C to 770 deg C, depending upon the carbon percentage), holding it at that temperature for sufficient time and than cooling it in the furnace to a temperature 600 deg C to 550 deg C, followed by slowly cooling it down to room temperature instill air.

2. Tool steels and high-alloy steels: Heating to a temperature of 750 deg C to 800 deg C, or even higher, holding at that temperature for several hours and then cooling slowly.

4. Diffusion annealing –  The purpose of diffusion annealing is to remove the heterogeneity in the chemical composition of steel ingots and heavy castings This process is mainly used before applying full annealing to steel castings. In this process, the metal is heated to a temperature between 1100 degrees C to 1200 degree C, where diffusion occurs and grains are homogenized. The metal piece being treated is held at the diffusion temperature for a short time to allow complete diffusion and then cooled down to between 800 degrees C to 850 degrees C by keeping it inside the shut-off furnace for a period of about 6 to 8 hours. Then it is removed from the furnace and cooled in the air down to the room temperature. Then full annealing is performed.

5. Isothermal annealing – The isothermal annealing consists of heating steel to austenite state and then cooling it down to a temperature of about 630 deg C to 680 deg C at a relatively faster rate. It is followed by holding it at this constant temperature (i.e isothermal) for some time and then cooling it down to the room temperature at a rapid rate. During the isothermal holding full decomposition to pearlite structure takes place and that is why the process is known as isothermal annealing. Because of the two rapid coolings the total annealing time is considerably reduced.

Normalizing

The normalizing process is similar to annealing in sequence but varies in the heating temperature range, holding time and the rate of cooling. Heating temperature of steel is 40 deg C to 50 deg C above the higher critical point, held at that temperature for a relatively very short period of time (about 15 min.) and then cooled down to room temperature in still air. This heat treatment is commonly used as the final heat treatment for such articles which are supposed to be subjected to higher stress during operation. Due to this treatment internal stress caused during previous operations are removed, the internal structure is refined to fine grains and mechanical properties of steel are improved. This process also improves the impact strength, yield point and ultimate tensile strength of steels. As compared to the annealed steels of the same composition the normalized steels will be less ductile but stronger and harder. For improvement of the mechanical properties normalizing process should be preferred and to attain better machinability, softening and greater removal of internal stress annealing process should be employed.

The main objects of normalizing are :

1. To refine the grain structure of the steel to improve machinability, tensile strength and
structure of weld.
2. To remove strains caused by cold working processes like hammering, rolling, bending,
etc., which makes the metal brittle and unreliable.
3. To remove dislocations caused in the internal structure of the steel due to hot working.
4. To improve certain mechanical and electrical properties.

Hardening

This process is widely applied to all cutting tools, all machine parts made from alloy steels, dies and some selected machine parts subjected to heavy-duty work. In hardening process steel is heated to a temperature within the hardening range, which is 30 degrees C to 50 degrees C above the higher critical point for hypereutectoid steels and by the same amount above the lower critical point for hypereutectoid steels, holding it at that temperature for sufficient time to allow it to attain austenitic structure and cooled rapidly by quenching in a suitable medium like water, oil or salt both.

In the process of hardening the steel is developed in such controlled conditions, by rapid quenching, that the transformation is disallowed at the lower critical point and by doing so we force the change to take place at a much lower temperature. By rapid cooling the time allowed to the metal is too short and hence transformation is not able to occur at the lower critical temperature.

The process of hardening consists of

(a) heating the metal to a temperature from 30 to 50°C above the upper critical point for hypoeutectoid steels and by the same temperature above the lower critical point for hypereutectoid steels.
(b) keeping the metal at this temperature for a considerable time, depending upon its thickness.
(c) quenching (cooling suddenly) in a suitable cooling medium like water, oil or brine.

Tempering

A hardened steel piece, due to martensitic structure, is extremely hard and brittle, due to which it is found unsuitable for most practical purposes. So a subsequent treatment is required to obtain a desired degree of toughness at the cost of some strength and hardness to make it suitable for use. It is especially true in the case of the tools. This is exactly what is mainly aimed at through the tempering of steel. This process enables the transformation of some martensite into ferrite and cementite. The exact amount of martensite transformed into ferrite plus cementite will depend upon the temperature to which the metal is reheated and the time allowed for the transformation.

The process involves reheating the hardened steel to a temperature below the lower critical temperature, holding it at that temperature for sufficient time and then cooling it slowly down to room temperature.

When the hardened steel is reheated to a temperature between 100 deg C to 200 deg C some of the interstitial carbon is precipitated out from martensite to form a carbide called epsilon carbide. This leads to the restoration of the BCC structure in the matrix. Further heating to between 200 deg C 400 deg C enables the structure to transform to ferrite plus cementite. Further heating to between 400 deg C and 550 deg C leads to the nucleation and growth of a new ferrite structure, rendering the metal weaker but more ductile. If steel is heated above 550 deg C the cementite becomes spheroidised, and if heating is continued even beyond the structure will revert back to the stable martensite. As such, if a good impact strength is desired reheating should not extend beyond 300 deg to 350 deg C. The section thickness of the components being treated also has a decisive effect on the results. Heavy components and thicker sections required longer tempering times then the lighter and thinner ones.

Surface hardening or case hardening.

In many engineering applications, it is desirable that a steel being used should have a hardened surface to resist wear and tear. At the same time, it should have soft and tough interior or core so that it is able to absorb any shocks, etc. This is achieved by hardening the surface layers of the article while the rest of it is left as such. This type of treatment is applied to gears, ball bearings, railway wheels, etc.

Following are the various surface or case hardening processes utilizing which the surface layer is hardened:

1. Carburising, 2. Cyaniding, 3. Nitriding, 4. Induction hardening, and 5. Flame hardening.

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