Heat treatment process of steel:

It is possible to change the properties of steel by heating and cooling the steel under controlled conditions. Steel heat treatment is used to refer to the process of heating and cooling solid steel to change the structural or physical properties of steel.

In the heat treatment process, the heating and cooling of the steel is carried out according to a strictly predetermined temperature schedule, resulting in structural changes in steel and specific mechanical properties.

Following are the purposes of the heat treatment of steel:

(i) to alter magnetic properties of steel,
(ii) to change the structure of steel,
(iii) to increase resistance to heat and corrosion of steel,
(iv) to increase surface hardness,
(v) to make steel easily workable, and
(vi) to vary strength and hardness.

The principal processes involved in the heat treatment of steel are as follows:

(1)Annealing
(2)Case hardening
(3) Cementing
( 4·) Hardening
( 5) Nitriding
(6) Normalising
(7) Tempering.

(1) Annealing:

The main purpose of this process is to soften the steel so that it can be easily worked out by the machine.

Annealing also causes the following effects:

(i) refinement of grain without serious Joss of ductility, and

(ii) release of internal stresses developed during previous operations in manufacturing.

Following is the procedure for annealing:

(i) Annealed steel is heated to the desired temperature. The temperature depends on the carbon content of the steel and ranges from about 50 ° C to 55 ° C above the critical temperature. The table shows the temperature to be kept during the annealing of the phases) with different carbon contents.

TABLE

ANNEALING TEMPERATURE

Sr.No. Range of annealing temperature Carbon content of sted

1. 871 to 925 Below 0·12 %
2. 843 to 870 0· 13 to 0·29 %
3. 816 to 842 0·30 to 0·49 %
4. 788 to 815 0·50 to l ·00 %

(ii) Once the desired temperature is achieved, the steel is held in annealing heat until it is completely heated. The time required to maintain annealing temperature depends on the type of furnace, the nature of the work, etc. Generally, this time can be referred to as enough time to dissolve carbon and diffuse through the material.

(iii) Then the steel is allowed to cool slowly in the furnace in which it was heated.

( 2) Case hardening:

In this treatment, the core of the specimen stays hard and soft and, at the same time, the surface hardens. Such a result is achieved by increasing the carbon content on the surface.

Following is the procedure of case hardening:

(i) The article to be carburized is held in a carburizing mixture at specific times and at a specific temperature. The timing and temperature depend on the required case depth and the composition of the steel. Its duration ranges between 6 and 8 hours and the normal temperature range is 900 ° C to 950 ° C.

(ii) After carburizing, the article is treated in the following ways:

(a) It is quenched directly from a box at carburizing temperature.

(b) It is cooled in the carburizing box and then reheated and cooled.

The above is a general process of hardening the case. Various useful case hardening processes such as sintering, induction hardening, nitriding, flame hardening have been developed.

Depth of case hardening :

Following factors affect the depth of case hardening:

( i) period of treatment,
(ii) the quality and nature of carburizing mixture, and
(iii) temperature of the furnace.

It is observed that at higher temperatures of the furnace, the depth of case hardening is more. Further, if the period of treatment is about 4 to 6 hours, the depth of case hardening is about 0·50 mm to 1 mm and if the period of treatment is increased around 18 hours or so, the depth may be about 3 mm or so.

carburizing mixtures:

The following are the carburizing used only in the case process

(i) animal charcoal,
(ii) bone and horn parings,
(iii) cyanides,
(iv) finely cut leather pieces, and
(v) wood charcoal and soda ash, the proportion is 95% and 5% respectively. The last one is more commonly used. Animal charcoal is also sometimes referred to as nitrogen contained in it helps the carbon to unite more rapidly with iron.

Precautions in case of hardening:

The following precautions should be taken during the case hardening process:

(i) If the articles are alloy steels, they must be quenched in oil.

(ii) Quenching should be carried out in the water. But for articles with uneven or uneven shapes or thicknesses, oil sanding should be adopted.

(iii) The article shall be kept in such a manner as to extend freely in all directions.

(iv) The article to be processed must be clean and free from dirt, grease, oil, corrosion, etc.

(v) The box in which the process is to be carried out must be cemented with a fire mud. It should be seen that the air is completely excluded from the box.

(vi) The thickness of the carburizing layer should be at least 25 mm around the article.

( 3) Cementing:

In this process or technique, the steel skin is saturated with carbon. This process involves heating the steel in a carbon-rich medium between 880 ° C to 950 ° C.

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(4) Hardening:

The purpose of this process is the reverse of the annealing process. Steel must be hardened by this process but it is softened by the annealing process. The process of hardening is the same as that of annealing, except that the cooling rate varies.

During the hardening process, cooling should be performed at a controlled rate. Such a controlled rate of cooling is called quenching

Following are the mediums of quenching:

( i) Air:

The hot material is allowed to cool in constant air. Mild quenching is obtained from this medium.

(ii) Oil:

The hot substance is left in the oil to cool. Quenching in oil is very slow. But the rapid expansion of the article helps prevent the quenching cracks that have developed.

(iii) Water:

This is usually the quenching medium. The hot substance is left in the water to cool. It is used for carbon steels and medium carbon low alloy steels.

( 5) Nitriding:

The process of saturating the surface layer of steel with nitrogen is called nitriding. Heating in the atmosphere of ammonia is carried out between 500 ° C to 700 ° C. The thickness of the nitriding layer can vary from 0 · 01 mm to l · 00 mm. The treatment hardens the steel and increases its resistance to corrosion, wear, and fatigue.

( 6) Normalising:

The purpose of this process is to normalize the steel structure and it is adopted when the steel structure is seriously disturbed for any reason. This process makes the material reasonably smooth without seriously affecting its strength.

Following is the procedure of normalizing:

( i) Steel is heated, the usual temperature range being 843°C to 954°C.

(ii) It is then allowed to cool in the air. As cooling becomes more rapid, less time is available to achieve equilibrium and, as a result, the material hardens than fully annealed steel.

(7)Tempering:

This process is applied to hardening process treated steels.

This process achieves the following objects:

(i) It develops the desired combination of hardness and ductility.
(ii) It relieves high residual stresses developed during the hardening process.

Following is the procedure adopted for tempering:

(i) After cooling in the hardening process the article is re-heated to the appropriate temperature. This temperature should be below the critical temperature.

(ii) Temperature is maintained for a given period. The duration period depends on the quality of the steel required and the composition of the steel.

(iii) Then the article is allowed to cool in steady air.

Also Read: Mechanical treatment of steel

Heat treatment process of steel | Detailed explanation