Lesson 3. Heat Treatment of farm equipment

Heat Treatment of Steels:

Properties of steel can be modified or altered by processes of heating and cooling. This is called “heat treatment”. Various heat treatments include hardening, tempering, annealing, case hardening etc

Glossary of Terms:





Heat Treatment

Combination of heating and cooling operations in a prescribed manner (w.r.t. time, temperature and rate of heating & cooling) to induce desired properties in metals and alloys in the solid state. The conventional heating for hot working does not come within the scope of heat treatment.


Rate of cooling

Temperature decrease per unit time



Rapid cooling


Full annealing

Heating to and holding at some temperature above the transformation range, followed by cooling slowly through the transformation range.

In the manufacture of forgings and castings, enormous internal stresses are set-up either by the actual forging forces or by unequal contraction on cooling. If not removed, these stresses may lead to cracking of castings on cooling. Annealing consists of heating the parts to about 900°c for half-an-hour and then taking them out of the furnace and allow the oven to cool down very gradually. This process is carried out before hardening if that is also required. It effectively disposes of the internal stresses in materials.


Case hardening

Hardening the surface by changing its composition followed by, if necessary, suitable heat treatment.

Wrought iron and carbon steel (c less than 0.5%) are relatively soft and can’t be appreciably hardened by ordinary heat treatments. They are hardened by case hardening. By this process, it is possible to produce articles that are both resistant to shocks and extremely hard at the surface. The objects to be hardened are packed in iron-boxes with carbonaceous material like animal charcoal or 3:1 wood charcoal + powered bones and are heated to about 900°c for about 5-10 hours according to depth of hard casing required. The carbon combines with iron of the material producing hard cement Fe3c. When the parts are cooled they are unpacked from iron boxes and reheated alone to about 800°c and quenched in cold water to harden the cases. There are many other case hardening methods.


Carburizing (case carburizing)

A process of introducing carbon into the surface of  solid pieces of steel by heating and holding above the transformation temperature in contact with a suitable carbonaceous medium.


Flame hardening

Rapid healing of the surface by means of an oxy-gas flame to temperature above the transformation range or cold working.



Any process which increases the hardness, for example, quenching from or above the transformation range or cold working. When steel with sufficient carbon (c more than 0.5%) is heated to 850°c (red colour) and quenched in water, brine (salt water) or oil, the steel is hardened. Higher the carbon content (up to 1.2%), the more the hardening effect. But after hardening the material becomes very brittle and it must be tempered to withstand the shock loads.

For given steel, more rapid the cooling greater is the hardness and more the brittleness. Oil has slower quenching effect than water, brine cooling is slower than water and air is the slowest, 


Induction hardening

Process of hardening by induction heating to the appropriate temperature and quenching in a suitable medium.



A process of surface hardening by introducing nitrogen into the surface in a suitable steel by heating and holding it at appropriate temperature in contact with cracked ammonia or other suitable nitrogenous medium.



A process of heat treatment for improving mechanical properties brought about by grain refinement and uniformity in structure. The process involves heating to and often holding for a specified time at a suitable temperature above the transformation range, followed by cooling freely in air.


Stress relieving (stabilizing)

Heating to, and if necessary, holding at a sufficiently high temperature below the transformation range followed by slow cooling to remove internal stresses only. Also, called stabilizing treatment.



Heating to elevated temperature but below transformation zone, of hardening steels and holding for specified time at temperature followed by slow cooling to develop desired mechanical properties in these steels. It consists of reheating of the hardened steel parts to about 500°c (blue colour) and quenching them from this temperature. In this way the material losses most of its brittleness and bit of its hardness.

Hardening and tempering of the parts of farm machines has become an important process in their manufacture. Steel plough shares, cultivator points teeth of rakes are all heat treated under carefully controlled conditions in order to produce the necessary resistance to abrasion or bending.

Different Case Hardening Methods:


What is done

Temp. to which heated

Case Hardening (2 mm depth)

Surface layer or case is hardened

Material is heated to critical temperature (800°c), exposed to carbon rich atmosphere, cooled and reheated to 200-500°c)


Steel is packed in pitch of charcoal

800°c for long period quenched and tempered


Finished heat treated steel parts are kept in an air-tight box

Heated to 800°c and NH3 is injected into the chamber



Heated to 800°c and carbon monoxide and ammonia are injected


Steel is dipped into molten bath of sodium cyanide for a short-time C&N are absorbed by the steel


Induction Hardening

High frequency alternating current is passed for a short period. Current is induced on the surface which causes localized heating. After heating surface is flooded with water to quench and harden the part


Flame Hardening

Oxy-acetylene torch is used to heat the surface quickly. Then quenched with water.

Above initial range 900°c

Last modified: Tuesday, 18 March 2014, 10:29 AM