It is possible to alter the properties of steel by heating and cooling steel under controlled conditions. The term heat treatment is used to indicate the process in which the heating and cooling of solid steel is involved to change the structural or physical properties of steel. Thus, in heat treatment process, the heating and cooling of steel are carried out according to a strictly predetermined temperature schedule with the result that the steel undergoes structural changes and acquires specific mechanical properties.
Following are the purposes of heat treatment:
(1) to alter the magnetic and electrical properties of steel,
(2) to change the structure of steel,
(3) to increase the resistance to heat and corrosion,
(4) to increase the surface hardness,
(5) to make the steel easily workable, and
(6) to vary the strength and hardness.
It may be noted that the heat treated steels, amount hardly to about 5 per cent of the total steel production. However the heat treatment is indispensable for tools and dies and a variety o special purpose of steels which are essential for the existence of the modern industry.
The heat treatment processes in general depend on the following factors:
(1) equipment for controlling the temperature;
(2) furnace employed;
(3) heating medium;
(4) methods employed in heating and cooling a given variety of steel;
(5) quenching mediums;
(6) rate of heating and cooling;
(7) variety or specification of steel; etc.
The heat treatment is generally an expensive proposition. But if it can be planned on a large scale, its cost can be justified.
The principal process involved in heat treatment of steel are as follows:
(2) Case hardening
(1) Annealing: The main object of this process is to make the steel soft so that it can be easily worked upon with a machine. The annealing reduces the tensile strength. But it increases ductility and brings back the steel to the best physical state to resis
The case hardening is important for the components like gears, bearings surfaces, etc., which require faces to be tough, shock resistant and capable of carrying high stresses. The steel used for this purpose is generally of low carbon content and does not respond appreciably to the heat treatment. The process of case hardening consists of increasing the carbon content of the case so that it will respond to the hardening and keeping the core soft and ductile.
Following is the procedure of case hardening:
(i) The article to carburized is held in the carburizing mixture for a definite time and at definite temperature. The time and temperature will depend upon the depth of case required and composition of steel. The usual period is 6 to 8 hors and the usual temperature range is 9000C to 9500C.
(ii) After carburizing, the article is treated in one of the following ways:
(a) It is quenched directly from the box at the carburizing temperature.
(b) It is cooled slowly in the carburizing box and then it is reheated and quenched.
(c) It is cooled slowly in the carburizing box and then it is reheated twice and also quenched twice.
The above is the general process of case hardening. The various other useful case hardening processes have been developed such as cyaniding, induction hardening, nitriding, flame hardening, etc. These processes adopt a specially prepared carburizing mixture and specially designed furnace.
Depth of case hardening: following factors affect the depth of case hardening
(i) period of treatment,
(ii) quality and nature of carburizing mixture, and
(iii) Temperature of furnace.