Steel components are used in many applications that require a hard and strong surface to resist wear and abrasion combined with a tough core to withstand shock and fatigue. This condition can be obtained by an alteration in the composition of the surface layer of a steel component followed by the application of a suitable “heat treatment. The surface region in which the properties and composition have been altered is known as the case, and the production of a hard surface is known as case hardening. In this process, the steel is caused to absorb such elements as carbon, nitrogen or a mixture of the two. When carbon alone is absorbed, the process is known as carburizing ; when nitrogen is added, the process is known as Nitriding, and when both are absorbed the operation is known as carbo- nitriding or cyaniding.
Purpose. The purpose of case hardening is as follows:
(a) To induce high carbon content on the metal surface while leaving the core of the metal soft.
(b) To increase the wear resistance of the metal.
(c) To increase the bending strength.
(d) To increase resistance to pitting.
(e) To increase resistance to scuffing or seizing.
Case Hardening Sequence. The various steps in the case hardening sequence are:
(a) Cementing (Carburising, Nitriding and Carbo-nitriding)
Cementing. It is the first operation to be carried out in case hardening to induce extra carbon onto the required surface. It is done by heating the job to 900° C to 950°C and burying (or dipping) it in the selected carburising media. The penetration depends upon the temperature, time allowed for reaction and diffusion to take place and upon the composition of carburising media.
Annealing. It is not carried out separately. After cementing, the articles are allowed to cool down slowly in the packed carburising media itself. This is done to anneal the newly formed case, which will have the effect of merging the outer high carbon case with the low carbon soft and tough inner core thus preventing the formation of a line between the two layers and reducing the tendency of the case to flake off or peel away during work.
Refining. This is most essential to counteract the ill effects of prolonged heating at high temperature during cementing. This consists of reheating the job to a temperature starting from 950°C and ending at 850°C and quenching each time either in oil or water. This is repeated twice or thrice depending upon the desired effects to be obtained, each time the heating temperature to be reduced by 50°C.
Hardening. Finally the newly formed high carbon steel is hardened by heating to about 760°C and quenching it in water or oil.
Tempering. Tempering is not carried out to all the case hardened jobs, unless it is liable to crack during usage. Plain carbon steels may be tempered by heating to its tempering temperature between 200° C to 320 °C and quenching it. In case of aero engine parts (alloy steels) the parts are to be heated up to 600° C and quenched.