HEAT TREATMENT OF CARBON STEELS
Object of heat treatment. Metals and alloys are heat treated to improve their mechanical properties, to relieve internal stresses or to improve their machinability. The properties of carbon steels can also be altered significantly by subjecting them to heat treatment processes.
Heat treatment consists of three basic steps:
(i) Heat the metal/alloy to a predetermined temperature. This temperature will, ideally, depend upon the actual composition of carbon steel (i.e. carbon percentage),
(ii) Soaking or holding the metal/alloy at that temperature for some time, so that the temperature across the entire cross-section becomes uniform, and
(iii) Cooling the metal/alloy at a predetermined rate in a suitable medium like water, oil or air.
The rate of cooling is the most important factor.
Kinds of Heat Treatments Given to Carbon Steels
Carbon steels are subjected to the following four basic heat-treatment processes:
(iii) Hardening, and
We shall now describe these processes very briefly.
The purpose of annealing is to soften the material. Along with softening, the internal stresses, if any, will also get removed.
The approximate temperatures to which the steel-sample should be heated will depend upon its carbon content. The recommended temperatures are shown in the following table:
Soaking time may be given at the rate of 3-4 minutes for everyone mm thickness of the cross-section of material.
In annealing, the work piece is allowed to cool inside the furnace only after switching off electrical power or oil supply to the furnace. This ensures that the work piece cools at a very slow rate.
This process results in softening of material and increase in ductility due to grain growth.
Normalizing entails heating to the same temperatures as recommended for annealing (except for high carbon steel specimens, which are to be heated to much higher temperatures than for annealing particularly as carbon percentage in sample increases), soaking and then cooling the sample in still air. Main object of normalizing is getting rid of internal stresses and grain-refinement.
Hardening involves heating (to the same temperatures as in case of annealing) and soaking. Thereafter, the work piece is taken out of the furnace and quickly cooled at a very fast rate in a tank of cold water or oil, agitating the water/oil vigorously. (This cooling operation is called ‘‘quenching.’’) The result is hardening of the work piece. However, in order to harden, the carbon content in the work piece should be at least 0.25%. Therefore, dead mild steel cannot be hardened in this way. Mild steel will also harden slightly for specimens containing over 0.25% carbon. Higher the carbon percentage, higher will be resulting hardness.
Hardened pieces become brittle and their extreme brittleness becomes a great disadvantage. They tend to fail in-service. Therefore hardening process is invariably followed by a tempering process.
Tempering means giving up a certain amount of hardness but shedding a great deal of brittleness acquired in the process of hardening. It is a trade off between hardness and brittleness, so that hardened component may give useful service without failure.
Tempering involves heating the carbon steel part to a temperature varying from 150°–600°C (depending upon how much trade off is required) and cooling the component in an oil or salt bath or even in air.
As mentioned above, only those carbon steels can be hardened whose carbon content is about 0.25% or more. How do we harden dead mild steel? The answer is by case hardening. In this process, the work piece is packed in charcoal and heated as in annealing. It is kept at that high temperature for a few hours. The result is that carbon enters into the surface of the work piece to the depth of a mm or two depending upon the heating time.
The work piece now has a case where carbon percentage is as per requirement for hardening. It is then heated and quenched in the usual manner. The result is a component whose surface acquires hardness, but core remains soft and tough.