Surface hardening:
In this process no chemical changes takes place. Large sized objects are subjected to heating followed by rapid cooling. The process is classified according to various methods of heating the steel. They are
i) Flame hardening:
It is the simplest form of surface hardening. The process consists of heating large work piece with oxy-acetylene or oxy-fuel flame, followed by spraying of jet of water as coolant. The hardened parts are reheated to a temperature of 180-200— C in a furnace or oil bath for stress relieving. This reheating will not result in reduction of hardness. The hardness obtained is due to martensite and lower bainite formation.
Over heating of the work has to be avoided since that may lead to cracking on quenching and excessive grain growth in the region just bellow the hardened surface. The carbon content of steel that can be flame hardened varies from 0.3 to 0.6%. High carbon steels also may be subjected to this process but greater care is required to avoid cracking. A case depth of 3 mm can be achieved in normal conditions. There are four different methods for doing flame hardening. They are
1. Stationary,
2. Progressive,
3. Spinning and
4. Progressive spinning.
There is little scaling, decarburization and distortion in flame hardening. Since the heating and cooling is too fast, the core remains unaffected.
ii) Induction hardening:
In the process of induction hardening, the heating of the component is achieved by electro magnetic induction. This process is generally used for hardening of crankshafts, camshafts, gears, axils etc. a conductor (coil) carries alternate current of high frequency, which is then induced in the enclosed steel part placed within the magnetic field. Thus induction heating takes place and the heat generated affects only the outer surface of the material. The heating process takes only few seconds. Immediately after heating the surface is quenched by using a cold-water jet. During quenching martensitic structure is formed, which makes the surface hard and wear resistant. The original toughness and ductility of the core remains the same. The hardening temperature for plain carbon steels is about 760— C. For alloy steels a higher temperature range is required.
iii) Electron beam hardening:
This process is used where other processes cannot be used due to associated distortion. The work is kept in vacuum at 0.06m bar pressure. An electron beam is focused on the surface to heat it. In the beginning the energy in put is kept high and with time it is gradually reduced to avoid melting. A case depth of 0.75mm can be achieved in this process. Control of voltage, current, beam, dwell time and focus are computer oriented.
IV) Laser hardening:
In this process as the name implies a laser beam is used to heat the work. Since the intensity of the laser beam is high the work may tend to melt if the high intensity beam is directly used. To avoid this a lens id used to reduce intensity by producing a defocused spot or scans of 1-25mm width. A 1kW laser produces a circular spot whose diameter may vary from 0.5 mm to 0.25mm. Industrial lasers are available up to 20kW. A case depth of 0.75mm can be achieved. The case depth is controlled by time and energy density. It takes lesser amount of time compared with induction and flame hardening methods. No separate quenching is required in laser beam hardening. Self-quenching takes place by the effect of surrounding unheated portion. The effect of heat in the surrounding surface is less and hence lesser is the chances of distortion. 
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