The selective hardening process differentially heats and treats the surface of metal components to produce a hardened case. There are several selective hardening methods including flame hardening, induction hardening, laser hardening, and electron beam hardening. Flame hardening involves heating specific areas of a component with an oxy-acetylene flame above the critical temperature before quenching in water. Induction hardening uses an induction coil to pass high frequency current and heat just the surface of an irregular part before quenching. These selective hardening processes allow for hardening of specific zones on a component without changing the internal composition.

1. SELECTIVE HARDENING PROCESS
Abin Basil Shaji

2. Introduction
Surface Treatments
 Industrial applications demand different properties for
surface and interior.
 The surface treatments can be classified as:
o Mechanical Treatments
o Protective Coatings
o Chemical Composition change
o Differential Heat Treatment

3. Differential Heat Treatment
 The process is ideal for heat treatable components
which are made of steel.
 The surface which is developed as a part of heat
treatment is called the ‘case’.
 Classified as:
o Diffusion methods
o Selective hardening methods

4. Selective Hardening methods
 These methods will not make any changes in surface
composition.
 Flame Hardening
 Induction Hardening
 Laser Hardening
 Electron Beam Hardening

5. Flame Hardening
 Mostly done where depth of the case is not serious
and where small zones has to be hardened.
 Process
 The areas of the component are heated above 850ºC( critical
temp) by Oxy-acetylene flame, followed by quenching with water
at high pressure.
 Tempering is done to relieve stresses.

7. Contd.
 Case hardening depends on:
• Temperature of the flame
• Heating time
• Temperature of quenching water.
• Time
 Applications
• Pulleys
• Gear teeth

8. Advantages
 Very rapid.
 Small zone can be hardened due to focused flame.
 Applicable to hardening of bulky parts that does not fit into
furnace
Disadvantages
Control of heat penetration is very hard.
Some steels can only be hardened.

9. Induction Hardening
 This type of hardening is done for irregular parts of
machine.
 Process
The part to be hardened is placed in an induction coil.
High frequency current is passed through coil.
This AC current will heat up the component’s surface.
Heated surface is quenched by spray of water.

11. Contd.
 Case hardening to a depth of 0.8mm can be obtained in 1-5s.
 Time of hardening depends on:
 Power
 Depth of hardening
 Frequency of current.
 Applications
• Roller bearings
• Pump shafts
• Piston rods

12. Advantages
 Rapid process and accurate, practical for automation.
 No warm time required.
 The depth of case can be controlled.
 Offers resistance to oxidation & scaling.
 Clean process.
Disadvantages
Medium carbon steels are suitable.
Hardness depends on carbon content.