Question

The machine tool guideways are usually hardened by
1. induction hardening
2. flame hardening
3. vacuum hardening
4. martempering

Answer 1

Correct Answer - Option 2 : flame hardening

Explanation

Flame Hardening:

• The surface is heated with an oxy-acetylene torch, then quenched with water spray or other quenching media.
• It is a type of surface/case hardening used for improving resistance to surface indentation, fatigue, and wear.
• There is practically no distortion of the workpiece because only small sections of a workpiece are heated.
• The work surface remains clean as the heating rate is very high 2400°C - 3300°C.
• The process is more efficient and economical for large work as compared to induction heating.
  • Example: Gear and sprocket, lathe beds, machine tool guideways, crankshaft, piston rod, etc.

Induction hardening:

• The process is employed to increase the hardness, wear-resistance, and endurance limit of the surface of the workpiece.
• The surface is heated to the austenite range and then quenched immediately to form martensite where the structure of the core remains unchanged.
• The workpiece should contain 0.4-0.5 % Carbon or alloying elements as Chromium, Ni, and Mo.
• The work is placed in copper induction coils and heated by high-frequency AC Current
  It is a type of surface/case hardening.
• Disadvantage: Each workpiece requires different fixtures for its holding.
• Mostly used in industry for hardening surfaces of a camshaft, gear, sprocket, crankshaft, piston rod, lathe bed, etc.

Martempering:

• Articles are first quenched in water to a temperature of 300 °C - 400 °C and then quickly transferred to a less intensive medium like oil or air where they are held until completely cooled.
• Widely used method for carbon-steel tools like taps, disc, milling cutters, etc.

Vacuum Hardening:

• Material is heated in the absence of oxygen by convection in the medium of inert gas or vacuum medium.
• Vacuum medium confers better hardening properties and prevents the occurrence of surface reactions such as oxidation, decarburization etc.
• Used for hardening tool steels and super-alloys.