Surface Hardening of Steels

Surface Hardening of Steels

• Purpose of surface hardening:
  • Improve wear resistance
  • Improve resistance to high contact stresses
  • Improve fracture toughness
  • Improve fatigue resistance
  • Improve corrosion resistance
• Components usually surface-hardened:
  • Gears
  • Bearings
  • Valves
  • Shafts
  • Bearing races
  • Cams
  • Hand tools
  • Rolls
  • Machine tools
  • Sprockets

Heat-Treating Methods

• Diffusional methods:
  • Carburizing
  • Nitriding
  • Carbonitriding
  • Nitrocarburizing
  • Boronizing
  • Chromizing
• Selective hardening methods:
  • Flame hardening
  • Induction hardening
  • Laser and electron beam hardening

Carburizing

• Process: adding carbon to steel surface
• Types of carburizing:
  • Pack carburizing
  • Vacuum carburizing
  • Gas carburizing
  • Plasma carburizing
• Carbon content achieved: 0.7 to 1.2 wt.%
• Suitable for: Low-carbon steels and alloy steels containing 0.08 to 0.2 wt.%C
• Carburizing temperature: 850-950 °C
• Carburizing time: 4 to 72 h
• Mechanism:
  • Surface hardness achieved: 55-65 HRC
  • Case depth: No technical limit; in practice, 0.5 to 1.5 mm
• Applications:
  • Gears
  • Cams
  • Shafts
  • Bearings
  • Piston rings
  • Clutch plates
  • Sprockets

Nitriding

• Process: diffusing nitrogen into steel surface
• Suitable for: Low-carbon alloy steels containing Al, Cr, Mo, V, Ni
• Nitriding time: Less than carburizing time
• Applications:
  • Gears
  • Valves
  • Cutters
  • Sprockets
  • Pump-boring tools
  • Fuel-injection pump parts

Carbonitriding

• Process: adding both carbon and nitrogen simultaneously
• Suitable for: Mainly for low-carbon steels; medium-carbon steels sometimes
• Temperature: 700-800 °C
• Carbonitriding time: Less than carburizing time
• Applications:
  • Gears
  • Bolts
  • Nuts

Nitrocarburizing

• Process: thermochemical low-temperature process
• Temperature: 482-593 °C
• Applications:
  • Wear/friction resistance
  • Similar to carburizing, but with lower distortion

Selective Hardening Methods

• Flame hardening:
  • Process: heating with combustible gas flame
  • Suitable for: At least medium-carbon steels containing ≥ 0.40 wt.%C, cast irons
  • Surface hardness achieved: 50-60 HRC
  • Case depth: 0.7-6 mm
  • Applications:
    • Lathe beds and centers
    • Crankshafts
    • Piston rods
    • Gear and sprocket teeth
    • Axles
    • Cams
    • Shear blades
• Induction hardening:
  • Process: heating with high-frequency alternating current
  • Suitable for: Medium carbon steels (wt.% C = 0.4), cast irons
  • Surface hardness achieved: 50-60 HRC
  • Case depth: 0.7-6 mm
  • Applications:
    • Similar to flame hardening

Recovery, Recrystallization, and Grain Growth

• Recovery:
  • First stage of annealing process
  • Reduces dislocation density
  • Relieves internal stresses
  • Partially restores properties
• Recrystallization:
  • Forms new, strain-free grains
  • Relieves internal stresses
  • Completes stress relief
  • Critical temperature and time depend on prior deformation, material composition, and purity
• Grain growth:
  • Reduces grain boundary area
  • Reduces system energy
  • Depends on temperature, time, and impurities
  • Normal vs. abnormal grain growth

Powder Metallurgy

• Advantages:
  • Produces parts with closed dimensional tolerance and good surface finish
  • Eliminates or minimizes scrap losses
  • Can be fully automated
  • Facilitates manufacture of complex shapes and unique compositions
  • High production rates
• Limitations:
  • High cost of powder material
  • Difficult to produce parts with intricate design
  • Residual porosity in sintered parts
  • Economically feasible for large volume production
  • Difficult to compress some metal powders
  • Health problems from atmospheric contamination
• Applications:
  • Production of porous parts (e.g., filters)
  • Tungsten and Molybdenum components
  • Automotive components (e.g., clutch plates, connecting rods, cam shafts, piston rings)
  • Grinding wheels
  • Nozzles for rockets and missiles
  • Complex-shaped parts
  • Electrical bushes for motors
  • Permanent magnets
• Production of metal powder:
  • Atomization
  • Crushing and milling
  • Electrolysis process
  • Chemical process
• Characteristics of metal powders:
  • Particle shape and size distribution
  • Density (true and apparent)
  • Flow rate
  • Compressibility and compression ratio
• Processing of powders:
  • Mixing and blending
  • Compacting
  • Sintering### Powder Compaction
• Green compact expands slightly due to elastic recovery when removed from the die.
• The expansion depends on the pressure and extent of plastic deformation in powder particles.

Pre-Sintering

• A process where green compact is heated to a temperature below the final sintering temperature to increase strength.
• Removes lubricants and binders added during blending.
• Performed only when machining is not required.

Sintering

• Heating material to a temperature below the melting point, allowing bonding or fusion of individual particles.
• Performed under a protective atmosphere to prevent oxidation.
• Continuous sintering furnace used, consisting of:
• Burn-off chamber: volatizes lubricants to improve bond strength and prevent cracking.
• High-temperature chamber: for bonding between powder particles.
• Cooling chamber: for cooling the sintered part.

Secondary Operations

• Performed to obtain desired dimensional tolerances and physical properties.
• Operations include:
• Finishing operations for better dimensional accuracy.
• Machining operations for specific shapes and sizes.
• Heat treating to improve hardness, strength, and wear resistance.
• Finishing operations to improve surface characteristics of the part.