Surface Hardening of Steels
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Questions and Answers

What is the primary purpose of surface hardening of steels?

  • To decrease fracture toughness
  • To reduce wear resistance
  • To improve fatigue resistance (correct)
  • To improve corrosion resistance only

    • Which of the following components is commonly surface-hardened?

  • Pumps
  • Pistons
  • Gears (correct)
  • Turbines

    • What is the mechanism of carburizing?

  • Heating of steel in a vacuum chamber
  • Coating of carbon-rich material onto steel
  • Electroplating of carbon onto steel
  • Diffusion of carbon atoms into steel (correct)

    • What is the temperature range for carburizing?

    <p>850-950 °C</p> Signup and view all the answers

    What is the typical carbon content achieved through carburizing?

    <p>0.7 to 1.2 wt.%</p> Signup and view all the answers

    What is the minimum carbon content required in steel for carburizing to work?

    <p>0.08 wt.%</p> Signup and view all the answers

    What is the preferred surface hardening method for heavy cases or selective hardening of large machine components?

    <p>Flame hardening</p> Signup and view all the answers

    What is the primary objective of recovery during the annealing process?

    <p>Reduction of Dislocation Density</p> Signup and view all the answers

    What happens when the heat-treating atmosphere is not controlled during carburization?

    <p>Decarburization occurs</p> Signup and view all the answers

    What is the driving force for recrystallization?

    <p>Stored energy from the deformed dislocation structure</p> Signup and view all the answers

    What is the temperature range for achieving a surface hardness of 50 to 60 HRC?

    <p>Not mentioned</p> Signup and view all the answers

    What type of steels are electron beam and laser hardening limited to?

    <p>Low alloy steels and plain carbon steels</p> Signup and view all the answers

    What is the primary purpose of the stress relief process?

    <p>To relieve the internal stresses introduced during plastic deformation</p> Signup and view all the answers

    What is the driving force for grain growth?

    <p>Reduction in grain boundary energy</p> Signup and view all the answers

    What is the primary benefit of powder metallurgy?

    <p>All of the above</p> Signup and view all the answers

    What is the primary step in the powder metallurgy process?

    <p>Preparation of powder or powders of desired composition</p> Signup and view all the answers

    What is the primary factor influencing the recrystallization behavior and grain size?

    <p>Prior deformation</p> Signup and view all the answers

    What is the primary advantage of powder metallurgy over other manufacturing processes?

    <p>Elimination or minimization of scrap losses</p> Signup and view all the answers

    What is the primary purpose of selective hardening methods?

    <p>To improve wear and friction resistance in specific areas of the part</p> Signup and view all the answers

    What is the temperature range for nitrocarburizing?

    <p>482 to 593 °C</p> Signup and view all the answers

    Which of the following is a characteristic of flame hardening?

    <p>Water quenching is applied as soon as the transformation temperature is reached</p> Signup and view all the answers

    What is the typical case depth for carbonitriding?

    <p>0.07 to 0.5 mm</p> Signup and view all the answers

    What is the primary difference between nitriding and nitrocarburizing?

    <p>Nitriding involves the diffusion of nitrogen, while nitrocarburizing involves the diffusion of both carbon and nitrogen</p> Signup and view all the answers

    What is the heating value of acetylene gas used in flame hardening?

    <p>53.4 MJ/m3</p> Signup and view all the answers

    What is a unique advantage of powder metallurgy in manufacturing?

    <p>Manufacturing of small and unique shape parts</p> Signup and view all the answers

    What is a limitation of powder metallurgy in terms of material properties?

    <p>All of the above</p> Signup and view all the answers

    What is a common application of powder metallurgy in the automotive industry?

    <p>Production of clutch plates and piston rings</p> Signup and view all the answers

    What is a method used to produce metal powders?

    <p>Atomization</p> Signup and view all the answers

    What is a characteristic of metal powders that affects their behavior?

    <p>Particle size and shape</p> Signup and view all the answers

    What is a benefit of powder metallurgy in terms of cost?

    <p>Low labor cost and absence of scrap</p> Signup and view all the answers

    What is the shape of particles that has excellent sintering property?

    <p>Spherical</p> Signup and view all the answers

    What is the ratio of initial volume of the powder to final volume called?

    <p>Compressibility</p> Signup and view all the answers

    What is the purpose of mixing and blending of metal powder?

    <p>To produce a uniform distribution of particle shape and size</p> Signup and view all the answers

    What is the purpose of adding lubricants to the metal powder?

    <p>To increase the flow characteristics of the powder</p> Signup and view all the answers

    What is the process of heating the green compact to a temperature below the final sintering temperature called?

    <p>Pre-sintering</p> Signup and view all the answers

    What is the purpose of the burn off chamber in the continuous sintering furnace?

    <p>To remove the lubricants and binders added during blending</p> Signup and view all the answers

    What is the process of forming the desired shape and size of the part called?

    <p>Compacting</p> Signup and view all the answers

    What is the density of the compact after compaction called?

    <p>Green Density</p> Signup and view all the answers

    What is the purpose of performing secondary operations?

    <p>To improve the strength and hardness of the compact</p> Signup and view all the answers

    What is the ratio of the mass of the loose powder to the unit volume of the powder called?

    <p>Apparent Density</p> Signup and view all the answers

    Study Notes

    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.

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    Description

    This quiz covers the process of surface hardening of steels, its purpose, and its applications. It also includes the components that are usually surface-hardened.

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