Surface Hardening of Steels
40 Questions
0 Views

Choose a study mode

Play Quiz
Study Flashcards
Spaced Repetition
Chat to lesson

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the recommended surface hardness achieved through carburizing?

  • 40 to 50 HRC
  • 30 to 40 HRC
  • 60 to 70 HRC
  • 50 to 60 HRC (correct)

    • Which of the following steels can be used with nitriding?

  • Low carbon steels
  • High carbon steels
  • Stainless steels
  • Chromium-molybdenum alloy steels (correct)

    • What is the primary objective of the recovery stage in annealing?

  • Relief of Internal Stresses
  • Formation of New Grains
  • Reduction of Dislocation Density (correct)
  • Relief of External Stresses

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

    <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 recommended 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 purpose of surface hardening?

    <p>To improve wear resistance and resistance to high contact stresses</p> Signup and view all the answers

    Which of the following components is usually not surface-hardened?

    <p>Electrical wires</p> Signup and view all the answers

    What is the minimum carbon content required for carburizing?

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

    What is the maximum surface hardness achieved through carburizing?

    <p>65 HRC</p> Signup and view all the answers

    What is the temperature range for carburizing?

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

    What is the maximum carburizing time?

    <p>No technical limit</p> Signup and view all the answers

    What is the primary function of the stress relief process?

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

    What happens to the grain boundary area during grain growth?

    <p>It decreases</p> Signup and view all the answers

    What is the driving force behind grain growth?

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

    What is the significance of temperature control in annealing?

    <p>It controls the stages of recovery, recrystallization, and grain growth</p> Signup and view all the answers

    What is a key advantage of powder metallurgy?

    <p>It eliminates or minimizes scrap losses</p> Signup and view all the answers

    What is a characteristic of the powder metallurgy process?

    <p>It is a chip-less working process</p> Signup and view all the answers

    What is the process of diffusing nitrogen into the surface of steel?

    <p>Nitriding</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

    What is the purpose of selective hardening?

    <p>To give inexpensive steels the wear properties of alloyed steels</p> Signup and view all the answers

    What is the typical hardness achieved by flame hardening?

    <p>50 to 60 HRC</p> Signup and view all the answers

    What is the process of heating the steel part in induction hardening?

    <p>Placing the part inside copper induction coils and heating by high-frequency alternating current</p> Signup and view all the answers

    What is the minimum carbon content required for flame hardening?

    <p>0.40 wt.%C</p> Signup and view all the answers

    What is the primary factor that determines the shape of particles?

    <p>Method of production</p> Signup and view all the answers

    What is apparent density defined as?

    <p>Mass of loose powder per unit actual volume</p> Signup and view all the answers

    What is the purpose of adding lubricants during mixing and blending?

    <p>To reduce friction between metal particles</p> Signup and view all the answers

    What is the purpose of compacting mixed powders in a die?

    <p>To form the desired shape and size of the part</p> Signup and view all the answers

    What happens to the green compact when it is removed from the die?

    <p>It expands slightly due to elastic recovery</p> Signup and view all the answers

    What is the purpose of pre-sintering?

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

    What is the purpose of sintering?

    <p>To allow bonding or fusion of individual particles</p> Signup and view all the answers

    What type of furnace is used for sintering?

    <p>Continuous sintering furnace</p> Signup and view all the answers

    What is the purpose of secondary operations?

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

    What is compressibility defined as?

    <p>Ratio of initial volume to final volume</p> Signup and view all the answers

    What is the benefit of powder metallurgy in terms of production rates?

    <p>500-1000 parts/hour</p> Signup and view all the answers

    What is a limitation of powder metallurgy in terms of product design?

    <p>Products with intricate designs are difficult to produce</p> Signup and view all the answers

    What type of parts are typically produced using powder metallurgy?

    <p>Small and unique shape</p> Signup and view all the answers

    What is a common application of powder metallurgy?

    <p>Production of filters</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 determines the behavior of metal powders?

    <p>Particle size and shape</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.

    Studying That Suits You

    Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

    Quiz Team

    Description

    Learn about the purpose and methods of surface hardening of steels, including improving wear resistance, corrosion resistance, and fatigue resistance. Understand the components that are usually surface-hardened.

    More Like This

    Use Quizgecko on...
    Browser
    Open
    Browser
    Browser