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What is the primary purpose of surface hardening of steels?
What is the primary purpose of surface hardening of steels?
What is the minimum carbon content required for carburizing?
What is the minimum carbon content required for carburizing?
What is the maximum case depth achievable through carburizing?
What is the maximum case depth achievable through carburizing?
What type of carburizing involves heating the part in a carbon-rich atmosphere?
What type of carburizing involves heating the part in a carbon-rich atmosphere?
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What is the temperature range for carburizing?
What is the temperature range for carburizing?
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What is the duration of the carburizing process?
What is the duration of the carburizing process?
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What is the typical case depth achieved through carburizing?
What is the typical case depth achieved through carburizing?
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What is the primary objective of the recovery stage during annealing?
What is the primary objective of the recovery stage during annealing?
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What is the name of the process that involves the nucleation of new, strain-free grains?
What is the name of the process that involves the nucleation of new, strain-free grains?
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What is the term for the removal of carbon from the surface of a steel part?
What is the term for the removal of carbon from the surface of a steel part?
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What is the temperature range in which the recovery stage typically occurs?
What is the temperature range in which the recovery stage typically occurs?
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What is the driving force behind the recrystallization process?
What is the driving force behind the recrystallization process?
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What is the primary purpose of selective hardening?
What is the primary purpose of selective hardening?
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Which of the following heat treatment processes involves the simultaneous diffusion of carbon and nitrogen into the surface of steel?
Which of the following heat treatment processes involves the simultaneous diffusion of carbon and nitrogen into the surface of steel?
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What is the typical temperature range for the nitrocarburizing process?
What is the typical temperature range for the nitrocarburizing process?
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Which of the following fuel gases has the highest heating value for flame hardening?
Which of the following fuel gases has the highest heating value for flame hardening?
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What is the principle of heat generation in induction hardening?
What is the principle of heat generation in induction hardening?
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What is the main factor that determines the shape of particles?
What is the main factor that determines the shape of particles?
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Which of the following heat treatment processes is suitable for low-carbon alloy steels containing Al, Cr, Mo, V, and Ni?
Which of the following heat treatment processes is suitable for low-carbon alloy steels containing Al, Cr, Mo, V, and Ni?
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What is the benefit of high apparent density in powders?
What is the benefit of high apparent density in powders?
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What is the purpose of adding lubricants to metal powder?
What is the purpose of adding lubricants to metal powder?
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What is the purpose of pre-sintering?
What is the purpose of pre-sintering?
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What is the primary function of the burn-off chamber in a continuous sintering furnace?
What is the primary function of the burn-off chamber in a continuous sintering furnace?
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What is the purpose of compacting metal powder in a die?
What is the purpose of compacting metal powder in a die?
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What is the benefit of using spheroidal shaped particles?
What is the benefit of using spheroidal shaped particles?
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What is the purpose of secondary operations in powder processing?
What is the purpose of secondary operations in powder processing?
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What determines the green density of a compact?
What determines the green density of a compact?
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What is the primary purpose of sintering?
What is the primary purpose of sintering?
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What is the primary driving force behind grain growth?
What is the primary driving force behind grain growth?
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What is the primary effect of recrystallization on material properties?
What is the primary effect of recrystallization on material properties?
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What is the primary advantage of powder metallurgy in terms of material waste?
What is the primary advantage of powder metallurgy in terms of material waste?
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What is the primary purpose of controlling the annealing temperature in heat treatment?
What is the primary purpose of controlling the annealing temperature in heat treatment?
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What is the primary effect of abnormal grain growth on material properties?
What is the primary effect of abnormal grain growth on material properties?
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What is the primary benefit of powder metallurgy in terms of labor cost?
What is the primary benefit of powder metallurgy in terms of labor cost?
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What is the main advantage of powder metallurgy over other manufacturing processes?
What is the main advantage of powder metallurgy over other manufacturing processes?
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What is a major limitation of powder metallurgy?
What is a major limitation of powder metallurgy?
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What type of components are typically made using powder metallurgy?
What type of components are typically made using powder metallurgy?
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What is the main goal of powder production methods?
What is the main goal of powder production methods?
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What is a characteristic of metal powders that affects their behavior?
What is a characteristic of metal powders that affects their behavior?
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What is a common application of powder metallurgy?
What is a common application of powder metallurgy?
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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 surface hardening process, its purposes, and components that undergo surface hardening, including gears and bearings. Learn about the benefits of surface hardening, including improved wear resistance and fatigue resistance.
