40 Questions
What is the purpose of surface hardening in steels?
To improve wear resistance and fatigue resistance
What type of steels are suitable for carburizing?
Low-carbon steels and alloy steels
What is the temperature range for carburizing?
850-950 °C
What is the result of heating high-carbon steel in a carbon-free furnace?
Decarburizing
What is the surface hardness achieved through carburizing?
55-65 HRC
What is the mechanism of pack carburizing?
Diffusion of carbon into the steel
What is the typical temperature range for nitrocarburizing?
482 to 593 °C
What is the purpose of selective hardening?
To improve the wear properties of inexpensive steels
What is the process of adding carbon and nitrogen simultaneously into the surface of steels?
Carbonitriding
What is the method of hardening that uses a combustible gas flame as the source of heat for austenitizing?
Flame hardening
What is the typical surface hardness achieved through carbonitriding?
55 to 65 HRC
What is the heating media used in flame hardening?
Oxygen acetylene, propane, or any other combination of fuel gases
What is the preferred method for heavy cases or selective hardening of large machine components?
Flame hardening
What is the process of removing carbon from the surface of steel?
Decarburization
What is the temperature range for achieving surface hardness of 50 to 60 HRC?
Not specified in the text
What is the primary objective of the recovery stage during annealing?
Reduction of dislocation density
What is the driving force for recrystallization?
Stored energy from deformed dislocation structure
What is the typical application of carburizing?
Low carbon steels
What is the primary purpose of completion of stress relief?
To completely relieve the internal stresses introduced during plastic deformation
What is the driving force for grain growth?
Reduction in grain boundary energy
What is the result of normal grain growth?
Uniform grain size enlargement
What is the primary advantage of powder metallurgy?
Elimination of machining
What is the purpose of compacting powder in powder metallurgy?
To provide strength to the parts
What is the benefit of powder metallurgy in terms of material utilization?
Minimization of scrap losses
What is the typical production rate of powder metallurgy?
500-1000 parts/hour
What is a limitation of powder metallurgy?
High cost of material in powder form
What type of components are typically made using powder metallurgy?
Components difficult to manufacture by other methods
What is a common method of producing metal powder?
Atomization
What is a characteristic of metal powders?
Varied particle size and shape
What is an application of powder metallurgy?
Production of filters
What is the primary factor that determines the shape of particles?
Method of production of powders
What is true density defined as?
Mass per unit volume of the solid material
What is the advantage of high apparent density powders?
They require lower compressive strike to produce a compact
What is the purpose of mixing and blending of powder?
To produce a uniform distribution of particle shape and size
What is the purpose of adding lubricants to the powder?
To reduce friction between metal particles
What is the purpose of compacting?
To form the size and shape of the desired part
What is pre-sintering?
The process of heating the green compact to increase its strength
What is sintering?
The process of heating the material to a temperature below the melting temperature
What is the purpose of secondary operations?
To obtain desired dimensional tolerances and physical properties
What are the three chambers of a continuous sintering furnace?
Burn-off, sintering, and cooling
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.
This quiz covers the benefits and limitations of Powder Metallurgy, a manufacturing process that achieves high production rates and produces unique parts with uniform structure.
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