40 Questions
What is the primary purpose of surface hardening of steels?
To improve fatigue resistance
Which of the following components is commonly surface-hardened?
Gears
What is the mechanism of carburizing?
Diffusion of carbon atoms into steel
What is the temperature range for carburizing?
850-950 °C
What is the typical carbon content achieved through carburizing?
0.7 to 1.2 wt.%
What is the minimum carbon content required in steel for carburizing to work?
0.08 wt.%
What is the preferred surface hardening method for heavy cases or selective hardening of large machine components?
Flame hardening
What is the primary objective of recovery during the annealing process?
Reduction of Dislocation Density
What happens when the heat-treating atmosphere is not controlled during carburization?
Decarburization occurs
What is the driving force for recrystallization?
Stored energy from the deformed dislocation structure
What is the temperature range for achieving a surface hardness of 50 to 60 HRC?
Not mentioned
What type of steels are electron beam and laser hardening limited to?
Low alloy steels and plain carbon steels
What is the primary purpose of the stress relief process?
To relieve the internal stresses introduced during plastic deformation
What is the driving force for grain growth?
Reduction in grain boundary energy
What is the primary benefit of powder metallurgy?
All of the above
What is the primary step in the powder metallurgy process?
Preparation of powder or powders of desired composition
What is the primary factor influencing the recrystallization behavior and grain size?
Prior deformation
What is the primary advantage of powder metallurgy over other manufacturing processes?
Elimination or minimization of scrap losses
What is the primary purpose of selective hardening methods?
To improve wear and friction resistance in specific areas of the part
What is the temperature range for nitrocarburizing?
482 to 593 °C
Which of the following is a characteristic of flame hardening?
Water quenching is applied as soon as the transformation temperature is reached
What is the typical case depth for carbonitriding?
0.07 to 0.5 mm
What is the primary difference between nitriding and nitrocarburizing?
Nitriding involves the diffusion of nitrogen, while nitrocarburizing involves the diffusion of both carbon and nitrogen
What is the heating value of acetylene gas used in flame hardening?
53.4 MJ/m3
What is a unique advantage of powder metallurgy in manufacturing?
Manufacturing of small and unique shape parts
What is a limitation of powder metallurgy in terms of material properties?
All of the above
What is a common application of powder metallurgy in the automotive industry?
Production of clutch plates and piston rings
What is a method used to produce metal powders?
Atomization
What is a characteristic of metal powders that affects their behavior?
Particle size and shape
What is a benefit of powder metallurgy in terms of cost?
Low labor cost and absence of scrap
What is the shape of particles that has excellent sintering property?
Spherical
What is the ratio of initial volume of the powder to final volume called?
Compressibility
What is the purpose of mixing and blending of metal powder?
To produce a uniform distribution of particle shape and size
What is the purpose of adding lubricants to the metal powder?
To increase the flow characteristics of the powder
What is the process of heating the green compact to a temperature below the final sintering temperature called?
Pre-sintering
What is the purpose of the burn off chamber in the continuous sintering furnace?
To remove the lubricants and binders added during blending
What is the process of forming the desired shape and size of the part called?
Compacting
What is the density of the compact after compaction called?
Green Density
What is the purpose of performing secondary operations?
To improve the strength and hardness of the compact
What is the ratio of the mass of the loose powder to the unit volume of the powder called?
Apparent Density
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 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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