40 Questions
What is the recommended surface hardness achieved through carburizing?
50 to 60 HRC
Which of the following steels can be used with nitriding?
Chromium-molybdenum alloy steels
What is the primary objective of the recovery stage in annealing?
Reduction of Dislocation Density
What happens when the heat-treating atmosphere is not controlled?
Decarburization occurs
What is the driving force for recrystallization?
Stored energy from the deformed dislocation structure
What is the recommended method for heavy cases or selective hardening of large machine components?
Flame hardening
What is the primary purpose of surface hardening?
To improve wear resistance and resistance to high contact stresses
Which of the following components is usually not surface-hardened?
Electrical wires
What is the minimum carbon content required for carburizing?
0.08 wt.%
What is the maximum surface hardness achieved through carburizing?
65 HRC
What is the temperature range for carburizing?
850-950 °C
What is the maximum carburizing time?
No technical limit
What is the primary function of the stress relief process?
To relieve internal stresses introduced during plastic deformation
What happens to the grain boundary area during grain growth?
It decreases
What is the driving force behind grain growth?
Reduction in grain boundary energy
What is the significance of temperature control in annealing?
It controls the stages of recovery, recrystallization, and grain growth
What is a key advantage of powder metallurgy?
It eliminates or minimizes scrap losses
What is a characteristic of the powder metallurgy process?
It is a chip-less working process
What is the process of diffusing nitrogen into the surface of steel?
Nitriding
What is the temperature range for nitrocarburizing?
482 to 593 °C
What is the purpose of selective hardening?
To give inexpensive steels the wear properties of alloyed steels
What is the typical hardness achieved by flame hardening?
50 to 60 HRC
What is the process of heating the steel part in induction hardening?
Placing the part inside copper induction coils and heating by high-frequency alternating current
What is the minimum carbon content required for flame hardening?
0.40 wt.%C
What is the primary factor that determines the shape of particles?
Method of production
What is apparent density defined as?
Mass of loose powder per unit actual volume
What is the purpose of adding lubricants during mixing and blending?
To reduce friction between metal particles
What is the purpose of compacting mixed powders in a die?
To form the desired shape and size of the part
What happens to the green compact when it is removed from the die?
It expands slightly due to elastic recovery
What is the purpose of pre-sintering?
To remove lubricants and binders
What is the purpose of sintering?
To allow bonding or fusion of individual particles
What type of furnace is used for sintering?
Continuous sintering furnace
What is the purpose of secondary operations?
To improve the strength and hardness of the final product
What is compressibility defined as?
Ratio of initial volume to final volume
What is the benefit of powder metallurgy in terms of production rates?
500-1000 parts/hour
What is a limitation of powder metallurgy in terms of product design?
Products with intricate designs are difficult to produce
What type of parts are typically produced using powder metallurgy?
Small and unique shape
What is a common application of powder metallurgy?
Production of filters
What is a method used to produce metal powders?
Atomization
What determines the behavior of metal powders?
Particle size and shape
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.
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.
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