5.2 Heat Treatment
5.2 Heat Treatment
1. Annealing, Quenching, Tempering
Annealing:
Purpose: Soften metal, relieve stresses, improve machinability.
Process: Heat above critical temperature → slow cool.
Types:
Full Annealing: Heat to austenite region → furnace cool.
Process Annealing: Heat below critical temperature → cool.
Stress Relief: Heat to ~600°C → slow cool.
Effects:
Reduces hardness and strength.
Increases ductility and toughness.
Refines grain structure.
Removes internal stresses.
Quenching (Hardening):
Purpose: Increase hardness and strength.
Process: Heat to austenite region → rapid cooling in medium.
Quenching Media:
Water (most severe cooling).
Oil (moderate cooling).
Brine (faster than water).
Air (slowest).
Outcome: Forms hard, brittle martensite.
Critical Cooling Rate: Minimum rate needed to form martensite.
Tempering:
Purpose: Reduce brittleness after quenching.
Process: Reheat quenched steel to 150-650°C → air cool.
Effects:
Increases toughness and ductility.
Reduces hardness slightly.
Relieves quenching stresses.
Temperature Effects:
Low temp (200-300°C): Increased toughness, slight hardness reduction.
Medium temp (300-450°C): Good balance of strength-toughness.
High temp (450-650°C): Maximum toughness, significant softening.
Complete Cycle: Annealing → Quenching → Tempering = Optimal properties.
2. Normalizing
Definition: Heating steel to austenite region → air cooling.
Purpose:
Refine grain structure after hot working.
Improve machinability.
Homogenize structure.
Slight hardening.
Process Details:
Temperature: ~50°C above critical temperature.
Cooling: Still air (faster than annealing).
Produces finer pearlite than annealing.
Compared to Annealing:
Faster cooling → finer grains.
Higher strength/hardness than annealing.
Less ductile than annealed steel.
Applications:
Prepare steel for hardening.
Improve properties of low-carbon steels.
Remove internal stresses from welding.
3. Surface Hardening
Purpose: Hard surface + tough core → wear resistance + impact strength.
Methods Without Composition Change:
Flame Hardening:
Surface heated by oxy-fuel torch → quenched.
For large parts, selective hardening.
Induction Hardening:
High-frequency current induces surface heating → quenched.
Fast, consistent, energy efficient.
Precise depth control.
Methods With Composition Change:
Carburizing:
Add carbon to low-carbon steel surface.
Pack (solid), gas, or liquid methods.
~0.8-1.0% C at surface.
Nitriding:
Add nitrogen to alloy steel surface.
Heat in ammonia atmosphere.
No quenching needed.
Higher hardness than carburizing.
Carbonitriding:
Add both carbon and nitrogen.
Lower temperature than carburizing.
Better hardenability.
Case Depth: Typically 0.5-2.0 mm.
Applications:
Gears, shafts, bearings.
Tools, dies.
Wear surfaces in machinery.
4. Powder Metallurgy
Process Steps:
Powder Production:
Atomization (liquid metal → droplets).
Reduction (metal oxides).
Electrolysis.
Mechanical comminution.
Blending/Mixing:
Different powders + lubricants.
Homogeneous mixture.
Compaction:
Press in dies under high pressure.
Green compact formed.
Sintering:
Heat below melting point.
Powder particles bond.
Increases strength.
Secondary Operations:
Coining, sizing, heat treatment.
Infiltration, impregnation.
Advantages:
Near-net-shape production.
High material utilization (>95%).
Complex shapes possible.
Controlled porosity.
Special compositions (immiscible metals).
Limitations:
High tooling cost.
Size limitations.
Lower strength than wrought materials.
Limited shape complexity compared to casting.
Materials Used:
Iron and steel powders.
Copper, bronze, brass.
Aluminum.
Tungsten carbide.
Special alloys.
Applications:
Self-lubricating bearings (porous).
Gears, cams, structural parts.
Electrical contacts.
Cutting tools (tungsten carbide).
Filters (controlled porosity).
Key Parameters:
Powder size and shape.
Compaction pressure.
Sintering temperature and time.
Atmosphere control.
Recent Developments:
Metal injection molding (MIM).
Additive manufacturing (3D printing).
Hot isostatic pressing (HIP).
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