Annealing Process

 Annealing Process

The Annealing Process is a heat treatment process used to make metals such as copper, aluminium, and steel softer, more ductile, and easier to draw or bend.

Purpose of Annealing

• Reduce hardness
• Increase flexibility and ductility
• Relieve internal stresses caused by drawing or rolling
• Improve electrical conductivity (especially for copper and aluminium)
• Prevent wire breakage during further processing

Basic Steps of Annealing

1. 
   
2. Heating
   • The material is heated to a specific temperature.
   • Copper wire: approximately 300–600°C.
   • Aluminium wire: approximately 250–450°C.
3. Soaking (Holding)
   • The material is held at the required temperature for a certain time to allow the internal structure to change.
4. Cooling
   • The material is cooled gradually (air cooling or controlled cooling) to achieve the desired softness.

Annealing Process Flow

Wire Drawing
      ↓
Heating in Annealer
      ↓
Holding at Temperature
      ↓
Controlled Cooling
      ↓
Soft Annealed Wire

Types of Annealing

1. Full Annealing
   • Produces maximum softness.
2. Continuous Annealing
   • Commonly used in cable and wire manufacturing lines.
3. Bright Annealing
   • Performed in a protective atmosphere to prevent oxidation and maintain a bright surface finish.

In Cable Manufacturing

After wire drawing, copper or aluminium conductors become hard. They are passed through a continuous annealing machine, where controlled heating and cooling restore softness and conductivity before bunching or stranding.

Simple Definition

Annealing is the process of heating and controlled cooling of a metal to reduce hardness, relieve stress, and improve ductility and conductivity.

1. Wire Drawing

• Copper rod (8 mm) is drawn through a series of dies.
• Final wire sizes are typically 0.10–1.50 mm.
• Drawing causes work hardening, increasing tensile strength and reducing elongation.

2. Electrical Resistance Heating

The wires pass through contact rollers connected to a DC power supply.

Principle

Heat generated:

$$Q = I^2Rt$$

Where:

• Q = Heat generated
• I = Current (A)
• R = Resistance (Ω)
• t = Time

The wire itself acts as a resistance element and is heated instantly.

3. Annealing Temperature

Material	Temperature Range
Copper	350–550°C
Aluminium	250–450°C

The exact temperature depends on:

• Wire diameter
• Drawing speed
• Required elongation
• Conductivity specifications

4. Cooling Section

Immediately after heating, the wire enters a cooling chamber containing:

• Deionized water
• Emulsion solution
• Steam protection (in some machines)

Rapid cooling prevents oxidation and stabilizes the metallurgical structure.

5. Drying

Air wipes or compressed air remove moisture before the wire reaches the take-up system.

Typical Operating Parameters

Parameter	Typical Value
Drawing speed	15–35 m/s
Annealing current	500–3000 A
Voltage	40–80 V
Copper temperature	350–550°C
Elongation after annealing	20–35%
Conductivity	100–102% IACS

Common Defects

Under Annealing

Cause

• Low current
• High line speed

Effect

• Hard wire
• Low elongation
• Frequent breaks during bunching

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Over Annealing

Cause

• Excessive current
• Low production speed

Effect

• Very soft wire
• Reduced tensile strength
• Surface discoloration

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Oxidation

Cause

• Improper cooling water quality
• Air leakage

Effect

• Dark wire surface
• Poor conductivity

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Quality Checks

Elongation Test

Measures ductility of the wire.

Tensile Strength Test

Ensures mechanical properties meet standards.

Conductivity Test

Measured with a conductivity meter and usually expected to be around 100% IACS for annealed copper.

Surface Inspection

Checks for scratches, oxidation, and discoloration.

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Example (Copper Wire)

Input Rod:           8.0 mm
Final Size:          0.20 mm
Drawing Speed:       25 m/s
Annealing Temp:      450°C
Current:             1500 A
Conductivity:        101% IACS
Elongation:          25%