Introduction
Mechanical working of metals is the process of shaping metals by applying mechanical force. In this process, metals are plastically deformed into required shapes and sizes without removing material. Mechanical working improves the mechanical properties of metals such as strength, hardness, and toughness, and is widely used in manufacturing industries.
Objectives of Mechanical Working
- To change the shape and size of metals
- To improve mechanical properties
- To refine grain structure
- To increase strength and hardness
- To produce components with better surface finish
- To reduce casting defects like porosity
Types of Mechanical Working
Mechanical working is broadly classified into:
- Hot Working
- Cold Working
1. Hot Working of Metals
Definition
Hot working is the process of deforming metals above their recrystallization temperature. At this temperature, new grains are continuously formed, preventing strain hardening.
Characteristics of Hot Working
- Large deformation is possible
- No strain hardening
- Low force required
- Improved ductility
- Refined grain structure
Advantages
- Improves mechanical properties
- Eliminates internal stresses
- Reduces brittleness
- Suitable for large sections
Disadvantages
- Poor surface finish
- Oxidation and scaling occur
- Less dimensional accuracy
Common Hot Working Processes
(a) Hot Rolling
- Metal is passed between rotating rolls to reduce thickness.
- Used to produce sheets, plates, and rails.
(b) Hot Forging
- Metal is shaped by hammering or pressing at high temperature.
- Produces strong and tough components.
(c) Hot Extrusion
- Heated metal is forced through a die to produce long sections of uniform cross-section.
2. Cold Working of Metals
Definition
Cold working is the deformation of metals below their recrystallization temperature, usually at room temperature.
Characteristics of Cold Working
- Causes strain hardening
- Requires high force
- Improves surface finish
- High dimensional accuracy
Advantages
- Increased strength and hardness
- Better surface finish
- Close dimensional control
- No oxidation
Disadvantages
- Reduced ductility
- Higher power requirement
- Residual stresses develop
Common Cold Working Processes
(a) Cold Rolling
- Reduces thickness and improves surface finish.
- Used for sheets and strips.
(b) Cold Drawing
- Metal is pulled through a die to reduce diameter.
- Used for wires and rods.
(c) Cold Forging
- Shaping of metal at room temperature.
- Used for bolts, screws, and fasteners.
Comparison Between Hot Working and Cold Working
| Hot Working | Cold Working |
|---|---|
| Done above recrystallization temperature | Done below recrystallization temperature |
| No strain hardening | Strain hardening occurs |
| Low strength, high ductility | High strength, low ductility |
| Poor surface finish | Good surface finish |
| Low dimensional accuracy | High dimensional accuracy |
Effects of Mechanical Working on Metals
- Grain refinement
- Increased strength and hardness
- Improved toughness
- Directional properties develop
- Reduction of casting defects
Applications of Mechanical Working
- Manufacturing of sheets, rods, wires, and tubes
- Production of automobile and aircraft parts
- Construction materials
- Tools, fasteners, and machine components
Advantages of Mechanical Working
- Better mechanical properties
- Economical for mass production
- Less material wastage
- Stronger products compared to casting
Limitations of Mechanical Working
- High initial equipment cost
- Not suitable for brittle materials
- Complex shapes are difficult to produce