Metal Forming Processes
Forging, Rolling, Extrusion & Drawing — Hot vs Cold Working, Formulas & GATE Prep
Last Updated: March 2026
Key Takeaways 📌
- Metal forming shapes metal by applying forces that cause plastic deformation — no material is removed.
- Forging: Compressive forces from hammers or presses. Produces superior grain structure and strength.
- Rolling: Metal passed between rotating rolls to reduce thickness. Most widely used forming process by tonnage.
- Extrusion: Metal forced through a die opening to produce continuous cross-sections. Extrusion ratio R = A₀/Af.
- Drawing: Metal pulled through a die to reduce cross-section (wire, tube, rod drawing).
- Hot working: Above recrystallisation temperature (lower forces, no strain hardening). Cold working: Below recrystallisation temperature (better finish, strain hardening).
1. What is Metal Forming?
Metal forming is a manufacturing process in which metal is shaped by plastic deformation — the material is permanently reshaped by applying forces that exceed its yield strength. Unlike machining, no material is removed; unlike casting, the metal remains solid throughout.
The key advantage of forming over other processes is the grain flow. When metal is formed, its internal crystalline grain structure aligns with the shape of the part, producing components with superior mechanical properties — especially fatigue strength and impact resistance. This is why critical components like crankshafts, connecting rods, landing gear, and turbine discs are forged rather than cast or machined from solid.
2. Hot Working vs Cold Working
The distinction is based on the recrystallisation temperature — the temperature above which deformed grains reform into new, strain-free grains.
| Feature | Hot Working | Cold Working |
|---|---|---|
| Temperature | Above recrystallisation temp. | Below recrystallisation temp. (often room temp.) |
| Forces required | Lower (metal is softer) | Higher (metal is harder) |
| Strain hardening | No (recrystallisation removes it) | Yes (material gets harder and stronger) |
| Ductility after forming | High (grains recrystallise) | Reduced (grains are elongated, work-hardened) |
| Surface finish | Poor (oxidation, scale) | Good (no oxidation) |
| Dimensional accuracy | Lower (thermal expansion/contraction) | Higher |
| Large deformations | Possible (no cracking) | Limited (may crack if too much deformation) |
| Examples | Hot rolling, hot forging, hot extrusion | Cold rolling, cold drawing, stamping, coining |
For steel, recrystallisation temperature ≈ 0.4 × Tmelting (in Kelvin) ≈ 400–450°C. Hot working is typically done at 900–1200°C for steel.
3. Forging
Forging shapes metal using compressive forces applied by hammers (impact) or presses (slow squeeze). It produces the strongest parts of any manufacturing process due to aligned grain flow.
| Type | Description | Applications |
|---|---|---|
| Open-die forging | Workpiece compressed between flat dies; free to deform laterally | Large shafts, rings, discs, custom shapes |
| Closed-die (impression) forging | Metal forced into shaped die cavities; produces flash | Crankshafts, connecting rods, gears, wrenches |
| Flashless forging | Precision closed-die with no flash; requires precise volume control | Precision parts, net-shape components |
Open-Die Forging Force (No Friction)
F = σf × A
Where σf = flow stress of the material, A = contact area at any instant.
With friction (barreling): F = σf × A × (1 + 2μr/(3h))
Where μ = friction coefficient, r = workpiece radius, h = height.
Barreling occurs in open-die forging because friction at the die-workpiece interface prevents the material from spreading freely. The centre bulges outward, creating a barrel shape. Lubrication reduces barreling.
4. Rolling
Rolling passes metal between two rotating rolls to reduce its thickness. It is the most widely used metal forming process — more than 90% of all metals produced go through rolling at some stage.
Rolling Parameters
Draft: Δh = h₀ − hf (reduction in thickness per pass)
Maximum draft: Δhmax = μ²R (where μ = friction coefficient, R = roll radius)
Contact arc length: L = √(R × Δh)
Roll separating force: F = σf × w × L (approximately, for flat rolling)
Where w = strip width
Condition for rolling to begin: The metal can only be pulled into the roll gap by friction. The maximum draft is limited by friction: Δhmax = μ²R. Higher friction (rough rolls, hot metal) allows greater reduction per pass. Larger rolls allow greater draft.
Volume Conservation in Rolling
h₀ × w₀ × v₀ = hf × wf × vf
Since width change is often negligible: h₀v₀ ≈ hfvf
The exit speed is higher than the entry speed (since the strip gets thinner).
Types of rolling: flat rolling (sheets, plates), shape rolling (I-beams, rails, angles), ring rolling (seamless rings), thread rolling (bolts), tube rolling.
5. Extrusion
Extrusion forces a billet of metal through a shaped die opening to produce a continuous length of uniform cross-section. Think of it like squeezing toothpaste — the metal comes out in the shape of the die.
| Type | Direction | Characteristics |
|---|---|---|
| Direct (forward) extrusion | Ram pushes billet in the same direction as product exit | Higher friction (billet slides against container). Most common. |
| Indirect (backward) extrusion | Die moves towards the stationary billet | Lower friction (no billet-container sliding). Lower force needed. |
Extrusion Ratio & Force
Extrusion ratio: R = A₀/Af
Where A₀ = initial billet cross-sectional area, Af = final product area
True strain: ε = ln(R) = ln(A₀/Af)
Ideal extrusion force: F = A₀ × σf × ln(R)
Actual force is 2–3× ideal due to friction and redundant deformation.
Extrusion can produce complex cross-sections that would be impossible to roll — aluminium window frames, heat sink profiles, structural channels with intricate shapes.
6. Drawing
Drawing pulls metal through a die to reduce its cross-section. Unlike extrusion (which pushes), drawing pulls — the drawing force is limited by the tensile strength of the drawn product.
| Type | Product |
|---|---|
| Wire drawing | Wire (0.01 mm to 10 mm diameter) |
| Rod drawing | Rods and bars (> 10 mm) |
| Tube drawing | Seamless tubes with controlled wall thickness |
Drawing Force (Ideal)
F = Af × σf × ln(A₀/Af)
Note: force acts on the FINAL (smaller) cross-section — this limits the maximum reduction per pass.
Maximum reduction per pass ≈ 63% (theoretically, when σdraw = σf). Practical: 20–45% per pass.
7. Process Comparison
| Feature | Forging | Rolling | Extrusion | Drawing |
|---|---|---|---|---|
| Force type | Compression | Compression (rolls) | Compression (ram) | Tension (pull) |
| Product shape | Discrete 3D parts | Flat sheets, profiles | Continuous profiles | Wire, rod, tube |
| Temperature | Hot or cold | Hot or cold | Hot or cold | Usually cold |
| Strength of product | Highest (aligned grain) | High | Good | High (strain hardened) |
| Volume | Low–medium | Very high | Medium–high | High (continuous) |
8. Worked Numerical Examples
Example 1: Rolling — Maximum Draft
Problem: Roll radius = 300 mm, friction coefficient = 0.2. Find the maximum draft and the minimum final thickness if initial thickness is 50 mm.
Solution
Δhmax = μ²R = 0.2² × 300 = 0.04 × 300 = 12 mm
hf,min = h₀ − Δhmax = 50 − 12 = 38 mm
Example 2: Extrusion Ratio & Force
Problem: A billet of diameter 100 mm is extruded to diameter 30 mm. Flow stress = 250 MPa. Find the extrusion ratio and ideal force.
Solution
A₀ = π(100)²/4 = 7854 mm², Af = π(30)²/4 = 706.9 mm²
R = A₀/Af = 7854/706.9 = 11.1
ε = ln(11.1) = 2.407
F = A₀ × σf × ln(R) = 7854 × 250 × 2.407 = 4,727,000 N ≈ 4,727 kN
Actual force would be 2–3× this due to friction.
9. Common Mistakes Students Make
- Confusing extrusion and drawing: Extrusion PUSHES (compressive force from ram). Drawing PULLS (tensile force on the product). In drawing, the force is limited by the product’s tensile strength; in extrusion, it is not.
- Using Δh instead of μ²R for maximum draft: The condition for rolling is Δh ≤ μ²R. Students sometimes forget the square on μ, which gives a dramatically wrong answer.
- Forgetting that hot working occurs above recrystallisation temperature, not melting temperature: Steel recrystallises at ~450°C but melts at ~1500°C. Hot working is done well below melting.
- Thinking cold-worked parts are weaker: Cold working actually increases yield strength through strain hardening. The part is harder and stronger but less ductile. This is often used intentionally (cold-rolled steel is stronger than hot-rolled).
10. Frequently Asked Questions
What is the difference between hot working and cold working?
Hot working occurs above the material’s recrystallisation temperature — deformed grains reform into new grains, so there is no strain hardening and large deformations are possible. Cold working occurs below recrystallisation temperature — the material strain-hardens (gets harder and stronger), surface finish is better, and dimensional accuracy is higher, but ductility is reduced and forces are greater.
Why is forging stronger than casting?
Forging aligns the internal grain structure of the metal along the shape of the part, creating a continuous grain flow. This aligned structure resists fatigue cracking and impact loading much better than the random grain structure of a casting. Castings can also have internal defects (porosity, shrinkage) that forging eliminates through compressive deformation.