| name | cold-forging |
| description | Cold forging — forward/backward extrusion, ironing, upsetting, flow stress, Forming Limit Diagram, strain hardening, cold forgeable materials, tool design, ICFG standards, tolerance and surface finish. |
| metadata | {"priority":7,"promptSignals":{"phrases":["cold forging","cold extrusion","cold heading","upsetting","backward extrusion","cold forming"],"minScore":3}} |
Cold Forging — Complete Skill
Cold Forging Processes
Definition: metal forming at room temperature (or slightly elevated for difficult alloys); below recrystallization temperature (T_form < 0.3 T_melt)
Result: work hardened; excellent surface finish; close tolerances; no oxidation scale
Typical processes:
- Forward (direct) extrusion: metal flows in direction of punch movement; area reduces
- Backward (indirect) extrusion: metal flows opposite to punch; cup or hollow form
- Upsetting (heading): height reduced; area increased; bolt heads, valve seats
- Ironing: wall thinning at constant diameter; achieves precision tolerances
- Coining: precision surface embossing; closes tolerances
Flow Stress and Strain Hardening
True stress-true strain (power law):
σ_f = C × ε^n [Pa; C = strength coefficient; n = strain hardening exponent]
Or: σ_f = K × (ε₀ + ε)^n [includes pre-strain ε₀]
Material constants (room temperature):
| Material | C [MPa] | n | σ_f at ε=1 [MPa] |
|---|
| 1020 steel (annealed) | 700 | 0.22 | 700 |
| 1045 steel | 850 | 0.20 | 850 |
| 6061-T0 Al | 250 | 0.22 | 250 |
| 2024-T0 Al | 380 | 0.20 | 380 |
| C10400 copper | 530 | 0.34 | 530 |
Maximum achievable strain before fracture:
ε_fracture ≈ (area reduction limit from formability); typically ε = 0.5–2.5 per pass before annealing needed
Forming Pressure Calculation
Forward Extrusion
Pressure at punch:
p_punch = σ_f_avg × (1 + μ × D / 4h) × ln(A₀/A₁) × correction_factors
Where: A₀ = billet area; A₁ = extruded section area; h = die land length; μ = friction
Approximate (slab analysis):
p = σ_f × [ln(A₀/A₁) + μ × L/r₁] [Pa; r₁ = exit radius; L = die land length; μ = friction ≈ 0.08–0.15 with lubricant]
Reduction ratio: R = A₀/A₁; ln R = true strain of extrusion
Backward Extrusion
Punch pressure:
p = σ_f_avg × (1 + 2μD/(3t)) [Pa; t = cup wall thickness; D = punch diameter]
Typically: p = 2–4 × σ_f (higher than forward due to recirculation zone)
Upsetting
Pressure:
p = σ_f × (1 + 2μr/(3h)) [r = billet radius; h = current height; μ = friction]
Without friction: p = σ_f; with friction: p increases as h decreases (barrel)
Buckling limit:
Slenderness ratio H₀/D₀ ≤ 2.5 (unsupported); H₀/D₀ ≤ 5 (with guide tools)
Tool and Press Selection
Press force:
F = p × A_tool [kN; A_tool = cross-sectional area being formed; p = forming pressure]
Die stress check:
Hoop stress in die at inner bore:
σ_θ = p × r_i² / (r_o² - r_i²) [MPa; r_i = inner radius; r_o = outer radius]
For hardened tool steel: σ_θ_allowable = 0.7 × σ_y = 0.7 × 1500 = 1050 MPa
Prestressed die (shrink-ring):
Required if p > 700 MPa; shrink-ring compresses die → reduces hoop tensile stress under pressure
Interference Δ ≈ p × r_i / (E_die × (1 - r_i²/r_o²)) [m]
Press type:
Hydraulic: adjustable stroke; slow; suitable for long strokes; 10–100 MN
Mechanical (crank): high speed (200–1000 parts/min); fixed stroke; most production
Servo press: flexible; high force at any position; preferred for complex parts
Material Selection for Cold Forging
Requirements: good cold formability (high n, high ε_f)
Coldformable steels:
Carbon: 1010–1020 (excellent); 1045 (marginal); > 0.45%C requires intermediate anneal
Micro-alloyed: 35MnB5 for high-strength without heat treatment
Stainless: 303Se, 304 (galling risk without lubrication)
Aluminum: 1xxx, 3xxx, 6061-O (excellent); 7xxx poor cold formability
Copper: C10100 to C26000 (excellent)
Titanium: Ti commercially pure (CP Ti); Ti-6Al-4V requires warm forming (250–350°C)
Lubrication
Critical for cold forging: friction > 0.15 causes scoring and die wear
Zinc phosphate + soap: standard for steel; excellent lubrication; applied to billet surface
Lubricant application:
Phosphating: immerse steel in zinc phosphate bath → porous phosphate layer → absorbs soap
Soap lubricant (sodium stearate): sprayed or tumbled; soap trapped in phosphate pores
Result: coefficient of friction μ = 0.04–0.08 (very low)
Aluminum: castor oil or lanolin; synthetic ester; μ = 0.05–0.1
Copper: mineral oil or soap; phosphating not needed
Tolerances and Surface Finish
Achievable tolerances (cold forging):
Diameter: ±0.05–0.15 mm; length: ±0.1–0.3 mm; weight: ±0.5%
After ironing: ±0.02–0.05 mm on wall thickness
Surface finish:
Ra = 0.4–1.6 μm (better than hot forging Ra 3–6 μm)
No scale; no decarburization (unlike hot forging)
Comparison to machining:
Cold forging: Ra = 0.8–1.6 μm without polishing; near-final form
Machined: Ra = 0.4–1.6 μm with turning/milling → cold forging competitive
ICFG Standards
ICFG (International Cold Forging Group):
Document 11/82: Tolerance recommendations for cold forged parts
Document 14/01: Die materials for cold forging
Document 18/02: Lubrication systems
Output
Provide: forging process type (forward/backward extrusion/upsetting/ironing), billet and part geometry (d₀, d₁, h₀, h₁ [mm]), true strain per pass ε = ln(A₀/A₁), flow stress σ_f at strain [MPa], forming pressure p [MPa], punch/press force F [kN], die design (prestressed check, interference Δ [mm]), number of forging stages, intermediate anneal required (yes/no), lubricant type, tolerances achievable [mm], surface finish Ra [μm], material selection, and applicable ICFG document.
