cold-forging

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.