Metal forming

In short

Metal forming shapes metal by plastic deformation - the volume stays the same and the material simply flows - instead of removing it as chips or melting it. The four bulk processes examined everywhere are rolling, forging, extrusion and drawing; between them they make nearly all steel plate, rails, TMT bars, crankshafts, aluminium window sections and wire used in India. It is a heavy-weight GATE topic (flow stress, maximum draft, forging load, extrusion ratio) and a standard interview area for Tata Steel, JSW, SAIL, Bharat Forge, Ramkrishna Forgings, Hindalco, Jindal Aluminium, Sundram Fasteners and every automotive Tier-1. Forming also improves properties - it breaks up cast structure, closes porosity and gives a favourable grain flow, which is why safety-critical parts are forged rather than cast.

Explained

Plastic deformation: the fundamentals

• Elastic vs plastic: below yield, strain is recoverable; above yield, dislocations glide on slip planes and the shape change is permanent. Forming always works above the yield point.
• Volume constancy: A₀l₀ = A l (incompressible plastic flow), so the sum of the three principal true strains is zero.
• True (natural) strain ε = ln(l/l₀) = ln(A₀/A) is additive over passes; engineering strain is not, so forming analysis always uses true strain.
• Flow curve σ = Kεⁿ: K is the strength coefficient (MPa), n the strain-hardening exponent (0.05-0.5). Tensile necking begins at ε = n.
• Mean flow stress Ȳ = Kεⁿ/(1+n) is the single value used for load and work calculations over a pass.
• Yield criteria: Tresca (maximum shear) and von Mises (distortion energy). In plane strain (wide rolling, strip, forging of long bars) the flow stress rises to 1.155 σ₀ = 2k.
• Strain rate sensitivity: hot flow stress σ = C ε̇ᵐ, with m ≈ 0.05-0.4 hot and ~0 cold; m ≈ 0.5-0.8 gives superplasticity.
• Workability/formability is the amount of deformation a metal takes before cracking - FCC metals (Al, Cu, austenitic steel) form best; HCP metals (Mg, Zn, Ti) need heating.
• Redundant work and friction: real forming needs more energy than ideal homogeneous deformation. Efficiency η = ideal work / actual work, typically 0.5-0.65 for drawing and extrusion.

Hot, cold and warm working

• Recrystallisation temperature ≈ 0.4 Tm (absolute). Working above it is hot, below it is cold, and 0.3-0.5 Tm is warm.
• Hot working: flow stress low, ductility high, unlimited deformation, no strain hardening (recovery and recrystallisation keep pace), cast dendrites and porosity broken down. Drawbacks: oxidation and scale, poor surface finish and tolerance, die wear, scale pits, energy for furnaces, shorter die life.
• Cold working: good finish and close tolerance, strain hardening raises strength (used deliberately in cold-drawn bar and cold-forged bolts), no scale. Drawbacks: high forces, limited ductility, need for intermediate annealing, directional (anisotropic) properties, residual stresses.
• Warm working: compromise - lower force than cold, better finish than hot; used in warm forging of gear blanks and CV joints.
• Hot-worked steel shows a fibrous macrostructure (grain flow lines). Flow lines following the part contour give high fatigue strength; flow lines cut by machining or exposed at a surface are a weak point.

Rolling

• Metal is squeezed between two rolls rotating in opposite directions; thickness falls, length increases, width spreads only slightly (spread is neglected for wide strip - plane strain).
• Draft Δh = h₀ − h₁; reduction r = Δh/h₀; contact (arc) length L ≈ √(R Δh).
• Bite condition: the roll must grip the slab - tan α ≤ μ, which gives maximum draft Δh_max = μ²R. Hence big rolls and rough (ragged) rolls for the first hot passes.
• Neutral (no-slip) point: entry speed of metal < roll surface speed, exit speed > roll speed. Friction acts forward before the neutral point and backward after it, giving the friction hill of roll pressure. Forward slip S = (v_f − v_r)/v_r.
• Roll flattening (elastic deformation of rolls) increases contact length and force; roll camber (slight barrel) compensates for roll bending so the strip is not thicker at the centre.
• Minimum rollable thickness falls with smaller roll diameter, better lubrication and applied front/back tension - hence 4-high and cluster (Sendzimir) mills with tiny work rolls backed up by large rolls for foil.
• Mill types: two-high (reversing or pull-over), three-high, four-high, cluster, tandem (continuous, many stands), planetary, universal (edging rolls).
• Shape and special rolling: blooms/billets/slabs from continuous casters, structural shapes through profiled grooves (rails, angles, channels), ring rolling (bearing races, flanges), thread and gear rolling (cold, chipless, stronger than cut threads), roll bonding (clad plate), skin pass/temper rolling.
• Defects: wavy edges, centre split and edge cracks (from roll bending and non-uniform flow), alligatoring (slab splits along mid-thickness), zipper cracks, laps, rolled-in scale, camber, roll marks.
• Indian practice: structural and plate mills work to IS 2062 (hot-rolled structural steel), reinforcement to IS 1786 (high-strength deformed/TMT bars) and IS 432; cold-reduced sheet to IS 513. Many rolled steel products carry mandatory BIS marking under the Steel and Steel Products (Quality Control) Orders - the covered list is revised from time to time.

Forging

• Compressive working between dies, in a hammer (impact) or press (squeeze).
• Open-die forging: flat or simple dies, metal free to flow sideways. Upsetting (height down, diameter up) and cogging/drawing out (length up) are the basic operations; friction at the die faces causes barrelling. Used for one-off large shafts, turbine rotors and rings.
• Impression (closed) die forging: the cavity shapes the part, surplus metal escapes as flash; the thin, cooled flash land builds back-pressure that forces the cavity to fill. Needs a sequence - fuller/edger, blocker, finisher.
• Flashless / precision forging: billet volume must be exact; near-net-shape connecting rods and gears, little machining.
• Related processes: roll forging, rotary swaging, radial forging, orbital forging, coining (closed die, no flash - coins and medals), hubbing, isothermal/hot-die forging (die heated to billet temperature, for titanium and superalloys), cold forging/heading of fasteners.
• Design rules: draft angles (about 3-7° for steel, 1-3° for aluminium), generous fillets and corner radii, a sensible parting line, machining allowance, a web thickness the metal can fill, and avoiding sharp changes of section. Upsetting without buckling needs free length ≤ about 3 × diameter.
• Equipment: board drop, power (air/steam) and counterblow hammers are rated by energy per blow; mechanical (crank/eccentric) presses by force at bottom of stroke and fixed stroke; hydraulic presses give slow, controlled, constant force over the full stroke; screw presses give energy and force.
• Forgeability tests: hot twist test, upset (compression) test to first crack, notched-bar forging test, wedge forging test.
• Defects: unfilled sections, cold shut/lap (two flowing surfaces fold together without welding, also from buckling), die shift, flash-line cracks, scale pits, internal bursts from secondary tensile stresses in cogging/upsetting, decarburised surface, flakes (hydrogen).
• Indian practice: forging stock and forgings to IS 1875 (carbon steel billets/blooms for forging) and IS 2004 (carbon steel forgings for general engineering); steel grades designated per IS 1570. Hammer and press guarding, noise and worker safety come under the Occupational Safety, Health and Working Conditions Code, 2020 (OSH Code, in force 21 November 2025, which repealed the Factories Act, 1948) and the State Factories Rules saved under it. Pune is India's forging hub - Bharat Forge is among the largest single-location forging plants in the world.

Extrusion

• The billet is pushed through a die so the section takes the die shape - a process of pure compression, so very brittle-prone materials can still be extruded.
• Direct (forward): ram pushes billet through the die at the far end; billet slides along the container so friction is high and force falls as the billet shortens; a dead-metal zone forms at the die corners.
• Indirect (backward): the die is carried on a hollow ram and moves into a closed container; no billet-container sliding, so force is lower and almost constant, but the ram must be hollow (load limit).
• Hydrostatic extrusion: fluid pressure surrounds the billet - no container friction, very high reductions, good for brittle metals; equipment is costly.
• Impact extrusion (cold, for tubes and cases), lateral extrusion, tube extrusion over a mandrel or through a spider/porthole/bridge die (which produces longitudinal weld seams in aluminium tube).
• Extrusion ratio R = A₀/A_f (15-50 typical, up to 100+ for aluminium); ideal pressure p = Ȳ ln R; practice uses Johnson's equation or an extrusion constant K_x.
• Hot extrusion lubrication: graphite and glass - the Ugine-Séjournet process uses a glass pad that melts into a thin lubricating, insulating film for steel.
• Defects: centre burst / chevron cracking (hydrostatic tension on the axis, large die angle and low extrusion ratio), surface (speed/fir-tree) cracking from too high a temperature or speed, piping / tailpipe / fishtail (oxide skin sucked into the centre at the back end - avoided by a dummy block or by discarding the butt), blisters.
• Aluminium extrusions in India are supplied to IS 733 (bars, rods, sections) and IS 1285 (round tube and hollow sections); builders' sections also follow IS 63-series alloy designations.

Drawing

• The workpiece is pulled through a die, so tensile stress is applied while the die imposes compression - reduction per pass is therefore limited by the risk of the wire breaking.
• Rod and wire drawing: hot-rolled rod is descaled (pickled or mechanically), coated (lime, borax, phosphate or soap), pointed, then drawn through a series of dies on a multi-block bull-block machine; cold-drawn wire needs patenting/annealing between groups of passes.
• Die geometry: bell (entry) → approach cone (half-angle 6-10° typical) → bearing/land → back relief. Dies are tungsten carbide for coarse wire and polycrystalline/natural diamond for fine wire.
• Practical reduction per pass is 20-30% for wire, 20-45% for rod; the theoretical maximum for a non-hardening metal without friction is 63% (ε = 1), because the required drawing stress then equals the flow stress.
• Tube drawing: sinking (no internal support, wall thickens slightly), fixed plug, floating plug, and moving mandrel (best wall control).
• Residual stresses: light drawing passes deform only the surface and leave longitudinal tensile residual stress at the surface - the cause of season (stress-corrosion) cracking in cold-drawn brass; heavier reductions or stress-relief annealing avoid it.
• Defects: centre bursts, seams, fins, laps, and die-pick-up marks. Mild steel wire is supplied to IS 280; steel wire ropes to IS 2266 (Usha Martin, Ranchi, is a major Indian producer).

Analysis methods, friction and newer processes

• Slab (free-body) method: equilibrium on a thin element plus a yield criterion - gives the friction hill in rolling and the pressure distribution in upsetting. Most examined.
• Uniform-deformation energy method (ideal work), slip-line field (plane strain, exact), upper- and lower-bound theorems (upper bound over-estimates load, safe for press selection), visioplasticity, and FEM (DEFORM, Forge, QFORM) for industrial die design.
• Friction: Coulomb μ at low pressure; at high interface pressure flow develops sticking friction and the shear stress saturates at τ = m·k (friction factor m). Typical μ: 0.05-0.1 cold with lubricant, 0.2-0.7 hot without.
• Lubricants: mineral/soluble oils and soaps cold; graphite in oil or water for hot forging; glass for hot steel extrusion; phosphate + soap conversion coatings for cold forging and drawing.
• Newer/special processes: thixoforming (semi-solid), equal-channel angular pressing (ECAP) for ultrafine-grained metals, incremental bulk forming, electromagnetic and explosive forming for high strain-rate work, superplastic forming of Al and Ti alloys.

Key points

• Metal forming - permanent shaping by plastic flow at constant volume; no chips, high material utilisation.
• True strain - ε = ln(A₀/A); additive across passes, unlike engineering strain.
• Flow curve - σ = Kεⁿ; mean flow stress Ȳ = Kεⁿ/(1+n) is used for forming loads.
• Recrystallisation temperature - about 0.4 Tm in kelvin; separates hot from cold working.
• Hot working - low force, unlimited strain, no hardening, but scale and poor tolerance.
• Cold working - high force, limited strain, but good finish, tight tolerance and strain-hardened strength.
• Plane-strain flow stress - 1.155 σ₀ (von Mises), applies to wide strip rolling and long forgings.
• Maximum draft in rolling - Δh_max = μ²R; bigger rolls or rougher rolls bite deeper.
• Neutral point / friction hill - roll pressure peaks where metal and roll speeds are equal; front and back tension lower the peak.
• Roll camber - a slight barrel ground on rolls to offset roll bending and keep strip gauge uniform.
• Flash in impression forging - deliberate surplus; its back-pressure makes the die fill, then it is trimmed.
• Barrelling - friction at the die faces restrains the metal, so an upset cylinder bulges.
• Hammer vs press - hammers are rated in energy per blow, presses in force; hydraulic presses give slow controlled flow for large forgings.
• Extrusion ratio - R = A₀/A_f; ideal pressure p = Ȳ ln R; direct extrusion force falls with stroke, indirect stays flat.
• Centre burst - axial cracking from hydrostatic tension; large die angle plus low extrusion ratio is the dangerous combination.
• Drawing limit - theoretical maximum reduction 63% per pass; practice 20-30% because drawing stress must stay below the exit flow stress.
• Grain flow - forged fibre lines following the contour give fatigue strength; cast or machined-through flow lines do not.
• Upper-bound solution - over-estimates the load, so it is the safe one for sizing a press.

Formulas and examples

1. Rolling - draft check, load and power. A 300 mm wide steel slab is rolled from 25 mm to 20 mm on 500 mm diameter rolls at 50 rpm; μ = 0.10, K = 240 MPa, n = 0.15.

• Draft Δh = 5 mm. Maximum draft = μ²R = 0.10² × 250 = 2.5 mm → the pass is *not* possible. The friction needed is μ = √(Δh/R) = √(5/250) = 0.141, so the rolls must be roughened (or the pass split in two).
• Taking μ = 0.15: L = √(250 × 5) = 35.36 mm; ε = ln(25/20) = 0.2231.
• Ȳ = 240 × 0.2231^0.15 / 1.15 = 240 × 0.7986 / 1.15 = 166.7 MPa.
• F = 166.7 × 300 × 35.36 = 1.77 × 10⁶ N ≈ 1.77 MN (about 180 tonnes).
• T = F L/2 = 1.77 × 10⁶ × 17.68 = 31.3 × 10⁶ N·mm = 31.3 kN·m per roll.
• Power = 2 × 2π × (50/60) × 31,300 = 327 kW for the two rolls.

2. Open-die upsetting - force and blows. A 50 mm diameter, 75 mm high medium-carbon steel billet is cold upset to 50 mm height; K = 600 MPa, n = 0.12, μ = 0.20.

• Volume = (π/4)(50²)(75) = 147,262 mm³. At h = 50 mm, A = 147,262/50 = 2,945 mm² → D = 61.2 mm.
• ε = ln(75/50) = 0.4055; Y_f = 600 × 0.4055^0.12 = 600 × 0.8973 = 538 MPa.
• K_f = 1 + 0.4 × 0.20 × 61.2/50 = 1.098.
• F = 1.098 × 538 × 2,945 = 1.74 × 10⁶ N ≈ 1.74 MN (about 177 tonnes) - so a 200 t press.
• Work = V × Kεⁿ⁺¹/(1+n) = 147,262 × 600 × 0.4055^1.12/1.12 = 147,262 × 194.9 = 28.7 kJ. A 1,000 kg tup falling 2 m delivers 1,000 × 9.81 × 2 = 19.6 kJ, so two blows minimum (more, allowing for friction and rebound losses).

3. Direct extrusion of aluminium. A 150 mm diameter billet is extruded hot to a 50 mm diameter round; mean flow stress 60 MPa.

• A₀ = (π/4)(150²) = 17,671 mm²; R_x = (150/50)² = 9; ε = ln 9 = 2.197.
• Ideal pressure = 60 × 2.197 = 131.8 MPa → F = 2.33 MN (238 t) - an under-estimate, since friction and redundant work are ignored.
• Johnson with a = 0.8, b = 1.5: ε_x = 0.8 + 1.5(2.197) = 4.096 → p = 60 × 4.096 = 245.8 MPa → F = 245.8 × 17,671 = 4.34 MN ≈ 443 tonnes. Select a 500 t press.

4. Wire drawing - stress, force and power. Steel wire is drawn from 5.0 mm to 4.0 mm diameter; K = 1,300 MPa, n = 0.30, efficiency η = 0.60, speed 5 m/s.

• A₀ = 19.63 mm², A_f = 12.57 mm² → reduction r = 1 − 12.57/19.63 = 36% (high but feasible).
• ε = ln(19.63/12.57) = ln 1.5625 = 0.4463; Ȳ = 1,300 × 0.4463^0.30/1.30 = 1,300 × 0.7851/1.30 = 785 MPa.
• Ideal σ_d = 785 × 0.4463 = 350 MPa; actual σ_d = 350/0.60 = 584 MPa.
• Exit flow stress Y_f = 1,300 × 0.7851 = 1,021 MPa > 584 MPa, so the wire will not neck - the pass is safe.
• P = 584 × 12.57 = 7,340 N; power = 7,340 × 5 = 36.7 kW.

5. Process comparison.

Where it is used

• Integrated steel plants: SAIL (Bhilai rail and structural mill, Rourkela, Durgapur wheel and axle plant), Tata Steel, JSW, AM/NS - hot and cold strip mills, plate mills, TMT bar mills (IS 1786), seamless tube mills.
• Automotive forging: Bharat Forge, Ramkrishna Forgings, MM Forgings, Happy Forgings - crankshafts, front axle beams, connecting rods, steering knuckles for Tata Motors, Ashok Leyland, Mahindra and global OEMs; cold-forged fasteners at Sundram Fasteners.
• Aluminium extrusion: Hindalco, Jindal Aluminium (Bengaluru), Bhoruka - window and curtain-wall sections, heat sinks, truck and metro-coach profiles, conductor and busbar.
• Wire and wire rope: Tata Wiron, Usha Martin - spring, tyre-bead, ACSR conductor, pre-stressing strand, elevator and crane ropes; drawn rods for fasteners.
• Heavy and power forgings: L&T Hazira, BHEL Haridwar - turbine rotors, generator shafts, pressure-vessel shells and flanges (open-die forging and ring rolling, with ASME/IBR inspection).
• Job roles: die designer (DEFORM/QFORM simulation), forge shop and rolling mill production engineer, process planner for pass schedules, quality engineer on grain-flow macro-etch and ultrasonic testing, tool and die maintenance.

Interview questions

• Q: Why is a forged component usually stronger than a cast one of the same material? A: Forging breaks up the cast dendritic structure, closes porosity and aligns the grain flow with the part contour, giving higher toughness and fatigue strength.
• Q: What decides the maximum draft in a rolling pass? A: Friction - the rolls must grip the stock, so Δh ≤ μ²R; larger-diameter or rougher rolls allow a bigger draft.
• Q: What is the neutral point in rolling? A: The point along the arc of contact where the metal and the roll surface move at the same speed; friction drags the metal forward before it and backward after it.
• Q: Why is flash deliberately created in impression-die forging? A: The thin, cooled flash land resists outflow and builds up pressure, forcing the metal to fill the thin and remote parts of the cavity; it is trimmed afterwards.
• Q: Why does an upset cylinder barrel? A: Friction at the die faces restrains the metal touching them, while the mid-height material is free to flow radially.
• Q: Direct or indirect extrusion - which needs less force, and why? A: Indirect, because the billet does not slide along the container wall, so container friction is eliminated and the force stays almost constant through the stroke.
• Q: Why can brittle materials be extruded but not drawn? A: Extrusion loads the metal almost entirely in compression (hydrostatic), which suppresses cracking; drawing applies tension to the already-deformed material.
• Q: Why is reduction per pass in wire drawing limited to about 20-30%? A: Because the pulling stress must stay below the flow stress of the exiting wire; the theoretical limit is 63% and friction, redundant work and a safety margin cut it well below that.
• Q: What causes centre burst in extruded or drawn bar? A: Hydrostatic tensile stress on the axis, promoted by a large die angle with a small reduction, which deforms the surface more than the core.
• Q: Which lubricant is used for hot extrusion of steel and why? A: Molten glass (Séjournet process) - it forms a thin, low-friction film that also insulates the hot billet from the die and container.
• Q: Why do we use true strain rather than engineering strain in forming? A: True strain is additive over successive passes and consistent with volume constancy; engineering strain is not, and it misrepresents large deformations.
• Q: A press or a hammer for a large open-die forging? A: A hydraulic press - its slow, controlled, constant force deforms the whole section right through, whereas a hammer blow deforms mainly the surface layers.

Common mistakes

• Mixing up engineering and true strain, or adding engineering strains across passes.
• Using the instantaneous flow stress Kεⁿ where the mean flow stress Kεⁿ/(1+n) is required for load or work, and vice versa.
• Forgetting the plane-strain factor 1.155 σ₀ for wide strip rolling and long forgings.
• Ignoring the bite condition and happily computing a load for a draft the rolls cannot grip.
• Using the initial area instead of the instantaneous area (and instantaneous D/h) in upsetting force calculations.
• Quoting ideal extrusion pressure Ȳ ln R as the answer and under-sizing the press - friction and redundant work can double it.
• Calling hot working "working at red heat": it is defined relative to the recrystallisation temperature, so lead is hot-worked at room temperature and tungsten is cold-worked at 1,000 °C.
• Unit slips - mm with m, MPa with N/mm² confusion in power terms, or rpm left un-converted to rev/s.

Related topics

Casting · Sheet metal and press tools · Powder metallurgy · Engineering materials and heat treatment · Strength of materials: stress, strain and yield criteria · Additive manufacturing (3D printing)