Forging shapes metal by compressive force in a die (press or hammer). Unlike rolling (continuous), forging makes discrete parts: bolts, rivets, connecting rods, gears, turbine shafts, propellers. Forged parts have good strength/toughness (favorable grain flow).

• Cold forging: greater forces, needs room-T ductility; good finish/accuracy.
• Hot forging: smaller forces; poorer finish/accuracy.

Open-Die Forging (Upsetting)

Workpiece compressed between flat dies. Interface friction opposes outward flow → barreling. Cogging (drawing out) reduces bar thickness in successive steps (lower force, less die wear, but slower).

Force on a solid cylinder:

$F = Y_f\,\pi r^2\left(1 + \frac{2\mu r}{3h}\right)$

$Y_f$ = flow stress, $\mu$ = friction coefficient, $r$ and $h$ = current radius and height.

General (simple or complex shapes):

$F = k\,Y_f\,A$

$k$ = shape factor (from tables), $A$ = projected area.

Closed-Die Forging

Workpiece fills die impressions; excess forms a flash — thin, cools fast, becomes strong, and blocks outflow so the cavity fills (enables complex shapes). Steps: fullering (distribute material away), edging (gather it locally), finishing (final shape). Precision forging needs little machining. Heading = upset the end of a rod (bolts, rivets, nails). Bearing balls: upset a cylinder, then polish.

Dies & Lubrication

Dies need high-T strength/toughness, hardenability, thermal/mechanical-shock and wear resistance — tool/die steels (Cr, Ni, Mo, V). Die inserts are replaceable. Lubricants cut friction/wear, lower force, aid flow, act as a parting agent — but excess lubricant traps in cavities (incompressible) → surface defects.

Forgeability

Ability to deform without cracking. Tests: upsetting (more deformation before cracking = better) and hot-twist (twist to failure at various T to find the optimum forging temperature).

Defects, Machines, Economics

• Defects: too little material → buckling → laps; too much → cracks.
• Presses: hydraulic (constant low speed, slow, costly), mechanical (crank/eccentric, high rate, automatable), screw (small/precision parts).
• Hammers: convert ram potential energy to kinetic — gravity-drop (up to ~120 kJ), power-drop (up to ~1150 kJ).
• Die failures: poor design, defective material, overheating, wear, overloading.
• Economics: material cost/part is constant; tooling cost/part falls as volume rises; labor moderate. Costly at low volumes, competitive at high volumes.
• Easier to cut cavities into a die (raised lettering on the part) than protrusions (recessed lettering).