Forging shapes metal by squeezing it between dies while it's hot and plastic. Unlike casting (which solidifies from liquid) or machining (which removes material), forging refines the internal grain structure by aligning the grain flow with the part's geometry. That grain flow is why forged parts dominate any application where fatigue matters more than cost.

The three main processes:

• Open-die forging: Hot billet hammered or pressed between flat dies. The smith (or 50,000-ton hydraulic press) shapes it by repositioning the workpiece between blows. Used for large shafts, rolls, and turbine rotors up to hundreds of tons.
• Closed-die (impression-die) forging: Heated billet driven into shaped cavities. Multiple stations progressively form the shape — blocker, semi-finisher, finisher. Excess metal squeezes out as flash, which is trimmed off. This is how connecting rods, crankshafts, and wrenches are made.
• Cold forging: Done at room temperature for small parts (bolts, fasteners, gears). Higher forces required, but excellent surface finish and dimensional accuracy. Every bolt head you've tightened was cold-headed.

Why grain flow matters: Cast parts have random grain orientation with internal porosity. Machined parts cut across the rolled grain of bar stock, exposing end grain at critical surfaces. Forging preserves continuous grain that follows the part contour — like wood grain following the shape of a baseball bat. A forged crankshaft's grain wraps around the journals and throws, putting the strongest direction where the bending and torsional loads concentrate.

Real-world example: A typical automotive connecting rod is closed-die forged from 4140 or microalloyed steel, then heat-treated and machined only where it must interface with bearings and the crank. The forging process gives it ~20-30% higher fatigue strength than an equivalent cast or fully-machined part. This is why aircraft landing gear, military rifle bolts, and rail axles are forged — nobody wants to be the engineer who specified a casting where a forging was needed.

Rule of thumb — forging force: For closed-die forging, required press force ≈ k × σ_flow × A_projected, where σ_flow is the material's flow stress at forging temperature (typically 50-150 MPa for hot steel), A is the projected area in the die parting plane, and k is 3-10 depending on shape complexity. A 200 cm² connecting rod in steel needs roughly 600-2,000 tons of press capacity — which is why forge shops are loud, hot, and built on massive foundations.