Hot and cold forging of steel: temperatures, grain flow and die types

Temperature ranges

Forging is classified by work temperature relative to the steel's recrystallization temperature.

• Hot forging: about 950–1250 °C for steel. The metal recrystallizes as it is worked, flow stress is low and large shape changes are possible in one stroke. Scale forms and dimensional tolerances are wider.
• Warm forging: below the recrystallization range. Press loads are lower than in cold forging and scale is less than in hot forging. It is often chosen for higher-carbon steels that are hard to cold form.
• Cold forging: at or near room temperature. Accuracy and surface finish are best and the steel work hardens, but press loads are high and the steel must be ductile, usually spheroidize annealed.

Grain flow and why forged parts perform better

Rolled bar has a fibre structure along its length, from elongated grains and inclusions. When a part is machined from bar, the cut surfaces cross this fibre and expose its ends. When the same part is forged, the fibre is bent to follow the outline of the part.

Forging also closes internal porosity and breaks up cast structure. Castings have no directional flow and can contain shrinkage porosity.

In parts loaded in fatigue or impact, such as hooks, eye bolts, shackles, crankshafts and pole-line hardware, a contour-following grain flow resists crack initiation and growth better than cut or cast structure. The benefit depends on the load direction; a forged part is not automatically stronger in every direction.

Open-die and closed-die forging

Open-die forging works the metal between flat or simple dies that do not enclose it. It suits large parts, shafts, rings and blocks and short runs, with machining to finish.

Closed-die or impression-die forging presses the metal into shaped cavities in a pair of dies. Excess metal escapes as flash, which is trimmed off. It produces near-net shapes with consistent dimensions in volume. Dimensional tolerances for closed-die steel forgings in Europe are given in EN 10243-1.

Upset forging increases the cross-section at one point of a bar, as in forming a bolt head or a flange on a rod.

Cold heading for fasteners

Most bolts, screws and rivets in common sizes are cold headed. Wire from coil is cut to length and the head is upset in one or more blows at room temperature, at high production rates and with little waste.

The wire is usually spheroidize annealed so it can take the strain without cracking. Cold-heading wire is covered by EN 10263 in Europe, JIS G 3507 in Japan and ASTM F2282 for fastener wire in the US.

Large diameters, long heads and high-alloy steels are hot forged instead, because press loads for cold heading become too high. Forged parts are usually heat treated afterwards, by normalizing or by quenching and tempering, to reach the specified properties.