Forging Practice

Forging is shaping metal by hammering or pressing it, generally hot (red or white heat), so that plastic flow produces the required shape. It improves the grain flow and strength.

Operations

• Upsetting: the length of a bar is reduced and the cross-section increased by compressing along its axis, e.g. making a bolt head or a rivet head. If the unsupported length exceeds about 3 times the diameter, the bar bends (buckles).
• Drawing down: the cross-section is reduced and the length increased by hammering between flat or round faces; the metal is turned after each blow.
• Fullering: grooves are made across the bar with a fuller to spread the metal in length (before drawing down) and to form shoulders.
• Bending: the hot bar is bent over the anvil horn or in a bending fork to make hooks, angles and rings.
• Punching: a punch drives a hole through the hot metal; the slug is removed through a pritchel hole (hardie hole). Drifting enlarges the hole.
• Cutting: a hot set or hot chisel cuts the hot bar; a cold set is used for cold work.

Other operations: welding (forge welding), swaging, flattening.

Common forging tools

• Anvil (with horn, face, hardie and pritchel holes), hammers (hand, sledge, ball peen), tongs of various jaws, flatters, set hammer, swages, fullers, drifts, punches, hot sets and chisels, and a forge or furnace.

 upsetting         drawing down
   | |               ___________
  _| |_             /__________/  length increased
  |___|  head

Forging hammers and presses

Drop hammer: ram falls under gravity (board or steam-lifted) on the heated billet in a die. Power hammer: steam or compressed air drives the ram down faster.

Advantages of forging

• Grain flow follows the shape, giving high strength, toughness and fatigue resistance.
• Removes internal voids and blowholes; gives dense structure.
• Good for small and large parts, produces a good surface and accuracy in closed dies.
• Material saving over machining from solid.

Limitations

• High cost of dies and equipment for small batches.
• Complex shapes with deep cavities and undercuts are difficult.
• Hot forging gives scale and poor surface and dimensional accuracy.
• Not suitable for brittle metals; needs heating and skilled operation; some safety risk.

Given: $d_1 = 40$ mm, $h_1 = 120$ mm, $h_2 = 50$ mm, $\sigma_f = 100$ MPa.

(a) Final diameter

Volume is constant:

$$\frac{\pi}{4}d_1^2 h_1 = \frac{\pi}{4}d_2^2 h_2 \Rightarrow d_2 = d_1\sqrt{\frac{h_1}{h_2}}$$ $$d_2 = 40\sqrt{\frac{120}{50}} = 40\times 1.549 = 61.97\ \text{mm}$$

(b) True strain

$$\varepsilon = \ln\frac{h_1}{h_2} = \ln\frac{120}{50} = 0.875$$

(c) Force at the end of the stroke

Final cross-section:

$$A_2 = \frac{\pi}{4}(61.97)^2 = 3016\ \text{mm}^2$$ $$F = \sigma_f A_2 = 100\times 3016 = 301{,}600\ \text{N} \approx 302\ \text{kN}$$

The force is highest at the end of the stroke as the area is largest. With friction the real force would be higher.

Buckling check

$$\frac{h_1}{d_1} = \frac{120}{40} = 3$$

This is at the limit of the rule that the unsupported length should not exceed 3 times the diameter in a single blow. The upsetting should be done in a die or by a series of light blows, or the free length should be reduced.

Answer: d₂ = 61.97 mm; true strain = 0.875; F = 302 kN; h/d = 3 is at the safe limit, so use a die or light blows.