Forging

Forging

Materials suitable for forging can be formed, mostly when hot, by applying heavy blows or pressure. The forming process causes a plastic change to take place in the solid material. ln contrast to workpieces machined from solid, forgings have a cohesive libre flow. Their structure is dense and their strength correspondingly high, which makes them suitable for items such as crankshafts and connecting rods. Forging is often cheaper than machining.

Forgeabllity of materials

Metals can only be forged if their plasticity (elongation) increases when they are heated; their strength decreases at the same time. The most important forgeable metals are steel, aluminium and its alloys, copper, brass and bronze. Cast iron cannot be forged, since it does not become ductile when heated.

The lower the carbon content of steel, the better it can be forged. Low carbon steels, in other words, are the most suitable for forging. The sulphur content makes the steel brittle when red-hot and leads to crack formation. Too high a phosphorus content makes the steel brittle when cold, so that workpieces tend to fracture when cold formed. Sulphur and phosphorus are therefore to be regarded as harmful components in a forging steel. When forging unalloyed steels, comply with the manufacturer's guidelines.

Forging steel

There are initial and final forging temperature limits which should always be complied with (note the manufacturer‘s recommendations). lf forming takes place within the specified temperature range, the

structure becomes dense and fine, and the strength is high, The lower the steel's carbon content, the higher the forging temperature must be. lf the workpiece is heated beyond the initial temperature and held at this temperature for some time, the steel will become overheated.

Overheated steel is coarse grained and brittle. The coarse grain can be eliminated by normalising. lf steel is heated until sparks appear, it is burnt and must be regarded as permanently unusable. Forged workpieces, particularly those of high-alloy steel, should be allowed to cool as slowly and uniformly as possible, to avoid internal stresses. Each glowing colour corresponds to a different temperature range, so that the colour of the workpiece is a good guide to the temperature it has reached.

Smith’s hearth

A health is used to heat small workpieces. A caking sulphur-free coal (smithy peas) or, in special cases, charcoal which is completely free from sulphur, should be used as fuel. To heat larger workpieces, closed forging furnaces heated by gas or oil are used.

Forging tools

Forging is usually carried out on IHS anvil. This rests on a wooden base which helps to absorb shock and vibration. The face of the anvil is hardened, and contains a round and a square hole. These holes are used to insen auxiliary tools such as the anvil chisel for cutting off, the filing block for rounding, the beak iron forcutting off and bending and various dies for the production of plugs. Forging tongs have various jaw shapes, and are used to hold the hot workpieces while forging. The forging hammer weighs 1 kg~2 kg. For more extensive stretching operations the smith works together with a hammer man, who stands to one side of him and uses the cross pane sledgehammer. A forging vice is used to clamp workpieces. There are forging machines to produce larger workpieces. The twomain categories are machine hammers and presses.

Forging operations

In free forging, the smith obtains the desired workpiece shape by means of hand hammers, machine hammers and forging presses, using simple tools.

ln drop forging, the workpiece is formed in a die to a high level of dimensional accuracy, with the aid of a die forging hammer or press. Die-forged workpleces are of high strength. Forged workpieces which are later to be machined by chip-removing methods include a machining allowance. This is about 3 mm per face on smaller items and 5 to 10 mm for larger ones.

By stretching, the length ol the workpiece is increased; its cross section decreases accordingly. Stretching is carried out with the peen of the cross pane sledge hammer or on the anvil beak, then finished off on the face of the anvil.

By flnishing, forged workpieces can be given a smoother surface.

ln stepping, a sharp·edged step is obtained at the transition from a smaller to a larger cross-section.

In upsetting, the cr0ss-section is heated and thenincreased, so that the length decreases.

When parting off, part of the workpiece is separated from the remainder. A notch is first produced with the cold chisel or anvil chisel, and the workpiece is then bent over the edge of the anvil and struck.

ln punching, the workpiece is struck with a drift over a perforated plate or hole.

In forge welding or pressure welding, the workpleces are welded together at high temperature when they are in a pliable state, by hammering or pressing.

Mass produced parts of high strength can be produced by hammering or pressing in forging dies, and lettled in a deburring die.