Hot and Cold working

Metals can be cast to shape by melting them and pouring them into moulds. Metals can also be cut to shape by using hand tools and machine tools. However, there is the alternative of working them to shape. This is how a Blacksmith shapes metal by hammering it to shape on the anvil. Metals that have been shaped by working (also called ‘flow forming’ ) are said to be wrought. Metals may be worked hot or cold. In either case the crystalline structure of the metal is distorted by the working process and the metal becomes harder, stronger and less ductile.

In cold-working metals the crystalline structure (grain) becomes distorted due to the processing and remains distorted when the processing has finished. This leaves the metal harder, stronger and less ductile. Unfortunately, further cold working could cause the metal to crack.
In hot-working metals the grain also becomes distorted but the metal is sufficiently hot for the grains to reform as fast as the distortion occurs, leaving the metal soft and ductile. Some grain refinement will occur so that the metal should be stronger than at the start of the process. The metal is easier to work at high temperatures and less force is required to form it. This is why the Blacksmith gets the metal red-hot before forging it to shape with a hammer.
Recrystallization is the term used when the distorted grains reform when they are heated. The temperature at which this happens depends upon the type of metal or alloy and how severely it has been previously processed by cold working.
Cold-working is the flow forming of metal below the temperature of recrystallisation (for example, cold heading rivets).
Hot-working is the flow forming of metals above the temperature of recrystallisation (for example, rolling red-hot ingots into girders in a steel mill).

Hot-working processes

Advantages		Limitations
Low cost Grain refinement from cast structure 3_ Materials are left in the fully annealed condition and are suitable for cold working (heading, bending. etc.) Scale gives some protection against corrosion during storage Availability as sections (girders) and forgings as well as the more usual bars, rods, sheets. strip and butt-welded tube		Poor surface finish – rough and scaly Due to shrinkage on cooling the dimensional accuracy of hot-worked components is of a low order Due to distortion on cooling and to the processes involved, hot working generally leads to geometrical inaccuracy Fully annealed condition of the material coupled with a relatively coarse grain leads to a poor finish when machined Low strength and rigidity for metal considered Damage to tooling from abrasive scale on metal surface

Cold-working processes

Advantages		Limitations
Good surface finish Relatively high dimensional accuracy Relatively high geometrical accuracy Work hardening caused during the cold-working processes: increases strength and rigidity improves the machining characteristics of the metal so that a good finish is more easily achieved		Higher cost than for hot-worked materials. It is only a finishing process for material previously hot worked. Therefore, the processing cost is added to the hot-worked cost Materials lack ductility due to work hardening and are less suitable for bending, etc. Clean surface is easily corroded Availability limited to rods and bars, also sheets and strip. solid drawn tubes

Technical