Temperatures can be judged by observing the colour of the metal as its temperature rises. This can only be learned by practice. The following guide uses terms typical of a smith's workshop.
This is the process of making metal longer and thinner and is usually carried out at near-welding heat. On heavy work this is more easily carried out either between top and bottom fullers or by using the top fuller only with the job on the anvil face.
UPSETTING OR JUMPING UP
This process makes metal shorter and thicker and is usually carried out at near-welding temperature. Metal can be either thickened at the ends of bars or swollen in the centre. The job should be carried out in stages keeping control of the heat by cooling with water in the appropriate places.
This can sometimes be carried out cold but is preferably done at a bright red heat. Bending can be carried out over the edge of the anvil, over the beak, by using a swage block or forked tools that can be hand-operated, held in a vice or placed in the tool hole of the anvil. During bending, the metal on the outside of the bend is subjected to a stretching action while that on the inside of the bend is subjected to compression or upsetting.
PUNCHING AND DRIFTING
Punching is best carried out at near-welding heat. If the hole is deep, the metal contracts around the punch and it then must be withdrawn and cooled after every two or three blows. The punch itself becomes hot during punching operations. If it is allowed to grow too hot and soften the end of the punch will enlarge, and it will become difficult if not impossible to remove from the work. When deep holes are to be punched, a little coal dust sprinkled into the hole from time to time generates gases that assist in the removal of the punch.
Punching should produce a hole of nearly finished size and shape but leaving work to be completed by drifting. Drifts should be of the correct size and shape required for the finished hole and are driven through the hole at a lower temperature. For very accurate holes, such as those needed for thread-tapping, the drift is driven through the work while the metal is barely at red heat. This minimizes loss of size owing to contraction of the metal. A little oil or grease applied to a drift will facilitate work and give an improved finish.
Hot-cutting is carried out using chisels or sets designed for the purpose. A bright red heat is usual and the chisel or set is placed in position on the work and struck with either a hand hammer or a sledgehammer. Chisels or sets may be straight-edged, curved or V-edged, depending on the work to be done. Cutting should be carried out on the cutting platform of the anvil or on a suitable metal shield placed over the anvil face. Chisel or set edges must not be allowed to impinge on the anvil face or they will damage both the tool and the anvil.
This is the procedure where two or more pieces of metal while in a plastic state are joined together by hammering. Much practice is needed before even the simplest weld can be carried out with success. Iron and most grades of mild steel can be welded without the use of a fluxing agent. Where difficulty is experienced and for higher carbon steels, borax or a commercial flux can be used. If these are unavailable, powdered white sand or glass can sometimes be of help. Sprinkled on to the work when it is nearing white heat, the sand or glass melts and forms a protective film on the metal, thus preventing oxidization. This film must be shaken off the metal before hammering the pieces together.
The skill of producing sound welds in various sections of metal is well worth mastering. The ability to join pieces together can frequently obviate the need for a great deal of heavy hammering. In light sections of metal, such as rods and bars, welds can often be carried out more efficiently by a smith than by electric-arc or oxyacetylene welding.
All blacksmithing work is based on the seven techniques described above. However, hardening and tempering must be added. These operations are not really blacksmithing operations, but the ability to harden and temper small tools is of great value to smiths. They should be able to harden and temper their own tools, and they will often be called upon to refurbish tools for other tradesmen, work which often includes hardening and tempering.
HARDENING AND TEMPERING
The smith is mostly concerned with plain carbon steels, which with a little knowledge and much practice can be successfully hardened and tempered in the forge. Iron is comparatively soft and ductile. The addition of carbon to iron produces steel. Carbon steel contains between 0.1 and 1.4 percent carbon. Beyond 1.4 percent carbon, steel is moving into the field of cast iron (fig 22).
Iron consists of crystals that are almost pure metal. When carbon is added at low percentages, it chemically combines with the iron and a difference in some of the crystals can be observed under a microscope. This difference is called pearlite. As the percentage of carbon is increased more pearlite is formed, and by the time 0.85 percent carbon is reached the whole matrix is pearlite. Another structure is also beginning to appear along the grain boundaries called cementite. By the time 1.4 percent is reached, considerable amounts of cementite become obvious (fig 23).
With carbon steels, the degree of hardness can be varied by heat treatment. Maximum softness can be obtained by heating above the upper critical limit (about 720°C for a 0.85 percent carbon steel) and then cooling slowly in lime, sand or ash. This is called annealing.
After working steel, it should be reheated and cooled normally in the atmosphere to relieve stresses produced by the working operations. This is called normalizing. To bring out maximum hardness in a normalized steel, it is again heated above its upper critical limit and cooled rapidly in brine, water or oil. In this state the steel is too hard and brittle for any useful purpose and must be tempered.
Tempering is the removal of some of the hardness and brittleness by reheating to a lower temperature. By the time 350°C is reached, all the effective and useful hardness has been removed. All tempering is carried out below 350°C. These lower temperatures can be judged quite accurately by observing the oxide colours formed on the polished surface of a steel when heated. They range from a very pale yellow to a dark blue. The darker the colour, the hotter and softer the steel. The more shock that a tool must withstand the softer we must make it, while at the same time leaving it hard enough to serve its intended purpose. For example, a tool for cutting wood can be harder than one for cutting steel by hammering. Students will have to practise and experiment, but with experience they can get good results. Most steels used by the smith for making tools are between 0.7 and 0.8 percent carbon. Maximum hardness is obtained by heating to a dull red (740-800°C) and quenching. Colour should be observed in the shade, not in bright light.