Iron, Steel and their Variations and Alloys by Brian Read - History of Woodworking Tools in UK

It has become apparent that to some members at any rate there is confusion about what the difference between wrought- &cast- iron, steel and the various alloy steels is. I have therefore put together a quick guide to the various types. It is very much a quick idiot’s guide – the literature of ferrous metallurgy occupies many Shelves in the British Library.

Wrought iron; Bloomery Iron

Although iron meteorites must have been available since before the stone-age it was not until around 1000 BC that man learnt to smelt iron from its ores.

At first he heated iron oxide with charcoal in small shaft furnaces and produced what is called Bloomery iron. The temperature rarely exceeded 1300deg Celsius and the iron never melted but accumulated in a pasty ball towards the bottom of the furnace. It consisted of particles of ore which had been reduced to fairly pure iron never melted but which were loosely sintered together in a matrix of viscous slag. This bloom had to be hammered while white hot to drive out most of the slag and consolidate the iron by welding. The resultant material still had strands of slag running through it but was strong enough to use. It had been wrought by the smith, and hence was known as "wrought iron."

Hammering a piece of this iron when cold hardened it to some extent but it was probably inferior to bronze for most uses. However it was found that if it was heated in close contact with red hot charcoal in the absence of air and then cooled rapidly it became much harder. Such iron is called "case hardened because the hard layer encases a soft core. It had in fact absorbed carbon into the surface layers and become steel.

Cementation Steel

This process was then extended to heating small diameter bar of iron with charcoal for a prolonged period in a sealed box, to yield "blister steel", so called because of the characteristic blisters on the surface. This steel was still very variable in quality because of varying amounts of carbon as one got further into the bars and required skilled forging to weld the thing pieces into a useable sized bar. Because the steel had to be welded, forged into a long thin bar, cut into lengths and re-welded to homogenize the steel it was known as "shear steel". Because of the amount of work needed to produce such steel it was expensive and usually welded onto a softer iron body or sandwiched between two wrought iron plates to conserve it. If the process was repeated you got "double shear" or even "triple shear" steel – more homogenous but also more expensive.

Crucible or Cast Steel

In the 18th century Benjamin Huntsman devised a method of heating blister steel in closed crucibles to give a much more homogenous, and molten steel, which could be cast into blocks for later forging – "cast steel". Such as steel has a carbon content from point one percent through to about one and a half percent.

Cast Iron

In the 12th century Europe it had been found that increasing the size of the smelting furnace and also increasing the blast of air through it caused an increase in temperature which allowed the production of molten iron for the first time. This melting allowed the iron to be run out of the bottom and then cast into ingots or pigs. This was because of a perceived similarity in appearance between the ingots running off the main casting channel and piglets feeding from a sow. Such ingots were "cast iron" or "pig-iron".

Unfortunately the highest temperatures, and hence the initial melting, occurred in the middle of the furnace and the molten iron trickled down through the hot charcoal/ coke absorbing carbon in the process. This lowered the melting point, making the casting process easier, but also took the carbon content to around four percent. Such material has none of the desirable properties of steel but instead is often extremely hard and brittle. It cannot be shaped in a forge in the same way as either wrought iron or steel but only cast into predetermined shapes.

Later a method of heat treatment was developed which allowed a certain amount of working to be done and this was "malleable iron". Castings made of this were not liable to break if dropped onto a hard surface. If the casting was done in such a way that the molten iron cooled very rapidly then it became extremely hard and wear resistant – hence its use in ploughshares. A plough made from recycled swords would wear away rapidly in stony soils!

Finery steels

Cast-iron is convertible to steel by burning out the excess carbon. This is the easiest done by heating the near molten iron in a blast of air. The first process was the "finery" method which used charcoal as fuel in an open furnace. Hammer scale was often added since it readily reacts with excess carbon in the cast iron to give iron and carbon dioxide. Hammer scale was a waste material from working hot iron so it was readily available.

Puddling process; Bessemer Steel

Much later came another of these processes, "puddling". Here a mixture of Pig-iron and hammer scale was heated in a coal – fired reverberatory furnace in a stream of air and stirred with wrought iron poles. The use of a reverbertory furnace kept the highly undesirable sulphur in the coal from contaminating the end product. As the carbon burnt out the melting point of the iron went up and the material became thick and pasty. It stuck to the end of the poles and could be removed. Great skill was needed to judge the temperature of the molten iron and hence the composition of what was being removed. Later Bessemer used a blast of hot air in a "converter". The reaction is exothermic and no fuel was thus needed but it was not suitable for all compositions of cast iron.

The chemistry/metallurgy behind these materials is both complex and yet simple. Put iron melts at 1530 Celsius, high carbon steel at 1430 Celsius, cast iron 1230-1150 Celsius. At these temperatures much of the carbon present is combined with the iron, in the form of carbides. Cold cast iron is essentially iron crystal plus carbides and phosphides separated by flakes of graphite. This makes for easy fracture along the graphite interface – the material is brittle! Malleable cast iron is cast-iron which has been heat treated so that the carbon forms as nodules or spheres rather than flakes in a pure iron matrix. This makes it much less likely to fracture and keeps the ductility of pure iron.

Above 1430C pure iron crystals have a structure and density known as delta-Fe. Any carbon present is in solid solution, along with any carbides and alloying elements. As it cools the crystal structure remains the same down to 1430C where it changes. The change in crystal structure is accompanied by volume changes so that internal stresses can build up in the solid material. Because we are dealing with a solid, the phase change process is not instantaneous and, if the rate of cooling is high, then the delta-Fe structure remains "frozen". It is unstable however and if re-heated to near the transition point will spontaneously change to the gamma iron structure.

Below 1430C the stable crystal structure is gamme-Fe which continues to exist down to 900 C. Ferrous materials held within this temperature rage will revert to this structure, the higher the temperature the faster the rate of conversion. Up to 1.7 percent carbon is soluble in gamma-Fe, excess precipitates either as carbides or as graphite. Under the microscope the material has a characteristic appearance and is known as "austenite". Any steel or iron with this general appearance is known as "austenitic" and it is associated with a definite range of properties. Alloying elements present can extend the stability range of this phase considerably. Rapid cooling from just over 900C can "freeze" the structure if these properties are required in the finished product, although as before it is unstable and may spontaneously change if maintained just below the transition temperature for a period of time.

Below 900C the stable crystal structure is alpha-Fe. Iron and other carbides also precipitate but may remain partially in solid solution in the iron.

Under the microscope the transition is marked by the disappearance of the austenite and its decomposition to complex structures of carbides and alpha-Fe the simplest of these is pearlite a distinct laminar structure of iron and carbide crystals. Formation of this requires the presence of an optimum, or eutectoid, concentration of carbon at the temperature of formation. If their material is not optimum then other crystalline phases are mixed in with the pearlite.

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