It was discovered that if a piece of iron were heated in contact with charcoal, the surface became quite hard, a process now called casehardening. The trouble was that only a very thin skin—as little as a few hundredths of a millimetre—was hard. When this wore off, as it would in time, the softer part of the iron was exposed.
By the early years of the eighteenth century it was well known that the longer iron was heated while packed in carbon, the deeper the carbon penetrated into the iron, giving a thicker hard skin. The process was known as cementation. Some carbon steel was made in this way but it took a long time to get the composition right. Iron could be in the furnace for as much as three weeks to produce what was called shear steel (because it was widely used for making shears for the textile industry). It was not of very good quality, but it was the best that could be done at the time.
Some further processing would improve the quality and this was practised to make the so-called double shear steel. Because it was so slow and costly to make, shear steel was only used when it had to be, and even then it was employed very economically. Such things as scythe blades and other tools were made by heating a thin strip of carbon steel and a thicker one of wrought iron together and welding them into one piece quickly under a power hammer. In this way the expensive steel was only used at the cutting edge, where it was essential. The process was satisfactory for some things but it was of little use for a type of tool which was going to be in increasing demand as the industrial revolution developed, the engineer’s cutting tool.
Cast and wrought iron were excellent materials for making the many types of machinery needed, but they could not be cast, forged or rolled to the precise shape and dimensions required. Parts of iron components, at least, had to be machined to shape. A steam engine cylinder, for example, could be as it was cast on the outside, but the inside had to be machined to fit the piston, and the piston itself had to be machined also.
The man who found the first way of making better carbon steel was neither an engineer nor an ironmaker. Benjamin Huntsman, a Doncaster clockmaker, was dissatisfied with the steel available for making clock springs. In 1740 he took some shear steel and melted it in a clay pot or crucible. He poured out the molten metal, let it solidify and tried working it: it was better than anything he had ever seen before.
Although the process was not understood at the time, the carbon had spread itself throughout the molten metal, and the resulting steel was much more uniform. Huntsman tried in vain to keep his discovery secret, but it was taken up widely in Sheffield, where Huntsman started a small works to make what soon became called crucible steel.
In time it was found that the quality of the steel could be not only controlled but varied according to need. Steel for making, say, a wood chisel, or a chisel for cutting metal, or a razor, needed different grades. Crucible steel could be made to suit the application, and the steelmakers of Sheffield in particular became very expert at supplying exactly what their customers needed.
Crucible steelmaking lasted all through the nineteenth century and into living memory, but it is now extinct. A crucible steel-melting shop has been preserved at Abbeydale industrial museum, Sheffield (where some very good examples of waterwheel-driven hammers can also be seen, in proper working order).
Ferrous Metals and New Stage in Technological Development in the World
