Heat treatment of steel

Copper melts at 1084°C, and if copper ore is smelted together with tin the resulting bronze alloy flows easily at about 950°C. Pure iron, however, melts only at 1537°C, which was beyond the means of the early metalworkers. At the temperatures that their furnaces could reach, iron was reduced from the ore in the solid state, producing a solid spongy mass mixed with slag and unburnt charcoal, called a 'bloom'. This became the raw material of the smith. By hammering on the bloom, which was kept soft by frequently re-heating it, he could drive out the slag and other impurities, eventually producing a bar of almost pure iron - 'wrought iron'.

Pure iron is, however, inferior to bronze in many respects - it is softer, and does not hold an edge as well. Why, then, go to all that trouble? During the hot-working process, while the bloom was being re-heated, small quantities of carbon from the charcoal in the forging furnace would diffuse into the metal. Alloying with carbon increases the hardness and strength of iron dramatically. This improved alloy is, in fact, a steel.

The hardness of steeled iron is increased still further if the red-hot metal is quenched - suddenly cooled by plunging it into water. Although this technique hardens the steel, it also makes it brittle and likely to break with use. However, if the quenched steel is re-heated for a short time, and then slowly cooled, it gives up some hardness but also becomes tough and springy. This process is known as tempering.

These metalworking techniques make use of the fact that the atomic structure of the metal varies with temperature. These changes are immensely important, for they enable metallurgists to vary the properties of steel according to the purpose for which it is intended. If they did not take place, our metals technology might still be based on copper and bronze.

The role of carbon

The amount of carbon that can be dissolved in iron depends on the arrangement of atoms in the crystal lattice. At normal temperatures, iron has a body-centred cubic (bcc) structure. This material, which is termed ferrite, can contain only a few hundredths of a per cent carbon. At temperatures higher than about 906°C, however, the atoms rearrange themselves to form a face-centred cubic (fcc) lattice. This structure, which can accomodate up to 1.7 per cent carbon in solid solution, is called austenite, after Sir William Roberts-Austen, a renowned 19th-century metallurgist.

What happens, then, when an iron-carbon solid solution is slowly cooled? At some temperature above 723°C (exactly which temperature depends on the carbon content), the iron atoms start to revert to the ferrite structure. The carbon atoms do not immediately come out of solution, but diffuse to where the lattice is still face-centred cubic, thus creating carbon-rich zones in the carbon-free ferrite. When these areas are saturated, carbon starts to precipitate from solution. This process is completed at 723°C, when all the austenite has been converted to ferrite.

The carbon that precipitates from solid solution is deposited in fine lamellae of 'cementite', an iron carbide with the formula Fe3C, which alternate with layers of surplus ferrite. This structure is called 'pearlite', due to its iridescent appearance under the microscope.

Cementite has similar properties to intermetallic compounds - it is extremely hard, but brittle. A steel consisting of pearlite alone (0.8%C), which combines the hardness of cementite with the ductility of ferrite, would be too hard for structural use. (Pearlite is, however, the basis of high-stength steel wire and rope.) The most common types of structural steels are low-carbon (less than 0.3%C), which have excellent ductility and moderate strength. Medium-carbon (0.3-0.7%C) and high-carbon steels are often hardened further by quenching, which produces an entirely different structure.
Martensite in steel

Cooling an iron-carbon solid solution precipitates a separate carbon- rich phase only if the rate of cooling is slow enough for the alloy structure to remain in equilibrium with the temperature conditions. With very rapid cooling, there is not sufficient time for the reaction that forms pearlite to take place, and the carbon remains in solution. In this case, an alternative transformation occurs. The alloy reverts to its low-temperature structure, which cannot easily accomodate the carbon, and as a result the lattice becomes severely distorted. It is this distortion that gives quenched carbon steels their great hardness.

Seen under the microscope, the transformed steel has a characteristic microstructure, consisting of interlocking laths of a new phase, which result from the stresses due to the transformation. This material is called martensite, after the German metallurgist Adolf Martens who first observed the texture in about 1890.

Unlike the transformation to pearlite, the martensite transformation involves a structural change only - no movement of the carbon atoms is involved. The process is sometimes referred to as a 'diffusionless' solid-state transformation.

Although the presence of martensite gives steel great hardness, it also reduces its toughness and makes it brittle. Few steels, therefore, are used in the fully hardeded martensitic condition. The quenching stresses are generally relieved by a carefully controlled tempering treatment, which involves heating the steel to a temperature between about 250 and 550°C, depending on the carbon content and the properties desired. This allows some of the carbon to precipitate from the supersaturated solid solution as a finely dispersed carbide phase.