As we already know low-carbon steels cannot usefully be strengthened by martensitic heat-treatment, but nevertheless they can be subjected to heat treatment that refines their grain structure or relieves the effects of workhardening. Low-carbon steels can be deformed and cold-worked in the same way as any other solid solution, but when further deformation would cause cracking, the material must be annealed.

Fig. 1: The effect of annealing below and above the critical temperature for a cold worked low carbon steel: (a) as cold worked before annealing; (b) sub critically annealed at 675°C (note pearlite stringers in recrystallized ferrite matrix); (c) fully annealed at 920°C (entire structure equiaxed): all magnifications 175x (in the book Alexander, et al [1])

The effect of the transformation can be both beneficial and detrimental to the properties of a steel. It is beneficial in the sense that no matter what the starting structure of the material the small y grains that form when the y phase first appears subsequently give the material the optimum combination of strength and toughness. If, however, the austenite grains are allowed to grow large, ductility and toughness in the product fall appreciably. Consequently, care must be taken to minimize the time available for grain growth.

There are two variations of the annealing process used with steels: Full annealing and Normalizing.

Full annealing is the description of the recrystallization process carried out above the critical temperature range and is used for the treatment of large complicated shapes, which are heated to the Y region for several hours to allow recrystallization. Cooling has to be slow and is usually achieved by leaving the parts in the furnace. This treatment is necessary to avoid the distortion that may be encountered if different parts of the metal cool at different rates. In such cases, it is found useful to have present in the structure particles that restrict the growth of austenite grains. This may be achieved with the addition of aluminum to the molten steel, when aluminum oxide particles form and obstruct grain growth. The aluminum is also added to lock up any residual oxygen in the steel.

Normalizing is an annealing process in which the steel is heated to a temperature 30-50°C above the critical temperature and for long enough for y to form and the temperature to equalize throughout the section. It is then cooled in still air. This technique produces a much finer grain structure than full annealing and as air cooling is used minimizes the time a furnace is occupied. Normalizing is, therefore widely used as a means of refining the grain structure of steels. It is suitable for both cold-worked and hot-worked structures where the desire is to reduce the grain size by taking the steel through the transformation range. Where sectional size variations exist, full annealing may avoid problems with distortion or residual stresses.

As with other engineering metals, steels may also be given heattreatment at low temperature (below 0.4T_m) to allow for the recovery and relief from elastic strains introduced by forming operations such as machining, bending or welding, but structural changes are on a submicroscopic scale and are net visible using an optical microscope. To all intents and purposes no microstructural change takes place, only the relaxation of locked-in stresses.

Two requirements must be satisfied to produce the hard, wear-resistant martensitic structure suitable for use for bearing surfaces, and similar applications. First, the carbon content must be sufficient to cause lattice strain on quenching and second, the steel must be heated to a temperature above that at which the transformation from a- to y-iron takes place so that all the carbon can be dissolved in the austentic solid solution.

The martensitic structure that has the desired hardness and wearresistance is also brittle and if the whole of the cross-section of a component were to be heat-treated the component would itself be wholly brittle. However, the production of a martensitic structure can be restricted to the surface layers, where its properties are in fact required while leaving the underlying material in a tough, but softer, condition. There are two ways of achieving this so-called case-hardening.

In the first method, known as case-carburizing, carbon, and sometimes nitrogen, atoms are diffused into the surface of a low-carbon steel until the carbon level reaches a value sufficient to form a hard martensite to the required depth. This is achieved by immersing the steel in a bath of molten chemicals of special composition at a high temperature or by heating the metal in an atmosphere of special composition, again at high temperature. The whole component is then quenched, and finally tempered to relieve the stresses induced by quenching. The central portions contain very little carbon and are not hardened by the quench. The outer case acquires its full martensitic hardness, however, and develops the required wear-resistance. By the use of nitrogen-containing chemicals in the molten bath or a furnace atmosphere containing nitrogen it is possible to incorporate nitrogen atoms in the surface layers of the steel along with the carbon. This process is known as carbo-nitriding, and gives additional hardness.

In the second case-hardening process, called induction hardening, the steel must initially have enough carbon throughout the section to form martensite when suitably quenched, i.e., more than 0.5% carbon. Before treatment, the steel must have a pearlite or tempered martensite structure to impart toughness and strength in the core material. The surface layer to be hardened, which may be quite localized, is then rapidly heated by electrical induction or by a gas flame to a temperature above the transformation point, which converts the surface layer into an austenitic solid solution. When this austenitic zone reaches the required depth, the whole component is quenched, causing the martensitic transformation. This transformation will occur only in the surface layers and not in the core because the latter did not reach the austenitization temperature. Hence, the final structure is hard martensite covering the original strong pearlite or tempered martensite. Because the core material has a high-carbon content this structure is stronger than a low-carbon steel carburized using the first method. A typical example of local case hardening - the tooth on a bandsaw blade - is shown in Fig. 2. The springy, medium-carbon steel of which it is made is cut to shape, and the teeth subsequently hardened by induction hardening. If the martensitic structure extended into the backing strip the blade would not flex properly, and unless the teeth are martensitic they would wear out very quickly as the saw was used. In this example, the tooth has a Vickers hardness of 900, and the backing a hardness of 280.

One other a4vantage of case-hardening is the volume expansion that takes place when martensite forms from austenite. When only the surface layers are subjected to the transformation as described above, they develop high residual compressive stresses when quenched, balanced by tensile stresses in the backing material. The net effect is that the compressive stress remains in the surface and any externally applied loads must

Fig. 2: Etched section of bandsaw blade showing locally hardened tooth on tempered steel blade: magnification 15x (in the book Alexander, et al [1])

overcome this residual stress before the surface layers themselves are subjected to tensile forces. Since fatigue cracking usually intiates at the surface under the action of tensile stress, fatigue life in particular is greatly improved by such treatment, and this is why numerous components used on motor vehicles, such as axle shafts and journals, are inductionhardened to depths of a millimetre or so. Not only do the martensitic surface layers improve the wear-resistance but the residual compressive stress greatly extends the life of the components.

1. W.O. Alexander, G.J. Davies, K.A. Reynolds and E.J. Bradbury: Essential metallurgy for engineers. 1985. Van Nostrand Reinhold (UK) Co. Ltd. ISBN: 0-442-30624-5