Although pure iron is a weak material, steels cover a wide range of the strength spectrum from low yield stress levels (around 200 MPa) to very high levels (approaching 2000 MPa). These mechanical properties are usually achieved by the combined use of several strengthening mechanisms, and in such circumstances it is often difficult to quantify the different contributions to the strength. These results should then be helpful in examining the behavior of more complex steels.

Like other metals, iron can be strengthened by several basic mechanisms, the most important of which are:

* Work hardening
* Solid solution strengthening by interstitial atoms
* Solid solution strengthening by substitutional atoms
* Refinement of grain size
* Dispersion strengthening, including lamellar and random dispersed structures.

The most distinctive aspect of strengthening of iron is the role of the interstitial solutes carbon and nitrogen. These elements also play a vital part in interacting with dislocations, and in combining preferentially with some of the metallic alloying elements used in steels.

Work hardening
Work hardening is an important strengthening process in steel, particularly in obtaining high strength levels in rod and wire, both in plain carbon and alloy steels. For example, the tensile strength of a 0.05% C steel subjected to 95% reduction in area by wire drawing, is raised by no less than 550 MPa while higher carbon steels are strengthened by up to twice this amount. Indeed, without the addition of special alloying elements, plain carbon steels can be raised to strength levels above 1500 MPa simply by the phenomenon of work hardening.

Basic work on the deformation of iron has largely concentrated on the other end of the strength spectrum, namely pure single crystals and polycrystals subjected to small controlled deformations. The diversity of slip planes leads to rather irregular wavy slip bands in deformed crystals, as the dislocations can readily move from one type of plane to another by cross slip, provided they share a common slip direction.

The yield stress of iron single crystals are very sensitive to both temperature and strain rate and a similar dependence has been found for less pure polycrystalline iron. Therefore, the temperature sensitivity cannot be attributed to interstitial impurities. It is explained by the effect of temperature on the stress needed to move free dislocations in the crystal, the Peierls-Nabarro stress.

Solid solution strengthening by interstitials
The formation of interstitial atmospheres at dislocations requires diffusion of the solute. As both carbon and nitrogen diffuse much more rapidly in iron than substitutional solutes, it is not surprising that strain ageing can take place readily in the range from 20°C to 150°C. Consequently the atmosphere condenses to form rows of interstitial atoms along the cores of the dislocations. These arise because the temperature is high enough to allow interstitial atoms to diffuse during deformation, and to form atmospheres around dislocations generated throughout the stress-strain curve. Steels tested under these conditions also show low ductility`s, due partly to the high dislocation density and partly to the nucleation of carbide particles on the dislocations where the carbon concentration is high. The phenomenon is often referred to as blue brittleness, blue being the interference color of the steel surface when oxidized in this temperature range.

The break away of dislocations from their carbon atmospheres as a cause of the sharp yield point became a controversial aspect of the theory because it was found that the provision of free dislocations, for example, by scratching the surface of a specimen, did not eliminate the sharp yield point. An alternative theory was developed which assumed that, once condensed carbon atmospheres are formed in iron, the dislocations remain locked, and the yield phenomena arise from the generation and movement of newly formed dislocations.

To summarize, the occurrence of a sharp yield point depends on the occurrence of a sudden increase in the number of mobile dislocations. However, the precise mechanism by which this takes place will depend on the effectiveness of the locking of the pre-existing dislocations. If the pinning is weak, then the yield point can arise as a result of unpinning. However, if the dislocations are strongly locked, either by interstitial atmospheres or precipitates, the yield point will result from the rapid generation of new dislocations.

Under conditions of dynamic strain ageing, where atmospheres of carbon atoms form continuously on newly-generated dislocations, it would be expected that a higher density of dislocations would be needed to complete the deformation, if it is assumed that most dislocations which attract carbon atmospheres are permanently locked in position.

Strengthening at high interstitial concentrations
Austenite can take into solid solution up to 10% carbon, which can be retained in solid solution by rapid quenching. However, in these circumstances the phase transformation takes place, not to ferrite but to a tetragonal structure referred to as martensite. This phase forms as a result of diffusion less shear transformation leading to characteristic laths or plates.

If the quench is sufficiently rapid, the martensite is essentially a supersaturated solid solution of carbon in a tetragonal iron matrix, and as the carbon concentration can be greatly in excess of the equilibrium concentration in ferrite, the strength is raised very substantially. High carbon martensites are normally very hard but brittle, the yield strength reaching as much as 1500 MPa; much of this increase can be directly attributed to increased interstitial solid solution hardening, but there is also a contribution from the high dislocation density, which is characteristic of martensitic transformations in iron-carbon alloys.