Iron and Its Interstitial Solid Solutions

The study of steels is important because steels represent by far the most widely used metallic materials, primarily due to the fact that they can be manufactured relatively cheaply in large quantities to very precise specifications. They also provide an extensive range of mechanical properties from moderate strength levels (200-300MPa) with excellent ductility and toughness, to very high strengths (2000 MPa) with adequate ductility. It is, therefore, not surprising that irons and steels comprise well over 80% by weight of the alloys in general industrial use.

Steels form perhaps the most complex group of alloys in common use. Therefore, in studying them it is useful to consider the behavior of pure iron first, then iron-carbon alloys, and finally examine the many complexities which arise when further alloying additions are made.

Pure iron is not an easy material to produce. However, it has recently been made with a total impurity content not exceeding 60 ppm (parts per million), of which 10 ppm is accounted for by non-metallic impurities such as carbon, oxygen, sulphur, phosphorus, while 50 ppm represents the metallic impurities. Iron of this purity is extremely weak: the resolved shear stress of a single crystal at room temperature can be as low as 10 MPa, while the yield stress of a polycrystalline sample at the same temperature can be well below 150 MPa.

The phase transformation: α- and γ- iron

Pure iron exists in two crystal forms, one body-centred cubic (bcc) (α-iron, ferrite) which remains stable from low temperatures up to 910°C (the A3 point), when it transforms to a face-centred cubic (fcc) form (γ-iron, austenite). The γ-iron on remains stable until 1390°C, the A₄ point, when it reverts to bcc form, (now δ-iron) which remains stable up to the melting point of 1536°C.

The detailed geometry of unit cells of α- and γ-iron crystals is particularly relevant to, for example, the solubility in the two phases of non-metallic elements such as carbon and nitrogen, the diffusivity of alloying elements at elevated temperatures, and the general behavior on plastic deformation.

The bcc structure of α-iron is more loosely packed than that of fcc γ-iron. The largest cavities in the bcc structure are the tetrahedral holes existing between two edge and two central atoms in the structure, which together form a tetrahedron.

It is interesting that the fcc structure, although more closely-packed, has larger holes than the bcc-structure. These holes are at the centers of the cube edges, and are surrounded by six atoms in the form of an octagon, so they are referred to as octahedral holes.

The α↔γ transformation in pure iron occurs very rapidly, so it is impossible to retain the high-temperature fcc form at room temperature. Rapid quenching can substantially alter the morphology of the resulting α-iron, but it still retains its bcc structure.

Carbon and nitrogen in solution in α- and γ- iron

The addition of carbon to iron is sufficient to form a steel. However, steel is a generic term which covers a very large range of complex compositions. The presence of even a small concentration of carbon, e.g. 0.1-0.2 weight per cent (wt%); approximately 0.5-1.0 atomic per cent, has a great strengthening effect on iron, a fact known to smiths over 2500 years ago since iron heated in a charcoal fire can readily absorb carbon by solid state diffusion. However, the detailed processes by which the absorption of carbon into iron converts a relatively soft metal into a very strong and often tough alloy have only recently been fully explored.

The atomic sizes of carbon and nitrogen are sufficiently small relative to that of iron to allow these elements to enter the α- iron and &gamma- iron lattices as interstitial solute atoms. In contrast, the metallic alloying elements such as manganese, nickel and chromium have much larger atoms, i.e. nearer in size to those of iron, and consequently they enter into substitutional solid solution.

However, comparison of the atomic sizes of C and N with the sizes of the available interstices makes it clear that some lattice distortion must take place when these atoms enter the iron lattice. Indeed, it is found that C and N in α-iron occupy not the larger tetrahedral holes, but the octahedral interstices which are more favorably placed for the relief of strain, which occurs by movement of two nearest neighbor iron atoms. In the case of tetrahedral interstices, four iron atoms are of nearest-neighbor status and the displacement of these would require more strain energy. Consequently these interstices are not preferred sites for carbon and nitrogen atoms.

The solubility of both C and N in austenite should be greater than in ferrite, because of the larger interstices available. It is, therefore, reasonable to expect that during simple heat treatments, excess carbon and nitrogen will be precipitated. This could happen in heat treatments involving quenching from the γ state, or even after treatments entirely within the α field, where the solubility of C varies by nearly three orders of magnitude between 720°C and 20°C.

Precipitation of carbon and nitrogen from α-iron. α-iron containing about 0.02 wt % C is substantially supersaturated with carbon if, after being held at 700°C, it is quenched to room temperature. This supersaturated solid solution is not stable, even at room temperature, because of the ease with which carbon can diffuse in α-iron. Consequently, in the range 20-300°C, carbon is precipitated as iron carbide. This process has been followed by measurement of changes in physical properties such as electrical resistivity, internal friction, and by direct observation or the structural changes in the electron microscope.

The process of ageing is a two-stage one. The first stage takes place at temperatures up to 200°C and involves the formation or a transitional iron carbide phase (ε) with a close-packed hexagonal structure which is often difficult to identify, although its morphology and crystallography have been established. It forms as platelets on {100}_α planes, apparently homogenously in the α-iron matrix, but at higher ageing temperatures (150-200°C) nucleation occurs preferentially on dislocations. The composition is between Fe_2.4C and Fe₃C.

Ageing at 200°C and above leads to the second stage of ageing in which orthorhombic cementite Fe₃C is formed as platelets on {110}_α. Often the platelets grow on several {110} planes from a common centre giving rise to structures which appear dendritic in character. The transition from ε-iron carbide to cementite is difficult to study, but it appears to occur by nucleation of cementite at the ε-carbide/α interlaces, followed by re-solution of the metastable ε-carbide precipitate.

The maximum solubility of nitrogen in ferrite is 0.10 wt %, so a greater volume fraction of nitride precipitate can be obtained. The process is again two-stage with a be tetragonal α" phase, Fe₁₆N₂, as the intermediate precipitate, forming as discs on {100}_α, matrix planes both homogeneously and on dislocations. Above about 200°C, this transitional nitride is replaced by the ordered fcc γ’, Fe₄N.

The ageing of α-iron quenched from a high temperature in the α-range is usually referred to as quench ageing, and there is substantial evidence to show that the process can cause considerable strengthening, even in relatively pure iron. In commercial low carbon steels, nitrogen is usually combined with aluminium, or present in too low concentration to make a substantial contribution to quench ageing, with the result that the major effect is due to carbon. This behavior should be compared with that of strain ageing.

Some practical aspects. The very rapid diffusivity of carbon and nitrogen in iron compared with that of the metallic alloying elements is exploited in the processes of carburizing and nitriding.

Carburizing can be carried out by heating a low carbon steel in contact with carbon to the austenitic range, e.g. 1000°C, where the carbon solubility, c₁, is substantial. The result is a carbon gradient in the steel, from c₁ at the surface in contact with the carbon, to c at a depth.

The diffusion coefficient D of carbon in iron actually varies with carbon content, so the above relationship is not rigorously obeyed. Carburizing, whether carried out using carbon, or more efficiently using a carburizing gas (gas carburizing), provides a high carbon surface on a steel, which, after appropriate heat treatment, is strong and wear resistant.

Nitriding is normally carried out in an atmosphere of ammonia, but at a lower temperature (500-550°C) than carburizing, consequently the reaction occurs in the ferrite phase, in which nitrogen has a substantially higher solubility than carbon.

Nitriding steels usually contain chromium (≈1%), aluminum (≈1%), vanadium or molybdenum (≈0.2%), which are nitride-forming elements, and which contribute to the very great hardness of the surface layer produced.