Steels and their properties

Iron is an important element of living organisms and one of our most heavily used industrial materials — especially its alloy with carbon, steel. The majority of the products made by industry are based on steel or an iron alloy. This is explained by the cost of production and by how widely the properties can be influenced. Although the development of the plastics industry and the use of cermets have created opportunities to replace steel in a number of fields, it can be stated that steel and iron alloys will remain a fundamental raw material in the near future as well. By iron alloys we generally mean those materials that are manufactured by large-scale industrial processes, are used in large quantities, and in which the proportion of Fe is greater than that of any other constituent. Industrially used iron-based alloys can be divided into two large groups: steels and cast irons. Their fields of application and their new types overlap in many cases, but in general it can be said that the toughness properties of steels are better than those of cast irons, while their processing is more costly, making them more expensive raw materials. Thanks to the development of technologies, cast irons are being used in an ever wider field and in many cases successfully compete with certain steel products as well.

The properties of steels

Steels are formable iron alloys, whose internationally accepted definition has been determined as follows: a material containing predominantly iron, generally less than 2% carbon, and other elements. Some chromium steels may contain more than 2% carbon, but 2% is the customary dividing line between steel and cast iron. This limit cannot be regarded as sharp on the basis of either the chemical composition or the formability. Formability decreases only gradually as the carbon content increases; even cast iron with a 3.5–4% C content can be rolled into a thin sheet or forged into a component if it has a graphitic structure. Formability fundamentally depends on the microstructural constituents present in the alloy, on their properties and on their arrangement within the structure, not directly on the carbon content.

The classification of steels

Steels, as very widely used raw materials, can be classified according to several criteria:

1. On the basis of their chemical composition we distinguish unalloyed and alloyed steels. A steel counts as unalloyed if, apart from carbon, it contains no deliberately introduced alloying element that would influence the mechanical or physical properties of the steel. The limit values separating unalloyed and alloyed steels are laid down in standards. Alloyed steels contain more of the always-present elements than the smallest quantity achievable in normal operation, or else contain further, deliberately added elements beyond these. It is also customary to distinguish, on the basis of the quantity of alloying elements, low-alloy (max. 5–6% total alloying content), alloyed (5–10% alloying content) and high-alloy (above 10% alloying content) steels.
   

   

2. According to an internationally accepted division, steel types are sorted into so-called quality groups on the basis of the care taken during steelmaking. The main quality types of steels are the following:
   • Base steel is every steel for which no quality requirement is prescribed that would demand special care during steelmaking. Only unalloyed steels belong to this group.
   • Quality steels: the requirements applying to them fall between those for base steels and stainless steels. They must be manufactured with special care, in which the most important parameters are grain size, sulphur and phosphorus content, and surface quality.
   • Stainless steels: they must be manufactured with special care. They may be of unalloyed or alloyed material. All steels suitable for heat treatment purposes belong to this group.
3. Grouping according to method of manufacture. During the manufacture of steel, different residual constituents remain behind depending on the production method, or their quantity and distribution changes. All of this is significant from the point of view of the further processing of the steel and the quality of the finished product.
   

   

4. On the basis of the deoxidation method applied:
   • Rimmed steel: good yield, good surface quality, readily cold-formable. These steels age quickly and contain max. 0.25% carbon.
   • Killed steels: the oxygen is bound to an element that forms compounds in the solid state. The enrichment of contaminants is lower, the yield is poorer, and some of the oxides remain behind as inclusions. With vacuum treatment the oxygen leaves in gaseous form and no inclusions are formed.
   • Specially killed steels: during deoxidation, deoxidising agents with a nitrogen-binding and grain-refining effect (Al, V, Nb, Ti) are also applied. These are more resistant to ageing and their tendency to brittle fracture is lower than that of unalloyed steels killed only with Si and Mn.
5. On the basis of their microstructure, which generally occur in combination: on the basis of their equilibrium microstructure we distinguish ferritic, semi-ferritic, hypoeutectoid, hypereutectoid (see: eutectic), ledeburitic, semi-austenitic and austenitic steels. In the non-equilibrium state there exist pearlitic, martensitic, austenitic, ferritic and bainitic steels.
6. According to their mode of use: Structural steels: such steels are utilised as raw material in the fields of machine and vehicle manufacture and the production of steel structures. In these cases, alongside strength we also require adequate toughness and elongation. This can be ensured with a carbon content below 0.25% or with a quenched and tempered microstructure. Tool steels: cutting tools and the tools of forming operations are made from these. It is essential that they withstand the stresses acting on them without deformation and that they resist wear. These properties are generally achieved by quenching and tempering, so they must be readily hardenable and must also meet toughness requirements. Special steel types and alloys: alloys with some pronounced property — e.g. heat-resistant, corrosion-resistant, non-thermally-expanding, etc. — can be classed here. They contain a larger quantity of alloying elements.

The effect of carbon on the properties of steel

The microstructure of steel, and thus its properties, vary as a function of the alloying and carbon content. We can state that the effect of carbon is fundamentally decisive for the properties of steel. Steel in the equilibrium state containing 0.8% carbon is purely pearlitic, steels with a 0…0.8% C content have a ferritic-pearlitic microstructure, and those containing 0.8…2.1% carbon have a pearlitic-cementitic microstructure. The strength and other physical properties of unalloyed steel, however, are determined by the quantity of the microstructural constituents and by their distribution and fineness. As the quantity of pearlite increases, the strength of the steel rises, but its elongation, contraction and impact energy decrease. The brittle microstructural constituent appearing in hypereutectoid steels, cementite16, still increases the strength up to a certain limit (approx. 0.9%). At a higher carbon content the cementite usually already appears in a network-like arrangement and thus also reduces the tensile strength of the steel, while increasing the yield point. The yield point and the tensile strength vary approximately in parallel. The strength of iron can be increased most cheaply with carbon. With increasing C content, however, the toughness of the steel decreases. The presence of carbon in steels is also decisive from the point of view of hardenability. The carbon content strongly reduces the critical cooling rate. The hardness of steel, similarly to the tensile strength, increases with increasing carbon content.

The effect of alloying elements on the mechanical properties of ferrite

The majority of alloying elements dissolve substitutionally in alpha-iron, in ferrite. As a result of the dissolution the lattice parameter of the mild steel changes, and following the lattice distortion the mechanical properties change. The greater the lattice distortion, the greater the increase in strength and in the fatigue limit. The strength of ferrite is increased most by Ti, Si, W, Mo, Mn and Ni. Introducing the quantity of alloying elements needed for the increase in strength raises the cost of the steel. The strength-increasing effect of alloying elements asserts itself better in the heat-treated state, yet the effect of carbon dissolving interstitially in iron is far more significant from the point of view of increasing hardness and strength than that of substitutionally dissolving alloying elements. With the exception of Si and Mn, alloying elements do not influence the toughness characteristics of ferrite. All the more varied is the effect of alloying elements on impact energy. With the exception of Ni, Cr and Mn, they reduce the specific impact energy. The effect of alloying elements on the ductile-to-brittle transition temperature is of great significance. Ni reduces this critical temperature, so the ductile range becomes larger, while the other alloying elements increase it. The mechanical properties of steels with a carbon content greater than 0.2% are determined even more by the method of heat treatment and by the microstructure that develops. In such steels the significance of the individual alloying elements must also be judged on the basis of their effect on the heat treatment processes of the steel.

The effect of alloying elements on the processes taking place during the heat treatment of steel

Alloying elements change the critical transformation temperatures of Fe. This must be taken into account during the heat treatment of steels. In addition, alloying elements strongly influence the rate of austenite formation during heating and slow down the transformation of pearlite into austenite. The effect of alloying elements on the critical cooling temperature is also considerable. From the point of view of heat treatments, the through-hardenability of the steel is very important — to what depth and diameter the steel hardens through. The through-hardenability of steels is determined by their critical cooling rate, which depends on the chemical composition and the grain size. The majority of alloying elements reduce the critical cooling rate, so the through-hardenability increases.

The field of application of steels:

Corrosion-resistant and acid-resistant steels

Unalloyed steels are not resistant to the corrosive effect of acids, the atmosphere or water vapour. The oxide film on their surface is not dense enough, it does not seal the metal off from the corrosive medium and thus does not prevent further corrosion. The corrosion resistance of steels can be increased with alloying elements, which is possible in the following way: alloying elements must be applied that create a thin, well-adhering film on the surface of the steel that separates it from the corrosive medium. Cr and Al are used for this purpose in a quantity of approx. 1%. Another possibility for a step-change increase in corrosion resistance is if the quantity of the alloying element in the steel is an integer multiple of 1/8 atomic weight part. Corrosion-resistant steel contains at least 12% chromium, generally nickel, and in some cases other alloying elements. In this way the alloyed steel durably resists the effects of temperature and pressure, and even certain chemical and electrochemical effects. Austenitic chromium-nickel steels are, because of their good corrosion-resistant properties, also called acid-resistant steels. These alloying elements exert their favourable effect alongside a low carbon content. Generally a C content greater than 0.12% is not permissible, but only steels with a carbon content of at most 0.03% are resistant to intergranular corrosion. The use of austenitic corrosion-resistant steels is very wide-ranging — they are used everywhere from the manufacture of fasteners, vessels, tanks and medical instruments to the chemical industry. It is important that their behaviour during machining differs significantly from that of commonly used steels. These steels can also be used at low temperatures without the risk of brittle fracture; they are cold-tough and heat-resistant, and generally contain 17–20% chromium and 8–24% nickel as alloying elements. Our NONOX A2 (stainless) and NONOX A4 (acid-resistant) fasteners are also made from such raw materials, and they provide a reliable solution over the long term both outdoors and indoors.