Steel is the most widely used metal in the world and consists of many thousands of different compositions, each offering a unique combination of properties, which are tailor-made to satisfy individual requirements. Steels are alloys of iron and carbon, but many steels have their properties enhanced by the addition of other alloying elements and by the application of different thermomechanical and heat treatments.
The wide range of available steels can mostly be categorized into one of three families: carbon steels, alloy steels and stainless steels.
Carbon steels are body-centered cubic (bcc) in structure. Those containing over about 0.1% C (depending on section thickness) can be hardened by heat treatment. The strength is primarily dependant on the carbon content. They are available in plate, tube, strip, bar rod, or wire and structural sections and are used in a wide range of industries and applications. Microalloying—additions of Vanadium, niobium or Titanium—can be utilized to increase strength further. It is especially useful when allied to controlled thermomechanical processing to give an excellent combination of strength and fabricability. Very high levels of strength to over 3000 N/mm2 can be introduced by cold work, e.g., in wire drawing of high carbon steel.
Alloy steels involve the addition of elements including chromium, Nickel, Molybdenum, and Vanadium. Such steels can be heat treated (often by quenching and tempering) to produce increased strength and hardness (i.e., tensile strengths above 750 N/mm2 and up to 2000 N/mm2), together with good ductility and toughness. Various surface treatments can be applied to give improved properties, fatigue performance and wear resistance. They are mainly used in automotive engine and transmission components and in the energy industries.
All the above steels are prone to Corrosion in certain environments. To prevent or reduce corrosion, they can be coated with metals such as zinc, tin, chromium or cadmium. A wide range of organic, painted coatings are also available.
Stainless steels are produced by adding at least 11% chromium to steel to produce a thin passive protective layer of Cr2Q3, which promotes corrosion resistance. This is improved by further increasing the chromium content. Four basic groups of stainless steel are available:
Ferritic grades, which contain between 11-17% chromium, they are body-centered cubic in structure and magnetic.
Martensitic grades contain similar amounts of chromium, but more carbon than ferritics and possibly other additions such as molybdenum to increase hardenability and strength.
Austenitic grades contain between 17-25% Cr, 7-20% nickel and in some instances molybdenum. They are face-centred cubic in structure, nonmagnetic and can be formed and welded more easily than ferritics.
Duplex stainless steels were developed to provide the strength of ferritics, but with improved corrosion resistance. They contain about 22% Cr, 5% Ni, and possibly molybdenum.
Stainless steels are used extensively in food and drink production and the chemical and energy industries; martensitics are used for cutlery and other cutting tool manufacture.
Further details of the metallurgy of steels and their applications are given by Llewellyn (1992), Pickering (1992), and ASM Metals Handbook Vol. 1 (1990).
Figures 1, 2, and 3 show that the thermal properties of the various steel compositions can vary considerably and must be taken into account when designing with steel. By special control of composition and microstructure, creep resistant steels have been developed to give enhanced performance at high temperatures. Further thermal data are available from Smithells (1992) and Roth-man (1987).
The hardness and strength of steels can be altered by heat treatment. The two most frequently used heat treatment processes are annealing, and hardening and tempering.
When steels are worked, either during manufacture or when the metal is manipulated to form a component, the hardness increases. This makes further working difficult; to facilitate further manufacturing, the steel structure must be softened by annealing at a temperature typically within the range 600-800°C. Treatments in the range 400-1000°C, depending on the steel type, can also be used to stress-relieve a component. This is required when a component has been machined or plastically deformed, heat treated or welded, which may result in distortion during the subsequent manufacturing procedures. Austenitic stainless steel components may also be stress-relieved to minimize the likelihood of stress-corrosion cracking in certain corrosive environments.
Alloy steels and martensitic stainless steels can be hardened by heating to temperature around 1000°C and cooling at a sufficiently fast rate to form a martensitic or bainitic structure. Quenchants can be chosen to cool at different rates and include brine, water, oil, air or molten salt. The choice of quenchant depends upon the type of steel and the component dimensions. For some highly alloy steels it may be necessary to cool to below 0°C to harden the steel completely.
After quenching the structure of the steel is hard but also brittle. In order to increase the toughness, albeit at the expense of a lower hardness and strength, it is necessary to temper, typically at temperatures between 500 and 650°C. The choice of tempering temperature depends upon the combination of strength and toughness required.
When steels are cooled to harden the structure, volume changes occur which are:
Expansion when fcc iron (austenite) transforms to bcc (ferrite or martensite).
Contraction when iron carbide is precipitated.
Normal thermal contraction.
When a steel is quenched these changes occur very quickly and unevenly. Because the outside cools more rapidly than the inside, thermal gradients are set up which cause stresses and hence distortion or cracking.
The likelihood of quench crack formation can be minimized by good practice including:
Tempering immediately after hardening.
Slow, even heating.
Choice of suitable quenchant.
Design of structure to minimize stress raisers including surface defects, sharp angles, internal nonmetallic inclusions and uneven sections.
A wide range of furnaces are available including gas or oil fired and electric. Controlled atmosphere or vacuum furnaces are used when decarburization, carburization or oxidation of the steel surface cannot be tolerated.
Furnace atmospheres can be selected intentionally to alter the composition of the outer layer of the steel, by the diffusion of carbon or nitrogen, usually to increase hardness by carburizing or nitriding. Steel compositions have been developed to be given this treatment.
Further details on all aspects of the heat treatment of steels are given in numerous publications including Thelming (1984) and ASM Handbook Vol. 4 (1991) whilst the heat treatment characterization of individual grades are detailed by Van der Voort (1991).
When using steels at elevated temperatures, the choice is governed by:
The strength and life required.
The corrosivity of the environment.
The thermal expansion characteristics.
Many steels have been developed specifically for use at elevated temperatures: these contain alloying additions of elements such as chromium, molybdenum, vanadium, tungsten. Although the strength of these steels decreases with increasing temperature, the reduction is not as pronounced as with other steels.
When metals are used, under stress, at a temperature greater than 0.5 TmK (the melting point in degrees absolute), creep can take place and lead to fracture, possibly after long periods of time (sometimes after 105 h or longer) at static stresses much lower than those which will break the specimen under a normal tensile test. Consequently, when specifying a steel for use at elevated temperatures, the design stress value is based on the 0.2% proof stress at lower temperatures or the creep rupture stress for higher temperatures. Examples are shown in Figure 4 for several high temperature steels.
Temperature gradients are inevitable in many components at elevated temperatures and with changes in temperature and hence stress level, localized expansion or contraction can occur. A single severe change can cause cracking due to thermal shock but repeated changes may give rise to cumulative damage, termed thermal fatigue.
Figure 1 shows that the coefficient of thermal expansion of austenitic stainless steels is considerably greater than for other steels. Consequently, when components are manufactured from austenitic stainless steel and another types of steel design, considerations must be taken to ensure warping or fracture will not occur.
Steels will oxidize when exposed to an oxidizing environment (e.g. air or flue gas) at elevated temperatures. The maximum operating temperature depends on the chromium content and consequently stainless steels offer greater oxidation resistance than plain carbon or low alloy steels.
Selected steels also possess excellent impact properties at extremely low temperatures. Fine grain C-Mn steels can be used down to about -50°C and a range of nickel steels (2.25, 3.5, and 9% Ni) can be used down to liquid nitrogen temperature (-196°C). For even lower temperatures, austenitic stainless steels can be used to hold liquid helium at 4.2 K, with the added bonus that the polished, highly reflective, low corrozivity surface finish can be obtained to reduce heat gains from radiation.
To prevent cracking of many alloy steels during welding it is necessary to pre- and post-heat to ensure a slow cooling rate. The necessity for pre-heat is dependent upon the section thickness and the carbon equivalent (CE) of the steel where
Although stainless steels are not prone to cracking, their low thermal conductivity and high thermal expansion necessitate the use of clamps or tack welding the joint prior to welding to minimize distortion.
Llewellyn, D. T. (1992) Steels: Metallurgy and Application, Butterworths Heinemann.
Pickering, F. B. (1991) Materials Science and Technology, Vol. 7 Constituents and properties of steels, Weinheim.
ASM Metals Handbook, 10th Edn. (1990) Vol. 1. Properties and Selection: Irons, steels and high performance alloys.
Smithells Metals Reference Book (1992) Brandes, E. A. and Brook, G. B., Eds., Butterworths Heinemann.
Rothman, M. F. (1987) High-Temperature Property Data: Ferrous Alloys, ASM International.
Thelming, K. E. (1984) Steel and Its Heat Treatment, 2nd edn., Butterworths.
ASM Handbook Vol. 4, 10th edn. (1991) Heat Treating, ASM International.
Van de Voort, G. F. (1991) Atlas of Time Temperature Diagrams for Irons and Steels, ASM International.