In metallurgy, stainless steel is defined as an iron-based alloy containing at least 10.5% chromium by weight. Chromium is the key element that protects the steel against rusting. The material takes its name from the fact that, unlike other steels, it does not readily stain, corrode or rust.
Without reference to a specific alloy type or grade, it is also called corrosion-resistant steel, particularly in the aerospace industry. Today, stainless steels are readily available in many grades and surface finishes that perform without problems throughout the product’s lifetime in the harsh environments in which they are used. In everyday life we encounter stainless steel products almost everywhere, from cutlery to watches.
Stainless steel offers high resistance to corrosion and oxidation in many natural and man-made environments. However, selecting the right type and grade for each application is essential. Grade selection starts at the design stage with a detailed analysis and definition of all the operating conditions the material may be exposed to.
Good oxidation resistance at room temperature in normal atmospheric conditions is achieved with about 13% chromium by weight; for very severe conditions, the chromium content can be increased to around 30%. When the chromium in stainless steel comes into contact with oxygen in the air, it immediately forms a passive layer based on chromium oxide (Cr2O3) on the surface. This layer, too thin to be seen with the naked eye, protects the steel by preventing water and gases from reaching the underlying metal. If the layer is scratched or damaged for any reason, it quickly re-forms in the presence of oxygen. This phenomenon is called passivation and is also seen in some other metals such as titanium.
Like molybdenum and vanadium, which are used in lower proportions, nickel also contributes to the stability of the passive layer.
Its resistance to corrosion and staining, low maintenance requirement, long service life and the resulting low life-cycle cost, together with its attractive appearance, make stainless steel indispensable in a wide range of commercial applications. Although there are more than 150 stainless steel grades in total, only around 15 are widely used and well known on the market.
Stainless steels are produced by hot and cold rolling in forms such as flat products, plates, bars, wire, tubes and sections, while complex parts are made by casting. Main fields of use include the food industry, medicine and surgical instruments, industrial equipment, the automotive industry, household appliances, buildings and structural elements. Stainless steel is also used in jewellery and watches that we keep with us every day; the most common grade in this field is 316L. Unlike silver, stainless steel does not oxidise and tarnish over time. It also has a considerably lower density than silver (about 7.9 g/cm³ compared with 10.5 g/cm³), which gives designers more freedom.
Stainless steel is 100% recyclable. About 60% of the stainless steel produced is made from material recovered from end-of-life products and production scrap.
Alloying elements determine the structure and properties of stainless steel. For example, adding nickel stabilises the austenitic microstructure of iron; this structure makes the steel non-magnetic in the annealed condition and less brittle at low temperatures. Manganese is also present in many grades in varying amounts and helps maintain the austenitic structure provided by nickel at a lower cost. In martensitic grades, on the other hand, the carbon content is increased for higher hardness and strength; after heat treatment these steels are used in products such as razor blades, knives and cutting tips.
Stainless steels are classified into five groups according to their crystalline microstructure:
The 300-series austenitic stainless steels account for about 60% of total world stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and enough nickel and/or manganese to keep the austenitic structure stable from very low temperatures up to the melting point.
The best-known type is AISI 304, which contains 18% chromium and 8% nickel and is also known as 18/8. Superaustenitic stainless steels such as AL-6XN and 254 SMO offer very high resistance to chloride pitting and crevice corrosion thanks to their high molybdenum (6% and above) and nitrogen additions, and to stress corrosion cracking thanks to their high nickel content.
The high alloy content of superaustenitic grades also raises their cost considerably. It should therefore be remembered that a similar, though not identical, performance can be obtained at lower cost with ferritic or duplex stainless steels. The most common austenitic grades are AISI 304 and AISI 316.
Austenitic stainless steels are non-magnetic in the annealed condition; cold-worked 304 can become slightly magnetic. They cannot be hardened by heat treatment and harden only through cold working. They have high ductility and very good corrosion resistance, formability and weldability. Because they tend to work-harden during machining, suitable tooling and cutting parameters are required. Their crystal structure is face-centred cubic (FCC).
Ferritic stainless steels are a group of stainless steels that generally contain no nickel, but contain 10.5% to 30% chromium together with carbide-forming and ferrite-stabilising alloying elements such as molybdenum, titanium and vanadium.
They have good general corrosion resistance and are more resistant than austenitic grades to stress corrosion cracking in chloride environments. Their mechanical and physical properties are close to those of carbon steels; unlike austenitic grades they are magnetic, cannot be hardened by heat treatment and are easily cold formed. The only heat treatment applicable to these steels is annealing.
Price increases in alloying elements, especially nickel, have accelerated the development of ferritic grades, and new, lower-cost ferritic grades with a wide range of uses have been developed that offer corrosion resistance close to austenitic grades in some applications.
These steels generally have a microstructure of roughly equal proportions of ferrite and austenite, and their corrosion performance varies with their alloy content.
In addition to higher strength than austenitic grades, duplex stainless steels offer better resistance to localised corrosion, in particular pitting, crevice corrosion and stress corrosion cracking. They owe these properties to their high chromium content of 19–28%, up to 5% molybdenum and a lower nickel content than austenitic grades.
The main limitation of duplex steels is the service temperature. When operated for long periods above about 280–300 °C, the ferrite phase undergoes hardening and embrittlement known as 475 °C embrittlement. At low temperatures, toughness drops markedly because of the ferrite phase, so use is generally limited to about −50 °C. The most common duplex grade is 2205. Their crystal structure is body-centred cubic (BCC) in the ferritic regions and face-centred cubic (FCC) in the austenitic regions.
Martensitic stainless steels are similar in composition to ferritic grades, but thanks to their higher carbon content they can be hardened and strengthened by heat treatment like carbon steels. Their main alloying elements are 12–15% chromium, up to 1% molybdenum and 0.1–1.2% carbon. With a few exceptions, they contain no nickel.
Martensitic stainless steels are magnetic. As the carbon content increases, their hardenability and strength increase while their toughness and ductility decrease. Depending on the carbon content and other alloying elements, they can be heat-treated to a hardness of up to 60 HRC. Optimum corrosion resistance is achieved by tempering after hardening.
Their corrosion resistance is lower than that of ferritic and austenitic grades. Formability and weldability are limited; the best machinability is obtained with free-machining grades such as 416. Depending on the alloying elements and their proportions, a small amount of retained austenite may be present in the structure.
Martensitic steels are used successfully where strength and resistance to mechanical wear are required together with corrosion resistance, and they are also used as tool steels. Their crystal structure is body-centred tetragonal (BCT).
Precipitation-hardening stainless steels, also called age-hardening stainless steels, contain mainly chromium and nickel and combine the properties of martensitic and austenitic grades.
Like martensitic stainless steels, they can achieve high strength through heat treatment, and like austenitic grades they offer good corrosion resistance. Hardening is achieved by adding one or more elements such as copper, aluminium, titanium, niobium and molybdenum. The most common grade in this group is 17-4 PH, also known as 630. Named after its content of about 17% chromium and 4% nickel, this grade also contains about 4% copper and 0.3% niobium.
One advantage of PH steels is that they can be supplied in the solution-treated (Condition A) state, ready for machining. After machining or fabrication, strength can be raised to the required level by an ageing heat treatment at a relatively low temperature. Because the treatment takes place at a low temperature, the part does not suffer thermal distortion.
Precipitation-hardening stainless steels are divided into three subgroups: martensitic PH, semi-austenitic PH and austenitic PH. Depending on the alloy content, they can offer corrosion resistance close to that of AISI 304. Because they can be susceptible to stress corrosion cracking in the solution-treated condition, they are generally not used without an ageing heat treatment. Depending on the subgroup, their crystal structure can be BCT, FCC or a combination of both.
For process equipment such as reactors, tanks and mixers, the material is selected according to the chemical nature of the product, its chloride content, the operating temperature and the cleaning conditions. At its Dilovası facility, Makfen manufactures stainless steel process equipment in AISI 304/304L, AISI 316/316L and, where required, duplex stainless steel; the grade is chosen for each project after assessing the operating conditions.
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