Ferrous metals or alloys are metals that consist mostly of iron (Fe). Steel is an iron-based alloy containing typically less than 1% carbon, where iron frequently contains 2% or more carbon. Iron and steel are widely available, strong, cheap, and can be shaped by casting. Their mechanical properties can be improved by heat treating and, in the case of steels, by working (i.e. rolling or forging). Stainless steels were developed to resist corrosion and generally contain 12% or more chromium, and may contain nickel in any amount up to or even exceeding the chromium content based upon the mechanical properties desired and application.
There are several types of stainless steel. When considering these alloys for use in a corrosive environment, the most widely used method for initial selection is to compare PREn ratings (pitting resistance equivalent number) across materials. This is calculated using the weight % of key alloying elements present in any particular grade of stainless steel. The formula is:
PREn = % Cr + (3.3 x % Mo) + (16 x % N)
Obviously, alloys containing the highest amounts of chromium, molybdenum and nitrogen are considered to possess better resistance to pitting corrosion. This may be accompanied by a corresponding increase in tensile and yield strengths.
Ferritic Stainless Steels
Stainless steels owe their ability to resist corrosion primarily to the presence of a passive film on their surface. Chromium is chiefly responsible for formation of this passive film. Iron ceases to rust at approximately 12% chromium content. Resistance to oxidizing corrosives increases rapidly with chromium content up to about 20%. Beyond that level, though, resistance increases at a more gradual and declining rate. Consequently, very few stainless alloys contain more than 27% chromium. These alloys are composed principally of chromium and iron, with low carbon content. Alloys are often categorized by their crystal structure, which is impacted by chemistry and processing, including heat treating. Ferritic stainless steels have a crystal structure that is chiefly ferrite.
Martensitic Stainless Steels
The addition of sufficient carbon to alloys of chromium and iron results in alloys that can be hardened and tempered. Corrosion resistance is reduced somewhat by the carbon level, but the reduction is minimized when these alloys are fully hardened and tempered. Consequently, the higher carbon alloys (over 0.15% C) are normally used only in the fully hardened and tempered condition. Their structure upon rapid cooling from temperatures above 1600°F / 870°C is chiefly martensitic.
Common martensitic grades include: MTEK 410, MTEK 416, MTEK 420, MTEK 431, and MTEK 440.
Austenitic Stainless Steels
Of all elements added to the basic alloy of chromium and iron, nickel is the most important. It not only improves corrosion resistance, but also changes the alloy’s structure and mechanical properties. As nickel is added in increasing amounts to a ferritic iron/chromium alloy, the structure of the alloy changes from ferrite, through mixed ferrite and austenite, to essentially all austenite. Most “18-8 grades” (a common description of 304 stainless that is made up of roughly 18% chromium and 8% nickel) are made with controlled amounts of ferrite for improved welding characteristics and higher strength. The change in structure is accompanied by a marked increase in ductility and toughness. Stainless alloys, predominantly austenitic in structure, make up a family of stainless steels that are by far the most widely used of all types.
Other alloy additions can be made to change performance of the metal in the range of environments. There are numerous alloys in this group which contain from 16% - 25% chromium and 6% - 25% nickel. Other commonly used alloying additions include molybdenum and nitrogen, which enhance both strength and corrosion resistance.
Common austenitic grades include: MTEK 303, MTEK 304, MTEK 309, MTEK 310, MTEK 316, MTEK 317, and MTEK 347.
Super Austenitic Stainless Steels
Under certain aggressive conditions, the general grades of austenitic stainless steel are more susceptible to pitting, crevice corrosion and stress corrosion cracking. This has led to development and additions to the austenite family, referred to as super austenitic stainless steel.
The use of super austenite is rapidly growing due to changing conditions in today’s process industries. A “fit it and forget it” approach is being adopted to employ alloys which do not require constant, regular replacement in conditions such as deep sea water and high-temperature closed circuit process plants.
Super austenitic stainless steels contain high levels of chromium and nickel together with significant additions of molybdenum and nitrogen. The result is a series of austenitics. They are up to 30% stronger than conventional 300 series stainless and offer superior resistance to pitting, crevice corrosion and stress corrosion cracking. An austenitic stainless steel is consider super austenitic when its PREn exceeds 40.
Common super austenitic grades include: MTEK 20, MTEK 20M, MTEK 6XN, and 254SMO®.
Duplex Stainless Steels
Duplex stainless steels are alloys with structures generally regarded as being approximately equal parts of austenite and ferrite, with a 60/40, 40/60-phase distribution being the envelope widely considered as acceptable.
The combination of austenite/ferrite produces alloys with twice the strength of conventional austenitic stainless steels.
Duplex stainless steels are virtually immune to stress corrosion cracking (the Achilles’ heel of common austenitic stainless steels) and are highly resistant to pitting and crevice corrosion. Possessing these characteristics, it is not surprising to find the majority of (but by no means all) applications to be seawater related. Duplex stainless steels have many uses in offshore oil and gas production and Naval equipment, particularly sub-surface.
Common duplex grades are: MTEK 2205, MTEK 29MN / CD4MCuN, Ferralium® 255, Zeron® 100
Precipitation Hardening (Age hardened) Stainless Steels
The need for stainless steels that would combine the excellent corrosion resistance of the austenitic types with the ability to be hardened by heat treatment led to the development of a family of stainless steels known as PH types. They can be precipitation (age) hardened by low temperatures (900°F / 480°C), minimizing distortion.
Common grades are: MTEK 17-4PH®, MTEK 15-5PH®, MTEK and 14-4PH®.
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