OVERVIEW OF STAINLESS STEELSThe demand for stainless steels has increased dramatically in recentyears as more and more companies specify these materials for theirproducts. Industries which utilize stainless steels extensively includechemical and power, food and beverage, pharmaceutical, andtransportation. Other major uses are in architecture, domesticappliances, household utensils and small electrical and electronicappliances.

Stainless steels owe their widespread use to a combination of uniqueproperties that have advantages over those of other steels andnonferrous metals for many applications. The principal outstandingcharacteristics are their resistance to corrosion in many environments,their good mechanical properties over an extremely wide range oftemperatures, and their superior resistance to oxidation and scaling atvery high temperatures.

These unique properties are achieved through substantial additions ofchromium to iron and carbon, with or without the addition of otherelements such as nickel, manganese, molybdenum, and nitrogen.Chromium, however, is the key ingredient since a dramatic increase incorrosion resistance occurs when a level of about 10.5% chromium isreached. Steels with at least 10.5% chromium have traditionally beendesignated as stainless steels.

These steels attain their stainless characteristics by virtue of their abilityto form an invisible, adherent, chromium-rich oxide film which stronglyresists attack by the atmosphere and many industrial gases andchemicals.

The chemical compositions of 21stainless steels and 3 high temperaturealloys are listed in this website, in addition to a general description ofeach alloy, its physical properties, its specified mechanical properties andtypical applications.

This website also includes a list of standard specifications, an overview ofstainless steels and a glossary of terms for stainless sheet, strip andplate products.

DESCRIPTION OF STAINLESS STEELSStainless steels may be divided into five main groups, the first four ofwhich are based upon a common crystalline structure of the steels withinthe group. The fifth group, known as precipitation hardening steels,consists of alloys that can be hardened by an aging treatment.

AUSTENITIC STAINLESS STEELSAustenitic stainless steels containing chromium and nickel are given a300 Series classification and austenitic steels containing chromium,nickel and manganese are given a 200 Series classification. It should benoted, however, that some austenitic stainless steels do not have these200 and 300 Series designations. The steels in this austenitic grouphave many compositions and properties, but they have manycharacteristics in common. They can be hardened by cold working butnot by heat treatment. In the annealed condition all are essentiallynonmagnetic, but some may become slightly magnetic by cold working.In the annealed condition they have excellent formability.

The various grades of austenitic steels have excellent corrosionresistance in many environments, resisting attack by the atmosphere andby many industrial gases and many chemicals. They have a resistanceto scaling at temperatures as high as 2000°F (1095°C). The resistanceof an individual austenitic alloy to corrosion or oxidation in a particularenvironment will depend upon that steel’s alloy content.

Many of these steels have good strength at high temperatures whichaccounts for their wide use at elevated temperatures. They are alsoamong the primary materials selected for use at extremely low

temperatures since they do not become brittle as with other types ofsteel. All austenitic stainless steels are sensitive, to some degree, tochloride stress corrosion cracking and fully austenitic types may besensitive to solidification cracking.

Some types are susceptible to carbide precipitation or intermetallic phaseprecipitation under adverse conditions of fabrication and use.

Type 304 stainless steel is the most widely used steel of the austeniticgroup. It has a nominal composition of 18% chromium and 8% nickel.

FERRITIC STAINLESS STEELSFerritic stainless steels are straight chromium 400 Series steels thatcontain from 11 to 27% chromium and little or no nickel. They cannot behardened by cold working. These steels are magnetic. They havemoderate ductility and resistance to corrosion and oxidation.

Ferritic stainless steels are relatively weak at high temperatures, and inheavier sections they may be lacking in toughness at below ambienttemperatures.

Type 430 is the general purpose steel of the ferritic group. It has anominal composition of 17% chromium and is used for highly polishedtrim applications in mild atmospheres. It is also used in food processingand for consumer goods. Type 409 and its recently standardizedvariations are the most widely used ferritic grades of all the nickel freestainless steels. They are among the least expensive of all stainlesssteels. These grades have a nominal composition of 11% chromium andtheir major use is in automotive exhaust systems.

MARTENSITIC STAINLESS STEELSMartensitic stainless steels, like the ferritic steels, are straight chromiumtypes in the 400 Series. They have from 11 to 18% chromium with orwithout small additions of nickel or other elements. These steels aremagnetic.

Unlike the austenitic and the ferritic steels, the martensitic steels arehardenable by heat treatment and are generally used in the hardenedand tempered condition.

Type 410 stainless steel is the general purpose grade in the martensiticgroup. It has a nominal chromium content of 12.5% and the carboncontent is limited to a maximum of 0.15%. A tensile strength as high as190 ksi (1380 MPa) can be obtained with Type 410 stainless steel afterheat treatment, but with reduced ductility.

DUPLEX STAINLESS STEELSDuplex stainless steels have an annealed structure which typicallyconsists of about equal parts of austenite and ferrite. These steels have18 to 29% chromium, 3 to 8% nickel and various other elements,particularly nitrogen and molybdenum.

The duplex grades are highly resistant to chloride stress corrosioncracking, they have excellent pitting and crevice corrosion resistance andhave about twice the yield strength of the common austenitic grades.Type 329 and Alloy 2205 are typical duplex steel grades.

PRECIPITATION HARDENING STAINLESS STEELSPrecipitation hardening steels are not characterized by a single crystallinestructure, but rather by a particular strengthening mechanism in one ofthe structures. They contain 11 to 18% chromium, 3 to 27% nickel andsimilar amounts of other metals. They combine high strength andhardness with corrosion resistance which is superior to that of themartensitic chromium stainless steels. They can be hardened by solutiontreating and aging to develop tensile strengths as high as 240 ksi (1655MPa) for the grade designated as UNS S17700 (17-7PH), for example.