Stainless steel alloys in detail: main elements, core features and stainless steel classification

Stainless steel is usually taught as five boxes: austenitic, ferritic, martensitic, duplex, and precipitation hardening. The boxes are real, and they are not the reason a grade behaves the way it does. A grade sits where it sits because of two numbers that can be calculated from its composition, and its corrosion resistance can be ranked by a third. Once those three numbers are on the page, the five boxes become regions on a map and not labels to memorise.

The first number is the chromium content, and it is a threshold and not a scale. The second pair is the chromium and nickel equivalents, which decide whether the structure that forms on cooling is ferrite, austenite, or a mixture. The third is the pitting resistance equivalent number, which converts chromium, molybdenum, and nitrogen into a single figure that can be compared across grades.

This page sets out the three, shows where the common grades land on each, and explains the one mechanism that removes the protection the first number provides: the chromium that forms the passive film can be taken away by carbon at the grain boundaries.

Chromium is what makes the alloy stainless. Above roughly 10.5 % in solid solution, the metal forms a thin and continuous chromium oxide film on its surface, and that film is what separates a stainless steel from a low alloy steel that merely contains chromium. Below that level the film is not continuous, and the alloy rusts like an ordinary steel.

The threshold is a statement about chromium in solution and not about chromium in the analysis. If the chromium is tied up in a precipitate, it is not available to form the film, and the alloy behaves as though it contained less. That distinction is the whole subject of the last section, and it explains why a grade that meets its certificate can still corrode at a weld.

Nickel does something different. It does not create the film, and it is not required for corrosion resistance in every family. Its role is to stabilise austenite, which is why the austenitic grades contain 6 to 22 % nickel in the composition ranges that are commonly sold, and why the ferritic grades contain little or none.

Chromium and molybdenum both favour ferrite. Nickel, carbon, and manganese favour austenite. The Schaeffler diagram reduces a full composition to two numbers that capture those tendencies, and a grade is placed on the diagram by calculating both.

The carbon term explains a result that surprises buyers. Two grades with the same nickel content can land in different regions if their carbon levels differ, because carbon is thirty times as effective as nickel at stabilising austenite in this expression. That is one reason a low carbon version of a grade is not simply a cleaner version of the same grade, and why the L suffix appears on drawings where welding is involved.

The austenitic row carries a qualification that the textbooks often drop. An annealed austenitic grade is non-magnetic, and a cold worked one is not entirely. Cold working can form martensite in a metastable austenitic grade, so a deep drawn or heavily rolled part can show a magnetic response that the annealed certificate does not predict. A specification that requires non-magnetic behaviour should say so at the finished condition and not at the annealed condition.

Pitting is the failure mode that decides a material in chloride service, and it depends on three elements and not one. The pitting resistance equivalent number combines them into a single figure, and the standard form is chromium plus 3.3 times molybdenum plus 16 times nitrogen.

The number places the common grades on a scale that runs from about 16 to above 70, and the scale is the reason a 316 is specified for a coastal site and a 304 is not.

Two thresholds are worth remembering because they appear in purchase specifications. A PREN of at least 32 is the figure commonly used to resist pitting in seawater, and a PREN of 40 or higher is the figure commonly used for hydrogen sulphide service and for seawater service where the risk is higher. Those are screening values and not guarantees, and the next paragraph explains why.

The limitation is stated by the association that publishes the calculation. PREN is useful for ranking grades within one family, and it cannot predict whether a particular grade will suit a particular application. A 2205 and a 904L with similar PREN do not behave identically, because one is duplex and the other is austenitic, and the comparison is only meaningful between grades of the same structure. Temperature, surface finish, flow, and the presence of deposits all move the answer, and none of them appears in the formula.

Chromium forms the film only while it remains in solution. Carbon removes it, and it does so at the grain boundaries. When an austenitic stainless steel is held in a temperature range of roughly 500 to 850 °C, the carbon in the alloy combines with chromium at the boundaries to form chromium carbides, and the metal immediately beside each carbide is left depleted in chromium.

The depleted zone is narrow and it is continuous along the boundary, so the alloy now has a path through it that is no longer stainless while the bulk of the grain still is. A corrosive medium attacks that path, and the result is intergranular corrosion, which looks like a joint that has lost its edges and not a surface that has rusted.

The two standard answers work by different mechanisms. The L grade reduces the carbon available, so less carbide can form. A stabilised grade adds titanium or niobium, which combine with carbon more readily than chromium does, so the chromium stays in solution. Both are correct, and they are not interchangeable: a stabilised grade can be used at a higher temperature than an L grade, and an L grade is usually cheaper.

A grade should be chosen by the mechanism that is expected to end the part. The table links the mechanism to the family and to the number that predicts it.

DLX supplies stainless steel and nickel alloy products from Changzhou, Jiangsu, in wire from 0.025 mm, round bar to 50 mm, strip from 0.50 mm to 2.5 mm in thickness, and tube and plate in the standard mill sizes. The range covers the austenitic, ferritic, duplex, and precipitation hardening families, together with the nickel alloys used where a stainless grade reaches its limit in chloride or in reducing acid.

Material is supplied against the standard named on the order, with an EN 10204 3.1 certificate on request and the heat number traceable to the test results. The useful question is not which family a grade belongs to, and it is which mechanism is expected to end the part and which number predicts that mechanism.

Because that is the level at which the chromium oxide film becomes continuous enough to protect the surface. Below it the film is incomplete and the alloy rusts like a low alloy steel. The threshold applies to chromium in solution, so an alloy whose chromium has been consumed by carbides can behave as though it were below the line even when the certificate says otherwise.

Molybdenum. The 316 grade adds it, and molybdenum carries a weight of 3.3 in the pitting resistance number, which moves the value from about 19 for 304 to about 27 for 316. That is enough to matter in chloride service and not enough to reach the seawater threshold of 32, which is why a coastal application often moves to 904L, to a duplex grade, or to a nickel alloy.

In the annealed condition it is, and in the cold worked condition it may not be. Cold working can form martensite in a metastable austenitic grade, so a deep drawn or heavily rolled part can show a magnetic response that the annealed certificate does not predict. A specification that requires non-magnetic behaviour should state the condition in which it is measured.

It is the loss of chromium from the grain boundaries when chromium carbides form in the range of about 500 to 850 °C. A weld and its heat affected zone pass through that range as they cool, so the boundary becomes a path that is no longer stainless while the grain interiors still are. The result is intergranular corrosion, and the standard answers are a low carbon grade or a stabilised grade.

It is a screening tool and not a selection method. It ranks grades within one family, and it cannot predict whether a particular grade will suit a particular application. A duplex and an austenitic grade with similar values do not behave identically, and temperature, surface finish, flow, and deposits all move the answer without appearing in the formula.

Low carbon. The carbon limit drops to about 0.03 %, which reduces the amount of chromium carbide that can form at the grain boundaries during welding. It is the standard answer for a fabricated part that cannot be re-solution annealed after welding, and it costs less than a stabilised grade.

Stainless steel is five families placed by two calculated equivalents, protected by one threshold, and ranked in chloride service by one number. Chromium above about 10.5 % in solution forms the film. The chromium and nickel equivalents decide whether the structure is ferrite, austenite, or a mixture. The pitting resistance number, chromium plus 3.3 times molybdenum plus 16 times nitrogen, ranks grades within a family and sets the screening thresholds of 32 and 40.

The protection can be removed without changing the certificate. Carbon takes chromium from the grain boundaries between about 500 and 850 °C, and a weld is the usual place where that happens. A low carbon grade or a stabilised grade prevents it, and the choice between them depends on the service temperature and on the cost.

Schaeffler diagram and equivalent expressions, as published in welding metallurgy references: chromium equivalent = Cr + Mo + 1.5 Si + 0.5 Nb, and nickel equivalent = Ni + 30 C + 0.5 Mn.

British Stainless Steel Association, calculation of pitting resistance equivalent numbers. Source for the expression PREN = Cr + 3.3 Mo + 16 N, the published ranges for 304, 316, 316L, 904L, 2205, and 2507, and the statement that PREN is useful for ranking and comparison but cannot predict whether a grade will suit a given application.

Published PREN comparison data for stainless and nickel alloys, giving single values of 19 for 304, 27 for 316, 35 for 904L, 36 for 2205, 43 for 2507 and for 254 SMO, 46 for AL-6XN, 51 for alloy 625, and 68 for Hastelloy C-276.

Published technical guidance on PREN thresholds, stating that a value of at least 32 is used to protect against seawater pitting and that 40 or higher is used for hydrogen sulphide and for more severe seawater service.

Published research on the sensitisation of austenitic stainless steel, describing chromium carbide precipitation at the grain boundaries and the resulting chromium depleted zone in the temperature range of about 500 to 850 °C.

Published welding metallurgy references describing sensitisation in the heat affected zone and the use of low carbon and stabilised grades to prevent it.

EN 10088 and ASTM A240, as the product standards that define the stainless grades named and the composition ranges quoted.

EN 10204, inspection documents, for the definition of the 3.1 certificate as a document carrying actual test results and endorsed by the manufacturer independent inspection representative.

The composition ranges and calculated values above are standard figures for the grades named, and they are not acceptance limits for a delivered lot. PREN is a ranking index and not a corrosion rate, and two grades with the same value in different families do not behave identically. Sensitisation depends on carbon content, on time in the range, and on the cooling rate, so a grade that is resistant in one fabrication can be attacked in another with the same analysis. Property values for a delivered part depend on its condition and on the direction of the test, and a certificate describes a heat and not a component. Product specifications are revised, and the edition named on the purchase order governs. Nothing in this page is a substitute for an engineering release or for the qualification testing agreed between buyer and supplier.