--- title: "Stainless Steel Composition: What Each Element Does" source: "https://www.laxconsteels.com/stainless-steel-composition/" description: "Stainless steel is iron with at least 10.5 percent chromium. What chromium, nickel, molybdenum, carbon and nitrogen do, with the windows for 304, 316 and 2205." --- # Stainless Steel Composition: What Each Element Does Send an enquiry Published 11 September 2026 9 min read Stainless steel is an alloy of iron with a minimum of 10.5 percent chromium and a maximum of 1.2 percent carbon, the definition used by EN 10088. The chromium reacts with oxygen to form a chromium oxide film a few nanometres thick on the surface, and that film, which re-forms whenever it is scratched, is what makes the steel stainless. Every other alloying element is added to change the structure, the strength, the corrosion resistance in a particular environment or the ease of working, and each grade is a composition window defined by a standard. Grade 304 is 18 to 20 percent chromium and 8 to 10.5 percent nickel with the balance iron; grade 316 adds 2 to 3 percent molybdenum; grade 430 has the chromium and no nickel. The composition of a heat is fixed in the melt shop, measured by optical emission spectrometry and printed on the mill certificate against the heat number. ## Iron, the base Iron is the balance of a stainless steel and is never specified as a number: the standards give every other element as a range or a maximum and leave the iron to make up the rest. In the common bar grades it comes to roughly 65 to 75 percent of the mass, is higher in the ferritic and martensitic grades that carry no nickel, and falls to about half in a high-alloy austenitic grade such as 904L. The iron supplies the lattice the other elements sit in, and its two crystal forms, body-centred cubic ferrite and face-centred cubic austenite, are what the alloying elements stabilise one way or the other. The stainless families are named after which structure the composition leaves at room temperature: ferritic, martensitic (austenite transformed to martensite on quenching), austenitic, duplex (both) and precipitation hardening. The [grade reference](https://www.laxconsteels.com/grades/) lists the families and every grade within them. ## Chromium Chromium is the element that defines the material. At 10.5 percent and above it forms the passive oxide film; the resistance of the film rises with chromium content, so 430 at 16 to 18 percent resists more than 409 at 10.5 to 11.75, and 310S at 24 to 26 percent resists more than 304 at 18 to 20. Chromium is a strong ferrite former: on its own with iron it leaves the steel ferritic and magnetic. It also raises the resistance to oxidation at high temperature, which is why the heat-resisting grades carry 22 to 26 percent. Chromium contributes to the pitting resistance equivalent number in full, one point per percent. ## Nickel Nickel is an austenite former. At about 8 percent beside 18 percent chromium it stabilises the face-centred cubic structure down to room temperature, and that structure is what gives the 300 series its formability, its toughness at cryogenic temperature, its non-magnetic condition and its weldability without hardening. Nickel raises the resistance to reducing acids and to stress corrosion cracking at high content (above about 25 percent), and in the austenitic grades its market price is the main source of the alloy surcharge on a quotation. The 200 series replaces part of the nickel with manganese and nitrogen for cost; the ferritic and martensitic grades contain little or none. ## Molybdenum Molybdenum raises resistance to pitting and crevice corrosion in chlorides. It is the difference between 304 and 316, at 2 to 3 percent, and it counts 3.3 times its percentage in the pitting resistance equivalent number, so two percent of molybdenum is worth as much as six or seven percent of chromium against chlorides. The duplex grades carry 3 to 3.5 percent (2205) and 3 to 5 percent (2507); 904L carries 4 to 5 percent. Molybdenum is a ferrite former, so grades that add it also add nickel to hold the austenite, which is why 316 carries 10 to 14 percent nickel against the 8 to 10.5 of 304. ## Carbon Carbon strengthens steel by interstitial solution and, in the martensitic grades, is what allows hardening by quenching: 410 at up to 0.15 percent hardens to about 40 HRC, 420 at 0.15 percent minimum to about 50, 440C at 0.95 to 1.2 percent to 60. In the austenitic grades carbon is a problem rather than an asset. Between about 425 and 815 degrees Celsius, as beside a weld, carbon combines with chromium to form chromium carbides at the grain boundaries, and the steel next to the boundary is left below 10.5 percent chromium and corrodes there: sensitisation. The answer is the L grades, 304L and 316L at 0.03 percent carbon maximum, or a stabilising element (below). The H grades, such as [304H](https://www.laxconsteels.com/grades/aisi-304h/) at 0.04 to 0.10 percent, use the carbon on purpose for creep strength above 500 degrees Celsius. ## Manganese Manganese is present in all of these grades as a deoxidiser and to tie up sulphur as manganese sulphide. The standards set it as a maximum rather than a range: 2 percent in the austenitic bar grades, 1 percent in the ferritic and martensitic grades of the table below, with the analysis of a given heat usually falling between 1 and 2 percent where the higher limit applies. In the 200 series (201, 202) it is raised to 5.5 to 10 percent as an austenite former in place of part of the nickel, together with nitrogen. The result has a similar structure to 304 but lower corrosion resistance, because the chromium is also lower, and it work hardens faster. ## Nitrogen Nitrogen is a strong austenite former and a strong interstitial strengthener, and it counts 16 times its percentage in the pitting resistance equivalent number. At 0.1 to 0.25 percent it gives the duplex grades their yield strength of 450 MPa and more, twice that of 304, and much of their pitting resistance; 316LN and 304LN use it to recover the strength that the low carbon gave up. It is deliberately controlled rather than removed in the argon oxygen decarburisation converter. ## Silicon Silicon is a deoxidiser, held to a maximum of 1 percent in the common bar grades and typically analysed between 0.3 and 0.7 percent. Raised to 1.5 to 3 percent it improves oxidation resistance at high temperature, as in 314 and some heat-resisting castings, and at about 4 percent it improves resistance to concentrated nitric acid. It is a ferrite former. ## Titanium and niobium Titanium and niobium have a stronger affinity for carbon than chromium has. They form their own carbides and leave the chromium in solution, so the steel does not sensitise beside a weld or in service at 425 to 815 degrees Celsius. Each standard sets the required amount as a multiple of the carbon the steel carries: ASTM A276 requires titanium in 321 at five times the sum of the carbon and nitrogen contents as a minimum, to a maximum of 0.70 percent, and niobium in [347](https://www.laxconsteels.com/grades/aisi-347/) at ten times the carbon as a minimum, to a maximum of 1.00 percent. The stabilised grades are used in exhaust systems, furnace parts and pressure equipment that run in that range; the L grades cover welding but not long service at temperature. Titanium is also the stabiliser in ferritic 439 and 441, where it improves weldability. ## Copper Copper improves resistance to sulphuric acid and is added at 1 to 2 percent in 904L for that reason. In the precipitation hardening grades 17-4 PH and [15-5 PH](https://www.laxconsteels.com/grades/15-5-ph/), copper at 3 to 5 percent is the hardening element: it precipitates as fine copper-rich particles during ageing at 480 to 620 degrees Celsius and raises the yield strength to over 1,000 MPa. In other grades it is a residual held to about 0.5 percent maximum. ## Sulphur and phosphorus Both are residuals held low, sulphur to 0.03 percent and phosphorus to 0.045 percent in most grades, because they lower toughness and, in the case of sulphur, hot workability and corrosion resistance. The exception is the free-machining grades: [303](https://www.laxconsteels.com/grades/aisi-303/) and [416](https://www.laxconsteels.com/grades/aisi-416/) carry sulphur at 0.15 percent minimum, deliberately, so that the manganese sulphide inclusions break the chip during machining. The price of the machinability is a lower pitting resistance than the parent grade and a limit on welding and cold forming. ## Composition windows of the common grades The table gives the windows of ASTM A276 for bar, except the 17-4 PH row, which is ASTM A564 because the precipitation hardening grades are specified there. The EN material number is given beside each grade for identification; the EN 10088-3 windows are not identical to the ASTM ones and the differences are noted below the table. Single values are maxima; the balance is iron. *Composition windows in percent by mass for nine stainless steel grades to ASTM A276, and ASTM A564 for 17-4 PH* | Grade | C | Cr | Ni | Mo | Mn | N | Other | | --- | --- | --- | --- | --- | --- | --- | --- | | [304](https://www.laxconsteels.com/grades/aisi-304/) (1.4301) | 0.08 | 18.0 to 20.0 | 8.0 to 10.5 | | 2.0 | 0.10 | | | [304L](https://www.laxconsteels.com/grades/aisi-304l/) (1.4307) | 0.03 | 18.0 to 20.0 | 8.0 to 12.0 | | 2.0 | 0.10 | | | [316](https://www.laxconsteels.com/grades/aisi-316/) (1.4401) | 0.08 | 16.0 to 18.0 | 10.0 to 14.0 | 2.0 to 3.0 | 2.0 | 0.10 | | | [316L](https://www.laxconsteels.com/grades/aisi-316l/) (1.4404) | 0.03 | 16.0 to 18.0 | 10.0 to 14.0 | 2.0 to 3.0 | 2.0 | 0.10 | | | [321](https://www.laxconsteels.com/grades/aisi-321/) (1.4541) | 0.08 | 17.0 to 19.0 | 9.0 to 12.0 | | 2.0 | 0.10 | Ti 5 x (C+N) min, 0.70 max | | [430](https://www.laxconsteels.com/grades/aisi-430/) (1.4016) | 0.12 | 16.0 to 18.0 | 0.75 | | 1.0 | | | | [410](https://www.laxconsteels.com/grades/aisi-410/) (1.4006) | 0.08 to 0.15 | 11.5 to 13.5 | 0.75 | | 1.0 | | | | [2205](https://www.laxconsteels.com/grades/duplex-2205/) (1.4462) | 0.03 | 22.0 to 23.0 | 4.5 to 6.5 | 3.0 to 3.5 | 2.0 | 0.14 to 0.20 | | | [17-4 PH](https://www.laxconsteels.com/grades/17-4-ph/) (1.4542, A564) | 0.07 | 15.0 to 17.5 | 3.0 to 5.0 | | 1.0 | | Cu 3.0 to 5.0, Nb 0.15 to 0.45 | Silicon is 1.0 percent maximum, phosphorus 0.045 and sulphur 0.03 in each of these unless the grade says otherwise; 2205 is held tighter, to 0.030 phosphorus and 0.020 sulphur. The EN windows are not the same as the ASTM ones: EN 1.4301 runs from 17.5 percent chromium against the 18.0 of A276 304, and EN 1.4401 allows 2.0 to 2.5 percent molybdenum against the 2.0 to 3.0 of A276 316. A bar certified to both standards is melted inside the overlap. The full windows, with the UNS and EN equivalents, are on each grade page and in the [equivalents table](https://www.laxconsteels.com/grades/equivalents/). ## How the composition is measured and certified A composition is a specification until it is measured, and two measurements of the same steel are recognised by the standards. The heat analysis is taken from a sample of the liquid steel, cast into a small mould, faced and read on an optical emission spectrometer: a spark excites the surface and the light emitted at each element's wavelengths gives its concentration to about 0.01 percent for the major elements, with carbon, sulphur and nitrogen read on the same instrument or on a combustion analyser. That result is the composition printed on the mill test certificate to EN 10204 against the heat number stamped on the bar. The product analysis is taken from the finished bar instead, and may be ordered separately; because the elements segregate slightly as the steel solidifies, the standards allow a product analysis a wider tolerance than the heat analysis, and ASTM A751 gives the permitted variation element by element. Two points read a certificate wrongly if they are not known: iron is not listed at all, and a single figure printed under an element that the standard states as a maximum is the measured value, not the limit. ## The composition as bar Laxcon Steels melts stainless steel in its own shop and reads every heat on the optical emission spectrometer before casting, so the composition on the certificate of a [hot rolled round bar](https://www.laxconsteels.com/products/hot-rolled-round-bars/) (16 to 125 mm) or a [bright bar](https://www.laxconsteels.com/products/bright-bars/) (5 to 115 mm) is the analysis of the heat the bar was rolled from. Austenitic, ferritic, martensitic, duplex and precipitation hardening grades are supplied to the ASTM and EN windows above. ## Frequently asked questions ### What is stainless steel made of? Iron with at least 10.5 percent chromium, which forms the protective oxide film. Most grades add nickel (8 to 14 percent in the 300 series) for an austenitic structure, and some add molybdenum for chloride resistance, nitrogen for strength, or titanium and niobium for stability at high temperature. Carbon is held low in most grades and raised only in the hardenable martensitic ones. ### What is the composition of 304 stainless steel? 18 to 20 percent chromium, 8 to 10.5 percent nickel, 0.08 percent carbon maximum, 2 percent manganese maximum, 1 percent silicon maximum, 0.045 percent phosphorus and 0.03 percent sulphur maximum, balance iron, to ASTM A276. The 18/8 name comes from the two main figures. ### What does nickel do in stainless steel? Nickel stabilises the austenitic (face-centred cubic) structure, which gives the 300 series its formability, toughness at low temperature, non-magnetic condition and easy welding. It also improves resistance to reducing acids. It is the element whose price most affects the price of stainless steel. ### Why do 304L and 316L have less carbon? Carbon above about 0.03 percent forms chromium carbides at the grain boundaries when the steel is held between 425 and 815 degrees Celsius, as beside a weld, and the metal next to the boundaries loses its corrosion resistance. The L grades hold carbon to 0.03 percent so that welded parts do not sensitise. ## Sources - ASTM A276/A276M, Standard Specification for Stainless Steel Bars and Shapes: the composition windows and the stabiliser multiples for 321 and 347. - ASTM A564/A564M, Standard Specification for Hot-Rolled and Cold-Finished Age-Hardening Stainless Steel Bars and Shapes: the 17-4 PH window. - EN 10088-1, Stainless steels, list of stainless steels; EN 10088-3, technical delivery conditions for semi-finished products, bars, rods, wire, sections and bright products. - ASTM A751, Standard Test Methods and Practices for Chemical Analysis of Steel Products. - EN 10204, Metallic products, types of inspection documents. - Nickel Institute, Design guidelines for the selection and use of stainless steels. ![Stainless steel hot rolled round bars produced by Laxcon Steels](https://www.laxconsteels.com/images/products/stainless-steel-hot-rolled-round-bars.webp) Enquire about **Hot Rolled Round Bars** First name Last name Email Phone Company Location Product - Bright Bars - Precision Shaft Quality (PSQ) Bars - Pipes and Tubes - Hexagonal Bright Bars - Square Bright Bars - Cold-Drawn Flat Bars - Forged & Proof Machined Bars - HRAP Flat Bars - HRAP Equal Angles - HRAP Unequal Angles - Threaded Bars - Hot Rolled Round Cornered Squares (RCS) Bars - Hot Rolled Round Bars - Continuous Cast Billets / Blooms - Channels - Forging Quality Ingots - Precipitation Hardening Steels - Duplex Steel - T-Profile - Others Message \* Required ## From the Mill - [![Stainless steel hot rolled round bars produced by Laxcon Steels](https://www.laxconsteels.com/images/products/stainless-steel-hot-rolled-round-bars.webp) Hot Rolled Round Bars 16 mm to 125 mm](https://www.laxconsteels.com/products/hot-rolled-round-bars/) - [![Stainless steel bright bars produced by Laxcon Steels](https://www.laxconsteels.com/images/products/stainless-steel-bright-bars.webp) Bright Bars 5 mm to 115 mm](https://www.laxconsteels.com/products/bright-bars/) - [![Optical emission spectrometer used to certify grade composition](https://www.laxconsteels.com/images/quality/laxcon-steels-optical-emission-spectrometer.webp) Grade reference](https://www.laxconsteels.com/grades/)