UNS S31603 Forgings

What UNS S31603 is

S31603 and 316L are the same steel under two naming systems. UNS gives it a number, AISI gives it a name, and ASTM A182 writes it into a forging specification as grade symbol F316L. A drawing calling for S31603 is asking for the chemistry. A drawing calling for F316L is asking for a forging certified to the testing, heat treatment and marking rules of A182 as well.

Two numbers define the grade against its neighbours. Carbon at 0.030 % maximum, against 0.08 % for standard 316, starves the reaction that throws chromium carbides onto the grain boundaries while a weld cools through roughly 450 to 850 C. That is the whole reason the L grade exists. The cost is strength, and it is why some designers move to F316LN and buy the yield back with nitrogen. Molybdenum at 2.00 to 3.00 % is the second number, and it is the one that separates 316 from 304.

UNS S31603 chemical composition

Limits below are weight percent, balance iron. Single figures are maxima. ASTM A182 applies to forgings and is the column that normally governs our product.

Nickel runs wider at the top in A182, 10.0 to 15.0 % against 10.0 to 14.0 % in A240, because extra nickel helps hold austenite stable at the lower carbon level. Note also that EN caps molybdenum at 2.50 % where ASTM allows 3.00 %, so a 1.4404 heat is not automatically interchangeable with an A182 F316L heat when the specification is tight.

PREN, taken as %Cr + 3.3 × %Mo + 16 × %N, puts a typical heat in the mid-20s. That sits above 304L and below the 6 % molybdenum and duplex grades. PREN is a comparison index rather than an acceptance requirement, so if the service is chloride-critical, state the minimum chromium and molybdenum you want on the order instead of relying on the grade name.

UNS S31603 mechanical properties

Acceptance minima differ by product form. Quoting plate figures on a forging drawing is a common source of argument at inspection, so both are set out here.

A182 sets no Brinell band for the austenitic grades, so hardness on a forging is reported as measured unless your own specification states a limit. The structure is austenitic and cannot be hardened by heat treatment; strength above the minima comes from cold work or from moving to a nitrogen-bearing grade.

Away from room temperature

Austenitic steels keep their ductility going down, and S31603 is used at cryogenic temperatures without an impact problem. Going up, yield strength falls steadily, so code calculations at 300 C and above work from much lower allowables than the room-temperature figure suggests. Sustained service above roughly 425 C brings carbide precipitation and sigma phase into play, and at that point a stabilised or H grade such as 321H or 347H is usually the better buy. Tell us the design temperature with the enquiry and we will say so rather than quoting what was asked for.

UNS S31603 physical properties

Heat treatment and forging practice

What decides the result is the cooling, not the soak. The quench has to carry the forging through roughly 450 to 850 C fast enough that carbides get no chance to reform at the grain boundaries, which on heavy sections means a real water quench started without delay. A piece that sat too long in transfer can still pass its tensile test and then fail an ASTM A262 Practice E test. There is no conventional stress-relief window of the sort a carbon steel has, because any extended time in the sensitising range undoes the solution anneal.

Austenitic stainless stays stiff at temperature and work-hardens quickly, so the press does more work for the same reduction and the low end of the window is narrower than on carbon steel. Finishing too cold risks cracking. Finishing too hot leaves a coarse grain that shows up later as poor ultrasonic response. Ring geometries are pierced and then rolled radially and axially, which runs the grain flow around the circumference instead of cutting across it.

UNS S31603 equivalent grades

Mechanical acceptance, permitted residuals, product-form scope and pressure-equipment approval all differ between ASTM and EN. A 1.4404 certificate does not by itself satisfy a drawing calling for A182 F316L. Dual certification is usually arrangeable but has to be agreed before melting, not after.

Choosing between the neighbouring grades

Corrosion behaviour, and where the grade stops

S31603 handles general corrosion across a wide span of process chemistry, and its recognised strength is resistance to intergranular attack in the as-welded condition. Published data shows it performing in phosphoric, nitric, citric, lactic and acetic acids, in hydroxides, in nitrate, chloride, acetate and sulphate salts, and in marine atmospheres.

The limits matter as much as the range. Chloride stress corrosion cracking is the classic failure mode for every standard austenitic and becomes a live risk above roughly 60 C in chloride solutions under tensile stress. Crevice geometry under gaskets and deposits will start attack at conditions the open surface tolerates. Strong reducing acids, hydrochloric in particular, are outside the grade. Where any of that describes your service, the answer is a duplex, super duplex or nickel alloy, and we would rather tell you that at enquiry stage than replace the forging later. See Hastelloy C-276, Alloy 20 or 904L.

UNS S31603 forged products

Forged to drawing or specification. Not held as catalogue stock. Minimum order one piece.

Where S31603 forgings go

The combination of low carbon and 2 to 3 % molybdenum makes this the default austenitic grade for wet corrosive duty. It goes into chemical and petrochemical pressure vessels, heat exchangers and piping; pulp and paper digesters, liquor tanks and paper machine parts; food, dairy and brewing equipment where weld-line corrosion cannot be tolerated; pharmaceutical and fine-chemical plant; offshore and subsea production hardware; compressor and nitrogen generator components; and valves, pumps and fittings across all of them.

Testing, inspection and certification

Ultrasonic examination of austenitic stainless forgings is carried out to EN 10228-4, which is the part written for austenitic and austenitic-ferritic steels, or to ASTM A388 where a US specification governs. EN 10228-3 covers ferritic and martensitic forgings and is the wrong reference for this grade, a detail worth checking on any supplier's paperwork including ours.

Available on request: liquid penetrant to ASTM E165 or EN ISO 3452, intergranular corrosion testing to ASTM A262 Practice E, ferrite measurement, positive material identification by XRF or OES, impact testing including sub-zero, and grain size determination. Material ships with an EN 10204 type 3.1 works certificate as standard, or type 3.2 endorsed by a nominated inspection body when the order calls for it.

Composition and property limits quoted from ASTM A182, ASTM A240 and EN 10088 as published. Figures are given for guidance; the governing documents are the purchase order specification and the material certificate issued with your forging. Last reviewed: 2026-10-05.