Austenitic stainless steels: 1.4301, 1.4307, 1.4401, 1.4404, 1.4571

What the number says

The material number to EN 10027-2 is a key, not a name: “1.” stands for steel, “43” and “44” for the stainless steels with nickel – the 43 group without molybdenum, the 44 group with it. The last two digits simply count. The steel name to EN 10027-1 is more telling: X5CrNi18-10 means a high-alloy steel (X) with about 0.05 % carbon (5), 18 % chromium and 10 % nickel. In X2CrNiMo17-12-2 you read the low carbon (2 = about 0.02 %), the molybdenum in the name and its share as the last figure. Once you can read the steel name, the table is only there for looking things up.

304 versus 316: what the molybdenum is for

The difference between the 304 and the 316 family is a single alloying element: molybdenum. It makes the passive layer more resistant to chlorides – that is, to pitting and crevice corrosion in seawater, swimming-pool water, salty air, chlorine-based cleaners and in every crevice under a gasket where chlorides concentrate. Without a chloride load, 1.4301 can do everything 1.4401 can – with chlorides, the 316 family is the safe choice. That is why drinking-water press systems run on 1.4401 or 1.4404, while a heating or cooling circuit manages with 1.4301 connectors.

What the number does not say: whether the material resists your medium. A statement on corrosion resistance needs concentration, temperature, oxygen and flow – manufacturers and material data sheets give that, not this profile. Where we name a system for a medium, it is the system manufacturer’s approval.

L or not L – and what titanium has to do with it

During welding, the zone next to the weld passes through temperatures at which chromium and carbon combine into chromium carbides at the grain boundaries. The grain boundary loses chromium, the passive layer loses its effect there – the material becomes susceptible to intergranular corrosion. Three ways out: little carbon, so that hardly any carbides form (the L grades 1.4307 and 1.4404, L for low carbon); binding the carbon to titanium, which takes it away from the chromium (1.4571, stabilised); or solution annealing after welding, which is not an option on site.

The rule of thumb that follows: if the line is welded and not annealed afterwards, an L grade or a stabilised grade belongs in it. If it is pressed, screwed or pushed together, the question is secondary – which also explains why a press connector may be made of 1.4301 or 1.4401 while the matching buttweld fitting is usually supplied in 1.4307 or 1.4404. The other way round, the L grade loses nothing: 1.4404 can be used wherever 1.4401 is specified, and today the mill almost always certifies both numbers at once (see below).

1.4571 or 1.4404? Both solve the welding problem, both are 316 grades. 1.4571 is the material of German practice – flanges, fittings and valves of the EN world have been made from it for decades, and it appears throughout the parts lists of older plants. Internationally it is the exception: the ASTM counterpart 316Ti is hardly available on the market, and projects to ASME call for 316L. For an enquiry this means: if 1.4571 is in your parts list, you get it in flanges and fittings; if you tender internationally, you write 1.4404/316L. Titanium stabilisation brings fine titanium carbide inclusions to the surface, which is why 1.4571 is not the first choice for mirror-polished or electropolished parts – there 1.4404 is specified anyway.

Where the material sits in our range

The register of the materials world lists, for each material, where it sits in our range: in which tube, fitting, flange or system we carry it and which page describes it.

For drinking water, what counts with press systems is the system manufacturer’s approval, not the material number alone: Sanpress carries the tube in 1.4401 or in ferritic 1.4521 with DVGW approval, NiroSan in 1.4404; Temponox with connectors of 1.4301 on 1.4520 tube is expressly not intended for drinking water and is marked brown. The system pages name tube, connector and marking in detail.

Recognising it on the part – in this order

1. Paper before testing. Certificate, order documents, isometric and piping class are more reliable than any test on the part. Tubes and fittings carry the material and the heat number in the stamp or the tube marking; the way to the certificate to EN 10204 is described on Kennzeichnung lesen (reading the marking) and Certificates to EN 10204.

2. The magnet separates the family, not the number. Austenitic stainless steels are practically non-magnetic in the annealed condition. Ferritic (1.4520, 1.4521) and duplex steels hold the magnet – so “sticks = not stainless” is wrong. And take care at the fitting: cold working produces strain-induced martensite, so the magnet pulls slightly at the extrados of an elbow, at the collar and at the branch of a tee although the part is austenitic. The reading is clearest on a straight, annealed length of tube.

3. The molybdenum spot test separates 304 from 316. A drop of test solution on a freshly ground spot changes colour within a few minutes on molybdenum-bearing steel; on 1.4301/1.4307 it stays light. That distinguishes the 304 and 316 families, but not 1.4401 from 1.4404 or 1.4571 – that takes an analysis.

4. The limiting sentence. A material check on the part names the probable family – not the material number, heat, heat treatment, pressure rating or suitability, and it does not replace a certificate. Where the paper is missing, Werkszeugnis verloren (lost certificate) helps; where the part is unknown, Bauteil bestimmen (identify a component).

Two worlds: EN number to UNS and ASTM grade

Each of the five numbers has an ASTM counterpart – and each counterpart is the same alloy, not the same material. The UNS number names the alloy (S31603 is the low-carbon molybdenum austenitic steel, whether it is called 1.4404 or 316L); the product standard sets composition ranges, heat treatment and the scope of testing. With the austenitic steels, dual certification is state of the art: 304/304L and 316/316L almost always leave the mill with both numbers on one certificate. The exception is 1.4571: its counterpart 316Ti (A182 F316Ti) is rare in the ASTM world, so 1.4571 is practically never dual certified. In detail, with all the limits of the comparison: Materials between the worlds.

The typical mix-ups

• “V2A” and “V4A” are old trial designations, not materials: today V2A means the 304 family (1.4301/1.4307), V4A the 316 family (1.4401/1.4404/1.4571). Whoever orders “V4A” has to say which number is meant – otherwise the supplier decides.
• 1.4401 instead of 1.4404 is equivalent for pressing or screwing, but not for welding without post-weld annealing. The other way round (1.4404 instead of 1.4401) is always permissible unless the specification expressly demands the higher carbon – which practically never happens.
• 1.4571 instead of 1.4404 is usual in the EN world and technically mostly unproblematic; in an ASME specification 316Ti is a foreign material and needs the client’s consent.
• 316 is not “seawater-proof”. The 316 family is more resistant to chlorides, not resistant in every seawater – with stagnant, warm or oxygen-poor water, duplex or higher-alloyed steels come into consideration. That is a question for the manufacturer, not for this profile.
• AISI 316 on a stainless threaded fitting is an alloy designation of the American pattern, not a certificate to EN – behind it stand 1.4401, 1.4404 or 1.4571. Which number is delivered is stated by the certificate; the range is on Stainless threaded fittings.

Further reading

The materials world: Materials · Materials between the worlds · Carbon and heat-resistant steels · Trinkwasser-Werkstoffe ab 2027 (drinking-water materials).

From our range: Tubes EN 10217-7 · Flanges · Stainless threaded fittings · Viega Sanpress · Sanha NiroSan · Viega Temponox · ASTM A182 profile · Certificates to EN 10204.