Carbon and heat-resistant steels: S195T, P235GH, P265GH, P250GH, 16Mo3

What the steel name says

For non-alloy steels the steel name to EN 10027-1 is built around the application, not the composition – which makes it readable. The first letter names the purpose: S for structural steel, P for pressure purposes, E for engineering steel. The figure is the minimum yield strength in megapascals: a P265GH yields at no less than 265 MPa at room temperature, an S195T at 195 MPa. The letters after it are additional symbols: T for tubes, G for “other characteristics” and H for high temperature – GH together marks the pressure-vessel steels whose strength at operating temperature is guaranteed in the standard. With 16Mo3 the system changes: that is a steel name by composition – about 0.16 % carbon, molybdenum as the main alloying element with the factor code 3 (about 0.3 %). Once you have read that, you can tell from a steel name what the steel is meant for.

Black, galvanized or coated

A non-alloy steel does not protect itself – unlike stainless steel it has no passive layer. What keeps it from rusting is either the medium (oxygen-poor heating water in a closed circuit), a coating or the insulation. That is why the same steel comes in three surfaces:

• Black means untreated – the dark mill scale, often with a thin film of oil. The threaded tube and the boiler tube for closed heating circuits, steam and compressed air. Inside, the lack of oxygen in the circuit protects; outside, the insulation or a coat of paint.
• Hot-dip galvanized means dipped in molten zinc: a zinc layer bonded to the steel that sacrifices itself where it is damaged. The threaded tube for open systems, fire-fighting water, compressed air with condensate and wherever the tube stays visible. For drinking water, galvanized steel tube is bound to rules and water quality – the recognised rules of technology and the tube manufacturer say so, not this profile.
• Coated in our range means above all the electroplated zinc-nickel layer of the Megapress connectors: a few micrometres that protect the connector against external corrosion, for instance condensation on cooling lines. It protects only the connector – the tube needs its own protection and the insulation according to the rules of technology.

For the order this means: material and surface are two pieces of information. “Threaded tube DN 50 medium” is only complete with “black” or “galvanized” – and for the press connector, the system page says on which tube surfaces it is approved.

Heat-resistant: what “GH” and molybdenum do

Every steel loses strength as the temperature rises. A heat-resistant steel is one for which the standard guarantees the yield strength at elevated temperature as well – the designer can calculate with it, the inspector can have it certified. With P235GH and P265GH this is the non-alloy case: the same composition as a structural steel, but closely controlled, tested and with guaranteed elevated-temperature values. The pressure-temperature ratings of flanges build on this: EN 1092-1 assigns the materials to groups, and for each group and PN rating it is fixed which pressure is permissible at which temperature. P250GH is the flange material of this logic; the figures belong on the pressure-temperature page of the flange world, not in this profile.

16Mo3 begins where non-alloy steel starts to creep under continuous temperature: about 0.3 % molybdenum raises the creep rupture strength and pushes the service limit up by a few dozen degrees – steam and hot-water lines in power stations and industry. After that come the chromium-molybdenum steels (13CrMo4-5, 10CrMo9-10), which we procure as special materials. For the trade, 16Mo3 is the material where dual certification ends: its ASTM counterpart A182 F1 / A234 WP1 is tested after a different heat treatment, and the mill issues a separate certificate for each world.

Where the material sits in our range

The register of the materials world lists, for each material, where it sits in our range – and, for the ASME side, the flange materials of the American world that we procure for international projects.

Two worlds: is A105 my P250GH?

No – but it sits in the same material group. The ASTM world names the steel by product form: A105 is the forged flange and fitting steel, A106 Gr. B the seamless pipe, A234 WPB the buttweld fitting, A350 LF2 the forging steel for low-temperature service. The EN world names the material the same across all products and only changes the product standard. That EN 1092-1 lists P250GH and ASME B16.5 lists A105 in pressure-temperature groups of similar position makes the two interchangeable in the way of thinking, not in the certificate: composition ranges, scope of testing and heat treatment are different. With non-alloy steels, dual certification is possible if the composition of the heat meets both standards (A106 Gr. B with P265GH, A105 with P250GH) – it has to be ordered, it cannot be obtained afterwards. All counterparts with their limits: Materials between the worlds; the flange page on this: EN 1092-1 versus ASME B16.5.

Low-temperature toughness is a question of its own. A350 LF2 is not a “better A105” but a steel with tested impact energy at low temperature – for refrigeration plants, liquefied gas, outdoor lines in cold regions. The EN counterparts P285NH/P285QH (EN 10222-4) test at a different temperature and with a different specimen; here the mill issues a separate certificate for each world. Whoever needs low-temperature toughness orders it expressly, in the world of their specification.

Recognising it on the part

Paper before testing: tubes to EN 10216-2 and EN 10217-1 carry material, standard and heat number in the tube marking, flanges the material and heat number in the stamp, ASME flanges the grade to MSS SP-25 – the way is described on Kennzeichnung lesen (reading the marking). Threaded tube to EN 10255 is often marked only with the manufacturer’s mark and the standard; the material there is unambiguously S195T anyway.

On the part, non-alloy steel is magnetic and rusts red at bare spots – both are secondary indications: ferritic and duplex stainless steels also hold the magnet, and foreign rust on stainless steel looks like rust. The spark test (bright, branched sparks with non-alloy steel) is workshop practice, attacks the surface and remains a last resort. What the eye cannot do: separate P235GH from P265GH, P250GH from S235 or 16Mo3 from a non-alloy steel – for that there are the stamp, the certificate and, if need be, the analysis. 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.

The typical mix-ups

• “St 37”, “C22.8”, “15Mo3” are the old DIN names of S235, P250GH and 16Mo3. They appear in many plant documents; you find the assignment in the standards comparison. Orders are placed with the EN steel name.
• Threaded tube is not boiler tube. S195T to EN 10255 is meant for threading and welding, without guaranteed elevated-temperature values; whoever has to prove pressure and temperature takes P235GH/P265GH to EN 10216-2 or EN 10217-1.
• Galvanized is not a material designation. Galvanized and black threaded tube are the same steel; galvanizing is surface protection. In combination with press connectors, the system manufacturer’s approval for the respective tube surface applies.
• A105 instead of P250GH is not a like-for-like swap under the Pressure Equipment Directive: only the EN material carries the presumption of conformity, the ASTM material needs a particular material appraisal. What that means for procurement is explained on Materials between the worlds.
• Welding on galvanized tube is a question of occupational safety and weld quality (zinc fumes, porosity), not a material question; the company’s welding procedure and the tube manufacturer’s data sheet are decisive.

Further reading

The materials world: Materials · Materials between the worlds · Austenitic stainless steels.

From our range: Tubes EN 10255 · Gewinderohre DIN 2440/2441 · Tubes EN 10216-2 · Tubes EN 10217-1 · Flanges · EN 1092-1 versus ASME B16.5 · Viega Megapress · ASTM A105 profile · ASTM A106 profile · ASTM A234 profile · Certificates to EN 10204.