P265GH vs P235GH Buttweld Fittings: Which to Specify for EN 13480 Steam Lines

Where these two grades come from

The names follow the EN 10027-1 designation system. "P" means steel for pressure purposes, the number is the specified minimum yield strength in MPa for the thinnest wall range, and "GH" means the steel has specified properties at elevated temperature. The same two grades appear across the product standards a steam line is built from: seamless pipe to EN 10216-2, welded pipe to EN 10217-2 and EN 10217-5, plate to EN 10028-2 and buttweld fittings to EN 10253-2. That is why a P235GH line will normally have P235GH pipe, fittings and plate-made components, and the same for P265GH.

Both grades are normally delivered in the normalised condition. Both are non-alloy steels, and both sit under EN 10253-2, the part of the fittings standard for non-alloy and ferritic alloy steels with specific inspection requirements.

Side-by-side comparison

The figures below are the commonly quoted values for seamless tube to EN 10216-2 in the thinnest wall range (up to 16 mm). Minimum yield strength falls for heavier walls, plate to EN 10028-2 has narrower tensile ranges (360–480 MPa and 410–530 MPa for thin plate), and values can change between editions, so always confirm against the product standard and edition named in your project specification.

What the difference means in an EN 13480 design

Allowable stress

For ferritic (non-austenitic) steels in the time-independent range, EN 13480-3 takes the design stress as the lower of the minimum yield or proof strength at the design temperature divided by 1.5 and the minimum room-temperature tensile strength divided by 2.4. At room temperature, using the thin-wall figures above, the comparison is simple. P235GH gives the lower of 235/1.5 (about 157 MPa) and 360/2.4 (150 MPa), so 150 MPa. P265GH gives the lower of 265/1.5 (about 177 MPa) and 410/2.4 (about 171 MPa), so about 171 MPa. On those nominal figures that is roughly 14% more allowable stress; the margin is different for heavier walls and at temperature.

At steam temperatures the proof-strength term usually governs, and the gap depends on the tabulated values at the design temperature, where P265GH's values are generally the higher of the two. Your piping designer will take the actual values from the edition of EN 10216-2 or EN 10217-2 the project uses, and check any amendment to EN 13480-3 that applies.

Wall thickness and weight

Required wall thickness for a straight pipe is roughly inversely proportional to the design stress. For the same pressure and diameter, a P265GH line can therefore use a thinner wall than a P235GH line, which reduces weight, support loads and welding volume. On small-bore, low-pressure lines the minimum practical wall often governs anyway, so the saving disappears. On large-bore headers and higher-pressure steam mains it can be worth having.

Creep range

At higher temperatures, time-dependent (creep) strength replaces proof strength as the governing value. Both grades have creep data in the product standards, but carbon steels lose strength quickly in this range, and long exposure can lead to graphitisation in some conditions. For long-term service toward the top of the carbon-steel range, designers usually move to a low-alloy steel such as 16Mo3 or 13CrMo4-5 (1.7335) rather than pushing P265GH further.

Matching fittings to the pipe

This is where most specification errors happen. An EN 10253-2 Type B fitting is made to have the same strength as the connecting pipe of the same material and wall thickness. The words "same material" are part of that promise.

• Same grade on pipe and fitting. The simple case. Order the fitting in the pipe grade, give the pipe OD and wall, and the Type B design does the rest.
• P265GH fittings on P235GH pipe. The fitting is the stronger part, so it is normally acceptable for strength. It is still a substitution: get the designer's written acceptance and make sure the material list in the PED documentation shows it.
• P235GH fittings on P265GH pipe. The fitting is the weaker part. A Type B fitting of the same wall no longer carries the guarantee, so the fitting needs a design check and probably a heavier wall.

If a heavier fitting wall is used, the bore can step down at the weld. Arshya can machine the fitting ends to match your pipe bore so that the root of the weld lines up. See our page on EN 10253 Type A and Type B fittings for how the Type B wall is distributed.

Welding and fabrication

Both grades are normally in group 1.1 of ISO/TR 15608 (steels with a specified minimum yield strength up to 275 MPa), so a welding procedure qualified on one will usually cover the other within the qualification ranges of EN ISO 15614-1. Filler metals are the same families. The practical difference is that P265GH allows more carbon and manganese, which can give a slightly higher carbon equivalent. On thin walls this rarely matters. On thick headers, high restraint or cold site conditions, preheat should be set from the actual carbon equivalent on the certificate, the combined thickness and the hydrogen level of the consumable. Our article on carbon equivalent and preheat for pipe fitting welds explains how to read those values from the 3.1 certificate.

Post-weld heat treatment, where required, follows the fabrication rules of EN 13480-4 for the thickness and material group. Because both grades are in the same material group, the requirements are normally the same for both at a given thickness.

How to choose: a short decision guide

1. Follow the pipe. If the pipe grade is already fixed, order fittings in the same grade unless the designer approves otherwise.
2. Check where the wall is governed. If minimum practical wall governs, P235GH is usually enough. If pressure governs on large bores, run the numbers for P265GH.
3. Check the temperature. If the line spends long periods in the creep range, ask whether a low-alloy grade is the better answer than either carbon steel.
4. Check availability of the pipe. The fitting is rarely the problem; the matching pipe size and wall in the chosen grade sometimes is.
5. Write it down. State grade name and number, EN 10253-2, Type B, and the pipe OD and wall on every line.

If you are still deciding between a piping code and a vessel code for parts of the system, our article on EN 13480 vs pressure vessel codes may help.

Supply from Arshya

We supply elbows, tees, reducers and caps in P235GH and P265GH to EN 10253-2, in Type A and Type B, with Type B as our specialism. Fittings are made at our group manufacturing facility, Arshya Engineers & Fabrications Pvt Ltd at Village Dundsa, Palwal, Haryana, with in-house heat treatment. The facility holds a PED 2014/68/EU material manufacturer certificate from TÜV Rheinland (notified body 0035), AD 2000-Merkblatt HP 0 and ISO 9001; certificates are on our quality page. Every order comes with EN 10204 3.1 certificates, or 3.2 with TÜV, Lloyd's Register, BV, DNV or SGS on request.

Frequently asked questions