Post-weld heat treatment: guide values by material to EN 13480-4 and ASME B31.3
Whether a pipe weld must be stress relieved, at what temperature and for how long depends on material, wall thickness and code. This overview compares the guide values of the European piping standard EN 13480-4 and the American ASME B31.3 – for planning and for discussions with the welding supervisor and the operator. The project’s qualified welding procedure specification (WPS) is always binding.
First: what the tables do – and what they don’t
The values below are minimum requirements and guide values from the codes. Designers, operators or the welding procedure specification may require heat treatment even below the thickness limits – for example where there is a risk of stress corrosion cracking, in hydrogen service or at low temperatures – and may specify narrower temperature ranges. Codes are revised continually; for a project, the edition agreed there applies.
Europe: EN 13480-4 (piping)
EN 13480-4 assigns steels to material groups according to CEN ISO/TR 15608. The holding temperatures are taken from EN 13480-4:2017, the holding times from ISO/TR 14745:2015 (t = governing wall thickness in mm).
| Group | Materials (examples) | Holding temperature | When required | Holding time (guide value) |
|---|---|---|---|---|
| 1.1 | P235GH, P265GH | 550–600 °C | from 35 mm wall thickness; below that only in special cases | up to 35 mm: 30 min · 35–90 mm: t min · over 90 mm: 40 + 0.5·t min |
| 1.2 | P355NH, 16Mo3 | 550–600 °C (16Mo3: 550–620 °C) | as 1.1 | as 1.1 |
| 5.1 | 13CrMo4-5 | 630–700 °C | always; exception small pipes up to Ø 114.3 mm and 7.1 mm wall with at least 200 °C preheat | up to 15 mm: 30 min · 15–60 mm: 2·t min · over 60 mm: 60 + t min |
| 5.2 | 10CrMo9-10 | 670–730 °C | as 5.1 | as 5.1 |
| 5.3 | X11CrMo5 (P5) | 700–750 °C | always | as 5.1 |
| 6.4 | X10CrMoVNb9-1 (P91) | 730–770 °C | always; before PWHT, intermediate cooling for martensite transformation | according to the WPS |
| 8.1 | austenitic steels (1.4301, 1.4404 …) | – | as a rule no heat treatment | – |
Simplified compilation, without guarantee. The holding times in EN 13480-4 may differ from ISO/TR 14745 in detail (e.g. minimum values); the text of the agreed edition of the standard is authoritative.
USA: ASME B31.3 (process piping)
ASME B31.3 assigns materials to P-numbers. Values according to the 2018 and 2022 editions (Tables 331.1.1, 331.1.3 and 330.1.1). Holding time for all P-numbers listed here except 15E: up to 50 mm 1 h per 25 mm, at least 15 min; over 50 mm 2 h plus 15 min for each additional 25 mm.
| P-No. | Materials (examples) | Holding temperature | When required | Minimum preheat |
|---|---|---|---|---|
| 1 | carbon steel (A106 B, A105) | 595–650 °C | exempt at all thicknesses if preheated to 95 °C from 25 mm and welded in multiple passes from 5 mm | according to the WPS; from 25 mm, 95 °C as a condition of the exemption |
| 3 | C-½Mo (A335 P1) | 595–650 °C | exempt up to 16 mm (C ≤ 0.25 %, preheat, multiple passes) | 10 °C up to 13 mm, otherwise 95 °C |
| 4 | 1¼Cr-½Mo (A335 P11) | 650–705 °C | exempt on butt welds up to 16 mm (C ≤ 0.15 %, preheat, multiple passes) | 120 °C |
| 5A | 2¼Cr-1Mo (A335 P22) | 675–760 °C | exempt on butt welds up to 16 mm (C ≤ 0.15 %, preheat) | 150 °C, for higher strength 200 °C |
| 5B | 5Cr-½Mo (P5), 9Cr-1Mo (P9) | 675–760 °C | always | 150 °C or 200 °C |
| 15E | 9Cr-1Mo-V (P91) | 705–775 °C | always; holding time at least 30 min | 200 °C |
| 8 | austenitic steels | – | neither required nor prohibited – according to the WPS | 10 °C |
Simplified compilation, without guarantee. The exemptions apply only if all conditions of the standard are met and neither the designer nor the welding procedure specification requires heat treatment. The 2024 edition renamed the tables; check new projects against the current edition.
Preheating
Preheat and interpass temperature are set by the welding procedure specification. They depend on chemical composition, wall thickness, welding process, hydrogen content of the weld metal and heat input, and also apply to tack welding. For non-alloy steels in Europe, the preheat temperature is determined from the carbon equivalent according to EN 1011-2; for creep-resistant steels the standard gives minimum values.
As an order of magnitude for creep-resistant steels over 15 mm wall thickness and low-hydrogen weld metal, EN 1011-2 gives around 75 °C for 16Mo3, 100 °C for 13CrMo4-5 and 10CrMo9-10, and 150 °C for 5 to 9 % chromium steels. For P91, filler metal manufacturers recommend at least 200 °C with an interpass temperature of no more than 250 °C.
Heating, holding, cooling and measuring
- ▸Heating and cooling rate: Above 300 °C, EN 13480-4 limits the rate to 5500/t °C per hour, at most 300 °C/h (t in mm). ASME B31.3 specifies, above 315 °C, at most 335 °C/h divided by half the wall thickness in inches.
- ▸Start of the holding time: The holding time only starts once all measuring points have reached the minimum temperature – and it is interrupted as soon as one measuring point falls below it.
- ▸Thermocouples: On the weld and at the edge of the heated zone; on larger diameters distributed around the circumference. Attaching thermocouples in duplicate safeguards the measurement against failure.
- ▸Width of the soak band: ASME B31.3 requires at least 25 mm on either side of the weld at holding temperature. EN 13480-4 sets a minimum width of the heated band depending on diameter and wall thickness, and requires at least half the peak temperature at its edge. Sufficiently wide insulation prevents excessively steep temperature gradients.
- ▸Supporting: Support the line during PWHT so that it does not deform under its own weight – otherwise there is a risk of distortion, for example at flanges.
- ▸P91: Allow to cool below the martensite finish temperature before PWHT, do not go straight from preheat into PWHT; the upper PWHT temperature must not exceed the Ac1 transformation point of the parent metal and the weld metal.
Flange distortion during welding →Cold cutting instead of flame cutting →
Where HSOS helps
We carry out preheating and stress relieving of pipe welds with electrical resistance heating – to your welding procedure specification, with calibrated measuring and control equipment and a complete temperature chart for each weld. If required, we also take care of cutting and bevelling beforehand and bolting afterwards.
Frequently asked questions
Does every weld on carbon steel have to be heat treated?
No. According to EN 13480-4, stress relieving of non-alloy steels in groups 1.1 and 1.2 is as a rule only required from 35 mm wall thickness, below that only in special cases. ASME B31.3 exempts carbon steel (P-No. 1) at all thicknesses if the conditions on preheating and pass sequence are met. Designers or operators may nevertheless specify heat treatment, for example in caustic or amine service.
Why do EN and ASME differ on temperature?
The two codes set their temperature ranges independently and matched them to their own material systems. The ranges usually overlap, for example for P91 with 730–770 °C to EN 13480-4 and 705–775 °C to ASME B31.3. The cycle is run within the overlap or within the window of the welding procedure specification.
How is the heat treatment documented?
With a temperature chart for each weld or control zone – heating, holding, cooling – with assignment of the measuring points, plus details of the heating pads, insulation and the equipment used, with calibration certificates.
Compiled from EN 13480-4:2017, ISO/TR 14745:2015, EN 1011-2, ASME B31.3 (2018/2022) and manufacturers’ data, without guarantee. The edition of the standard agreed in the project, the welding procedure specification and the requirements of the designer and operator are authoritative.
Heat treatment to the WPS – with a chart for each weld
Send us the material, dimensions and welding procedure specification – we will plan heating zones, measuring points and personnel.