Welding Preheat Temperature Calculator — EN 1011-2 CEN Method B, CE(IIW), and Pcm
Selecting the correct minimum preheat temperature is one of the most consequential decisions in structural and pressure vessel welding. Too low, and the risk of hydrogen-induced cold cracking (HICC) in the heat-affected zone or weld metal becomes unacceptable; too high, and cost escalates, productivity suffers, and metallurgical consequences such as grain coarsening and sensitisation can degrade joint toughness. This calculator implements the three most widely used preheat determination methods — EN 1011-2 Annex C (CEN Method B), CE(IIW) (International Institute of Welding carbon equivalent), and the Ito-Bessyo Pcm parameter — with full step-by-step results and interpretive guidance.
Key Takeaways
- EN 1011-2 CEN Method B gives the minimum preheat Tp0 as a function of the CEN carbon equivalent Cp, combined plate thickness, and diffusible hydrogen class HD.
- CE(IIW) = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 is valid for steels with C > 0.18%; values above 0.45 signal increasing cold-crack risk; above 0.60 mandatory preheat is always required.
- Pcm = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B is the preferred parameter for modern low-carbon HSLA grades (C < 0.15%) such as S460–S960 and API 5L X70–X100.
- Hydrogen class H5 (properly baked low-H electrodes or GTAW/SAW) gives the lowest required preheat; H35 (cellulosic electrodes) requires the highest.
- Combined (heat-sink) thickness is the sum of all member thicknesses meeting at the joint — a T-joint uses the plate plus the flange thickness, not just one plate.
- Preheat must be maintained as a minimum interpass temperature throughout the entire weld sequence; it is measured 75 mm from the weld preparation in EN 1011-2 practice.
- Post-heat (dehydrogenation hold at 200–300°C for 2–4 h) is required for CE > 0.55 or thick sections (> 50 mm) and is distinct from PWHT stress relief.
Preheat Temperature Calculator
EN 1011-2 Annex C (CEN Method B) • CE(IIW) • Pcm — carbon and low-alloy steels
The Physics of Cold Cracking: Why Preheat Works
Hydrogen-induced cold cracking (HICC) — also called hydrogen-assisted cracking (HAC) or delayed cracking — is the most common and insidious form of weld defect in structural steel fabrication. Unlike hot cracking which occurs in the solidifying weld pool, cold cracking occurs after the joint has reached ambient temperature and can appear hours to days after welding is complete, making it particularly dangerous because it may not be detected by in-process inspection.
The Three-Condition Requirement
Cold cracking requires all three of the following conditions simultaneously. Eliminating any single condition prevents cracking.
1 — Susceptible microstructure: Hard, hydrogen-trapping microstructures — primarily untempered martensite and coarse-grained upper bainite in the coarse-grained HAZ (CGHAZ) — provide the crack initiation sites. Martensite hardness above approximately HV 350–400 is conventionally taken as the threshold for cracking susceptibility in carbon and low-alloy steels.
2 — Dissolved hydrogen: Hydrogen enters the weld pool from moisture in consumables, shielding gas impurities, surface contamination, or the base metal itself. On solidification, solubility drops sharply from ~30 ml/100g in liquid iron to ~1–2 ml/100g in ferrite, driving supersaturation that forces hydrogen to diffuse outward. Any hydrogen trapped at dislocations, grain boundaries, and martensite lath interfaces acts as a local embrittlement agent.
3 — Tensile stress: The residual tensile stress field from weld thermal contraction acts as the driving force for crack propagation once a hydrogen-weakened microcrack forms at a susceptibility site.
How Preheat Prevents Cracking
Preheat reduces the cooling rate through the critical temperature range between Ar3 (~850°C) and Mf (~150–300°C). Slower cooling achieves two protective effects: it favours transformation to softer ferrite, pearlite, and tempered bainite rather than martensite (reducing HAZ hardness below HV 350); and it extends time at elevated temperature where hydrogen diffusion is faster (DH at 200°C is ~13× greater than at 20°C), allowing diffusible hydrogen to escape before the cracking temperature range is reached.
Effect of preheat on t8/5 cooling time (butt weld, 20 mm plate, Q = 1.5 kJ/mm): T_preheat = 20°C → t₈/₅ ≈ 6 s → CGHAZ hardness ~450 HV (martensitic) T_preheat = 100°C → t₈/₅ ≈ 10 s → CGHAZ hardness ~380 HV (mixed bainite/martensite) T_preheat = 150°C → t₈/₅ ≈ 15 s → CGHAZ hardness ~340 HV (bainite dominant) T_preheat = 200°C → t₈/₅ ≈ 22 s → CGHAZ hardness ~300 HV (tempered bainite) Hydrogen diffusion coefficient in BCC ferrite: D_H(T) = 7×10⁻⁷ × exp(−7500/RT) m²/s At 20°C: D_H ≈ 3×10⁻¹⁰ m²/s At 200°C: D_H ≈ 4×10⁻⁹ m²/s → ~13× faster diffusion at preheat temperature
Carbon Equivalent Formulas Explained
CE(IIW) — International Institute of Welding Formula
CE(IIW) = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
Validity: Best for C > 0.18% (medium carbon and alloy steels)
Widely used in EN 1011-1, AWS D1.1, ISO 17671
Risk interpretation:
CE < 0.35 → Very low risk; no preheat required for most conditions
0.35 ≤ CE < 0.45 → Low risk; preheat conditional on thickness and HD class
0.45 ≤ CE < 0.55 → Moderate risk; preheat typically required; HD class critical
0.55 ≤ CE < 0.65 → High risk; significant preheat always required; post-heat advised
CE ≥ 0.65 → Very high risk; mandatory preheat + post-heat; WPS qualification essential
Pcm — Ito-Bessyo Cracking Parameter
Pcm = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B
Validity: Best for C < 0.15% (low-carbon HSLA steels: S460–S960, X70–X100)
Developed at Yawata Steel by Ito and Bessyo (1968)
Risk interpretation:
Pcm < 0.20 → Low susceptibility
0.20–0.25 → Moderate; preheat conditional on HD and thickness
0.25–0.30 → High; preheat required
> 0.30 → Very high; mandatory preheat and post-heat
CEN Carbon Equivalent Cp (EN 1011-2 Annex C)
Cp = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 [numerically identical to CE(IIW)] EN 1011-2 Annex C — Minimum preheat temperature T_p0: T_p0 (°C) = 697 × Cp − 0.175 × √t + 0.571 × t × Cp − 26 × √HD − 296 where: Cp = CEN carbon equivalent (= CE IIW) t = combined (heat-sink) thickness (mm) HD = diffusible hydrogen content (ml/100g deposited weld metal) If T_p0 ≤ 0°C → no preheat required (provided steel surface > 5°C and dry) Round up calculated T_p0 to nearest 25°C increment in practice Heat input correction (EN 1011-2 Annex C Note 2): When Q > 1.5 kJ/mm: t_eff = t × √(1.5 / Q)
Hydrogen Content Classes and How to Select Them
| HD Class | Max HD (ml/100g) | Typical Processes and Consumable Conditions | Preheat Penalty vs H5 |
|---|---|---|---|
| H5 | ≤ 5 | GTAW (all), SAW with dried flux, properly baked E7018 (300–350°C / 1 h min.), GMAW solid wire, FCAW-G metal-cored wire | Baseline (lowest preheat) |
| H10 | ≤ 10 | Standard low-hydrogen SMAW electrodes (E7016, E7018) stored correctly, short-arc GMAW, FCAW-G with acceptable moisture | +15–30°C typical |
| H15 | ≤ 15 | Rutile electrodes (E6013), basic electrodes without baking, FCAW-S self-shielded, SMAW in humid conditions | +25–50°C typical |
| H25 | ≤ 25 | Semi-automatic processes with higher ambient humidity, SMAW electrodes from partially used packets | +40–70°C typical |
| H35 | ≤ 35 | Cellulosic electrodes (E6010, E6011) — inherently high hydrogen | +55–100°C typical |
Combined (Heat-Sink) Thickness
| Joint Type | Combined Thickness Formula | Example |
|---|---|---|
| Butt weld (plate-to-plate) | t = plate thickness t1 | 25 mm plate: t = 25 mm |
| T-joint or cruciform (fillet weld) | t = t1 + t2 | 12 mm web + 20 mm flange: t = 32 mm |
| Corner joint | t = t1 + t2 | 15 mm + 15 mm: t = 30 mm |
| Nozzle-to-vessel | t = nozzle neck + shell thickness | 25 mm nozzle + 40 mm shell: t = 65 mm |
| Pipe girth weld | t = pipe wall thickness only | DN 300 SCH 80 (t = 18.3 mm): t = 18 mm |
| Three-plate cruciform (double T) | t = tweb + 2 × tflange | 10 + 2×25 = 60 mm |
Preheat Verification and Monitoring in Practice
| Method | Accuracy | Range | Advantages | Limitations |
|---|---|---|---|---|
| Temperature-indicating sticks (Tempilstik) | ±1% of melting point | 38–1650°C | Fast, cheap, no calibration required | Discrete temperature points only; leave residue on steel |
| Contact thermocouple (digital) | ±1–2°C | -50 to 1350°C | Accurate, continuous reading, data logging possible | Requires clean contact surface; calibration required |
| Infrared (IR) thermometer | ±2–3% | -50 to 1600°C | Non-contact; fast; suitable for difficult access | Emissivity correction required for polished/scaled surfaces |
| Thermocouple welded to plate | ±1°C | -50 to 1300°C | Most accurate; continuous data logging; permanent record | Expensive to install; introduces a weld; damages surface |
| Thermal imaging camera | ±2°C | -20 to 2000°C | Full-surface temperature map; records variation across joint | High cost; emissivity calibration; specialist interpretation |
EN 1011-2 specifies preheat measurement at a minimum distance of 75 mm from the weld preparation edge. The measurement must be taken before each pass to confirm the interpass temperature remains within the specified minimum-to-maximum range throughout the full multi-pass sequence.
Post-Heat Treatment (Dehydrogenation)
- On completion of the final pass, do not allow the joint to cool below the preheat temperature.
- Raise to post-heat temperature (typically 200–300°C as specified in the WPS).
- Hold for 2–4 hours (longer for thickness above 50 mm).
- Wrap in insulating blankets; cool at maximum 50°C/hour.
- Do not carry out NDE until ambient temperature and a minimum 24-hour delay period have elapsed.
AWS D1.1 vs EN 1011-2: Comparison
| Feature | AWS D1.1:2020 | EN 1011-2:2001 |
|---|---|---|
| Primary method | Table 4.5 tabulated by steel group and thickness; or Annex I analytical method | CEN Method B (Annex C) analytical formula; or Method A (Annex B) nomogram |
| Carbon equivalent | CE(IIW) for grouping; Annex I uses Pcm-based index | CE(IIW) = Cp for Annex C |
| Hydrogen classification | H4, H8, H16 per AWS A4.3 | H5, H10, H15, H25, H35 per EN ISO 3690 |
| Preheat — 20mm plate, CE 0.42, standard LH | ~10°C (Group I per D1.1 Table 4.5) | ~25°C (EN 1011-2 Annex C) |
| Preheat — 50mm plate, CE 0.55, standard LH | ~107°C (Group II per D1.1) | ~125°C (EN 1011-2 Annex C) |
| Measurement location | Within 75 mm (3") of fusion line | At least 75 mm from weld preparation edge |
| Post-heat | Commentary only; WPS qualification governs | Explicitly required in Section 5.4 for CE > 0.55 |
| Ambient temperature minimum | Welding prohibited when base metal < -18°C without preheating to 38°C | Steel surface must be above 5°C; no absolute minimum stated |
Frequently Asked Questions
What is the EN 1011-2 CEN Method B preheat formula?
EN 1011-2 Annex C (CEN Method B) calculates minimum preheat using: Tp0 (°C) = 697 × Cp − 0.175 × √t + 0.571 × t × Cp − 26 × √HD − 296, where Cp is the CEN carbon equivalent (identical to CE IIW), t is combined thickness in mm, and HD is diffusible hydrogen in ml/100g. If the result is below 0°C, no preheat is required provided the steel surface is above 5°C and dry.
What is the difference between CE(IIW) and Pcm carbon equivalents?
CE(IIW) = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 is best for steels with C > 0.18% and is the standard in EN 1011-2 and AWS D1.1. Pcm = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B was developed specifically for modern low-carbon HSLA grades (C < 0.15%) where CE(IIW) underestimates the effect of boron, silicon, and nickel. For S460–S960 and API 5L X70–X100 pipeline steels, Pcm gives a more accurate assessment.
What is the hydrogen content (HD) scale and which class should I use?
EN ISO 3690 defines HD in ml H2 per 100g deposited metal: H5 (≤5, GTAW/baked LH/SAW), H10 (≤10, standard LH SMAW properly stored), H15 (≤15, rutile/basic no bake), H25 (≤25, semi-auto with higher moisture), H35 (≤35, cellulosic E6010/E6011). Always use the HD class declared on the filler metal certificate. Electrodes stored incorrectly typically rise to H15–H25; rebaking at 300–350°C for 1 hour restores H5 classification.
Why does preheat prevent hydrogen-induced cold cracking?
Preheat slows HAZ cooling through the martensite transformation range (Ms to Mf), allowing partial decomposition to softer ferrite and bainite and reducing peak HAZ hardness below the cracking threshold (~HV 350). Simultaneously, slower cooling extends time at elevated temperature where hydrogen diffusion is ~13× faster than at ambient, allowing diffusible hydrogen to escape before the joint cools into the cracking range (<150°C). Tensile residual stress — the third leg of the cold cracking triangle — is not reduced by preheat; only PWHT addresses that.
What is the combined (effective) plate thickness for preheat calculation?
Combined thickness accounts for all members meeting at the joint as the heat sink. Butt weld: t = plate thickness. T-joint/fillet: t = web + flange. Nozzle-to-vessel: t = nozzle neck + shell. The larger the combined thickness, the faster heat is extracted, the higher the HAZ cooling rate, and the greater the hardening risk — hence the higher required preheat.
What is the maximum interpass temperature in welding?
Typical maximum interpass limits: carbon and low-alloy steels 250°C; P91 (9Cr-1Mo-V) 300°C; austenitic stainless steels 150°C (to limit sensitisation); duplex stainless steels 150°C (to prevent sigma phase formation). Exceeding the maximum causes HAZ grain coarsening and reduced impact toughness. The interpass temperature must remain above the minimum preheat throughout the welding sequence.
Do I still need preheat if CE(IIW) is below 0.40?
Not necessarily, but preheat may still apply depending on thickness and hydrogen class. EN 1011-2 waives preheat when Tp0 ≤ 0°C, provided steel is above 5°C and dry. AWS D1.1 sets a nominal minimum of 10°C for Group I steels (CE ≤ 0.45) up to 19 mm, rising to 66°C for heavier sections — confirming even low-CE steels require thermal management on thick sections.
What is post-heat treatment and when is it required?
Post-heat holds the weld at 200–300°C for 2–4 hours immediately after welding before any cooling, allowing residual diffusible hydrogen to diffuse out before the joint cools into the cracking range (<150°C). EN 1011-2 Section 5.4 requires it when CE(IIW) > 0.55, thickness > 50 mm, or preheat > 200°C. It is distinct from PWHT: post-heat is a hydrogen effusion treatment at 200–300°C, while PWHT is a stress relief at 595–760°C.
How does heat input affect the required preheat temperature?
Higher heat input slows HAZ cooling (increases t8/5), reducing martensite formation and allowing more hydrogen to diffuse out — the same protective effects as preheat. EN 1011-2 Annex C provides a heat input correction that reduces effective combined thickness when Q > 1.5 kJ/mm, lowering the calculated Tp0. However, excessive heat input degrades HAZ toughness through grain coarsening, so heat input and preheat must be balanced in WPS development.
Recommended Reference Books
EN 1011-2:2001 Welding Recommendations — Arc Welding of Ferritic Steels
The primary European standard containing CEN Method A and B preheat calculation methods, hydrogen class guidance, post-heat requirements, and interpass temperature limits.
View on AmazonAWS D1.1/D1.1M Structural Welding Code — Steel
The AWS structural welding code containing Table 4.5 preheat tables, Annex I analytical preheat method, and WPS qualification requirements for prequalified joints.
View on AmazonSteels: Microstructure and Properties — Bhadeshia & Honeycombe
Graduate-level coverage of HAZ microstructure evolution, hydrogen embrittlement mechanisms, martensite transformation, and the physical basis of carbon equivalent formulas.
View on AmazonWelding Engineering and Technology — Lancaster
Comprehensive practical coverage of preheat practice, electrode baking, post-heat procedures, and cold cracking prevention across all major structural steel grades and welding processes.
View on AmazonDisclosure: MetallurgyZone participates in the Amazon Associates programme. If you purchase through these links, we may earn a small commission at no extra cost to you. This helps support free technical content on this site.
Further Reading
