Preheat Temperature Calculator — ASME P-Number and Carbon Equivalent Method
Preheat temperature is one of the most consequential variables in any welding procedure. Applied correctly, it eliminates hydrogen-induced cold cracking (HICC) — the leading cause of in-service weld failures on carbon and alloy steels. Applied incorrectly — too low, too narrow, or not maintained — it offers no protection while creating a false sense of compliance. This calculator delivers minimum preheat values via two parallel routes: the ASME P-Number method (using B31.3 Table 330.1.1 and B31.1 Table 131.4.1-1 mandatory code limits by material group and thickness) and the Carbon Equivalent (CE) method (using the IIW CE formula or the Pcm formula for low-carbon HSLA steels, per EN ISO 1011-2 and AWS D1.1 Annex B). Both routes also output interpass temperature limits, post-weld hydrogen bake-out guidance, and full step-by-step formula workings — so you understand not just what the number is, but why.
This guide covers the metallurgical basis for preheat, the correct formula to use for each steel type, the ASME P-Number table in full, measurement procedures per code, and every practical situation where the code minimum is insufficient. Whether you are writing a WPS under ASME Section IX, checking an in-progress weld in the field, or reviewing a PQR for a pressure vessel or piping system, the tools and reference tables here give you everything in one place.
Preheat Temperature Calculator
ASME P-Number (B31.3 / B31.1) · CE / IIW Method · Pcm Method — choose your input route
Enter steel chemistry from the MTC. The calculator computes CE (IIW), then applies EN ISO 1011-2 Method A (combined thickness method) for preheat. Best for C > 0.18%.
Pcm formula (Ito-Bessyo) — use for modern HSLA / fine-grain steels with C < 0.18%. Enter chemistry from MTC. Preheat from: T = 1440×Pcm − 392 °C.
Minimum code values only. Higher preheat may be required by the WPS, client specification, or actual restraint conditions. Always verify against the applicable construction code edition in effect and the qualified WPS/PQR. Temperature measuring instruments must be calibrated.
Why Preheat Matters — The Metallurgical Basis
Cold cracking in welds — also called hydrogen-induced cold cracking (HICC), hydrogen-assisted cracking (HAC), underbead cracking, or delayed cracking — is responsible for more in-service pressure equipment failures than any other single weld defect type. Unlike hot cracking, which forms during solidification while the weld is still near melting point, cold cracking typically initiates hours to days after welding has been completed and the joint has fully cooled. This delayed onset means it can pass visual inspection and even initial NDE, only to manifest as a leak or fracture under operating load.
Three conditions must simultaneously be present for HICC to occur, sometimes called the “cold cracking triangle”:
| Factor | What It Means | How Preheat Addresses It |
|---|---|---|
| Susceptible microstructure | Hard martensite in the HAZ (HAZ hardness > 350 HV is a common threshold) | Slower cooling rate reduces martensite fraction; more bainite and ferrite form instead |
| Diffusible hydrogen | Hydrogen picked up from moisture, flux, or atmosphere during welding (measured in mL/100g) | Elevated temperature accelerates hydrogen diffusion out of the weld metal and HAZ before the joint becomes brittle |
| Residual tensile stress | Thermal contraction stresses locked into the joint during cooling | Reduced thermal gradient during cooling lowers residual stress magnitude; preheat alone cannot eliminate this, but reduces its severity |
Remove any one of these three factors and cold cracking cannot occur. Preheat attacks the first two directly and reduces the third indirectly. This is why low-hydrogen consumables (E7018-H4, E9018-G-H4) and preheat are always specified together for high-CE steels — each reduces a different leg of the triangle.
ASME P-Number System — Material Classification for Preheat
The P-Number system is defined in ASME Section IX, Table QW/QB-422. It groups materials with similar composition, weldability, and mechanical properties to reduce the number of welding procedure qualifications needed. Critically for preheat purposes, each P-Number group has defined minimum preheat requirements in the construction codes (B31.3, B31.1, Section VIII) that are mandatory minimums, not recommendations. When a code says “minimum preheat 150 °C”, it means no weld may be made on that material at any lower temperature — not even tack welds.
The P-Number assignment also feeds directly into ASME Section IX QW-406, which classifies preheat as an essential variable for most processes. A decrease of more than 55 °C (100 °F) from the PQR-qualified preheat temperature requires requalification of the WPS. For more on qualification ranges and essential variables, see our P-Number, F-Number, and A-Number guide.
ASME B31.3 Table 330.1.1 — Preheat Requirements by P-Number
This is the governing preheat table for process piping fabricated under ASME B31.3. Thickness is the nominal wall thickness of the thicker component at the joint. When two materials with different preheat requirements are joined, the higher preheat applies.
| P-Number | Material (typical) | Thickness | Min. Preheat (°C) | Min. Preheat (°F) | Max. Interpass | Status |
|---|---|---|---|---|---|---|
| P-1 | Carbon steel: SA-106B, SA-516-70, SA-105, A36 | ≤ 25 mm (1″) | 10 | 50 | — | Recommended |
| P-1 | Carbon steel: SA-106B, SA-516-70, SA-105 | > 25 mm (1″) | 80 | 175 | — | Required |
| P-3 | C-0.5Mo, 1Cr-0.5Mo: SA-335 P1/P2, SA-182 F2 | All | 120 | 250 | 300 °C | Required |
| P-4 | 1.25Cr-0.5Mo, 2.25Cr-1Mo: SA-335 P11/P22, SA-182 F11/F22 | All | 150 | 300 | 300 °C | Required |
| P-5A | 5Cr-0.5Mo: SA-335 P5, SA-182 F5 | All | 200 | 400 | 300 °C | Mandatory — critical |
| P-5B | 9Cr-1Mo: SA-335 P9, SA-182 F9 | All | 200 | 400 | 300 °C | Mandatory — critical |
| P-15E | Grade 91 (9Cr-1Mo-V): SA-335 P91, SA-182 F91 | All | 200 | 400 | 300 °C | Mandatory — critical |
| P-6 | 13Cr Martensitic SS: SA-182 F6a, AISI 410 | All | 200 | 400 | — | Mandatory |
| P-7 | 17Cr Ferritic SS: SA-182 F429, AISI 430 | All | 50 | 125 | — | Required |
| P-8 | Austenitic SS: 304L, 316L, 321, 347 | All | None | — | 175 °C max | None required |
| P-10I | Duplex SS 2205, 2507 | All | None typically | — | 150 °C max | Interpass limit critical |
| P-31 to P-35 | Nickel alloys: Inconel 625, 718, Alloy 20 | All | None typically | — | 175 °C max | None required |
| P-41 to P-49 | Aluminium alloys: 5083, 6061, 5052 | All | None | — | — | None required |
Carbon Equivalent Formulas — IIW Method and Pcm Method
When you have the steel chemistry from the Mill Test Certificate (MTC), you can calculate a carbon equivalent value that quantifies hardenability more precisely than P-Number alone. There are two primary CE formulas in use, and using the wrong one for the steel type will give a misleading result. See our dedicated Carbon Equivalent Calculator and Guide for full details on both formulas and their code references.
IIW Carbon Equivalent — EN ISO 1011-2 Method A (C > 0.18%)
Pcm Formula — Ito-Bessyo Method (C < 0.18%, HSLA steels)
Hydrogen Scale and Its Effect on Preheat
The hydrogen content of the deposited weld metal is classified by the “H” designator on the consumable packaging — for example, E7018-H4 means a maximum of 4 mL of diffusible hydrogen per 100 g of deposited weld metal. The effect on required preheat is significant:
| Hydrogen Class | Max H2 (mL/100g) | Preheat Adjustment | Typical Consumables |
|---|---|---|---|
| H16 | 16 | Full code minimum preheat required | Cellulosic E6010/E6011, basic uncontrolled |
| H8 | 8 | Preheat may be reduced 25–30 °C vs H16 | E7018 (standard), ER70S-2 (TIG) |
| H4 | 4 | Preheat may be reduced 40–50 °C vs H16 | E7018-H4, ER308L, ER316L (TIG/MIG) |
| H2 | 2 | Preheat may be reduced 50–75 °C vs H16 | E7016-1H2, premium filler metals, orbital TIG |
Preheat Requirements by Material — Detailed Engineering Notes
P-1 Carbon Steel — SA-106 Gr. B, SA-516 Gr. 70
Carbon steel is the most widely used pressure piping material and has the most nuanced preheat rules. The code table sets a threshold at 25 mm (1 inch) wall thickness, but this does not tell the whole story. A high-carbon P-1 material (C = 0.28%, CE = 0.52) with 20 mm wall thickness technically requires no preheat per the code table — but the CE method would indicate preheat of 100–120 °C. Always use the CE method as a check alongside the P-Number table when the carbon content is above 0.22%.
For P-1 materials, SMAW with E7018-H4 is the most common choice for high-restraint joints or when ambient temperature is below 10 °C. The combination of low-hydrogen consumables and even moderate preheat (50–80 °C) provides reliable HICC prevention even on carbon-heavier heats. For routine work with normal chemistry (C < 0.22%, CE < 0.42), no preheat is needed with low-hydrogen electrodes in a heated workshop.
P-3 and P-4 — Chrome-Moly Steels
Chrome-moly steels from P-3 (1Cr-0.5Mo) through P-4 (2.25Cr-1Mo, SA-335 P22) require 120–150 °C preheat and exhibit a critical additional constraint: they must not be allowed to cool below the preheat temperature between passes or during any interruption to welding. If welding is interrupted, the joint must be either maintained at preheat temperature (using local electric resistance heating or gas torch) or immediately given a post-weld hydrogen bake-out at 250–300 °C before cooling to ambient. Failure to maintain preheat on P-4 materials produces classic underbead cracking in the coarse-grained HAZ, which is often not detected until the joint fails in service.
P-15E — Grade 91 (9Cr-1Mo-V)
Grade 91 is the most demanding common pressure piping material from a preheat and PWHT perspective. It requires 200 °C minimum preheat (all thicknesses), a maximum interpass temperature of 300 °C, and must proceed directly to PWHT at 730–775 °C without intermediate cooling. If the joint cools to room temperature before PWHT, the untempered martensite that has formed is extremely brittle — the weld will frequently crack during or immediately after cool-down to ambient. For a comprehensive P91 welding guide including preheat, interpass, PWHT, and metallurgical requirements, see our P91 welding guide.
P-8 Austenitic Stainless Steel — Interpass Temperature Critical
Austenitic stainless steels (304L, 316L, 321, 347) require no preheat — but they impose a maximum interpass temperature limit of 175 °C (some stricter specifications use 150 °C). Exceeding the interpass temperature risks sensitisation — carbide precipitation in the HAZ — which depletes chromium from grain boundaries and destroys corrosion resistance. The mechanism is the same as the weld decay that occurs in unstabilised grades exposed to the sensitisation temperature range of 425–850 °C. Use interpass temperature as strictly on stainless steel as you would preheat on alloy steel.
P-10I Duplex Stainless Steel — Heat Input Window
Duplex stainless steels require neither a minimum preheat nor a mandatory interpass temperature minimum, but they impose a heat input window: typically 0.5 to 2.5 kJ/mm, with interpass temperature limited to 150 °C maximum (100 °C in some client specifications). Outside this window, the austenite/ferrite balance — the defining feature of duplex metallurgy — is destroyed. Too little heat input produces excessive ferrite; too much produces secondary austenite phases (sigma phase, chi phase) that embrittle the weld. For the full technical background, see our duplex stainless steel welding guide.
Fully Worked Example — P-4 Material, 2.25Cr-1Mo Pipe
Problem: Butt weld on SA-335 P22 (2.25Cr-1Mo) pipe, 38 mm wall thickness, under ASME B31.3. SMAW process, E9018-B3 electrode (H4 designation). Ambient temperature 8 °C.
Preheat Measurement — Methods and Code Requirements
ASME B31.3 Para. 330.1.1(a) specifies that preheat temperature shall be checked using temperature indicating crayons (Tempilstiks), thermocouple pyrometers (contact or embedded), or other suitable calibrated means. Each method has specific application considerations:
| Method | Description | Accuracy | Best For | Limitations |
|---|---|---|---|---|
| Tempilstik / Temperature Crayon | Wax-based crayon with a calibrated melt point; mark melts when surface reaches rated temperature | ±1 % of rated temperature | Field preheat verification, quick check | Go/no-go only — cannot read over-temperature; some crayons affected by surface contamination |
| Contact Thermocouple Pyrometer | Calibrated thermocouple probe pressed against workpiece surface | ±1–2 °C when calibrated | All applications; can verify interpass; records data | Must be calibrated; probe pressure affects reading; damaged probes give false readings |
| Infrared Pyrometer (Non-contact) | Measures surface thermal radiation; reading depends on emissivity setting | ±2–5 °C (when emissivity correctly set) | Hot pipe in service, inaccessible surfaces | Emissivity must be set for metal surface (0.1–0.3 for bare steel); highly reflective surfaces give inaccurate readings |
| Thermocouple Welded to Surface | Type K or J thermocouple tack-welded or capacitor-discharge welded to workpiece surface | ±1 °C | PWHT, long-duration preheat monitoring, automated recording | Requires capacitor discharge welding for attachment without WPS qualification on P-4 and above |
Post-Weld Hydrogen Bake-Out and PWHT Quick Reference
Post-weld hydrogen bake-out (PWDH) is performed immediately after welding at 150–250 °C for 1–4 hours, before the joint cools to ambient, to accelerate hydrogen diffusion out of the weld metal and HAZ. It is specified on the WPS and is mandatory for P-4 and above materials when PWHT cannot be performed immediately. It is distinct from PWHT, which is performed at much higher temperatures to relieve residual stress and temper the HAZ microstructure.
| P-Number | PWHT Temp. (°C) | Min. Hold Time | Max Brinell (Post PWHT) | Bake-Out Required? |
|---|---|---|---|---|
| P-1 (t > 19 mm) | 593–635 | 1 hr/25mm, min 1 hr | — | Recommended if delay >48 hr |
| P-3 | 593–663 | 1 hr/25mm, min 1 hr | 225 HB | Yes — 200–250 °C, 1–2 hr |
| P-4 | 675–720 | 1 hr/25mm, min 1 hr | 241 HB | Yes — 250–300 °C, 2–4 hr |
| P-5A | 704–760 | 1 hr/25mm, min 2 hr | 241 HB | Yes — 300 °C, hold at preheat |
| P-5B | 732–788 | 1 hr/25mm, min 2 hr | 241 HB | Yes — proceed direct to PWHT |
| P-15E (Grade 91) | 730–775 | 1 hr/25mm, min 1 hr | 225 HB | Must not cool — direct to PWHT |
| P-6 (Mart. SS) | 650–760 | 1 hr/25mm, min 1 hr | — | Yes — 200 °C min, PWHT promptly |
Recommended References
AWS D1.1 Structural Welding Code — Steel
Contains Annex B hydrogen control method for preheat determination, CE tables, and mandatory preheat requirements for structural steel. Essential reference for weld engineers and CWIs.
View on AmazonWelding Metallurgy — Sindo Kou (2nd Edition)
The definitive graduate-level reference on weld HAZ microstructure, martensite formation, hydrogen solubility, and cold cracking mechanisms. Covers CE formulas and their metallurgical basis in depth.
View on AmazonASME B31.3 Process Piping Code
The primary construction code governing process piping fabrication. Table 330.1.1 preheat requirements and Table 331.1.1 PWHT requirements are reproduced in every WPS for process piping.
View on AmazonPreheat for Welding — TWI Best Practice Guide
TWI’s practical guide covering all preheat determination methods, hydrogen scale, measurement techniques, and worked examples for common engineering steels — ideal for CWIs and welding supervisors.
View on AmazonDisclosure: WeldFabWorld participates in the Amazon Associates programme (StoreID: neha0fe8-21). 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.
Frequently Asked Questions
What is the purpose of preheat in welding?
Preheat slows the cooling rate of the weld and heat-affected zone, giving diffusible hydrogen time to escape before the joint cools to ambient temperature where it becomes trapped and can initiate cracking. It also reduces the proportion of hard, brittle martensite that forms in the HAZ during rapid cooling, replacing it with tougher bainitic and ferritic microstructures. Additionally, preheat reduces the thermal gradient across the joint, lowering residual stress and minimising distortion. Preheat is most critical for materials with a carbon equivalent above 0.40 (IIW formula) and for sections thicker than 25 mm — and is always required when welding ASME P-3 through P-15E materials.
What P-number is carbon steel in ASME?
Carbon steel is P-Number 1 in the ASME classification system (ASME Section IX Table QW/QB-422). P-1 covers carbon and low-carbon manganese steels such as SA-106 Gr. B (seamless pipe), SA-516 Gr. 70 (pressure vessel plate), SA-105 (pipe flanges and fittings), and structural grade A36. ASME B31.3 Table 330.1.1 requires no minimum preheat for P-1 materials up to 25 mm wall thickness (though 10 °C is recommended), and 80 °C (175 °F) for walls above 25 mm. For a full list of P-Number assignments and their material specifications, see our P-Number guide.
What is the difference between the IIW CE formula and the Pcm formula?
The IIW formula (CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15) is best suited for steels with carbon above 0.18% and is referenced in AWS D1.1, ASME, and BS EN 1011-2. The Pcm formula (Pcm = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B) is more accurate for modern low-carbon HSLA and fine-grain steels below 0.18% C, as it explicitly accounts for silicon and boron. For preheat using IIW CE: T = 697 × CE_eff – 273 °C. Using Pcm: T = 1440 × Pcm – 392 °C. Using the wrong formula — IIW on a low-C HSLA steel — will overestimate the required preheat. Always check the carbon content of the MTC to select the appropriate formula. For full details and a standalone calculator, see our carbon equivalent guide.
What is hydrogen-induced cold cracking and how does preheat prevent it?
Hydrogen-induced cold cracking (HICC) occurs when three conditions are simultaneously present: a susceptible microstructure (hard martensite in the HAZ), sufficient diffusible hydrogen, and residual tensile stress. It typically initiates 24–72 hours after welding is completed — hence “delayed cracking.” Preheat prevents HICC by slowing the cooling rate to reduce martensite formation and by keeping the joint warm long enough for hydrogen to diffuse out. The critical escape temperature is approximately 150 °C — below this, hydrogen becomes trapped. Using low-hydrogen consumables (H4 or H2 class) reduces the hydrogen input, allowing lower preheat temperatures on borderline materials. The two strategies — preheat and low-hydrogen consumables — together provide overlapping protection: each removes a different leg of the cold cracking triangle.
What is the ASME B31.3 preheat requirement for P-4 (1.25Cr-0.5Mo) material?
ASME B31.3 Table 330.1.1 requires a minimum preheat of 150 °C (300 °F) for all P-4 materials (including SA-335 P11 / 1.25Cr-0.5Mo and SA-335 P22 / 2.25Cr-1Mo pipe) regardless of wall thickness. This preheat must be maintained throughout welding and at all times when the joint is above ambient temperature until PWHT is performed. PWHT for P-4 is required at 675–720 °C per B31.3 Table 331.1.1. The maximum interpass temperature for P-4 is typically limited to 300 °C (572 °F), though some client specifications set this at 250 °C. If welding is interrupted, the joint must be heated to 250–300 °C for a hydrogen bake-out before being allowed to cool. For in-depth P22 and P91 welding notes, see our P91 welding guide.
Does tack welding require preheat under ASME codes?
Yes. ASME B31.3 Para. 330.1.1 explicitly states that preheat requirements apply to all welding operations including tack welds, repair welds, and seal welds on threaded joints. Tack welds on alloy steel (P-3 and above) without preheat are a common source of HAZ cracking in fabrication yards and are the starting point for many in-service failures. If the tack weld will be incorporated into the final weld, it must meet all the same preheat requirements as the production weld and must be executed using a qualified WPS with the appropriate electrode type and storage protocol. Tack welds on P-15E Grade 91 are particularly critical — a cold tack on this material will almost always crack, creating a discontinuity that propagates into the production weld.
What is the preheat zone extent required by ASME B31.3?
ASME B31.3 Para. 330.1.4 requires that the preheat zone extend at least 25 mm (1 inch) beyond each edge of the weld preparation on all sides. Temperature must be verified on the workpiece surface at a distance of 4 times the workpiece wall thickness from the weld edge, with a maximum measurement distance of 50 mm. For wall thicknesses greater than 50 mm, the minimum measurement distance increases to 75 mm in each direction from the weld preparation. When materials having different preheat requirements are welded together (dissimilar P-Number joints), the higher preheat temperature of the two materials must be applied across the full preheat zone. Temperature crayons (Tempilstiks), contact thermocouple pyrometers, or non-contact infrared thermometers are acceptable measurement instruments under the code.
What is post-weld hydrogen bake-out and when is it required?
Post-weld hydrogen bake-out (PWDH) consists of heating the weld joint to 150–250 °C immediately after welding, before allowing the joint to cool to ambient, and holding at temperature for 1–4 hours. This accelerates diffusion of hydrogen out of the weld metal and HAZ at a temperature still above the 150 °C hydrogen trapping threshold. It is specified on the WPS for P-3, P-4, P-5, P-6, and P-15E materials when PWHT cannot be performed immediately. Bake-out is distinct from PWHT — it does not relieve residual stress, temper the HAZ, or satisfy PWHT requirements. For P-15E Grade 91, bake-out is not a substitute for PWHT; the weld must proceed directly to PWHT at 730–775 °C without cooling to ambient. If there is any possibility of delay before PWHT, the weld should be kept wrapped in insulation blankets to maintain temperature above 200 °C until furnace treatment begins.
What P-number is Grade 91 chrome-moly steel?
Grade 91 (9Cr-1Mo-V) — supplied as SA-335 P91 seamless pipe, SA-182 F91 forgings, and SA-217 C12A castings — is classified as P-Number 15E in ASME Section IX and ASME B31.3/B31.1. P-15E requires a minimum preheat of 200 °C (400 °F) for all thicknesses per B31.3 Table 330.1.1. Maximum interpass temperature is 300 °C (572 °F). PWHT must be performed at 730–775 °C (1340–1430 °F) for a minimum of 1 hour per 25 mm wall thickness (minimum 1 hour regardless of thickness) without intermediate cooling to ambient. P91 welding is extremely sensitive to preheat, interpass, and PWHT — deviations produce delta ferrite, type IV cracking susceptibility, or premature creep failure. Our full P91 welding and material requirement guide covers all these requirements in detail.
Applicable Standards and References
- ASME B31.3 — Process Piping: Para. 330 (Preheat), Table 330.1.1, Table 331.1.1 (PWHT)
- ASME B31.1 — Power Piping: Para. 131.4 (Preheat), Table 131.4.1-1
- ASME Section IX — QW-406 (Preheat as essential variable), Table QW/QB-422 (P-Number assignments)
- ASME Section VIII Div. 1 — UCS-56 (PWHT requirements), Non-mandatory Appendix R (preheat guidance)
- AWS D1.1 — Structural Welding Code: Annex B (Hydrogen Control Method for preheat), Clause 4
- EN ISO 1011-2 — Welding recommendations for metallic materials: Pt. 2 arc welding of ferritic steels (Methods A and B for preheat)
- BS EN 1011-2 Annex A — Combined Thickness Method and CE Method for preheat calculation
- JIS Z 3158 — Pcm formula (Ito-Bessyo) for HSLA steels
- NACE MR0175 / ISO 15156 — Materials for use in H2S-containing environments: HAZ hardness limits