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Preheat Temperature for Carbon Steel Welding — Complete Guide per AWS D1.1 & ASME

Published 06 January 2025 · Fast Group Engineering · 8 min read

Skipping preheat when welding carbon steel can cause hydrogen-induced cold cracking — a metallurgical defect that is not visible immediately after welding but causes joint failure or accelerated degradation over time. In pressure vessel fabrication, pipeline construction, and heavy industrial structures, preheat is a mandatory technical requirement defined by specific parameters in the approved WPS.

Field preheat temperature verification on carbon steel pipe using Tempilstik temperature indicating crayon
Verifying preheat temperature directly on a carbon steel pipe weld using Tempilstik® — quick field check, result can be photographed into the QC record.

What is Preheat and Why is it Mandatory?

Preheat is the process of heating the base metal surrounding the weld zone to a specified minimum temperature before striking the arc. It is a direct intervention in the metallurgical behavior of the heat-affected zone (HAZ) — not an optional precaution.

Technical PurposeMechanism
Prevents hydrogen-induced cold cracking (HIC)Removes surface moisture and slows cooling — eliminating the primary hydrogen source that diffuses into the weld pool and causes HAZ cracking
Reduces HAZ cooling rateHigher base temperature slows post-weld cooling, preventing brittle martensite formation in the HAZ and maintaining joint toughness
Promotes hydrogen diffusionSlower cooling gives residual hydrogen sufficient time to diffuse out of the weld metal before solidification is complete
Reduces residual stress and distortionReducing the thermal gradient between the weld pool and the surrounding base metal limits shrinkage and warping
Key distinction: Preheat is performed before and during welding. PWHT (Post Weld Heat Treatment) is a completely separate thermal cycle applied after the weld is complete — typically at 600–750°C to relieve residual stress. Preheat does not substitute for PWHT, and PWHT does not eliminate the need for correct preheat.

AWS D1.1 Preheat Table — Quick Lookup

AWS D1.1 (Structural Welding Code — Steel) is widely applied in Vietnam for bridges, industrial buildings, and steel structures. Minimum preheat temperature depends on material thickness and Carbon Equivalent (CE).

Steel Category (AWS D1.1) Carbon Equivalent (CE) t ≤ 20 mm t 20–38 mm t 38–65 mm t > 65 mm Tempilstik® Part No.
Category I
SS400, A36, SM400		 CE ≤ 0.40 Not required* Not required* 66°C (150°F) 110°C (230°F) #28006 / #28312
Category II
SM490, A572 Gr.50		 CE 0.41–0.45 Not required* 66°C (150°F) 110°C (230°F) 150°C (302°F) #28006 / #28312 / #28318
Category III
SM520, A572 Gr.65		 CE 0.46–0.60 66°C (150°F) 110°C (230°F) 150°C (302°F) 200°C (392°F) #28006 / #28312 / #28318 / #28327
Category IV
HY-80 and equivalent		 CE > 0.60 110°C (230°F) 150°C (302°F) 200°C (392°F) 230°C (446°F) #28312 / #28318 / #28327 / #28332

* Not required per standard — preheat to minimum 10°C recommended when ambient temperature is below 5°C.
CE formula (IIW): CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. Obtain values from the steel Mill Test Certificate.

Practical example: Welding SM490 plate (Category II) at 30 mm thickness → AWS D1.1 requires 66°C → use Tempilstik® #28006. When the crayon melts on contact with the surface, welding may begin.

ASME B31.3 Preheat Table — Process Piping

ASME B31.3 applies to process piping systems in power plants, refineries, and chemical facilities. Unlike AWS, ASME classifies materials by P-Number — found in the approved WPS or the pipe Mill Certificate.

P-Number (ASME B31.3) Typical Steel Wall < 25 mm Wall ≥ 25 mm Tempilstik® Part No.
P-No.1 (Gr.1,2,3) Carbon steel: A106 Gr.B, A53, A333 Gr.6 10°C (50°F) 79°C (175°F) #28009
P-No.1 (Gr.4,5) High-carbon steel: A106 Gr.C 79°C (175°F) 79°C (175°F) #28009
P-No.3 Alloy steel: A335 P1, A335 P2 79°C (175°F) 79°C (175°F) #28009
P-No.4 1.25Cr–0.5Mo: A335 P11, P12 150°C (302°F) 150°C (302°F) #28318
P-No.5A 2.25Cr–1Mo: A335 P22 200°C (392°F) 200°C (392°F) #28327
P-No.15E (P91) 9Cr–1Mo-V: A335 P91 200°C (392°F) 200°C (392°F) #28327 — PWHT mandatory

For P91 full requirements see: P91 Steel Welding — Preheat 200°C & Mandatory PWHT.
For complete P-Number guide see: ASME B31.3 Preheat — Process Piping.

Field Verification with Tempilstik®

Infrared thermometers (IR guns) frequently give erroneous readings on heated steel surfaces because the emissivity coefficient changes continuously with temperature — compounded by the effects of mill scale, rust, and weld spatter. The physical crayon method provides significantly higher reliability in field conditions.

Tempilstik® operates on a simple principle: mark the crayon on the steel surface — if it melts immediately to a clear liquid, the surface has reached or exceeded that rated temperature. If the chalk-like mark stays solid, that zone has not yet reached the required temperature. No calibration required. No emissivity errors. Instant result.

Preheat MinimumTempilstik® Part No.Typical Application
66°C / 150°F#28006AWS Cat. I structural steel — best-selling SKU in Vietnam
79°C / 175°F#28009ASME P-No.1 pipe wall ≥ 25 mm (A106 Gr.B)
110°C / 230°F#28312AWS Cat. I heavy section, Cat. II–III thick plate
150°C / 302°F#28318AWS Cat. III–IV, ASME P-No.4 (P11)
200°C / 392°F#28327ASME P-No.5A (P22), P91 — mandatory minimum

Tempilstik® (Made in USA) meets ±1% Fahrenheit (±3% Celsius) accuracy and is recognized by AWS D1.1, ASME Section I, III, VIII, B31.1, and B31.3 as a valid direct-contact temperature measurement method. Full 116-SKU range: Tempilstik® product page.

Common Field Mistakes

The standard requires checking the full HAZ — extending at least 75 mm on each side of the weld centerline. A single measurement near the heat source is not representative of the actual weld zone temperature.

The correct measurement position is 50–75 mm from the weld edge. This confirms that heat has penetrated through the full section thickness — not just the surface immediately exposed to the torch.

An ambient temperature of 35–40°C does not substitute for preheat. Preheat requirements are metallurgical limits — independent of how the weather feels. The base metal must reach the specified temperature at the weld zone regardless of ambient conditions.

After the first pass, interpass temperature must also be monitored — it must not exceed the WPS maximum. Skipping this step can destroy the microstructure of already-deposited weld metal.

For thick plate, the surface reaching the preheat threshold does not mean the core has heated through. Starting welding too early allows the cold core to draw heat back, negating the preheat effect. Allow adequate soak time — typically 2–5 minutes after the surface reaches the target for sections over 50 mm thick.

Frequently Asked Questions

When drawings or specifications do not explicitly specify preheat, the industry norm is 50–65°C as a minimum for plain carbon steel under standard conditions. This removes surface moisture and reduces initial thermal shock to the arc, and is typically included in a contractor's own WPS.

Yes. The temperature indicating crayon method is recognized by AWS and ASME as a valid direct-contact temperature measurement method. Tempilstik® achieves ±1% Fahrenheit accuracy, requires no periodic calibration, and satisfies AWS D1.1, ASME Section I, III, VIII, B31.1, and B31.3.

Preheat is performed before and during welding at 60–250°C to prevent hydrogen-induced cold cracking. PWHT (post weld heat treatment) is performed after welding at 600–750°C to relieve residual stress and improve HAZ properties. The two steps are completely independent — neither substitutes for the other.

Need Tempilstik® for carbon steel preheat verification?

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