Preheat Temperature Quick Reference — AWS D1.1 & ASME B31.3
Preheat temperature is one of the most important parameters in weld quality control — particularly for thick carbon steel, low-alloy steel, pressure piping, and offshore structural work. This article consolidates a practical quick-reference table covering common industrial materials including A36, A572, SS400, SM490, P11, P22, P91, P92, and Duplex 2205, with Tempilstik® part numbers for direct field use.
Why Preheat is Technically Mandatory
Skipping preheat when welding carbon steel can lead to hydrogen-induced cold cracking (HIC) — 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.
Preheat removes surface moisture and slows cooling — eliminating the moisture-derived hydrogen source that diffuses into the weld pool and causes HAZ cracking.
A higher base temperature slows the post-weld cooling rate, preventing brittle martensite formation in the HAZ and maintaining joint toughness.
Slower cooling gives residual hydrogen enough time to diffuse out of the weld metal before the joint solidifies completely.
Reducing the thermal differential between the weld pool and the surrounding base metal limits shrinkage and warping after welding.
Quick-Reference Table — Common Industrial Materials
Designed for welding engineers, supervisors, and maintenance teams as a first-pass reference. These figures represent an initial selection range — not a substitute for the approved WPS. When selecting Tempilstik®, choose the rating closest to the minimum control threshold specified.
| Material | Common Applications | Reference Preheat Range | Tempilstik® Part No. |
|---|---|---|---|
| A36 / SS400 | Carbon structural steel | 50–100°C (depending on thickness) | #28002 (50°C) / #28016 (100°C) |
| A572 / SM490 | High-strength structural steel | 75–125°C (depending on thickness) | #28009 (79°C) / #28016 / #28019 |
| P11 (1.25Cr-0.5Mo) | Steam piping, power plant alloy piping | 150–200°C | #28318 (150°C) / #28327 (200°C) |
| P22 (2.25Cr-1Mo) | High-temperature process piping | 200–250°C | #28327 (200°C) / #28019 |
| P91 (9Cr-1Mo-V) | High-pressure steam lines, power plants | 200–300°C (min 200°C mandatory) | #28327 (200°C preheat) + #28053 (interpass guard) |
| P92 (9Cr-1.8W-Mo-V) | High-efficiency power plant systems | 200–300°C | #28327 / #28039 / #28053 |
| Duplex 2205 | Corrosion-resistant pipe, vessels, equipment | Control interpass ≤ 150°C — typically no minimum preheat required | #28318 (interpass max check) |
For complete data by steel category, thickness, and CE — see the full preheat reference table (AWS D1.1, ASME B31.3, API 1104).
Notes on AWS D1.1 and ASME B31.3 Application
For structural steel, AWS D1.1 classifies preheat requirements by steel category (based on Carbon Equivalent and thickness), welding process, and ambient conditions. For process piping, ASME B31.3 combines the P-Number system with WPS requirements and owner specifications to define preheat and interpass limits.
The key practical point: preheat is not a single fixed number for a given material. The same carbon steel will have different requirements at 10 mm versus 50 mm wall thickness. Similarly, outdoor welding in wind or high humidity requires more rigorous control than shop welding in controlled conditions.
- Always verify the WPS/PQR before selecting the preheat temperature
- Confirm material grade, wall thickness, and welding process
- Clean the measurement area before applying the Tempilstik® crayon
- Heat the full zone around the weld uniformly — not just a single point
- Re-verify if the joint has been left to cool before welding resumes
Example — SS400: C=0.18, Mn=1.2 → CE = 0.18 + 1.2/6 = 0.38 → Category I → preheat by thickness.
Obtain C and Mn values from the steel Mill Test Certificate (MTC) for the specific heat being welded.
Offshore and Heavy Industrial Conditions
In offshore environments, refineries, power plants, and outdoor maintenance sites, ambient conditions can cause surface temperatures to drop rapidly after heating. Wind, humidity, rain, extended outdoor exposure of the material, or heavy-section components all affect heat retention.
For this reason, QC inspectors should always measure at the actual weld zone — adjacent to the weld edge — rather than at a point removed from the heat source. For pipe or heavy plate, ensure the heated zone is wide enough: reaching surface temperature while the core is still cold defeats the purpose of preheat.
Practical tools for these conditions:
- Tempilstik® crayon set covering the range of WPS temperatures on the project
- Tempilaq® Advanced liquid indicator — for marking large surface areas or complex geometry where crayon access is limited
How to Select the Correct Tempilstik® Rating
The simplest approach: select the rating that matches the WPS minimum preheat temperature. If the WPS specifies a minimum of 150°C, use Tempilstik® rated at 150°C (Part #28318). When the crayon melts on contact with the surface, that location has reached the required threshold.
When the WPS also specifies a maximum interpass temperature, add a second higher-rated crayon for interpass monitoring — if that crayon melts, the joint is too hot and welding must pause.
| WPS Minimum Preheat | Tempilstik® Part No. | Notes |
|---|---|---|
| 66°C (150°F) | #28006 | AWS Cat. I–II structural steel — best-selling SKU in Vietnam |
| 79°C (175°F) | #28009 | ASME P-No.1 pipe ≥ 25 mm wall |
| 110°C (230°F) | #28312 | AWS Cat. I heavy section, Cat. II–III thick plate |
| 150°C (302°F) | #28318 | AWS Cat. III–IV, ASME P-No.4 (P11) |
| 200°C (392°F) | #28327 | AWS Cat. IV, ASME P-No.5A (P22), P91 — mandatory minimum |
Always record the Tempilstik® part number used, measurement location, and time in the QC report when project traceability is required. → Full 116-SKU listing: Tempilstik® product page.
Need Tempilstik® for preheat verification on site?
Fast Group Engineering — authorized Tempil® distributor in Vietnam. C/O + C/Q + VAT invoice. Stock in HCMC and Vung Tau. Individual crayons or box of 10.
📞 +84 938 888 958 | ✉ sales@tempil.vn