Preheat Temperature Reference Table — AWS D1.1, ASME B31.3, API 1104
This page consolidates minimum preheat temperatures from the three most widely applied international welding standards in industrial projects in Vietnam and Southeast Asia: AWS D1.1 (structural steel welding), ASME B31.3 (process piping), and API 1104 (oil & gas transmission pipelines). Each table includes the corresponding Tempilstik® part number for direct field use, in both °C and °F.
Bilingual Glossary
Technical terms used throughout all tables below, with Vietnamese equivalents for cross-referencing with local WPS documents:
| English Term | Vietnamese | Definition |
|---|---|---|
| Preheat temperature | Nhiệt độ tiền hàn / nung sơ bộ | Minimum base metal temperature before welding begins |
| Interpass temperature | Nhiệt độ liên hàn | Base metal temperature between passes — expressed as a maximum limit |
| PWHT | Xử lý nhiệt sau hàn | Post Weld Heat Treatment — independent thermal cycle after weld completion |
| Carbon Equivalent (CE) | Carbon tương đương | Formula combining C, Mn, Cr, Mo, V — governs hardenability and preheat requirement |
| HAZ | Vùng ảnh hưởng nhiệt | Heat-affected zone — base metal adjacent to weld altered by welding heat |
| WPS | Quy trình hàn | Welding Procedure Specification — approved document defining welding parameters |
| P-Number | P-Number (ASME) | ASME material group number — basis for WPS qualification and PWHT requirements |
| Wall thickness / t | Chiều dày thành | Component thickness at the weld joint — governs preheat threshold selection |
Table 1 — AWS D1.1: Structural Steel Welding
Applicable to: bridge structures, industrial buildings, offshore structural steel, topside modules, jackets, and general structural fabrication.
Source: AWS D1.1 Structural Welding Code — Steel, Table 3.2 (Prequalified preheat and interpass temperature minimums). Classification by Steel Category based on Carbon Equivalent (CE) and thickness.
| Steel Category | Carbon Equivalent (CE) | Typical Grades | ≤ 20 mm ≤ ¾" |
20–38 mm ¾"–1½" |
38–65 mm 1½"–2½" |
> 65 mm > 2½" |
Tempilstik® Part No. |
|---|---|---|---|---|---|---|---|
| Category I | CE ≤ 0.40 | SS400, A36, SM400 S235, S275 (EN equiv.) |
Not required* | Not required* | 66°C / 150°F | 110°C / 230°F | #28006 #28312 |
| Category II | CE 0.41–0.45 | SM490, A572 Gr.50 S355 (EN equiv.) |
Not required* | 66°C / 150°F | 110°C / 230°F | 150°C / 302°F | #28006 #28312 #28318 |
| Category III | CE 0.46–0.60 | SM520, A572 Gr.65 S420, S460 (EN equiv.) |
66°C / 150°F | 110°C / 230°F | 150°C / 302°F | 200°C / 392°F | #28006 #28312 #28318 #28327 |
| Category IV | CE > 0.60 | HY-80, A514 High-strength alloy |
110°C / 230°F | 150°C / 302°F | 200°C / 392°F | 230°C / 446°F | #28312 #28318 #28327 #28332 |
* Not required per standard — but preheat to minimum 10°C is recommended when ambient temperature is below 5°C or the surface has moisture.
Table 2 — ASME B31.3: Process Piping (P-Number Classification)
Applicable to: refineries, chemical plants, gas processing facilities, LNG terminals, and industrial process piping systems.
Source: ASME B31.3 Table 330.1.1. P-Number classification per ASME Section IX. Preheat depends on wall thickness at the weld joint — use the greater thickness when two components of different thickness are joined.
| P-Number Material Group |
Typical Material | Common ASTM Grades | Thickness < 25 mm < 1" |
Thickness ≥ 25 mm ≥ 1" |
Tempilstik® Part No. |
|---|---|---|---|---|---|
| P-No.1 Gr.1,2,3 | Carbon steel | A53 Gr.B, A106 Gr.B A333 Gr.6 |
10°C / 50°F | 79°C / 175°F | #28009 |
| P-No.1 Gr.4,5 | High-carbon steel | A106 Gr.C A53 Gr.B (high-C) |
79°C / 175°F | 79°C / 175°F | #28009 |
| P-No.3 | Low alloy steel (0.5Mo) | A335 P1, A335 P2 A161 T1 |
79°C / 175°F | 79°C / 175°F | #28009 |
| P-No.4 | 1¼Cr–½Mo steel | A335 P11, A335 P12 A161 T11 |
150°C / 302°F | 150°C / 302°F | #28318 |
| P-No.5A | 2¼Cr–1Mo steel (P22) | A335 P22 A213 T22 |
200°C / 392°F | 200°C / 392°F | #28327 |
| P-No.5B Gr.1 | 5Cr–½Mo steel | A335 P5 A213 T5 |
200°C / 392°F | 200°C / 392°F | #28327 |
| P-No.15E | 9Cr–1Mo-V (P91) | A335 P91 A213 T91 |
200°C / 392°F | 200°C / 392°F | #28327 PWHT mandatory |
| P-No.8 | Austenitic stainless steel | A312 TP304, TP316 A358 TP304L |
Not required | Not required | — |
| P-No.10H | Duplex stainless steel | A790 UNS S31803 A928 UNS S32750 |
Not required* | Not required* | — |
* No preheat required — but control maximum interpass temperature strictly: ≤ 150°C for duplex (see Table 4).
For P91 full welding requirements see: P91 Steel Welding — Preheat 200°C & Mandatory PWHT.
For B31.3 case study (pipe to flange) see: ASME B31.3 Preheat — Process Piping Guide.
Table 3 — API 1104: Oil & Gas Transmission Pipelines
Applicable to: onshore and offshore transmission pipelines, gathering lines, and distribution systems. Preheat requirement depends on pipe grade and wall thickness.
Source: API 1104 Welding of Pipelines and Related Facilities. CE per IIW formula: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15.
| Pipe Grade (API 5L) | Yield Strength | Carbon Equivalent CE (IIW formula) |
Wall ≤ 25.4 mm ≤ 1" |
Wall > 25.4 mm > 1" |
Tempilstik® Part No. |
|---|---|---|---|---|---|
| API 5L Gr.B | 241 MPa (35 ksi) | CE ≤ 0.43 | Not required* | 50°C / 122°F | #28002 |
| API 5L X42 | 290 MPa (42 ksi) | CE ≤ 0.43 | Not required* | 50°C / 122°F | #28002 |
| API 5L X52 | 359 MPa (52 ksi) | CE 0.40–0.43 | 50°C / 122°F | 80°C / 176°F | #28002 #28009 |
| API 5L X60 | 414 MPa (60 ksi) | CE 0.42–0.46 | 80°C / 176°F | 100°C / 212°F | #28009 #28016 |
| API 5L X65 | 448 MPa (65 ksi) | CE 0.43–0.47 | 100°C / 212°F | 120°C / 248°F | #28016 #28019 |
| API 5L X70 | 483 MPa (70 ksi) | CE 0.43–0.47 | 100°C / 212°F | 120°C / 248°F | #28016 #28019 |
| API 5L X80 | 552 MPa (80 ksi) | CE 0.46–0.50 | 120°C / 248°F | 150°C / 302°F | #28019 #28318 |
* Not required per standard at this thickness — verify against project WPS and any supplementary specification. CE values are indicative; use actual MTC data for final selection.
Table 4 — Maximum Interpass Temperature
Interpass temperature is a maximum limit — must not be exceeded between weld passes. The values below are typical; always confirm against the approved WPS for the specific material and project.
| Material | Maximum Interpass Temp. | Standard | Notes | Tempilstik® Part No. for Limit Check |
|---|---|---|---|---|
| Carbon steel | ≤ 250–300°C ≤ 482–572°F |
AWS D1.1 | Verify in WPS — typically 250°C for structural steel | #28039 (450°F/232°C) #28053 (900°F/482°C) |
| P11 (1¼Cr–½Mo) | ≤ 300°C ≤ 572°F |
ASME B31.3 | Strict control — overheating reduces HAZ toughness | #28053 (900°F/482°C) |
| P22 (2¼Cr–1Mo) | ≤ 300°C ≤ 572°F |
ASME B31.3 | Combined with mandatory PWHT after welding | #28053 (900°F/482°C) |
| P91 (9Cr–1Mo-V) | ≤ 300°C ≤ 572°F |
ASME B31.3 EN 1011-2 |
Mandatory strict control — exceeding limit causes carbide dissolution, reduces creep strength | #28053 (900°F/482°C) |
| Duplex stainless | ≤ 150°C ≤ 302°F |
API, NORSOK | Maintain γ/α phase balance — overheating causes sigma phase precipitation | #28318 (302°F/150°C) |
| Austenitic stainless 304/316 | ≤ 175°C ≤ 347°F |
ASME | Reduces sensitization risk — chromium carbide precipitation at HAZ grain boundaries | #28025 (294°F/146°C) |
For the two-crayon technique (simultaneous minimum preheat + maximum interpass control), see: Interpass Temperature Control — AWS & ASME.
Table 5 — °C ↔ °F Quick Conversion with Tempilstik® Part Numbers
Common preheat and interpass temperature checkpoints with their nearest Tempilstik® part number. Formula: T°F = T°C × 9/5 + 32.
| °C | °F | Tempilstik® Part No. | °C | °F | Tempilstik® Part No. |
|---|---|---|---|---|---|
| 38°C | 100°F | #28000 | 177°C | 350°F | #28025 |
| 50°C | 122°F | #28002 | 193°C | 380°F | #28030 |
| 66°C | 150°F | #28006 | 200°C | 392°F | #28327 |
| 79°C | 175°F | #28009 | 232°C | 450°F | #28039 |
| 100°C | 212°F | #28016 | 260°C | 500°F | #28043 |
| 110°C | 230°F | #28312 | 300°C | 572°F | #28053 |
| 121°C | 250°F | #28019 | 316°C | 600°F | #28047 |
| 150°C | 302°F | #28318 | 371°C | 700°F | #28057 |
→ Full 116-SKU listing filterable by °C or °F: Tempilstik® product page.
Frequently Asked Questions
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