Preheat Temperature Lookup Table by Steel Type — AWS D1.1, ASME B31.3, API 1104

This page consolidates minimum preheat temperatures from the three most widely applied international welding standards in Vietnam's industrial sector: AWS D1.1 (structural steel welding), ASME B31.3 (process piping), and API 1104 (oil and gas pipeline welding). Data is presented bilingual English with corresponding Tempilstik® part numbers for rapid field look-up at the work site.

Bilingual Technical Glossary

Table 1 — AWS D1.1: Structural Steel Welding

Applied to: bridges, industrial buildings, offshore structures, module topsides, jackets.
Apply to: structural steel welding — bridges, buildings, offshore structures, jackets.

Carbon Equivalent formula (CE) — AWS D1.1:

All element values (C, Mn, Cr, Mo, V, Ni, Cu) are taken from the Mill Test Certificate (MTC) / Material Test Report (MTR) supplied with the steel lot. CE values must come from the actual material documentation — not assumed from the grade alone.

AWS D1.1 Table 4.5 (2020 Edition) — Minimum Preheat Temperature

* Not required per standard — preheat to minimum 10°C (50°F) recommended when ambient is below 5°C or surface carries moisture.
* Not required per standard — but AWS recommends preheat to 10°C minimum when ambient temperature is below 5°C or surface has moisture.

Table 2 — ASME B31.3: Process Piping

Applied to: process piping in refineries, chemical plants, power generation, and industrial gas facilities. Classified by P-Number — not by Carbon Equivalent.

What is a P-Number? ASME assigns a P-Number to weld base materials based on chemical composition and mechanical properties. The P-Number for a specific pipe is found in the approved WPS or on the Mill Certificate supplied with the pipe.

ASME B31.3 Table 330.1.1 — Minimum Preheat by P-Number and Wall Thickness

* Duplex stainless: preheat generally not required — but maximum interpass temperature ≤ 150°C (302°F) is mandatory to maintain austenite/ferrite phase balance. Use #28318 to verify interpass ceiling is not exceeded.

Table 3 — API 1104: Oil & Gas Transmission Pipeline Welding

Applied to: transmission pipelines carrying oil, natural gas, and condensate. Classified by API 5L pipe grade.

API 1104 Section 5.3 — Minimum Preheat by Pipe Grade and Wall Thickness

* API 1104 does not require preheat but recommends 50°C minimum when ambient is below 0°C or the surface is wet or carries moisture.

Table 4 — Maximum Interpass Temperature Reference

Controlling interpass temperature is equally important as preheat. Exceeding the maximum interpass limit promotes excessive grain growth in the HAZ and reduces impact toughness — especially critical for alloy steels and stainless grades.

Quick °C / °F Conversion & Tempilstik® Part Number Reference

View the full 116-rating range at tempil.vn/en/tempilstik/ — searchable from 100°F (38°C) to 2000°F (1093°C).