2.3 Preheat and Interpass Temperature

Definitions and Purpose

Preheat is heat applied to the base metal before welding begins (and maintained during welding). Interpass temperature is the temperature of the weld zone between successive passes of a multi-pass weld — measured immediately before the next pass is started. Preheat maintenance temperature is the minimum that must be held throughout. These three controls govern the cooling rate and are the inspector's primary tools for preventing hydrogen-induced cracking and managing HAZ microstructure.

Preheat serves four purposes, all of which an inspector should be able to recite:

1. Slows the cooling rate through the 1,500–900°F range → less martensite → lower cracking risk.
2. Allows diffusible hydrogen to escape the weld and HAZ before it can accumulate.
3. Reduces residual stress by lowering the thermal gradient between the weld and the surrounding metal.
4. Drives off surface moisture (and condensation), removing a hydrogen source.

Carbon Equivalent (CE)

The carbon equivalent converts a steel's alloy chemistry into a single number that predicts its hardenability and cracking susceptibility — the higher the CE, the more preheat the steel needs. The most widely used formula is the IIW (International Institute of Welding) formula, where each element is its weight percent:

CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

Manganese is the most significant alloying contributor after carbon (divided by 6). Chromium, molybdenum, and vanadium are grouped (divided by 5); nickel and copper are the weakest contributors (divided by 15). For thin sections and lower strengths, the Pcm formula is sometimes used instead, but the IIW CE is the one most CWI candidates are asked to apply.

Worked example: a steel with 0.18% C, 1.20% Mn, 0.10% Cr, 0.05% Mo, 0.20% Ni, 0.15% Cu gives CE = 0.18 + 1.20/6 + (0.10+0.05+0)/5 + (0.20+0.15)/15 = 0.18 + 0.20 + 0.03 + 0.023 ≈ 0.43 — "good," but preheat is prudent on thick sections.

AWS D1.1 Preheat Requirements

AWS D1.1, Structural Welding Code — Steel, specifies prequalified minimum preheat and interpass temperatures in a table (Table 5.8 in the 2020/2025 editions; the same content was Table 3.2 in older editions). The required temperature is selected from four inputs:

• Steel category (steels grouped by ASTM specification and strength)
• Thickness of the thickest part at the point of welding
• Welding process and hydrogen level (e.g., SMAW with low-hydrogen electrodes vs. non-low-hydrogen)
• Diffusible hydrogen designator of the consumable, under the alternative (Annex H) hydrogen-control method

Higher-strength or higher-carbon steel categories, or use of non-low-hydrogen consumables, raise these values substantially. The exact numbers come from the code table on exam day (Part C is open-book), but the trend — preheat rises with thickness and with steel strength/hardenability — must be second nature.

Measuring Preheat and Interpass Temperature

Common measurement tools:

• Temperature-indicating crayons (Tempilstik): rated to melt at a set temperature; a smear that liquefies confirms the minimum is met.
• Contact pyrometer / surface thermocouple: gives a direct numerical reading.
• Infrared (non-contact) thermometer: convenient but unreliable on shiny/scaled surfaces unless emissivity is corrected.

Where to measure (AWS D1.1): where the heating torch is applied to one side, measure on the surface opposite the heated face to confirm the full thickness is hot. Where heating one side and measuring the opposite face is not possible, after the heat source is removed, measure at a distance at least equal to the thickness of the thickest part but not less than 3 in. (75 mm) from the joint, allowing time for equalization. A localized torch reading taken right at the joint on the heated side overstates the temperature and is a classic exam trap.

Interpass Temperature

Interpass temperature has both a floor and a ceiling:

• Minimum interpass = minimum preheat. The required preheat must be maintained between every pass; letting the joint cool below it reintroduces the cracking risk preheat was meant to prevent.
• Maximum interpass temperature is imposed (by the WPS or code) to protect mechanical properties. Running too hot causes:
  • Excessive austenite grain growth in the HAZ (lower toughness)
  • Loss of strength and impact toughness, especially in quenched-and-tempered (Q&T) and thermomechanically-controlled-processed (TMCP) steels
  • Typical maximum interpass values fall around 400–600°F (200–315°C), but Q&T steels may be capped lower to preserve their heat-treated properties.

The inspector's job is to verify, with a calibrated instrument and at the correct location, that the joint stayed between the WPS minimum and maximum throughout welding — and to document it.

For the Exam: Know the IIW CE formula cold and that higher CE means more preheat. Know that minimum interpass equals minimum preheat, that a maximum interpass exists to protect toughness, and the D1.1 measurement rule: opposite the heated face, or at least the thickness but not less than 3 in. (75 mm) from the joint.