What's the minimum preheat for ASTM A36 plate?

Per AWS D1.1:2025 Table 5.8, preheat depends on thickness and filler hydrogen designator. For A36 (Group I) up to 3/4 in thickness with low-hydrogen filler, the minimum is typically 32°F [0°C]. For thicker plate or higher hydrogen, minimums rise to 70°F or higher.

Is interpass temperature a maximum or minimum?

Interpass temperature has BOTH a minimum (don't drop below preheat) and a maximum (don't exceed the qualified limit). The minimum prevents cold cracking between passes; the maximum controls HAZ size and toughness.

How do I verify preheat in production?

Surface temperature is checked with a calibrated temperature-indicating crayon, a contact thermocouple, or a non-contact infrared thermometer. Verify at the joint, at least 3 in [75 mm] from the heat source, just before striking the arc.

If the maintenance schedule slips and the plate cools below preheat, what do I do?

Re-preheat to minimum and continue. Document the interruption in the production record. The WPS dictates the rules; the production log captures compliance.

Preheat and interpass temperature: how to document on a WPS

Preheat prevents cold cracking. Interpass temperature controls HAZ size. Both belong on every WPS — here's how to specify and verify them.

Preheat and interpass temperature are not interchangeable. Preheat sets the starting condition before the arc strikes. Interpass temperature governs the cooling state between passes. Both belong on every WPS and both are tracked separately in production.

Why preheat matters

Preheat slows the cooling rate of the weld and HAZ. Slower cooling:

Reduces the formation of hard, brittle martensite in the HAZ
Allows diffusible hydrogen to escape before it can cause cracking
Reduces residual stress

For low-hydrogen welding on low-carbon steels under 3/4 in, preheat can sometimes be 32°F [0°C] — basically just "not freezing." For higher-strength steels, thicker sections, or non-low-hydrogen filler, preheat rises substantially.

AWS D1.1:2025 Table 5.8 specifies preheat as a function of:

Base metal category (varies by Group and listed grade)
Thickness of the thickest part
Filler metal hydrogen designator (H4, H8, H16, or no designator)
Welding process

How to document preheat on a WPS

Three lines:

Minimum preheat: specific temperature, with units
Method of measurement: contact thermocouple, temperature crayon, IR thermometer
Where to measure: at the joint, distance from the heat source, on which side of the plate

Example WPS preheat block:

Minimum preheat: 70°F [21°C]. Method: temperature-indicating crayon, calibrated per Section X.X. Measurement location: at joint, 3 in [75 mm] from the joint centerline on the heat-applied side, prior to striking the arc.

If preheat is by torch, specify the torch (oxy-acetylene, propane, etc.) and the heating pattern.

Interpass temperature

Interpass temperature is the temperature of the plate between passes. It has two limits:

Minimum: must equal or exceed the preheat minimum
Maximum: specified by the WPS, derived from the supporting PQR

Why the maximum matters: high interpass slows cooling and grows grain. Toughness drops. For CVN-required work, the maximum is tight.

AWS D1.1:2025 Table 6.6 (baseline) and Table 6.8 (CVN supplementary) list interpass as an essential variable. An increase beyond the qualified maximum requires requalification. (Note: the 2025 edition consolidated the interpass rules — the standalone interpass-decrease rule from 2020 Table 6.5 Row 36 was dropped.)

How to document interpass on a WPS

Minimum interpass: = minimum preheat
Maximum interpass: specific temperature
Method and location of measurement: same approach as preheat
Time between passes: sometimes specified for high-volume production

Production verification

In a typical structural shop, preheat is verified visually with a temperature crayon. The welder marks the plate, watches the indicator melt at the marked temperature, then strikes the arc. Simple, fast, audit-defensible.

For higher-stakes work — pressure vessels, AISC seismic, fracture-critical — verification escalates:

Calibrated contact thermocouple
Heating-pattern documentation (preheat blanket, induction heating, oxy-fuel torch with specified flame setting)
Time-temperature charts retained as a production record

Common preheat/interpass mistakes

WPS lists a preheat number without specifying the method or location. Auditor flags the gap.
Production drops below preheat overnight and the next shift starts welding without re-preheating. Catastrophic for high-restraint joints.
Welder pushes interpass above the WPS maximum to keep up with schedule. Toughness drops in CVN-required work.
WPS preheat is below Table 5.8 minimum for the thickness and hydrogen designator. Direct audit fail.
Method of measurement isn't traceable to calibration. A temperature crayon without a calibration record is a finding.

A rule engine that knows Table 5.8 by category, thickness, and hydrogen designator catches mistake #4 at draft time. The others are production-discipline problems.