4.2 Preheating, Interpass Temperature & Heat Treatment

4.2 Preheating, Interpass Temperature & Heat Treatment

Thermal control decides whether a pipeline girth weld cools into a tough, crack-free joint or a brittle, hydrogen-cracked one. API 1104:2021 takes a deliberate approach: Section 7.10 requires the preheat, interpass, postheat, and PWHT practices on the qualified WPS to be followed, while Table 1 turns thermal deviations into essential variables that force requalification. The standard prints almost no universal temperature numbers — the qualified procedure carries them.

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1. Why Preheat Works (Metallurgy)

Preheating the joint before welding:

1. Slows the cooling rate, preventing hard, brittle untempered martensite in the heat-affected zone (HAZ).
2. Promotes hydrogen effusion — dissolved atomic hydrogen diffuses out of the steel before it can collect at stress concentrations and cause delayed underbead cracking.
3. Reduces thermal gradients and residual stress between the weld zone and the cold pipe body.

The need for preheat rises with carbon equivalent (CE_IIW = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15), wall thickness, restraint, and low ambient temperature. As industry practice, CE_IIW ≤ 0.38% welds readily; 0.39-0.45% calls for preheat on heavy wall or cold work; above roughly 0.45% preheat and low-hydrogen practice are mandatory.

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2. The Table 1 Thermal Essential Variables (22nd Edition)

Exceeding maximum interpass temperature degrades the joint the opposite way: grain growth, loss of yield strength, and poor HAZ toughness. Interpass temperature is measured immediately before starting the next pass, near the arc start position (3.1.29).

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3. Postheat vs. PWHT — Definitions That Are Examined

• Postheat (3.1.37): heating a completed weld to temperatures intended to accelerate hydrogen diffusion. Section 7.10 notes postheat is particularly beneficial when using cellulosic (EXX10-type) electrodes. Postheat is not PWHT.
• PWHT (3.1.38): heating a completed weld to temperatures intended to produce stress relief, tempering, normalizing, or other metallurgical change. When PWHT is used, 5.3.2.19 requires the WPS to carry the full procedure: application method, heating rate, temperature range, time at temperature, and cooling rate. Specific soak temperatures and hold times come from the company specification or governing design code — not from API 1104.

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4. Field Verification Practice

CWIs verify thermal control with calibrated instruments — temperature-indicating crayons, contact pyrometers/thermocouples, or emissivity-calibrated infrared devices — taken on the pipe surface near (not within) the weld zone:

• Confirm minimum preheat before arc start and re-verify whenever conditions change (wind, rain, night temperature drop).
• Confirm interpass temperature immediately before each subsequent pass; if the joint has cooled below the minimum, reheat first.
• Record preheat/interpass values on the daily welding report for traceability (see Section 7.3 of this guide).
• Confirm any postheat or PWHT cycle against the WPS parameters, with thermocouple records where specified.

5. Worked Heat-Input Check

Heat input ties preheat and interpass control together in Table 1. For a non-waveform-controlled process:

$HI = \frac{60 \times V \times A}{1000 \times S}$

Example: a mechanized fill pass runs at 24 V, 180 A, travel speed 10 in./min → HI = 60 × 24 × 180 / (1000 × 10) = 25.9 kJ/in. If the qualification record shows 22 kJ/in., the ±20% band is 17.6-26.4 kJ/in. — 25.9 stays inside. At 28 kJ/in. the procedure would be outside its qualified envelope and the WPS would need requalification (5.4.2.8 c).

6. Deliberate Cooling and Postheat in the WPS

Two WPS items complete the thermal picture (5.3.2.17-5.3.2.18): if forced cooling (e.g., water) is used, the WPS designates the cooling type and the maximum weld temperature before deliberate cooling starts; if postheating for hydrogen diffusion is used, the WPS states the minimum temperature and the time-at-temperature range. Adding, deleting, or changing either one is an essential variable under Table 1.