7.1 Preheat, Interpass & PWHT

7.1 Preheat, Interpass & PWHT

Quick Answer: Preheat raises the joint temperature before welding so cooling is slower, hydrogen can diffuse more readily, and thermal gradients (hence residual stress peaks) are moderated. Interpass temperature keeps multipass welds inside a defined warm band. PWHT is a controlled post-weld thermal cycle—typically for stress relief and/or tempering—verified by method, thermocouples, soak temperature/time, and cooling rules. Module WT2.7 expects inspectors to know why these controls exist and what to check on the floor.

Chapter 6 showed that hydrogen cold cracking needs diffusible hydrogen, a hard susceptible microstructure, tensile stress, and low temperature after cooling. Preheat, interpass, and PWHT are the systematic thermal tools that attack those conditions. IWI-S candidates must connect the metallurgy to the inspection checklist—not only memorise a table of “typical °C.”

Three Purposes of Preheat

Training and shop practice usually group preheat benefits into three linked purposes:

1. Slow cooling of the weld and HAZ

Raising the starting temperature of the parent metal reduces the thermal gradient between the weld pool and the bulk plate. The joint therefore spends less time racing through critical transformation ranges at very high cooling rates. On hardenable carbon and low-alloy steels, slower cooling lowers the chance of forming hard martensite in the coarse-grain HAZ—directly reducing one of the four cold-cracking conditions.

Cooling rate is also governed by thickness, heat input, joint geometry, and fixture heat sinks. Preheat does not replace a correct heat-input window on the WPS; it works with that window. Thin sheet of low carbon equivalent may need little or no preheat; thick restrained plate of higher CE often needs a documented minimum.

2. Hydrogen diffusion

Hydrogen mobility in steel is strongly temperature-dependent. Holding the joint warm (preheat and minimum interpass) gives diffusible hydrogen more opportunity to escape to free surfaces or harmless sites before the structure cools into the embrittled “cold” regime. Combined with low-hydrogen consumables, dry storage/baking, and joint cleanliness, thermal control is a primary hydrogen-management strategy.

Some critical procedures add a post-heat or hydrogen bake-out hold after welding—still elevated, often for a defined time, but usually not the full high-temperature PWHT cycle used for stress relief. Inspectors must not confuse labels: follow the WPS wording exactly.

3. Reduce residual stress severity from thermal gradients

Large temperature differences between the weld zone and cold bulk material drive differential expansion/contraction and high residual tension after cooling. Preheat reduces those gradients. It does not eliminate residual stress completely—full residual-stress relief, when required, is the job of PWHT or other engineered methods. Still, preheat is part of the residual-stress story for crack-sensitive fabrications.

Setting and Measuring Preheat

The required preheat temperature comes from the applicable WPS, fabrication standard, or client specification—often informed by carbon equivalent, thickness, hydrogen scale of the process, and joint restraint. The inspector’s job is compliance verification, not free redesign of temperatures on the shop floor.

Common heating methods:

• Oxy-fuel or propane torch heating (with care against local overheating and incomplete coverage)
• Electric resistance heating mats / pads
• Induction heating systems
• Furnace preheat of assemblies (shop or site, when practical)

Measurement practices the inspector should expect to see defined:

• Where temperature is measured (distance from the weld edge, both sides of thick joints, root side when accessible)
• When (before arc start, after interruptions, after thermal soak)
• How (contact thermocouple, calibrated digital thermometer, temperature-indicating crayons only if the procedure allows them)
• Minimum temperature must be achieved through the thickness zone of interest—not only a hot surface skin

Exam tip: If welding is interrupted and the joint cools below the minimum, preheat must be restored before restarting—unless the WPS explicitly allows another path.

Interpass Temperature Control

Interpass temperature is the temperature of the weld zone immediately before depositing the next pass (or next layer, depending on procedure language).

Minimum interpass

Often aligned with the preheat philosophy: keep multipass joints warm enough that cooling rate and hydrogen diffusion remain under control throughout the weld. Losing minimum interpass mid-joint can recreate cold-start conditions on a partially filled groove.

Maximum interpass

Many procedures also set a maximum interpass temperature. Reasons include:

• Protecting toughness in some C–Mn and fine-grain steels (overheating can degrade properties)
• Controlling microstructure in quenched-and-tempered or TMCP materials
• Limiting heat accumulation in stainless and other alloys (covered in later materials chapters)
• Maintaining dimensional control on precision fabrications

Inspectors verify that welders wait when the joint is still too hot, not only that they reheat when it is too cold. Both directions matter.

Practical multipass checks:

1. Confirm WPS states min and/or max interpass and measurement location.
2. Watch production: successive passes without cooling can overshoot max interpass on thick, high-heat-input welds.
3. After long breaks, reconfirm minimum temperature before restart.
4. Record temperatures on travellers when the quality system requires objective evidence.

Post-Weld Heat Treatment (PWHT)

PWHT is a deliberate thermal cycle applied after welding is complete (or after defined stages), under controlled heating rate, soak temperature, soak time, and cooling rate. It is not a casual torch warm-up after the last pass.

Main purposes at IWI-S level

Other specialised cycles (normalising, solution annealing, precipitation hardening) appear for specific materials; always treat them as code- and WPS-defined, not generic “PWHT.”

What PWHT is not

• Not a substitute for low-hydrogen practice when the WPS requires both
• Not automatic for every structural weld—many building-steel joints are as-welded
• Not identical to post-heat / hydrogen bake-out (usually lower temperature / different intent)
• Not a licence to ignore hardness limits, impact tests, or material certificates

Inspector Verification of Heat Treatment

IWI-S competence includes verifying that thermal controls are planned, executed, and recorded. Core checkpoints:

Heating method and setup

• Method matches WPS/client approval (torch vs mats vs furnace vs induction).
• Coverage is uniform for the band width required around the weld (especially for circumferential pipe PWHT).
• Insulation, supports, and thermocouple attachment do not create cold spots or local overheating.
• For furnace work: load arrangement allows free air/gas flow; parts do not rest in ways that block soak.

Thermocouples and temperature measurement

• Thermocouples (or approved sensors) of correct type, calibrated within the quality system interval.
• Attachment locations per procedure (often multiple points: top/bottom, inside/outside, heat source side and far side).
• Controlling thermocouple vs monitoring thermocouples clearly identified on charts.
• Recording devices (chart recorders, digital logs) capture the full cycle with time stamps.

Soak temperature, soak time, rates

• Heating rate limits respected (too fast can cause distortion or thermal stress cracking on thick sections).
• Soak temperature within the qualified band (both minimum for effectiveness and maximum to protect material).
• Soak time meets the requirement (often linked to thickness—e.g. minutes per mm—per the governing code/WPS).
• Cooling rate controlled when required (still air, controlled cool, furnace cool)—especially important for hardenable and creep-resistant steels.

Documentation and hold points

1. Review the WPS/WPQR PWHT range before production starts.
2. Witness or review thermocouple placement at a hold point when the ITP requires it.
3. Review the heat-treatment chart/log against soak min/max and time.
4. Confirm material identity of the load matches the job (wrong grade in the furnace is a critical NCR).
5. After PWHT, verify any required hardness surveys, NDT, or dimensional checks specified by the procedure.
6. Raise an NCR if soak was short, temperature undershot/overshot, thermocouples failed, or cooling violated the procedure—do not “accept by visual” alone.

Link to Cracking Prevention (WT2.5 ↔ WT2.7)

When delayed cracks appear, the investigation should include actual preheat/interpass records and consumable hydrogen practice—not only the NDT report. When hardness fails after welding of alloy steel, check whether PWHT occurred and whether the cycle matched the qualified range.

Exam Focus for IWI-S

Expect questions that ask you to:

• List the purposes of preheat (slow cooling, hydrogen diffusion, residual-stress moderation)
• Distinguish minimum vs maximum interpass
• Separate post-heat / hydrogen bake-out from full PWHT
• Name stress relief and tempering as primary PWHT purposes
• Describe inspector checks: method, thermocouples, soak time/temperature, cooling, records
• Connect thermal controls to hydrogen cold cracking prevention

Exam tip: Preheat is process control before and during welding; PWHT is a qualified thermal cycle after welding. Both must be measured and recorded—guesswork is not inspection.