§2.3A · Under Heat control and metallurgy
Preheat vs interpass temperature for the CWI
Preheat and interpass temperature are both numbers the WPS makes you hit, and the difference is timing. Preheat gets checked before the first arc strikes. Interpass gets checked before every pass after that, and it can carry a top limit as well as a bottom one. Either way, the point is to slow down how fast the joint cools.
- Exam
- CWI
- Clause
- §2.3A
- Tags here
- 8
- Parent
- Heat control and metallurgy
2.3A.1Four temperatures on one joint
- Preheat
- Base metal temperature around the joint immediately before welding starts. It's a minimum. The steel can be warmer, never colder.
- Minimum interpass
- The floor the weld area must stay at or above before each new pass. For prequalified D1.1 work, the minimum preheat and interpass are given together.
- Maximum interpass
- A ceiling some WPSs and job specs add, typically where strength or toughness would suffer from heat building up pass after pass.
- Postheat
- Holding the joint warm after the last pass. It's a separate step for hydrogen control, covered on hot vs cold cracking.
2.3A.2One joint, from cold steel to the last pass
Before the arc
Bring it up and let it soak
Torch, induction or resistance blankets heat the joint. Give the heat time to get through the thickness, then read the temperature at the distance from the joint the code or WPS sets.
First pass
Hold it along the length
Preheat has to cover the stretch being welded. A joint that's hot where the torch sat and cold a foot away isn't preheated.
Before every pass
Read the interpass
Too cold, heat it back up. Too hot against a maximum, wait. Write the reading down.
Breaks and shift change
Re-check before restarting
A half-filled joint left over lunch can drop below the minimum. It gets brought back up before anyone strikes an arc.
After the last pass
Cool as the WPS says
Slow cooling, insulation or postheat if the procedure calls for it.
2.3A.3What pushes the number up
Preheat buys time. Warmer steel cools slower after the arc passes, which means a softer heat-affected zone and more time for hydrogen to work its way out. The metallurgy behind that is in heat control and metallurgy. What raises the requirement:
- Thickness. A heavy plate pulls heat out of the weld fast, like a big heat sink.
- Chemistry. Steels that harden easily, the ones with a high carbon equivalent, need more help.
- Restraint. A stiff, locked-in joint has nowhere to give as it shrinks.
- Hydrogen. Damp consumables or a dirty joint add the third ingredient for cracking.
Heat input works on the same cooling rate from the other side. More heat from the arc slows cooling too, so the two get balanced on the WPS. The heat input page has the formula.
Checking it, and where
You'll see temperature-indicating crayons, contact thermometers and infrared guns. A crayon only tells you whether you're above or below one temperature, so bracketing a range takes two: one stick for the minimum, one for the maximum. An IR gun can read low on shiny or bright metal, so know its limits before you trust it.
Preheat is a before-welding check and interpass is a during-welding check. That timing is part of the inspector duties you'll be tested on.
2.3A.4Temperature checks to sign off
Definitions first, then shop situations where you decide what the reading means for the next pass.
0 of 8 tagged · 0 accepted
Tag 01
The term 'interpass temperature' refers to:
Remarks on every option
- A Ambient is the surrounding air. It affects how much preheat you need but isn't interpass temperature.
- B Correct: It's read on the joint right before the arc restarts, so it captures the heat built up by earlier passes, not just what preheat put in.
- C Before the first pass is preheat. Interpass applies between passes.
- D After welding is done is postweld territory, like postheat or PWHT.
Pick an option. The remarks on all 4 open here.
Tag 02
What is the primary purpose of preheating before welding?
Remarks on every option
- A Speed isn't the reason. Preheat is about how fast the weld cools.
- B Arc stability depends on the process and settings, not base metal temperature.
- C Correct: Preheat slows cooling, which limits hard HAZ structure and gives hydrogen time to escape, cutting the risk of cracking.
- D Preheat doesn't change how much filler the joint needs. Joint design sets that.
Pick an option. The remarks on all 4 open here.
Tag 03
What is the primary purpose of interpass temperature control during welding?
Remarks on every option
- A Arc stability depends on process settings. Interpass limits are about the metal's properties.
- B Electrode use depends on joint volume, not interpass temperature.
- C Interpass limits often make the welder wait, the opposite of skipping the wait.
- D Correct: A minimum keeps cooling slow enough to avoid hard, crack-prone structure. A maximum protects strength and toughness from too much heat.
Pick an option. The remarks on all 4 open here.
Tag 04
What is the effect of excessive interpass temperature on weld properties?
Remarks on every option
- A Too much heat does change the metal. That's why many WPSs set a maximum interpass.
- B Higher interpass doesn't improve properties. Past the limit it coarsens the grain.
- C Correct: Running too hot between passes slows cooling too much, coarsening grain and cutting toughness and strength.
- D The damage is to the metal's properties, not just its looks.
Pick an option. The remarks on all 4 open here.
Tag 05
What indicates proper control of interpass temperature during multiple pass welding?
Remarks on every option
- A Steel only changes color at very high temperatures. Color can't confirm interpass limits.
- B Correct: Interpass control is shown by measuring before each pass with crayons or a thermometer and recording the readings.
- C A fixed wait ignores air temperature and joint size. Only a measurement shows the real temperature.
- D Looking at the bead tells you nothing about its temperature.
Pick an option. The remarks on all 4 open here.
Tag 06
Which temperature measurement method is most commonly used for monitoring preheat?
Remarks on every option
- A Infrared guns are common, but emissivity and distance throw off readings. The standard preheat check is the crayon.
- B Correct: Temperature-indicating crayons are the classic preheat check. They're cheap and simple, and they melt at a stated temperature, so the result is clear.
- C Contact thermocouples are accurate but used more for PWHT and recorded heating than routine preheat checks.
- D Optical pyrometers are mainly for very high temperatures, not the usual preheat range.
Pick an option. The remarks on all 4 open here.
Tag 07
What effect does increasing carbon content have on the required preheat temperature?
Remarks on every option
- A Carbon affects more than cooling rate. It raises hardenability, and that's what drives the preheat requirement.
- B More carbon means more preheat, not less.
- C Correct: Higher carbon raises hardenability and crack risk, so a higher carbon or carbon equivalent calls for more preheat.
- D Carbon is the biggest single term in carbon equivalent, which is used to set preheat.
Pick an option. The remarks on all 4 open here.
Tag 08
Which aspect of welder performance requires immediate correction if observed?
Remarks on every option
- A Small angle changes as position shifts are normal technique, not a violation.
- B Speed variation inside the WPS range is allowed. It isn't a nonconformance.
- C Correct: Preheat below the WPS minimum is a direct procedure violation that raises cracking risk. Stop and correct it right away.
- D Arc length variation that stays within the WPS voltage range is normal.
Pick an option. The remarks on all 4 open here.
2.3A.5Heard at the fit-up table
Is the interpass minimum the same number as preheat?
Often, yes. D1.1 gives the minimum preheat and interpass together for prequalified procedures. Any maximum comes from the WPS or the job spec.
Can I start welding when only one end of the joint is up to temperature?
No. The minimum applies over the area being welded, measured where the code or WPS says. One warm spot doesn't count.
Does thin steel ever need preheat?
Thin, low-carbon, low-restraint joints may need nothing beyond the code's floor. That floor still bites on cold or wet steel. Check the clause in your edition rather than relying on a number from memory.
2.3A.6Sources
- AWS Exam References and Editions Guide, August 2025 — D1.1:2020 or D1.1:2025 for Part C, checked October 2026
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