Interpass Temperature in Welding: A Practical Guide

A fabricator runs three passes on a 50 mm bracket, walks away for coffee, comes back, drops a cap on the joint and finishes the job in forty minutes. Two days later the UT report shows a toe crack running the length of the weld — and the inspection report says the interpass temperature was never brought back up to the preheat minimum before welding resumed. The arc was never the problem. The temperature of the steel between passes was.

Interpass Temperature in Welding: The Definition Most Shops Get Wrong

You will meet this term on a drawing, in a WPS, and eventually on an inspection report. The definition is short. The mistakes that grow out of it are not.

Interpass temperature is the temperature of the weld area measured immediately before the next weld pass is deposited. It may be specified as a minimum, a maximum, or both. That is the whole definition. What makes it expensive is that many shops treat it as a formality on a travel document, when in reality it is a live measurement taken between every pass on a multipass weld.

Worth saying out loud, because the term gets stretched in shop talk. Canadian Fabricating & Welding frames it the same way: a temperature reading taken on the joint immediately prior to restarting the arc for the next run. The international standard says it more formally. ISO/TR 17671-1 defines interpass temperature as “Ti: temperature in a multi-run weld and adjacent parent metal immediately prior to the application of the next run” (ISO/TR 17671-1). Notice it says multi-run weld. If the joint gets one pass only, there is no interpass temperature to worry about.

Preliminary temperature: what preheat actually covers

Preheat temperature is the minimum temperature of the base material in the weld zone before any arc is struck. The part people forget is the second half of that sentence. Per the AWS and ESAB explainer on preheat and interpass temperatures in welding, and the same ground covered in The Welder’s Consumables Corner series, preheat applies to welding, tack welding and carbon arc gouging. Every arc. So a tack made on cold steel in a windy bay still needs its preheat, and so does the gouging step before a repair. More on that process in our guide to air arc welding and carbon arc gouging.

Why the minimum interpass usually equals the minimum preheat

Here is the part that trips people up. In most procedures, the minimum interpass temperature and the minimum preheat temperature are the same number. TWI puts it plainly: keeping the base metal from cooling too quickly between passes is what stops martensite forming, and the easiest way to do that is to hold the same floor you started with.

So if your WPS calls for 150 °C minimum preheat on a 20 mm carbon steel plate, the joint must be at 150 °C or above every time you are about to lay down another pass. Same rule repeated, pass after pass, and it is the rule most often broken.

Where the limits live: WPS, code, PQR

Your limits are not on the drawing unless the drawing says so. They sit in the Welding Procedure Specification, written to satisfy the governing code (AWS D1.1 for structural steel, ASME Section IX for ASME work, D1.6 for stainless) and the qualification record behind it. The AWS Preheat Guide is a useful single-page reference for what those codes expect and how the numbers were arrived at.

Three consequences follow. The WPS governs, not a rule of thumb. A WPS written to a different code edition may carry different numbers, so check the edition named in the contract. And if the WPS and the code disagree, the shop cannot quietly pick the easier one.

What interpass temperature is not

• Not arc temperature. Your arc column is thousands of degrees. The interpass reading is on the steel, not in the arc.
• Not heat input. Heat input (kJ/mm) describes energy put in per unit length and it sets the cooling rate. Interpass temperature describes where the next pass starts from. They interact, but they are not the same variable.
• Not PWHT. Post-weld heat treatment is a deliberate furnace or induction operation after the final pass to relieve residual stress. Interpass control happens while the torch is still live.
• Not a target to reach. When only a minimum is specified, you are looking for a floor, not a number to chase. Running the joint warmer than required adds distortion and cost for nothing.

The 75 mm maintenance zone

AWS D1.1 requires that preheat and all subsequent minimum interpass temperatures be maintained for a distance at least equal to the thickness of the part, but not less than 3 in (75 mm), in all directions from the point of welding. That is the D1.1 rule every fabricator should know by heart: on a 12 mm plate you need a 75 mm heated band, on a 100 mm plate you need 100 mm. You are not heating a strip of metal. You are keeping a zone around the joint hot enough that the HAZ does not quench itself between passes.

And if the job stops

The minimum interpass temperature should never be less than the initial preheat temperature. If welding is interrupted long enough for the joint to drop below preheat, it must be reheated before welding resumes, exactly as it would have been heated before the first pass (MaJIC). That one rule, followed, removes most toe cracking from a shop’s inspection reports.

Why Interpass Temperature Changes the Microstructure You Get

Every temperature limit printed on a procedure is metallurgy written as a number. Preheat and interpass temperature govern two things: how fast the weld zone cools, and how much heat the base metal has absorbed by the time the next pass goes down. Those variables set the microstructure you end up with, and microstructure sets strength, toughness, hardness and corrosion resistance. Cool the joint too fast and you get hard, crack-prone martensite. Run it too hot and you coarsen grains, over-temper or sensitize the metal.

The Minimum Side: Slowing the Cooling Rate

When weld metal and the surrounding base metal cool quickly, carbon in the austenite has no time to diffuse out, so the austenite transforms into hard, brittle martensite. That is why preheat and minimum interpass temperatures exist: they slow the cooling rate and reduce martensite formation. The Welder’s introduction to preheat and interpass temperatures and AWS cover the same ground, but the mechanism is worth understanding.

Heat also keeps hydrogen on the move. Hydrogen is diffusible in austenite at welding temperatures, so while the metal is hot it can migrate to a free surface and escape. Freeze the joint quickly and it stays trapped. That is the logic behind the CEPA Foundation guidance on hydrogen-induced cracking, which lists four benefits of elevated preheat and interpass temperature: it removes surface moisture, accelerates hydrogen diffusion away from susceptible microstructures, slows cooling so hard microstructures do not form, and reduces residual weld stress on cooling. In plain terms, a warm joint is a joint that can breathe.

The Maximum Side: When the Joint Runs Too Hot

The ceiling exists for the opposite reason. Heat accumulating in the heat-affected zone (HAZ) promotes grain growth. Coarsened grains have less grain boundary area to block crack propagation, so strength and impact toughness drop. Different base metals punish an over-temperature in different ways:

• Carbon and low alloy steel: grain coarsening in the HAZ reduces strength and lowers Charpy V-notch toughness. Grain growth near the weld does not re-refine on subsequent passes.
• Quenched and tempered base metal: a high maximum interpass temperature can excessively temper the desirable quenched-and-tempered microstructure of the parent metal, greatly reducing strength, as noted by MaJIC through Canadian Fabricating & Welding. The maximum can be the binding constraint even when the minimum is easy to hold.
• Austenitic stainless steel: at elevated temperatures chromium carbides precipitate at the grain boundaries. This sensitisation strips chromium from the boundary region, reduces corrosion resistance and creates susceptibility to intergranular corrosion. Maximum interpass temperatures of about 150 °C (300 °F) are generally recommended for low-carbon grades.
• Duplex stainless steel: the HAZ can precipitate sigma phase, which reduces both toughness and corrosion resistance.
• Aluminium: interpass temperatures that are too high cause recrystallization and grain growth in cold-worked alloys, and over-aging in precipitation-hardened alloys. The HAZ softens and hot cracking susceptibility rises during solidification. Practical aluminium procedures are covered in our aluminium MIG welding guide and in aluminium boat welding.

What the Research Shows: the 315 °C Ceiling

A 2024 study in the Journal of Manufacturing and Materials Processing (Dornelas et al., 8(6), 248) tested this on 2.25Cr-1Mo steel welded by submerged arc welding. Raise the interpass temperature above the ASME B31.3 limit of 315 °C and the Charpy V-notch impact energy of the simulated coarse-grained HAZ fell significantly. The dominant mechanism was effective grain size coarsening, and the relationship between effective grain size and CVN impact energy was inversely linear. The grains got bigger and the steel got weaker in the notch.

One nuance is worth keeping: even in the degraded condition, measured values still stayed above the 42 J minimum required by NORSOK M630. Exceeding the interpass limit is a toughness reduction, not an automatic failure. The code limit is not a technical cliff but a margin you spend for reasons that only show up on a bad day, in a bad lot, in service years later.

The Root Pass Exception

Do not assume one number covers the whole multipass weld. TWI notes that the root pass may require a higher preheat than the minimum interpass temperature of the filling passes, because the root region carries a higher stress concentration. The root is where the joint is thinnest, where the geometry concentrates load, and where the first thermal cycle does the most damage. If a procedure lists a separate root preheat, that is not an inconsistency. It is the procedure acknowledging where the risk lives.

Get this right and the numbers on the WPS read for what they are: the temperature range that preserves the microstructure the design engineer calculated for. Get it wrong and the weld still passes a visual inspection. That is what makes the failure mode so expensive. The microstructure has already changed, quietly, and it will only speak up later.

Interpass Limits by Material, Process and Welding Code

The interpass limit isn’t a single number for a material. It’s dictated by the steel, the welding process, the diffusible hydrogen level of the consumable, and the code governing the job. While minimums help prevent hydrogen cracking, the maximums are just as critical to prevent grain growth, loss of toughness, or distortion. With multipass welds, all of these factors work together to set the range your procedure must stay within.

AWS D1.1: The Starting Table

For structural carbon steel, minimum preheat and interpass values are found in the prequalified table. In the AWS D1.1/D1.1M:2025 edition, this is Table 5.11. Previous editions used Table 5.8 (2020) or Table 3.2 (before the 2015 reorganization). Many older WPS packages still reference these historic numbers, so always verify the edition named in the contract.

The table changes based off of steel specification, thickness band, process, and hydrogen condition. These are shall requirements, not suggestions. However, the D1.1 Commentary is clear that they are not all-encompassing. Factors such as restraint, hydrogen level, and service conditions may call for higher preheat than the table dictates. The Fabricator’s consumables corner on preheat and interpass temperatures breaks down these practical considerations in more detail.

Going Below the Table: Annex B and AWS D14.8M

Not every job has to rely solely on the conservative table. AWS D1.1 Annex B is normative, meaning it carries the same weight as the body of the code, and provides two alternate routes for preheat and interpass limits:

• HAZ hardness control method: Limited to fillet welds, this method caps the heat-affected zone hardness instead of holding the steel above a fixed temperature.
• Hydrogen control method: Uses the Pcm carbon equivalent and Figure B.1’s three zones (base metal, heat-affected zone, and weld metal) to establish calculated limits.

Similarly, AWS D14.8M:2009 (an identical reproduction of ISO/TR 17844:2004) utilizes four carbon equivalent methods: IIW CE, CET (CEN), Japanese CEN, and Pcm. These cover SMAW, self-shielded FCAW, gas-shielded FCAW, GMAW, and SAW for consumables with diffusible hydrogen levels from 3 mL/100 g up to over 15 mL/100 g.

As noted by the AWS Welding Digest for June 2025, these calculated approaches can drastically lower required temperatures. However, qualification testing is still mandatory for any values below the table. These are calculated limits, not shortcuts.

Material and Service Ceilings

For many alloys, the limit that is most restrictive is the maximum interpass temperature. Exceeding it is often the culprit for lost toughness, sensitization, or a drop in strength.

Material or Service	Typical Maximum Interpass	What It Controls
Structural-grade carbon steel	Up to 300 °C (575 °F)	HAZ grain growth, impact toughness, and strength
Austenitic stainless steel (AWS D1.6)	350 °F (175 °C)	Sensitization and ferrite control
2.25Cr-1Mo (ASME B31.3)	315 °C (600 °F)	Coarse-grained HAZ toughness, as shown by a 2024 MDPI study
Hardox 400-600 and HiAce (SSAB)	225 °C (437 °F)	Hard HAZ and loss of wear-plate strength
Most Strenx grades (SSAB)	300 °C (572 °F)	Strength retention
Thin wear-plate sheet (SSAB)	100 °C (212 °F)	Distortion and burn-through

For stainless, hydrogen cracking is not the primary concern, which is why AWS D1.6 takes a much lower ceiling. The AWS preheat guide goes over these family-based differences further. For Cr-Mo, the 315 °C (600 °F) ceiling is backed by measurable drops in Charpy V-notch energy when exceeded. Selecting the correct filler for these services is covered in our guide to alloy steel welding electrodes.

Pipeline and Wear Plate Specific Limits

For mainline pipeline work, the CEPA Foundation guidance document sets its recommendations based off of the weakest link in the consumable chain. For wall thicknesses below 12.7 mm (0.500 in) with all low-hydrogen passes, a minimum preheat and interpass of 50 °C (122 °F) is recommended. For 12.7 mm and above where a cellulosic root or hot pass is used before low-hydrogen fill and cap passes, that recommendation jumps to 121 °C (250 °F).

Wear plate is even more restrictive. SSAB’s published limits are 225 °C (437 °F) for Hardox 400-600 and HiAce, 300 °C (572 °F) for most Strenx grades, and 100 °C (212 °F) for thin sheet. These apply to tack welds and root runs just as much as fill and cap passes, with tacks generally kept to about 50 mm (2 in). If heat input falls between 1.0 to 1.69 kJ/mm, or if ambient humidity is high or below 5 °C (41 °F), an extra 25 °C (77 °F) should be added. Gouging operations also demand the same discipline, which is outlined in the guide to air arc welding and carbon arc gouging.

How to Measure Interpass Temperature Correctly

Interpass limits are only as useful as the reading that triggers them. Get the instrument, the location or the timing wrong and you end up in one of two bad places: rejecting perfectly good steel, or passing a joint that was never actually at temperature. Here is the procedure that turns the numbers in your WPS into a repeatable shop routine.

Choosing the right instrument

AWS guidance on preheat and interpass measurement is worth reading in full, because the trade-offs between instruments are not intuitive. The AWS Welding Digest lays out the three common shop options plainly, and the summary below follows that logic.

Instrument	Strength	Watch out for
Temperature-indicating crayon (Tempilstik type)	Instant go/no-go, no power required, works where you cannot see a display	You need one crayon per limit temperature
Contact thermometer / pyrometer	More accurate than infrared, reads through a probe	Must be calibrated; recalibrate after dropping or rough handling
Infrared thermometer	Inexpensive, fast, no contact, good for scanning a seam	Must be calibrated; shiny, rusty or painted surfaces each give a different reading
Thermocouple / thermal camera	Continuous logging of the full thermal history	Cost, setup, and a data review someone has to actually do

Crayon marks stay solid below their rated temperature and melt the moment the temperature is exceeded, which makes them ideal for a welder with gloves on and slag flying. They are calibrated to their own melting point, so they need no calibration and no battery. Contact devices are more accurate than infrared but demand a calibration regime, and an instrument that gets knocked off a scaffold should go straight back for recalibration. Infrared guns are the cheapest option and the easiest to misuse: emissivity changes with surface condition, so mill scale, fresh grinding and paint will all skew the reading unless you set the gun for the surface you actually have.

The two-crayon method

If your procedure specifies both a minimum and a maximum interpass, carry a crayon at each limit. Mark the base metal near the joint with the lower-temperature crayon first. If it does not melt, the joint is below the floor: go back and reheat. Then mark with the higher-temperature crayon. If that one melts, the joint is above the ceiling: wait, let it cool, and re-measure. One crayon cannot police two limits, and a single mark that stays solid is ambiguous, because it might mean the joint is too cold or safely within range.

Where to measure

Minimum interpass is checked at the same location used for preheat. AWS D1.1 requires preheat to be measured 3 in. (75 mm) in all directions from the edge of the weld joint, and through the thickness of the material where it is less than 3 in.

Maximum interpass is measured differently, and this is where most shops go wrong. Following ESAB and AWS guidance, the maximum should be checked roughly 1 in. (25 mm) from the edge of the weld joint, because taking the reading at the preheat location can under-report the actual peak. Wider industry practice, described in trade guidance such as The Fabricator’s consumables column, is to take interpass readings 1 to 3 in. (25 to 75 mm) from the weld centreline at multiple locations along the seam, because a single reading tells you very little about a long joint.

For continuous welding on rotating pipe or vessels, the recommended location is the weld start. Check it each time the workpiece rotates and the new pass overlaps the previous bead, since that overlap is where the heat builds fastest.

Timing and technique

A reliable field sequence runs like this:

1. Remove slag with a chipping hammer and wire brush. Slag is an insulator and will inflate the surface reading.
2. Wait roughly 30 to 60 seconds for heat to redistribute into the surrounding base metal.
3. Mark the crayon on the base metal about 25 mm from the weld toe.
4. Read or check the mark immediately before striking the arc, never on the freshly deposited bead.

If the job involves back-gouging, remember that carbon arc gouging and air arc work add their own heat and their own preheat requirement, so a gouged joint is rechecked before the next pass goes in.

Recording the readings

Log the pass number, the time, the reading, whether it passed, and what you did about it. Where the contract or quality plan calls for it, those interpass readings belong in the weld inspection records. Internationally, ISO/TR 17671-1 points to ISO 13916 as the reference standard for measuring preheat and interpass temperature. The Canadian Metalworking review of the same guidance is a good practical read if you want the sequence in one place.

Common Interpass Mistakes and How to Fix Them

Most interpass mistakes come down to habits, not complex metallurgy. The good news is that nearly all of them can be caught on the bench with a thermometer and a bit of discipline.

Assuming Preheat Covers the Entire Job

Preheat is a one-time check, not a blanket pass for the whole weld. The minimum interpass temperature must be verified before every pass. If a job is interrupted for lunch, a crane delay, or fit-up changes, the joint can cool below the required floor temperature without anyone noticing.

If welding resumes after a drop below the initial preheat temperature, the joint must be reheated to the minimum before striking an arc, as the MaJIC preheat and interpass guidance states. This is a direct safeguard against hydrogen-assisted cold cracking. By the time that risk shows up, it’s usually in the form of a toe or root crack found under NDT.

Measuring in the Wrong Place or With the Wrong Tool

Even with the right limits, a bad reading can give a false sense of security. The AWS guidance on preheat and interpass points to three of the most frequent errors:

• Reading the bead. Fresh weld metal is far hotter than the base metal that the next pass will bond to. Always take your reading at the same location used for preheat verification on the base metal.
• Leaving slag behind. Slag acts as an insulator and will inflate your temperature reading. Knock it off, and allow the heat to redistribute for 30 to 60 seconds before measuring.
• Ignoring emissivity. Shiny, freshly ground, or mill-scale covered steel can reflect enough energy to throw an infrared thermometer off by tens of degrees. Without emissivity adjustments, contact thermometers or temperature indicating crayons are often the more reliable option for these surfaces.

Only Checking the Minimum

Not every WPS has just a floor. If your procedure sets both a minimum and a maximum interpass, one crayon simply isn’t enough. Use a crayon for the minimum to confirm it’s met, and a separate crayon for the maximum to ensure it’s not exceeded. If the lower crayon won’t melt, reheat. If the upper crayon melts, stop and let the joint cool before re-measuring. Most importantly, the crayons in your box must match the exact limits written in the WPS.

Exempting Tacks, Roots, and Gouging

Small work is still covered work. SSAB is explicit that tack welds and root runs must follow the same minimum preheat and maximum interpass values as the rest of the weld. A cold tack can become a hydrogen trap once it’s buried under a hotter fill pass. This also extends to cleanup: the preheat zone must remain within limits during carbon arc gouging, as the process still applies localized heat and can disrupt the required temperature range.

Letting Heat Input Run Away

Fat beads, wide passes, and a slow travel speed add more heat than a procedure was qualified for. Pass after pass, the joint temperature will climb until the maximum interpass limit is breached, often without the welder realizing it. The fix isn’t to rush cooling, it’s to control heat input and pass size. For steels, this can lead to a grain-coarsened HAZ with reduced impact toughness. In aluminium, the first warning sign is often excessive distortion, as outlined in our aluminium hull welding guide.

Using Unauthorized Forced Cooling

Fans, wet rags, or compressed air may seem like a quick fix to beat the maximum interpass, but they change the cooling rate of the weldment. Forced cooling methods should not be used to bring a joint down to the maximum interpass limit without first consulting a qualified welding engineer. On carbon and alloy steels, this can produce a hard HAZ with elevated hardness readings. On austenitic stainless steels, it can promote sensitization. The potential for compromised mechanical properties, reduced service life, repairs, or catastrophic failure far outweighs any time saved.

Checking Only One Spot

A single pass at the start of a long seam tells you nothing about conditions hours later at the far end. The AWS preheat guide recommends checking at multiple locations along the joint. More importantly, every reading should be logged against its pass number, not left on a scrap of paper. This simple log is often the first piece of evidence to prove compliance should a question arise.

For those looking to tighten up their consumable selection against hydrogen risks, our guide to alloy steel welding electrodes is a useful companion. Additionally, for arc energy management on non-ferrous work, the aluminium MIG welding guide can help keep pass size and heat input under control. Each of the mistakes above is preventable, and prevention is always the only option that doesn’t require a repair.

Preheat vs Interpass vs PWHT: Where the Confusion Starts

Three temperatures are almost always listed together on a work instruction: preheat, interpass, and PWHT. While all involve heat, they serve three entirely different purposes. Preheat is the base metal temperature in the weld zone before the first arc is struck. Interpass temperature is the temperature of the weld area measured immediately before the next pass on a multipass weld. PWHT, or post weld heat treatment, is a separate operation carried out after the final pass to treat the completed weld as a whole.

Variable	Preheat	Interpass	PWHT
When it applies	Before the first arc, including tacking and carbon arc gouging	Between passes, on multipass welds only	After the final pass, as a separate operation
What it controls	Cooling rate of the first pass and the risk of hydrogen cracking	Cumulative thermal history, HAZ grain structure and hardness	Residual stress relief and dimensional stability
Typical magnitude	0 to several hundred degrees Celsius	0 to several hundred degrees Celsius	Several hundred degrees Celsius, held and soaked for a specified time
Who sets it	Code minimum table plus the WPS	Code minimum and maximum limits plus the WPS	The WPS and engineering specification, not the welder
How it is measured and recorded	Crayon, calibrated contact, or infrared thermometer at the specified distance, logged on the weld record	Same instruments, read immediately before striking the arc, logged pass by pass	Not a shop measurement; furnace chart or thermocouple log attached to the heat treatment record

PWHT Cannot Fix a Hot Weld

PWHT is a controlled furnace operation. It cannot undo damage done during welding. If the interpass ceiling is exceeded on a multipass carbon steel weld, the heat-affected zone will have already coarsened. Running the part through a furnace will not reverse those grains or restore the toughness demonstrated by the procedure qualification. It only adds cost and distortion risk while creating a false sense of compliance.

For some alloys, this distinction is even more critical. The temperatures used to stress relieve steel will anneal and soften the heat-affected zone of aluminium, which is why most aluminium welds are never PWHT’d. For alloys in this family, proper heat control is handled differently, as shown in our aluminium MIG welding guide.

Interpass Temperature Is Not Heat Input

The two are related, but not interchangeable. Heat input, measured in kJ/mm, dictates the cooling rate of a single pass. Interpass temperature sets the starting condition for the next pass. A high heat input slows cooling on its own, and a hot interpass temperature slows the cooling of the following deposit even further. Both must be controlled together, which is why the AWS Welding Digest guidance on preheat and interpass treats them as a pair.

This cumulative heat can come from more than just welding passes. Operations like carbon arc gouging and air arc work add heat to the joint, and a gouge-and-grind repair mid-sequence can push a region past its interpass ceiling just as easily as an extra weld pass would.

Interpass Limits Are an Essential Qualification Variable

The biggest compliance factor is qualification. In AWS D1.1/D1.1M, preheat and interpass temperatures are listed as essential variables under Subclause 6.8.4. The same principle applies to ASME codes. As TWI states, a maximum interpass temperature is specified to control weld metal microstructural development and keep production welds similar to those made during procedure qualification. Changing these limits is not a shop adjustment, but a change outside of what was qualified.

According to CEPA guidance, minimum preheat and interpass temperatures must be maintained until welding is complete. This means keeping a heating source between passes when needed, not performing heat treatment. The discipline applies across industries, from pipelines to the much tighter windows seen in aerospace TIG welding, where interpass control is logged per pass with no room for assumption.

Controlling Interpass Temperature Without Killing Productivity

Here is the objection you will hear in almost every fabrication shop: “Interpass limits slow the job down.” And there is truth in it. Holding a joint at 150 °C while a fitter waits for a crayon to confirm it costs real money in labour, fuel and schedule. The mistake is treating the limit as a fixed number you must endure rather than a constraint you can engineer around. There are several documented, practical ways to control interpass temperature without paying a productivity penalty.

Start by proving you need a lower limit

The biggest lever is often in the calculation, not the shop. The prequalified table values in AWS D1.1 are conservative defaults. The alternative methods in AWS D1.1 Annex B and AWS D14.8M calculate limits from the actual composition, diffusible hydrogen content, thickness and heat input of the joint rather than from carbon equivalent alone.

According to the AWS Welding Digest (June 2025), because these methods consider actual composition, diffusible hydrogen, thickness and heat input, preheat and interpass temperatures “can be dramatically lower than the table values,” with the potential to reduce the cost of time, labour and energy. The caveat that keeps engineers honest is that qualification testing is still required. You are not skipping the test, you are running a smarter one.

Control the heat instead of chasing the thermometer

Most interpass exceedances are caused by too much heat going in, not by a badly calibrated instrument. The SSAB welding guide recommends welding with heat input as low as possible for thin gauges, minimising the cross-sectional area of the weld, using symmetrical welds, decreasing the spacing between tack welds, and using back-step or step welding techniques. Every one of those reduces local heat accumulation at the point where your interpass limit binds.

Design the sequence before you strike an arc

Sequence planning is the cheapest productivity tool in welding, and it directly governs how much heat accumulates in one spot. ISO/TR 17671-1 advises that parts should be assembled so joints are accessible and visible to welders and operators, and that the sequence of assembly and welding should allow all welds to be examined. It also notes that pre-setting joints or pre-bending parts, and specifying the weld sequence, may be necessary to control distortion and residual stress.

• Balance the passes. Alternate sides so no region of the joint absorbs the whole thermal cycle.
• Back-step and skip welding. Stepping back along the seam distributes heat instead of concentrating it at the stopping point.
• Work from rigid to loose ends. Fix the constrained end first and let the free end absorb the shrinkage.
• Use shorter, more closely spaced tacks. A grid of small tacks replaces one long, heat-soaked tack, and tacks count against interpass limits too.

On repair or field work where cycle time is already dominated by access and weather, the case for a slower process is still strong. Our guide to why SMAW still wins covers the practical angle.

Manage the environment, not just the metal

Ambient temperature, wind and moisture all push the joint toward the lower limit. SSAB recommends increasing the interpass requirement by about 25 °C for cold or humid ambient conditions, and specifically calls out temperatures below 5 °C. CEPA guidance notes that maintaining minimum preheat and interpass temperatures until welding is complete may require applying a heating source between passes on large-diameter line pipe. That is a planned, budgeted cost, and far cheaper than a rejected girth weld. Shielding the seam from wind and moisture is usually the cheapest intervention of all.

Instrument the process

Once you stop relying on feel, the interpass limit stops costing time. Thermocouples with a data logger give a continuous, auditable temperature trace. Thermal cameras cover long seams and large joints where a spot check every few minutes is inadequate. On mechanised cells, interpass monitoring can be wired into the weld cell so the machine refuses to strike an arc below the floor and pauses when the ceiling is approached. This is where the labour saving really lands: the welder welds, the logger documents, and the inspection file writes itself.

Know where the shortcut stops working

Raising interpass temperature to shorten cycle time is risk-free only up to a point. In the 2.25Cr-1Mo study published in Journal of Metallurgical and Materials Engineering (MDPI, 2024), raising interpass temperature above the ASME B31.3 limit of 315 °C measurably reduced coarse-grained HAZ Charpy V-notch impact energy, with effective grain size coarsening as the main mechanism. Even a deliberate excursion with margin costs toughness in a way that a UT report will never show you.

Worked Examples: Interpass Control in Six Real Shop Scenarios

Interpass temperature isn’t just a number on a procedure—it plays out differently by material, process, and code. The examples below reflect common calls welders and inspectors make in the shop, and the fix that would change the outcome.

1) Structural Steel Shop: 50 mm A36 Bracket, SAW, Multipass

For a 50 mm thick A36 bracket welded with submerged arc welding (SAW), the bracket’s mass acts as a heat sink and drives a fast cooling rate in the heat-affected zone (HAZ). That raises the risk of higher hardness and hydrogen cracking on a multipass joint.

The crew pulls the preheat and interpass values from AWS D1.1. For thicknesses over 2½ in. in the thickest part being welded, both minimum preheat and minimum interpass increase based on steel specification, process, consumable hydrogen condition, and thickness band. The table is a code minimum, not a recommendation, and must match the contract edition.

They maintain that minimum for at least the part thickness (never less than 75 mm) in all directions, checking the back side on single-sided joints before each pass. Using the two-crayon method when a range is listed, they clean slag, allow 30–60 seconds for heat redistribution, and take a reading before striking the arc.

The one thing the welder would have done differently: Treat the minimum interpass as a hard floor at every pass—not just at the start. On heavy sections like this, it’s easy to skip a measurement after a break.

2) Pipeline Mainline Spool: Cellulosic Root, Low-Hydrogen Fill

A cellulosic root followed by low-hydrogen fill and cap is common on pipeline spools, but hydrogen introduced at the root can become trapped if the joint cools too quickly between passes.

Following CEPA guidance, the crew maintains the recommended minimum interpass to keep hydrogen diffusible and observes the required post-weld delay before nondestructive examination. For higher-strength line pipe, that delay is longer to allow time for delayed hydrogen cracking to appear. On rotating pipe, readings are taken at the weld start—the spot where the next bead will overlap and best represents interpass temperature.

The one thing the welder would have done differently: Not rush to NDT. Maintaining interpass is only half of hydrogen control—respecting the post-weld delay is just as critical.

3) Quenched-and-Tempered Attachment: When the Ceiling Becomes the Constraint

On a repair attaching a bracket to quenched-and-tempered (Q&T) base metal, the minimum preheat is often easy to meet. The real constraint is the maximum interpass temperature. Excessive heat accumulation can over-temper the Q&T base metal, coarsen grains in the HAZ, and reduce strength and impact toughness.

With the ceiling binding the joint, the crew uses smaller, well-spaced passes, watches cumulative heat, and pauses to cool if temperatures near the limit before the next pass.

The one thing the welder would have done differently: Monitor the maximum interpass just as aggressively as the minimum—going over the ceiling is the error most likely to show up in mechanical testing.

4) Austenitic Stainless Piping: Fast Cooling, Tight Ceiling

For austenitic stainless, the main concerns are sensitization and ferrite control—not hydrogen cracking. Per AWS D1.6, many procedures set a ceiling around 150–175 °C. Exceeding it allows chromium carbides to precipitate at grain boundaries, increasing the risk of intergranular corrosion.

Since stainless cools quickly and passes can come in rapid succession, the crew measures interpass before each pass, watches for heat tint, controls heat input and root purge where required, and plans for pickling or cleaning if heavy heat tint forms.

The one thing the welder would have done differently: Not rely on touch alone. Damage from time above the ceiling occurs in range, not just while the metal feels hot—a calibrated reading is the only reliable check.

5) Hardox Bucket Liner Repair: Thin Zones, High Wear Material

Repairing a Hardox bucket liner demands tight control. SSAB specifies that the maximum interpass temperature is measured on top of the weld metal or immediately adjacent to the joint at the start position just before the next pass. For many grades, that ceiling is around 225 °C and can drop further with thickness or cold/humid conditions.

Exceeding it risks forming a hard, brittle HAZ that defeats wear resistance. The crew keeps preheat thickness-driven, limits heat input with small beads, avoids concentrating heat in one spot, and reads at the exact start position before each pass.

The one thing the welder would have done differently: Respect the 225 °C ceiling at the joint start position—not somewhere farther away. Location of measurement is part of the limit.

6) Aluminium Trailer or Boat Hull: Sequence Over a High Floor

For most common aluminium alloys, there’s typically no high minimum interpass to chase. The concern is cumulative heat: excess interpass drives recrystallization, grain growth, and over-aging of heat-treatable alloys—softening the HAZ and raising hot cracking risk.

Using resources like Aluminum MIG Welding: The Complete Practical Guide, Aluminum Boat Welding: The Complete Guide for Strong Hull Joints, and Aluminum Cleaner for Welding: How to Prep Aluminum Right, the crew focuses on cooling rate: small passes, balanced or skip/back-step sequences, adequate cooling between passes, and avoiding long, uninterrupted runs that heat soak the panel.

The one thing the welder would have done differently: Prioritize sequence and pass size over holding a temperature. With aluminium, interpass control is a cooling strategy—taking an extra minute between passes beats rushing the next bead while heat-soaked.

A Shop-Ready Interpass Temperature Checklist

Most shops don’t fail at interpass control from lack of knowledge, they fail from lack of routine. The limits live in the welding procedure, but the decision is made on the floor between passes with a schedule to keep. Print this checklist and run it at three points: before the first arc, between every pass, and at close-out.

1. Before the Arc: The Pre-Weld Briefing

• Verify the material. Confirm the grade against the drawing and mill certificate. The preheat table only applies to the material you actually have.
• Pull the correct code table. Identify the welding category, process, and thickness band used in AWS D1.1. Confirm you are using the edition named in the contract, as table numbers have been renumbered across editions.
• Get both limits. Read the minimum and maximum interpass temperatures from the WPS out loud. For most procedures the minimum equals preheat, but for quenched-and-tempered steel, austenitic stainless, or HSLA, a maximum may exist. Both need to go to the floor.
• Check the consumable. Confirm the electrode or wire classification and its hydrogen rating. A cellulosic root and a low-hydrogen fill are not interchangeable on a critical joint. See our guide to alloy steel welding electrodes for more on matching hydrogen levels.
• Mark the maintenance zone. AWS D1.1 requires preheat and minimum interpass to be maintained for at least the thickness of the part, never less than 3 in / 75 mm, in all directions from the weld. Refer to the AWS preheat guide for exact measurement locations.
• Plan for hot work. Tacks are welds, and carbon arc gouging is hot work. If the job calls for air arc gouging, include it in the plan. Follow the sequence guidance found in ISO/TR 17671-1.
• Assign the log. Name the person responsible for readings before the first arc, along with where they will be recorded.

2. Between Passes: The Repeatable Loop

1. Clean first. Remove all slag. Slag insulates the surface and will inflate your reading.
2. Let it redistribute. Give the joint 30 to 60 seconds before measuring to allow heat to move into the base metal.
3. Measure in the right place. Never measure off of the freshly deposited bead. Readings must be taken from the base metal, not the weld metal.
4. Use the two-crayon method. Have a crayon for the minimum and one for the maximum. If the lower crayon won’t melt, reheat. If the upper crayon melts, stop and cool.
5. Correct excursions. If below the minimum, reheat with a torch, induction heater, or oven until the lower crayon melts. If above the maximum, allow for natural cooling. Never quench with a wet rag or compressed air to beat the clock.
6. Log every pass. Record the pass number, time, reading, pass or fail, and any action taken.

Reading	Where to measure	Instrument note
Minimum interpass	Same location as preheat, roughly 3 in / 75 mm from the joint edge, or through the thickness if thinner than that.	Crayon at the minimum rating, or a calibrated contact thermometer.
Maximum interpass	Roughly 1 in / 25 mm from the weld toe. The preheat location tends to understate the peak temperature here.	Crayon at the maximum rating, or an infrared thermometer with emissivity set for the surface.

For shops running multipass SMAW, this loop is the bulk of interpass control. It’s the same discipline that makes SMAW so reliable when held to its procedure.

3. End of Job: Close-Out

• Verify the log. Ensure all passes have readings, times, instrument used, welder identification, and pass or fail status.
• Document excursions. Every breach must be noted with its corrective action, including any section that was removed and re-welded.
• Escalate maximum breaches. Any excursion above the maximum interpass is not the welder’s call to absorb. Notify the engineer of record before moving to inspection.
• Mind the delay for inspection. For hydrogen-sensitive joints, allow for the required time before NDT. Inspecting too early can pass a weld that is prone to delayed cracking.
• File the records. Submit the completed interpass log with the weld inspection package per the quality plan or contract requirements.

Interpass temperature isn’t paperwork, it’s prevention. Nothing changes visibly when it goes out of limits, but the grain structure of the heat-affected zone and trapped hydrogen will. Those changes can show up weeks later as toe cracks, a hard HAZ, or delayed failures. Controlling it only costs minutes between passes. Ignoring it can cost a repair, a re-test, and a customer who won’t forget.

Frequently Asked Questions

Is interpass temperature the same as preheat temperature?

Usually, yes. In most welding procedures the minimum interpass temperature equals the minimum preheat temperature, which keeps the base metal from cooling too quickly between passes. The exception is that the root pass may need a higher preheat than the filling passes because of the higher stress concentration at the root, and some procedures specify a separate minimum interpass value.

What happens if the interpass temperature is too low?

The joint cools faster than the procedure expects, which raises the risk of hard martensitic microstructures in the heat-affected zone and hydrogen-assisted cold cracking. The correct response is to reheat the joint back to the minimum interpass temperature before striking the next arc, not to continue welding cold.

What happens if the interpass temperature is too high?

Excess heat accumulates and coarsens the HAZ grains, reducing impact toughness and strength; on quenched-and-tempered steel it over-tempers the base metal, and on austenitic stainless it can cause sensitization. You must stop, allow the part to cool naturally, and re-measure before depositing the next pass.

Where should I measure the interpass temperature?

Measure the minimum at the same location used for preheat — AWS D1.1 uses 3 in. (75 mm) from the joint edge — and measure the maximum roughly 1 in. (25 mm) from the weld toe, because the peak is not at the preheat location. Industry practice is to take readings 1–3 in. (25–75 mm) from the weld centreline at several points along the seam.

Can I use an infrared thermometer instead of a temperature-indicating crayon?

Yes, but emissivity settings matter enormously — polished, freshly ground or mill-scale-covered steel can read tens of degrees low compared with the true temperature. A common shop compromise is an infrared gun for continuous monitoring with temperature-indicating crayons as the spot check at critical passes.

Does interpass temperature matter on a single-pass weld?

Interpass temperature only exists between passes, so a single-pass weld has no interpass condition to control. Preheat still applies and still controls the cooling rate of that one pass, so it remains the critical variable for hydrogen cracking on thin or highly restrained joints.

What interpass limit applies to austenitic stainless steel?

Much lower than for carbon steel, because the controlling mechanism is sensitization rather than hydrogen cracking. AWS D1.6 sets a maximum interpass temperature of 350 °F (175 °C) for austenitic grades, and many shops work to roughly 150 °C (300 °F) for low-carbon stainless to protect corrosion resistance.

Can I speed up cooling with a fan or wet rag to meet the maximum interpass limit?

Not without approval. Forced cooling changes the cooling rate and therefore the microstructure and residual stress of the joint, so a qualified welding engineer should review the procedure before fans, wet rags or compressed air are used on a production weld.

Can I use a lower interpass temperature than the code table shows?

Sometimes, yes. AWS D1.1 Annex B and AWS D14.8M provide methods based on carbon equivalent, diffusible hydrogen, thickness, heat input and restraint that can produce lower values than the table. Any lower value must be covered by qualification testing before production welding begins, and the procedure must be written into the WPS.

Why do we have to wait between passes on stainless steel when carbon steel is fine?

Stainless has a much lower thermal conductivity than carbon steel, so heat concentrates in the weld rather than spreading into the plate, and the joint climbs quickly toward its maximum interpass limit. The waiting period is the stainless steel’s own cooling rate working against you, not a rule borrowed from carbon steel practice.