Heat Stick for Welding: Types, Uses, and How to Use One

A pipefitter on a refinery turnaround is two hours into a 16-inch carbon steel weld when the inspector asks for a temperature reading on the base metal. Nobody has a calibrated thermometer on the truck, the torch preheat is “about right,” and the only instrument left is a stub of blue wax in a plastic tube. That stub — a heat stick — is the cheapest and most underrated tool in the box, and also the one most often misused. Here’s what it actually tells you, what it doesn’t, and why searching for “heat stick for welding” can land you in two completely different industries.

Heat Stick for Welding: What It Actually Is

For many welders, confirming base metal temperature comes down to a small stick of wax. However, when searching for a heat stick for welding, it’s easy to run into two entirely different tools. One verifies temperature, the other creates it. Knowing which one you’re after is the first step to using either correctly.

With preheat being such a critical step for crack resistance, it’s no surprise that checking it is built into many procedures. Getting that temperature right is a major part of welding plate steel joint prep and preheat guide practices, making a reliable verification method just as important as the heat source itself.

The Temperature-Indicating Heat Stick

This is the tool most welders have tucked in their box. A temperature-indicating heat stick is coated in a thermochromic compound that melts irreversibly once it touches a surface at or above its rated temperature. Simply put, if the mark melts, the metal has reached at least that rating. If it doesn’t, it’s below it. It cannot give an exact reading above its rating, nor can it tell you what the temperature was at any point in the past.

These indicators come in many forms, from solid crayons and sticks to liquid paints, adhesive labels, and tags. Some are single-rating, melting at one specific temperature, while multi-rating versions show a window between two melt points to help narrow down a range. Common ratings run anywhere from 110°F up to 1,000°F, with Celsius options widely available as well.

As Miller’s preheat guidance notes, crayons are one of four common ways to verify base metal temperature between passes, alongside thermocouples, infrared thermometers, and thermal imaging cameras. The biggest appeal is accessibility, as they only cost cents per use compared to far more expensive equipment.

The Electric Resistance Heat Stick

The second tool that shares the name is an active heating element. An electric resistance heat stick is a powered rod or flexible cable that is clamped or strapped directly to a joint. When connected to a power source with controls, electrical current passes through the resistance element to generate heat and conduct it into the base metal.

These powered heaters are most often used on heavier, more restrained sections where an open flame is undesirable. They’re controllable, produce no combustion gases, and can heat larger areas more uniformly. However, they require power, take more time to set up with insulation, and aren’t always practical in tight, remote, or unpowered job sites.

Knowing the Difference Before You Buy

The overlap between the two is the main source of confusion. The temperature-indicating stick verifies heat, while the electric resistance heat stick creates it. This distinction is crucial when working to any procedure or specification, including those set by the American Welding Society. For indicators specifically, trusted manufacturers such as TEMCo Industrial, Tempil, and distributors like Lawson Products are widely used.

Tool	What it does	How it’s used	Best for	Typical cost tier
Temperature-indicating heat stick (crayon/wax)	Verifies surface temperature by melting at a rated point	Rubbed on cleaned base metal; melt = at or above rating	Field preheat and interpass checks	Cents per use
Electric resistance heat stick (powered element)	Actively heats the base metal to a target temperature	Clamped or strapped to the joint with a power source and controls	Controlled preheat on heavy or restrained sections	Hundreds to thousands (equipment + power)

Why Preheat Matters: The Welding Metallurgy Behind Cracking

A heat stick is cheap insurance, but only once you know what it insures against. Codes demand preheat temperatures for metallurgical reasons. Grasp those and the job sheet checks stop feeling like paperwork.

As Miller explains in its preheat guide, preheating reduces the cooling rate of the weld and drives out moisture, helping prevent hydrogen buildup and cracking. Here is the detail behind those two sentences.

What Preheat Actually Does to the Weld Metal

Prewarming the base metal before you strike an arc does four jobs.

1. It slows HAZ cooling. The heat-affected zone is base metal that changed microstructure but never melted. On cold steel the arc heats the first few inches fast and unevenly, and that is where most toe and root cracks begin. Warming the parent metal gentles the gradient so the HAZ transforms instead of shock-freezing.
2. It lets diffusible hydrogen escape. Molten weld metal can vent hydrogen. Solid weld metal cannot. Preheat buys holding time for hydrogen to get out before the puddle locks it in.
3. It reduces residual stress and distortion. Heat fed into a plate slowly and evenly has less shock value than heat dumped into cold steel by an arc. That matters on restrained joints and on anything downstream of a dimensional inspection. See our guide to weld distortion causes and correction.
4. It improves fit-up, bead shape, and fusion. A heated plate expands slightly, closing the root opening so the root pass lands properly on heavy plate. Driving off moisture and contaminants also cuts porosity risk and gives the puddle a cleaner surface to fuse with.

Miller also notes preheat is common before welding steel or steel alloy pipes and plates 1 inch thick or more, across oil and gas, transmission pipeline, power plant, structural construction, mining, shipbuilding, and heavy equipment. If your job has a preheat requirement, somebody has already run that analysis.

Hydrogen, Hardness, and Cold Cracking in Plain Language

Hydrogen-assisted cracking is simpler than it sounds. Molten weld metal is a sponge for hydrogen, arriving from electrode coating, mill scale, cutting oil, grease, airborne moisture, and a wet ground. While the metal is liquid some bubbles out and escapes. As the weld solidifies the escape route closes, and the trapped hydrogen collects at grain boundaries and inclusions in the hardened HAZ, where the pressure it builds can pull the steel apart.

Three conditions must line up, which is why cracks appear hours later:

• A susceptible microstructure. The HAZ has to be hard enough to be brittle. High carbon content and high cooling rates both push hardness up.
• Diffusible hydrogen. Enough of it, still in the metal, still mobile.
• Stress. Tensile residual stress from welding, restraint, or service load pulling the grain boundaries open.

Remove any one of the three and you remove most of the risk. Preheat attacks two at once: it slows cooling, keeping HAZ hardness down, and gives hydrogen time to diffuse out before the metal locks it in. Our guides to welding metallurgy explained and why welds crack and fail cover the background. AWS D1.1 frequently caps maximum HAZ hardness in structural work, commonly 22 to 26 HRC depending on grade and service condition, and preheat is the primary lever a welder has over that value.

Hydrogen cracking is also time dependent, so the delay between preheating and welding matters. A joint that cools below the required minimum before the arc is struck can be as bad as never preheating, because you paid for the heat and let it escape. That is the best argument for checking the metal right before you start and again between passes.

Where Preheat Becomes Mandatory

Preheat gets specified by the welding procedure, the governing code, and the project specification. A typical preheat specification from Miller requires the part to be held between 250°F and 400°F for 30 minutes before welding begins, with base metal temperature maintained in that range between passes. Note what that means: a floor, a ceiling, a soak time, and an interpass rule. A heat stick rated at 400°F tells you the top of the window was reached, and nothing about the hold or the floor.

AWS D1.1 drives preheat from carbon content and thickness. ASME B31.3 and API 1104 drive it from wall thickness and specified minimum yield strength, and they want readings recorded. That is the honest case for the tool: a heat stick does not create heat and does not cure a cold joint. It converts a guess into evidence, turning “I think it’s hot enough” into a documented number on a preheat log an inspector can sign.

How a Temperature-Indicating Heat Stick Actually Works

A temperature-indicating heat stick doesn’t create any heat of its own. Instead, it reacts to the heat of the base metal. The tool utilizes a thermochromic lacquer or wax coating that is bonded to a solid stick, crayon, or supplied as a liquid to be painted onto the workpiece. As the surface of the steel reaches that coating’s specific transition point, the coating softens and melts, leaving behind a visible, permanent mark. Once melted, that mark cannot be reversed, which is one of the most important facts to understand about this tool.

The key insight that most beginners miss is directionality. A melted stick proves the surface reached at least its rated temperature. It can never prove that the metal was cooler than that rating, and it cannot tell you how much hotter it got past it. In other words, it is a lower-bound indicator, not a thermometer that will give you an exact number. This is also why having clean contact is so vital. Much like the importance of surface prep when working with reactive metals such as those covered in the aluminum cleaner for welding guide, any mill scale, rust, grease, or moisture can block the transfer of heat and give a false “no melt” result.

The Rating Series and Single vs. Multi-Rating Sticks

Not every job calls for the same preheat temperature, so heat sticks come in a graded rating series to cover a wide range. On the common Fahrenheit market, these are typically offered in steps such as 110°F, 165°F, 220°F, 275°F, 330°F, 385°F, 440°F, 495°F, 550°F, 600°F, 650°F, 700°F, 750°F, 800°F, 850°F, 900°F, 950°F, and 1000°F, with matching 50°C to 500°C increments available for metric work. Each individual stick carries only one of these ratings.

There are two styles to choose from when it comes to window reading. Single-rating sticks give a simple yes or no: if it melts, you’ve hit or exceeded that temperature. Multi-rating (temperature range) sticks are a little different. These utilize two coatings, one that melts at the lower end of the window and one at the upper end. If the bottom coating melts away but the top coating remains intact with an unbroken band in the middle, that means the metal is sitting squarely inside of the required preheat window. This makes range sticks extremely convenient for jobs that have both a minimum and a maximum interpass limit.

Physical Formats of Temperature Indicators

While most people picture the classic wax crayon, the “heat stick” family is far more versatile than that. Depending on the job, welders will use different formats to get the evidence they need. The breakdown below covers the most common ones used in the trade:

Format	Best for	Accuracy style	Notes
Solid stick	Quick spot checks on flat plate	Very close to melt point, often within a few degrees	Durable and easy to carry in a welders’ kit.
Crayon (Tempilac-style)	General shop and field use	Narrow tolerance band, typically within a few °F	The most recognizable form. Easy to mark with, but can break if mishandled.
Liquid-applied paint (Tempilaq-style)	Monitoring multiple spots at once	Consistent for the rated coating	Paint a dot on the part and watch for it to liquefy. Great for large weldments.
Adhesive label	Pre-checks on parts or pressure equipment	Varies by brand	Self-adhesive and great for spots that are hard to hand-mark.
Temperature tag	Post-weld heat treatment (PWHT) verification	Irreversible, up to around ±10°F on some formulations	Commonly used to document soak temperatures for pressure piping and vessels.

Liquid-applied coatings, labels, and tags are especially useful when you need to keep an eye on several locations simultaneously, without having to touch each one during an active heating cycle.

Ultimately, the beauty of the temperature-indicating heat stick is its simplicity. By utilizing an irreversible thermochromic reaction, it converts a guess into evidence. However, the proof that it gives you is not “the metal is X degrees”, it’s “the metal is at least Y degrees”. Understanding that one distinction is what separates a correct preheat verification from a misleading one.

How to Use a Heat Stick for Welding, Step by Step

There is no use guessing when the weld procedure says the base metal has to be hot enough before the first arc strikes. A temperature-indicating heat stick takes all of the “feels warm enough” out of preheat and turns it into evidence. The trick, however, isn’t just rubbing it onto the plate. It’s doing it the right way every time.

Before You Start: Pick the Right Rating

Before you touch the stick to the metal, your job is already on the paperwork.

• Check your requirements: Confirm the minimum preheat temperature and the interpass temperature limit directly from your WPS or applicable code. Never go off memory.
• Select a stick just below your target: Pick a stick rated at or slightly below your required minimum. If it melts, you have met or exceeded that minimum. A stick rated far above your target will likely never melt, tricking you into thinking the metal is cold.
• Plan your check zone: Know where the code expects you to check within the joint. A reading only counts if it lands in a code-defined zone, not somewhere random across the plate.

The Five-Step Check

Once the rating is picked, run this simple, repeatable process for every preheat check:

1. Clean the check point. Wire brush or grind off mill scale, rust, paint, grease, and moisture. Scale and grease block contact for the coating, and both can release fumes when heated.
2. Find a representative spot. Choose a flat spot on the base metal roughly 1 to 2 inches back from the joint prep, or directly on the prepared joint itself. Avoid the bead, slag, and any built-up area.
3. Apply perpendicular pressure. Hold the stick perpendicular against the clean metal with firm, steady pressure for several seconds so the thermochromic coating can respond. Don’t swipe and snatch it away.
4. Read the result. If the coating melts or liquefies, the surface reached at least the rated temperature of that stick. If it stays intact, it did not. This proves one thing only: the temperature at that exact contact point.
5. Record it. Log the time, check location, stick rating, and pass number in your preheat log. That record is what makes the reading defensible on code work.

Where Exactly to Touch the Metal

Location is everything. A point sitting in the arc’s radiant heat or the torch flame plume will read drastically hotter than the rest of the joint and will almost always hand you a false pass. Aim for a cooler, representative location on the base metal that still falls inside the code-defined zone.

It also helps to understand how hot is a weld arc to see just how deceptive radiant heat can be to the naked eye. Small temperature differences play a massive role in the risks that lead to weld distortion when preheat is overlooked or guessed.

Monitoring Interpass Temperature Between Passes

Meeting minimum preheat is only half the battle. Many WPS’s set an interpass temperature limit, with many carbon steel applications capping it anywhere from 300°F to 400°F. Letting the joint cool too little between passes can be as damaging as not preheating at all, with a much higher risk of hydrogen induced cracking.

Between every pass, clean and re-check the interpass location with the correct rating stick. Don’t wait until the end of the joint to verify it, and never assume the metal is within range just because it stopped glowing.

Storing and Discarding Used Sticks

Heat sticks are cheap for a reason, but they are heat sensitive and easily ruined if mistreated.

• Store properly: Keep the tube sealed, out of direct sunlight, and out of a hot truck or trailer. On a sweltering summer day, an unprotected tube can exceed the rating of the sticks inside and melt them before they ever reach the joint.
• Only use untouched sticks: Never return a used stick to the tube. Once the coating has melted, smeared, or been contaminated with slag, grease, or moisture, it will give ambiguous, unreliable readings next time.
• Dispose safely: Discard melted or damaged sticks in a designated metal scrap container. The coating can still be hot, and tossing them into a regular plastic bin creates an unnecessary burn or fire risk.

Discipline is the whole trick. A heat stick turns a guess into evidence, but only when it is the right rating, checked in the right spot, and logged every time. Follow your WPS over any shortcut, and you will have one of the most powerful crack prevention tools in your box working for you.

Heat Stick vs. Thermocouple, IR Thermometer, and Thermal Camera

There is no single instrument that wins every preheat or interpass check. As noted in Miller’s preheat guidance, crayons, thermocouples, infrared thermometers, and thermal imaging cameras all see daily use in the field. The real skill is knowing what each tool can prove, and what it cannot, before you lean on it for a code requirement. For a broader look at the gear these checks depend on, see our equipment for welding guide.

The Side-By-Side Breakdown

Instrument	What it measures (surface vs. bulk)	Accuracy	Response Time	Logging	Cost	Power Source	Best Job	Code Acceptance
Heat stick / thermochromic crayon	Surface contact reaction	Good for pass/fail at its rated temperature. No exact readout above or below.	Seconds	None (manual, physical mark)	$3 – $25 for a set	None	Quick preheat and interpass pass/fail checks in the field	Often accepted as a working check, however many QA plans still require a traceable instrument for records.
Thermocouple with meter	Metal temperature at the probe contact point (closest to bulk temperature)	Very high when calibrated correctly	Seconds	Manual or digital with saved readings depending on meter	$50 – $400+	Battery or plug in meter	Code work that requires a documented, calibrated temperature	The reference method for the majority of codes requiring a temperature record.
Infrared (non-contact) thermometer	Surface radiation	Varies drastically based off of emissivity	Instant	Some models allow storage, however most are basic point readings	$30 – $250	Battery	Fast spot checks on large, accessible, and properly prepared surfaces	Sometimes accepted, however often scrutinized without emissivity correction and a calibration record.
Thermal imaging camera	Surface radiation over an area	Good with correct emissivity settings, however still a surface measurement	Instant, real time	Yes, images, videos, and temperature data	$1,000 – $10,000+	Rechargeable battery	Mapping gradients across a large weldment to find cold spots or uneven preheat	Valuable for QA verification, however usually used as a supplemental tool rather than a substitute for a code mandated probe reading.
Pyrometer (contact or optical)	Varies by type (optical surface, or contact probe based)	Varies. Contact pyrometers can be accurate, optical rely heavily on emissivity	Instant to seconds	Model dependent	$80 – $1,000+	Battery or plug in	Specialized applications, often in shop preheat setups	Case by case based off of calibration and the code being used

Emissivity, Contact, and What Each Tool Tells You

The biggest trap with any non-contact instrument is emissivity. Bare or shiny steel reflects much of the infrared energy it receives, so an infrared thermometer aimed at mill-scale-free, mill-finish plate will read low, sometimes by a wide margin. The fix is a high-emissivity target, such as a small patch of electrical tape or paint, placed on the surface you are checking, or setting the correct emissivity value for your material on a thermometer that allows it.

This is exactly where a heat stick earns its keep. It is a contact method, so emissivity never enters the picture. It reacts directly to the temperature of the metal it is pressed against. It still is not perfect. Note: A heat stick measures exactly what it touches, and on a scaled or rusted surface that reading may sit a few degrees off the true clean-metal temperature. Clean your check point with a wire brush before you press the stick to it.

For traceability, a thermocouple with a spot-welded or magnetically attached probe gives the truest bulk measurement. That is precisely why it stays the reference method for codes demanding a documented, calibrated temperature. At the other extreme, thermal imaging cameras are unbeatable for scope. A single-point check can miss a cold tail at the end of a long seam, whereas a thermal image shows the entire gradient and exposes it within seconds.

How To Pick The Right One

For the majority of everyday fit-up checks, a heat stick is the cheapest, fastest, and most practical option. For code pressure piping, structural work under a documented QA plan, or any weld that needs a signed preheat log, use a calibrated thermocouple and meter as your record keeping tool. When you are mapping a massive weldment or chasing down inconsistent heating, a thermal imaging camera earns its cost. And for a rough, non-critical spot check on an already prepared, high-emissivity surface, an infrared thermometer can suffice, as long as you know its limits.

The smart approach is not picking one tool to rule them all. It is knowing when to use each one. A heat stick tells you whether you are above its rating at the moment of contact, a thermocouple produces the number that goes on the log, and a thermal camera shows you where to look in the first place. Choose based on the question you are actually trying to answer.

The Other Heat Stick: Electric Resistance Preheat Sticks and Preheaters

While the wax or lacquer indicator is what most people picture for a heat stick for welding, there’s a second tool that shares that exact name: the electric resistance heat stick. For heavy fabrication, pipe welding, or any job where bringing a large, thick joint up to preheat temperature needs to be controlled, this powered version is often the right call.

How an Electric Resistance Heat Stick Works

An electric resistance heat stick creates its own heat, rather than indicating the temperature of the metal like its crayon counterpart. A resistance element, often a tubular heating element or a coiled element sheathed in flexible cable or a rigid bar, generates heat as current flows through it. This element is clamped or strapped directly to the joint, and is often covered with ceramic or fibre insulation to trap as much of that heat as possible. From there, the heat conducts into the base metal from the outside in.

Power for these setups can come from a site 120V/240V supply with a control box, or in some cases, a welding machine can be used as a power source. With the addition of a temperature controller, the goal isn’t just to get hot, it’s to reach and hold the exact preheat temperature called for on the job.

Why Shops Adopt Them

Resistance preheaters aren’t brought out for every job, but when they’re needed, they solve a lot of headaches:

• Cleaner heating: No open flame, no soot, no combustion gases, and in many setups, no fire watch required strictly for the heating step.
• More uniform heat: A properly insulated resistance setup distributes heat far more evenly than trying to hand-balance a rosebud across a joint.
• More capability: These units can reach preheat temperatures that a rosebud torch would struggle to hold on very heavy, large, or heavily restrained sections.
• Greater control: With temperature monitoring and insulation, it’s much easier to hold within the window required for code work.

The Limitations to Know

As useful as they are, electric resistance heat sticks come with tradeoffs. They need a power source, they heat from the outside in so thick sections take much longer to soak, and heat is lost at the ends of a joint. Insulation adds both cost and setup time, they’re useless on non-conductive workpieces, and tight or awkward access can make clamping them impossible altogether.

Comparing the Four Standard Preheat Methods

According to Miller’s preheat guide, there are four standard preheating methods for welding: induction, open flame torch, resistance heating with ceramic pads, and convection ovens. Each one fits a very different job.

Method	Strengths	Limitations	Typical Notes
Induction	Uniform heated area, setup often under five minutes, coils stay cool for the operator, no need for personnel to watch it.	Higher initial cost, operator training needed for wrapping technique.	Great for production or jobs where speed and consistency matter.
Open Flame Torch	No extra heating equipment if a torch is already owned, little training needed.	Inconsistent uniformity, slower time to temperature, soot and smoke, gas storage, fire watch required. Roughly $40 to $60 per hour in consumable fuel.	Most common in the field, but the hardest to hold within a tight preheat window.
Resistance Heating (Ceramic Pads)	Consistent temperature when pads are intact, well suited for larger parts.	Usually contracted out, fitting and securing pads can take an hour or more, long cool-down time. Insulation and ceramic pads are costly consumables, broken pads create hot and cold spots, elements get extremely hot, and electricians with a dedicated electrical drop are often required. Requires continuous supervision.	Reliable when done right, but can be schedule-heavy for field work.
Convection Ovens	The most uniform heating overall, ideal for batch processing many parts at once.	Permanently installed, requires a substantial electrical connection, takes several hours to preheat, and heats the surrounding area.	Best reserved for the shop, not a portable option for most field turns.

At the end of the day, the electric resistance heat stick fills a specific gap: when flame isn’t practical, and precision matters. For portability and simplicity, the indicator heat stick will always be the cheapest way to verify temperature. For soaking massive, code-driven joints, a controlled resistance preheater is often the tool that makes that requirement realistically achievable.

Where Welders Use Heat Sticks: Materials, Jobs, and Codes

Not every weld needs preheat, but when it does, a heat stick is the fastest way to prove the metal is in the window before striking an arc. Verification needs change dramatically from job to job, so knowing which materials call for it saves wasted effort and costly cracking.

Carbon Steel Plate and Structural Work (AWS D1.1)

On structural steel governed by AWS D1.1, preheat requirements come from both the carbon content of the steel and its thickness. Thick, higher carbon plate cools slowly, leaving the heat-affected zone harder and far more prone to cold cracking. Preheat also buys time for diffusible hydrogen to escape before the weld metal locks in.

The Welding Procedure Specification (WPS) states minimum and maximum preheat temperatures along with the interpass limit. Many procedures allow alternatives such as the HAZ hardness control method or hydrogen control method, but where a tabulated preheat is required, a heat stick verifies the surface falls inside the exact range before and between passes. This is routine on the heavy sections covered in our heavy metal welding guide.

Transmission Pipeline and Process Piping (API 1104 & ASME B31.3)

For code pressure work, documentation matters as much as the heat itself. For cross country pipelines under API 1104, minimum preheat is a function of pipe wall thickness and the specified minimum yield strength (SMYS). As an illustrative guide for carbon steel, preheat is typically not required below roughly 3/4 inch wall, around 200°F for walls above that, and up to 300°F for the heaviest wall. Raise the SMYS and those minimums climb with it, sometimes well above what thickness alone would suggest.

For facility and process piping, ASME B31.3 takes a procedure-based approach. Preheat and interpass temperatures must be specified in the WPS and, where the specification calls for it, recorded. Inspectors often want those readings on pipe jobs, which makes a heat stick a practical tool for spot checks on the line, following the practices in our industrial pipe welding codes guide.

Cast Iron, Stainless, and Aluminium

These metals punish mistakes far more readily than mild steel, and preheat here targets their unique tendencies rather than cooling rate alone.

• Cast Iron: Repairs and castings usually get austenitic stainless or nickel-alloy filler metals, since the graphite in the iron behaves like cracks. Larger castings often see preheat between 200°F and 400°F, followed by slow, controlled cooling. Our cast iron guide covers more repair practice.
• Stainless Steel: Thin austenitic stainless rarely needs preheat. Heavier sections and grades such as 316 and 347 may call for preheat and tight interpass limits, because overheating stainless promotes distortion and, in sensitizing grades, raises the risk of intergranular corrosion.
• Aluminium: A phenomenal heat sink, aluminium rarely needs preheat. Heavy sections and alloys high in magnesium and silicon may benefit from modest preheat. High-strength alloys such as the 2000 and 7000 series are more crack prone and often call for restraint control over preheat entirely.

Hardfacing, Hardened Steels, and Heavily Restrained Sections

The most expensive, time sensitive welds lean on preheat the hardest. Here a heat stick helps prevent dilution, lost hardness, and the most catastrophic cracking.

• Hardfacing, Cladding, and Overlay: Preheat limits base metal dilution into a wear resistant deposit and reduces cracking inside the surfacing weld metal. Our hardfacing and overlay guide expands on those controls.
• Hardened and Quenched-and-Tempered Steel: These steels are heat treated for strength. Poor preheat either cracks them or overheats and negates the temper, so it must be controlled tightly to respect tempering temperature limits. Our hardened steel guide covers those boundaries in depth.
• Thick, Heavy, and Heavily Restrained Sections: Mass and restraint multiply cracking risk. A thick, fixed joint contracts harder than a small free plate, building residual stress, so preheat slows cooling and lets that stress distribute. Heat stick checks at multiple points along the joint confirm consistency.

Material or Job	Preheat Approach	Why the Heat Stick Matters
Carbon steel pipeline (API 1104, illustrative)	Not required below roughly 3/4 in wall; around 200°F above that; up to 300°F for the heaviest wall	Confirms the code minimum before the root pass
Cast iron repair	200°F to 400°F on larger castings, then slow controlled cooling	Keeps the HAZ above the graphite cracking range
Stainless steel	Rarely on thin austenitic; limited preheat plus strict interpass on heavy 316 and 347	Shows you are not overheating the stainless
Aluminium	Rarely; modest preheat on heavy or high magnesium and silicon alloys; restraint control on 2000 and 7000 series	Balances the heat sink against burn-through risk
Hardfacing and quenched-and-tempered steel	Set by the WPS to control dilution, hardness, and tempering limits	Documents the preheat actually delivered on the joint

Common Heat Stick Mistakes and How to Troubleshoot Them

Most failed preheat checks are not the fault of the heat stick itself, they are the fault of how it gets used. Small habits on the shop floor can convince a welder the metal is in range when it is not, or hide the fact that it is already outside it.

The Most Common Mistakes

• Stick rated too high: Symptom: it never melts, no matter how long you hold it on the joint. Fix: drop to a lower rated stick or re-check your check point. It is one of the easiest ways to falsely conclude the metal is “cold.”
• Touching slag, the weld bead, or scale: Symptom: an erratic, smeared mark with no clean melt. Fix: slag, weld deposit, and thick mill scale are not the base metal that AWS D1.1 or your WPS cares about. Always check clean base metal, never the deposit.
• Checking in radiant heat: Symptom: the stick melts far faster than expected when the part should be below target. Fix: never check in the rosebud plume, near the arc, or where radiant heat bathes the spot. Move 1 to 2 inches away from the joint prep.
• Only checking the middle of a long seam: Symptom: the center passes but the far end is still below preheat or drops fast between passes. Fix: heat loss is uneven, especially when heating from one end. Verify both ends and at least one middle point. A frequently overlooked cause of cold ends.
• Using one stick for a whole multi-pass job: Symptom: no logged proof of interpass temperature on later passes. Fix: interpass heat climbs as welds accumulate. Re-verify between passes with the correct rated stick and log the readings.
• Treating the melt as an exact number: Symptom: assuming “it melted” means it is exactly at that rating. Fix: a heat stick proves the surface reached at least its rated temperature and no more. If the WPS sets a maximum preheat or interpass, use a higher rated stick or a calibrated instrument.
• Using a heat stick where the code demands a calibrated reading: Symptom: an inspector rejects a log of stick checks with no traceable readings. Fix: a heat stick is an excellent working verification tool, but it is not always acceptable as sole traceable evidence. Check your WPS and the applicable code for recording requirements.
• Confusing a heat stick with a thermite heat source: Symptom: mixed up terminology on rail or heavy exothermic work. Fix: a thermochromic heat stick has no exothermic reaction at all. It is an indicator, not a heat source, and the two should never be used interchangeably.
• Ignoring the interpass limit while chasing minimum preheat: Symptom: the joint came up to preheat, but between the third and fourth pass the metal crept above the allowable interpass. Fix: preheat is a starting point and interpass is a window you must maintain. Watch for heat buildup, allow controlled cooling, and verify it.
• Skipping the check because it “feels hot”: Symptom: gauging temperature by hand distance or touch. Fix: “feels hot” is not evidence. Clean metal can sit well below target while perception tricks you, or the reverse. Always verify with the proper stick.

Troubleshooting Table

Symptom	Likely Cause	Fix
Stick will not melt	Rated too high, or dirty metal with no contact.	Try a lower rated stick and clean down to bare base metal with a wire brush or grinder.
Stick melts instantly	Torch plume, radiant heat from the arc, or an overheated spot.	Move to a cooler spot away from the heat source and verify with a lower rated stick.
Stick smears with no definite melt	Loose scale, rust, paint, grease, or moisture.	Strip to clean base metal. If it still smears, an infrared thermometer or thermocouple may be more reliable.
Readings vary along the joint	Uneven heating, large thermal mass, or heating from one end.	Check ends and middle, watch the far end, and heat slowly and evenly.
Readings jump between passes	Heat accumulation with no interpass check.	Check and log after each pass within WPS limits and cool in a controlled way if it climbs too fast.

Ultimately the heat stick is only as good as the discipline behind it. A clean contact point, the right rating, verification between passes, and knowing when to step up to a calibrated instrument prevent most of these mistakes. If the readings do not make sense, question the point of contact first.

Safety, Compliance, and Recording Preheat Temperatures

A heat stick is a ten-dollar stick of wax, and the liability it can create is not ten dollars’ worth. Get safety or paperwork wrong on a code job and the cost lands elsewhere: a rejected weld, a re-do, an insurance question. This section is about what keeps you safe, what has to be recorded, and what a heat stick reading actually proves.

OSHA and the Fire Watch Reality

The federal standard governing US shop and field welding is 29 CFR 1910.252, Welding, Cutting, and Brazing General Requirements: fire prevention, ventilation for fumes, eye and face protection, protective clothing, and training. It applies to an arc, a torch, or a resistance preheater.

The heat stick itself creates no flame, no heat, and no fumes; it is coated wax. But nobody uses one in isolation. It verifies preheat that something else created, usually an open flame torch inches from combustible material. The safety rules follow the heat, not the instrument.

So the moment a torch comes out on a refinery, ship deck, or structural steel job you are in hot work territory: fire watch, permit, cleared area. A rosebud on a 24-inch line is a moving source of radiant heat, not a point source, so a bigger preheat area means more site to clear and watch. Miller’s preheat guide confirms it: flame preheating brings comfort and safety issues around the soot, smoke, and gases of open flame, and personnel must be on fire watch.

Switch to resistance heating or induction and the picture improves: no open flame, no soot, no combustion gases, no fire watch for the heating step. That is a real safety upgrade, and part of why shops move to electric preheat on code work. The trade-off is a power source and someone who understands the control box.

Fumes, Soot, and the Heat You Cannot See

Open flame preheat adds a second exposure on top of arc fume. A propane or acetylene torch produces carbon monoxide, soot, and combustion products. Soot you can see; carbon monoxide you cannot. It is colourless, odourless, and binds to haemoglobin with roughly two hundred times the affinity of oxygen. Ventilation sized for arc fume may not clear a low flame against a plate.

Preheat is never the only source on a hot job. Stainless or hardfacing adds hexavalent chromium exposure from base metal and filler, and in enclosed work controlling the plume takes more than a respirator. Our guides to safe indoor welding ventilation and welding fume control and workplace air quality cover engineering controls, local exhaust placement, and monitoring. The preheat step is where those controls get skipped.

Add radiant heat and it gets worse. Preheating raises the ambient temperature long before the arc is struck, and heat stress arrives before anyone thinks to hydrate: shade, water, rotation. Remember the surface too. Mill scale, paint, grease, and oil are combustible, and a torch held inches off a scaled plate can flash scale or grease into an ember, so the check point and surrounding metal get cleaned before preheat.

When a Heat Stick Reading Is Not Enough Evidence

A heat stick is a working tool, not evidence. It does not appear in 1910.252, and it is not named in AWS D1.1 or ASME B31.3 as an approved measuring device. Codes require preheat and interpass temperatures recorded from calibrated instruments or from verification methods named in the WPS. Where a code job demands a documented number, a melted stick is a hint, not a record.

Nobody needs a thermocouple for every weld on a back fence. On non-code work the heat stick is often enough. On code work it is a sanity check that keeps you and the inspector in the right zone before you burn hours of wire on a cold joint.

• Calibrated thermocouple and meter — the record: traced, logged, signed. What a QA plan and an inspector want to see.
• Irreversible temperature indicators or tags — common on post-weld heat treatment of pressure equipment, applied at the soak location to show the achieved temperature and soak duration afterwards. The tag changes permanently at its rated temperature.
• Heat stick or thermochromic crayon reading written on a form — indicative, often accepted on non-code work or as a supporting entry.

On pressure equipment repair work, the ordering catches people out. Work typically cannot proceed until the preheat range is documented and the inspector confirms the joint is at or above the minimum temperature. That is a hold point, not a formality: the documentation must exist before the permit to weld is issued.

The documentation stack on a coded job, and where the heat stick touches it:

1. Welding procedure specification — required preheat minimum, interpass maximum, specified verification method.
2. Preheat log — rating, location, time, and pass number for every check.
3. Welder or inspector sign-off — a named person confirming the joint was at or above minimum before welding began.
4. Weld map — which joint is which, and which preheat record belongs to it.
5. Post-weld heat treatment chart — the soak curve and the irreversible tag evidence at the soak location.
6. NDE reports — radiography, ultrasonic, or MPI results closing the loop back to the preheated joint.

Verify with the heat stick so you do not weld cold. Record with the instrument the specification demands. On most code work the stick is in your hand while the thermocouple makes the official entry, and both point to the same answer.

Choosing and Buying Heat Sticks: A Practical Field Guide

Buying heat sticks is one of the easiest purchasing decisions in a welding shop, which is exactly why it gets done badly. A tube of indicators costs less than a shop lunch, so nobody agonises over the choice. Then the tech grabs a 400°F stick for a 250°F requirement, presses it into mill scale and calls the plate cold. The usual field failure isn’t a bad tool. It’s a good tool used in the wrong place, on a dirty surface, at the wrong moment in the pass sequence.

The useful buying question isn’t which brand wins. It’s what this box will let you prove when the inspector wants a number.

The Buyer’s Checklist

• Single rating or range series? A single-rating stick answers one question: has this metal reached X? A range stick melts at a lower and a higher point, and the intact band between them proves the metal sits inside the window, not merely above the floor. Worth it when the WPS gives a minimum and a maximum.
• Format. Stick, crayon, liquid, label, or tag. Match the format to the job, not the price tag.
• Accuracy band. Check the published transition tolerance and pick a rating at or slightly below your target, so a melt means something.
• Coating durability. Will it survive a truck bed, a dropped toolbox and grinding dust? Cheap sticks smear before they read.
• Shelf life and storage range. Heat-sensitive consumables have a rated shelf life. Check the packaging date and stated storage range before buying a case.
• Cost per verified check. Divide what you pay by the number of checks the tube supports, not by the sticker price.

Format	Buy it for	Watch for
Solid stick	Field work, one check per stick	Evidence is gone once used; carry spares
Thermographic crayon	Frequent checks, marking hot metal	Wears fast, brittle when cold
Liquid-applied paint	Wide band on a part	Needs a clean, dry surface and drying time
Adhesive label	Checks recorded on the part	Backing may not survive handling
Tag or card	Long-cycle, one-and-done checks	Least information of the five

Before you settle on a format, it helps to know what the tool can and cannot do. This primer on preheat practices and tool selection is a good companion read.

Building a Starter Kit

For general fabrication and structural work, a practical shop kit is four single-rating sticks: 200°F, 300°F, 400°F and 600°F. That set covers most carbon steel preheat and post-weld heat treatment checks and fits in a shirt pocket. If you weld code pressure piping under ASME B31.3 or structural work governed by AWS D1.1, add a range series so you can bracket the specified window instead of chasing the minimum. Shops running thick plate should also review our guide to heavy metal welding for thick sections, where preheat windows get tight.

Storage: The Cheapest Way to Waste Money

Heat sticks are heat-sensitive consumables. A tube left on a truck dash in July can soften, bleed, or deform, and a melted stick is a melted stick. Store them cool and shaded, out of direct sun, away from sparks and grinding dust, with the tube capped. Grit and grease from a truck bed ruin coatings faster than heat does. A shop drawer or an insulated pouch in the toolbox is enough protection for a small investment.

A Ten-Step Routine for Every Preheat Job

1. Read the WPS or code for minimum preheat and maximum interpass.
2. Clean the check point to remove mill scale, rust, paint and grease.
3. Pick the stick rated at or just below the required temperature.
4. Place it perpendicular on flat base metal near the joint, clear of radiant heat.
5. Wait a few seconds for the coating to respond, then read the result.
6. Log it with the time, location and stick rating.
7. Re-check between passes for interpass compliance.
8. Watch the interpass ceiling while welding. It is a limit, not a target.
9. Protect and cool the joint so the preheat is not undone.
10. File the log with the weld package.

When a Heat Stick Stops Being Enough

Conclusion

A heat stick is the cheapest way to turn a guess into evidence. It only earns its keep if you know what it can prove: the surface reached at least its rating, it can never report a number above that rating, and it says nothing about the clean metal beneath the scale you checked.

Buy the ratings you weld at, store the tube like it matters, and run the same ten steps every time. When the WPS demands a recorded temperature, follow the WPS, not the tool. The stick confirms the heat. The procedure and the paperwork turn it into proof.

Frequently Asked Questions

What is a heat stick used for in welding?

A heat stick is a temperature-indicating tool whose coating melts when the metal it touches reaches a rated temperature. Welders use it to confirm that a joint has reached the minimum preheat temperature required by the welding procedure specification, and to check that the metal stays within its interpass temperature limit between passes. Some shops also use indicator labels and tags to document post-weld heat treatment soak temperatures.

Does a melted heat stick mean the metal is exactly that temperature?

No. A melted stick only proves the surface reached at least the rated temperature. It cannot tell you how far above the rating the metal is, it cannot report a temperature below its rating, and it never records a maximum. If your WPS sets both a minimum and a maximum, you need a second higher-rated stick or a calibrated instrument.

What temperature heat sticks should I buy?

Match the stick to the job. Most carbon steel fabrication and structural work calls for single-rating sticks in the 200°F to 600°F range, while pipeline and pressure work often needs a 200°F, 300°F, and 400°F set. If you work to codes with a specified range rather than a single minimum, buy a multi-rating range series so you can confirm the metal is inside the window.

Are heat sticks accurate enough to satisfy a welding code?

For shop work and field verification they are reliable and widely used, but most codes and project specifications expect the recorded temperature to come from a calibrated instrument such as a thermocouple, and some will not accept a temperature indicator as the record. Treat the heat stick as the working tool and the thermocouple as the evidence, and always check the specific specification in force on your project.

Where on the weldment should I place the heat stick?

Touch the stick to clean base metal at or very near the joint, typically about 1 to 2 inches back from the edge of the joint prep, and use the same location every time you check. Avoid the arc’s radiant heat and the torch flame path, because those can read far hotter than the joint itself. Brush the spot to remove scale, rust, paint, and grease first.

How is a heat stick different from a preheat torch?

A heat stick measures; it does not heat. A preheat torch (a rosebud-style fuel gas torch) supplies the heat, and the heat stick then confirms the metal reached the required temperature. Open flame heating is slower, less uniform, and more expensive per hour than electric induction heating, and it requires a fire watch while in use.

Can I use heat sticks on stainless steel or aluminium?

On stainless, thin austenitic sections usually need no preheat at all, and where preheat is specified on heavier sections the interpass limit is usually low, so a modest stick in the 200°F to 300°F range is usually enough. On aluminium, preheat is rarely required, but if your procedure calls for it on heavy sections, use a low-rating stick and check the procedure, because some alloys must not be preheated at all.

Why won’t my heat stick melt even though the metal feels hot enough to weld?

The most common causes are that you selected a stick rated above your target temperature, or you touched a scaled, dirty, or coated surface so the coating never made clean contact with the metal, or you checked a cold location away from where the heat actually is. Clean the point with a wire brush, use a stick rated just at or below the target, and check again on the base metal close to the joint.

Do I need a fire watch when using a heat stick?

A heat stick by itself creates no flame, so it does not trigger fire watch requirements on its own. The heating method it is verifying does: torch and open flame preheating both require fire watch precautions and hot work controls under OSHA 1910.252 whenever combustible material is in the vicinity. Electric resistance and induction preheating avoid the flame entirely, which is one reason shops adopt them.

Are heat sticks reusable, and how should I store them?

No. A stick that has melted or been used as a test has given up its evidence and should be discarded so it cannot be mistaken for an unused one. Store the tube sealed, out of direct sun, away from sparks and hot vehicles, and away from heat. Heat sticks left in a truck in summer can degrade or melt before they reach the job site.

When is a heat stick not good enough?

When the job needs a recorded numeric temperature, when it needs proof that the metal did not exceed a maximum, or when it needs bulk or through-thickness measurement rather than a surface reading. In those cases step up to a thermocouple and meter, a thermal imaging camera for mapping gradients, or an induction or resistance preheat system for the heating side of the job.