How To Check Interpass Temperature In Welding – For Stronger
To check interpass temperature, use a temperature-indicating crayon (Tempilstik) or a contact pyrometer at a distance of 1 inch (25mm) from the weld bead. Ensure the metal has cooled to the specified range before starting your next pass to prevent cracking or loss of mechanical strength.
For DIYers, an infrared thermometer is a quick alternative, but you must adjust for surface emissivity on shiny metals like aluminum or stainless steel to get an accurate reading.
Have you ever spent hours meticulously laying down beads on a heavy project only to notice small cracks forming as the metal cools? It is a common frustration for many garage fabricators, and it usually points back to one critical oversight: heat management. Learning how to check interpass temperature in welding is the key to moving from “sticking metal together” to creating professional-grade, structurally sound fabrications.
I understand that when you are in the flow of a project, stopping to wait for metal to cool feels like a waste of time. However, the internal structure of your steel changes with every degree of heat you add. If the base metal stays too hot between passes, you risk grain growth and reduced toughness; if it is too cold, you risk hydrogen-induced cracking.
In this guide, we are going to break down the exact methods I use in the workshop to monitor heat levels. We will look at the tools you need, where specifically to take your readings, and why this simple step is the difference between a weld that lasts a lifetime and one that fails under pressure.
Understanding the Importance of Interpass Temperature
Before we dive into the “how,” we need to understand the “why.” Interpass temperature refers to the temperature of the base metal in the welding zone immediately before the next pass is started in a multi-pass weld. It is a balancing act between two extremes.
If your interpass temperature is too high, the cooling rate of the weld becomes too slow. This can lead to a coarse grain structure in the metal, which significantly reduces the impact strength and toughness of the joint. In stainless steels, excessive heat can even cause a loss of corrosion resistance, a process known as sensitization.
Conversely, if the temperature is too low, especially on thick sections of high-carbon steel, the weld cools too quickly. This rapid quenching can create a brittle microstructure called martensite. When hydrogen is present from moisture or coatings, this brittleness leads to underbead cracking, which might not show up until hours after you finish the job.
Essential Tools for Measuring Weld Heat
You cannot accurately judge temperature just by looking at the color of the steel unless it is glowing cherry red, and by then, it is often too hot for most standard procedures. You need dedicated tools to get the job done right.
Temperature-Indicating Crayons
Often referred to by the brand name Tempilstiks, these are the gold standard for many workshops. These crayons are engineered to melt at a very specific temperature. You simply swipe the crayon on the metal; if it remains a dry mark, you are below the target temperature. If it melts into a liquid smear, you have reached or exceeded that temperature.
Infrared (IR) Thermometers
These “laser guns” are incredibly popular because they are non-contact and provide instant digital readouts. They are excellent for quick checks, but they have a major weakness: emissivity. Shiny surfaces like aluminum or polished stainless steel reflect infrared energy, giving you a false low reading. If you use one, ensure it has an adjustable emissivity setting.
Contact Pyrometers and Digital Thermocouples
A contact pyrometer uses a physical probe that you touch against the metal surface. These are highly accurate and are not affected by the shininess of the metal. For a serious DIYer working on critical automotive frames or structural repairs, a digital thermometer with a K-type thermocouple probe is a fantastic investment.
how to check interpass temperature in welding
To get an accurate reading, you must follow a consistent procedure. Measuring directly on top of the previous weld bead will give you a “false high” because that area hasn’t had time to dissipate heat into the surrounding plate.
First, identify your target temperature range. For most common mild steels (A36), you generally want to stay below 500°F (260°C) unless a specific procedure dictates otherwise. Once you finish a weld pass, allow the piece to sit for a few minutes. Clear any slag or heavy oxidation from the area where you intend to measure, as these materials act as insulators and will skew your results.
Next, apply your measurement tool at a distance of 1 inch (25mm) away from the weld fusion line. You should measure on the base metal, not the weld itself. If you are using a Tempilstik, make a small mark. If the mark remains solid, you are safe to begin the next pass. If it melts instantly, you must wait. If you are using a digital probe, hold it firmly against the cleaned surface until the reading stabilizes.
Finally, check the temperature at multiple points along the joint if the weld is long. Heat tends to build up more in the center of a plate than at the edges. Consistently checking ensures the entire assembly stays within the safe thermal envelope, preventing localized weak spots in your project.
Where to Measure: The 1-Inch Rule
One of the most common mistakes beginners make is measuring the temperature in the wrong spot. If you measure too far away, the reading will be too low. If you measure the weld bead itself, it will be much hotter than the actual base metal heat-affected zone (HAZ).
The industry standard is to measure 1 inch away from the weld groove. For very thick materials (over 1 inch thick), some codes require measuring at least 3 inches away, but for the average garage project, the 1-inch rule is your best bet. This ensures you are measuring the heat that the base metal is actually “soaking” in.
By measuring at this specific distance, you are monitoring the Heat Affected Zone. This is the area of the base metal that didn’t melt but had its microstructure altered by the heat. Keeping this area within the correct temperature range ensures that the transition between the weld and the base metal remains strong and ductile.
Managing Heat in Different Materials
Different metals have vastly different thermal properties. How you check and manage heat in mild steel is not the same as how you handle it in aluminum or stainless steel.
- Mild Steel: Generally forgiving. The main goal is to prevent the interpass temperature from climbing so high that the metal becomes “mushy” and difficult to control, or so low that it cracks.
- Aluminum: This metal is a massive heat sink. It draws heat away from the weld very fast. However, if the interpass temperature gets too high (usually above 250°F), the strength of the tempered aluminum (like 6061-T6) can drop significantly. Use an IR gun with caution here.
- Stainless Steel: Stainless has low thermal conductivity, meaning heat stays localized. It is very easy to overheat stainless, leading to “sugar” (oxidation) on the backside and a loss of corrosion resistance. Checking interpass temperature is mandatory for high-quality stainless work.
Common Pitfalls to Avoid
Even with the right tools, it is easy to get an inaccurate reading if you aren’t careful. One major pitfall is ignoring the ambient temperature. If you are welding in a freezing garage in the middle of winter, your metal will lose heat much faster than it would in the summer. You might need to use a torch to provide preheat just to keep the interpass temperature from dropping too low.
Another mistake is using a dirty measurement surface. Grease, oil, or heavy mill scale can interfere with both crayons and electronic probes. Always use a stainless steel wire brush to clean a small spot for your measurement. This ensures you are getting the temperature of the actual metal, not the contaminants sitting on top of it.
Lastly, don’t rush the process. It is tempting to see a “close enough” reading and start the next arc. However, heat builds cumulatively. If you start a pass at 450°F and your limit is 500°F, you will almost certainly exceed that limit by the time you reach the end of the bead. Always give yourself a buffer zone of at least 50 degrees.
Advanced Techniques for Critical Projects
If you are working on something where safety is paramount—like a roll cage, a suspension component, or a pressurized vessel—you might want to use dual-temperature crayons. You use one crayon for your minimum preheat temperature and another for your maximum interpass temperature.
For example, you might use a 250°F crayon to ensure the metal is warm enough to prevent cracking, and a 500°F crayon to ensure it isn’t getting too hot. This creates a “go/no-go” window that is very easy to follow visually. It removes the guesswork and provides a physical record that you followed a proper welding procedure.
Another pro tip is to use chill bars. If you find that your interpass temperature is climbing too fast and you are spending more time waiting than welding, you can clamp large blocks of copper or aluminum near the joint. These act as heat sinks, drawing excess thermal energy out of the workpiece and allowing you to maintain a consistent temperature more easily.
Frequently Asked Questions About how to check interpass temperature in welding
What happens if I exceed the maximum interpass temperature?
If the metal gets too hot, the cooling rate slows down significantly. This leads to grain coarsening, which makes the metal more brittle and less resistant to impact. In certain alloys, it can also cause elements like chromium to precipitate out, leading to rust in stainless steel.
Can I use a cheap laser thermometer from the hardware store?
You can, but you must be aware of its limitations. Most cheap IR thermometers have a fixed emissivity of 0.95. This works well for dull, black iron but will be wildly inaccurate on shiny steel or aluminum. For better accuracy, paint a small black spot on the metal with a high-temp marker and measure that spot.
Is interpass temperature the same as preheat temperature?
Not exactly. Preheat is the temperature you bring the metal up to before the first arc is struck. Interpass temperature is the temperature maintained between subsequent passes. Often, the minimum interpass temperature is required to be equal to the preheat temperature.
How long should I wait between weld passes?
There is no set time, as it depends on the thickness of the metal and the ambient air temperature. The only way to know for sure is to use your measurement tools. Thinner materials cool faster, while thick plates can hold heat for a very long time.
Do I need to check temperature on single-pass welds?
No, interpass temperature only applies to multi-pass welds. However, you should still be mindful of your heat input to prevent warping or burning through thin materials.
Summary of Best Practices
Mastering the art of heat management will instantly elevate the quality of your shop projects. By taking the time to learn how to check interpass temperature in welding, you are ensuring that the internal chemistry of your metal remains exactly where it needs to be for maximum strength.
Remember the core steps: clean your measurement area, measure 1 inch away from the weld, and use the right tool for the material you are working with. Whether you prefer the simplicity of a Tempilstik or the high-tech feedback of a digital pyrometer, the goal is the same—consistency and safety.
Don’t view the cooling time as a chore. Use those minutes to clean your previous bead, inspect for defects, or plan your next move. A patient welder is a successful welder. Now, grab your gear, fire up the machine, and start building something that is built to last!
