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Measuring interpass temperatures for arc welding

Q: What is your preference when it comes to measuring weld preheat and interpass temperatures? Right now I am using seven different temperature-indicating crayons with various ratings. Is that enough to get the job done?

The infrared pyrometer that I have is unreliable at best. If anything, I'm intrigued by the digital contact pyrometers out on the market.

As always, thanks for your insight to age-old questions for us newbies!

Preheat and Interpass Temperatures Defined

A: First, let’s define preheat temperature and interpass temperature. A preheat temperature is the minimum temperature the base material needs to be before any welding takes place. Interpass temperature is whatever the base material’s temperature is before the next pass is laid.

The thing about temperature-indicating crayons is that they are considered the “accepted” method, especially when dealing with governmental agencies like the Army Corps of Engineers and the Department of Transportation, just to name a couple.

Temperature-indicating crayons come in a range of temperature ratings. A typical structural steel welder or weld inspector should keep crayons rated at 150 F, 225 F, 300 F, 400 F, and 450 F around at all times. (It's hard to keep the 32 F, 50 F, and 70 F around, even in my neck of the woods!)

I use a rosebud torch to heat my weldment to the proper preheat temperature and strike the appropriate crayon onto the surface. If it melts, I know that surface temperature exceeds the crayon’s rating. If it does not melt, I know the surface has not yet reached the temperature rating of the crayon.

Keep in mind that when we check preheat or interpass temperature, we don't check it at the weld face; we check it a minimum of 3 in. outside the weld joint in all directions.

Honestly? I use my infrared gun to determine which crayon to use. It is also a great tool to use when monitoring a construction site. I can check to ensure interpass is being maintained from a considerable distance from the moment connections, typically from the floor below, as they're being welded.

If I can prove to myself that my infrared gun is accurate—and oftentimes that depends on the surface condition—I’ll rely on its readings. But if I’m the least bit concerned about its accuracy, I use the crayons.

How to Write a PQR

Here’s how I’ll write a PQR when using the crayons:

  • Pass 1: Greater than 125 / Less than 175
  • Pass 2: Greater than 175 / Less than 225
  • Pass 3: Greater than 225 / Less than 300
  • Pass 4: Greater than 300 / Less than 500

It’s kind of messy, but I don’t know a better way to do it and it’s never been questioned. In fact, when they see the ranges, they almost automatically know I used a temperature-indicating crayon and don’t question it.

I used to have a contact pyrometer and spent a lot of money to keep it calibrated. After going into business for myself, I simply wrote a calibration procedure that stated, “Place probe in ice water (50 percent crushed ice/50 percent water) and let it stand for 10 minutes. Ensure that the temperature reads 32 degrees F, plus or minus 5 degrees.”

I hope this helps.

About the Author
Braun Intertec

Paul Cameron

Braun Intertec

4210 Highway 14 East

Rochester, MN 55904