Good points about the IR thermometer. They are calibrated for black body emissions. They are fairly accurate if you are checking carbon steel that still has mill scale or "rust" on the surface, but on materials such as aluminum, stainless steel, or nickel alloys, they don't perform as well.
I was working with a nickel alloy and checked it with both the IRT and a thermal couple, there was about a 175 degree difference in the temperatures recorded by the two methods.
I usually use the thermal couple because it leaves no residue and thus, no contamination. Overall, the thermal couple is less expensive than the Tempil sticks and the crayons tend to break down over time.
Different materials require different preheat, if any. It is best to consult the appropriate welding standard to make sure you don't preheat something that doesn't require preheat or something that shouldn't be preheated.
You are absolutely correct about the failure of some welders to understand why we preheat carbon steels. I would place a sizable bet on the line that these are the same individuals that still believe the "sweat" is moisture entrapped within the metal and that preheat is needed to "dry" the steel before welding. They further believe that once the steel is dried, it is fine to allow it to cool before welding it. I've made good money over the years repairing their mistakes when I still earned my living with a welding hood and still make money witnessing the repairs as an inspector.
I've said it before and it still is true, the contractor's (or welder's) ignorance is the basis of my profit.
The Navy is on the right track with their requirement that the welders be trained and tested on the workmanship requirements of the fabrication standards before being turned loose on a project. It's like the old advertisement for oil filters, "Pay me now, or pay me later." Thorough training, while expensive, is less expensive than the repair of rejected work.
More and more fabricators are starting to see the validity and need for welder training. It isn't enough that the welder can deposit a "sound weld" any longer. They need to understand the reason "why" they are required to do things in a certain way. The number of training classes I teach is on the rise and the fabricators wouldn't ask me back if it didn't make a difference in their "bottom line" at the end of the year.
The training offered by people like Lawrence and Henry can not be appreciated until their students go to work for different companies and they take on leadership roles. "Education pays" is an understatement.
Best regards - Al
I agree with Al as far as where to check pre-heat and it should be spelled out in the WPS. 250 is a good rule of thumb for carbon, but I always make my guys use a temp stick nothing else. I had a guy one time came in with a little infra-red temp gun he bought at Auto Zone for 30 bucks. The AI on the job measured a cup of coffee right out of the microwave at two different distances about 4" away and about 2' away. Over 75 degrees difference in the readings, the moral of this story is a temp stick in most cases is more reliable of course. He told me the really good infra-red guns are accurate, but only if they are used correctly etc. I'll stick with temp sticks, the biggest problem is the guy who preheats than runs a couple of beads, goes out for a smoke and takes a bathroom break, then BS's with Joe for 10 minutes on the way back. Of course he doesn't stop to preheat again before he starts welding. . . . . . .
