The applicable welding standard should state where both preheat and interpass temperatures are measured. Some welding standards are more definitive than others. In some cases, the type of base metals being welded, etc. the interpass temperature may not be a major concern.
When the welding standard does not provide the necessary information, best judgement comes into play. I usually look to a welding standard or code that is recognized by industry for guidance. For me, that means I go to either AWS D1.1, D1.2, D1.6, or NAVSEA TP278 or 1688 for guidance.
When the information is not provided, I typically measure the preheat a distance equal to the thickness of the thicker base metal away from the anticipated toe of the weld, i.e. edge of the groove or toe of the fillet weld, but not less than 3 inches away from the toe of the weld. Interpass temperature is usually measured about one inch from the location where the next weld pass will be initiated just before initiating the weld. I do not measure where the weld was just terminated nor do I measure in the groove or on the weld because there is the potential for contaminating that location, especially a problem if temperature indicating crayons are used.
The welding code or standard prevails, if it is stated where the temperatures are to be measured, that is what I do. If there is a question as to where they are to be measured, I try to refer to some standard recognized by the industrial sector I am working with.
A short story on the subject of discussion. Back in the days when welding was just invented, I was welding on a moment connection in the field. As I terminated the weld the inspector ran up behind me, reached over my shoulder and rubbed his temperature indicating crayon (Tempil Stick to the novice) on the weld crater and pronounced the weld as "Rejected!"
"What the hxll are you doing?" I asked him.
"Checking your interpass temperature and you're way too high! So, I'm rejecting it!" he replied with a big grin.
"I guess I'm all done here!" I quipped. "Hey, boss, get my money, we're all done welding on this job!" I yelled to the foreman.
"What the hxll are you talking about Moore?" the foreman yelled back.
"This inspector says we can't weld on this steel." I said with a smirk.
"I didn't say that!" yelped the inspector. "I told Moore the interpass temperature was exceeded, so the weld is rejected!"
"Get the carbon arc ready. I'm going to have to cut out all the welds. This inspector caught me dead to rights! All these welds exceeded the interpass temperature so we better tell the engineer so we can get'em fixed. We'll probably have to glue'em together!" I yelled to the foreman.
Now you have to know the foreman and I had worked many jobs together and he was one of my mentors when it came to welding. His brother was professor of Metallurgy at Worcester Polytechnic Institute that answered our never ending stream of questions about welding metallurgy, so my good friend knew right away I was up to something and when along with my lead.
"Yep, we got caught with our pants around our ankles on this one!" he yelled back with a big grin on his face.
By now the inspector was getting a bit unnerved. What are these Ironworkers talking about? Why are they saying they are going to cut out all the welds? He didn't reject all the welds!
"I only rejected Moore's weld. He exceeded the interpass temperature, so it's rejected. I didn't say anything about the other welds!" he said.
To that I said, "How hot do you think these welds get?
"I don't know, but they can't get any hotter than the maximum interpass temperature!" he replied.
"That's what I'm driving at. The maximum temperature pass is well below the melting temperature of the steel. That must mean all the welds are rejected because all of them exceeded the interpass temperature!" I said.
By now my foreman was nearly ready to roll off the iron trying not to laugh.
"OK, what am I missing here?" asked the inspector.
"Interpass temperature is measured just before starting the next pass. It is a way of limiting the width of the heat affected zone and the problems associated with grain coarsening and loss of toughness. The weld has to be hotter than the melting point of the base metal inorder to get fusion. When the upper interpass temperature is exceeded, you simply wait until it cools off below the maximum interpass temperature listed on the WPS." I told him.
By now all the welder are laughing and the inspector didn't know if I was serious or just pulling his leg. Off he goes to the job office where he calls his office and gets the "low down" from his boss.
When he comes back he says, "You got me good on that one, but I'll catch you yet. One of these days you're going to fxxk up and its my job to catch you!"
"Good luck buddy, you're going to have to do some more reading before that will happen." I said with a grin.
Now you have to understand that this is the same inspector that I had previously told, "You can't do MT on a weld unless it's still red hot! The defects won't show up. See, the magnetic powder won't stick to the welds because they're already cooled off"
He got a real axx chewing when he told his boss none of the defects would show up because the welds were too cold.
It was great sport to screw with the inspectors that shouldn't have been on the job to begin with. What a turn of events, now I'm one of the enemy!
Best regards - Al
That is priceless Al!! LOL :-)
Now You know what You will have to put up with when You get Your CWI job.
Exactly!.....LOL...great story AL.....I'll have to make sure that I have my stuff in order before I come inspect behind you on the next job ;-)
<got my code book under my arm as I type this>....LOL
Al,
All I can say is, 'What goes around, Comes around'. You gave it, now you get it. Don't we all. Just stepped into a dandy myself. At least their pre-heat and interpass temps aren't the problem.
Nothing to add to some of the great responses already posted.
Have a Great Day, Brent
If I might throw a little fly in the ointment here. The farther away from the HAZ that you measure interpass temp the more meaningless it becomes. Maximum inteprass temp controls are in place for the weld and HAZ. This is not exclusively the case with mins especially related to hydrogen induced cracking where adequate heat in a distance away from the HAZ only enhances the evolution of hydrogen away from the weldment. But it all depends as well on the temperature gradient transverse to the weld. If there is 50 deg difference between the distance you have measured and HAZ, or the weld itself, just how important is this to you?
