formula to help us quote a job that has 500°F max interpass

Date 06-16-2009 19:33

It depends on a lot of factors John. The heat input is going to influence the cooling rates because the larger the electrode, the higher the amperage, the higher the heat input. Then there is the issue of the base metal thickness (mass) and the coefficient of thermal conduction into the adjacent base metal. I assume these are moment connections that will form T-joints, so the thermal cooling will be by conduction in three/four directions. The ambient temperature will also influence the situation because the temperature will ultimately dissipate into the air by conduction, but for most part the temperature variation from morning to the afternoon will be less than 40 degrees. In the grand scheme of things, that isn't too great a difference. A larger influence may be the wind. Windy days will cool the joint quicker than calm days.

I would weld a mock-up in your shop. Use pieces large enough to replicate the actual conditions to get a realistic idea of how the heat will dissipate into the surrounding steel. Base your estimates on the shop conditions since they will be representative of the worst case.

I would be surprised if the upper interpass temperature is a serious concern on the thicker materials. There will be cooling periods between each pass as the welder chips and cleans between each weld bead.

I would question the maximum interpass temperature restriction if there is no notch toughness requirement and the structure is a frame for a building. Who imposed the IPT limitations and why? Bridge structures are a different matter and your PH and IPT would be controlled by your qualified WPS. For a typical structural frame using materials typically used, an interpass temperature as low as 500 degrees doesn't make a lot of sense. Were toughness an issue, you would have to qualify the WPS anyway.

Best regards - Al

By jwright650 (*****)

Date 06-17-2009 11:18

Al,
I was thinking along the same lines as you mentioned with all of those factors making it a difficult guess at best. These are seismic connections in a very seismic active area and I suppose the EOR is just covering his bases, just in case....but man the erector has hundreds of them in this building, so I understand his concern about having an accurate estimate.

I told the erector the same thing about mocking up a few joints of different thickness and widths to get some sort of average just so he'd have a number to throw at his estimate.

Thanks for the reply, you confirmed all of my suspicions regarding this. I also asked the erector to ask the EOR if he can use compressed air between passes to help control the heat if he sees the interpass temps creeping up close on the welder so that he can continue to weld....I don't know if he even bothered to ask the EOR that question or if the EOR has given him an answer about that yet.

By Shane Feder (****)

Date 06-17-2009 11:55

Hi John / Al,

John,
I am a little bit confused about this statement.

"I know to use the tempsticks in 50° increments to let the welder know that he's sneaking up on the max temp without crossing over. "

What does 50 degree increments mean ?
The maximum interpass temp is a fixed temp that is measured before the next pass is applied. It is either above 500F/260C (In which case you will have to let it cool) or it is below 500F/260C (In which case the weld can be continued immediately) One 500F temp crayon would be all you would need.
The most critical thing which you haven't mentioned is the length of the welds (passes).
A long run/weld and the beginning of your weld will be much cooler than if it was a short run/weld.If the pass is only 2" long it will be much harder to comply than if it was 12" long.
I have seen a large amount of WPSs that have a maximum interpass of 250C for plain structural steel so I don't think 500F/260C is going to be that hard to achieve.
The other thing to consider is if there are a lot of joints in the one area just put 1 pass on one joint, another pass on another joint and so on and there will be no problem at all with exceeding the max IPT.
Hope that helps,
Cheers,
Shane

By jwright650 (*****)

Date 06-17-2009 13:08

Hi Shane,
My comment about using the tempilsticks in 50°F increments was to use a 400, 450, 500 or some such scheme.....mark with all 3 at 3" from the joint and as the temp increases the marks start to melt, so you can "see" that the temp is climbing towards the 500° mark.....that's all that I intended there.

450, 475, 500 might be better choices...IDK....but you get the idea.

By Shane Feder (****)

Date 06-17-2009 13:58

John,
I think the statement in D1.1 is very misleading (unless I am missing something major)
As js55 explained in another similar post the reason for maximum IPT is to control the mechanical / metallurgical properties of the weld and HAZ. The main reason for the preheat is to assist in slowing the cooling process in the surrounding area of the weld (hence the requirement for preheat 3" from the weld).
IPT for most codes (except for D1.1 ) is measured on the weld or a maximum 1" from the weld.
If your welders are checking for 500 F 3" from the weld before they put the next pass in you can virtually guarantee that the temperature 1" from the weld is over 500 F and on the weld is well and truly over 500 F.
IMHO the reason the D1.1 committee have grouped minimum preheat and minimum IPT together is so you will not be getting IPT less than the minimum preheat temps.
Your thoughts ?
Regards,
Shane

By jwright650 (*****)

Date 06-17-2009 15:59

Never really put much thought into the rational of measuring from 3" from the joint...just took it as something that AWS has historical data to prove that it is good enough for them, otherwise they would have spelled out a different approach for measuring the IPT vs min. preheat. I understand that if the steel is at 500°F at 3" away, that surely the temp within the joint will be much higher...but then as AL stated in his story in another thread that the weld has to be at a temp so that it melts and fuses with the surrounding steel and to accomplish that the weld metal has to be heated by the arc way above 500°F.

By CHGuilford (****)

Date 06-17-2009 16:31

John,
I understand your concern about exceeding the 500 degrees - but I'm wondering if you will actually get close to max in actual production.

As an example, our latest PQR plate was 1" thick by 15" long, FCAW-G, at 260 amps & 35 KJ/inch . Preheat was 140F; interpass climbed to 290 by pass #5; the peak was 360F when it was about 75% welded out. Cap passes were at about 300F. We did not wait for any cooling, but we did grind a bit where needed- but no delays over 5 minutes.

A vertical PQR 1" x 30" ran 225 amps, 49 KJ.inch, 13 passes, and never exceeded 320F. Records on some 25 PQRs show about the same - in fact we usually have to make a special effort to get the interpass to go higher when needed.

So unless you have a situation where they will poiund in the wire, I wouldn't think you'll hit 500F.

By 803056

Date 06-17-2009 20:31

Hello John;

I can see where the EOR would want to control the interpass temperature for the application you mentioned. Toughness counts!

I would hesitate to use compressed air to hasten interpass cooling. There is a couple of problems to think about, the primary one being that the compressed air is going to have water and oil entrained in it. There will be a lot of condensate in the air if the job conditions are anything like the one's I used to seeing. The compressor will be an old clunker that allows oil to pass freely from the sump into the air stream. That's great if you are trying to keep the "air-guns, yo-yos, and needle guns" well lubed, but not so good for the welds.

With a little planning the welder can work both the top and bottom flanges so one is cooling while he is welding the other.

Best regards - Al