Thank you, this additional explanation is helpful...
Ken A. Nisly-Nagele
-----Original Message-----
From: Al [mailto:alwynk@shaw.ca]
Sent: Friday, December 17, 2004 5:12 PM
To: PipingDesign@yahoogroups.com
Subject: [PipingDesign] Prev Q GMAC Short Arc Welding Limitations
GMAC Short Arc Welding Limitations - couldnt find aes email
MOst processes have a relatively large puddle which self regulates the rate
of cooling.
the welding process variables are designed around optimizing this.
it was orginally developed for sheet metal to keep the heat input down , but
is widely liked for "out of position" welding because the puddle is reduced
and herefore more manageable.
you can see its achilles heel.
GMAW-S a unique ability to weld out of position, to weld thin base metals
and to weld open butt root passes without backing and without "blowing
through." The bad aspect of the short circuit process is that it can
cool/freeze more much quickly than continuous arc welding eg the normal
spray arc. Short circuit means the electric arc current is repeatedly
on/off.
Hence the quick freeze puddle has a tendency to "cold lap" and this is a tight "squished" defect thats really hard to see with Radiaography (sorry no tech definition for squished).
Because of the bad characteristic, ASME Section IX applied special variables when the GMAW-S process is used and many specs exclude it completely. The thickness limit is because of the quicker cooling. Not only is there a greater tendency for cold shorting or cold lap but also shrink cracking (and other problems)as the material gets thicker due to the higher delta temp and rate of change. The cooling rate is less mitigated but even thru heating with continuous processes.
Its quite difficult to determine if the process is actually short cicuiting,and how much . As the voltage goes up a little, the tansfer mechanism goes into more globular transfer and less Sht cct. Somebody elected a magic number of 3/4inch as a limit. This is not definitive because it really depends on the welder skill. IN developing the procedure and approval the PQr will determine if its making the grade. The NDt testing will determine subsequently if the welders are up to the mark.
al
-----Original Message-----
From: Paul Bowers [mailto:pbowers@pipingdesign.com]
Sent: Wednesday, March 03, 2004 5:16 PM
To: PipingDesign@yahoogroups.com
Subject: [PipingDesign] Good Ques; GMAC Short Arc Welding Limitations
>Why would short arc welding not be allowed in pipe material thickness
>greater than ¾ inch?
Normally the reason certain weld processes aren't permitted is based on weld
quality. I seem to recollect that short arc welds tend to be brittle. You
might want to check at the Lincoln <http://www.lincolnelectric.com/> web
site to see whether short arc is recommended for low temperatures or whether
there are guaranteed Charpy values for shrt arc welding. That'd be the
giveaway.
Why would short arc welding not be allowed in pipe material thickness greater than ¾ inch?
This is for a B31.3 code application which specifies that short arc welding is allowed on for full thickness butt joints and fillet welds in pressure parts if the thickness of either material at the joint does not exceed ¾ inch.
Ken A. Nisly-Nagele, P.E.
Project Engineer, Mechanical
Applied Engineering Services, Inc.
7999 Knue Road
Indianapolis, IN 46250
317-585-8920
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