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Up Topic Welding Industry / Inspection & Qualification / confusion for years
- - By mike li Date 06-06-2008 00:56
I did lots of this kind of welding , for example. I weld 1/2 inch thickness carbon steel 6 inch pressur pipe butt joint. on positioner 1G mig root 4 more passes mig cap. X-ray test required cant clean inside root pass slag because the pipe is too deep to catch. use one shot to finish one cap pass ,non-stop ,that means the start point does not clean slag throughly.welder in my company all did same way. but  test will pass .my confusion is 1 -the surface slag doea not affect the test why do I need to clean the slag interpass. 2-one shot to finish the round weld ,stop the weld at the start place why dont have slag inclusion. please tell me why. thank you.
Parent - By raptor34 (**) Date 06-06-2008 01:30
you being a welder should notice the slag when you come back to your start burns away, this slag does not get caught in the weld because it can escape outside the cap, if you are welding inside the bevel the slag cant escape and gets caught in the weld and you get slag inclusions.
Parent - - By swsweld (****) Date 06-06-2008 02:23
You shouldn't have slag inclusion with GMAW. Do you mean FCAW?
Parent - - By mike li Date 06-08-2008 01:57
I mean mig (GMAW) it has slag.could catch slag inclusion easily.
Parent - - By swsweld (****) Date 06-08-2008 03:49
I'm not trying to be argumentative but GMAW Gas Metal Arc Welding (same as MIG Metal Inert Gas a non standard term) has no flux. Therefore produces no slag.  Slag is a nonmetallic product resulting from the mutual dissolution of flux and nonmetallic impurities in some welding and brazing processes. AWS A3.0.
You may be dealing with silicon inclusion but you are not getting slag from GMAW process.  Maybe I'm misunderstanding your post.
Parent - - By jrw159 (*****) Date 06-08-2008 12:48
swsweld,
  While you are correct in the fact that GMAW does not produce "slag" in the way most think about it, it does produce a thin "film" that , if not removed, can lead to lack of fusion in multi pass welds.

I have failed, and seen others fail, GMAW groove weld tests because of improper interpass cleaning, or lack of interpass cleaning.

jrw159 :-)
Parent - - By 803056 (*****) Date 06-08-2008 13:51
I agree the islands of silica/silicates are not technically slag products, but act in a similar manner. Many welders assume the glassy islands will be melted by the next weld pass. That isn't always the case. While spray mode transfer is a high heat process, short circuiting transfer is not. The spray mode transfer is more forgiving of those welders too lazy to properly remove the oxides from the weld surface, short circuiting is not forgiving.

It is interesting to test welders using bend tests to evaluate welds made by GMAW short circuiting transfer. It is an eye opening experience to see the bend straps snap in half because the weld simple "cast" into the shape of the groove without fusing to the bevel face. I try to make it a point to have the welder present when the coupon is evaluated. Since I use a manually operated wrap-around testing machine, I usually have the welder provide the brawn to pull the handle. Their eyes tell the whole story when the sample snaps and there are two pieces left in the testing machine. A very valuable training aid and a must for anyone that trains welders. The broken straps usually reveal the problem as improper/incomplete cleaning or failure to properly manipulate the arc. Incomplete fusion, incomplete joint penetration, oxide inclusions, etc. are typically observed within the fracture.

I don't fail many welders, they do it themselves when they bend their own "straps". ;)

Best regards - Al 
Parent - - By jrw159 (*****) Date 06-08-2008 14:28
Al,
  That has been something I too have incorporated into testing with the GMAW process. I have even had them start the bend test, and right at the point that the weld starts to fail, I stop them and release the pessure, pull the sample out and have them break it by hand. That is generally enough to help them understand the importance of interpass cleaning. :-)

jrw159
Parent - - By mike li Date 06-10-2008 02:04
thank you all very much,may be when I stop the weld ,spending a little more time to fuse the start point totally is better. however I still thinking about :the surface slag doea not affect the test (because inside root pass cant clean, surface pass normally just brush it)why do I need to clean the slag interpass, for X- ray test.
thank you
MIke
Parent - - By 3.1 Inspector Date 06-10-2008 11:24
Do you ask why you have to clean the interpass slag and not the slag on the weld surface?
If that is what you mean, you should read the other posts more carefully....

The interpass slag can cause lack of fusion between the weld layers, where as slag on the surface.....well, its on the surface of the completed weld :)
Parent - - By eekpod (****) Date 06-10-2008 12:42
Al and jrw159
Is there ever any time when a short circuit weld will pass a bend test?
We don't use it here, so I don't deal with it.  I am familier with how it's not pre-qualified and such, I'm just curiuos, will it usually always fail a bend test.
Thanks Chris
Parent - By js55 (*****) Date 06-10-2008 13:26
Depends on the skill of the welder. There is nothing inherent in short circuit transfer that makes it prone to bend problems other than the arc energy being imposed on the puddle contributing to a lack of fusion. If welder skill provides good fusion then a short circuit transfer weld is every bit as ductile and strong as any other transfer, given minor variances due to the same thing, low arc energy.
A 70S-6 deposit is a 70S-6 deposit.
Parent - By jrw159 (*****) Date 06-10-2008 18:24
IMHO, js55 answered this quite well. I agree with his statement. FWIW :-)

jrw159
Parent - - By 803056 (*****) Date 06-15-2008 21:24
Yes, I have seen welders pass the bend tests using GMAW(S), myself included. Which just proves anyone can weld if they have proper instruction.

The "trick" is not to depend on the electrode melting and filling the groove and assuming the molten weld puddle has sufficient superheat to fuse to the bevel face(s). The welder has to limited the weave width and direct the electrode to the point where fusion is desired. In other words, most welders will oscillate the electrode from side to side, but they do not direct the arc directly toward the bevel face. The oscillation will produce a weld bead with the proper width and volume of weld, but the molten metal will not have sufficient superheat to fuse to the bevel face, instead, the groove acts like a metal mold and the weld metal simply casts to the shape of the groove. In extreme cases, the original groove face will still be evident when the bend samples fail.

I've taken fillet tests, 3/8 inch and 1 inch grooved plate tests and passed all three in multiple positions. The metal has to be ground to bright metal, i.e., no mill scale, and the proper manipulation must be used to ensure the molten metal fuses to the bevel face properly. Every pass has to be completely cleaned and wire brushed between passes.

This is where proper training is important to teach the welder the proper techniques.

Best regards - Al
Parent - By jwright650 (*****) Date 06-17-2008 11:59
I've tossed out five gallon buckets filled with bend straps that failed and welded with GMAW where the welders didn't take the silicon islands seriously......
Up Topic Welding Industry / Inspection & Qualification / confusion for years

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