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Up Topic Welding Industry / Technical Discussions / Preheat for Carbon arc gouging??
- - By nitrorocket1 Date 05-22-2008 14:51
Our shop does weld and crack repair on large forge dies anywhere from 2000 to 25000 lbs. The material is D6 Die steel that requires a minimun 450 preheat to weld using Cormet F25 & F35 flux core welding wires.
  Anyway, for crack repair we use carbon arc in any size from 1/4" to 3/4" copper clad rod. In the past preheat of the die was NOT done prior to air arcing, only before re-welding. I have enforced preheat now on all dies to be backgouged for crack repair. We now preheat to the same temp the die will be welded at( about 450 degrees.

Is there a standard or any data on this? I am being heavily challenged based on the fact the guys do not like backgouging while the die is hot for discomfort reasons. Is preheat required? Does it not matter at all? Should we or should'nt we preheat for backgouge??

Thanks for any input.
Parent - By pipewelder_1999 (****) Date 05-22-2008 15:14 Edited 05-22-2008 15:19
There are a few posts on the subject. Take a look at the information already listed. Though I don't agree that it is required for a sound gouge, John Wright I believe indicates that that a AWS document does suggest preheating when gouging material required to be preheated when welding. I too would like to see some evidence that this is necessary.

http://www.aws.org/cgi-bin/mwf/topic_show.pl?pid=98682;hl=gouging%20preheat

http://www.aws.org/cgi-bin/mwf/topic_show.pl?pid=75738;hl=gouging%20preheat

http://www.aws.org/cgi-bin/mwf/topic_show.pl?pid=5059;hl=gouging%20preheat

In the cases in which you DID not preheat before, did any cracks occur? If yes, then preheat, If NO- Why preheat ? Any localized hardened areas that are very thin and near the surface of the material will be subsequently remelted during welding and thus any residual stress removed.

I too would love to get a answer for this question. I have based all of my opinions on other information I have read ofver the years and am not an expert. I would like to see some "FACTS".

A simple "Gouge and PT/MT may answer all of your questions. There are also variables related to the size of the gouge, size of the rods, number of passes etc all that could affect the size of a hardened area left by arc gouging.

I would imagine if you are doing tool and die work you may have access to a hardness tester of some type. A hardness profile on the gouge befer welding after welding with and without preheat would be a pretty good source of information. The highly hardenable materials would be much more sensitive that carbon steels and HSLA materials I have worked with.

Gerald Austin
www.weldingdata.com
Parent - By OBEWAN (***) Date 05-22-2008 16:45
If you wind up doing it and have trouble with your welders, you might consider getting one of those fixtured gouging torches on the screw slide with a hand crank.  That way, they would not have to hold the torch over the hot material.
Parent - - By chall (***) Date 05-23-2008 14:03
AWS C5.3 is a Standard Guide For Arc Gouging. 

https://www.awspubs.com/product_info.php?products_id=154&language=en

It's been awhile since I looked through it, but I seem to remember it saying something to the effect that if you would have to preheat for a weld, you should preheat for the arc gouge. 

That is the practice we employ.

Charles
Parent - By pipewelder_1999 (****) Date 05-23-2008 14:50 Edited 05-23-2008 14:59
I recall your quote and agree that that statement exists regarding preheat in the practice guide you refer to. I just wonder if that is a general statement to cover all materials as opposed to going into details for various materials.

It is odd that in the case with D1.1 there are specific prohibitions on Oxygen gouging (4 paragraphs) yet no mention is made regarding preheat that I can find. I do not think the max time at temperature is reached but I am guessing. I have seen plates get hotter from grinding than from arc gouging. The thermal gradient may be different but the peak overall temperature of the base metal did not seem to get as high.

I need a lab of my own to play in.
Parent - - By nitrorocket1 Date 05-23-2008 19:13
O.k.... The material is called D6. It is a die steel designed for high impact forging.

I am trying to understand exactly why preheat is required or recommended for backgouge if the material should be heated for weld. Could this be because if the material is already crack prone and or very thick to require preheat in the first place, then it should be done for gouge on the basis that it cools slower thus minimizing embrittlement of the gouged surface from to fast cooling or quenching??
Parent - By aevald (*****) Date 05-23-2008 19:51
Hello nitrorocket1, I believe you may have somewhat answered your question on your own. Temperature differential may further explain the stresses and possibility of additional cracking. If you have a cold casting, plate, or weldment that has a fair amount of mass and you apply heat to it, whether that heat be from a weld bead or the removal of metal you can still introduce stresses. With weld beads I believe it would be a compounded stress, the fast cooling weld metal will introduce stress from the cooling associated with it's cross section and additional stresses will be caused from expansion and contraction of the weld metal boundaries as they cool rather quickly due to normalizing in temperature caused by the surrounding mass. In the case of removal of metal with a carbon arc gouging process the stresses will be introduced as the surfaces that are exposed by the carbon arc process will normalize and likely harden up because of the rapid return to the starting temperature of the part. In a sense you could probably compare an arc strike to the type of condition left by the exposed excavation from carbon arc use. So by preheating before using the carbon arc process there would be less likelihood of surface stresses and hardening in the area of metal removal. A few additional thoughts for your consideration. Best regards, aevald
Up Topic Welding Industry / Technical Discussions / Preheat for Carbon arc gouging??

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