backgouge preheat

Date 12-17-2014 15:18

Mwilson,

I see some common kindnesses were missed on your first visit here:

WELCOME TO THE AWS WELDING FORUM!!

Now, please check out this thread for our answer recently to the same question:

https://www.aws.org/cgi-bin/mwf/topic_show.pl?pid=263577;hl=preheat

I believe that will help you without repeating all the same material.

He Is In Control, Have a Great Day, Brent

By mwilson230 Date 12-18-2014 03:37

Thanks for the link welderbrent,
and yes it was almost an identical question approx. a month ago. The answer seems to be it is not required unless job or contract specific as the only reference in AWS D1.1 is for access holes and/or beam copes and that is 150 degrees minimum for above 2". So the code itself does not require preheat for air carbon arc backgouging on any thickness. This is how I interpreted this.
MW

By welderbrent (*****)

Date 12-18-2014 13:00

That is the way most of us would also interpret it.

Brent

By ssbn727 (*****)

Date 12-19-2014 00:02

Just because the code doesn't have anything to say about whether or not to preheat prior to backgouging doesn't mean that you don't need to especially with certain steel compositions...
You have every right to disregard the use of preheat for backgouging if it's not required according to the code and you may think that it's also not necessary to preheat the second side of the joint which would only be true if the preheat was maintained and the metal wasn't left to cool down to ambient temperature... But according to your description, you did just that which means that once you strike an arc with the CAC torch @ whatever amperage setting chosen on to the metal @ ambient temperature you introduce heat input that is more concentrated than when you first welded the opposite side where you did preheat the metal to 225 degrees f prior to welding...

So don't you think that if preheat was required prior to begin welding on the first side in order to avoid HIC or HAC (Hydrogen Induced or Assisted Cracking) and other problems besides HIC or HAC... I'll mention it again...Remember the purpose of preheat:

Okay, we already know that preheat will reduce the risk of hydrogen cracking...Preheat also reduces the hardness of the weld heat affected zone which means that part of the heat affected zone on the other side will be exposed to sudden thermal shock from backgouging without preheat... So it makes common sense to used preheat on the side to be backgouged in order to lower the possibility of producing problems where the heat affected zone is that would extend slightly over to the opposite side of the joint which was already welded - near or just @ the root of the groove on the opposite side of the weld, of increasing the hardness of the root or in the worst case scenario, causing embrittlement or even worse which would be hydrogen embrittlement...

And then there's the shrinkage stresses that can accumulate internally after welding the one side and then the residual stress distribution to consider also... By preheating the side to be gouged, the reduction of those factors I just mentioned regarding what preheat reduces and improves on will occur... If you don't you'll be vulnerable to the possibilities that preheating will help you avoid... But heck, who am I to tell you how to do your job... I mean you have welded many double bevel groove joints on 3" thick steel before and didn't have any problems afterwards right? And I only backgouged around a couple of miles worth of metal during my career so what do I know... :roll:

So I guess TWI has it all wrong eh?

http://www.twi.ir/wp-content/uploads/2012/04/Job-knowledge-8.pdf

And in this Arc-Air .pdf brochure is incorrect also eh?

http://training.victortechnologies.com/index.php?p=asset&asset_id=84

And KEY to METALS has it all wrong also...

http://www.keytometals.com/Articles/Art130.htm

In any event, please let us know how you made out after completion and inspection... Oh I almost forgot - Best of luck to you! :grin:

Respectfully,
Henry

By mwilson230 Date 12-19-2014 14:16

Henry,
I appreciate your detailed lecturing on the importance of preheat, and the problems that can occur without it, you certainly have knowledge. I feel you are being a bit defensive, why I am not sure. I just wanted to get some other opinions ON THE AWS D1.1 CODE INTERPRETATION of backgouging preheating requirements, which I believe I did. Perhaps AWS should include this in the code so there would not be any questioning on this item, and therefore I would not have posted the question on the forum to begin with. As you said it makes common sense to preheat the other side, is not a code interpretation, its left to common sense. If a fabricator or erector doesn't want to preheat the back side in A572 gr. 50 with a E71T-1C-H8 electrode and the code doesn't require it, and the WPS only shows minimum preheat and interpass requirements, then how can a inspector enforce this?
Regards,
MW

By welderbrent (*****)

Date 12-19-2014 15:39

I'm not answering for Henry, he is more than capable.

In my opinion, the inspector can't ENFORCE that. We can't MAKE a fabricator or erector go beyond the code. But, we can have a talk with lead people and try to explain the nature of the D1 code as being a MINIMUM standard of requirements.

Then, using items as Henry has sited with references to back up the position one can make his case for REQUESTING and SUGGESTING that the contractor go above and beyond. But we can't enforce. Usually not even wise to write a report that says the fabricator did not do something that is not mandated by code. But, we can say in a report what they did that went above and beyond to let the engineer know they have found a contractor sincerely interested in quality and safety of the final product.

Example: (not meaning to toot my own horn) If you read my example in IT of my personal research on arc strikes, I use that often and find some real co-operation with many welders and their lead personnel and QC staff for handling arc strikes more carefully and at the very least getting them ground down as the code mandates instead of leaving them and saying 'It's only and arc strike'. I recently encountered an inspector who became a QA inspector after having been in a shop for years. During a discussion one evening on swing shift he read my article and said 'I know this article, I used it in training sessions at the shop I used to be at.' He just hadn't put the name together and realized I had written it, but the point was that people are using it and it does make a difference when people can see that going above and beyond has benefits so that after I have the knowledge that the way I used to do it is really questionable and that there is a better way, those who truly care, go the extra mile and sleep better at night.

Just my two tin pennies worth.

He Is In Control, Have a Great Day, Brent

By mwilson230 Date 12-20-2014 01:44

Welderbrent,

Thanks for the response, I concur we cannot (inspectors) make a fabricator or erector go beyond the code. I also concur that the code is minimum requirements, there are far too many instances that production dictates the erectors doing the absolute minimum or cutting corners such as minimum preheat and interpass temps.

So I received the input I was looking for and am thankful for your and Henry's responses.

The bottom line is it is not a minimum code requirement, but it would be a good idea to implement the minimum preheat when using the example I gave. I agree totally with you and Henry.

MW

By TimGary (****)

Date 12-20-2014 14:34

Thus the need for well thought out Contract Specifications, to spell out requirements over and above referenced Code minimums, to be agreed on or taken exception to by the Contractor, before the works begins.

Tim

By welderbrent (*****)

Date 12-19-2014 15:45

Oh, again, not defending Henry, BUT:

Henry does get passionate about quality, safety, and correct ways of doing things. Remember also that it is hard to really know the intention of words on the internet. Sometimes we think someone is ...attacking if you will, when they are not.

I think you see his passion here more than defensiveness. He just wants to make sure you have all the available info and references and understand where the science really takes you on this matter.

Brent

By ssbn727 (*****)

Date 12-18-2014 00:45 Edited 12-18-2014 01:55

Weldcome mwilson230,

"Say its 3" thick and one side was welded the previous day, and cooled to ambient temperature, would you need to preheat to 225 before backgouging. Obviously you would before welding the other side but it appears to be unclear per D1.1."

When I read this, I had to ask this question: If the side that was already welded and 225 deg, F was the preheat temperature required, then the same preheat temp is to be used for the other side that needs to be backgouged... AWS D1.1 does address min and max preheat temps for some specific grades of steel in table 3.2 of AWS D1.1 2010.... Granted, there are less steel grades that do require minimum preheat temps, and yet there are also at least 15 grades or more that do require minimum preheat temps, and it depends on the thickness, the welding process and of course the grade of steel...

So my next question would be this... what grade of steel are you referring to and which welding process are you using? by answering those 2 questions, you can go to Table 3.2 and figure out what the preheat temp is for the steel in question... Who knows? If the steel grade you're referring to doesn't have a required min and max preheat temp then I would agree with what you posted but, if the grade does require it, then refer to Tables 3.1 & 3.2 in AWS D1.1 2010 (page 69 & 70). Btw, is there anything in the contract docs that shows min and max preheat temps?

Preheat and interpass temperatures lower than required per Table 3.2 or calculated per Annex I may be used provided they are approved by the EOR and qualified by WPS testing.
One can also look @ 4.8.4 - Pre heat and Interpass Temperature. and then look @ Annex I, I1 to explain the purpose, tables I1 & I2, Commentary Annex I,

Respectfully,
Henry

By mwilson230 Date 12-18-2014 03:27

Henry,
I should have been more clear. It would be a group 1 or group 2 material and yes 225 would be the minimum preheat required prior to welding and for interpass temp., but I am only questioning the requirement of preheat for backgouging not for welding.
MW

By ssbn727 (*****)

Date 12-18-2014 08:21 Edited 12-18-2014 20:49

I realize that you were talking about backgouging and not welding MW...

Consider this... As you know, Carbon arc gouging is a concentrated brutal fast heating of metal that is blown out from the metal with pressurized air to form a groove where the metal used to be... The heat from the CAC is the same or more concentrated than the heat from welding... The sudden heating and cooling caused by the gouging is sure to cause some surface micro cracks if the metal isn't preheated to the same temperature used to preheat for welding on the other side with 3 inch thick steel and then left to rapidly cooling back down to ambient temperature... The preheating and flame cutting or gouging are parts of the preparation for welding but can be considered part of the welding operation... When flame cutting or gouging is required, preheating should be the same as when welding... It might not be quite as important since stresses are much smaller; however, the thermal shock on the metal can occur in gouging as well as in welding...

Then there's the possibility of moisture in either the CAC electrodes if they're not properly stored, and the possibility that the air itself being supplied to the torch not being properly filtered for moisture also can be a source to introduce Hydrogen into the gouged groove... A lot of variables to consider...

Now you only mentioned that the steel is a group 1 and group 2 grade which narrows the particular grade to only a few grades and since you don't want to be specific that's your perogative... So could you tell us if you're using a low hydrogen type of filler metal to weld with and if you are, what is the diffusable Hydrogen rate? Not that it really matters because what I'm trying to get you to understand is that you're asking for trouble if you don't use the same preheat temp that was used to weld the joint...

Sure the requirement to preheat when backgouging isn't listed in AWS D1.1 because it makes common sense to use preheat on both sides even to backgouge on the side opposite to the side where most of the weld deposit for the joint is located... And that's only provided that the person gouging doesn't find any porosity or trapped slag or even the possibility of voids caused by lack of fusion because if there is any of that appearing during the backgouging process, more metal would have to be removed... Meaning more heat input for an extended period of time will be required in order to gouge all of the crap that could be found... There's also the possibility that the gouging will be uneventful and for a much shorter period of time at temperature but nobody really knows until the gouging is performed...

Finally, when you mentioned: "one side was welded the previous day, and cooled to ambient temperature, would you need to preheat to 225 before backgouging"you do realize that in order to properly diffuse hydrogen from welded metal that has been left to cool down to ambient temperature requires that at least 48 hours waiting for adequate H diffusion to occur before NDT... This is especially true with Q&T steels... The sudden heat input could effect the hardness of the weld HAZ and possibly the weld also... If you weld one side, then the following day backgouge the opposite side before all of the H is diffused from the weld metal you might cause more work to complete or repair the joint than what you bargained for... A hydrogen crack can take anything from a few hours to 24 hours to occur. After 24 hours cracking is still possible but less likely, although there have been some reported cases of cracking at 72 hours...

Remember the purpose of preheat: Reduce the risk of hydrogen cracking, Reduce the hardness of the weld heat affected zone, Reduce shrinkage stresses during cooling and improve the distribution of residual stresses... If preheat is locally applied it must extend to at least 3"/75mm from the weld location and be preferably measured on the opposite face to the one being welded...

So, this is just my opinion but there are many who would also be better to be safe than sorry afterwards... Because the lack of preheating, the opposite side to be back gouged wasn't performed properly resulting in cracking and/or excessive hardness requiring a repair procedure to be followed in order to fix something that could have easily been avoided and then costing significantly more to complete the joint than originally anticipated... Not a good position to be in for sure. :eek:

Respectfully,
Henry

By welderbrent (*****)

Date 12-19-2014 00:31

And just for info, I included a reference to Henry's responses here, and agree with his reasoning, on the thread the link takes you to when Shawn asked an additional question about this over there.

Brent