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> Sub arc root pass cracking right up the middle.
TonyMc
post Dec 1 2016, 02:19 AM
Joined: 23-Feb 10



In work we have 2 sub arcs having the same problem on the same shells. These shells are 516-70, 6ft long and 40" dia. The thickness is 1 1/4". I don't recall which wire and flux we're using but it's bog standard every day Lincoln consumables that we use on a regular basis. The prep is a double bevel with a 1/4" landing halfway.

Our problem is with the root pass on the long seam, it's cracking right up the middle everytime leaving is with a big deep back gouge on the second side to dig it out. The root pass is on the inside of the shell and we've been preheating too. We've used 1/8" and 5/32", same results with both.

Anybody have an idea of what's causing these cracks?

Thanks.
Ballbearing
post Dec 1 2016, 11:04 AM
Joined: 9-Dec 07



Tony,
A quick guess would be preheat.
What temp ?
If they are 6 ft runs are you preheating the whole joint and then starting to weld or are you preheating the whole joint and then following the SAW welder with the preheat torch ?
Centre-line cracking is usually due to high restraint or not enough preheat or a combination of both,
Hope that helps,
Cheers,
BB
TonyMc
post Dec 1 2016, 02:34 PM
Joined: 23-Feb 10



Hi B.B

We preheat the entire length and it's vicinity of the seam prior to welding. We've tried various temperatures too. Same results.

Technic Al
post Dec 1 2016, 05:31 PM
Joined: 14-Oct 03



When is it cracking?
You say its Centre Line cracking. Is it a straight line or jagged?
Can you estimate how big your weld is
Do you know the welding parameters Amps Volts Speed

PS knowing the Consumables might help....can you find out the names
TonyMc
post Dec 2 2016, 12:42 AM
Joined: 23-Feb 10



Thanks for showing some interest Al. I'll post the parameters etc tomorrow morning.

The "crack" or split maybe a better term is a straight line right up the middle of the root pass. We've tried a deep penetration root as well as one with higher deposition.

There's no strong backs or tie plates across the seam. It's only tacked on the opposite side. I'm beginning to think that there's a lot of stress from rolling in the shell.
Technic Al
post Dec 2 2016, 09:49 AM
Joined: 14-Oct 03



Its difficult to say anything without seeing it. Just a few comments / pointers

Straight lines are more often lack of fusion rather than a crack (although sub arc doesnt usually suffer from this)

The bigger the weld the longer it takes to freeze in the centre so the forces could be pulling apart the soft / molten centre

It could be plate chemistry related. I doubt if its the consumables unless its completely the wrong choice. High impurities coupled with high heat input (sub arc) arent a good combination. Do you know the origin of the plate.

From the plate spec and the measurements you quote...are you in the USA?
TonyMc
post Dec 2 2016, 07:38 PM
Joined: 23-Feb 10



L 56 wire and 882 flux. Lincoln brand.

5/32" wire 31 - 33 V, 550 - 600 A 22"min travel.

1/8" wire 29 - 30 V, 450 - 480 A 18"min travel.

These are the parameters off our weld procedures, the ones that cracked or split!!

I'm based in Toronto, Ontario. Originally hailing from Glasgow. It's supposed to be metric here in Canada but the American influence is unmissable, hence the imperial dimensions.
Technic Al
post Dec 2 2016, 08:01 PM
Joined: 14-Oct 03



Maybe a few more volts would spread the weld and avoid a peaky shape. But I doubt if that alone would make it crack.

Maybe also drop the current a bit or increase the travel speed

You need to avoid a bead shape thats more than 1.5 x higher than its wide

The consumables look right so they shouldnt be the problem.

As I said before its difficult to say without seeing it.

Would it be possible to try one using a MIG root?

Have you got an actual analysis of the plate

lacka fusion
post Dec 4 2016, 12:55 PM
Joined: 22-Jan 10



why not try a single bevel on both plates with a 5 mm nose on each plate. then a good high juice on the 2nd side.
fullpen123
post Jan 1 2017, 03:27 PM
Joined: 13-Nov 10



Hi Tony MC,
What is the set up ? Only experienced centre line cracking to this extent when joint was over restrained. If shell has been tack welded to a fixture, this will cause centre line cracking. Joint needs to be allowed to expand and contract without restraint
Boilerbuster
post Jan 1 2017, 04:40 PM
Joined: 23-Aug 08



We only ever had a few bracings on the inside of the shells to help keep them circular when doing the sub Arc.
TonyMc
post Jan 5 2017, 12:58 AM
Joined: 23-Feb 10



Finished the job today. No joy resolving the root cracking issue. We tried everything from playing with parameters, rooting outside first rather than inside, laying a mig pass first, using strong backs and so on... we even sent a shell out for stress relief, before welding, just to see if it solved the issue. The MTR for the plate was good too. We had that checked out. We ended up just living with the cracks, filling and capping then gouging the hell out of the second side until we found sound metal. Seams were full RT and we got good results. Still annoying though that we never solved the problem.

I had a feeling it was to do with 50/50 joint prep. On thick plate, in the past, I've found that you will get better results with offset bevels, 2/3 1/3 for example.

Anyway, all of your inputs and suggestions were much appreciated.

Cheers.
everton
post Jan 5 2017, 03:43 PM
Joined: 26-Mar 09



Why don't you try a stick root which will cool more slowly and won't crack?
boilermaker
post Jan 12 2017, 11:55 PM
Joined: 3-Jul 07



My guess would be the Depth to Width ratio of the root pass.
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