Striking to shear tack weld - would this affect the integrity of the steel?

[kauri]

Our contractors have removed temporary positioning brackets that were tack-welded to steel reinforcement by striking them with a hammer. My thinking is this wouldn't affect the integrity of the steel in any way (I have already found the tack welds alone are an unfortunate defect). What do you think?

 

[kingnero]

Depends on the application. If there are non-critical surfaces on mild steel, I wouldn't think twice about it.
However, on high-strength steels like most rebar (yield = 500 MPa and elevated Carbon content), I would not allow it. Grinding smooth + PT/MT is an option, but that still doesn't tell you anything about the hardening that might have occured during cooling down after welding. But if the zone hasn't cracked after breaking the tack with a hammer (=> PT/MT will tell you this), it is not very likely it will crack afterwards in static situations.

 

[Agent666]

Based on previous thread stating you are in NZ (I think?), remember a tack weld in accordance with AS/NZS1554 needs to be min of 40mm long and deposited in exactly the same manner as the/a prequalified FW. If less length then the heat input might not be sufficient (cools to fast) unsure if this would have any impact on the base metal (probably but others can comment on this aspect). But it would impact on the weld strength (essentially you are outside the standard if not depositing a weld of at least 40mm length), but does this matter for a temporary weld presumably more for holding things in place, well I'm not sure but hopefully it gives you some things to think about. Might pay to discuss with Hera welding center or SCNZ if your company is a member.

In reality the length matters, but you don't see the minimum length applied very often in practice for temp tack welds. It's probably more critical if the tack weld is incorporated in a final weld somehow without being ground out or gouged out. EDIT or higher strength steels like previous poster noted.

If the breakout/tearout of the weld is into the base metal then the surface would need to be repaired. If not any remaining weld would need to be ground flush.

 

[Agent666]

Another similar thing, you wouldn't expect tack welded runoff or backing plates to be removed in any other way but being ground or back-gouged off the base metal. So why is this case different I have to ask myself.

 

[Agent666]

Take a look at section 5.9 of the welding standard, it's pretty clear there are some limitations involved for temporary attachments. Biggest being they are not allowed to tension flanges of beams and need magnetic partical examination to confirm soundness after removal!

 

Tacking is welding, and from a thermal cycle point of view it is worst-case.
(1) Tacking should follow a WPS, and preheat is even more important than for the subsequent welds (see above).
(2) Personnel doing tacking should be qualified.
(3) Grinding smooth + PT/MT should always be done to tacks and arc strikes (there's not much practical difference between them).
(4) Verify minimum wall thickness after performing step (3).

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[matschka]

The welding standards of my previous contractor, B.....l instructed the craft to avoid striking
directly a bracket that was tack welded as this method would pull out some base metal and then
this would have to be weld repaired and of course NDE'd. They instructed the craft to
grind a 'groove'or thin the tack weld by grinding first to weaken it. Then striking it with a ball pean hammer would remove it without pulling out base metal. My 1st. Contractor, 'Tank Builder'. that I worked for simply hammered off the bracket along with some base metal. The area
was circled with soap stone for weld repair.

 

[kauri]

Thankyou to everyone for the advice. To clarify it is in fact G500E, we stopped the tack welding as soon as we found out. Unfortunately it's probably past the stage of remedy, so we will consider not issuing a producer statement for this one.

 

[Agent666]

OK firstly I misread and you are actually welding reinforcing. Your reinforcing is useless now. Needs to be replaced unless it's welded correctly. And even then only microalloy bar can be welded, welding of QT is forbidden anyway. Basically it will fracture as high liklihood of hydrogen embrittlement if not following a welding procedure. I've seen it in the field where someone has hit grade 500E reinforcement with a hammer that has been tack welded and fractured it or even putting it down off the truck has caused some bars to fracture. A proper welding procedure limits the rate of cooling in the bars for g500E.

Do not accept any of this reinforcement in your structure

 

[kauri]

It's MA, we've had tensile tests performed on tack welds (but not hit by hammer, hence the question) samples show minor reduction in yield stress, greater reduction in elongation but above 10% (G500E classification). Failure occurs at the weld which is unaccepted in standard AS/NZS 1554.3. This issue has been picked up too late; at this point it may be impossible to remedy this, hence why I say, after reading everyone's comments, we will probably not issue a PS4 for this.

 

[Agent666]

Yeah I agree but it still needs to be remedied, even if it means tearing it down. The building isn't good for anything basically because of the contractors actions.