Agreed, once the contract is signed, the conditions are locked.
However, keep in mind that if the contractor's work is clearly below standard and it is suspected the contractor isn't complying with the code requirements, the Owner can request additional NDT. If it is demonstrated the work is substandard, it would not be easy to make the Owner pay for the NDT and subsequent rework. Remember who is holding the golden check. The Owner can withhold final payment until such time as he is satisfied the work is done properly.
The contractor can file against the Owner, but court costs can easily exceed the monies owed. If the number of welds found to be defective is substantial , it is doubtful the contractor would prevail in court.
I recently had such a case. The Engineer requested a visual inspection and UT of all moment connection after the structure was erected. Every connection was rejected visually or by UT. The contractor would have a difficult time justifying a 100% reject rate in court. I can predict the contractor's defense; "But judge, I would have done a better job if I know it was going to be inspected!"
Attached is a photograph of just one of the connections visually rejected. Not only was the top flange offset by half the thickness of the flange, but the entire beam was too low because the web stiffener was installed upside down. The bottom flange overlapped the column cap by half the thickness of the beam flange (about 1/4 inch).
Steel fabrication and erection isn't rocket science. If something doesn't fit properly, fix it. If it is the fabricator's mistake, back charge the fabricator for the cost of the repair. if it is the erector's fault, suck it up little buckaroo and fix it. It shouldn't be a case of "All is fair if no one is watching!"
Best regards - Al