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Re: welding nuts follow-up

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            Steve Gordin said in part, “My feeling is that the heat of such tack welding should not be able to affect the properties of a relatively massive nut.”



                        Even a tack weld melts the base metal. Just because the rest of the nut is does not become molten is immaterial. The properties of the nut have changed. The metallurgical notch that may have been created can propagate into the rest of the nut.

                        A tack weld is the worst kind of heat input into a piece of metal. It is more akin to an arc strike than a weld. It heats a small section of the metal then that volume cools as rapidly as it can, because it is the only part of the metal that was heated. This rapid heating and cooling can cause cracking. The reason for preweld and postweld heat treating is that they help the metal maintain a consistent crystal lattice, with less interruption by the molten weld or even the changed properties of the heat-affected zone. A tack weld has the opposite effect. And that is the reason that tack welds are to be incorporated into a qualified weld.

                        I have not gone back through the entire thread to see the main thrust of your posts, but the statement above got my attention this morning.

            Jim Getaz

            Precast Concrete Engineer

            Winchester, Virginia