There are two types of nuts to consider that are commonly used in construction applications, structural and non-structural. Structural nuts, such as A563 Grade DH or A194 Grade 2H, gain their strength through the heat treating process and, in general, should not be welded. Non-structural nuts are not heat treated, but are often manufactured from steel that is not weldable due to its high carbon content. This is typical of nuts larger than 1” diameter. For non-structural nuts, such as A563 Grade A, the carbon equivalency should be considered if welding is planned.
AISI Design Guide 21 mentions the following about welding to bolts and nuts:
“Occasionally, it becomes desirable to weld bolts to structural steel, or bolts to nuts. As a general principle, welding should not be done on bolts or nuts. However, if essential, the composition of the bolt (and nuts if involved) must be carefully considered.”
AISI takes the stance that welding to nuts and bolts should be avoided unless absolutely necessary. The simple solution is to use an anchor bolt with a forged head in lieu of a threaded rod with a welded nut.