Incorrect. Consult a good welding text such as "Modern Welding" by Althouse and Turnquist (the most widely used, and most authoritative, welding textbook).
It is true that fusion welding produces a HAZ (Heat Affected Zone) around the actual weld joint. This can significantly alter the properties of *some* materials, namely medium and high carbon steels, some alloy steels, and some aluminum alloys. But *part of the welding process* in those cases is post-weld heat treatment to restore those properties to their original pre- weld state. In other words, you haven't completed the welding process for those materials until you've done the post heat treatment.
For materials such as mild steel, the most commonly welded material, there is no such concern. The HAZ doesn't affect the material properties. That's because mild steel has too little carbon in the solid solution to produce the phase changes that could alter its crystaline structure. A *competent* welder will also choose an appropriate alloy filler material so that the fusion zone won't have different properties from the parents either.
It is well to note too that different welding techniques produce differing size HAZ. TIG welding produces less than arc, MIG produces less than either, and exotic techniques such as laser welding produce practically none at all.
Now you are postulating *cold welding* for the gage blocks, and that produces *no HAZ at all*. So the material properties surrounding the weld joint would not be altered *at all*. Of course cold welding isn't what's actually happening when you wring gage blocks together, but if it were, you'd still be wrong.