it is that expansion that breaks the rust. when it cools enough it becomes free from the block. if you hit it with penetrating oil or wax while still hot it will thin out the oil supper thin and allow it to wick in. also the faster cooling helps break the rust.
I have used water, Solvent, motor oil, soap, wax, but the one i like best is thin penetrating oil. they all seem to work but the penetrating oil seems to do the best job.
I am not totally convinced that a washer pre-welded on is needed. many of the ones i have welded where on strong enough to be like the head of a bolt. the hardest ones are the ones that break flush or below the surface and i dont see how a washer is going to make a difference here.
Just put a nut on it, leave it slightly high, start welding from the center getting the stud hot and the weld penetrating the steel then slowly work it out to the nut going in a circle . done this dozens of times. I weld them when ever i can before i attempt vice grips or drilling it out.
my two cents