I question how the vise was made and yes where, china. The cast iron part of my vise didn't break, it was the round metal, about 2 inches in diameter and about 1/4 inch thick. The round metal was brittle and broke, not the cast. There was no signs of where the round part was welded to the cast. So going out on a limb, is it possible the two parts were heat fitted? The cast heated and the round part cooled. So did the heat from the cast make the round part brittle? Or did they use the wrong metal to make the round part?
In my case, I cranked the amps up and on the first pass blew back the cast and welded the broken round part to the press fitted round part.
So, I'm guessing there is a flaw in the manufacturing process and not so much what rod and heat process you should use to fix it. Nothing is going to fix the brittle round part of the vise, except replacing it with a new vise.