You could be dealing with the type of armor that does require a certain amount of preheat and is prone to cracking on short stitch welds, tack welds or in areas where the cooling rate is to quick. Are the cracks in the weld or in the base material? Some armors do require preheat and some do not. They are usually welded with a low hydrogen undermatched filler. We use a low hydrogen flux cored wire on our armor. Do you know how the plate manufacture recomends this stuff to be welded?