Plate/Blade Welding
I have been working in a welding repair shop for a few years, so this is what I would suggest.
Although I haven't welded armour plate, we do have to repair cutting edges on loader buckets and such. The normal procedure is to "V" it out properly, then fill it in with narrow "stringer" beads using 1/8" dia. low hydrogen ( _ _18 series, i.e.: 7018, 8018, 11018 ) electrodes at around 125 amps.. If you have a DC machine, they are always run DC Reverse. If you have an AC machine, and it has sufficient open circuit voltage to start and run an _ _18 series rod (many of the lighter ones do NOT) then use a 7018AC rod which has ionisers in the flux to allow it to run properly on an AC machine. On plate that thin, preheat is not essential, but would help prevent possible cracking. A preheat of 200°-250°F is ample, and mostly used to dry out any moisture that my contribute to hydrogen embrittlement.
A better choice is a more "forgiving" low hydrogen rod (higher percentage-of-elongation) such as an ArcTec 223 from ArcTec Alloys. This allows the weld to stretch more than an _ _18 series rod, and so puts less stress in the base metal. We use this rod for the long welds when attaching cutting edges to buckets as it causes less warpage and has higher impact resistance.
Stainless rods ARE nickel/chromium, and the 18-8 they speak of would be a standard 304 grade. All of the "300" grades of stainless are "austenitic" meaning nonmagnetic and nonhardenable. Higher nickel (first number) makes it tougher, higher chromium makes it harder. If using this type of rod, a 304L, or 309L would be good choices as the "L" signifies "Low Carbon", and it will not pick up as much hardness from the base material. The drawback is the price, and that they do not weld very well out of position. A better choice in this category would be ArcTec 265 Unichrome, which has a much higher strength rating, and welds some what better out of position.
If wanting to use a MIG, I would lean towards a Flux core, Dual Shield, or Metal core type of wire, for added weld ductility. If using solid wire, a type ending with S2 or S3 would be preferable to an S6 wire because of a higher elongation value. If Mig welding, I would definitely use a preheat to prevent the weld from cooling to fast and causing hardness/brittleness problems.
A high nickel rod designed for use on cast iron, like an ArcTec Supercast 90, would likely give very good results as well.
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