stainless to csat iron?

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I've got a casting to repair, and it's pretty well soaked with drugs-it's a pill press. When I welded the crack that needed to be repaired-across a set screw hole-on a surface that has to be PERFECT-no pits or flaws, I got some nasty porosity at the knit line. After grinding it out & re-welding it several times  I came to the conclusion that even though the surface wasn't grease & oil soaked, like a standard cast iron repair, it was contaminated with a 50 year acumulation of 'inorganic' compounds-kinda like some people I know....  The solution is simple-mill the surface down further, and install a 3/16"-1/4" stainless steel plate-problem solved! Except for the fact that there is no way to drill & tap the surface. It's under another 'ledge' with about 2" to 2-1/2" clearance.  I've heard of welding, or cold stitching cast iron with SS rods-but I've never done it. Welded ALOT of castings with nickel rod, or brazed them, but no SS. I want to TIG this plate on, as we'll have to finish line bore the plate to match the casting after it's welded in place, and we need as pretty of a weld as possible. The FDA don't want no stinkin' pits in it.....    Is any alloy preferrable over another for this?(all I seem to have right now is 309 & 316L) Preheat to 400 deg F or so? I'd think that would help, & I'm sure post heat & draw down are going to be critical too. I want to use a 304 SS plate.   Anybody tried this before?? I'm thinking I'll only get 1 shot at getting this right.Thanks
Reply:I'd use 316L before 309, but would use 309L given the chance.
Reply:Also with very low heat input, this will help keep in the porisity.   John G.SMAW,GMAW,FCAW,GTAW,SAW,PAC/PAW/OFCand Shielding Gases.  There all here. :
Reply:I don't  THINK any S S  rod will do. SS is too hard , no give. if it's going to cast IRON ,you need a soft rod to join the metals . I'd recommend a high knickel rod, or a bi-metalic ,if you can get some .[SIZE="5"Yardbird"
Reply:Try using hastelloy rodI r 2 a perfessional
Reply:Originally Posted by Roy HodgesI don't  THINK any S S  rod will do. SS is too hard , no give. if it's going to cast IRON ,you need a soft rod to join the metals . I'd recommend a high knickel rod, or a bi-metalic ,if you can get some .
Reply:Originally Posted by SupeWhat are you basing this on?  The low carbon versions of several stainless rods have ample ductility for use with cast iron, and is done so frequently.  The stainless will be even more preferable for use on contaminated material to minimize the presence of porosity in the joint.Like weldgault said, keeping heat input relatively low with the stainless is important, but mostly to minimize the dilution factor and to keep the weld ductile, moreso than porosity.
Reply:BTW, I can spell 'cast iron', just can't type! Thanks for the tips-we'll find out tomorrow. There's a little room under the base to practice on. I hadn't thought of the hastelloy rod-definitely worth a try. I may have an old roll of 309L mig wire back in a corner-I'll try that too. I guess it's 1 of those deals where you keep your arc on the SS & feather it over on the cast just enough to get a good knit-I'm sure it'll be fun!
Reply:Get us some pics man.