Robert,
This is how I do it. I'm assuming your talking about the little cast shoes that go on the bottom of the newel posts? Anyhow, I use my mig, er70s with 75/25. I clear out a space on the shop floor and get prepared. First, I heat the cast piece to 400.....500 degrees. I usually heat until my temp gun won't read anymore, max's out at 600 degrees, set your cast piece where you want it and weld to steel post. Then I immediately throw some heat back on it for a minute or two and then the most important step of all. I have about 40 pounds of old sandblaster sand that I quickly pour all over the piece and pile around it. Your must completely cover it, your looking for a very slow cool. Then I walk away and do something else for a few hours.
Sounds nutty like squirrel poop I know but I have yet to have one that I do this way crack and have had handrail out there now for several years used daily. They have been dropped on the trailer and miss handled and hold up nicely. Before I started doing this "procedure" and just welding them straight......a mouse could fart and blow the cast piece off! Now, I don't know if I'd anchor a complete railing using this method. Usually for what I build I have solid anchors either steel plates with concrete anchors or core drilled and the cast shoe is in the middle for.....well for whatever reason it was required. Also have welded cast iron volutes on some decorative tube rail using this method. However I try and grind a bit of a bevel/groove in the volute where it meets the tube so I can get full pen on the tube as I typically grind it smooth after it cools.
You can substitute sand for floor dry, cat litter....not used of course!!
Shawn