An angle grinder with a metal cutting wheel should do the trick. If you can’t get it clean on the first pass, switch to a grinding wheel and grind it right down to the concrete. Watch out for sparks (there will be many) and wear safety glasses.
if you are scrapping the old railing you can hacksaw an inch above the floor height. chip some concrete away from around each point. then use an angle grinder with cut off wheel and cut in at an angle from the sides so that no metal comes about 1/4 inch from the old surface. patch these holes with a patching material (either mineral or plastic).
if you are putting in a new railing you will have to drill holes for anchors using a masonry bit. if these need to be in the same locations as the old ones then you are better chipping out the old ones with a chisel and 3 pound hammer.
If the rails are close enough to the edge of your steps a sawz-all could work. Otherwise making a square cut could be difficult.
You could also cut the iron posts long and use them as an internal support for the wood posts by drilling a hole in the bottom of the wood ones for the iron to slide up into.
As far as new tools go an angle grinder is neat but they don’t get much use. I have 2 of them. I rarely use them. I can’t remember the last time I used one at my own home. I use them to cut up old pressure tanks to large to fit out doors and to take sharp edges off of well casings.
I second the Saws-All and angle grinder. I did virtually the same job for a friend last year, except it was a ham radio tower set in concrete. Cut with the saw as close to the surface as I could, maybe 1/4" off and grind it down to the surface with a 60 grit disk. Kicked some dirt over the top and planted some flowers.
Hubby used a 20-lb sledge to break up the tile on our old shower as we were remodeling. He was well satisfied with the work until he walked out and back around through the bedroom and saw the dimples in that wall. :smack:
The sawz-all didn’t work. Got the metal blade but it dulled very fast, didn’t make it through the first post. The posts were set back too far from the edge so I could only saw with a limited piece near the end of the blade. It did the accordion thing a couple times, too.
I went out and got a $30 angle grinder and a cut-off disk. It was perfect for the job. Out of six iron posts, only two had to be ground farther down. The others were close enough to need no grinding.
The iron went to the back alley, the scavengers had snared it by morning, and two of four replacement posts are in place. I’ll get the other two up tonight, I hope, and then it’ll be paint, decking, and finally rails.