stick welding wrought iron railing

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So i got pulled into fixing a friends wrought iron railing, ive welded cast iron years ago so i though this would go ok. I bought some nomacast rods and cleaned the broken peices and started to weld. As soon as i stopped my 1/2" long weld i peened it with a slag hammer. As it cooled it would either crack in middle weld or right next to weld. I dont know what im doing wrong. Im set at 100 amps D/C EP on a miller thunderbolt 225. Do i need to pre/post heat? Or do i need to use 99nickle rod? Maybe more/less peening? Im about to resort to brazing. Some welds hold great others just crack. Or would a flux core wire feed do the trick. Any input would be greatly appreciated, thanks.
Reply:nomacast has always been hit or miss for me.  I gave up on nomacast the  years ago,  it is nonmachinable.  I use nickel rods.  I prefer to  braze many cast repairs when in the shop, since it will be undetectable as a repair after painting and easier to shape than nickel.  I use stick for quicker / onsite repairs.
Reply:Any tips for brazing?
Reply:Step #1:  Is the piece really "wrought iron"?http://en.wikipedia.org/wiki/Wrought_ironAlmost anything called 'wrought iron' nowadays is really just mild steel.As such, just weld it like any mild steel.  Clean it of rust, paint, oil, etc and then weld it up using mild steel filler electrodes (stick or MIG, depending on what machine you have available and how thick the parts are).  Done.And the railing is steel, right?  Some 'black railing' stuff is aluminum.  And aluminum stuff doesn't weld too well using steel fillers.  Last edited by MoonRise; 03-28-2013 at 11:23 AM.  The best laid schemes ... Gang oft agley ...
Reply:It is a cast iron from the 50's
Reply:And no its not aluminum, thanks
Reply:Most things I've read about cast says to pre-heat.  I've only welded one cast iron piece and I preheated it.Dave J.Dave J.Beware of false knowledge; it is more dangerous than ignorance. ~George Bernard Shaw~ Syncro 350Invertec v250-sThermal Arc 161 and 300MM210DialarcTried being normal once, didn't take....I think it was a Tuesday.
Reply:True wrought iron has a grain to it almost like wood does. If you break it it tends to peal like green wood does many times. Cast iron tends to crack and leave a different almost "sandy" looking surface ( I don't want to use the term "grainy" here and confuse it with wrought iron which has a completely different type of "grain". Cast iron/steel usually also has a casting line that may still be visible depending on the quality of the casting and prep done to the piece..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:The weld probably wouldn't have cracked if it were wrought iron.  What are you working on, photo?  I just bevel and braze.  Build it up proud, then grind.  I bevel flats, rather than radially if parts are round.    Nomacast is hit or miss even with preheat, for me.