brazing cast iron

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I am repairing a cook top from an old trailer and the grates which the pots go on has a couple cracks and old repairs in it they were not done so well so I am grinding em out and starting fresh. I have alot of stick welding under my belt but never any on cast iron, and I dont have a dc machine at hope so Im going to have to braze it. My main question is do I need to pre and post heat the material or can I just heat and braze where it is needed and all will be good. I am using radnor general purpose brazing alloy, the guy at my local welding shop recomended it.Just looking for some help, thanks.
Reply:On thin part like that typically you just make sure the part is at room temperature before starting, and let it cool slowly afterwards. Also warming up the part slowly and taking you time by its very nature limits the stresses on the part. What flux did you get to use? Or are the rods coated?-Aaron
Reply:the rod is already coated. Thanks
Reply:hmmm... never used the coated rods.. always wondered how they'd be..if you're not livin on the edge, you're takin up too much room..
Reply:You might want to get some asbestos blankets to maintain temp. Do not rely on the flux on the rod. Use a high temp. flux becouse you will have to put lots of heat into the metal. Also do some pre/post heating and allow to cool very slowly. Sometimes cast iron will crack along the heat effected zone. Also the higher the silver content of the rod the better results you will get.