welding a fireplace grate?

Here's my 2 cents.
I would do all of your joint prep and grinding off paint etc. first.
Then i would start the fire up and get it going good.
Set the grates on there and let them warm up to a dull cherry red.
Have your table or bricks set as up close to the fire as possible as well as your welder.
Weld using Ni-Rod or stainless about 1/2'' at a time starting with the ends and middle.
Put back on the fire immediately, and reheat.
Do this until you are satisfied with the strength that you require.
Leave welded grate on the fire until it burns out and the hot coals will keep it warm for hours and let them slowly cool the part.
Don't expect the welds to be pretty, the quality of the cast iron that they use for grates is low and there maybe voids of carbon etc. in the casting itself.
After it is completely cooled off overnight, then you can clean up the welds, paint, etc.
Then you're done!