welding a fireplace grate?

Totally agree with the post heat. The guy I watched do it, used an oxyacetylene torch to do this, lots of patience to preheat, a larger area, not just where he was brazing. He would braze, reheat, braze, reheat a bigger area than just the braze. When complete, he still kept a good size area warm for a bit. I think a fire would work for the post heat.
In a previous life I was a metalworker.