My first thought was induction heating blankets, but that probably doesn't fit well with manufacturing large quantities.
You'll probably need to be concerned with slow cooling and stress relieving as well.

I'm picturing 2 or three elongated ovens with a conveyor belt in between.
First oven - preheat, then a conveyor fed gap for welding, which won't take long.
Then conveyed into a  stress relief oven, followed by a slow cool oven.

I guess this would only work for a continued large quantity, rather than a large quantity batch.
Not much help, I guess...

Tim