Quote:
Originally posted by Peryn
When you hit the handle, adjust it so teh flame stays blue, but doesn't change shape/color very much. When you hit the handle, you dont want a significant change in teh flame. This way you blow out the metal, rather than altering the heat and flame characteristics every time you pull the O2 on. Also make sure to aim the tip in the direction your going to preheat the metal. Makes a smoother, faster cut then going straight up and down. I have found a 15degree angle works well, as much as 30 or even 40 in some instances will work nicely.
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Peryn's got it down. I've been using cutting torches for over 30 years. And I worked in Welding Supply for about 8 years. Use the methods that he described, and you should be fine. Keep a good steady speed going, without too many jerky movements. If you hold the torch in one place to long, the slag underneath the cut will "rollback" and refill your kerf.
So far as pre-heating metal prior to welding goes...I've only ever done it to cast iron. Cast iron is tricky to work with. Get it too hot, too fast, and it'll crack. If it cools too fast, it'll crack. I always slowly pre-heated the work with a torch (the key being slowly), effected the repair (use nickel alloy rod), then covered the piece with sand allowing it to cool overnight. Works like a charm.