No to OXY that will just make your local Fire brigade happy, try a propane gun, about £20 from your local DIY shop, hot air gun won't even get it hot enough. You want to get it almost red but not to the point of glowing, give the outer section the heat then cool the inner with water, that way,as Drewpy said, it gets a thermal shock and the outer stays hot while the inner pipe cools rapidly, then give it a tweek but NOT a full blooded heave, you want to separate it not distort it.