Durable method for producing finned tubing

A method of making operable diameter internally finned tubing comprising the steps of:

• (a) placing a metal tube containing at least 30 wt % nickel and 10 wt % chromium at least partially over an elongated mandrel having an oversized diameter, oversized land portions projecting radially outward from said mandrel and extending longitudinally along said mandrel, and valley portions located between said land portions extending longitudinally along said mandrel,
• (b) inserting said metal tube and said oversized mandrel between a pair of opposed roller dies in a tube reducer,
• (c) reciprocating said roller dies over said metal tube at a stroke rate of at least 40 strokes per minute and a feed rate of at least 0.635 cm per stroke to form oversized diameter internally finned tubing with said oversized mandrel, said internally finned tubing having fins corresponding to said valley portions and radially overspaced depressions corresponding to said overspaced land portions, said fins having a height substantially equal to a finished height, and having said depressions laterally overspaced, and
• (d) sinking said oversized diameter internally finned tubing to a finished diameter to reduce radial spacing between said fins and to elongate said oversized diameter internally finned tubing in a manner which does not substantially reduce the height of said fins.