Abstract: Elongated steel tubes are grouped together and bound into bundles with bing sufficiently tight that there is no clearance or play between the tubes during heat treatment. Such binding provides a rigid mechanical assembly which prevents any lateral deformation of the tubes with respect to each other. Heat treatment is carried out while the tubes are tightly bound in bundles, and each bundle is supported in the furnace at least at several locations along its length to prevent deformation. The bundles are preferably of hexagon shape in section but can be of other geometric shapes. This technique results in heat treated tubes which are perfectly straight.

Abstract: Circumferential stresses in tubes are relieved by exerting an axial pull on tube sufficient to elongate the tube at least 0.2%, and maintaining the pull on the tube for a predetermined period of time. The extent of elongation can be as much as 1% and the axial pull can be maintained for several minutes. The technique is particularly useful for substantially reducing or eliminating circumferential stresses in thin wall tubes which are created by straightening the tube with hyperbolic profile rollers.