Expanding of Boiler Tubes into a Tube Nest

Abstract

This paper describes a series of tests in which tubes have been rolled into tube nests under carefully controlled conditions using the test rig previously developed for a general investigation of the tube expanding process. The tube spacing used corresponds closely to the minimum reported in industry.

It is shown that considerable interference of the plastic zones associated with the joints can occur in the ligaments between the tube holes and the strains developed in the plate are larger than for a single-tube expanding. Nevertheless, the expanding of subsequent tubes tended to relax the strain around previously rolled joints and these joints had a low strength.

The strengths of all the joints in a nest were lower than that of a tube rolled, to the same degree of expanding, into a single-hole plate owing to the interaction of the plastic zones or to the greater flexibility of the tube nest plate. There appears to be an optimum degree of expanding in a tube nest which corresponds closely to that obtained for single-tube expandings.

The results found from these tests depend upon the geometry of the tube nest and the manner in which the joints are rolled.