A. Comparison of hydraulic and roller expansion

Hydraulic tube expansion was initially developed in association with the nuclear industry to improve production methods and avoid the problems inherent in conventional tube rolling. Development began in the early 1970s, and the technique is now available as a general industrial method.

(a) Roller expansion

Conventional rolling methods subject the tube to repeated plastic strains as the rollers pass around the tube, resulting in work hardening effects and changes in the grain structure of the material. During the process a considerable amount of wall thinning occurs with a consequent increase in tube length. Since this lengthening of the tube occurs continuously as the tube is rolled, overrolling can lead to a reduction of joint strength and leaktightness due to shearing of the keys that form between the tube and the tubesheet as the rolling proceeds. This effect also limits the length of joint that can be rolled at one time to about 50 mm (2 in.).

Completed expansions can be uneven around the circumference because of the sudden stop and reversal of the rollers or the presence of out-of-round holes. The use of tapered self-feeding rollers results in sliding contact between the rollers and the tube material and can result in flaking of the tube surface. These effects can result in problems of contamination and both stress and crevice corrosion. The large amount of friction inherent in the rolling process gives control and repeatability problems. Successful roller expanding is a skilled job, and despite the introduction of automatic torque-controlled systems, operator technique can still be a significant factor in obtaining a first-time seal. Rerolling of leaking joints can often disturb adjacent joints, so that leakers are "chased" around the tubesheet.