The Material Basis: ASTM A179 Tube
The parent tube for hot expanding is normally a seamless cold-drawn low-carbon tube made to ASTM A179/A179M, the standard for heat exchanger and condenser tube. The grade is deliberately low in carbon, 0.06-0.18%, with manganese 0.27-0.63% and phosphorus and sulphur each 0.035% maximum. Minimum tensile strength is 325 MPa, minimum yield strength is 180 MPa, and minimum elongation is 35%, with a hardness ceiling of 77 HRB for the boiler tube grade family. That combination of high elongation and low hardness is what makes the tube suitable for a forming operation rather than only for pressure service.
How Push-Type Hot Expanding Works
Push-type hot expanding uses three elements: a conical inner mandrel, an induction heating coil surrounding the tube at the deformation zone, and a hydraulic pusher that drives the tube forward. The cold parent tube is placed over the mandrel and pushed through the coil. As the tube advances, the heated section passes over the expanding cone and the diameter increases progressively to the mandrel size, so the wall is deformed and thinned while the metal flows along the cone.
The mechanism that makes the process workable is metal flow compensation. Material compressed on the inner arc flows toward the thinner outer region, which partially offsets the wall thinning caused by the increase in diameter. Without that flow, expansion would thin the wall in direct proportion to the diameter increase and the finished tube would fail the pressure duty it was ordered for.
Process Parameters That Control Quality
| Parameter | Effect on the product |
|---|---|
| Expansion ratio | Sets the starting parent tube diameter and the amount of wall thinning to be compensated |
| Induction heating temperature | Controls flow stress; too low and the metal tears, too high and grain growth reduces toughness |
| Pushing speed | Controls soak time in the heated zone and therefore temperature uniformity round the circumference |
| Mandrel cone angle | Controls the strain rate per unit length and the surface finish of the expanded section |
| Billet length and stop position | Controls the length of the expanded portion and the transition zone |
The expansion ratio is the single most important number. It is the ratio of finished outside diameter to parent outside diameter. Workshop practice for carbon steel keeps this ratio in a moderate band, because the compensation mechanism loses effectiveness as the ratio rises and the wall thinning can no longer be recovered. When a large step-up in diameter is needed, it is safer to specify a thicker parent wall and accept a heavier finished tube than to push a thin parent tube through an aggressive expansion.
Sizing, Tolerances and Tooling
Because the tooling set consists only of the mandrel, the tube, and locating fixtures, the process converts quickly between sizes. Core heads can be exchanged for multi-specification, small batch and short lead time work, which is why hot expanded tube is common for non-standard heat exchanger sizes. The trade-off is that diameter and wall thickness tolerances must be agreed on the drawing: the expanded section behaves differently from the cold-drawn parent, and the transition zone between expanded and unexpanded portions needs a defined length if the tube is to be rolled into a tube sheet.
Inspection of the Finished Expanded Tube
The finished tube is inspected for wall thickness by ultrasonic measurement at defined positions, because a local thin spot cannot be detected by a diameter check. Mechanical testing verifies that tensile strength, yield strength, elongation and hardness remain inside the requirements of ASTM A179/A179M after the thermal cycle, and the non-destructive test is either a hydrostatic or an eddy current test under the general requirements of ASTM A1016/A1016M. Straightness and ovality are measured after cooling, since both are affected by the thermal cycle and the restraint applied during pushing.
Frequently Asked Questions
Q: What is a hot expanded tube?
A: A tube formed by pushing a cold-drawn parent tube over a conical mandrel while an induction coil heats the deformation zone, increasing the diameter to a defined size.
Q: Which standard covers the parent tube?
A: ASTM A179/A179M for seamless cold-drawn low-carbon heat exchanger and condenser tube, with general requirements from ASTM A1016/A1016M.
Q: Why does wall thickness change during expanding?
A: Increasing the diameter stretches the circumference and thins the wall; part of the loss is recovered by metal flow along the mandrel cone, but the compensation is incomplete.
Q: What is the minimum elongation of A179 tube?
A: 35% minimum, with minimum tensile strength of 325 MPa and minimum yield strength of 180 MPa, which is what allows the forming operation.
Q: How is wall thinning verified on a finished tube?
A: By ultrasonic wall thickness measurement at agreed positions, supplemented by dimensional checks on straightness and ovality after cooling.