Parallel vs. Tapered Roll Expansion: Impact on Stress Corrosion Risk

Looking at fabrication, there are several operations that impart stress on the tube material, a primary source being tube expansion.

Two common types of mechanical expansion are parallel and tapered roll expanders. Parallel Roll, also referred to as Parallel Pin, is designed so the roll and expander are aligned with a zero-degree feed angle. This orientation requires an outside force to act on the mandrel to drive it forward, typically done using a hydraulic or electrically driven system. Alternatively, tapered roll expanders use a feed angle on the roll orientation, allowing the mandrel to draw forward as the tool is rotated.

Figure 1. Parallel Roll v Tapered Roll Expanders

To understand what role tube expansion might have in stress corrosion cracking, Elliott conducted a study to look at the amount of stress these two methods of expansion might produce by looking at the presence of shear bands. Shear bands are a microstructure feature that develops in the material grains as a result of plastic deformation, appearing as lines inside grain boundaries.

The material selected for testing was SA213 316 Stainless Steel, ¾” x 14BWG minimum wall tubing expanded into 2” thick 316 Stainless Steel tube sheets. The tube sheets for this experiment had 19 tube holes each and were manufactured by Elliott Tool Technologies to meet TEMA standards for triangular pitch and tube sheet hole grooves, as seen in Figure 2.

All tubes were all expanded in one operation using Elliott’s Ultra Hawk assisted rolling system at 600 RPM. The tested tools utilized common components, with roll orientation being the only functional difference.

Figure 2. Tube Sheet Design

After the tubes were expanded, the tube sheets were sectioned using a wire EDM and sent out for review. Metallographic samples revealed at 3% and 4% wall reduction the difference in identifiable shear bands is roughly the same between tapered and parallel pin. However, the differences become more pronounced at 6% wall reduction. Tapered pin expansion saw deeper shear banding, and as a result more residual stress, at a higher wall reduction compared to parallel pin.

Figures 3 & 4 show etched microstructures at 200x magnification. Both samples indicate some level of shear banding, but the tapered pin image shows a higher amount of shear banding on the ID surface of the tube. Elliott attributes this to be a result of the tapered roll expander generating drag along the length of the tube across the surface during expansion.

Figure 3. Parallel Pin Expansion At 6% Wall Reduction

Figure 4. Tapered Pin Expansion At 6% Wall Reduction

Premature tube failure is one of the leading causes of downtime in the field. Reducing any avenue for tube failure can help reduce downtime and costs. Since stress corrosion cracking failures can affect many tubes simultaneously, finding methods to reduce the likelihood of cracking would greatly reduce the cost of emergency repairs or retube efforts.

Based on Elliott’s research, stress corrosion cracking is more likely to occur at higher wall reduction percentages (≥ 6%), with tapered roll expansion producing more pronounced shear banding. As a result, parallel pin expansion could reduce the likelihood of this type of failure from occurring by limiting the residual stress imparted on to the surface during expansion.

Read more about the implications of stress corrosion in our earlier series publication.