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Corrosion
Figure 5 . Work hardening increases with wall reduction . Image courtesy of Elliott Tool Technologies .
These initial findings , combined with the research conducted by Bai et al ., Elliott determined that further testing was necessary .
Testing methods & reasoning The material to be tested 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 4 . 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 difference .
Figure 6 . Parallel pin expansion at 6 % wall reduction . Image courtesy of Elliott Tool Technologies .
Test results : greater evidence of shear banding in tapered pin at higher wall reduction After the tubes were expanded , the tube sheets were sectioned using a wire EDM to cut the full length in half . Samples were then evaluated a location 90 ° relative to the EDM cut and at the midpoint between the grooves . Surface hardness curves were observed as being almost identical between the two types over the 19 samples of each that were evaluated . Metallographic samples showed at 3 % and 4 % wall reduction the difference in identifiable shear bands is roughly the same . At 6 % wall reduction , the differences become more pronounced , aligning with previous results . Figures 6 & 7 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 on the surface during expansion . Based on the metallograph results it suggests the shear banding difference at the surface will cause an increase in the potential for stress corrosion cracking .
Conclusions & industry implications 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
30 Heat Exchanger World February 2025 www . heat-exchanger-world . com