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Corrosion
Figure 3 . Parallel roll v tapered roll expanders . Image courtesy of Elliott Tool Technologies .
Figure 4 . Tube sheet design . Image courtesy of Elliott Tool Technologies .
Indicators of stress in metals There are many different sources of residual stress in heat exchanger manufacturing , from welding to tube trimming . Due to the method of failure identified in the customer ’ s Titanium vessel , Elliott wanted to explore how the residual stress produced during the tube expansion process might impact tube material . There are two indicators of stress in metals : hardness and shear banding . All metals will see some level of hardness change with cold working . The permanent change to the grain structure from plastic deformation will increase the hardness in affected areas . Since tube expansion causes plastic deformation in the grain structure of the metal , it will result in a higher hardness and potentially cause defects in the metal . Shear banding is another method to determine how much stress was imparted on the metal . Shear bands are a microstructural feature that develops in the grains as a result of deformation . Shear banding increases with the degree of deformation . A 2013 research conducted by Bai et al , showed a correlation between increased shear banding and stress corrosion cracking , specifically in 300 series stainless steel ( 2013 ). This type of testing is destructive as a cross section must be cut through the material in order to identify it . Shear banding presents as lines appearing inside the grain boundaries , as shown in Figure 2 .
The experiment : does tube expansion method impact hardness & shear banding ? Two types of expansion methods Two types of mechanical expanders 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 .
Initial research : explanation of initial testing and results In 2023 , Elliott Tool Technologies presented findings of research comparing parallel and tapered roll expansion on SA213 304L ¾ ” x 14 BWG average wall tubes . Using these existing samples , Elliott decided to further investigate the differences in the micro hardness through the inner wall of the tube , as well as the metal structure after expansion . The samples were then sent out to Metallurgical Solutions Inc and Webco Industries for evaluation . While this test was not statistically significant , it did show some differences in both hardness and structure between the two methods .
www . heat-exchanger-world . com Heat Exchanger World February 2025
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