A manufacturer of industrial heat exchangers discovered cracks in welded Incoloy 825 tubes during bending operations. The tube supplier blamed these cracks on improper fabrication during the bending operation. Examination at Metallurgical Associates indicated the tubes contained cracks that extended around approximately one-third of the circumference of the tubes and that all cracks were located 180° from the weld seam. This indicated that the cracking was not associated with the welding of the tube by the supplier.
Examination of an opened crack using a scanning electron microscope (SEM), revealed revealed that the fracture occurred at the grain boundaries. However, the features exhibited unusually smooth and rounded grain boundary edges.
Longitudinal metallographic cross-sections were through a typical crack and adjacent intact tube wall. Optical microscopic examination of this cross-section revealed extensive separations at the grain boundaries. Many of these connected to adjacent grain boundary separations, forming continuous internal cracks which extended partially through the tube wall.
Cause and Prevention
Evaluation of the evidence revealed that the cracks exhibited by these tubes occurred due to intergranular rupture. There were no defects in the Incoloy 825 material or the welds used in the production of these tubes. Examination of the opened crack by SEM revealed smooth rounded grain boundary facets and secondary cracks that are characteristic of incipient melting. This condition was also observed in the microstructure along the length of the tube in line with the visually observed cracks. No cracking was present on the opposite side of the tubes.
Incipient melting occurs when a metal is exposed to temperatures that approach, but do not meet or exceed its melting point. At these temperatures, the metal becomes soft and “mushy”, but not quite liquid. Stresses applied to material in this weakened semi-solid state will produce a separation, or crack, which exhibits smooth rounded features rather than the sharp angular appearance that is typical of fractures at lower temperatures.
This evaluation of the tube had now determined the “How”, or mode of failure. The next objectives of the analysis were the identification of what had caused the incipient melting and how to prevent it.
Metallurgical Associates review of the tube mills manufacturing process confirmed that long strips of the Incoloy 825 material were rolled into tubular form, the longitudinal seam was welded, and the completed tube was passed through an induction heating coil. This final induction heating process anneals the tube, eliminating hard brittle material properties resulting from the welding process. All standard tube fabrication procedure. However, closer examination revealed that the cylindrical induction heating coil was misaligned. As the welded tube was pulled through the coil, this misalignment placed one side of the tube closer to the induction coil, over heating it to near melting point temperatures. The stress required to generate cracking under these conditions was provide by the “pull”, or tensile stress generated by pulling the tubes through the induction coil.
With the identification of the mode and root cause of the cracking, the failures were easily prevented by realigning the induction coil. The tube supplier quickly provided new defect free tubing, and the heat exchanger manufacturer was quickly back in production.
