Fatigue cracking of titanium tubes in an industrial heat exchanger

Abstract

The fatigue fracture of commercially pure (cp) titanium tubes at relatively low temperatures [120° C (250° F)] in a shell-and-tube heat exchanger was studied. The cp titanium tubes were subjected to flow-induced vibration and compression axial loading in service. The combined stresses resulted in numerous intergranular cracks in circumferential direction. Some cracks reached a size that resulted in complete rupture of tubes. Fractographic analyses indicated that the fracture was caused by high-cycle fatigue. A large portion of the tube wall indicated a lack of characteristic ductility of cp titanium. The reduced ductility was a result of second-phase particle inclusion and hydrogen absorption in tube material. The secondphase particles provided easy sites for initiation and coalescence of microvoids. Hydrogen absorption of titanium resulted in an increase in hydride content and thus a decrease in ductility.