Sustained Load Cracking of Titanium Alloy Weldments

Failure of critical titanium parts, including some offshore components, has drawn attention to delayed cracking in Ti-6Al-4V alloys, but, given good design and alloy variant selection, such failures are avoidable. Delayed cracking, or sustained load cracking (SLC), can occur at low to moderate temperature (approximately: −50 to 200°C), depending on the titanium alloy and condition. Appropriate testing methods are required to generate stress intensity threshold values (K_ISLC) that can be incorporated into the design of titanium structures and recommendations are needed on the optimum chemistry and microstructure for greatest resistance. In the present work threshold stress intensity factor data (K_ISLC) were generated for Ti-6Al-4V alloy sheet, forgings, pipe and weldments using two different rising stress intensity factor test methods. It is concluded that material with a beta-annealed microstructure and low oxygen content (i.e. extra-low interstitial material such as ASTM Grades 23 and 29), has high resistance to SLC and that weld metal and transformed heat-affected zone also perform well, before and after postweld heat treatment, provided interstitial element pick-up during welding is prevented. Purchasing material in a general ‘mill annealed’ condition is not recommended without specifying acceptable microstructures. Further refinement of test method is also recommended for defining K_ISLC.