CREEP AND CREEP CRACKING OF A HEAT EXCHANGER COMPONENT OF SUBMARINE MADE OF ASTM PURE TITANIUM GRADE 4 AND GRADE 2

摘要

Following a routine inspection, cracks were detected on bottom or tube-type heat exchanger inlet-outlet shells, made of grade 4, commercially pure titanium. Ultrasonic tests revealed that all devices were concerned by this problem and that cracks had propagated to half the thickness (～10 mm). The material analysis has shown that it was actually a 《 sustain load cracking 》 phenomenon. The mechanical driving force behind the phenomenon is linked to residual welding stresses, not stress-relieved after manufacturing. A material analysis applied to grade 4, grade 2, grade 23 and Ti 5111 titanium was carried out. Results show that all grades are more or less sensitive to the 《sustain load cracking》 (SLC) phenomenon, which leads to SLC thresholds lower than toughness K1c.