Metallurgical Analysis of Failed AISI 304L Stainless Steel Tubes Used in Launch Vehicle Applications

Abstract

Two numbers of AISI 304L stainless steel tubes with Ø6 and Ø4 mm used in pressurisation system module of a liquid engine of a satellite launch vehicle were found to crack after the qualification level random vibration test. The module has undergone resonance survey, sinusoidal vibration, and random vibration along two different axes before the occurrence of failure. The tubes failed during third-axis random vibration test. The failed tubes were subjected to detailed metallurgical analysis to understand the reasons for failure. The location of failure in both tubes was at the cross-section change area in the nipple. Cross-section change was provided in the design to reduce the diameter of the nipple to suit to tube diameter for welding. The cross-section change in the nipple acted as stress concentration resulting in the initiation of fatigue cracks. The presence of sharp-cornered inclusions in the material helped easy crack initiation. Smaller radius and angle at the weld–parent interface resulted in additional stress concentration. The presence of striations on the fracture surface and slip bands near to the fracture edge confirm that the failure of tubes to be due to vibration induced fatigue.