Metallurgical Analysis of Defects in the Weld Joints of Large-Sized Maraging Steel Rocket Motor Casing

18 Ni maraging steels (250 grade), due to their high strength and excellent fracture toughness, are candidate materials for pressure vessel applications such as rocket motor, since these steels are easily weldable over a wide range of thicknesses. These rocket motor segments with a diameter of 3,200 mm and length of 8,400 mm are fabricated with maraging steel plates of 8 mm thick and rolled rings. During the fabrication of one such segment, unacceptable fluorescent dye penetrant indications were observed after proof pressure testing in the weld joints which were further confirmed by ultrasonic testing (UT) and x-ray radiography. None of these indications were detected in previous stages of UT conducted prior to proof pressure testing. In order to understand the reasons for cracking and assess the suitability of the segment for repair, non-destructive, on-site, in situ metallography has been employed. Extensive in situ metallographic analysis was carried out on these segments at several locations where cracks were found, and this work describes the details of the work carried out on these segments with the goal of understanding the mechanism of cracking and recommending measures to avoid recurrence of such defects. Weld tensile specimens fabricated along with motor segment were also analysed to study the behaviour of welded joints. Two types of cracks were noticed on the rocket motor segments namely longitudinal cracks at the weld fusion boundary and cracks at HAZ–parent metal interface. Based on detailed analysis, the reasons for cracking of the segment are attributed to (a) stress corrosion cracking (for longitudinal cracks) and (b) HAZ–parent material interface cracking (for transverse cracks).