K. Sree Kumar

Metallurgical analysis of cracking in weldment of propellant tank

Abstract Medium strength Al–Zn–Mg aluminium alloy is extensively used in propellant tanks of satellite launch vehicles. The cylindrical tanks are fabricated by TIG welding of sheets and rings into larger diameter pressure vessels. End domes are fabricated by welding petals, and nozzle openings are provided by shrink fit nozzles, welded to the petals. A crack, partly on the overlap region was noticed on one such nozzle weld. A detailed analysis revealed that the crack initiated from the root of pits, propagated under the high longitudinal restraint at the weld stop area, through solute enriched dendrites at the weld fusion line, resulting in an intergranular mode of failure. This is typical of stress corrosion cracking (SCC).

Metallurgical analysis of failed SS 304 bolts used in flanged joints of a liquid oxidiser pipe line

Abstract Austenitic stainless steel (AISI 304 grade) bolts in flanged joints of a fuel and oxidizer pipeline have been found cracked after 4 years in service. The failed bolts have been studied through standard metallographic techniques to analyse the cause of the failure. Deposits on the bolts were analysed using ion chromatographic techniques. The study revealed corrosive attack from the oxidizing agents, which caused grain dissolution under the stress existing in the assembled bolts.

Microstructural aspects of weld repair in 18 nickel 1800 MPA maraging steel

The study of the microstructural aspects of the maraging steel weld repair has become important as this material finds extensive applications in aerospace and other strategic areas. The details available on these at present are very limited. During one of the fabrication trials of a large pressure vessel made of 18 Ni 1800 MPa maraging steel plates, 7.8 mm thick, defects were observed in the welds which were to be repaired. This called for simulated experiments. The microstructural variations observed in the repaired areas were the result of the repeated thermal cycles they had undergone. Based on this microstructural evidence, a new diagram depicting the newly formed weld zones is constructed from which the mechanical properties of the repair welds can be predicted.

A study of reaction kinetics in Al – 2% MWCNT composites

A composite obtained from pure aluminum and multiwall carbon nanotubes by mechanical milling in a high-power mill and ultrasonic mixing is studied. The apparent activation energy and the order of the reaction between carbon nanotubes and aluminum are determined using the Kissinger and Crane equations and differential scanning calorimetry under non-isothermal conditions.

Fabrication of high-strength Ti – 6% Al – 4% V titanium alloy by multipass warm rolling

A high-strength Ti – 6% Al – 4% V titanium alloy is studied after warm rolling at 650 – 750°C in several passes. Tensile mechanical tests are used to determine the rupture strength, the yield strength, and the elongation. The microstructure of the rolled alloy is investigated. Warm rolling is shown to be effective for forming superfine grains in the Ti – 6% Al – 4% V alloy and raising considerably the strength characteristics without lowering the ductility.