Metallurgical analysis of a failed maraging steel shear screw used in the band separation system of a satellite launch vehicle

AbstractMaraging steels have excellent combination of strength and toughness and are extensively used for a variety of aerospace applications. In one such critical application, this steel was used to fabricate shear screws of a stage separation system in a satellite launch vehicle. During assembly preparations, one of the shear screws which connected the separation band and band end block has failed at the first thread. Microstructural analysis revealed that the crack originated from the root of the thread and propagated in an intergranular mode. The failure is attributed to combined effect of stress and corrosion leading to stress corrosion cracking

Microstructure and texture evolution in AZ31 magnesium alloy during caliber rolling at different temperatures

Magnesium alloy AZ31 was caliber rolled at different temperatures viz. 523K, 573K, 623K, 673K and 723K imposing a total cumulative reduction of 92% and a cumulative strain of 2.6. The associated microstructure and texture evolution were studied using Electron Back Scattered Diffraction (EBSD) and X-ray macro-texture observations respectively. Grain refinement was observed in all the caliber rolled bars indicating the occurrence of dynamic recrystallization. Asymmetry parameter and texture index were used to study the evolution of texture. Near doubling of the yield strength at room temperature was observed by caliber rolling at a temperature of 573K. The improvement in mechanical properties was explained on the basis of the fine grain microstructures and suitable development of crystallographic texture. Based on the present study, warm caliber rolling can be used as a good production method for obtaining long bars of high strength magnesium alloy AZ31. Keywords: Magnesium alloy AZ31, Microstructure, Caliber rolling, Textur

Grain growth in calibre rolled Mg–3Al–1Zn alloy and its effect on hardness

Calibre rolling of Mg–3Al–1Zn alloy at 300 °C led to development of fine grain size of 3 µm. Subsequent annealing, from 5 to 6000 minutes at 300–450 °C, revealed faster grain growth initially up to 60 minutes, which became sluggish on prolonged annealing. The time exponent for grain growth kinetics (n) suggests bi-linear behaviour with n = 0.11 and 0.008 over these time scales. The activation energy, based on various n values, varied over wide ranges that made the understanding of the mechanisms for grain growth difficult. This problem is explained by concurrent evolution of texture and grain boundary structure. The effect of grain growth on hardness at ambient temperature was found to follow the H–P type relationship

Failure analysis of a vibration isolator used in launch vehicle applications

Vibration isolators are used in launch vehicles to protect sensitive electronic equipments from the rigors of flight structural vibrations during the lift-off and ascent phase. Failure of one such vibration isolator made of AISI 304 is reported in the present study after ground level testing has been completed. The vibration isolator had a crack of length approximately 42 mm at the fore end corner. Detailed optical and scanning electron microscopy was conducted on the failed vibration isolator to understand the reasons for cracking. Finally, it was concluded that micro cracks formed during the deep drawing operation (due to large strains at the fore end corner) propagated during vibration testing resulting in the failure of the body of the vibration isolator

Microstructure, properties and hot workability of M300 grade maraging steel

This paper presents isothermal uniaxial compression test results of M300 grade maraging steel over a wide range of temperatures (900–1200 °C) and strain rates (0.001–100s−1) to examine hot deformability and concurrent microstructural evolution. Processing map is generated and indicated the optimum processing parameters in the temperature range of 1125°-1200 °C and strain rate range of 0.001–0.1 s−1. High values of the efficiency of power dissipation, microstructural observations and EBSD results indicate softening mechanism to be the occurrence of dynamic recrystallisation. Material constants in a constitutive relation are evaluated from the flow stress data useful in computer modelling