Nickel-base alloy 625 is a solid solution matrix stiffened face- centred-cubic alloy containing Cr, Mo and Nb as alloying additions. This alloy is widely used in chemical industry due to its excellent combination of properties, viz. high temperature strength, corrosion resistance and fabricability. After long term exposure at elevated temperature, precipitation of orthorhombic Ni3Nb phase is reported to take place in this alloy. Age-hardening due to this precipitate causes decrease in ductility and toughness. This in turn results in considerable decrease in critical flaw size necessitating either removal from service or restoration of properties by in-situ heat treatment. Therefore, there is a need to standardise a non-destructive test method to monitor the extent of ageing degradation so as to take corrective measures in time and avoid costly failures. Microstructure of a material controls its mechanical properties and also affects its interaction with ultrasonic waves - thus providing the basis for ultrasonic materials characterisation. The objective of present study was to establish correlation between degree of age hardening in alloy 625 and ultrasonic parameters. Material for the present study was drawn from ammonia cracker tubes used in chemical plant operating at 873 to 927 K. The samples included virgin tube and tubes exposed for 50, 000, 75, 000 and 1, 00, 000 hours. Metallurgical characterisation involved tension test, hardness determination, optical microscopy and scanning electron microscopy. Ultrasonic parameters monitored included attenuation at different frequencies and velocity (longitudinal and shear). Service aged samples were heat treated at 973 K for 6 hours and were characterised to ascertain the recovery of properties.
Results obtained during this study indicate:
- ageing degradation of alloy 625 is taking place during long term elevated temperature exposure,
- it is feasible to monitor the extent of hardening non-destructively by ultrasonics,
- properties can be recovered by heat treatment and the same can also be ensured by monitoring ultrasonic parameters.
NDTnet
