Characterisation of Microstructure in 9 % Chromium Ferritic Steels Using Ultrasonic Measurements

Ultrasonic measurements have been performed for comprehensive microstructural characterisation in 9 % Cr ferritic steels. The variation in grain size, the effect of solution annealing temperature on microstructure developed, the variation in microstructure of the weld joints, the effect of post weld heat treatment and precipitation behaviour during thermal ageing and creep testing have been successfully evaluated by ultrasonic measurements. The ultrasonic velocity has been correlated with the hardness and the volume fractions of ferrite and martensite. For the first time, it has been shown that ultrasonic velocity measurements can be used for the identification of Ac1 and Ac3 critical temperatures. Ultrasonic attenuation and a new ultrasonic parameter, spectral peak ratio (SPR), have been found useful for the assessment of grain size and identification of various metallurgical features affecting the grain size such as dissolution of carbides and formation of d-ferrite. The benefits of SPR over conventional attenuation measurements have been demonstrated. The study revealed that a single ultrasonic parameter cannot characterise the solution annealing behaviour completely, and hence ultrasonic velocity, attenuation and spectral analysis of the first backwall echo need to be used in a complementary manner for comprehensive characterisation of the solution annealing behaviour. The ultrasonic velocity measurements performed on different solution-annealed specimens have been used for the imaging of weld metal and heat affected zone, obtaining the weld profile and also for assessment of the post weld heat treatment. The study revealed that the ultrasonic velocity could also be used effectively for investigating the microstructural degradation occurring due to thermal ageing and creep. The present study has provided a better insight into the ultrasonic wave/microstructure interaction and established new methodologies for non-destructive assessment of various microstructures in 9% Chromium ferritic steels useful for practical situations. TP 1