NONCONTACT ULTRASONIC MEASUREMENTS ON STEEL AT ELEVATED-TEMPERATURES

Non-contact ultrasonic measurements have been made on ferritic and austenitic steel specimens as a function of temperature from ambient to 1200-degrees-C, using a pulsed laser to generate and a reference beam laser interferometer to receive the ultrasound. The. generation efficiency is found to remain surprisingly constant in both thermoelastic and ablation regimes over a wide temperature range. The sensitivity of the laser interferometer is also found to be temperature independent to a first approximation. However, it is typically reduced by 3-6 dB by convection currents above approximately 900-degrees-C. Both the compression and shear velocities decrease with rising temperature. The former is measured with a precision of 1 in 10(3), the latter rather less accurately with the present configuration. Compression wave attenuation increases steadily below 600-degrees-C in both materials. There is a peak in attenuation in ferritic steel between 600 and 750-degrees-C, which is absent in austenitic steel. It coincides with a steeper decrease in ultrasonic velocity and is believed to be due to the martensitic structural phase transformation. The attenuation rose more rapidly in both materials as 1000-degrees-C was approached. The material attenuation varied with heat treatment, a value in the range 1-1.5 dB cm-1 being recorded at 1000-degrees-C. Complicated effects were observed during heat treatments at 1000-degrees-C and above. Both attenuation and forward scattering data were consistent with some annealing out of sub-structure, in addition to austenitic grain growth. Finally, there was evidence of lattice softening at the highest temperatures investigated. The data suggest that thicknesses of steel in the range 100-250 mm should be inspectable with a scaled-up system, depending upon various factors such as the presence of oxide scale, provided high power lasers are employed for generation and reception and an optimum bandwidth is chosen.