NDE of High Temperature Stainless Steel Used in Biomass Gasifier

A biomass gasifier was in operation for sometime and later failed near the weld portion. A portion of the sample near the failed region was cut and used for this investigation. Study based on ultrasonic velocity measurements, metallography and X-ray diffraction was carried out at different portions of samples of stainless steel used in the biomass gasifier. Precise ultrasonic velocity measurements were performed by pulse echo method using cross correlation technique. Longitudinal ultrasonic wave generating probes of 5 and 20 MHz were used. It is found that there is an increase in ultrasonic velocity in samples obtained from the exposed portion as compared to the virgin samples. Metallographic studies revealed that there is a microstructural reformation taking place due to prolonged temperature treatment at the regions away from the failed portion. Very near the failed portion, there is intergranular type crack propagation as a result of partial sensitization and embrittlement. X-ray diffraction study is in support of the metallographic observations. The increase in ultrasonic velocities in service exposed specimen with and without intergranular cracks was attributed to the continuous changes taking place in the microstructures. It is suggested that periodic monitoring of the critical parts especially the combustion zone by NDE techniques would help avoid premature failure of the biomass gasifier plant.