Analysis of Degradation in KANUPP's Extraction steam piping due to Ageing

KANUPP belongs to those 25% of nuclear power plants which are in operation for more than 30 years. Assessment of degradation due to ageing in KANUPP's Extraction Steam Piping has been done. Samples of twelve years old seamless carbon steel SA 106B pipe have been taken and subjected to metallographic examination, hardness testing, radiographic examination and ultrasonic thickness measurement to investigate and analyze ageing in piping material. Examination of outer surface under microscope revealed corrosion pits having depth up to 0.07mm. Radiograph of aged pipe revealed material removal and wall thinning due to erosion on inner surface of pipe. The extent of erosion has been checked by measuring reduction in thickness via ultrasonic testing, which is 8.7%. Reductions in hardness and strength have also been observed which is 20.4% and 0.7% in inner and outer layer of the pipe material respectively. Metallographic examination showed that initially present bands or pearlite and ferrite have been eliminated due to prolong exposure to high temperature and microstructure of aged steel has uniform mixture of both phases. Irregularly distributed micro graphite nodules have occurred in ferrite grains and on grain boundaries as well, because of detachment of carbon from iron in pearlite, this has reduced pearlite content from 40% to 25%, the reduction in pearlite is more in steel of internal layer of pipe as this layer was facing hot and pressurized steam directly. This Transformation in microstructure reduces hardness and strength of steel. Extent of degradation in the samples provided has not been found very high, if the condition of whole extraction steam piping is found same as the condition of samples provided, then it could be used further. Heat treatment has been done on SA 106B samples with different soaking time to produce different structures having different hardness, and then the samples were inspected via eddy current. For the samples of same material, the curves are different because of different microstructure and hardness. It suggests that ageing can be assessed by eddy current test which is a non-destructive test. This technique could also be helpful in investigation of ageing affects on hardness and microstructure of inner layer of pipe which is not possible by UT and radiography. (author)