Micromagnetic Barkhausen emissions in 2 center dot 25 wt% Cr-1wt% Mo steel subjected to creep

Micromagnetic Barkhausen emissions have been measured from a number of specimens of Cr-Mo steel in order to study the effects of creep. Creep causes changes in the microstructure of materials as a result of exposure to a high temperature and pressure. Scanning electron micrographs showed that all the specimens were at an early stage of creep. The number density of cavities in specimens which were taken from the outer surface of the pipe was lower than in specimens taken from the inner surface, indicating lower creep on the outer surface. Rms and peak-to-peak voltages of Barkhausen emissions were found to increase from the outside to the inside of the pipe, indicating that these emissions increase with increasing level of creep. Segregation of impurities, which takes place during creep, was considered to be the primary cause of the increase in Barkhausen emission signals. A model has been developed which showed that Barkhausen voltage should increase up to a critical value of the number density of cavities, beyond which it should decrease. As the specimens measured in this study were in the initial stage of creep, only an increase in Barkhausen voltage (rms and peak to peak) was observed with creep across the range of specimens investigated. In the experimental measurements an angular variation in rms voltage was also observed which was explained by the orientation of the easy magnetization axis due to the development of long-range residual stresses in a direction different from the pipe axis.