MICROMAGNETIC BARKHAUSEN EMISSIONS IN 2-CENTER-DOT-25 WT-PERCENT CR-1WT-PERCENT 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. Cree p causes changes in the microstructure of materials as a result of exp osure to a high temperature and pressure. Scanning electron micrograph s showed that all the specimens were at an early stage of creep. The n umber 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 inc rease with increasing level of creep. Segregation of impurities, which takes place during creep, was considered to be the primary cause of t he 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 decre ase. As the specimens measured in this study were in the initial stage of creep, only an increase in Barkhausen voltage (rms and peak to pea k) 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 m agnetization axis due to the development of long-range residual stress es in a direction different from the pipe axis.