EVALUATION OF CASE DEPTH ON STEELS BY BARKHAUSEN NOISE MEASUREMENT

Use of eddy currents to measure the depth of surface modified layers i n ferromagnetic materials has been the subject of numerous studies. Th is technique, however, is limited by surface permeability which consid erably reduces skin depth value. The Barkhausen noise technique, used to evaluate residual stresses, has also been used to evaluate case dep th in carburised steels (AISI 8620 and 9310) and ion nitrided tool ste els (AISI P20, D2, and H13). A method employing digital signal process ing, which includes a fast Fourier transform has been applied at frequ encies ranging from 0 to 200 kHz. Experiments show good correlation be tween magnetic Barkhausen noise and hardening depth values between 100 and 1000 mu m for carburised steels, and between 25 and 200 mu m for nitrided steels.