The AC magnetic testing method of high exciting frequency (60kHz) by using a coaxial probe is proposed to evaluate the Post Weld Heat Treatment (PWHT) temperature, nondestructively. Low alloy steel welded joint specimens (1.25Cr-0.5Mo and 2.25Cr-1Mo) and pipes (1.25Cr-0.5Mo) under several PWHT temperature conditions were investigated. It is found that the normalized 3rd harmonic component intensity (Fh{sub}3) of the detected signals, which is defined as the ratio of the 3rd harmonic component intensity to the fundamental harmonic component intensity, changes considerably with the PWHT temperature. The normalized 3rd harmonic component intensity is related to the pinning strength k in the JA model. The regression curve based on JA model agrees with the experimental results of 1.25Cr-0.5Mo weld joint pipes.
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