High chromium steel is widely used in ultra-super critical thermal power plants for its high creep strength. However, in structural testing and analysis of welds extracted from weld joints run in actual plants for a long time, abnormal structures with lower hardness than base material had been observed in welded joints of Mod. 9Cr1Mo steel. Experimental results suggest that such abnormal structures result in a decrease in creep strength, therefore it is crucial to develop a nondestructive examination method to evaluate them quantitatively.

In this study, an ultrasonic testing method based on the frequency and scattering noise analyses was proposed to quantitatively evaluate the area ratio of abnormal structures (ARAS). Nine specimens with different thickness and weld shapes were used in the study. Three of them are abnormal structure free and the rest extracted from actual plants contain different ARASs. Three transducers with frequencies of 5, 10 and 15 MHz were used. Experiment results show that the frequency of 10 MHz is most sensitive to abnormal structures. Signals from backwalls were acquired and their Fourier transforms were performed. Results for welds with abnormal structures were compared to those for abnormal structure-free welds and for base material. The comparison shows that ultrasonic waves severely attenuate while they travel through welds containing abnormal structures and the frequency of backwall echo decreases due to the abnormal structure. Besides frequency analysis, the comparison of backscattering noise between abnormal structure-free welds and welds with abnormal structures was conducted as well.

The dependence of the frequency of backwall echo and backscattering noise on ARAS was revealed. Experimental results show that the backscattering noise level linearly increases with the increase of ARAS, however, the variation of frequency has more complicated behavior (Figure 1). According to the experimental results, a method was proposed to determine the ARAS and it was verified by a blind test. The determined ARAS is in good agreement with the macrophotography result.


Quantitative Evaluation of Abnormal Structures in High Chromium Steel Welds Using Ultrasonic Testing Technique

No proceedings papers in 2020. Video only.

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