Abstract：During the scanning analysis of steel sample by laser-induced breakdown spectroscopy (LIBS), MnS inclusions would cause the abnormal signal both for S and Mn at the same time. Moreover, the signal intensity had linear relationship with the inclusion area. According to the above characteristics and the inspection requirements in German standard DIN 50602, a rating method for MnS inclusions by LIBS was proposed. In experimental method, the fitted curves between signal intensity and inclusion area for both S and Mn elements were obtained based on specimen statistics, and the curves were used as calibration curves. Then the field of view with highest total signal intensity of characteristic elements in inclusion was found within the scanning region and used as the field of view with most inclusions. The inclusion area in the field of view was obtained by inversion calculation with the signal intensity of inclusions according to the calibration curve. Finally, the rating grade was calculated according to the relational expression between rating grade and inclusion area. The contrastive analysis of the proposed LIBS method and traditional metallographic method was implemented on 34CrNiMo6 steel specimen for the main shaft of wind generator. The results showed that, when the content of MnS inclusion was low (n<3), the results of Mn obtained by the LIBS method had significant difference with those obtained by the traditional metallographic method. But the results of S were basically consistent for both methods. When the content of MnS inclusion was high(n≥3), the analysis results of S and Mn by the proposed LIBS method were in good agreement with those obtained by the traditional metallographic method. The proposed method could meet the precision requirements of conventional metallographic examination.
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