Study on the evaluation of the aging grade for industrial heat-resistant steel by laser-induced breakdown spectroscopy

Heat-resistant steel is widely used in various types of industrial pressure-bearing equipment, and the changes in the metallographic structure and properties of the steel are of great importance for the function and safety of this equipment. In this work, laser-induced breakdown spectroscopy (LIBS) was applied to estimate the aging grade of industrial heat-resistant steel. First, representative spectra of the metal matrix of an industrial pipe were obtained via depth profile analysis, and the effects of surface oxides on the matrix plasma parameters were evaluated and compared. The results indicated that correlation analysis of the emission intensities along the radial direction can accurately identify the boundary between the surface oxide and the matrix. The presence of surface oxides would decrease the plasma temperature and increase the electron density. Secondly, multiplicative scattering correction (MSC) was applied to correct for the interference of surface oxides, and K-fold-support vector machine-recursive feature elimination (K-SVM-RFE) was exploited to select the optimal spectral feature subset. Finally, estimation of the aging grade model was established based on the support vector machine (SVM) and the optimal feature subsets. The estimation accuracy of the validation set after spectral correction was improved from 80.56% to 84.72%, and the model exhibited similar capability for the industrial pipe and the corresponding samples with mechanical polishing to eliminate surface oxides. The work demonstrates that LIBS is suitable for the in situ estimation of steel aging grade for industrial pipe.