Non-metallic inclusions (NMI) are simple or complex compounds that result from chemical interaction between alloy or impurity elements and gases dissolved during steel welding processes. Morphology, size, distribution and predominantly chemical nature of NMI affect the microstructure of welded metals and, as a consequence, their mechanical properties. Therefore, awareness of the type of inclusion which could be produced during welding of steels, is particularly important for prediction or interpretation of their performance.

In the present work, it has been developed a software based on equilibrium thermodynamic calculations that consider multicomponent interactions in diluted liquid steel solutions, with the aim to construct stability diagrams that could describe satisfactorily NMI formation during welding. The first stage of the project involved the establishment of the main reactions that give place to the formation of NMI. Then, applying Van't Hoff isotherms, the equilibrium composition for each compound was determined, considering the effects of chemical composition of alloys and interaction between alloying elements. The nonlinear equations system was solved by Newton method. Graphs were generated as a result of iterations. The numerical results obtained, were compared with NMI observed in different High Strength Low Alloy (HSLA) steels, which are particularly sensible to the presence of inclusions. The analysis of NMI was performed by energy dispersive spectroscopy (EDS) adapted to a scanning electron microscopy (SEM), on metallographic prepared or fracture surface of the alloys.

It was observed a good correspondence between predictions from the program and the type of inclusions suggested by chemical analysis over the different alloys studied, even those with complex character.

Keywords: thermodynamic diagram, non-metallic inclusion, HSLA steels, welding.