Finite Element Modelling of Porosity in Cobalt Base-Molybdenum-Chromium-Silicon Coatings with Gurson Damage Model

This work presents the finite element modelling of porosity in super alloys coatings, developed with cobalt-base/chromium/molybdenum/silicon metallic powders, which were thermally sprayed on oil & gas steel pipeline substrates, with the aim to protect the steel against H 2 S and CO 2 corrosive environments. Therefore, in the developed finite element models, a small level of porosity, identified and analysed on the cross-section of the developed coatings, was incorporated in the developed models in order to perform a more realistic analysis of the structural response of the coating with some level of porosity by the local damage modelling technique. The porosity was incorporated in the developed finite element models with the micromechanical Gurson-Tvergaard-Needleman damage model, consequently the damage model parameters of Gurson-Tvergaard-Needleman model were calibrated against the true stress-strain material curve of the coating. The damage model was applied only on the finite elements subjected to higher bending loads. The values of and damage parameters are in the range of those published in the literature, for different type of steels, however value was lower, showing that for super alloy coatings, is quite lower than for steels. For the case of the initial and critical void volume fraction, the best calibrate values are higher compared to steels values reported in the literature. The relative density was similar compared to data published in the literature. Once the damage model parameters were properly calibrated, the modelling was employed to evaluate the stresses and strain states in the coating/substrate structure and in coating-substrates interface. The developed models were able to properly simulate the hardening material response of the coating with good agreement with material data. The results showed that Gurson-Tvergaard-Needleman damage modelling technique was able to model porosity damage in cobalt-base/chromium/molybdenum/silicon hard coatings, since numerical results agree well with true stress-strain material curve of coating material.