Microstructural and local electrochemical characterization of gr. 91 steel welded joints as a function of Post Weld Heat Treatments

摘要

The gr. 91 steel is widely used as a structural component in the energy production field. Many ofthese components are joined by welding and subjected to both high temperature and high internalpressures in service conditions. The welds are the main candidates to be severely affected bycorrosion due to both chemical and microstructural heterogeneities present in the joint.Plates were welded by gas tungsten arc welding technique. The joints were subjected to twodifferent Post Weld Heat Treatments (PWHT): tempering at 760°C or normalization at 1080°Cfollowed by tempering at 760°C. Welded joints without heat treatment were also studied as areference.The welded joints underwent to microstructural (Scanning Electron Microscope and LightMicroscope) characterization (micro HV).The corrosion resistance of the specimens was evaluated by means of different corrosion testsand by electrochemical micro-cell technique, that enables to determine the electrochemicalbehaviour of each zone of the weld joint (Base material, HAZ’s and weld bead). In particular, thetests have been performed at micro-scale with electrochemical micro cell and the corrosionmorphologies have been evaluated by means of SEM analysis. The joints have been evaluatedalso on macro scale, by performing a potentiodynamic test on the HAZ. The corrosionmorphologies have been then evaluated by SEM.It was found that both the as welded and the tempered samples presented a lack of corrosionresistance in the HAZ where the Cr carbides precipitated at grain boundaries during the weldingprocess. The normalizing plus tempering treatment is the unique PWHT that reconstructed themicrostructure, producing a material with no zones of grain boundary precipitation of Cr carbides.Generally speaking, the corrosion resistance of material is strongly related to shape, distributionand chemical composition of precipitates produced by heating and cooling the material (weldingprocess or heat treatment).