Microstructural characterisation and micromechanical investigation of the heat-affected zone in multi-pass welding of 9Ni steel pipes

Abstract

9Ni steels have been recently adopted in supercritical CO2 injection systems in deepwater oil fields. The manufacture of these reinjection systems involves multi-pass welding procedures, which produce an Heat-Affected Zone (HAZ) with a high heterogeneity level regarding the microstructural features and the local mechanical properties. An extensive microstructural and micromechanical characterisation was performed over the HAZ of three welded joints with different heat-input conditions to evaluate the effects of the reheating cycles and the welding parameters on the microconstituents. Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD) analyses were performed to identify microstructural features that correlate to the local mechanical responses evaluated through an extensive microhardness mapping. Regarding the Coarse Grained HAZ (CGHAZ), the highest microhardness values for all welding conditions are found at the Supercritically Reheated CGHAZ (SCR-CGHAZ), characterised by its refined microstructure and a quite low area fraction of coarse martensite laths. The Subcritically Reheated CGHAZ (SC-CGHAZ) and the Intercritically Reheated CGHAZ (IC-CGHAZ) – regions where wider martensite blocks and higher coarse martensite lath area fractions were observed – composed the softer zones of the microhardness map. It was also found that reheating at intercritical temperatures induces the formation of supersaturated fresh martensite and may contribute to retained/reversed austenite particles’ C-enrichment, which may degrade the mechanical properties at the IC-CGHAZ.