Evaluation of the microstructure and localized corrosion behaviour of AISI 2507 super duplex stainless steel welds

Duplex stainless steels are a category of stainless steels with a microstructure containing approximately equal amounts of ferrite and austenite. They have desirable properties of both austenitic and ferritic stainless steels, such as high tensile strength, low temperature toughness and excellent resistance to corrosion, particularly chloride stress corrosion cracking). Poor welding has induced adverse microstructural features and consequently increased the susceptibility to various forms of corrosion such as stress corrosion cracking and intergranular corrosion. The focus of this research study was to (i) conduct an in-depth microstructural analysis of the various weld passes of gas tungsten arc welded (GTAW) AISI-2507 super duplex stainless steel plate, in terms of the weld morphology, phase transformations within the weld regions and distribution of major alloying elements and (ii) determine the susceptibility to various forms of localised corrosion to include the degree of sensitisation (inter granular corrosion), susceptibility to pitting corrosion in terms of the critical pitting temperature and corrosion rate assessment using potentiodynamic scanning. Microstructural variations were observed not only between the base metal and weld regions, but between the different weld regions themselves. Phase identification using X-ray diffraction and electron back scattered diffraction (EBSD) techniques confirmed the absence of detrimental secondary phases and intermetallic precipitates in the base metal, while in contrast emphasized the presence of sigma, chi, and chromium carbides in the weld regions. Vickers hardness measurement showed relatively higher values for the root pass and filler pass compared to the base metal while the cap pass showed the lowest hardness values. This has been correlated primarily with differences in the ferrite content between these regions and also other factors such as grain size, presence of intermetallic precipitates, and concentration of interstitial atoms such as C and N. A reproducible DL-EPR test method was successfully developed to characterize the degree of sensitization in the super duplex stainless steel grade 2507. This DL-EPR test induced the most favourable selective attack conditions for studying sensitisation, without inducing other forms of corrosion such as pitting or general corrosion. The critical pitting temperature was shown to be a maximum for the base material and lowest for the cap region. The pitting corrosion resistance (elevated CPT) was observed for the weld regions with increased ferrite (fill region). Likewise, increased amounts of austenite (cap and root) were accompanied by a reduction in the CPT. This has been confirmed by corrosion rate measurements where lower corrosion rates were observed for studies conducted at temperatures lower than the CPT and higher corrosion rates were observed for studies conducted at temperatures higher than the CPT. Correlations existed between microstructural variations and corrosion behaviour of welded components and the un-welded base material, revealing that the corrosion performance was strongly influenced by microstructure features, such as ferrite/austenite ratio, grains shape and size, and the presence of chromium rich intermetallic precipitates and secondary phases.