Mechanical property and microstructure characterization of incoloy 800H alloy and its welds after corrosion testing in high temperature steam

Abstract

Incoloy 800H is currently being considered as one of the near-term candidate materials for design and construction of components in the very high temperature reactor (VHTR) and supercritical water reactor (SCWR), both expected to operate at high temperatures. Many of the components deployed in the nuclear industry are fabricated using conventional arc welding processes. Such welding processes can significantly degrade the mechanical properties of components. The objective of this work was to study mechanical properties of Incoloy 800H base metal (BM) and its weldments as affected by corrosion testing in high temperature steam. Tubular Incoloy 800H samples were welded using a gas tungsten arc weld (GTAW) technique. Steam corrosion testing of the samples was conducted in an autoclave at 575 °C and 23.5 MPa for 500 h. Tensile properties of the samples were measured before and after corrosion testing. Both the BM and its weldments displayed an increase in strength but a decrease in ductility after corrosion testing. The degree of strength enhancement of the weldments was much higher than that of the BM; the ultimate tensile strength (UTS) increased by 12.5 % for the weldments in comparison to 1.8 % for the BM. Conversely, the ductility of both the BM and weldments was dramatically degraded after corrosion testing; elongation decreased around 35 % for both the BM and weldments. Microstructural characterization of the samples before and after corrosion testing was performed by optical and scanning electron microscopy. The microstructures showed indications of second phase precipitation and changes in grain morphology produced by the processes of welding and corrosion testing. The relationship between the mechanical properties, microstructural evolution, and thermal ageing are discussed.