Oxidation evolutions in different regions of 9Cr3W3Co martensitic steel welded joints exposed to oxygenated supercritical water at 650 °C

Abstract

The oxidation behaviors of 9Cr3W3Co steel welded joints exposed to supercritical water with dissolved oxygen of 300 ppb at 650 °C, 30 MPa for 50–1500 h were studied. The results showed that the grain boundary density of the weld metal was slightly greater than that of the equiaxed grain zone and significantly greater than that of the inter-critical zone and the base metal, but the fine grain band of the inter-critical zone had a larger percentage of the prior austenite grain boundaries. Different from the base metal and heat affected zone, the weld metal was virtually free of Cu. The higher grain boundary density promoted the growth of the internal oxide layer at the initial stage of oxidation, and accelerated the formation of continuous FeCr₂O₄ layer with the extension of oxidation time. Cu promoted the growth of outer layer of oxide film in heat affected zone and base metal, and delayed the formation of the continuous FeCr₂O₄ layer. Consequently, at the initial stage of oxidation, the oxide film formed in the equiaxed grain zone was the thickest. After long-time oxidation, the continuous FeCr₂O₄ layer was first formed in the weld metal, resulting in the thinnest oxide film of weld metal. In addition, the FeCr₂O₄ band formed along various boundaries led to the formation of wavy oxidation fronts in each sub region of the welded joint after long-time oxidation.