Solidification phenomena in creep resistant 9Cr weld metals and their implications on mechanical properties

Abstract

AbstractMartensitic 9% Cr steels play an important role in the implementation of modern and efficiency-enhanced power generation technologies. In the presented study, metallurgical solidification phenomena in heat-resistant 9% Cr filler metals and their effects on the mechanical properties of the weld metal were analysed. The focus was on welded joints of steel grades P91 and CB2, which were welded with flux-cored wires. The investigation of welded joints and weld metals in the creep-damaged and unloaded condition provided detailed inspects into the formation and development of inhomogeneous areas. The microstructure of the individual weld metals was characterized in detail in the as-welded and in the heat-treated condition. It became apparent that inhomogeneities were formed in large areas of the weld metal. In particular, EDX measurements made it possible to explain these solidification phenomena and trace their development within the manufacturing process of the welded joints. It was found that even a slight uneven distribution of chromium and a diffusion of carbon caused extensive negative effects on the development of weld metal microstructures. In addition, the influence of these microstructural inhomogeneities on both the mechanical weld metal properties and the creep rupture strength is discussed. Finally, test welds were performed to optimize the microstructure of the flux-cored wire weld metal and possibilities to avoid microstructural inhomogeneities were derived. The results show that the short-term and long-term properties of the weld metal are affected by the inhomogeneous areas within the weld metal. In summary, it can be assumed that for steel grades P91 and CB2 the safety of plants in high-temperature operation is not endangered.