Experience with 9Cr3W3CoVNbBN base material and crosswelds at 650°C for implementation in USC power plants

Abstract

This manuscript gives an overview on recent developments of a martensitic steel grade based on 9Cr3W3CoVNb with controlled additions of boron and nitrogen. Alloy design by thermodynamic equilibrium calculations and calculation of boron-nitrogen solubility is discussed. Two melts of a 9Cr3W3CoVNb steel were produced. The oxidation resistance was examined at 650°C in steam atmosphere. The alloy exhibited high resistance to steam oxidation at 650°C for at least 5.000 hours of exposure. Microstructural evolution during welding within the heat-affected zone was investigated by physical weld simulation, thermo-dilatometry and in situ X-ray diffraction using high energy synchrotron radiation. The heat-affected zone microstructure is characterized by optical microscopy. Two multilayer welds, one with a nickel based filler and one with a newly developed flux cored filler of matching chemical composition, were fabricated. All weldability experiments showed that within the heat-affected zone the initial base material microstructure is retained to a large extent. The formation of a uniform fine-grained heat-affected zone region is suppressed. Uniaxial creep tests of base material, weld metal and crossweld specimens were started at different levels of stress at 650°C. Up to the established maximum testing time of 20.000 hours, the base material shows significant improvement compared to standard 9 wt. % Cr grades P91 and P92. Crossweld specimens show an improved strength level, higher than that of grade P92 base material. Preliminary results of an extensive testing program on a 9Cr3W3CoVNbBN steel show significant improvement compared to the state-of-the-art 9 wt. % Cr martensitic steel grades.