Weldment Evaluation of High Pressure Steam Rotors

Abstract

Life extension of high pressure and intermediate pressure rotors by weld repair is a viable alternative considering the latest developments. Our objective is to select a filler wire that will produce a weld with mechanical properties compatible with those of the rotor material. Three filler metals, (CrMoV, 12 percent Cr and 5 percent Cr) were used in the investigation. Multipass submerged arc groove welds were produced, post weld heat treated at 677° C (1250°F), and submitted to a series of mechanical tests at room and elevated temperatures. The tests samples were machined parallel to the weld direction (longitudinal), which included only weld metal, and perpendicular to the weld metal (crossweld) so that the test sample includes portions of the weld metal, heat-affected zone (HAZ) and base metal. A limited metallurgical evaluation was also performed. The room temperature tensile properties of the CrMoV and 12 percent Cr crossweld samples exceeded those of the rotor metal, but the tensile properties of the 5 percent Cr crosswelds did not match those of the rotor metal. The 12 percent Cr crossweld samples failed in the weld metal during hot tensile and stress rupture testing, and these failures were attributed to slag entrapment; yet the yield and tensile strengths of these weldments just exceeded those of the rotor base metal. The CrMoV crossweld specimens performed the best at the high temperature testing; the failure for the hot tensile specimens occurred in the weld metal and the values matched those of the all weld metal (longitudinal) specimens which greatly exceeded the rotor base metal strengths. The crossweld stress rupture samples failed outside the weld metal at the heat affected zone (HAZ) near the unaffected base metal with rupture lives lower than the rotor base metal. Microhardness evaluations in the CrMoV and 12 percent Cr cross weldments about the HAZ/base metal boundary identify a soft region at this location. The mechanical properties of the 5 percent weldment were in general inferior to the rotor base metal. The weld metal’s hardness was lower than the rotor base metal given its superior Charpy energy values; however, the creep rupture strength was inferior.