Structural stability of dissimilar weld between two Cr-Mo-V steels -: Results are presented of an experimental study and modeling of the structural stability of 6Cr-Mo-V 8-3-2 and X12Cr-Mo-Nb 10-1 steels in a temperature interval from 600° to 900°C

Heterogeneous weld joints of creep-resistant steels are used with increasing frequency in the construction of new or repair and reconstruction of existing power-generating and chemical industry facilities. These weld joints show structural instability during postweld heat treatment (PWHT) and in the course of subsequent operations. Carbon redistribution and subsequent sequence of changes in the structure lead to the formation of a carbon-depleted zone (CDZ) in low-alloy steel. On the contrary, a carbon-enriched zone (CEZ) appears in high-alloy steel, with the differences in chromium content having the greatest significance. This paper presents the results of a study of structural changes in laboratory welds of 6Cr-Mo-V 8-3-2 (T25) and X12Cr-Mo-V-Nb 10-1 (P91) steels annealed at temperatures from 600 degrees to 900 degrees C (1112 degrees to 1652 degrees F). Carbon redistribution measurements by the EPMA method were complemented with detailed structural analyses aimed at the phase and chemical compositions of coexisting carbides and carbonitrides. The results of experimental work were compared with thermodynamic and kinetic calculations using the Thermo-Calc and DICTRA software. A very good agreement between the calculations and the experiments was obtained, in particular for the phase composition of individual areas of the weld joints.