Microstructural evolution of a 12Cr-2W-Gu-V-Nb steel during three-year service exposure

Microstructural evolution of 12Cr-2W-Cu-V-Nb steel tubes (ASME SA213-T122) after one-year and three-year service exposure tests in a Japanese practica l boiler has been investigated from a standpoint of the phase stability of precipitates. The test tubes consist of tempered martensite and delta -ferr ite, and the main precipitates are MX-type carbonitride. M23C6 carbide and Laves phase. TEM observations on thin films show that the MX has precipitat ed in a plate-shaped with a coherent or semi-coherent relationship with the matrix inside grains. An estimation of the lattice misfit between MX and t he matrix suggests that the coherent strain has been high enough to enhance the shear stress and then strongly interact with dislocations. Another imp ortant point is that morphology and compositions of MX have been stable und er the present service conditions, thereby the creep strength as well as te nsile strength has kept high after long-term service exposure. The long-term exposure to the present service temperature has enhanced the precipitation of Fe-2(W, Mo) Laves phase inside grains, resulting in a mark ed reduction in the dissolved W and Mo in matrix. It is found that the kine tics of W-partitioning between matrix and Laves phase can be successfully e xpressed by the Johnson-Mehl-Avrami type equation and applied to estimate t he actual temperatures of the exposed tubes. It is concluded that the kinetics of Laves phase precipitation and morpholo gy of MX have mainly controlled a microstructural stability in the 12Cr-2W- Cu-V-Nb steel, and also give helpful suggestion to increase the creep resis tance during the long-term service exposure.