Microstructural evolution of modified 9Cr-1Mo steel due to annealing and high-temperature fatigue

The microstructural evolution of Modified 9Cr-lMo steel has been investigated for a variety of conditions, including normalizing, tempering and subsequent annealing. The alleged advantages of the modified alloy for high temperature service, when compared to the standard 9Cr-lMo composition, have been examined for high temperature fatigue conditions; the only difference between the two alloys is the presence of micro additions of V and Nb in the modified alloy.Air cooling (normalizing) of Modified 9Cr-lMo from 1045°C results in the precipitation of fine (FeCr)3C particles within the martensite laths. Additional carbide precipitation and changes in the dislocation structure occur during the tempering of martensite at 700°C and 760°C after normalizing. The precipitation of M23C6 carbides occurs preferentially at lath interfaces and dislocations. The formation of Cr2C was detected during the first hour of tempering over the range 650-760°C, but was replaced by VC within one hour at 760°C. During prolonged annealing at 550°C-650°C, following tempering, the lath morphology remains relatively stable in the absence of simultaneously applied stresses; partitioning of the laths into subgrains and some carbide coarsening are beginning after 400 hr. annealing at 650°C (Fig. 1), but the lath morphology persists. The martensite lath stability is attributed primarily to the VC precipitates distributed along the lath interfaces (Fig. 2) and promotes improved performance of the Modified 9Cr-lMo alloy over the the standard alloy under static tensile and creep conditions.