The quantitative evaluation of the relationship between creep deformation and microstructure is important in order to improve the accuracy of the residual creep life evaluation of power boilers and turbines. In this study to characterize the creep deformation mechanism of a Modified 9Cr-1Mo Steel used in newly constructed boilers, stress change tests were conducted during creep tests. It was confirmed that the dislocation behavior during the creep tests were in viscous manner because of no instant plastic strain observed at stress increments and transient backward creep behavior after stress reduction. Mobilities of dislocation evaluated by observed backward creep behaviors after stress reductions and internal stresses evaluated by the changes of creep strain rate in stress increments were stable during creep deformation. Mobile dislocation densities were evaluated with the estimated mobilities of dislocation and the changes of creep strain rate in stress increments. Variation of evaluated mobile dislocation densities with creep strain showed same tendency of variation of creep strain rate. Therefore mobile dislocation density is the dominant factor that influences the creep strain rate in creep deformation of this steel. The internal stress of mobile dislocation can be considered to be originated from the line tension of bowing mobile dislocation in this steel.
