Evaluation of Creep Damage and Dynamic Recrystallization in FGHAZ of Mod. 9Cr–1Mo Steel with Fractal Dimension of Grain Boundaries, GOS and KAM

Abstract

In the Fine Grain Heat Affected Zone (FGHAZ) of Mod. 9Cr–1Mo steel, the shape of grain boundaries is complex. It becomes simpler with creep and grain growth. The complexity of the grain boundaries is expressed with fractal dimension. Prior research found that the fractal dimension of FGHAZ decreased with creep damage, but then saturated or slightly increased, which might imply dynamic recrystallization.

The objectives of this research are to clarify the cause of the phenomenon and to confirm that the fractal dimension of grain boundaries can be used as the indicator of creep damage and dynamic recrystallization.

In this research, creep interruption tests were conducted with the specimens of simulated FGHAZ of Mod. 9Cr–1Mo steel. Two types of specimens with different stress multiaxiality and the same maximum principal stress were used. After the tests, the microstructures were observed by EBSD to measure the fractal dimension of grain boundaries, GOS average, the area ratio of recrystallized grain, KAM average, and the average grain size. Vickers hardness was also measured.

In the specimen of the higher stress multiaxiality, the average grain size and the area ratio of recrystallized grain increased and other indicators decreased with creep. However, in the specimen of the lower stress multiaxiality, these trends were different at mid-life. They were considered to represent dynamic recrystallization.

It was concluded that the fractal dimension of grain boundaries can be used as one of indicators of creep damage and dynamic recrystallization and that the dynamic recrystallization was affected by stress multiaxiality.