Microstructural evolution of as-rolled modified 9Cr-1Mo steel during friction stir welding

Friction stir welding was tried on a modified 9Cr-1Mo (wt.%) ferritic steel in an as-rolled condition. The microstructure of the resultant weld was divided into four distinct regions according to each thermo-mechanical history experienced during welding; i.e., stir zone, thermo-mechanically affected zone, inner heat-affected zone, and outer heat-affected zone. The first three zones showed distinct martensite morphologies depending on the different recrystallization phenomena during the heating cycle in the single-phase austenite region. In the outer heat-affected zone, however, only tempering occurred without phase transformation owing to a relatively low heating temperature. Hardness distribution of the weld closely reflected such microstructural differences, indicating that a considerable softening occurred in the thermo-mechanically affected zone and outer heat-affected zone owing to the coarsening and tempering effects, respectively.