Stabilization of retained austenite in Cr-Mo steel by microalloying and friction stir welding

The stabilization of retained austenite in an alloyed steel was attempted by the combination of the microalloying of Nb and the intense plastic deformation by friction stir welding (FSW). The Nb-bearing Cr-Mo steel sheet (0.18wt%C- 1.14%Cr- 0.17%Mo-0.09%Nb- bal. Fe) and the Cr-Mo sheet without Nb (0.20%C- 1.07%Cr- 0.16%Mo- 0.00%Nb- bal. Fe) were prepared and friction stir welded. Using the Nb-added Cr-Mo steel, the two types of pre-treatments before the final FSW process were imposed in order to change the state of the NbC. The electron back scattering diffraction (EBSD) analysis clarified that it is possible to significantly promote the stabilization of the retained austenite during the FSW if the NbC is resolved and refined. However, the amount of the retained austenite is rather decreased when the coarse NbC is formed. This result indicated that the accumulation of dislocations is the key phenomenon for stabilization of the austenite since it is well known that the fine NbC or solute Nb can prevent the dynamic recovery.