Welding Metallurgy of Martensitic 9% Cr Steels and Implications on Crossweld Creep Properties

Welded structures of martensitic 9-12% Cr steels are widely used for high temperature applications in energy and processing industry. Compared to the base materials, welded joints of such grades often experience severe damage and premature failures in the heat-affected zone during service.  Based on the results of in-situ synchrotron X-ray diffraction, in-situ high temperature confocal laser scanning microscopy, and equilibrium phase diagram calculations, the welding metallurgy of these martensitic Cr steels is explained. Special emphasis is placed on the suppression of grain refinement in the heat-affected zone of boron and nitrogen balanced experimental 9% Cr steels. These steels have the potential to overcome Type IV cracking observed in the fine-grained heat-affected zone of commonly used 9-12% Cr steels.  Within this work, phase transformations and mechanical properties of welded joints and HAZ simulated material of conventional 9% Cr steels and that of advanced experimental steel grades are compared. Results show that while phase transformation temperatures are similar for all 9% Cr steels, transformation mechanism are different and result in different heat-affected zone microstructures. In general, welded joints with suppressed uniform fine-grained heat-affected zone microstructures show improved creep properties. These results validate the basic concept of improving long-term crossweld creep strength through suppression of grain refinement.