Effect of δ-Ferrite Evolution and High-Temperature Annealing on Mechanical Properties of 11Cr3W3Co Ferritic/Martensitic Steel

An 11Cr3W3Co ferritic/martensitic steel was annealed at 1100 degrees C for different time to gradually dissolve delta-ferrite, and then conducted tensile, hardness, and short-term creep tests in combination with microstructural characterization to study the effect of delta-ferrite on the mechanical properties of high-Cr ferritic/martensitic steels. The amount of delta-ferrite gradually decreased to a minimum value with increasing annealing time up to 10 h, and then tended to an ascending tendency when annealed for 15 and 20 h. Accordingly the tensile strength at 300 and 650 degrees C, and Vickers hardness of the steel had an increase and a decrease tendency when delta-ferrite amount decreased down to its minimum value and increased again, respectively. The short-term creep property at 210 MPa at 650 degrees C of the steel exhibited a serious degradation as annealing time gradually increased to 15 h. The morphology and orientation of delta-ferrite grains seriously affected the short-term creep property of the steel. delta-ferrite with a continuously bamboo-like shape parallel to loading direction effectively improved the short-term creep property of the steel at high temperature, while delta-ferrite with a granular or block shape seriously damaged the short-term creep property of the steel. These findings have also been discussed.