MICROSTRUCTURE AND CREEP PERFORMANCE OF WIRE ARC ADDITIVE MANUFACTURED GRADE 91 STEEL

In this work, creep performance of an as-built Grade 91 steel using wire-arc additive manufacturing (WAAM) was tested using a unique creep testing system integrated with in-situ high-temperature digital image correlation (DIC) strain mapping capability. The test results show creep lifetime of this as-built Grade 91 steel is much higher than that of Grade 91 steel weldments, even slightly above the mean lifetime of the conventional Grade 91 steels. In addition, highly nonuniform creep deformation in the as-built specimen was clearly captured by the DIC measurements, and correlated to the characterized inhomogeneous microstructures in multi-scales. Microstructure heterogeneities, including different martensite lath sizes, tempering degree of martensite matrix, and variations in precipitates, exist in two distinctive regions. A soft region, considered as the reheated “heat affected zone” (HAZ), has fine equiaxed grains, smaller martensite blocks, more precipitates, and a lower hardness. These microstructure features made these reheated HAZ regions more vulnerable to creep strength degradation, showing a larger creep deformation in the DIC measurements. A hard region with coarse columnar grains, less-tempered martensite matrix, and finer precipitates, exhibits a higher hardness and a better creep resistance. This study has clearly demonstrated that microstructure and creep strength variations at the meso-scale exist in the WAAM builds and should be carefully considered in WAAM process and material qualifications.