Insights into creep properties and microstructure of Gr. 91 steels with varied impurity contents

The creep properties and microstructures of two types of Gr. 91 steel with different impurity (Sb, As, Sn) contents were investigated. The steel with higher impurity concentrations (Steel 1) ruptured in slightly shorter time than the steel with lower impurity concentrations (Steel 2). The reduction of area for Steel 1 was slightly lower than that for Steel 2 due to higher density of creep voids in Steel 1. However, no clear correlation was observed between impurity contents and either elongation or creep intolerance parameter (λ). Microstructural observation revealed that the effect of impurities on the initial microstructure was considered negligible. During creep, the crystallographic relationship between common rotation axes and rotation angles at block/packet boundaries gradually disappeared. Among block boundaries, <011>53° and <011>60° were found to be more susceptible to creep deformation, whereas <111>60° was relatively less sensitive. This disappearance of crystallographic relationship was more pronounced in Steel 1 than in Steel 2, implying that the difference in creep strength between the two steels could be attributed to the ease of migration and merging of block/packet boundaries.