Segregation and its effect on creep rupture strength in Grade 92 steels

The reason for the heat-to-heat variation of the creep strength of Grade T92 steels was investigated, focusing on segregation of alloying elements. This variation was clearly confirmed at 650 degrees C and 700 degrees C. Layered contrasts were observed along the longitudinal direction of the boiler tube, which were considered to be due to solidifying segregation in the manufacturing process. The prior austenite grain size, packet size and block size seemed to be finer in the segregation band than in other areas. The segregation of alloying elements such as Cr, Mo and W was observed along the longitudinal direction of the tube. The difference between the maximum and minimum Cr content, Delta Cr, at a distance (x) was correlated with the difference of degree of Cr segregation among the steels. A correlation between Delta Cr and time to rupture at 650 degrees C was found. The decrease in number density of M23C6 and MX particles, length of high-angle boundaries, and average KAM value during creep exposure was fast in strongly segregated material. This indicates that precipitation and martensitic structure strengthening rapidly decrease during creep exposure in steels with strong segregation.