Creep behavior of a 10%Cr heat-resistant martensitic steel with low nitrogen and high boron contents at 650 °C

The purpose of this study is to identify the relationships between the creep behavior of the 10% Cr martensitic steel with 0.008% B and 0.003% N and changes in microstructure and precipitations of secondary phases. Creep tests were carried out at 650 degrees C under an applied stress ranging from 180 to 120 MPa. The 10% Cr steel exhibits a linear stress vs. time to rupture dependence without creep strength breakdown up to approximately 40 000 h. The 100 000 h creep rupture strength at 650 degrees C is predicted to be 110 MPa. The high creep strength of the 10% Cr steel is attributed to high threshold stress of 111.5 MPa. Threshold stress is associated with the detachment stress, i.e. the stress required for detachment of dislocations from M23C6 carbides after finishing the climb, mainly. Analysis of the pinning pressures exerted from different precipitates shows that M23C6 carbides also play the main role in preventing the lath boundary migration and therefore, provide high stability of the tempered martensite lath structure under creep conditions. Coarsening of the lath structure under long-term creep conditions is associated with growth of Laves phase particles, mainly.