Origin of Threshold Stresses in a P92-type Steel

The creep behavior of a P92-type steel was examined at 923 K. The threshold shear stress of 51 MPa was revealed by an analysis of steady state creep behavior at applied stress ranging from 118 to 200 MPa. The tempered martensite lath structure stabilized by M23C6-type carbide particles with an average size of about 110 nm and MX-type carbonitrides with a size of 40 nm located within ferritic matrix evolved after tempering. The transient creep at an applied stress of 118 MPa was accompanied by two-fold decrease in internal elastic stress and dislocation density. The threshold stresses matched the values of internal stresses, which were measured by means of the lattice curvature within individual laths, and those corresponded to Orowan stressor detachment stress originated from M(C,N) carbonitrides. A dispersion of M(C,N) carbonitrides played a crucial role in superior creep resistance of P92-type steels because of dispersion hardening and hindering the relief of internal stresses originated from lath boundaries.