Dynamic strain ageing of P91 grade steels of varied silicon content

Abstract Plate materials of modified 9Cr–1Mo steel (P91) containing four levels of silicon (Si) exhibited dynamic strain ageing (DSA) phenomenon characterized by gradually reduced failure strain and serrations in the engineering stress–strain diagrams under tensile loading within a temperature range of ambient to 400 °C. Since parameters including dislocation density ( ρ ), activation energy ( Q ) and work-hardening index ( n ) can play important roles in developing a mechanistic understanding of DSA, efforts have been made to evaluate them. A maximum value of ρ was calculated at 400 °C due to the formation of critical sized precipitates near grain boundaries, thus impairing the dislocation motion. The average value of Q ranged between 64 and 80 kJ/mole depending on Si content. The magnitude of n was gradually enhanced with increasing temperature up to 400 °C, followed by significant drop at 550 °C. Except for the alloy with 2.0 wt.% Si, the other tested materials exhibited a combination of intergranular cracking and cleavage failures at the primary fracture surface.