Mechanical response of 9% Cr heat-resistant martensitic steels to abrupt stress loading at high temperature

Microscopic deformation behavior upon abrupt stress loading at high temperature and the effects of microstructural factors on the behavior were investigated in 9% chromium martensitic steels with a variety of microstructures. Anelastic deformation occurs upon abrupt stress loading in steels with a martensitic lath structure. The anelastic deformation originates from the lath structure; that is, the cell structure independent of the presence of a high dislocation density, precipitates, or solute atoms of W or Mo. The lath boundaries, which correspond to sub-boundaries, elastically bend under applied stress, and the motion of the lath boundary requires time due to its complicated structure. This is the reason for the anelastic deformation in the martensitic lath structure. The anelastic strain of extremely low-carbon 9% Cr steel is smaller than that of conventional 9% Cr steels, indicating the difficulty of lath boundary motion in the low-carbon steel. The many MX precipitates present at the lath boundaries effectively retard the elastic bending of the lath boundary in the extremely low-carbon 9% Cr steel. The difficulty of lath boundary motion expected from the analysis of anelastic strain can reasonably explain the improvement of creep strength in extremely low-carbon 9% Cr steel reported in the literature.