This work presents a residual life assessment method based on the quantitative analysis of changes in the fraction and width of a precipitate free zone (PFZ) around grain boundaries during long-term creep. Such changes resulted from precipitation and coarsening of carbides on grain boundaries and within the ferrite matrix during long-term creep exposure. Testing was conducted at creep temperatures ranging from 450 to 650 °C, for a maximum time of 142415.1 h. A relationship between the fraction of grain boundary with PFZ, PFZ width, and the Larson-Miller Parameter (LMP) was established, and the mechanism of PFZ formation was discussed. The increase in width, governed by the diffusion of alloy elements to grain boundaries, was mathematically modeled. The relationship between the width and creep exposure time was fitted to a square root law, and the PFZ broadening rate constant and activation energy for width growth were calculated. The proposed model will be applicable to determine the operating parameters of 2.25Cr–1Mo steel components in high-temperature equipment.

Heat-to-heat variation of long-term creep strength and microstructural changes was investigated in ASME Gr. 91 steels (heat name: MGC heat and MgC heat) by focusing on the effects of minor elements. The heat-to-heat variation of creep strength was not observed at 500 °C and 550 °C. At 600 °C and 650 °C, there was no significant difference in the creep strength up to 10,000 h in the two heats. However, at 600 °C, the creep strength of the MGC heat was clearly lower than that of the MgC heat at around 100,000 h. The effect of Ni content (MGC heat: 0.28 mass%, MgC heat: 0.04 mass%) in the range of the specification (Ni ≤0.40 mass%) on microstructural changes was examined. There was no significant difference in the dislocation structure after creep rupture in the two heats. No effect of Ni content was observed on the coarsening of M23C6 particles during the creep. In the MGC heat, a significant number of modified Z-phase particles were formed, consuming a large number of MX particles during the creep. On the other hand, the number of modified Z-phase particles of the MgC heat was lower after the creep rupture than with the MGC heat. An increase in the Ni content promotes modified Z-phase formation and eliminates MX particles during the creep, indicating that the deformation resistance can decrease after a long time in steel with high Ni content even in the range of the specification.

The microstructures near to and remote from the tip of a crack in ASME Gr.92 steel were investigated after creep crack growth at 873 and 898 K, focusing on the martensitic lath, the dislocation structure, and precipitates. After creep, the mean lath width near the crack tip was obviously larger than that of the virgin material, whereas the lath width remote from the crack tip was only slightly larger than that of the virgin material. The mean dislocation density near the crack tip markedly decreased after creep, whereas only a small change was observed in the dislocation density remote from the crack tip. The mean size of M23C6 particles near the crack tip after creep was larger than that of the virgin material, whereas their mean size remote from the crack tip was almost the same as that of the virgin material.► Multiaxiality of stress causes Type IV failure in high Cr steel. ► The multiaxiality of stress and stress is higher at the region near crack tip. ► Recovery of dislocation structure was rapid near crack tip. ► Coarsening of M23C6 carbide was promoted near crack tip during creep crack growth.

Creep behavior of ASME P23/T23 steels was investigated and analyzed, focusing on creep strength degradation in the long-term. Creep rupture strength at 898 and 923 K dropped in the long-term in both P23 and T23 steels. The stress exponent of minimum creep rate at 898 and 923 K was 7.8–13 for higher stresses and 3.9–5.3 for lower stresses in the P23/T23 steels. The change of stress exponent with stress levels was consistent with the drop in creep rupture strength in the long-term. The Monkman–Grant rule was confirmed in the range examined in P23 steel, while the data points deviated from the rule in the long-term in the case of T23 steel. The creep ductility of P23 steel was high over a wide stress and temperature range. On the other hand, in T23 steel, creep ductility at 898 and 923 K decreased as time to rupture increased, and then recovered again in the long-term. Fracture mode changed from transgranular to intergranular fracture in the long-term at 898 and 923 K. At 923 K, the rapid tertiary creep was observed under lower stresses in T23 steel, comparing with P23 steel.

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.

The creep rupture behavior and the effects of oxidation on the creep strength of ASME T23 steel have been investigated. At 625 °C and 650 °C, a greater degradation of long-term creep strength was observed than expected from conventional master curve descriptions in the literature. Oxide scales were observed around the surface of the crept samples tested in air at 625 °C and 650 °C but no oxide scales were observed on samples tested in helium. However, at 625 °C, the creep rupture life was almost the same both in helium and in air, indicating that factors other than oxidation are responsible for the reduction in long-term creep strength. At 650 °C, the creep rupture life was longer in helium than in air, indicating an influence of oxidation, but the long-term strength in helium was still below the master curve. Reasons for the long-term degradation of strength, including microstructural changes, are discussed.