Changes in microstructure and mechanical properties of 1Cr-0.5Mo and 2.25Cr-1Mo steels during long-term service have been investigated. The study includes inspection of a 1Cr-0.5Mo steel reactor vessel which operated for 20 yr and tests on specimens that had been exposed to service environments. The reactor vessel, exposed at 490 to 530°C for 170,000 hr (about 20yr), showed appreciable decrease in the room temperature yield strength, impact toughness and creep rupture strength compared with the original properties. The service exposure caused changes in carbide morphology and species, forming M2C and M7C3 carbides and transferring significant amounts of Cr and Mo from the matrix to carbides; only 32 percent of the total Mo and 72 percent of the total Cr remained in the matrix. The microstructure of 2.25Cr-1Mo steel showed higher stability than that of 1Cr-0.5Mo steel, although a similar transfer of Mo and Cr from the matrix to carbides took place. The Mo and Cr contents remaining in the 2.25Cr-1Mo steel matrix after a 2-yr exposure were only 25 and 74 percent, respectively, of the total quantities in the steel. The partitioning of Mo and Cr to carbides could increase the stability of carbides and consequently reduce the carbon content in the matrix. The mechanical properties are influenced by both the change in the composition of the matrix and the change in carbide morphology. The metallographic examination provides useful information, although qualitative at this stage, on the degree of deterioration of materials.

Low-carbon/medium-nitrogen 316 stainless steel (316FR) is a principal candidate for the high-temperature structural materials of a demonstration fast reactor plant. Thermo-mechanical fatigue damage is one of critical issues to be known for the design and reliability of the high-temperature materials subjected to thermal cycles. Early growth of small cracks in order of micron-meters in size can provide some essential information for life and the remaining life prediction to these failures. Thus, many efforts have been made; however, there are a lot of matters to be understood. This paper is dealing with how cracks propagate under thermo-mechanical fatigue conditions. Special attentions are paid to the roles of strain rate and thermal cycles on small crack propagation behaviors in 316FR stainless steel. The experimental results revealed a difference in crack growth rates between the small and long cracks under the creep-fatigue condition : the small cracks exhibited growth rates remarkably higher than long cracks at a given fatigue J integral range. The results also indicated that the small crack propagation rate under the isothermal low cycle fatigue increased with the decreasing of the strain rate due to creep effect. It was also shown from the results that the role of irreversible creep strain, as well as that of irreversible plastic strain, was essential in small crack propagation process under the in-phase type thermo-mechanical loading.

In this paper, the crack propagation behavior of naturally initiated small crack in in low-carbon/medium-nitrogen 316 stainless steel under thermo-mechanical fatigue loading was investigated. The experimental results indicated that the importance of the investigation of small crack propagation behavior because the information on the basis of physically long crack growth rate provides a dangerous evaluation on reliability to actual components. The small crack exhibits high growth rate under the In-phase TMF loading because of irreversible creep and plastic strains. However, the growth rates of small crack under the Out-of-phase TMF loading were lower because the effect of creep deformation became negligible in such condition.

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