The transformation behavior and microstructural changes of Ti 4Fe 7Al alloy during tempering were investigated by performing a hardness test and using X ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. Within a brief time of 30 s during tempering at 450°C, the hardness rapidly increased to a maximum value of 600 Hv. The transformation behavior estimated from the increase in hardness indicated``a lower half of C curve'' in the TTT diagram, which suggests that the transformation occurs through certain thermal activation processes. However, it is difficult to assume that the transformation is controlled by the diffusion of substitutional atoms, since the mean diffusion distance of Fe atoms during tempering at 300°C for 1×10 3 s, which also maximized the hardness, is much less than the distance of the nearest neighbors in the b structure. The structure of the a″phase formed by tempering depends on both the temperature and holding time; a high temperature and a long time bring the a″phase close to an hcp structure. As determined by XRD measurement, the sample tempered at 150°C for 60 min was composed of the b phase. However, the v and a″phases were also detected in selected area diffraction (SAD) patterns. The sample tempered at 450°C for 60 min consisted of a single a″phase and exhibited a tweed structure in the TEM observation. Although the tweed structure seemed to contain only one a″ variant in the SAD patterns, HR TEM observation revealed that it was composed of multiple nanoscale variants. It was considered that the marked grain refinement due to the nanoscale variants led to the extreme hardening of the sample and the significant broadening of XRD peaks. When the tempering temperature was increased to 600°C, the a phase and finely dispersed TiFe precipitates were formed after 60 min by the diffusion of atoms, which resulted in the softening of the sample and the sharpening of the XRD peaks.
To establish a remaining life assessment standard for aged thermal power boilers, internal pressure creep tests of representative low alloy pipe steels were conducted. The tested materials were 2.25Cr-1Mo steel, 1Cr-0.5Mo steel, and 0.5Mo steel. Specimens with and without welded parts in their trunks were made of each material and were φ 70 mm-L400mm-t15mm in dimension. Creep test conditions were 570–620°C × 59MPa and internal pressures were applied by Argon gas or water vapor. Creep tests were interrupted at intervals to acquire data on the crept material as creeping progressed. The observations of replicas from the specimen surface showed the characteristic features of each and the following results were obtained from the comparison of them. (1) The specimens of 2.25Cr-1Mo steel and 1Cr-0.5Mo steel suddenly expanded at about 80% of creep life ratio, and that of 0.5Mo gradually expanded from 60%. They finally ruptured with 15–20% expansions in diameter. (2) Although 2.25Cr-1Mo steel witnessed few creep voids near the end of its creep life, 1 Cr-0.5Mo steel and 0.5Mo steel witnessed many voids such as microcracks from mid-creep life onward. (3) Deformation of ferrite grains contributed to the expansion of 2.25Cr-1Mo steel specimens. On the other hand, partial separations of grains seem to have been owed to the expansions of 1Cr-0.5Mo steel and 0.5Mo steel. In 2.25Cr-1Mo steel, the strength of grain boundary versus grain interior seems to be higher than those of 1Cr-0.5Mo steel and 0.5Mo steels. (4) The amount of creep voids in the HAZ of 1.5Cr-0.5Mo steel was roughly three times that of 2.25Cr-1Mo steel.
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