For coal-fired power plants, the technology of the Advanced-Ultra Super Critical (A-USC) boilers is being developed with the purpose of enhancing generation efficiency and reducing CO₂ emissions [1] - [6] . The steam temperature of A-USC boilers will be 100˚C higher than that of the conventional 600˚C-class Ultra Super Critical (USC) boilers [7] . Therefore, Ni-based alloys are planned to be used in A-USC boilers. As the Ni-based alloy, currently, Ni-23Cr-7W Alloy (HR6W) is receiving attention as a candidate material for A-USC boilers [8] . The creep strength of HR6W is enhanced by precipitation strengthening on the Laves phase and M₂₃C₆ carbide. HR6W has excellent high-temperature ductility and thermal fatigue characteristics compared to other Ni-based alloys [9] . Boiler tubes are generally subject to bending in manufacturing process. Austenitic stainless steels are used for boiler tubes of USC boilers. There are many reports about the effect of cold working on the creep strength of austenitic stainless steels [10] [11] [12] [13] [14] . At the short-time conditions, the creep rupture strength of cold-worked austenitic stainless steels is significantly higher than that of the non-cold-worked austenitic stainless steels. On the other hand, at the long-time conditions, the creep rupture strength of cold-worked austenitic stainless steels is similar to that of non-cold-worked austenitic stainless steels [11] . It has been clarified that this characteristic results from aggregation and coarsening of carbide. Conversely, with regards to HR6W, Okada et al. reported that creep rupture time of HR6W that was subjected to cold working by up to 30% at room temperature increases approximately 100 times than HR6W that was not subjected to cold working and the creep strength is stable for a long time [15] . Saito et al. reported that the increase in the creep rupture time of HR6W that was subjected to cold working of 20% was maintained in a long-term test of 70,000 h or more [16] . In addition, Kubushiro et al. reported that the increase in creep rupture time of cold-worked HR6W is remarkably large as compared with other cold-worked Ni-based alloys [17] . However, there has been almost no discussion on the mechanism of this increase in strength.

The purpose of this study is to clarify the reason why the creep rupture time of cold-worked HR6W is longer than that of the non-cold-worked HR6W. For this purpose, the pre-strained HR6W was prepared via interrupted tensile test and microstructures of HR6W after a series of creep tests were investigated.

In this study, two heats of HR6W tube materials were used as test materials. The tubes were subjected to solution heat treatment. The chemical composition of HR6W is shown in Table 1. For simulating the cold work, among the test materials, Alloy A was processed into arc-shaped tensile test specimens, i.e., cross-section of the specimens is arc-shaped. The arc-shaped specimens were parallel portion with a width 15 mm, a thickness 8 mm, and a gauge distance 70 mm. Tensile interrupted tests were carried out on them. The specimens were subjected to pre-strain of 10% and 15%. They are referred to as the pre-strained samples. After the interrupted tensile test, the parallel portion of each test specimen was re-processed