The influence of holding time during tempering on the long-term creep rupture strength of mod.9Cr-1Mo steel was investigated in this study, so as to elucidate proper heat treatment for boiler applications. Tempering was conducted at 770°C for 0.5h, 1h, 3h, 10h and 100h for the test materials, after re-normalization at 1050°C for 1h in all cases. Creep rupture tests were conducted at 600°C, and ruptured specimens were investigated to better understand the microstructural changes, including changes in the number density of precipitates, in order to observe and discuss their creep strength. All creep rupture test results for materials tempered within 10h exceeded the average creep strength of T91. Shorter tempering times such as 0.5h and 1h were clearly correlated with longer time to rupture at 600°C under 80MPa to 100MPa stress conditions. Reduction of area in creep-ruptured specimens decreased principally with lowered creep stress. Materials tempered for 0.5h and 100h showed the lowest reduction of area at 90MPa and 100MPa respectively, and their reduction of area recovered at lower than those stress levels. These stresses, showing minimum reduction of area, met inflection stress in the creep rupture strength curve.

Grade 91 steel has achieved broad acceptance within the modern boiler industry to fabricate a variety of critical pressure components including tubing, piping and headers, particularly in Ultra Super Critical (USC), Advanced Ultra Super Critical (A-USC) and Combined Cycle Power Plants (CCPP). The applications for which this material is used enforce severe requirements on strength, corrosion, creep properties and thermal stability during service. The properties of Creep Strength Enhanced Ferritic steels (CSEF) such as Grade 91 are critically dependent on manufacturing factors like steelmaking, heat treatments and welding: poor control of these parameters can severely compromise material properties. In scientific literature, several studies correlate low creep ductility to high content of trace elements such As, Sn, Sb, Pb, Cu, P and S. Since the current reference Codes, namely ASTM/ASME, don’t require particular restrictions for these elements, Electric Power Research Institute (EPRI) has issued guidelines for grade 91 which enforce a significant reduction of impurities and trace elements. This paper discusses steelmaking operating challenges to produce Grade 91 steel with very low contents of the above mentioned residual elements, starting from the furnaces charges, up to the chemical composition measuring equipment used in the steel shop laboratories.

ASME Grade 91 steel seam-welded elbow pipe, which has been used in a USC plant (A-Plant) for about 6 × 10 4 h, was investigated to clarify the microstructure and remaining creep life of the material at long-term region. SEM and TEM observations were conducted on specimens cut from the welded portions of the intrados and extrados of the elbow, and the number density of creep voids in fine-grained HAZ was measured in the wall-thickness direction. Then, creep rupture tests were performed to examine the remaining life of each portion of the base metal and welded joint. On the basis of the results, it was suggested that the microstructural changes were small and that the cumulative creep damage was also small for the elbow pipe during its use at the USC plant for about 6 × 10 4 h. The present result was compared with the result of an investigation on Grade 91 steel elbow used in another USC plant (B-Plant) for about 5 × 10 4 h. The A-Plant material had a creep life about ten times longer than that of the B-Plant material for not only the base metals but also the welded joint. Through the comparison of the investigation results, it was suggested that the difference in the creep deformation property between the base metals of the elbows was the main reason for the difference in their creep lives.

In order to clarify the effect of stress and strain on microstructural changes during creep for T91 steel, creep interrupted tests were performed at 600°C for 10000h, 20000h, 30000h, 50000h and 70000h. The steel studied was T91 steel with high Ni content (0.28mass%) in the range of specification. Changes of dislocation structure and precipitates distributions were observed for the grip and gauge portions of creep interrupted samples. The subgrain size gradually increased with increasing creep time up to 50000h in both the grip and gauge portions. However, the subgrain size abruptly increased after 50000h in the gauge portion as compared with the grip portion. Decrease in dislocation density inside subgrain was promoted in the gauge portion as compared with the grip portion. The size of M 23 C 6 gradually increased with increasing creep time up to 50000h in both the grip and gauge portions. The increase in M 23 C 6 size was accelerated after 50000h in the gauge portion as compared with the grip portion. The Z phase formation was promoted in the gauge portion as compared with the grip portion. The number density of all kinds of particles gradually decreased with increasing creep time in the gauge and grip potions. After 50000h, the number density rapidly decreased in the gauge portion as compared with the grip portion.

A methodology is developed for evaluating its creep rupture life from analysis of an on-going creep curve with the aid of an Ω creep curve equation. The method is applied to on-going creep curves of grade 91 steel for evaluating their rupture lives. Quick decrease in creep rupture strength has been reported recently in long-term creep of grade 91 steel. The quick decrease of the steel is discussed by using the rupture lives evaluated. The quick decrease is confirmed in the present study in the time range longer than 3 x 10 4 h at 600°C.

Structural changes in P92-type steel after creep at temperature of 600°C under a stress of 140 MPa were investigated. The steel was solution treated at 1050°C and tempered at 780°C. The structure in the grip portion of the creep specimen changed scarcely after creep exposure for 6876 h. In contrast, the structural changes in the gage and neck sections were characterized by transformation of the tempered martensite lath structure into relatively coarse subgrain structure. The formation of a well-defined subgrain structure in the gage and neck sections was accompanied by the coarsening of M 23 C 6 carbides and precipitations of Laves phase during creep. Mechanisms of grain boundary pinning by precipitates are discussed.

In order to evaluate long term creep strength of modified 9Cr ferritic steels, the system free energy of creep ruptured specimens at both 650 and 700 °C is evaluated as the sum of chemical free energy, strain energy and surface energy, which are obtained by a series of experiments, i.e., chemical analysis using extracted residues, X-ray diffraction, and scanning transmission electron microscopy. Change ratio of the system free energy and creep stress showed the relationship with one master curve irrespective of creep conditions, indicating that the steel ruptures when the applied stress exceeds a limited stress depending on the microstructural state expressed by the change ratio of system free energy. Furthermore, it was found that dominant factor of the change ratio was the chemical free energy change. On the basis of these results, long term creep strength of the steel was evaluated at 700 °C, for example, 19MPa at 700 °C after 10 5 h. It is concluded that long term creep strength of modified 9Cr ferritic steels can be predicted by the system free energy concept using the ruptured specimens with various creep conditions.