This study evaluates various nondestructive testing methods for detecting creep damage and assessing residual life in Grade 91 steel welds. Three primary detection techniques were investigated: phased array ultrasonic testing (PAUT), eddy current testing with high-temperature superconductor direct current and superconducting quantum interference device (ECT•HTS-dc-SQUID), and replica observation. PAUT detected creep damage between 60-80% of creep life, while ECT•HTS-dc-SQUID showed detection capability between 80-90% of creep life. Replica observation revealed creep voids only in the final stages before rupture. Additionally, three strain measurement methods were evaluated: capacitive strain sensors (providing continuous monitoring during creep exposure), laser displacement meters (used during test interruptions), and SPICA strain measurement. Both capacitive sensors and laser meters produced results comparable to conventional extensometer measurements. The SPICA method proved particularly effective in measuring heat-affected zone (HAZ) strain after creep exposure, revealing higher strain values in the HAZ compared to base and weld metal, with a consistent increase during creep exposure.

A newly developed ferritic heat-resistant steel; 9Cr-3W-3Co-Nd-B steel has higher creep rupture strength both in the base metal and welded joints than the conventional high-Cr ferritic heat-resistant steels. The creep rupture strengths of 9Cr-3W-3Co-Nd-B steel welded joints were below the lower limit of the base metal in long-term creep stage more than 20,000 hours. The creep rupture position was heat-affected zone (HAZ) from 1.0 to 1.5 mm apart from the fusion line on the welded joint specimen ruptured at 34,966 hours. The equiaxed subgrains and coarsened precipitates were observed in HAZ of the ruptured specimen. In order to clarify the creep fracture mechanism of the welded joints, the microstructures of HAZ were simulated by heat cycle of weld, then observed by EBSD analysis. Fine austenite grains formed along the prior austenite grain boundaries in the material heated just above A C3 transformation temperature, however there were no fine grains such as conventional steel welded joints. The prior austenite grain boundaries were unclear in the material heated at 1050 °C. The creep rupture life of the material heated at just above A C3 transformation temperature exceeded the lower limit of base metal and there was no remarkable degradation, although it was shorter than the other simulated materials. It is, therefore, concluded that the creep fracture of 9Cr-3W-3Co-Nd-B steel welded joint in long-term stage occurred at HAZ heated at from just above A C3 transformation temperature to 1050 °C. It is speculated that the fine austenite grains formed along the prior austenite grain boundaries and inhomogeneous microstructures cause the coarsening precipitates and recovery of lath structure during long-term creep deformation.

Development of the advanced USC (A-USC) boiler technology has been promoted in recent years, which targets 700°C steam condition. HR6W (Ni-23Cr-7W-Ti-Nb-25Fe) and HR35 (Ni-30Cr-6W-Ti-15Fe) have been developed for A-USC boiler tubes and pipes. The former alloy is mainly strengthened by Fe 2 W type Laves phase. The latter one employs precipitation strengthening of α-Cr phase in addition to Laves phase. Characteristic alloy design of both alloys, which does not use precipitation strengthening of γ′ phase (Ni 3 Al), leads to superior ductility and resistance to stress-relaxation cracking. Stability of creep strength and microstructure has been confirmed by long-term creep rupture tests. The 100,000h average creep rupture strength of HR6W is 85MPa at 700C. That of HR35 is 126MPa at 700°C which is comparable with conventional Alloy617. Tubes of both alloys have been evaluated by the component test in Japanese national A-USC project with γ′ hardened Alloy617 and Alloy263. Detailed creep strength, deformation behavior and microstructural evolution of these alloys are described from the viewpoint of the difference in strengthening mechanisms. Capability of these alloys for A-USC boiler materials has been demonstrated by the component test in the commercial coal fired boiler as the part of the A-USC project.

Materials with a higher creep strength and sensitization resistance superior to those of 347AP are required in complex refinery such as delayed coker. To optimize material designing, the effect of B addition on the Z phase-strengthened steels has been investigated. B addition significantly improved the creep strength of steel containing Nb despite the absence of M 23 C 6 , whereas the creep strength of steel without Nb showed only a slight change with B addition. The size of Z phase was smaller in the Nb-containing steel with B content than that in B-free steel. It was suggested that the refinement of Z phase contributed to the creep strengthening by B addition in steels containing Nb.

Developed 9Cr-3W-3Co-Nd-B heat-resistant steel SAVE12AD (Recently designated as ASME Grade 93) pipes and tubes have higher creep strength in both base metal and welded joints than conventional high Cr ferritic steels such as ASME Grades 91, 92 and 122. The welded joints of SAVE12AD tubes with commercial filler wire for W62-10CMWV-Co (Gr. 92) or Ni base filler wire ERNiCr-3 (Alloy82) also have much better creep rupture strength than those of conventional steels because of suppression of refining in the Heat-Affected-Zone (HAZ). However, the creep rupture strength of weld metal of W62-10CMWV-Co was marginal. Additionally, the hot cracking susceptibility of weld metal using Ni base filler wire ERNiCr-3 was occasionally below the required level. Similar welding consumable for SAVE12AD has been developed to solve these problems. Optimization of nickel, neodymium and boron contents on similar welding consumable enables to obtain both the good long-term creep rupture strength and low enough hot cracking susceptibility of weld metal. Consequently, SAVE12AD welded joint is expected to be applied of piping and tubing above 600°C in USC power plants because of its good properties with similar welding consumable.

Recent advances in materials technology for boilers materials in the advanced USC (A-USC) power plants have been reviewed based on the experiences from the strengthening and degradation of long term creep properties and the relevant microstructural evolution in the advanced high Cr ferritic steels. P122 and P92 type steels are considered to exhibit the long term creep strength degradation over 600°C, which is mainly due to the instability of the martensitic microstructure strengthened too much by MX carbonitrides. This can be modified by reducing the precipitation of VN nitride and by optimizing the Cr content of the steels. An Fe-Ni based alloy, HR6W strengthened by the Fe2W type Laves phase is found to be a marginal strength level material with good ductility at high temperatures over 700°C and to be used for a large diameter heavy wall thick piping such as main steam pipe and hot reheat pipe in A-USC plants, while Ni-Co based alloys such as Alloys 617 and 263 strengthened by a large amount of the y’ phase are found to be the high strength candidate materials for superheater and reheater tubes, although they are prone to relaxation cracking after welding and to grain boundary embrittlement during long term creep exposure. A new Ni based alloy, HR35 strengthened by a-Cr phase and other intermetallic phases has been proposed for piping application, which is specially designed for a good resistance to relaxation cracking as well as high strength and a good resistance to steam oxidation and fire-side corrosion at high temperatures over 700°C.