This paper investigates creep rupture and damage behaviors of HR6W weldment using full thickness specimen cut from the circumferentially welded pipe. Creep tests were conducted at 750°C for durations up to 8,000 hours, and damage morphology of weldment during creep was characterized. The applicability of several nondestructive detection methods to the creep damage evaluation was discussed. It was found that full thickness specimen was broken at the base metal and main crack was inclined approximately at 45 degrees to the axial direction of the specimen. Times to creep rupture of full thickness specimen were comparable with those of the standard specimen. In addition, a small crack in base metal on the outer surface was first observed at life fraction of 35% by replication. PT can detect the crack in about half of the life. The crack whose length is longer than 3mm can be detected by UT in latter half of the life.

Advanced power systems that operate at temperatures higher than about 650°C will require nickel-base alloys in critical areas for pressure containment. Age-hardened alloys offer an additional advantage of reduced volume of material compared with lower strength solid solution-strengthened alloys if thinner tube wall can be specified. To date, the only age-hardened alloy that has been approved for service in the time dependent temperature regime in the ASME Boiler and Pressure Vessel Code is INCONEL alloy 740H. Extensive evaluation of seamless tube, pipe, and forged fittings in welded construction, including implant test loops and pilot plants, has shown the alloy to be fit for service in the 650-800°C (1202-1472°F) temperature range. Since, nickel-base alloys are much more expensive than steel, manufacturing methods that reduce the cost of material for advanced power plants are of great interest. One process that has been extensively used for stainless steels and solution-strengthened nickel-base alloys is continuous seam welding. This process has rarely been applied to age-hardened alloys and never for use as tube in the creep-limited temperature regime. This paper presents the initial results of a study to develop alloy 740H welded tube, pipe and fittings and to generate data to support establishment of ASME code maximum stress allowables.

The mechanisms of recent cracking failures of HR3C super heater pipes of a fossil power plant in the Netherlands were investigated. Initial failure investigations showed that pitting corrosion of the sensitized HR3C initiated subsequent stress corrosion cracking (SCC). It was concluded that magnesium chloride hydrates from condensed seawater had initiated pitting corrosion as well as SCC similar to the standard ASTM G36 SCC test. By experimental application of the ASTM G36 procedure, this tentative mechanism is reproduced and confirmed by a series of laboratory tests with pure magnesium chloride as well as with synthetic seawater. It included the effects of temperature, magnesium chloride concentrations of the evaporating water and applied bending moments on cracking. As a result for the 175h testing period in MgCl2*6H 2 O cracking increases significantly above 100°C up to 120°C but is reduced slightly at temperatures up to 155°C. With increasing bending moments, the U-shaped test pieces revealed increasing crack depths up to total fracture of the 5mm thick sections. Lower magnesium chloride concentrations as in concentrated seawater provided identical cracking, however, to a lower extent. It is therefore concluded that the operational failure of the sensitized HR3C super heater pipes was initiated in presence of condensed seawater and followed the same mechanism as found in the experimental investigation. As a conclusion, the presence of seawater saturated air at temperatures between 100° and 155°C should be avoided.

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.

Type IV damage was found at several ultra-supercritical (USC) plants that used creep-strength-enhanced ferritic (CSEF) steels in Japan, and the assessment of the remaining life of the CSEF steels is important for electric power companies. However, there has been little research on the remaining life of material that has actually served at a plant. In this study, the damage and remaining life of a Gr.91 welded elbow pipe that served for 54,000 h at a USC plant were investigated. First, microscopic observation and hardness testing were conducted on specimen cut from the welded joint; the results indicated that the damage to the elbow was more severe in the fine-grain heat-affected zone near the inner surface. Furthermore, creep rupture tests were performed using specimens cut from the welded joint of the elbow, and from these results, the remaining life was evaluated using the time fraction rule as almost 110,000 h. Finite-element analysis was also conducted to assess the damage and remaining life, and the results were compared with the experimental results.

An internal pressure creep test has been carried out on a Gr. 91 steel longitudinal welded pipe at 650°C to examine the type IV failure behavior of actual pipes, using a large-scale experiment facility “BIPress”, which can load internal pressure and bending force on large diameter pipes at high temperatures. The creep test was also interrupted three times to measure hardness and voids density in the HAZ region of the outer surface of the test pipe. Results of the measurement of the hardness and voids density at the interruption did not indicate creep damage accumulation. The welded pipe suddenly ruptured with large deformation, which caused crushing damage to the surrounding facility. Type IV cracking occurred in the longitudinal welded portion of the test pipe, and the length of the crack reached 5000mm. SEM observation was carried out at the cross section of the welded portion of the test pipe and voids density was measured along the thickness direction in the HAZ region. To clarify the stress/strain distribution in the welded portion, creep analysis was conducted on the test pipe, where the materials are assumed to consist of base metal, weld metal and HAZ. After stress redistribution due to creep deformation, stress and strain concentrations were observed inside the HAZ region. Then, the authors' creep life prediction model was applied to the creep test result to examine its validity to actual size pipes. It was demonstrated that the life prediction model can evaluate damage of the Gr. 91 steel longitudinal welded pipe with sound accuracy.

In order to improve thermal efficiency of fossil-fired power plants through increasing steam temperature and pressure high strength martensitic 9-12%Cr steels have extensively been used, and some power plants have experienced creep failure in high temperature welds after several years operations. The creep failure and degradation in welds of longitudinally seam-welded Cr- Mo steel pipes and Cr-Mo steel tubes of dissimilar metal welded joint after long-term service are also well known. The creep degradation in welds initiates as creep cavity formation under the multi-axial stress conditions. For the safety use of high temperature welds in power plant components, the complete understanding of the creep degradation and establishment of creep life assessment for the welds is essential. In this paper creep degradation and initiation mechanism in welds of Cr-Mo steels and high strength martensitic 9-12%Cr steels are reviewed and compared. And also since the non-destructive creep life assessment techniques for the Type IV creep degradation and failure in high strength martensitic 9-12%Cr steel welds are not yet practically established and applied, a candidate way based on the hardness creep life model developed by the authors would be demonstrated as well as the investigation results on the creep cavity formation behavior in the welds. Additionally from the aspect of safety issues on welds design an experimental approach to consider the weld joint influence factors (WJIF) would also be presented based on the creep rupture data of the large size cross-weld specimens and component welds.

The rising global energy demand has led to a surge in the construction of high-efficiency power plants with advanced steam parameters. National and international projects indicate that fossil fuels will continue to be the primary source of power generation in the coming years, despite significant efforts and progress in utilizing alternative energy sources. Economic pressures and climate protection concerns necessitate more cost-efficient and environmentally sustainable energy production. Achieving this requires reducing specific fuel and heat consumption per kilowatt-hour, making it essential to improve the efficiency of new power plants beyond those commissioned in Germany between 1992 and 2002. While new construction and process innovations contribute to efficiency gains, the primary factors driving improvement are increased steam pressure and temperature. Current design parameters include steam temperatures of 605 °C (live steam) and 625 °C (hot reheat steam), along with pressures of 300 bar (live steam) and 80 bar (hot reheat steam), which have become critical for obtaining building and operating licenses in Germany. However, the European Creep Collaborative Committee’s (ECCC) 2005 reassessment of the creep strength of steel T/P92 (X10CrWMoVNb9-2) has placed limitations on further increasing steam temperatures beyond 625 °C.

In conventionally fired power plants, appropriate materials are required which correspond to the different temperature and oxidizing conditions in the boiler and in the superheater sections. Pipe steels with 2 1/4 Cr (P22; T23; T24) must be welded to 9 - 12 % Cr steels (P91; E911; P92; VM12). In this area, the choice of the appropriate welding filler material is vital for the quality of the weldment. This report highlights the possibilities for achieving optimal properties in differing dissimilar metal welds under conditions of reduced carbon diffusion.

Dissimilar joints between modern 10% chromium steels and low-alloy steels are unavoidable in new installations or upgrades of steam turbine components. Welds between 10CrMo9-10 (P22) and X10CrMoVNb9-1 (P91) steel pipes are often required. This paper studies this heterogeneous weld from a steam turbine manufacturer's practical perspective. Two types of filler materials were used: P22- and P91-based weld metals. The integrity and mechanical properties of the prepared heterogeneous welds were evaluated according to the welding standard EN 288-3. Both approaches yielded satisfactory results. Additionally, creep rupture strength was evaluated. The creep rupture strength of both joints fell within the -20% scatter band of the P22 base material's creep rupture strength. The weld design with P91 filler material appeared to slightly outperform the P22-based approach for longer exposure times.

A major cost contributor of P91 pipe welding is the vital requirement of ensuring proper protection of the root or first pass of the weld from oxidation through the use of an inert gas blanket, i.e. backing gas. The necessity for oxidation protection negatively impacts the cost of both weld set-up and the actual welding process of P91 pipe fabrication. In an effort to decrease the associated costs of welding P91, Fluor Corporation has invested in significant research and extensive field-testing to develop the wire/gas mixture that contributes to the breakthrough in welding P91 with “No Backing Gas (NBG)”. Combining this novel technique with the semiautomatic GMAW-S (using inverter technology with a controlled transfer) eliminates all cost associated with the need to provide a backing gas, including installation of purge dams, backing gas, and man-hours associated with implementing these activities.

In the late 1980s, the domestic utility industry experienced failures in dissimilar metal welds (DMWs) between low-alloy ferritic tubing and austenitic tubing in superheaters and reheaters. Extensive research by EPRI found that nickel-based filler metals provided significant service life improvements over 309 stainless steel filler metals. Improved joint geometries and additional weld metal reinforcement were determined to extend service life further. A new nickel-based filler metal was also developed, exhibiting thermal expansion properties similar to the low-alloy base metal and a low chromium content that would result in a smaller carbon-depleted zone than currently available fillers. However, this new filler metal was never commercialized due to a tendency for microfissuring, resulting in less than desired service life. This paper discusses further investigation into the filler metal microfissuring issue and examines long-term testing to determine the filler's suitability for high-temperature applications.