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.

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.

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.

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.

Grade 92 steel has been widely applied in the power generation industry for use as steam pipes, headers, tubes, etc. owing to a good combination of creep and corrosion resistance. For the welding of thick section pipes, a multi-pass submerged arc welding process is typically used to achieve sufficient toughness in the weld. To relieve the internal stress in the welds and to stabilise their microstructures, a post weld heat treatment (PWHT) is commonly applied. The heat treatment conditions used for the PWHT have a significant effect on both the resulting microstructure and the creep behaviour of the welds. In this study, interrupted creep tests were carried out on two identical Grade 92 welds that had been given PWHTs at two different temperatures: 732°C and 760°C. It was found that the weld with the lower PWHT temperature had a significantly reduced stain rate during the creep test. In addition, microstructural examination of the welds revealed that the primary location of creep damage was in the heat affected zone in the sample with the lower PWHT temperature, whereas it was in the weld metal in the sample with the higher PWHT temperature. To understand the effect of the different PWHT temperatures on the microstructure, initially the microstructures in the head portions of the two creep test bars were compared. This comparison was performed quantitatively using a range of electron/ion microscopy based techniques. It was apparent that in the sample subjected to the higher PWHT temperature, larger Laves phase particles occurred and increased matrix recovery was observed compared with the sample subjected to the lower PWHT temperature.

Welding of collector pipes, flat heads, dished ends and connector pipes performed with high temperature and creep-resistant steels most often has been performed using GTAW process combined with MMA processes. Progress in GMAW process and availability of high quality filler materials (solid wires) enables welding of the above connections also using this method. In order to prove its efficiency, this article presents the results of related tests. The range of tests was similar to that applied during the qualification of welding procedure. The investigation also involved microscopic and fractographic examinations and creep tests. The results reveal that welding with GMAW is by no means inferior to a currently applied SMAW method yet the time of the process is shorter by 50%. The article presents the world’s first known positive results in welding of P92 grade steel using GMAW welding method.

Components such as tubes, pipes and headers used in power generation plants are operated in a creep regime and have a finite life. During partial replacement, creep exhausted materials are often welded to virgin materials with superior properties. The aim of this study was to identify a suitable weld filler material to join creep aged X20CrMoV12-1 to a virgin P91 (X10CrMoVNbV9-1) steel. Two dissimilar joints were welded using the gas tungsten arc welding (GTAW) process for the root passes, and manual metal arc (MMA) welding for filling and capping. The X20 and the P91 fillers were selected for joining the pipes. The samples were further heat treated at 755°C to stress relief the samples. Microstructural evolution and mechanical properties of the weld metals were evaluated. The average hardness of X20 weld metal (264 HV10) was higher than the hardness measurement of P91 weld metal (206 HV10). The difference in hardness was attributed to the high carbon content in X20 material. The characterisation results revealed that the use of either X20 or P91 weld filler for a butt weld of creep aged X20 and virgin P91 pipes material does not have a distinct effect on the creep life and creep crack propagation mechanism. Both weld fillers (X20 and P91) are deemed to be suitable because limited interdiffusion (<10 μm) of chromium and carbon at the dissimilar weld interface was observed across the fusion line. The presence of a carbon ‘denuded’ zone was limited to <10 μm in width, based on the results from local measurements of the precipitate phase fractions using image analysis and from elemental analysis using EDS. However the nanoindentation hardness measurements across the fusion line could not detect any ‘soft’ zone at the dissimilar weld interface. The effect of the minute denuded zone was also not evident when the samples were subjected to nanoindentation hardness testing, tensile mechanical testing, Small Punch Creep Test (SPCT) and cross weld uniaxial creep testing.

The piping stress and thermal displacement corresponding to different types of riser rigid support and hanger devices in different installation directions have been calculated by means of finite element analysis, to further analyze the impact on cracking of adjacent steam tee welds exerted by the constraint effect of riser rigid hangers on angular displacement. It can be seen from the analysis that a riser rigid hanger has a constraint effect on angular displacement, and such a constraint effect, however, is weak and limited on the piping stress and thermal displacement, so the piping stress and supports and hangers are not the main reasons for the cracking of tee welds. In addition, the calculation results alert that for an axial limiting hanger of riser with a dynamic axial pipe clamp and rigid struts, its constraint effect on angular displacement has a significant impact on the piping stress and thermal displacement.

Investigations on welded joints made from a modified parent material and welding consumables are described. Tubes and pipes with typical dimensions have been welded using different welding processes and consumables (GTAW, SAW, SMAW, modified filler metals). The influence of melting loss and chemical composition of the consumables on the weld performance was studied. Short-term tensile and long-term creep tests on cross weld specimens were carried out in order to evaluate strength. The results obtained so far show that the properties of the welded joints are rather optimistic, it could be assumed that the modified Alloy 617 and the welding consumables used will meet the requirements for use in a plant operated at ultra critical steam conditions with live steam temperatures up to 720°C and pressure up to 300 bar. This allows for first practical applications in test loops of plants. These applications including the Welding Procedure Qualifications are described.

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.

This paper reports on a study that investigated how low cycle fatigue (LCF) and fatigue crack propagation (FCG) properties of P92, P122, and P23 steels vary between 600°C and 700°C depending on the location relative to a cross weld. Microstructure analysis was also performed on fractured specimens. Due to its higher yield strength, P122 exhibited the best continuous LCF life. However, creep-fatigue interaction (CFI) in the weld heat-affected zone (HAZ) of P122 and P23 steels significantly reduced their lifespans compared to continuous LCF tests. This reduction is attributed to the effect of weld thermal cycles on fine precipitates. FCG tests revealed that the base metal consistently outperformed the HAZ in all tested steels and temperatures. P92 and P122 showed similar FCG rates except for P92's behavior at 600°C, which resembled P23. In both steels, the HAZ exhibited faster FCG rates at 600°C and 700°C compared to the base metal, particularly at lower stress intensity factor ranges (ΔK). Within the HAZ, the region 1 mm from the fusion line displayed the slowest FCG rates, followed by the base metal, while the fusion line and the region 2 mm from it showed the fastest. Fracture surfaces near the fusion line displayed cleavage-like features, while the region 1 mm away exhibited features associated with higher crack growth resistance.

The paper describes methods for practical high temperature weldment life assessment, and their application to the analysis of notable high energy piping weldment failures and interpretation of cross-weld data. The methods described in the paper are simplified versions of full continuum damage mechanics (CDM) analysis techniques which have been developed over the last 20 years. The complexity of the CDM methods and their data requirements has been a barrier to their more widespread use. The need for simplified methods has been driven by the need for risk assessment of in-service high temperature welded piping and headers around the world, the need to connect cross-weld data to weld joint design and assessment, and in general, the need to develop suitable guidelines for evaluating the strength of weldments relative to that of base metal.

Current nondestructive examination (NDE) technology detection capabilities limit our ability to detect stress corrosion cracking (SCC) damage until it has progressed significantly. This work describes the continued development of an in-situ monitoring technique to detect and characterize mechanical damage caused by SCC, allowing the detection of the incipient stages of damage to components/piping. The application of this study is to prevent failures in the primary cooling loop piping in nuclear plants. The main benefit to the industry will be improved safety and component lifetime assessment with fewer inspections. The technique utilizes high resolution fiber optic strain gages mounted on the pipe outside diameter (OD). This technique has successfully detected changes in the residual stress profile caused by a crack propagating from the pipe inside diameter (ID). The gages have a resolution of < 1 με. It has been shown experimentally for different crack geometries that the gages can readily detect the changes of approximately 10-60 με caused on the OD of the pipe due to crack initiation on the ID. This paper focuses on the latest in the development of the technology. Details of the previous work in this effort may be found in References 1 through 3. A short summary is provided in this paper. The main recent development was the full scale accelerated SCC cracking in boiling magnesium chloride (MgCl 2 ) experiment. In conjunction with experimentation, both 2D and 3D finite element (FEA) models with thermal and mechanical analyses have been developed to simulate the changes in residual stresses in a welded pipe section as a SCC crack progresses.

Welded joints of Ni-base alloys are often the critical part of components operated under high temperature service conditions. Especially welds in thick-walled structures are susceptible to various crack phenomena. Creep rupture and deformation behavior of different similar welds of Alloy 617B, both circumferential and longitudinal, were determined in many research German projects with the aim to qualify the nickel alloys and its welded joints for the use in highly efficient Advanced Ultra Supercritical (AUSC) power plants. Damage mechanisms and failure behavior have also been investigated within these projects. In order to reduce the welding residual stresses in thick-walled components a post weld heat treatment (PWHT) for Alloy 617B is recommended after welding. This PHWT reduces not only residual stresses but causes changes in the damage mechanisms and failure behavior of welded joints of Alloy 617B. Improving effects of PWHT have been investigated in this study and results of microstructural investigations were correlated with the material behavior.

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.

Grade 92 steel, a creep strength-enhanced ferritic (CSEF) steel, is used in supercritical steam fossil power plants for boilers and piping systems. While its creep strength is crucial, understanding the interaction between creep and fatigue damage is also vital for assessing component integrity under cyclic loading. Despite existing studies on its creep-fatigue behavior, additional data under creep-dominant conditions relevant to plant evaluations are needed. Girth welds, critical to piping system integrity, are particularly important in this context. EPRI and CRIEPI initiated a project to develop a comprehensive database on the creep-fatigue behavior of Grade 92 steel's base metal and welded joints and to establish a suitable life estimation procedure. Key findings include: (i) a thick pipe with submerged arc welding (SAW) was manufactured for testing; (ii) base metal and cross-weld specimens showed similar behavior under short-term creep and cyclic loading; (iii) these specimens had lower creep strengths than average literature values for this steel class in the short time regime, with differences decreasing as stress decreased; and (iv) the fatigue and creep-fatigue behavior of these specimens were similar to those of Grade 91 and 122 steels, with common characteristics in creep-fatigue failure prediction models across the three CSEF steels.

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.

CO 2 emission reduction from coal power plants is still a serious issue to mitigate the impact of global warming and resulting climate change, though renewables are growing today. As one of the solutions, we developed A-USC (Advanced Ultra Super Critical steam condition) technology to raise the thermal efficiency of coal power plants by using high steam temperatures of up to 700℃ between 2008 and 2017 with the support of METI (Ministry of Economy, Trade and Industry) and NEDO (New Energy and Industrial Technology Development Organization). The temperature is 100℃ higher than that of the current USC technology. Materials and manufacturing technology for boilers, turbines and valves were developed. Boiler components, such as super heaters, a thick wall pipe, valves, and a turbine casing were successfully tested in a 700℃-boiler component test facility. Turbine rotors were tested successfully, as well, in a turbine rotating test facility under 700℃ and at actual speed. The tested components were removed from the facilities and inspected. In 2017, following the component tests, we started a new project to develop the maintenance technology of the A-USC power plants with the support of NEDO. A pressurized thick wall pipe is being tested in a 700℃ furnace to check the material degradation of an actual sized component.

In recent years continuous and extensive research and development activities have been being done worldwide on 700°C A-USC (Advanced Ultra Super Critical) power plants to achieve higher efficiency and reduce the CO 2 emission. Increasing steam temperature and pressure of such A-USC boilers under consideration require the adoption of Ni based alloys. In the Japanese national project launched in 2008, Ni based alloy HR6W (45Ni-23Cr-7W-Ti, ASME Code Case 2684) is one of the candidate materials for boiler tube and pipe as well as Alloy617, Alloy263 and Alloy740H. The most important issues in A-USC boiler fabrication are the establishment of proper welding process for thick wall components of these alloys and verification of the long term reliability of their weldments. In our previous study, the weldability of HR6W was investigated and the welding process for Ni based thick wall pipe was established with the narrow gap HST (Hot wire Switching TIG) welding procedure originally developed by Babcock-Hitachi K.K. In this paper, creep rupture strengths of HR6W weldment were verified by the long term test up to 60,000 hours for tube and 40,000 hours for pipe. In Japanese national project, narrow gap HST welding process was also applied to the welding test for the other Ni based candidate pipe materials. Furthermore, as the practical A-USC boiler manufacturing trials, header mockup test was conducted and qualified for HR6W.

Welding processes and fabrication techniques have been studied in the development of Advanced USC boilers. Advanced 9Cr steels, Fe-Ni alloy (HR6W) and Nickel base alloys (HR35, Alloy 617, Alloy 263, Alloy 740 and Alloy 740H) have been selected as candidate materials for the boiler. The weld joints of these alloys were prepared from plates, small diameter tubes and large pipes, and welding procedure tests were performed. In this study, TIG and SMAW were applied. Both welding process produced good weld joints, and they showed good results in bending tests, tensile tests and the Charpy impact test. To select the annealing conditions for stress relief, stress relaxation tests and hardness tests were conducted on the weld joints after various heat treatments. The microstructure was also evaluated by SEM and TEM. Creep rupture tests are being performed for the weld joints with and without heat treatment. The maximum creep rupture tests are expected to take over 100,000 hours. In the study of fabrication techniques, hot bending tests by high frequency induction heating for large pipes and cold/hot bending tests for small diameter tubes were established. After the bending tests, mechanical property tests such as tensile tests, impact tests and creep rupture tests were conducted. The effect of pre-strain on creep strength was studied to take the creep test results after bending into consideration. The creep rupture test will be continued for specimens from weld joints and bending pipes to show their long term reliability.