A longitudinal crack and window opening type failure occurred in neutral zone that is applied to least plastic deformation in the bent TP347H tube during operation. From the analysis of residual stress and plastic deformation during the tube bending, there is low creep strength and high residual stress in neutral zone as compared other regions like intrados and extrados. Therefore, failure occurred in neutral zone due to stress relaxation concentrated in grain boundary during operation.

During commissioning of recently built modern, and highly efficient coal-fired power plants, cracks were detected after very short time of operation within the welds of membrane walls made from alloy T24. The root cause analysis revealed transgranular and mostly intergranular cracks adjacent to the heat affected zone beside weld joints. At that time, the degradation mechanism was rather unclear, which led to an extended root cause analysis for clarification of these failures. The environmentally assisted cracking behavior of alloy T24 in oxygenated high-temperature water was determined by an experimental test program. Hereby, the cracking of 2½% chromium steel T24 and 1% chromium steel T12 were determined in high-temperature water depending on the effect of water chemistry parameters such as dissolved oxygen content, pH, and temperature, but also with respect to the mechanical load component by residual stresses and the microstructure. The results clearly show that the cracking of this low-alloy steel in oxygenated high-temperature water is driven by the dissolved oxygen content and the breakdown of the passive corrosion protective oxide scale on the specimens by mechanical degradation of the oxide scale as fracture due to straining. The results give further evidence that a reduction of the residual stresses by a stress relief heat treatment of the boiler in combination with the strict compliance of the limits for dissolved oxygen content in the feed water according to water chemistry standards are effective countermeasures to prevent environmentally assisted cracking of T24 membrane wall butt welds during plastic strain transients.

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.

Finite element (FE) modeling has been applied to a stress relaxation cracking (SRC) test in order to evaluate the effects of changing sample geometry and material type. This SRC test uses compressive pre-straining to create a tensile residual stress in modified compact-tension specimens and has been used to test 316H stainless steel. The FE model is first used to verify that sample integrity will not be compromised by modifying the geometry. The FE model is then applied to candidate Advanced Ultra Supercritical nickel-base alloys 617, 740H, and 800. It is determined that this stress relaxation test will be appropriate for these alloys.

The use of the bainitic class of creep strength enhanced ferritic steels T/P23 and T24 has increased over the last decade in a wide range of applications including replacement headers, superheater and reheater tubing and in waterwall tubing. Many issues have been reported in one or both of these materials including hydrogen induced cracking, reheat cracking and stress corrosion cracking. To appropriately address these issues, work has been initiated that includes a literature review, development of a database of phase transformation temperatures, investigation of tempering behavior, and an analysis of the effect of phase transformation on residual stresses. Such information will be provided in the context of understanding why these two materials appear highly susceptible to these cracking mechanisms.

The inspection and evaluation of defects in the welds of P92 high temperature reheater header with a diameter of about 1000mm and a wall thickness of about 100 mm have been done by means of hardness test, nondestructive testing on the surface, ultrasonic testing, metallographic and component sampling. By analyzing the results of on-site test and samples removed from the component, it is found that cracks existing in the welds are hydrogen induced delayed cracks. During the welding process and post-heating treatment (hydrogen bake-out), dehydrogenation was insufficient. This fact, combined with welding residual stresses resulted in the observed hydrogen induced cracking.

Advanced 700°C-class steam turbines demand austenitic alloys for superior creep strength and oxidation resistance beyond 650°C, exceeding the capabilities of conventional ferritic 12Cr steels. However, austenitic alloys come with a higher coefficient of thermal expansion (CTE) compared to 12Cr steels. To ensure reliability, operability, and performance, these advanced turbine alloys require low CTE properties. Additionally, for welded components, minimizing CTE mismatch between the new material and the welded 12Cr steel is crucial to manage residual stress. This research investigates the impact of alloying elements on CTE, high-temperature strength, phase stability, and manufacturability. As a result, a new material, “LTES700R,” was developed specifically for steam turbine rotors. LTES700R boasts a lower CTE than both 2.25Cr steel and conventional superalloys. Additionally, its room-temperature proof strength approaches that of advanced 12Cr steel rotor materials, while its creep rupture strength around 700°C significantly surpasses that of 12Cr steel due to the strengthening effect of gamma-prime phase precipitates. To assess the manufacturability and properties of LTES700R, a medium-sized forging was produced as a trial run for a turbine rotor. The vacuum arc remelting process was employed to minimize segregation risk, and a forging procedure was meticulously designed using finite element method simulations. This trial production resulted in a successfully manufactured rotor with satisfactory quality confirmed through destructive evaluation.

In this paper, the performance of T23 and 12Cr1MoVG water wall tubes as well as their welded joints in engineering applications is reported. It was found that the T23 water wall tube may have water leak problems during its operation. In order to make sure the safe operation, leakage reasons of T23 water wall tube were analyzed and improvement measures were taken. Recommendations on the choice of water wall material of 1000MW USC tower boiler are given.

Steam-side oxidation and the resultant exfoliation of iron-based scales cause unplanned shutdowns at coal-fired power generation plants. Exfoliate removal is currently limited to frequent unit cycling to minimize the volume of exfoliated scale, upgrading a plant with a “blow down” system, or installing a higher alloy. This paper discusses the rate of steam-side oxidation on Type 304H stainless steel (304H) tube after shot peening the internal surface with commercially available techniques. Shot peening the ID of Type 304H austenitic stainless steel superheater tubes has been shown to improve the overall oxidation resistance in steam. Decreasing the oxidation rate directly impacts the volume of exfoliated scale. The adherent spinel scales are thinner and more robust than non-shot peened tubes of the same alloy. Most of the improved oxidation resistance can be attributed to the presence of a spinel oxide layer combined with a continuous chromia layer formed near the steam-touched surfaces. The presence of a continuous chromia layer vastly reduces the outward diffusion of iron and minimizes the formation of iron-based scales that exfoliate. This work showed that a uniform cold-worker layer along the tube ID has a profound effect on oxidation resistance. Incomplete coverage allows oxidation to proceed in the non-hardened regions at a rate comparable to the oxidation rate on unpeened Type 304H.

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.

Based on the research and development of Ni-based alloy of 700°C steam turbine bolts and blades worldwide, the process, microstructure, properties characteristics and strengthening mechanism of typical 700°C steam turbine bolts and blades materials Waspaloy are discussed in this study. The result shows that Waspaloy has higher elevated temperature yield strength, creep rupture strength, anti-stress relaxation property and good microstructure stability. The Waspaloy alloy could meet the design requirements of 700°C steam turbine bolts and blades.

INCONEL 740H has been developed by Special Metals for use in Advanced Ultra Super Critical (A-USC) coal fired boilers. Its creep strength performance is currently amongst the ‘best in class’ of nickel based alloys, to meet the challenge of operating in typical A-USC steam temperatures of 700°C at 35 MPa pressure. Whilst the prime physical property of interest for INCONEL 740H has been creep strength, it exhibits other physical properties worthy of consideration in other applications. It has a thermal expansion co-efficient that lies between typical values for Creep Strength Enhanced Ferritic (CSEF) steels and austenitic stainless steels. This paper describes the validation work in support of the fabrication of a pipe transition joint that uses INCONEL 740H pipe, produced in accordance with ASME Boiler Code Case 2702, as a transition material to join P92 pipe to a 316H stainless steel header. The paper gives details of the material selection process, joint design and the verification process used for the joint.

The Cr and W effect on the creep strength of ferritic steels were studied using the new strengthening hypothesis, precipitation strengthening mechanism, by examining the residual aligned precipitates consisting of W and Cr. In 2 mass% W-containing steel, the increase in Cr content up to 10 mass% resulted in the creep life extension. However, the Cr content higher than 11 mass% decreased the creep life. In 9 mass% Cr-containing steel, the increase in W content decreased the creep deformation rate with creep time. However, it also shortened the time to reach the minimum creep rate. Therefore, optimum Cr and W contents possibly resulted in the optimum alloy design. To understand the effect of W and Cr contents on creep strength, the precipitation strengthening hypothesis by the precipitates at the block boundary must be introduced. The residual aligned precipitation line is supposedly an effective obstacle for the dislocation motion at the interparticle space of the aligned precipitates. The new hypothesis will be activated after block boundary migration. It occurs during the acceleration creep period. On the basis of the hypothesis, creep strength was expressed as the summation of threshold creep stress and effective internal creep stress. According to the experimental data of microstructure recovery, the effective internal stress decreased with creep deformation and consequently vanished. In such cases, creep strength is decided only by the threshold stress of creep. Integrating all, we concluded that the creep deformation mechanism of ferritic creep-resistant steel possibly transits from the viscous dislocation gliding mode to the microstructure recovery driven type mode during the acceleration creep.

The application of cold weld repair techniques in the power industry has been well documented. This type of repair is only considered when a conventional repair (involving post-weld heat treatment) is impracticable or the penalties of time and cost for conventional repair are sufficiently high. A typical cold weld repair in the UK has involved low alloy ferritic steel (½Cr½Mo¼V, 2¼Cr1Mo) components welded with nickel based SMAW consumables or ferritic FCAW consumables. Modified 9Cr steel components have been used in UK power plant since the late 1980’s for a number of applications, such as superheater outlet headers, reheat drums and main steam pipework. The problems associated with this material have also been well documented, particularly premature type IV cracking of welds on creep weakened modified 9Cr steel. RWE Generation UK have developed modified 9Cr cold weld repairs on headers, pipework and tubes. These repairs have been underwritten with extensive testing. This paper will describe the work performed on developing T91 cold weld repairs and where they have been applied on power plant.

Research and development of 700°C A-USC steam turbine unit needs to be supported by materials with excellent overall performance. Waspaloy is a kind of γ′ phase precipitation hardening superalloy developed by the United States in the 1950s. In the 700°C R&D Plan of Shanghai Turbine Plant, it was selected as a candidate material for high temperature blades and bolts. The composition, microstructure, properties, blade die forging process and bolt rolling process of Waspaloy alloy were researched in this paper. Simultaneously, Shanghai Turbine Plant successfully manufactured Waspaloy alloy trial production for high temperature bolts and blades. The results show that Waspaloy not only has excellent processing performance, but also has good high temperature strength, long-term performance, stress relaxation resistance and long term aging performance stability at 700°C. It can fully meet the requirements of high-temperature blades and bolts of 700°C A-USC unit. It shows that the 700°C A-USC unit high temperature blades and bolts were successfully developed by Shanghai Turbine Plant.

This study aims to examine the effects of grain boundary oxidation and creep on crack initiation and fracture behaviors in cold worked surface layer, under static tensile stresses in air. To determine these effects in relation to percent cold work and hardness scale, cold-rolled plates with a reduction ratios between 10% and 50% were prepared. Uniaxial constant load (UCL) tests were conducted at elevated temperature in air using smooth round bar specimen. UCL tests with a load of 0.9σy (926MPa) at 550°C show that rupture time for all cold- rolled materials were shorter than that of as-received material. From cross-sectional observation after UCL testing, surface crack at grain boundary and voids were observed in as-received material, whereas creep cracks were also observed in cold-rolled materials. This implied that crack initiation was assisted by cold working. Comparing test results with a load reduced to 0.8σy (823MPa), difference of rupture time was expected as a factor of 5 for as-received material, and measured as 2-3 for cold-rolled materials. It was suggested that cold worked layer was more sensitive to creep than base metal.

The current study proposed a new method that utilizes digital image correlation (DIC) techniques to measure in-situ full field strain maps of creep resistant material welds. The stress-rupture test is performed in a Gleeble thermal mechanical simulator. This technique successfully captured a significant difference in the local creep deformation between two Grade 91 steel welds with different pre-welding conditions (standard and non-standard). Strain contour plots exhibited inhomogeneous deformation in the weldments, especially at the heat-affected zone (HAZ). Standard heat-treated specimens had significant creep deformation in the HAZ. On the other hand, non-standard heat treated specimens showed HAZ local strains to be 4.5 times less than that of the standard condition, after a 90-hour creep test at 650°C and 70 MPa. The present study measured the full field strain evolution in the weldments during creep deformation for the first time. The proposed method demonstrated a potential advantage to evaluate local creep deformation in the weldments of any creep resistant material within relatively short periods of time.

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.

The effect of Cr content on the creep strength at 650°C was examined with high Cr heat resistant steels for the USC high-temperature rotor shafts. The amount of Cr was varied from 8.5% to 11.5%, and then, the alloying effect of Cr was investigated on the stability of the precipitates at 650°C. Within the present range of the Cr content, the short-term creep rupture life under the higher applied stress increased with the Cr content in the steels, whereas the long-term creep rupture life under the lower applied stress decreased with the Cr content in the steels. For example, under the applied stress of 98MPa, the 9%Cr steel exhibited the longest creep rupture life among the experimental steels. Also, it was found from the experiment using the extracted residues that the degree of solution strengthening and the sorts of precipitates scarcely changed regardless of the Cr content in the steels. The Laves phase precipitated finely in the lath was enlarged in the 11.5%Cr steel even after a short-term creep. This result indicates that the coarsening of precipitates such as the Laves phase promotes the recovery of the lath in the early stage of creep deformation. It was suggested that 9%Cr is desirable content in the ferritic steel for suppressing the degradation of creep strength in 98MPa at 650°C.