Search Results for residual stress

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.

Extended storage of spent nuclear fuel (SNF) in intermediate dry cask storage systems (DCSS) due to lack of permanent repositories is one of the key issues for sustainability of the current domestic Light Water Reactor (LWR) fleet. The stainless steel canisters used for storage in DCSS are potentially susceptible to chloride-induced stress corrosion cracking (CISCC) due to a combination of tensile stresses, susceptible microstructure, and a corrosive chloride salt environment. This research assesses the viability of the cold-spray process as a solution to CISCC in DCSS when sprayed with miniature tooling within a characteristic confinement in two different capacities: cleaning and coating. In general, the cold-spray process uses pressurized and preheated inert gas to propel powders at supersonic velocities, while remaining solid-state. Cold-spray cleaning is an economical, non-deposition process that leverages the mechanical force of the propelled powders to remove corrosive buildup on the canister, whereas the cold spray coating process uses augmented parameters to deposit a coating for CISCC repair and mitigation purposes. Moreover, both processes have the potential to induce a surface compressive residual stress that is known to impede the initiation of CISCC. Surface morphology, deposition analysis, and microstructural developments in the near-surface region were examined. Additionally, cyclic corrosion testing (CCT) was conducted to elucidate the influence of cold-spray cleaning and coating on corrosion performance.

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.

As many nuclear power plants are in the license renewal operating period and some are entering subsequent license renewal, there is increased probability that repairs will be needed on components that have been exposed to significant neutron fluence. The neutron-driven transmutation of nickel and tramp boron in austenitic materials commonly used in reactor internals can lead to the generation of trapped helium and the associated risk of helium-induced cracking (HeIC) during weld repairs. In the weld heat affected zone, where temperatures are insufficient to allow the helium to diffuse out of the material, the helium can remain trapped. Upon cooling, the residual stresses, combined with weakened grain boundaries due to helium coalescence, can lead to cracking. The current ASME limit for helium content for Code repairs is 0.1 appm. Prior work has demonstrated a strong inverse correlation between helium content and permissible weld heat input for avoidance of HelC. The helium concentration in the material to be repaired is thus a critical input to the development of weld repair processes to be applied to these materials. The reliable measurement of helium in irradiated materials at concentrations relevant for the evaluation of HeIC risk is a specialized process. It is important to demonstrate that the capability is available and can be practically leveraged to support emergent repairs. This paper presents on the execution and results of a multi-laboratory test program aimed at demonstrating the industry capability of acquiring accurate, repeatable, and timely measurements of relatively low concentrations of helium (< ~20 appm) within austenitic materials commonly used in reactor internals. Participating laboratories were supplied with equivalent specimens extracted from boron-doped coupons that were irradiated to drive the boron-to-helium transmutation reaction. The results and lessons learned from the program are expected to support the development of industry guidance for the acquisition of similar measurements supporting nuclear component repairs.

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.

The incore instrumentation system of a pressurized water reactor (PWR) facilitates neutron flux mapping and temperature measurements at specific core locations. A guide conduit, extending from the seal table to the lower reactor pressure vessel head, guides and protects each incore guide thimble between the table and the lower reactor vessel head. Each flux thimble houses a detector and drive cable. Once filled with reactor coolant, the conduit becomes an extension of the reactor coolant pressure boundary. This paper reports the examination results of cracking detected in a TP304 stainless steel guide conduit adjacent to a fillet weld at the upper surface of a TP304 seal table. The cracking resulted in reactor coolant leakage that was detected by the presence of boric acid deposits on the exterior of the conduit and table. Failure analysis including dimensional measurements, chemical analysis, stereomicroscopy, metallography, and scanning electron microscopy showed that extensive cracking of the conduit and seal table material occurred due to stress corrosion cracking (SCC). Assessment showed that chlorine-containing deposits were present on the exterior of the conduit and on the surfaces of the seal table and were due to the design and operation of HVAC systems at the coastal plant. Stainless steels are susceptible to SCC in environments with elevated temperatures, chloride contents, and increased tensile stress – particularly in non-post weld heat treated (PWHT) weld regions and the heat affected zone (HAZ). This was the apparent primary cause of the failure. However, chloride-induced SCC of such materials typically results in transgranular crack propagation, whereas the observed cracks were indicative of intergranular stress corrosion cracking (IGSCC). Microstructural analysis showed that the observed cracks initiated in sensitized areas of material adjacent to the weld. Sensitization of the material caused chromium depletion from adjacent areas and increased susceptibility of the depleted areas to IGSCC. In this case, the most probable source of sensitization was related to welding and the long-term growth of grain boundary carbides nucleated during welding. This was considered a contributing cause to the failure.

Simplified or reference stress techniques are described and demonstrated for high temperature weld design and life assessment. The objective is the determination of weld life under steady and cyclic loading in boiler headers and piping systems. The analysis deals with the effect of cyclic loading, constraint and multiaxiality in a heterogeneous joint. A common thread that runs through most high temperature weld reports and failure analyses is the existence of a relatively creep-weak zone somewhere in the joint. This paper starts with the assumption that the size and creep strength of this zone are known, in addition to parent metal properties. Life prediction requires an efficient analysis technique (such as the reference stress method), which separates the structural and material problems, and does not require complex constitutive models. The approach is illustrated with a simple example of an IN617 main steam girth weld, which could be present in an advanced plant concept with 700°C steam temperature.

Steels have a proven track record of safe operation in steam power plants for decades. Interest in developing supercritical CO 2 power cycles as a more efficient and sustainable alternative to steam cycles has driven a need to understand steel performance in these new environments. In particular, the potential of the high temperature CO 2 environment to influence the creep behavior of the steel must be determined. Prior research on this topic between the 1960s and 1980s found conflicting conclusions, but nevertheless raised the possibility that carburization during CO 2 exposure may strongly affect the creep behavior. This raises concerns particularly for thin-sectioned components such as compact heat exchangers, where even small rates of carburization can become problematic over long operating lifetimes. To shed light on this issue, this research investigates the creep behavior of austenitic stainless steel 347H and 309H (a higher Cr alternative) at 650°C. Specimens of 0.5, 1.0, and 2.0 mm thickness were tested to further assess the effect of steel thickness. Both steels show a reduction in creep life in CO 2 relative to air, with 309H showing slightly better performance than 374H. Analysis is ongoing to determine the reason for degraded creep properties.

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.

Advanced ultra-supercritical fossil plants operated at 700/725 °C and up to 350 bars are currently planned to be realized in the next decade. Due to the increase of the steam parameters and the use of new materials e.g. 9-11%Cr steels and nickel based alloys the design of highly loaded components is approaching more and more the classical design limits with regard to critical wall thickness and the related tolerable thermal gradients. To make full use of the strength potential of new boiler materials but also taking into account their specific stress-strain relaxation behavior, new methods are required for reliable integrity analyses and lifetime assessment procedures. Numerical Finite Element (FE) simulation using inelastic constitutive equations offers the possibility of “design by analysis” based on state of the art FE codes and user-defined advanced inelastic material laws. Furthermore material specific damage mechanisms must be considered in such assessments. With regard to component behavior beside aspects of multiaxial loading conditions must be considered as well as the behavior of materials and welded joints in the as-built state. Finally an outlook on the capabilities of new multi-scale approaches to describe material and component behavior will be given.

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.