Thick-walled Components Research Articles

A Net Shape Profile Extraction Approach for Exploring the Forming Appearance of Inclined Thick-Walled Structures by Wire Arc Additive Manufacturing

Wire arc additive manufacturing (WAAM) offers a viable solution for fabricating large-scale metallic parts, which contain various forms of inclined thick-walled structure. Due to the variety of heat dissipation conditions at different positions, the inclined thick-walled structure is a major challenge in fabrication that may produce collapses and defects. However, there is a lack of effective sensing method for acquiring the forming appearance of individual beads in the structure. This paper proposes a novel approach for extracting individual bead profiles during the WAAM process. The approach utilizes a structured-laser sensor to capture the morphology of the surface before and after deposition, thereby enabling an accurate acquisition of the bead profile by integrating the laser stripes. Utilizing the proposed approach, the research investigated the forming mechanism of beads in inclined thick-walled components that were fabricated by various deposition parameters. The width of the overlapping area at the overhanging feature decreased as the layer number increased, while the height of the same area increased. The height of the overlapping area in each layer increased with an increase in deposition current and decreased when the deposition speed was increased. These phenomena suggest that the heat input is a major factor that influences the formation of the overhanging feature. Both the deposition current and deposition velocity influence heat input, and thereby have an effect in enhancing the geometrical accuracy of an overhanging feature. The experimental results indicate that the proposed approach facilitates morphology change investigation, providing a sufficient reference for optimizing deposition parameters.

Dynamic simulation and control strategy development of molten salt steam generation system for coal-fired power plant flexible retrofit

The coal-fired power plant (CFPP) coupled with the molten salt thermal energy storage system is a potential way to improve its flexibility and peak-shaving ability. The steam generation system (SGS) is a suitable choice to convert the feed water into hot steam by using the heat from the molten salt. In this paper, the schematic of an SGS coupled with the CFPP is presented. A dynamic model of the SGS basing lump parameter method is established and validated to investigate its dynamic response characteristics. Five different disturbance experiments including feed water inlet parameters, molten salt inlet parameters and medium pressure steam valve, are conducted and analyzed. The dynamic response curves of molten salt and steam are obtained. Moreover, the load adjustment process of SGS with three different load changing rates (3 %, 6 %, 10 % Pe/min) under the rated conditions is compared and its influence on the SGS is analyzed. The temperature changing rates in the thick-wall components of SGS are within 2 °C/min which meets the safe operating standard. Based on the above results, the three elements control strategy with excellent robustness for the SGS safety operation is proposed and demonstrated. The results show that the response time for the SGS load regulation process and the water level fluctuation have been significantly improved as the three elements control strategy is imposed. The water level can be stabilized in 200 s with tiny fluctuation when the SGS loads down 10 % thermal load with 10 % Pe/min load changing rate. These results could provide useful references for the design and control strategy making of the CFPP coupled with the molten salt thermal energy storage system.

Passive wall thickness monitoring using acoustic emission excitation

Erosion-corrosion is a problematic damage mechanism for the oil and gas industry. To manage the risk of erosion-corrosion networks of particle impact monitoring systems have been installed on pipelines in order to detect acoustic emission from abrasive sand particles impacting the inside surface of the pipe. It would be of value if the existing network of particle impact monitoring systems were not only capable of detecting particle impact, but also sizing the remaining wall thickness. Particle impact monitoring systems are passive and are not generally equipped for excitation. This paper explores the feasibility of using passive acoustic emission transducers for wall thickness measurement, utilizing the fact that active pulse-echo measurements can be approximated by autocorrelating diffuse acoustic waves, such as those generated by particle impact. Two measurement modalities are presented: a) time-of-flight measurements and b) resonant ultrasound spectroscopy measurements. The more usual time-of-flight based measurement is limited by the fact that acoustic emission transducers typically have sensitive bandwidths limited to <1 MHz. The relatively low frequency operation limits the use to thick wall components where the component thickness ≫ ultrasonic wavelength. In thinner walled components a resonant ultrasound spectroscopy approach is required. Experimental measurements are shown that are truly passive (with no purposeful excitation at all), and semi-passive, utilizing acoustic emission from sand impact or compressed air as the excitation source. Results show very good agreement with active measurements.

Cold-Source Composite Welding Repair of 9Cr2Mo Thick-Walled Parts: Microstructure, Mechanical Properties, and Finite Element Simulation.

The restoration of large support rollers poses an industrial challenge due to the high energy consumption of traditional repair methods. Consequently, a novel approach for repairing support rollers has been introduced and tested on thick-walled components. Finite element simulations aided the study of welding repairs for large thick-walled components, examining element distribution, microstructure, mechanical properties, and residual stress distribution across various processes. The results indicate that employing ABAQUS2023 finite element software to analyze stress variations under different working conditions, the Dynamically Controlled Low-Stress No-Distortion method effectively mitigates residual stresses both during and post welding, reducing average transverse residual stresses by 14.5% and average longitudinal residual stresses in the weld zone by 29.1%. The Dynamically Controlled Low-Stress No-Distortion method narrows the high-temperature range of the heat source, consequently decreasing the size of the heat-affected zone by 33.3% compared to conventional welding. The microstructure featured dendrites and equiaxed columnar crystals, with the Dynamically Controlled Low-Stress No-Distortion method capable of grain refinement, transforming some equiaxed columnar crystals into cellular structures. As grains were refined, microhardness improved, with the covering layer's microhardness rising by 14.68%. A comparison between simulated and measured values of lateral and longitudinal residual stresses at corresponding points revealed discrepancies of 14.6% and 20.5% in accuracy, respectively.

Contribution to the Fracture Mechanics Characterization of Repair Welding of Thick-Walled Ductile Cast Iron Components

AbstractFor the spheroidal graphite cast iron materials, also known as ductile cast iron (DCI), EN-GJS-400-18-LT and EN-GJS-450-18, different welding procedures were examined for potential repair welding. A repair weld was performed on thick cast iron plates using the optimum procedure in each case. The weld was evaluated for all areas of the welded joint (weld metal, fusion line/heat-affected zone and base metal). In addition to the metallographic microstructural analysis, the characterization of the repair weld was carried out by means of hardness distribution measurements, static tensile testing and notched bar impact testing as well as fracture mechanics investigations under static and under cyclic loading in a wide load ratio range. On this basis, a generalized description of cyclic crack growth could be made according to the NASGRO® MODEL. NASGRO® is a fracture mechanics and fatigue crack growth software. In accordance with the possible operating conditions of repair-welded, thick-walled components, the static and impact tests were performed in the temperature range down to $$-$$ - 40 $${}^{\circ }\text {C}$$ ∘ C .

Comparisons of ray and finite element simulations of ultrasonic wave fields in smoothly inhomogeneous austenitic welds of thick-walled components

This work addresses the crucial challenges of simulating ultrasonic inspections in welded parts. We focus on the simulation of radiated ultrasonic fields in welded materials, incorporating spatially varying anisotropy and damping properties. Using macroscale descriptions to capture variations in weld properties as a function of grain orientation, two simulation methods are evaluated: a ray method and a hybrid finite element solution within the CIVA software. Simple simulations validate both models, but complexities emerge in real-world scenarios, notably caustic phenomena and beam splitting due to specific grain orientations. The proposed hybrid FE model can therefore provide reliable reference solutions, even for 3D configurations, allowing the biases arising from radius approximations to be quantified.

Linking the microstructure with strain-life curves for improved utilization of the lightweight potential of thick-walled nodular cast iron

The assessment of the fatigue life of critical, highly stressed component areas is an important aspect in the design of thick-walled components for wind turbines made of nodular cast iron. With the demand for resource efficiency through lightweight design combined with a certain capability to withstand extreme loads and special events, this issue is becoming increasingly important. However, there are still gaps in the description of local strain-based material behaviour. For the design and local strain-based fatigue assessment of large cast components, possibilities to determine the fatigue life as accurately as possible must be provided by standards and guidelines. The paper aims to provide a wide range of material and fatigue data from one common source for general design interests. During research projects at Fraunhofer LBF, material properties for the PRAM-based fatigue life assessment and the elastic-plastic material behaviour have been derived from 9 nodular cast iron grades with 44 microstructural variations. To describe the influence of the technological size effect on cyclic stress-strain and PRAM-N curves the quasi-static parameters (tensile strength and elongation at break) and the microstructure (nodularity and graphite particle density) are considered. In addition, suggestions for scatter bands in the strain-based design approach are given to the designers of thick-walled GJS components.

Design of Multi-Input Multi-Output Non-linear Model Predictive Control for Main Steam Temperature of Super Critical Boiler

Flexible operation of coal-fired power plants is becoming increasingly necessary for successful integration of large-scale renewable power generation into the power grid. The maximum ramp rate and the number of load cycles are generally limited by the thermal stress experienced by the boiler pressure parts, turbine metallurgy and creep and fatigue of critical thick-walled components Main steam temperature is a critical operating parameter that must be controlled within acceptable limits for safe operation. Main steam temperature deviation beyond acceptable limit has impact on boiler pressure parts and turbine material of construction due to creep and fatigue effect. Base load operating units do not require steep ramp rate and hence recommended ramping rates are kept low within the safe operating zone in comparison to the flexible operation of the units with wide range load change width. Thermal stresses are caused by the temperature changes inside the thick-walled components and turbine steam admission parameters. Hence, the quality of main steam temperature control plays a vital role in flexible operation of the coal fired units. Conventional cascaded PID temperature control loop architecture performs well at steady state condition within a limited variation of load change at low ramp rate but it acts slowly and performs poorly at transient operating conditions of flexible operation of the boiler turbine with wide range load variation and load cycle with high ramp rate and remains far from rated conditions. In this paper, a Multi-Input Multi-Output (MIMO) Non-linear Model Predictive Control (MPC) design for regulation of the main steam temperature of a Once-Through supercritical Boiler is proposed. The controller is based on a non-linear dynamic model which incorporates dynamics of the variables of interest. It has the capability to operate effectively across a wide load range while maintaining main steam temperature within acceptable limits. A notable advancement in this design of MPC is the incorporation of coal flow demand and feedwater flow demand as additional control inputs alongside primary and secondary spray flows. In simulation test cases, the MPC controller demonstrates satisfactory performance and computational efficiency.

Optimisation of the cooling of pressurised thick-walled components operating with fluid at saturation temperature

Determining the optimum heating or cooling times for thick-walled pressure elements allows a steam boiler to reduce start-up or shutdown times. A new method has been developed to determine the optimum fluid temperature change to shorten the steam boiler start-up and shutdown. The fluid temperature curve is designated so that the total circumferential stress at the hole edge does not exceed the allowable stress. The total stress at the hole edge where it is concentrated consists of thermal and pressure stress. The example of the optimum cooling of the steam boiler drum illustrated the method used. A three-dimensional finite element method (FEM) analysis was carried out for the connection between the drum and the downcomer to determine the transient temperature and stress distributions. It was shown that the total circumferential stress due to pressure and thermal loading at the hole edge did not exceed the allowable stress when optimal fluid temperature changes were used. The method shown in this paper was compared with the boiler standard EN 12952-3, which permits the determination of allowable heating and cooling rates of boiler pressure components as a function of pressure. The comparison shows that the cooling/heating time determined by the new method can be significantly shorter than that determined by the method in EN 12952-3. By using the new method for designating the optimum fluid temperature, the flexibility of the boiler and the power unit can be improved as the heating or cooling of critical pressure elements can be more rapid. It can also be used to accidently cool PWRs (Pressurised Water Reactors) and BWRs (Boiling Water Reactors) pressure vessels.

The effect of microstructure on the fatigue crack growth behavior of Ni-Cr-Mo-V steel weld metal

Fatigue crack growth (FCG) rate, which could significantly affect fatigue lifetime of thick wall components, has been studied extensively in forgings with homogeneous microstructure. However, there were few studies on the effect of inhomogeneous microstructure on FCG rate in weld metal. In this work, the effect of microstructure on FCG behavior of Ni-Cr-Mo-V weld metal was investigated. The FCG rate in the Paris regime was similar to that reported in the literature. However, in the near-threshold regime, the FCG rate decreased rapidly at first with the decrease of stress intensity factor range (ΔK) and then increased slightly. Finally, the FCG rate slowly decreased with the decrease of ΔK. It was found that when the monotonic plastic zone size around the crack tip was equivalent to the ferrite grain size in reheated grain zone at a certain ΔK, the crack propagated along the ferrite grain boundaries. The occurrence of intergranular fracture increased the fracture surface roughness, and enhanced the roughness-induced crack closure effect, which increased the FCG resistance and reduced the FCG rate drastically at the early stage. At a lower ΔK level, frequent deflection and crack branching were observed and the mechanisms were investigated by crystallographic analysis.

Study on Porosity Defect Detection in Narrow Gap Laser Welding Based on Spectral Diagnosis.

As an advanced connection technology for large thick-walled components, narrow gap laser welding has the advantages of small heat input and high efficiency and quality. However, porosity defects are prone to occur inside the weld due to the complex welding environment. In this study, the influence of the process parameters and pollutants such as water and oil on the porosity defect were explored. The action mechanism of water on the electron temperature and spectral intensity of the laser-induced plasma was analyzed. The results showed that the spectral intensity during narrow gap laser welding was weaker than that of flat plate butt welding. Under the optimal welding process conditions, the electron temperature during narrow gap laser self-fusion welding was calculated as 7413.3 K by the Boltzmann plot method. The electron density was 5.6714 × 1015 cm-3, conforming to the thermodynamic equilibrium state. With six groups of self-fusion welding parameters, only sporadic porosity defects were observed according to the X-ray detection. When there was water on the base metal surface, a large number of dense pores were observed on the weld surface and in the weld through X-ray inspection. Compared with the spectral data obtained under the normal process, the relative light intensity of the spectrometer in the whole band was reduced. The electron temperature decreased to the range of 6900 to 7200 K, while the electron density increased. The spectrum variation during narrow gap laser wire filling welding was basically the same as that of laser self-fusion welding. The porosity defects caused by water and oil pollutants in the laser welding could be effectively identified based on the intensity of the Fe I spectral lines.

Fatigue inhomogeneity of thick 316L stainless steel joints fabricated by ultra-high power laser-MIG hybrid welding

The ultra-high power laser-MIG hybrid welding technique provides an effective method for thick-walled components joining, but the fatigue properties of joints are rarely reported. This research aims to unveil the fatigue behavior evolution of thick weldments along penetration direction. The fitted S-N curves are derived and results show that the fatigue strength presents a downtrend and failure location is transferred from heat affected zone to weld center in depth direction. The mechanism of fatigue crack initiation and propagation are discussed, and find that the shrinkage cavity in lower layer will reduce fatigue performance. The research results provide theoretical guidance for the service of ultra-high power laser-MIG hybrid welded joints.

The flexible boiler operation in a wide range of load changes with considering the strength and environmental restrictions

Thermal steam units cooperating in one power system with wind and photovoltaic farms must have high flexibility, i.e., operate over a wide range of load variations, typically between 40 and 100%. In addition, the start-up and shutdown time of the unit should be short. Thick-walled pressure components that limit the fast start-up of the units are turbines, steam boilers, and pipeline branches due to the occurrence of high thermal stresses in them. This paper determines the allowable heating rates of thick-walled components of a 97.22 kg/s natural circulation steam boiler. The boiler drum is the critical element, with a heating time of about 6000 s from the cold state. Computer simulations of the boiler combustion chamber were also performed at three different loads, i.e., 100, 60 and 40%. The calculations show that the maximum emissions of harmful substances do not exceed the permissible values given in European regulations: CO≤140mg/mn3 and NOx≤100mg/mn3. The maximum content of CO = 136.8 mg/mn3 occurs at 60% boiler load. The maximum content of NOx = 217.3mg/mn3 in the exhaust gas upstream of the SCR (Selective Catalytic Reduction) system occurs at 40% load, but downstream of the SCR system is reduced below the limit value 100mg/mn3.

Quality Monitoring of Micro-shrinkage Defects in Thick-walled Injection Molded Components

Sink mark defects are commonly observed in thick-walled injection molded components as a result of under-packing in the hot-spot areas and an insufficient compensation of the polymer melt as the plastic begins to cool. These defects degrade the surface quality of the molded parts and are a reliable indicator of excessive volume shrinkage during the molding process. Consequently, effective methods for quantifying sink marks in thick-walled components, and predicting the occurrence of such defects as a function of the injection molding process parameters, are of significant interest in improving the molding quality. Accordingly, taking an injection molded plastic handle with an average thickness of 11.57 mm for illustration purposes, the present study proposes a method for measuring the micro-shrinkage (ΔΩ) of thick-walled injection molded components using a simple image-processing technique. Simulation and experimental trials are performed to investigate the effects of the injection molding parameters on the quality of the polymer melt (as evaluated using five quality indexes (PA, PB, ηA, ηB, and CFi,p) extracted from the pressure profiles within the chamber and the tie-bar elongation, respectively) and the quality of the final molded component (as quantified by two metrics, namely the weight variation of the molded parts and the micro-shrinkage). It is shown that the quality of the polymer melt and final thick-walled component is determined mainly by the magnitude of the holding pressure applied in the molding process. By contrast, the injection speed, V/P switchover point, and barrel temperature exert only minor effects. The experimental results obtained under various values of the holding pressure show that the five injection molding process quality indexes are all highly correlated with the micro-shrinkage. The correlation is particularly strong for the pressure-based quality indexes (PA, PB, ηA, and ηB). Hence, it is suggested that these indexes provide a viable means of predicting the formation of sink mark defects and excessive micro-shrinkage in thick-walled injection molded components.

Clay 3D printing as a bio-design research tool: development of photosynthetic living building components

Within architecture, microalgae are employed to address sustainability issues and mitigate the impacts of anthropogenic carbon dioxide (CO2) emissions. This study proposes digital fabrication of ceramic ‘living’ building components as an investigative tool for design conditions. The health of the chlorophyte (green) microalga Chlorella vulgaris was monitored over two-week periods when immobilized in kappa carrageenan and clay binder-based hydrogels, and grown on a range of digitally fabricated ceramic components. The use of 3D printing is presented in relation to laboratory testing of controlled substrate variables including the impact of ceramic firing temperature, component wall thickness, three types of geometry for exploring cell growth, surface patterns to investigate cell migration, internal chamber subdivisions and clay type. The experiments reveal the benefits and limitations of creating micro-ecologies for algae growth through the introduction of geometry variation. In this study, the natural organismal sensing abilities are explored as a means for cell distribution.

Increase the flexibility of steam boilers by optimisation of critical pressure component heating

The paper presents a new method for determining the optimum fluid temperature changes during heating cylindrical thick-walled components weakened by holes. An inverse heat conduction problem was solved to determine the temperature of the fluid so that the equivalent total stress due to pressure and thermal load at the edge of the hole at the point of concentration does not exceed the allowable stress. The stress concentration factor at the edge of the hole was determined using the Finite Element Method (FEM) to represent the actual shape of the junction between the element and the stub. Optimum temperature changes of the steam-water mixture in the boiler drum and live steam temperature in the outlet header of the final superheater during the boiler start-up were determined. Based on the optimum time temperature and pressure variations determined for all boiler pressure elements, it is possible to identify the critical element. The heating time for the critical element is the longest. The critical boiler element governs the process of boiler start-up. The flexibility of the boiler can be significantly improved using the proposed method as boiler start-up times can be shortened by approximately one-third compared to current practice.

A metastable jet model for leaks in steam generator tubes

The modeling and prediction of flow rates through leaks in steam generator tubes requires an understanding of the rapid decompression of a subcooled fluid flowing through a small and short flow path. The thin walls of steam generator tubes may lead to the phenomenon that the coolant water cannot evaporate within the leak flow path, which is a key difference compared to leak flow in thick-walled components. Experimental investigations in the literature were investigated and additional laboratory tests on dedicated specimens were performed, including mass flow measurements and optical measurements, which study the flow pattern at the outlet of the leak. These experiments cover a ratio of flow length (wall thickness) to hydraulic diameter L/dh range from 0.4 to 12. Based on the findings, a metastable jet model is proposed, which is based on the formation of a jet of metastable liquid water. The model parameters are fitted with the experiments. The model is compared with alternative approaches, which demonstrates that it is suitable to approximate the mass flow of leaks in steam generator tubes with a sufficiently large leak width or cross section. Due to the small absolute wall thickness of steam generator tubes only very narrow flaws are found to be successfully modeled with ordinary two-phase flow models.

Extension of the process limits for laser beam welding in vacuum of thick-walled steel sheets

Welding thick-walled steel sheets with established joining processes such as gas metal arc welding and submerged arc welding is often associated with very long production times and high-energy input due to the low welding depth and welding speed. Laser beam welding in vacuum enables efficient joining of thick-walled components with only one weld seam at moderate working pressure of 0.2 mbar. This paper demonstrates the extension of the known limits of laser beam welding in vacuum using up to 40 kW laser power.

Optimisation of heating and cooling of pressure thick-walled components operating in the saturated steam area

The paper presents a new method of determining optimum temperature variations of the fluid during heating and cooling of cylindrical thick-walled components weakened by holes. The temperature of the fluid located or flowing through the pressure component is equal to the saturation temperature.The stress concentration factors at the edges of the holes were determined using the Finite Element Method (FEM) to represent the real construction of the junction between the element and the spigot.Optimum fluid temperature variations were determined so that the maximum equivalent stress at the edge of the opening is equal to the permissible stress determined concerning thermal fatigue. The equivalent stress at the hole edge at the point of its concentration includes thermal stress and stress due to pressure. Optimum temperature changes of hot water and steam-water mixture were determined during heating up and cooling down of the boiler drum, i.e., during boiler start-up and shutdown.Three-dimensional computations of the transient field of temperature and stress were carried out for the junction of the drum and downcomer to show that the equivalent stress at the edge of the hole does not exceed the allowable stress if the optimum temperature of the fluid was estimated by the proposed method.

Extension of the process limits in laser beam welding of thick-walled components using the Laser Multi-Pass Narrow-Gap welding (Laser-MPNG) on the example of the nickel-based material Alloy 617 occ

The joining of thick-walled components using beam-based joining techniques is content of worldwide research and development activities, but has not yet been established in industry. State of the art to weld nickel super alloys is currently a TIG narrow-gap welding. The present paper is focusing on a new specific laser beam welding process, the so-called Laser Multi-Pass Narrow-Gap welding (Laser-MPNG). It first explains the process principle based on 2D beam oscillation, the use of fiber lasers and the multi-pass principle. The potential of the Laser-MPNG welding process is demonstrated using the technically significant nickel-based material Alloy 617 occ. As a result, it was possible for the first time to realize a weld with a wall thickness of 140 mm free of cracks or bonding defects. Promising results of creep and low-cycle fatigue tests are used to show the potential that Laser-MPNG welded joints would have for future industrial applications.