Modeling and Simulation of Optimal Blank Design and Hot Pressing Process for Manufacturing Large Francis Turbines Blades From Very Thick Plates

Hot pressing process is widely used in automotive, shipbuilding, energy production and civil engineering. However, the trial and error technique that is intensive time and energy consuming is still used. Particularly, the design of Francis turbines of hydropower plants is not standard, but variable from site to site due to hydraulic conditions and cost of energy. As a result, the blade hydraulic profile of each Francis turbine is different. The blades, one of the key components of Francis turbine runners, are produced in small batches and the setup of the dedicated punch and die increases significantly the unit production costs. In this paper, the blade unfolding process for optimal blank design will be firstly presented, and then a hot pressing process for very thick plates is proposed. The pressing process of high strength steel at hot temperature is characterized by thermo-mechanical behaviors, three-dimensional unsteady deformation, high nonlinearity, continuous local forming. The analyses of residual stress distribution and applied forces are carried out.

Creep Crack Growth Prediction of Very Long Term P91 Steel Using Extrapolated Short-Term Uniaxial Creep Data

Practical time frames in newly developed steels, and technical and financial restrictions in test durations means that extrapolation of short-term laboratory test results to predict long-term high temperature service component failure is an area of concern when conducting a fitness for service or remaining life assessment. Recent literature presenting uniaxial creep and crack growth tests indicate that some materials show lower failure strains during longer term laboratory tests. The constraint based remaining failure ductility based NSW model crack prediction model has been shown to be capable of predicting upper/lower bounds of creep crack growth in a range of steels when data are obtained from relatively short to medium-term laboratory experiments (< 10,000 hours). This paper compares and analyses the response of the NSW model to predict long term creep crack propagation rates using a wide database of modified 9Cr material over s range of temperatures. The paper employs extrapolation methods of available uniaxial data to make viable conservative predictions of crack growth at high temperatures where at present no data is available.

Extrapolation of Creep Strength by Fracture Energy for 316FR Stainless Steel at 823K

316FR stainless steel is a candidate material to be used for a reactor vessel and internals for fast reactors with a design life of 60 years at an operating temperature of 823K. This paper describes an extrapolation approach based on fracture energy for calculating creep strength. A change in fracture energy is assumed to be expressed as a power-law function of time to failure and energy density rate. The energy density rate is calculated using initial stress, rupture elongation, and time to rupture. It is important to evaluate a change in rupture elongation for the extrapolation of creep strength at 823K. The time to rupture at 823K is estimated and extrapolated on the basis of the fracture energy approach. This paper shows the validity of extending the design life to 60 years by using the Larson–Miller parameter compared with the estimation by the fracture energy approach.

A New Generalized Nonlinear Superposition Theory and its Applications in Modeling Corrosion-Fatigue

Corrosion-fatigue and stress corrosion cracking have long been recognized as the principal degradation and failure mechanisms of materials under combined corrosive environment and sustained/cyclic loading conditions. These phenomena are strongly material and environment dependent, and cycle-dependent fatigue and time-dependent matter diffusion/chemical reaction at the crack tip can be operational simultaneously. How to include these cycle-dependent and time-dependent phenomena in a single model and how to study the failure mechanisms interaction are big challenges posed to material scientists and engineers. In this paper the current linear superposition theories for modeling cycle-dependent and time-dependent corrosion-fatigue and stress corrosion cracking phenomena are reviewed first. Subsequently, a generalized nonlinear superposition theory is proposed to incorporate possible nonlinear interaction or synergistic effect among the underlying mechanisms. The unified model derived from the new theory, depending on the specific materials and loading condition and environment, can be reduced to pure corrosion, pure fatigue, stress corrosion cracking and corrosion-fatigue. Finally, for the first time, a new breakthrough parameter is defined in this paper to quantitatively describe the interaction or synergistic effect between two different operating mechanisms, such as time- and cycle-dependent mechanisms.

Dependence of Tensile Strength on Geometrical Interface Conditions of Bonded Dissimilar Materials

Many engineering structures applied for generating energy are said to have been requiring high strength under high temperature conditions. Fine ceramic is expected to be useful in structural applications in various industries by joining to metals. Ceramic can be used in structural parts for engineering where resistance to high temperature and/or high strength are required from the viewpoint of the optimum structural design. Use of ceramic for engineering structures by joining to metal generates a bonded interface between the ceramic and metal.

Simulating Residual Stresses Using a Modified Wedge-Loaded Compact Tension Specimen

Residual stresses are induced in components when fabrication processes produce internal stresses or local deformation and cause accelerated creep damage and cracking during service at elevated temperatures. A method of inducing residual stresses in laboratory fracture specimens is proposed where an oversized wedge is inserted into the crack mouth of a compact tension, C(T), type specimen. In this way the extent of internal stresses can be controlled in order to minimise the level of crack tip plasticity which inherently reduces the remaining strain to failure. Numerical simulations of wedge insertion into specimens made of 316H austenitic stainless steel have been developed to calibrate the wedge insertion process. These models have been experimentally validated using surface strains measured during the wedge insertion, using Digital Image Correlation (DIC), and Neutron Diffraction (ND) measurements. The validated Finite Element (FE) model is used to determine the wedge insertion depth required to maximise the residual stresses without causing significant crack tip plasticity. The validated numerical simulation is used to determine the wedge insertion depths of further wedge-loaded C(T) specimens made from uniformly pre-compressed 316H stainless steel. The reduced creep ductility of this material increases the rate of crack growth under creep conditions. This method of inducing residual stresses with limited crack tip plasticity allows creep crack growth under simulated secondary loading conditions to be investigated without the influence of non-uniform creep ductility caused by work hardening.

Investigation of Adhesion Formation in New Stainless Steel Trim Spring Operated Pressure Relief Valves

Examination of proof test data for new (not previously installed) stainless steel (SS) trim spring operated pressure relief valves (SOPRV) reveals that adhesions form between the seat and disc in about 46% of all such SOPRV. The forces needed to overcome these adhesions can be sufficiently large to cause the SOPRV to fail its proof test (FPT) prior to installation. Furthermore, a significant percentage of SOPRV which are found to FPT are also found to “fail to open” (FTO) meaning they would not relief excess pressure in the event of an overpressure event. The cases where adhesions result in FTO or FPT appear to be confined to SOPRV with diameters ≤ 1 in and set pressures < 150 psig and the FTO are estimated to occur in 0.31% to 2.00% of this subpopulation of SS trim SOPRV. The reliability and safety implications of these finding for end-users who do not perform pre-installation testing of SOPRV are discussed.

The Correction of Stress Intensity Factor for Crack Growth Evaluation Under Steep Residual Stress Distribution and Steep Yield Strength Distribution

Welds and heat affected zones have the distribution of the residual stress or the yield strength. The crack growth evaluation is conventionally conducted using stress intensity factor in those regions. However, the stress intensity factor may be invalid when the residual stress distribution or yield strength distribution changes in the vicinity of a crack tip. The reason is that the distributions around the crack tip affect the plastic zone size and the stress intensity factor inappropriately represents the stress state in the vicinity of a crack tip. In this study, the residual stress distribution and yield strength distribution was assumed along the crack propagation path and the validity of the stress intensity factor was discussed on that condition. As a result, the stress intensity factor tended to be invalid when the steep residual stress distribution or the steep yield strength distribution. When the steep distribution exists, the crack growth evaluation should be conducted using a parameter considering the elastoplastic behavior near the crack tip. For that purpose, the authors proposed new method of the plastic zone correction using a differential term of the stress intensity factor. The new method was demonstrated through the case study for stress corrosion cracking of nuclear power plants.

Development of Pipe Wall Thinning Evaluation Method and Prediction Tool

Flow accelerated corrosion (FAC) and liquid droplet impingement erosion (LDI) are the main pipe wall thinning phenomena in piping system of power plants in Japan. Authors have promoted the development of prediction method to evaluate local thinning trend by FAC/LDI. To apply the method to pipe wall thinning management in power plants, it is required to be transformed into practical tools for easy usage. In Japan, discussion is being made to considerate the introduction of prediction tools into wall thinning management based on wall thickness measurement at present. Authors have simplified their FAC/LDI models to predict wall thinning trend one-dimensionally along piping layout, and applied to actual thinning data of power plants. With PWR’s FAC data and BWR’s LDI data, maximum thinning rate for each pipe elements were roughly predictable with considerable accuracy. Especially for high thinning rate data, which is important in plant management, the model was able to evaluate within the factor of 2. By installing this model, prediction software “FALSET” was developed, equipped with practical functions for the management. With the further verification and improvement of each function, there are prospects for this software to be utilized as a management tool in power plants.

Simplified Prediction of Creep Void Density in Heat-Affected Zone of Grade 122 Steel Considering Multiaxial Stress State

This paper deals with a simplified method for approximately predicting creep void growth in heat-affected zone (HAZ) of ASME grade 122 (11Cr-2W-0.4Mo-Cu-Nb-V) steel weldments. Authors have proposed a simplified prediction method based on the relationship between creep void density increasing rate and multiaxial stress state. This method has been applied to prediction of creep void growth behavior for grade 91 (9Cr-1Mo-Nb-V) tubular specimens with longitudinal weldments. In this study, the method has been also applied to grade 122 steel to clarify the applicability of the method. Internal pressure creep tests of grade 122 tubular specimens with longitudinal weldments subjected to several internal pressures have been conducted to reveal creep void growth behavior in HAZ. In addition, finite element creep analyses for the specimens at different creep strain rates in base metal, weld metal and HAZ have been carried out to investigate distribution of stresses and stress triaxiality factor in HAZ. A comparison between stress distributions and creep void distributions revealed that stress triaxiality factor affects growth behavior of creep voids. From the result, the relationship between creep void density increasing rate and the parameter as a function of principal stress and triaxiality factor was established. It was found that the slope of this relationship for 122 steel has a tendency to be slightly small compared with grade 91 steel. To demonstrate the applicability of the proposed simplified prediction method, the method was applied to the internal pressure creep test specimens at different experimental conditions. As a result, the predicted void distribution and void density increasing rates for grade 122 steel were in good agreement with the experimental results.