Alfred Scholz

Robust Methods for Creep Fatigue Analysis of Power Plant Components Under Cyclic Transient Thermal Loading

The aim of this paper is to apply robust mechanisms-based material laws to the analysis of typical high-temperature power plant components during an idealized start-up, hold time and shut-down sequence under a moderate temperature gradient. Among others a robust constitutive model is discussed, which is able to reflect inelastic deformation, hardening/recovery, softening and damage processes at high temperature. The model is applied for a creep analysis of advanced 9–12%CrMoV heat resistant steels and calibrated in particular case against experimental data for 10%CrMoV steel type. For a steam temperature profile transient heat transfer analysis of an idealized steam turbine component is performed providing the temperature field. From the subsequent structural analysis with the inelastic constitutive model local stress and strain state variations are obtained. As an outcome a multi-axial thermo-mechanical fatigue (TMF) loading loop for one or several loading cycles can be generated. They serve as input for a fatigue life assessment based on the generalized damage accumulation rule, whose results come close to reality. In addition, the accuracy of a simplified method which allows a rapid estimation of notch stresses and strains using a notch assessment rule (NAR) [1] based on Neuber approach is examined.Copyright

Creep and creep rupture behaviour of 650°C ferritic/martensitic super heat resistant steels

Abstract The investigations of high-temperature creep properties of advanced ferritic/martensitic steels for 650 °C power plant components focus on the chemical composition as well as on the heat treatment conditions. First experiments on various modifications of the 9% Cr model piping steel P92 demonstrate a negative influence of 12% Cr, 5% Co and low tempering conditions on the creep rupture strength up to 104 h operating time. Low tempering conditions promote the precipitation of the modified Z-phase Cr(V, Nb) N during creep. This phase was recently identified as a major cause for premature breakdown in creep strength of some 9 – 12% Cr martensitic steels. The aim “650 °C/100 MPa/100 000 h” in creep life was not achieved in this investigation so far. The effect of boron on the improvement of creep behaviour depends on the interaction of chemical composition and heat treatment. Further activities focus on the stabilization of M23C6, a fine distribution of VN and measures for avoidance of Z-phase.

Creep crack behaviour of a coarse grain nickel-base super alloy

Abstract The paper presents creep crack growth data, obtained for cast nickel-base super alloy IN-738 LC, which represents a typical large coarse grain structure. Long term creep crack growth experiments were performed at 700 and 850°C on side-grooved compact tension and corner crack specimens. In parallel, uniaxial creep tests on smooth specimens were conducted. The coarse grain structure has been examined using electron backscatter diffraction (EBSD). Creep fracture in polycrystalline materials is observed to be typically intergranular and dominates along grain boundaries. These observations were confirmed for the investigated alloy. From metallographic examinations, an effect of grain orientation on creep crack behaviour, however, was not observed. The Nikbin – Smith –Webster (NSW) model, which is based on the creep fracture mechanics parameter C*, and a damage accumulation model, which considers only nominal stress in the ligament, lead to reasonable results when recalculating creep crack growth.

Transient liquid phase bonding of pairings of parent superalloy material with different composition and grain structure

Joining of different nickel-base superalloys could simplify the manufacturing of turbine blades. The used technique of choice is transient liquid phase bonding, which is an established repair technology for high temperature components. Two nickel-base superalloys with distinct composition and grain structure are bonded and the joints are analysed regarding the microstructure. To quantify the mechanical properties of these joints, tensile and short term creep rupture tests were performed at room and elevated temperatures.

Validation of a Constitutive Material Model with Anisothermal Uniaxial and Biaxial Experiments

Abstract The dynamic behaviour of steam turbine components is currently a key issue in terms of the discontinuity in the power supply from regenerative energy systems like wind-mills. These systems call for a combined and flexible use of conventional steam power plants. The influence on fatigue behaviour with superimposed creep on lifetime of large plant components is still unknown. Fatigue loading of such components is induced by temperature transients during start-up and shut-down processes. This requires an anisothermal inspection of the thermo-mechanical fatigue (TMF) loading at design. Hence, a viscoplastic constitutive material model of the Chaboche type was adapted to a 10% Cr forged steel. As a verification of the constitutive material model, uniaxial TMF experiments were performed. These experiments were derived from a temperature cycle on the surface of a turbine rotor during a hot start. The mechanical loading depends on the start temperature and the temperature rate of the start up, and the induced loading of the surface is biaxial. Therefore, a biaxial experiment with variable amplitude loading was also used for the purpose of material model verification. In order to recalculate deformation and to predict lifetime, the constitutive material model was implemented in a FEM calculation. In the present contribution it is shown that the constitutive material model of type Chaboche introduced can be applied to the recalculation of the deformation with an implicit evolution of damage under service-type conditions. Further efforts are necessary to investigate innovative parameter identification procedures and routines for the extrapolation of the inner variables of the constitutive material model under cyclic loading.

Crack behaviour of 10Cr-steels under creep and creep-fatigue conditions

Abstract High-temperature components with notches, defects and flaws may be subjected to crack initiation and crack propagation under service conditions. To study these problems and to support an advanced remnant life evaluation, fracture mechanics procedures are required. Due to a more flexible service mode of power plants an increase of start up and shut down processes can be observed. Therefore knowledge of life assessment and integrity of such components is decisive. In order to enlighten these problems for steam power plant components, crack initiation time and crack growth rate of modern martensitic steels of type 10CrMoWVNbN were determined at 550 and 600°C in forged and cast conditions. For the experiments, side grooved compact tension, C(T), specimens were used as well as side grooved double edge notched tensile, DEN(T), specimens. Creep/creep-fatigue crack initiation and crack propagation can be described with the usual fracture mechanics parameters C* and KI and a modified two-criteria method was established in order to describe creep fatigue crack initiation.

A Method to reduce calculation time for FE simulations using constitutive material models

Abstract The utilisation of constitutive material models is a widely applied approach to simulate the deformation and damage evolution under creep–fatigue loading. However, the application of such kind of models is mostly very time consuming. For a practical and efficient use in daily industry business, answers have to be found to reduce calculation time. In this paper, the potential of the usage of an extrapolation method will be presented. This method is based upon a forward and backward extrapolation algorithm of inner state variables required for the description of both, deformation and damage. In the end, only a small fraction of load cycles have to be simulated. This leads to a significant reduction of calculation time with a negligible influence on result preciseness. Discussing the potential of the application of a developed extrapolation method in combination with a developed unified viscoplastic material model is the main goal of this paper.

Results of a low cycle fatigue inter-laboratory comparison on 1CrMoNiV rotor steel at elevated temperature

Abstract Following an initial intercomparison investigation carried out under a VAMAS programme, a similar venture has been carried out on a single source of 1Cr rotor steel at 525°C at a single total strain range of 0.5% and reversed strain rate of 6%/min (103/s). A key difference was that, for each specimen prior to testing at high temperature, bending measurements were performed at room temperature as a check on unixial alignment since this parameter had previously been suggested as a source of scatter in endurance. Fifteen laboratories participated in the comparison and 67 test results have been subjected to statistical analysis. There appeared to be no consistent effect of the magnitude of bending strain but it is concluded that a minimum of five repeat tests are required from each establishment for meaningful analyses to be made.

Influence of metastable tetragonal ZrO2-reinforcements on the properties of MoSi2-composites

Abstract In this work, MoSi2 – ZrO2-composites with different volume fractions (15-, 25-, 35 vol.% ZrO2) were examined. Zirconium oxide was successfully tetragonal stabilised (metastable state) through a high temperature isostatic pressure process, where the tetragonal – monoclinic phase transformation is inhibited by the pressure of the MoSi2-matrix. Stabilisers, e. g. Y2O3 were not used. Increasing ZrO2 content enhances strength and fracture toughness at room temperature. For temperatures above 900°C up to 1500°C no influence of the volume fraction of ZrO2 was observed with respect to the compression tests. Wear of pre-oxidised samples (20 min at 1000°C) is higher for the 25 and 35 vol.% than non-oxidised material. Oxidation resistance is observed to be poor and decreases with increasing ZrO2 content in the composite. Early crack initiation leads to poor creep behaviour and thermo-shock behaviour. This is probably due to the oxygen diffusing through the ZrO2-particles. Consequently no continuous protective SiO2-layer can be built as known from pure MoSi2-materials. In general the investigated material is not suitable for high temperature applications because of the poor oxidation behaviour.

High Temperature Fatigue of Welded Joints: Experimental Investigation and Local Analysis of Butt Welded Flat and Cruciform Specimens

Austenitic stainless steel of type X6CrNiNb18-10 exhibits advantageous mechanical and chemical properties and is a common material for numerous applications in the nuclear power plant and chemical industries. Besides the mechanical strain induced by high pressure, the fatigue life in welded pipelines is affected by additional thermomechanical strains due to thermal loading. The welding process mainly determines the geometry and metallurgical constitution of the welded joint. Therefore, the butt welds additionally influence the strain gradient along the component and reduce its lifetime.While the base and weld material are similar, they show different softening and hardening behavior, especially at ambient temperature. Cyclic hardening occurs in the base material, whereas cyclic softening can be observed in the weld material. The hardness distribution along the welded joint reveals no clear differentiation of base material, the heat affected zone and weld material. The attributes of the individual materials cannot be transferred to the welded joint automatically. Thus, the analysis of the interaction between the materials along the welded joint is a main topic of this research. To this end, digital image correlation is used for different kinds of specimens and load conditions. The position along the testing area at which fatigue failure occurs depends on the specimen type and the load condition but not on the temperature. Further, isothermal and anisothermal fatigue tests on welded cruciform specimens are presented. The common practice of the effective strain is discussed for the analyzed conditions.© 2015 ASME