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Manufacturing-induced defects are a key source of crack initiation and component failure under high temperature cyclic loading. In this work, 3D X-ray micro-computed tomography and microstructural analysis of manufacturing-induced defects is presented for forged and cast MarBN martensitic-ferritic steel, along with high temperature, low cycle fatigue testing, for assessment of the comparative effects of two manufacturing processes. Forging is found to significantly reduce the volume fraction and complexity of manufacturing defects, compared to the cast material, resulting in approximately double the fatigue life. A voxel-based finite element methodology for experimentally-identified cast and forged manufacturing defects is presented, in conjunction with a multiaxial, critical-plane damage model, within a unified viscoplastic user-material subroutine. The effect of the complex morphologies of the manufacturing defects on high temperature fatigue crack initiation is thus quantified, highlighting the relative effects of the two different manufacturing processes. The authors gratefully acknowledge funding from the Irish Research Council and GE Power under the Enterprise Partnership Scheme (EPSPG/2015/55), and the SFI Research Centre for Advanced Manufacturing, I-Form (SFI/16/RC/3872). The authors would like to express their gratitude to those in the IMPEL consortium. The authors acknowledge the facilities and technical assistance of the Centre for Microscopy & Imaging at the National University of Ireland Galway. The authors wish to acknowledge the DJEI/DES/SFI/HEA Irish Centre for High-End Computing (ICHEC) for the provision of computational facilities and support. peer-reviewed 2021-05-22

The cyclic oxidation behaviour of fine-grained Type 347 stainless steel (TP347H FG) at 650 °C in air saturated steam and deoxygenated steam environments for 100–1000 h has been investigated. Electr...

Spallation of oxides grown on TP347H FG in air-saturated steam at 923 K for 50–1000 hours was investigated using a newly developed technique termed: Simultaneous Thermographic and Optical Recording Media Examination (STORME), recording the temperature disturbance resulting from delamination and spallation, during cooling, using Infra Red and video cameras. For the first time, the radius of the delamination site was measured experimentally at the point of buckle formation and followed during growth to the point of spallation. Collection of the exact spalled fragment was possible. The strain energies at each stage were calculated and a Spallation Map constructed.

Neural networks can be a useful tool to analyse the oxidation and corrosion behaviour of materials at high temperature. Examples are given of the use of neural network models to analyse datasets of material behaviour after exposure to combustion, gasification and steam atmospheres. The use of networks to identify changes in mechanism, additional significant experimental parameters and the onset of spallation is demonstrated.The limitations of neural network modelling are briefly discussed. Although they can be trained to fit any existing dataset, care must be taken in using the networks to predict a time sequence of events.

The excellent steam oxidation resistance of iron aluminide coatings on ferritic steels at 650 °C has been demonstrated both by laboratory tests and field exposure. These coatings are formed by the application of an Al slurry followed by diffusion heat treatment at 700 °C for 10 h. The resulting microstructure is mostly composed of Fe2Al5 on top of a much thinner FeAl layer. This coating exhibits perpendicular cracks due to thermal expansion mismatch between coating and substrate. However, these stress relieving cracks do not seem to have an effect on the mechanical properties of the substrate. Cyclic oxidation, creep resistance and TMF testing of these coatings at 650 °C indicate excellent performance.

International audience; A novel mechanistic approach is proposed for the prediction of the life of thermal barrier coating (TBC) systems. The life prediction methodology is based on a criterion linked directly to the dominant failure mechanism. It relies on a statistical treatment of the TBC's morphological characteristics, non-destructive stress measurements and on a continuum mechanics framework to quantify the stresses that promote the nucleation and growth of microcracks within the TBC. The last of these accounts for the effects of TBC constituents' elasto-visco-plastic properties, the stiffening of the ceramic due to sintering and the oxidation at the interface between the thermally insulating yttria stabilized zirconia (YSZ) layer and the metallic bond coat. The mechanistic approach is used to investigate the effects on TBC life of the properties and morphology of the top YSZ coating, metallic low-pressure plasma sprayed bond coat and the thermally grown oxide. Its calibration is based on TBC damage inferred from non-destructive fluorescence measurements using piezo-spectroscopy and on the numerically predicted local TBC stresses responsible for the initiation of such damage. The potential applicability of the methodology to other types of TBC coatings and thermal loading conditions is also discussed.

Low velocity burner rigs have been used to perform cyclic oxidation tests. Due to the nature of the equipment there are limitations on the cycle frequency that can be used and there are also several other parameters that must be defined. The importance of these parameters on metal wastage in the martensitic steel, P91 has been investigated. Salt deposition rates are independent of fuel and salt composition under the conditions investigated. However the observed metal wastage in the martensitic steel P91 is influenced both by the fuel and salt compositions. Intermittent washing of the specimens has little influence whilst the corrosion product remains intact but causes additional loss of corrosion product after spallation has been initiated.

A survey of existing testing practice and pre-existing data was conducted to provide a starting point for the COTEST project on cyclic oxidation testing. The main parameters within the test that need to be controlled were identified as: control of test environment; temperature stability during hold periods; heating and cooling rates; specimen preparation; and post test evaluation. Existing experimental facilities were surveyed to establish the full range of variability within these parameters.

This study has used thermogravimetric, acoustic emission and finite-element modelling techniques to examine the tensile cracking of a chromia layer on a 20Cr25Ni steel during thermal cycling from an oxidation temperature of 900 °C. It is found that the process is highly sensitive to the temperature amplitude of the thermal cycle and that hold periods at the bottom temperature result in enhanced oxide cracking whereas hold periods at the peak temperature can inhibit the onset of cracking. For standard saw-tooth cycles, the finite-element analysis shows that these effects can be explained by a creep hysteresis effect. This arises from stress relaxation during the cooling half-cycle, which results in the development of in-plane tensile stresses within the oxide layer on the return to temperature. Low-temperature hold periods in the creep regime increase the in-plane tensile stress in the oxide on the return to temperature. High-temperature hold periods tend to reduce the peak tensile stress produced in subsequent cycles but, for the example examined here, the effect is small.

Important efforts to develop new steels or to protect high creep strength steels in order to allow operation of steam turbines at 650 °C are being carried out world-wide to increase efficiency. Within the European Project “SUPERCOAT” (Coatings for Supercritical Steam Cycles), work has been concentrated in the development of coatings to withstand 50,000–100,000 h of operation at 650 °C under high pressure steam. Aluminide coatings on ferritic–martensitic steels produced by applying an Al slurry followed by a diffusion heat treatment, have shown to be protective at 650 °C under steam for at least 32,000 h of laboratory steam exposure under atmospheric pressure. Although the “as diffused” coatings present through thickness cracks, these do not propagate during exposure to steam or thermal cycling and no new cracks seem to develop. Moreover, no changes in residual stresses could be observed after thermal cycling. Microstructural characterization of samples at different periods of exposures has been carried out by SEM-EDS and XRD. The principal mechanism of coating degradation is loss of Al at the surface due to inwards diffusion. Microhardness as well as Young's modulus and fracture strength were measured using well established techniques. The coatings show reasonable ductility (∼1.6%) when stressed in tension between room temperature and 400 °C which further increases at higher temperatures providing evidence that the coatings should withstand the mechanical conditions likely to be encountered in service.

The fracture and adhesion properties of five different commercial high velocity oxygen fuel (HVOF) coatings have been evaluated as a function of temperature. Tests were carried out on coatings in the as-received, aged and densified conditions. Under all conditions the adhesion of the coatings was excellent. The strain to cracking was generally low for coatings in the ‘as-coated’ condition, however this was improved by densification or thermal ageing. It was observed that in some cases the crack path changed with treatment.

Laboratory exposures cannot reproduce all the features present in service conditions. The experimentalist is faced with the conflict between increasing the complexity of laboratory tests to replicate service more closely and keeping testing costs low by maintaining a simple procedure. The influence of various experimental parameters, which can be controlled in the laboratory, on the steam oxidation response of materials is reviewed and recommendations for best practice are proposed.

Specimens of ferritic and martensitic steels have been exposed to steam atmospheres at 550 °C using three different experimental procedures. Slight differences in scale growth kinetics were observed but the major differences were in the morphology of the oxide scales that were formed. The microstructures of the scales have been compared with those formed on similar materials during service.

Within the European COST522 programme, there are extensive project groupings investigating the degradation of materials, both alloys and coatings, in a wide variety of power plants. Two project groupings are addressing materials degradation in (a) gas turbines and (b) the hot gas paths/heat exchangers of boilers and gasifiers. In both of these broad topics there is a need to generate and compile quantitative information on materials degradation that is appropriate to components operating in systems using new/dirtier fuels, as well as to components operating at higher metal temperatures.The data generated from research into both of the topics are being compiled into databases. For the boiler/gasifier heat exchanger fireside, the database contains corrosion damage information generated from: (a) a range of plant and pilot plant operations (using biomass, waste and coal fuels), (b) laboratory tests (targeted at particular environments to investigate different specific degradation effects in more deta...

The development of de-alloyed zones during oxidation of martensitic and austenitic steels and Ni based superalloys has been reviewed and the influence of de-alloying on local creep strength has been assessed. The de-alloyed zones in martensitic steels have similar, possibly higher, strength than the bulk material, whereas in Ni based superalloys the de-alloyed zone is significantly weaker than the bulk alloy. The effect in austenitic steels varies according to the strengthening phases present in the alloys: the de-alloyed zone is weaker in alloys strengthened by chromium carbides and/or γ′ but has similar strength in alloys strengthened by niobium carbide.

9 Cr martensitic steels are widely used in high temperature steam environments for their combination of creep strength and oxidation resistance. These materials are pushed to their limit in the quest for higher efficiencies in boilers and steam turbines and loss of the protective oxide scale may result in premature failure of critical components.The major cause of oxide scale spallation is a temperature drop. The mechanism behind the spallation process is the generation of internal stresses in the oxide scale due to the mismatch of thermal expansion coefficients between the substrate and the haematite, magnetite and spinel layers in the oxide scale. These scales can be partially relaxed due to creep processes if the cooling rate is sufficiently slow.Specimens of P92 have been oxidised in flowing steam for periods up to 2000 h and cooled at a linear rate of 100°C h–1. Acoustic emission (AE) of the samples was monitored during the oxidation and cooling periods. Specimens were sectioned and prepared ...

Three methods have been used to measure the adhesion of oxide scales and coatings on materials used in turbines. All three methods were suitable for oxide scales grown on 9- and 12-Cr steels, whereas for coated materials the methods were not universally applicable. The data generated by each method were not directly applicable to current predictive models for oxide scale failure. However, a route has been proposed that would allow future workers to correlate data obtained by testing with the input required by the modelling approaches. This could be achieved by an extensive assessment of finite element modelling coupled with critical experiments.

This chapter discusses materials selection and the main degradation mechanisms for each major component of the high temperature and low temperature cylinders used in steam turbines. The selection of appropriate materials and manufacturing processes allows efficiency improvements arising from enhanced design of steam turbines to be achieved with low risk of component failure.

Deflection tests have been used to estimate the stresses developed in the alumina layer formed during short-term oxidation of a Fe-22Cr-5Al-0.3Y Fecralloy steel at 1000°C. Elastic analysis of the deflecting specimen is inappropriate under these test conditions because of the low creep strength of the alloy. Accordingly, a recent creep analysis has been used in this work using currently determined creep properties of the alloy substrate. The results of the analysis show that for the thin oxides produced (

The behaviour of Incoloy 800H and AISI 310 stainless steel has been studied in a simulated coal gasification atmosphere (CGA) with and without the presence of deposits at 550°C. The samples were also exposed to two test procedures to simulate downtime corrosion (DTC); one in which the samples were simply withdrawn to a cooler region of the furnace were condensate formation took place, and the other type of DTC was the EPRI test. Two types of deposit were used; one was a typical coal char and the other consisted of the char with added FeCl2 and NaCl. The EPRI DTC test induced the most severe attack when combined with exposure to the CGA, but the addition of salt to the deposit appeared to make little difference to the aggressiveness the test. In contrast to many laboratory tests, chlorine-containing compounds were found at the scale—alloy interface of the most severely attacked samples, and thus represented a good simulation of corrosion encountered in operating plant. After 530 h exposure a maximum metal ...

Physically-based creep Continuum Damage Mechanics (CDM) is briefly reviewed to introduce the formalism necessary for mathematical modelling of creep and fracture in the presence of a chemically-interacting fluid environment. A recently developed theory of creep in precipitation-hardened alloys is presented in the form of a constitutive equation and two important creep/environment interactions are discussed in detail. Spallation of oxide scales during creep of a low-alloy ferritic steel is one and is modelled using the new creep constitutive equation which incorporates particle-ageing as the principal intrinsic damage mechanism. Lack of experimental data has meant that only qualitative support for the model predictions could be given. Carbon dioxide gas bubble formation along grain boundaries in nickel alloys is the other form of environmental attack considered; a new kinetic model based upon oxyen diffusion-control is described and shown to be in quantitative agreement with a large experimental dataset.

Materials technology is key to enabling improvements in steam turbine performance arising from retrofit. Materials selection, the main degradation mechanisms and material issues, together with any special processes required, are discussed for each major component of the high temperature and low temperature cylinders. It is demonstrated that the selection of appropriate materials and special processes allows the efficiency improvements arising from retrofit of steam turbines to be achieved with low risk of component failure.

Several mechanisms of creep/oxidation interaction have been discussed and placed within the framework of a creep-corrosion interaction diagram by modelling the individual mechanisms, using continuum creep damage mechanics. Although validation of the models is difficult because experimental data are rare, some agreement with model predictions has been achieved where appropriate data are available.

This paper describes the results of an investigation of the effect of trace elements on the creep and stress rupture performance at 850°C of the high-chromium alloy IN939 which is used for blades and nozzle guide vanes in land-based gas turbines. The results showed that both Pb and Bi had a seriously detrimental effect on creep performance which was independent of the ductility of the base alloy. The presence of impurity caused increased amounts of grain boundary cavities which resulted in premature failure, and in this respect the results were consistent with those obtained for other alloys of this type. However, the creep resistance was reduced in the material doped with Pb and with Bi and this behaviour had not been observed in previous work on similar alloys. The results have been analysed in the context of recent developments in the understanding of mechanisms of creep deformation and fracture.

Many nickel-base superalloys exhibit creep embrittlement after exposure to oxygen-containing environments at high temperatures. This embrittlement is characterized by a region of profuse grain boundary cavitation immediately below the surface, and four mechanisms have been proposed in the literature to account for it. Since each mechanism has microstructural consequences, these have been checked metallographically. From the results given in this paper it is concluded that, for two nickel-base superalloys, cavitation is nucleated by bubbles of a gaseous internal oxide of carbon.

In modern gas turbines thermal barrier coatings (TBCs) provide the key technology controlling the performance and lifetime of key components. It is therefore critical to be able to understand the mechanisms controlling the failure of TBCs and to predict these events. Most current approaches relate TBC failure to stress generation at or near the thermally grown oxide (TGO) that forms between the bond coat and the ceramic topcoat. A full model of TBC failure requires characterisation of the coating system, identification of the key failure mechanisms, quantitative description of stress evolution in the key areas within the coating system and robust failure criteria for each failure mechanism. Thus the solution to lifetime prediction invariably requires massive effort both in terms of determining the appropriate input parameters for the model and in computing the solution. This position can be alleviated through the use of interpolation techniques. Recently a model for TBC lifetime of an IN738/MCrAlY/EB-PVD system has been developed that predicts stress evolution at and near the TGO and relates these local stresses to several possible failure mechanisms. The stress distribution is dependent on time and temperature of exposures as well as the geometry of the bondcoat/ceramic interface and requires FE calculation for each specific set of conditions. In order to reduce the need for extensive calculation a software tool has been developed that interpolates the key stress values for each failure mechanism from a matrix of previously calculated values. Thus the failure of the TBC system can be readily calculated for other service conditions. The paper will describe the principles of the software tool and validation of the approach through comparison with non-destructive measurements on the coating system.

Traditional component design procedures consider service lifetimes to be limited by either stress or chemical attack from reactive components in the fluid working environment. However, these processes are often coupled; stress can affect the rate of environmental attack and vice versa. The synergy resulting from this coupling is frequently detrimental with regard to service performance, and creep lifetimes, for example, may be reduced dramatically; the paper reviews the evidence and identifies the underlying mechanisms. Physically based equation sets for coupled creep and environmental attack are also presented and some of their predictions are compared favourably with data published previously.MST/1126

In order to develop databases of creep information for design purposes for steels and other alloys, large-scale test programmes were undertaken involving several casts , product forms, etc. Little attempt was made to separate materials variables from those due to testing, and, even for simple measures of performance like life to rupture, considerable variability occurred and scatter-bands were large. Design information was obtained using statistical methods and curve fitting to predict long-term performance. The simple design rules incorporated safety factors which, along with the relatively inaccurate materials data , resulted in very conservative designs for power plant and pressure vessels.

A round-robin has been carried out to validate calibration and measurement procedures for various designs of depth-sensing indentation instrument. The procedures used in instrument calibration and for the round-robin exercise are detailed. Hardness and Young's modulus values were obtained for the test materials used in the round-robin experiments over the load range 20–1000 mN. Both hardness and Young's modulus values showed a dependence on load with increasing values at lower loads. The effect was more marked for hardness than for Young's modulus. Uncertainties (95% confidence) in measurement varied greatly between the participants, the best being ±5% and in the worst case this was as great as ±100%. These differences were related both to the calibration accuracy achieved by the different participants and to the repeatability of measurements for the various instruments. Where accurate calibration was possible and instrumentation was adequate, agreement with accepted hardness and Young's modulus ...

Revue et classification des plus recents travaux experimentaux sur les interactions entre la corrosion et le fluage dans le cas des superalliages base nickel. Modelisation de ces phenomenes permettant d'obtenir une approche valable du comportement des composants en service

The influence of a coating on the creep properties of nickel-base superalloys is not fully understood: possible mechanisms are reviewed and mathematical models of creep and fracture are given for coated and uncoated superalloys, both in air and under sulphidising conditions. The trends of behaviour predicted by the models show broad agreement with the limited experimental database found in the literature, although anomalies were observed which could be explained only qualitatively.

The microstructure and the tensile and stress-rupture properties of the nickel-base superalloy M A R-M002 have been studied using material of normal commercial composition and three melts with additions of 0·0024 and 0·0050%N and of 0·16%Si. Increasing the nitrogen content resulted in a change in carbide morphology, from ‘Chinese script’ to a blocky form, and to increased microporosity. Interference-film microscopy revealed characteristic centres in some of the blocky carbides that had high Ti contents consistent with carbide nucleation on Ti(C, N) particles formed in the melt. The increased nitrogen content was associated with a significant decrease in the rupture life at 760°C and 695 MN m−2, and a change in the fracture morphology. In the Si-doped alloy, which contained little microporosity, there were increased amounts of a phase rich in Ni–Hf (probably Ni5Hf) that also contained Si, but there was only a slight decrease in stress-rupture properties.

The fracture characteristics specimens of IN-939 tested in creep at 850°C for times up to 12 000 h have been examined using optical metallography. In addition to the normal composition, specimens doped with small amounts of bismuth and of iron were also tested. The results show that creep fracture occurred by cavitation at the grain boundaries similar to that frequently reported for wrought alloys. The presence of the trace elements resulted in the formation of larger numbers of smaller cavities compared with those observed in the base material.

The effects of microstructural variations arising from the casting conditions on the creep performance of alloy IN939 have been assessed in order to determine the structural features having the greatest influence on creep properties. The results of creep tests at 850°C show that while grain size has an effect, the dendrite spacing is a more significant parameter in influencing performance. In particular, the life-to-rupture was greatest in specimens with fine dendrites as a consequence of the improved ductility with this type of structure. Thus the optimum combination of creep properties was obtained for material with fairly coarse grain size (~1mm) and fine dendrites (~20 μm). The structural changes associated with the various stages of the commercial heat treatment have been determined by optical metallography and electron microscopy of thin foils, and evidence has been found of dislocation bowing and shearing of γ′ particles in fractured creep specimens. The creep results are discussed in the c...

In this paper the authors compare the creep and low-cycle fatigue properties of conventional, directionally solidified, and single-crystal castings produced from nickel-base superalloys. A brief historical review describes the reasons for the evolution from wrought to cast product through directionally solidified to modern single-crystal (‘monocrystal’) castings. The influence of microstructural variations produced by the casting conditions, such as porosity and grain size, on creep and low-cycle fatigue properties are illustrated. The important aspects of postsolidification heat treatment, hot isostatic pressing, and the damaging effects of impurities are described for conventional castings. The results of controlling the microstructures produced by directional solidification especially by high temperature gradient solidification are demonstrated by comparing the creep properties of directionally solidified materials with those of the conventionally cast alloys in long-term tests. The creep and l...