Estimating the remanent creep life of power plant components

This paper examines the current status of remanent creep life assessment methods for power plant components. Consideration is given mainly to predictive techniques based on post-service examination and testing with application to low alloy ferritic components in fossil plant. The requirements for producing methodologies, namely the development of mechanistic and parametric models for creep damage and failure, are discussed together with aspects on the measurement of the relevant creep damage feature or property. Techniques considered include physical and mechanical property measurement, metallographic examination, strain measurement, and accelerated creep and rupture testing. Methods based on accelerated testing are discussed in detail; extrapolative techniques and application of the life fraction rule are considered both from an experimental and mechanistic viewpoint. Finally, attention is given to the choice of representative stress to apply to uniaxial data. The influence of material parameters on the representative stress is emphasized and upper and lower bounds appropriate to creep brittle and creep ductile material states are considered.     

Influence of postweld heat treatment on creep properties of 1CrO.5Mo and 2.25Cr1Mo weldments

The creep properties of simulated heat-affected zone, weld metal and cross-weld specimens of tCr0.5Mo and cross-weld specimens of 2.25Cr tMo have been studied with reference to the effect of postweld heat treatment (PWHT). A reduction in the creep rupture strength of up to 20% has been observed after PWHT. Contrary to what has been previously proposed, PWHT was in the present investigation found to have only a marginal influence on the creep ductility. The creep rate can increase by up to a factor of 2-4 after postweld heat treatment, probably because of-carbide coarsening during the heat treatment. Preheating at 200°C did not affect the creep properties of 2.25Cr lMo. Overheating, on the other hand, at 650°C for 1000h reduced the rupture time of tCr0.5Mo by almost an order of magnitude. The tensile strength and yield strength of tCrO.5Mo were significantly reduced after postweld heat treatment. Reductions of up to 100-200MPa were observed both at room temperature and at 550°C in some cases.     

Rationalization of creep rupture data of a cold worked steel

A method of presenting creep rupture data in parametric form is described, and this method is illustrated by application to 347 stainless steel.     

Long-term creep strength predictions from short-term creep test data for high Cr creep-resistant steels and microstructural evolution origin of over-predictions

A new tensile creep model that integrates the tensile strength at creep temperature is investigated for its generic applicability in predicting the long-term creep strengths from short-term creep test data for high Cr creep-resistant steels using creep and tensile strength data measured for a grade of 11Cr steel. The results show that, when the long-term creep strengths are specified by stresses producing the required minimum creep rate, they can be accurately predicted using short-term creep test data. However, when they are specified by stresses giving the required creep rupture time, using only short-term creep test data will lead to over-predictions. The microstructure evolution origin of such over-predictions is traced to the Z-phase precipitation during creep in creep-resistant steels with more than 9 wt.% Cr. The conventional concept on the relationship between creep test stress and creep mechanisms is also re-evaluated in light of the new results.     

Influence of aluminium on creep strength of 9–12% Cr steels

The influence of aluminium on creep strength of 9% Cr steels is predicted by a fundamental model for creep. Through thermodynamic modelling the particle structure is determined for a temperature and composition range. This shows how AlN is formed at the expense of MX carbonitrides of VN character when the aluminium content is increased. The remaining MX particles are of NbC type and have approximately one fifth of the original phase fraction. The evolution in microstructure such as particle coarsening is included in the model as well as the recovery. Rupture time is predicted using a modified Norton equation including back-stresses calculated from microstructure. The predictions show correspondence to some of the creep data for the steel P91 over a temperature and stress range. Furthermore, simulation with high Al content verifies the observed early failure of Al rich components. Overall, the simulations show a decrease in rupture time by a factor 6 due to Al additions of up to 0.2%.     

A micro-continuum model for the creep behavior of complex nanocrystalline materials

A nanocrystalline material which has an average grain size of less than 100 nm is characterized with a significant portion of atoms residing in the grain boundaries or in the grain-boundary affected zone (GBAZ), while nanocrystalline materials with a more complex structure may contain additional strengthening nanoparticles or nano pores. In this article we develop a micro-continuum model to capture the creep response of such a complex nanocrystalline system. We make use of the concept of a three-phase composite with the GBAZ serving as the matrix, and grain interiors and dispresed particles (or voids) as two distinct types of inclusions. Both the grain interior and the GB zone are capable of undergoing the rate-dependent plastic deformation, but the strengthening nanoparticles or pores are taken to deform only elastically. During deformation the porosity will continue to evolve; its evolution is also addressed. In addition, the effect of temperature on the overall creep response is also accounted for. Several important features of creep characteristics in light of grain size, and nanoparticle and nanopore concentrations, are illustrated, and it is also demonstrated that the calculated results are in reasonable agreement with available experimental data.     

Influence of Pd addition on the creep behavior of AZ61 magnesium alloy

The effect of Pd addition (0, 2, and 4 wt%) on the microstructure and creep properties of permanent mold AZ61 (Mg-6Al-1Zn) alloy has been studied. The results indicate that Pd addition introduces a lamella-shaped Al₄Pd phase at the grain boundary, in addition to the Mg₁₇Al₁₂ (β) phase. The addition of Pd also suppresses the precipitation of the Mg₁₇Al₁₂ phase and residual Al at grain boundaries during solidification. These effects lead to an improvement in the creep behavior of AZ61. Moreover, extended steady-state creep and reductions in both the minimum creep rate and total creep strain are also observed in the case of 4 wt% Pd addition.     

High temperature corrosion of alloys containing rare earth or refractory elements: a review of current knowledge and possible future developments

The beneficial effects of minor (≤1%) additions of rare earth/reactive elements on the oxidation of HT alloys have been known for many years. Much interest has recently centred on the influence of certain refractory elements present at levels of several weight percent in a number of such alloys. The reasons why minor additions of these reactive elements, especially the rare earths, have beneficial effects have been the subject of systematic studies over the past 10–15 years. Ideas have been influenced by the understanding of the mechanisms of the development of protective chromia or alumina scales; in particular, the role such additions play in assisting the more rapid nucleation and growth of oxides on HT alloys, reducing the period of transient oxidation and the earlier attainment of steady-state scaling conditions. Much attention has also been paid to the mechanisms whereby such additions appear to promote improved scale adherence.

The various theories of high temperature corrosion of such alloys and supporting experimental evidence are reviewed in this paper. The reported information concerning the beneficial role or otherwise of the refractory metals, particularly with respect to hot corrosion in gas turbines, is appraised. Attention is drawn to the potentially beneficial effects of additions of certain of these Group III-VI elements in the development of novel alloys or coatings to resist corrosion in the complex environments of low oxygen potential, which occur in plants to produce synthetic natural gas from oil or coal. Areas for further research are also identified.     

In-plane and out-of-plane constraint effects on creep crack growth rate in Cr–Mo–V steel for wide range of C*

Abstract

In this work, the stress dependent creep ductility and strain rate model have been implemented in a ductility exhaustion based damage model and the creep crack growth (CCG) rates of a Cr–Mo–V steel in compact tension (C(T)) and middle tension (M(T)) specimens with different thicknesses and crack depths have been simulated over a wide range of C*. The effects of in-plane and out-of-plane constraints on CCG rates are examined. The results show that the in-plane and out-of-plane constraint effects on CCG rate are more pronounced for the high constraint specimen geometry (C(T)), while such effects are less significant for low constraint specimen geometry (M(T)). The constraint effects on CCG rates mainly occur in low and transition C* regions and the CCG rate increases with increasing in-plane and out-of-plane constraints. There exists interaction between in-plane and out-of-plane constraint in terms of their effects on CCG rate. The higher in-plane constraint strengthens the out-of-plane constraint effect on CCG rate and higher out-of-plane constraint also strengthens the in-plane constraint effect on CCG rate. The constraint effects on creep crack growth behaviour for a wide range of C* mainly arise from the interaction of crack-tip stress states and stress dependent creep ductility of the steel in different C* levels.     

Elevated temperature mechanical properties and microstructures of high pressure die cast magnesium AZ91 alloy cast with different section thicknesses

This paper investigates the influence of variations in the microstructure of high pressure die cast AZ91 on the elevated temperature mechanical properties of the alloy. Thinner-walled high pressure die castings show an improvement in elevated temperature strength, ductility and creep resistance. Further improvements to the creep resistance were achieved by ageing the alloy prior to creep testing. It appears that an increased proportion of fine grained ‘skin’ region in the thinner castings contributed to the improved properties. Also, it appeared that the presence of supersaturated solute Al in the eutectic α-Mg contributes to the poor creep properties, probably due to the microstructural instability. Final failure is associated with the growth of voids either from porosity in the alloy or nucleated from discontinuous precipitates.     

Rapid determination of fatigue life based on temperature evolution

This paper presents an approach to rapidly predict high cycle fatigue life based on the temperature evolution of a specimen under cyclic loading. The initial slope of the temperature evolution was proved as an indicator for rapid estimation of fatigue life theoretically. Meanwhile, a real-time temperature measurement system based upon high-precision semiconductor temperature sensors was developed to detect the temperature evolution based upon high-precision semiconductor temperature sensors. In order to verify the presented approach, constant-amplitude fatigue tests were carried out on A7N01 aluminium alloy and the welded joints, respectively. The predicted fatigue life based on proposed approach was in good agreement with experimental results.     

Microstructure,tensile properties and creep behavior of Al-12Si-3.5Cu-2Ni-0.8Mg alloy produced by different casting technologies

The relationship between the as-cast microstructure and mechanical properties of the Al-12Si-3.5Cu-2Ni-0.8Mg alloys produced by permanent mold casting (PMC) and high pressure die casting (HPDC) is investigated. The alloys in both PMC and HPDC consist of Al, Si, Al₅Cu₂Mg₈Si₆, Al₃CuNi, and Al₇Cu₄Ni phase. However, the microstructure of the HPDC alloy is significantly refined. Compared to the PMC alloy, the ultimate tensile strength of the HPDC alloy is significantly increased from 244 MPa to 310 MPa, while the elongation shows a reverse trend at room temperature. At low stress and temperature range, slight variations of stress exponent and activation energy indicate that the minimum creep rate is controlled by the grain boundary creep. Then the minimum creep rate is higher for the specimen with the smaller grain size, where grain boundary creep is the dominant creep mechanism. At high stress region, the stress exponent for the PMC alloy and HPDC alloy is 5.18 and 3.07, respectively. The different stress exponents and activation energies measured at high stress and high temperature range indicates that the creep mechanism varies with the casting technologies.     

Viscoplastic behavior of a FeCrAl alloy for high temperature steam electrolysis (HTSE) sealing applications between 700 °C and 900 °C

High temperature steam electrolysis (HTSE) is one of the most promising technologies for the industrial production of hydrogen. However one of the remaining problems lies in sealing at high temperature. The reference solution is based on glass seals which presents several drawbacks. That explains why metallic seals are under development. The expected seal will be submitted to creep under low stresses between 700 °C and 900 °C, possibly involving complex loading and thermal history. The candidate material investigated in this work is a FeCrAl (OC404, Sandvik) supplied as a 0.3 mm thick sheet. The ability of this material to develop a protective layer of alumina was studied first, as well as grain size growth during thermal ageing. Creep and tensile tests were performed between 700 °C and 900 °C to determine its mechanical properties. This database was used to propose and identify an elasto-viscoplastic behavior for the material. Creep was described by the Sellars-Tegart law. This law was then used to simulate and predict creep indentation tests performed in the same range of temperatures.     

Effect of consolidation route on structure and property control in rapidly solidified Al–Cr–Zr–Mn povvder alloy for high temperature service

Abstract

In this paper the authors describe a new rapidly solidified alloy which is capable of meeting the projected requirements of the aerospace industry. Initial studies using splat quenched particulates were carried out on the Al–Cr–Zr system. Microhardness tests indicated that these alloys age–hardened between 350 and 400°C and showed excellent thermal stability. Further alloy development and studies of fabrication behaviour were carried out using air atomized powder. Powders were consolidated by either conventional or hydrostatic extrusion. Microstructural changes during fabrication were identified and correlated with mechanical property data. The alloy can achieve the requirements of the aerospace industry provided microstructural development is controlled during fabrication.

MST/233     

Laser shock processing of aluminium alloys. Application to high cycle fatigue behaviour

Subjecting target metallic samples to a very short pulse (about 20 ns) of intense (GW cm⁻²) laser light generates, through a surface plasma, a high-pressure stress wave propagating to the first millimetre in depth, which is commonly called laser shock processing (LSP). The purpose of this work was to evaluate the role of this novel process on the cyclic properties of A356, Al12Si and 7075 aluminium alloys. Major contributors to the fatigue performance improvements were investigated in order to determine the optimum shock conditions. These were mainly compressive residual stress (RS) levels for which a large range of incident shock conditions was performed. We showed that stress levels were very sensitive to the laser fluence and the number of local impacts, and experimental RS measurements were found to be in good agreement with analytical modelling results. In comparison, a conventional shot peening (SP) treatment was found to lead to higher surface hardening and RS levels, but with a very detrimental roughening not observed after LSP. High cycle (10⁷) fatigue tests carried out on laser- processed, shot-peened and untreated notched samples illustrated the efficiency of LSP as a new, promising method to improve the fatigue limits σ_D of structures, especially in comparison with enhancements displayed by SP (+22% vs. +10%). According to crack detection electric measurements, fatigue performance improvements with LSP mainly occurred during the crack initiation stage.     

Corrosion behavior of 5Cr0.5Mo steel in sulfur‐bearing solution at high temperature

Corrosion behavior of 5Cr0.5Mo steel in sulfur‐bearing solutions as a function of temperature, test time, and sulfur content was investigated by weight loss measurements in this study. The results indicate that the corrosion rate of the steel increases with increasing test temperature. Besides, the corrosion rate increases during the initial test time and then decreases with longer test duration. Moreover, the corrosion rate rises with a higher amount of sulfur in the solution, and consequently a lower amount of sulfur leads to a decreased corrosion rate. In addition, the effect of Cr and Mo elements has been discussed.     

Microstructure and elevated temperature mechanical and creep properties of Mg–4Y–3Nd–0.5Zr alloy in the product form of a large structural casting

In order to save the invaluable heavy rare earth (HRE) elements for important functional applications, a modified version of the WE43 magnesium alloy, Mg–4Y–3Nd–0.5Zr (wt.%), free of the HRE elements, has been designed. As part of the alloy development program, a large complex component of the alloy (net product weight: 80 kg) was made via differential pressure casting. The large component was then subjected to the T6 treatment (solid solution and ageing) following established commercial practice for the T6 treatment of the WE43 alloy. A significant number of samples were prepared from the thickest section (58 mm) of the T6-treated component for both microstructural characterization and detailed property assessment. The alloy showed noticeably higher tensile strengths than did the HRE-containing WE43 alloy over the temperature range of 473–573 K. The creep resistance of the alloy was superior to that of the WE43 alloy at 473 K while being similar at 523 K. The microstructures of the alloy in the as-cast, solution treated and then aged states were characterized. The component-based detailed assessment suggests that the idea of using neodymium (Nd) to replace the HRE elements in the WE43 alloy is promising for structural applications at elevated temperatures.     

Microstructure,phase formation and properties of rapid solidified Al–Fe–Cr–Ti alloys

Improvements in the mechanical strength of Al–Fe–Cr–Ti alloys have been demonstrated when non-equilibrium microstructures are developed. This paper investigated the effect of cooling rate and composition on the phase formation, microstructure and properties of new Al_96.6Fe_1.5Cr_1.7Ti_0.2 and Al_91.6Fe_4.9Cr_2.2Ti_1.3 (at.-%) alloys. Wedge-shaped samples produced by suction casting were characterised by optical, scanning electron microscopy, differential scanning calorimetry, X-ray diffraction, energy dispersive X-ray spectroscopy and microhardness. The results showed that the morphology and size of the phases precedent of the flower-like phases change from small, spherical particles to large flower-like phases with decreasing the cooling rate. The presence of intermetallic phases Al₁₃Fe₄, Al₁₃Cr₂ and Al₃Ti in the Al_91.6Fe_4.9Cr_2.2Ti_1.3 alloy, resulted in a hardness 1.6 times higher compared to the Al_96.6Fe_1.5Cr_1.7Ti_0.2 alloy.