Reliability analysis and life prediction of HK40 steel during high-temperature exposure

This paper is concerned with reliability analysis and residual life prediction of HK40 austenitic steel based on their creep rupture data by introducing a Z-parameter method. A normal distribution is supported for the value of the Z-parameter and the distribution curves of tubes after service exposure have been shown to deviate to the right with an increase in service time. According to creep rupture data both from different segments of a long-term service tube and from the same segment locations after different service time, it is proven that the Z-parameter reflects the deterioration in creep rupture properties. Residual life prediction and damage evaluation by Z-parameter method based on confidence level are also presented.     

Creep life assessment of an overheated 9Cr-1Mo steel tube

Crude oil heater 9Cr–1Mo steel tubes from a refinery plant were studied, after 24 years of service at nominally 650 °C and 27 MPa, to predict their remanent lives. The investigation included dimensional, hardness and tensile measurements in addition to accelerated stress rupture tests between 650 °C and 700 °C and microstructural examination. Tube specimens were taken from two sections, the overheated side and the side which only saw the nominal operating temperature. The method employed involved the prediction of the increase in temperature with increasing sediment deposition during the operating life times using an FEM model. In addition the predicted temperatures are used to derive appropriate creep properties at relevant temperatures in a 3D pipe FEM creep analysis to predict the pipe deformation rate. All compare well with the actual service exposed pipe measurements and layer deposition. The overheated side revealed a small loss of creep strength in a stress rupture test. A layer of sediment (appr. 10 mm thickness) consisting basically of sintered carbon (coke) spread over the inside of the tube was acting as a thermal barrier causing the temperature to rise above 650 °C. Analysis for the overheated side predicted an upper bound temperature of ≈800 °C and a life of about 50 h suggesting that failure by creep rupture could occur rapidly in the sediment region.     

“服役条件-持久强度”干涉模型及其在T91/P91钢可靠性评估中的应用

分析了基于Z参数的用于高温材料持久寿命可靠性预测的服役条件-持久强度干涉(SCRI)模型,建立了适用于该模型的Monte-Carlo随机模拟分析算法.利用SCRI模型对T91/P91钢进行了不同可靠度下的持久寿命预测,在综合考虑性能数据分散性及服役条件波动性的情况下实现了材料持久寿命的可靠性预测,为材料的可靠性设计提供了新的思路和方法.     

High-temperature tensile and creep properties of a ferritic stainless steel for interconnect in solid oxide fuel cell

The purpose of this study is to investigate the high-temperature mechanical properties of a ferritic stainless steel (Crofer 22 APU) for use as an interconnect material in planar solid oxide fuel cells (pSOFCs). Tensile properties of the Crofer 22 APU steel are evaluated at temperatures of 25-800 °C. Creep properties are evaluated by constant-load tests at 650-800 °C. Several creep lifetime models are applied to correlate the creep rupture time with applied stress or minimum creep rate. Experimental results show the variation of yield strength with temperature can be described by a sigmoidal curve for different deformation mechanisms. The creep stress exponent, n, has a value of 5 or 6, indicating a power-law creep mechanism involving dislocation motion. The apparent activation energy for such a power-law creep mechanism is estimated as 393 kJ mol⁻¹ through some thermally activated relations. Creep rupture time of the Crofer 22 APU steel can be described by a Monkman-Grant relation with a time exponent, m = 1.11. The relation between creep rupture time and normalized stress is well fitted by a universal simple power law for all of the given testing temperatures. Larson-Miller relationship is also applied and shows good results in correlating the creep rupture time with applied stress and temperature for the Crofer 22 APU steel. Fractographic and microstructural observations indicate most of the creep cavities are nucleated along grain boundaries and a greater amount of cavities are formed under high stresses.     

Reliability-based assessment of polyethylene pipe creep lifetime

Lifetime management of underground pipelines is mandatory for safe hydrocarbon transmission and distribution systems. The use of high-density polyethylene tubes subjected to internal pressure, external loading and environmental variations requires a reliability study in order to define the service limits and the optimal operating conditions. In service, the time-dependent phenomena, especially creep, take place during the pipe lifetime, leading to significant strength reduction. In this work, the reliability-based assessment of pipe lifetime models is carried out, in order to propose a probabilistic methodology for lifetime model selection and to determine the pipe safety levels as well as the most important parameters for pipeline reliability. This study is enhanced by parametric analysis on pipe configuration, gas pressure and operating temperature.     

Prestrain effects on creep ductility of a 316 stainless steel light forging

Specimens were machined from a section of 316 stainless steel header which was in service for approximately 30 000 h. Constant stress creep tests were performed at 450 MPa and 575°C on materials subjected to various levels of prestrain to determine the effect of prior deformation on subsequent creep behaviour. The results show that, provided the prestrain exceeds the initial strain on loading to the creep test conditions in the un-prestrained condition, a significant reduction in rupture life, minimum creep rate and creep ductility was obtained by increasing prestrain. The Monkman–Grant relationship proved to be a reliable indicator of the effect of prestrain on creep properties and showed that the reduced rupture lives were a consequence of the progressive exhaustion of creep ductility with prestrain.     

Linear matching method for creep rupture assessment

The recently developed linear matching method (LMM), which is easily implemented within commercial FE codes, has been successfully used to evaluate elastic and plastic shakedown loads. In this paper, the method is extended to the prediction of the creep rupture life of a structure, based upon a bounding method currently used in the life assessment method R5. The method corresponds to the requirement that, for the operating load history, the structure should shakedown where the yield stress is given by the lesser of the plastic yield stress and a high temperature rupture stress corresponding to a rupture time. A holed plate subjected to cyclic thermal load and constant mechanical load is assessed in detail as a typical example to confirm the applicability of the above procedures. The examples show that the method remains numerically stable, even when the method is inverted.     

基于参数回归模型的2．25Cr-1Mo钢蠕变断裂时间统计分析

基于多种寿命数据分布,包括指数（Exponential）、对数正态（Log—Normal）、威布尔（weIbull）、广义GaITIFna以及对数罗吉斯提克（Log—I。ogistic）分布,采用MansonHaferd时间一温度参数模型对2．250r1Mo（T22）铜的蠕变断裂时间进行了回归分析,对比研究了基于不同分布模型的蠕变断裂时间与温度和应力的关系、模型拟合优度以及各种模型蠕变断裂时间中位值与5％分位值的差异．结果表明：除Exponential分布模型不适用以外,Log—Normal、we．hull、广义G8mma和Log—I。ogistic分布模型均适用于蠕变断裂时间数据;这4种模型对蠕变断裂时间中位值的估计均相近,但是对低分位值的估计有所不同,特别是在数据分散度较大时,蠕变断裂时间低分位值的差异较为显著,其中Wemull分布模型的蠕变断裂时间低分位值显著低于其他模型．因此,宜采用多种分布模型对2．25Cr-1Mo铜进行综合统计分析,以正确预测其蠕变寿命或长时服役应力．     

Using small punch test data to determine creep strain and strength reduction properties for heat affected zones

Abstract

The small punch (SP) test is a miniature technique that can provide information on creep performance of local features in welded components. However, the multiaxial stress and deformation history in SP test means that it can be a challenge to interpret the test results in a way that is comparable to uniaxial standard creep testing. This work aimed to compare SP test results from as new and service exposed P91 (9Cr–1Mo–V–Nb) base material (BM) and heat affected zones (HAZ) to uniaxial creep testing results from welded new and service exposed material. Two methods are proposed for predicting the uniaxial strain response for any zone of the weld: one alternative is to apply the SP rupture data and to accommodate the shape of the uniaxial base material creep curve to that of the zone of interest in the SP test. The other alternative is to use the SP deflection data and an appropriate translation function to the uniaxial creep curve. In both cases, the Wilshire creep equations have been used as the rupture model. The approach will also predict the stress reduction factors of welds and its constituent parts (including the subzones of HAZ). It is proposed that the approach is used to provide the local constitutive creep models for component assessment by finite element analysis (FEA).     

Creep oxidation behaviour and creep strength prediction for Alloy 617

Creep experimental data was obtained by a series of creep tests with different stress levels at 950 °C for Alloy 617. Oxidation behaviour was investigated by observing the microstructures of fractured specimens after the creep tests. Oxidation thickness was measured quantitatively with the creep rupture times, and the oxidation microstructures were represented by a SEM image. In addition, the long-term creep strength for Alloy 617 was predicted by using a multi-constant method with two C instead of the conventional one with a unique C in the Larson-Miller (LM) parameter. For 10⁵ h at 950 °C, the creep strength for the conventional method was 7.2 MPa, but for the multi-constant method it was reduced to 4.7 MPa. The conventional method did not thoroughly match with the creep rupture data, and revealed an overestimation for the prediction of the long-term creep strength. On the other hand, the multi-constant method revealed a good agreement with the creep rupture data, and its method was thus more accurate than the conventional one. This multi-constant analysis can be used to accurately predict the long-term creep rupture of Alloy 617.     

Creep deformation,rupture and ductility of Esshete 1250 weld metal

Abstract

Esshete 1250 is an austenitic steel with high creep strength and is well established as a superheater boiler tube material in UK power stations. In addition, Esshete 1250 is readily welded with either inert gas welding or metal arc welding and has been used for piping and headers in super critical power stations. The latter thick section components are either solution heat treated after welding or enter service in the as-welded condition. Components that enter service in the as-welded condition contain high residual stresses, which pose a threat to the structural integrity of these welds. In order to perform life assessments of as-welded Esshete 1250 welds, it is necessary to have data on the creep deformation, stress relaxation, rupture and ductility of the weld metal, which are described in this paper.     

A stochastic computation model for the creep damage of furnace tube

The service life of high temperature furnace tubes varies significantly in engineering practice. Predicting the service life of the tubes has thus long been a concern. Due to the difficulties in defining variability of creep data, previous research has mainly concentrated on deterministic creep damage models. In order to study the random nature of service life, a new stochastic creep damage model is proposed in this paper. A comparison with results calculated by use of the Monte Carlo method verifies the creep damage model. The randomness of the creep damage is demonstrated with a calculation on HK-40 furnace tubes which provides an effective means to assess the reliability of the furnace tubes.     

运行中的X20CrMoV12.1主蒸汽管道直管段剩余寿命评估

上海某热电厂X20CrMoV12.1材质的主蒸汽管道经23年运行后,材料的力学性能明显退化,表现出脆化倾向;相应的组织结构产生明显的热损伤,显现出蠕变损伤特征,但尚未发现蠕变空洞或表面裂纹等动态缺陷形成.服役材料的高温持久强度试验应力计算分析结果表明,该主蒸汽管道在正常工况下仍满足继续安全运行10万小时的基本条件.建议在下一个运行周期应加强金属监督,进行中期评估,以确保运行安全.     

Time-dependent deformation and fracture of multi-material systems at high temperature

The present work reports several recent research activities on the time-dependent deformation and fracture performance of multi-material system and structures at elevated temperature. A micro region deformation measuring technique is developed to achieve the full creep strain fields of multi-material systems at high temperature in light of the long-distance microscope and digital speckle correlation method. The mean field approach based on the self-consistent method and the generalized self-consistent method are introduced to predict the time-dependent deformation of the dual-components material system under the creep condition. For the pressure vessels with functionally graded materials under both internal and external pressures, an asymptotic solution for creep stress and strain is derived based on the Taylor expansion series. For the time-dependent fracture issue of multi-material structures, a modified creep crack fracture parameter prediction method for C^∗ integral in the mismatched weldments and the particle-reinforced composite is proposed based on the generalized equivalent homogenous model. Finally, time-dependent failure assessment diagram (TDFAD) for multi-material system is derived for defect assessment of structures with various combinations of materials.     

Remaining life assessment of carbon steel boiler headers by repeated creep testing

The condition of carbon steel boiler headers that have been in service for over 25 years has been assessed periodically by NDT, dimensional measurements, replication and accelerated creep testing. Historical temperature records were limited, so estimates of effective header temperatures were made from replicas. These estimates were compared with header stub thermocouple readings. At about 280,000 service hours, samples were chain-drilled from the headers for accelerated creep testing. These test results indicated that the headers had satisfactory remaining life. Nine years after the original samples were taken, additional samples were removed from one header at 337,000 service hours. The creep rupture properties measured from the repeated tests were almost identical to the initial results. A mild degree of random, nodular graphite was found in the samples and its effect on creep properties is discussed.     

Improved approach for predicting weld creep strength factors of ferritic steels

Abstract

The cross-weld (CW) creep strength of ferritic steels is typically lower than that for parent metal (PM), and in the past the ratio of CW to PM creep strength (weld strength factor – WSF) was assumed to be limited to ～80%. For newer Cr steels WSF can be significantly lower for a typical design life of 100 000 h or more. The possibility of low WSF is also accommodated in the current design codes such as EN 13445, but no suggested WSF values are given for guidance. Assuming a too high WSF for such welds obviously results in an unsafe (too long) predicted creep life. Unfortunately, as a further complication the WSF of the newer Cr steels can decrease when the operating temperatures are increased for improved efficiency of future power plants. It is hence important that reliable and sufficiently high values of WSF can be guaranteed. However, there is often much less extensive data on the creep strength of welds than on parent steel, and also the extrapolation to long term values of WSF can add more relative uncertainty than what is expected in extrapolating the long term creep strength of parent steel. Here an improved approach is proposed to predict WSF using the Wilshire creep model to obtain the relationship between the CW creep strength and the corresponding parent material (PM) strength. The Wilshire model directly provides the WSF value for each CW data point, when the expected normalised stress is based on the CW time to rupture at stress and temperature. The corresponding master curve parameters are those for PM, when the PM hot tensile strength is also known. The WSF data points for each CW test can then be fitted for temperature and stress dependence. This approach avoids fitting distortion in WSF, unlike the traditional assessment where a master curve is first obtained for the CW creep strength. As an example, WSF of welded P91 steel at 100 000 h is here predicted in the temperature range of 550–650°C.     

The integrity of materials in high temperature components; Performance and life assessment

Engineering components operating under conditions where time dependent degradation processes occur (creep, corrosion, oxidation, microstructural degradation, embrittlement etc.) have generally been designed on a relatively simplistic basis: the best available experimentally determined mechanical property data characterising the long term properties (creep and rupture strength, oxidation and corrosion rates etc.) are assembled and lower bound properties from the data sets adopted as the relevant values. These are then combined with conservatively calculated values for the operating conditions (temperature, stress etc.) and a suitable engineering safety factor to define dimensions which should, if the assumed properties and conditions are realised during the operational lifetime of the component, lead to a satisfactory and safe behaviour. The designer must therefore be vigorous in his application of the relevant design codes, confident he can properly determine the operating conditions of stress, temperature and environment and sure that the materials properties on which his design is based will in fact be realised in practice. Unfortunately, for a wide variety of reasons, it has been the case that one or other of these criteria has not been met in practice and the resultant failures have led to very significant, and occasionally catastrophic, financial penalties. Very often the loss of production costs are greater than those involved in replacement. Such events have attracted the wide interest of the metallurgical fraternity and the investigations initiated after these incidents have greatly improved the understanding of both high temperature material properties and the impact of operating factors. This knowledge, when appropriately applied, can certainly reduce the incidence of similar failures in future.

It is important to note that, for the majority of remaining life assessments of plant of all types operating at elevated temperatures, the life limiting process is rarely controlled simply by the creep behaviour of the parent material. More usually it is the effects of factors not considered in the original design, for example weldments, stress concentrators, environmental attack and so forth, which lead to premature failure. It is invaluable, therefore, to have a prior knowledge of the consequences of these factors derived from failure investigations in detecting the early stages of failure and the component remaining life.     

3D thermomechanical behaviour of solid oxide fuel cells operating in different environments

Hermetic sealing and long-term structural reliability of fuel cell stacks depend strongly on the thermomechanically induced stress–strain behaviour. These are especially affected by the environment; the fuel cell is operating in. Most of the research and development studies, as well as laboratory studies are conducted within electrically heated furnaces rather than operating in an insulated system environment. The thermomechanical comparison of them is not fully understood, yet. The present study utilises a previously developed full scale three dimensional planar type 6-cell SOFC short stack model to shed light on the thermomechanical response of high temperature fuel cells operating in system and furnace environments. The physically resolved coupled computational fluid dynamics and computational structural mechanics model has been improved, accounting for the rate dependent creep strain, as well as including the furnace domain and thermal radiation to fully describe the thermal and deformation behaviour of the stack. The non-linear elastoplastic behaviour of the metal components as a function of temperature is considered. The results are validated using creep strain data from the literature and in-house post-mortem images. The study gives an insight about the critical regions prone to failure due to creep strain operating in different environments and the long-term fuel cell behaviour. Moreover, the critical locations appear to be prone to high creep strain after 1000 h operation time.