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.     

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).     

Development of a new methodology for validating thermal storage media: Application to phase change materials

Long‐term stability and long‐term performance of thermal storage media are a key issue that should be thoroughly analysed when developing storage systems. However, no testing protocol or guideline exists up to now for validating storage media, so that authors apply their own criteria, not only for designing testing procedures but also for predicting the material behaviour under long‐term operation. This paper aims to cover this gap by proposing a methodology for validating thermal storage media; in particular, phase change materials (PCMs). This methodology consists of different stages that include PCM characterization, preliminary assessment tests, and accelerated life testing. For designing the accelerated life tests, lifetime relationship models have to be obtained in order to predict PCM long‐term behaviour under service conditions from shorter tests performed under stress conditions. The approach followed in this methodology will be valid for materials to be used as sensible or thermochemical storage media, too.     

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.     

Residual stresses in a CrMoV-2CrMo pipe weld: Part 1—The as-welded condition

There is a complete lack of reliable data concerning the through thickness distribution of residual stress in welds, both for the as-welded and the stress relieved conditions. As a result, the assessment of defects in welds and current stress relieving practice are largely based on surface measurements of residual stress.To improve this situation a technique has been established which permits the complete distribution of residual stress to be determined. This technique has been applied to a ferritic main steam pipe weld, in the as-welded condition. In establishing this technique, which uses experimental measurements combined with finite element analyses, three different methods of solution were investigated involving line loads, band loads and triangular loads. Only the last was found to give acceptable accuracy.The results of the analysis show that, in general, the residual hoop stresses over the outer two-thirds of the weld are tensile, being compressive over the inner third. Axial stresses are tensile over the outer third and compressive over the inner two-thirds.The approximations and assumptions used in the analysis are discussed, together with the implications that the results have on defect assessment and stress relieving.     

Comparison between predicted and actually observed in-service degradation of a nickel pigmented anodized aluminium absorber coating for solar DHW systems

The actual service degradation in optical performance of a nickel pigmented anodized aluminium absorber coating has been investigated in order to better validate predicted service life data from accelerated life testing. Samples from the coating taken from collectors used in solar DHW systems for time periods of 10 years or more were analysed for that purpose. The study, which was performed by the IEA Working Group: Materials in Solar Thermal Collectors, utilized results from a comprehensive joint case study on accelerated life testing previously performed in Task X of the IEA Solar Heating and Cooling Programme. It could be concluded from the present study that the agreement between degradation data determined for the absorber samples from the DHW systems and that from accelerated life testing from the Task X study was astonishingly good both from a quantitative and a qualitative point of view. For the anodized aluminium coating the results of the present study strongly point to the fact that the design of the solar collector with respect to airtightness is the most crucial factor in determining service life. The service life was defined as the period during which the optical performance is not less than 95% of its original value. The estimated service life is of the order of 30–40 years for the coating in an airtight solar collector with controlled ventilation of air, whereas in a non-airtight collector with essential uncontrolled ventilation of air, the corresponding life is around 5–10 years. The general conclusion from the study is that the accelerated life testing method as developed by the Task X group is an efficient tool in predicting expected service life of absorber coatings and is therefore to be recommended for qualification of durability of new kinds of absorber coatings.     

基于加速蠕变断裂试验的12%Cr钢高温螺栓安全可靠性评价研究

火力发电厂汽轮机高温螺栓是汽轮机组的重要部件,在长期运行过程中,高温螺栓要承受蠕变、疲劳及其交互作用的影响,机组的启停、振动以及螺栓的制造质量等均对螺栓的使用寿命产生重要影响.在初步了解不同运行时间段的12%Cr钢高温螺栓材料性能状况的基础上,采用加速蠕变断裂试验法对运行35×104h的高温螺栓安全使用状况进行了综合评估,在评价高温部件安全可靠性方面进行了新的尝试.     

Hardness–fracture toughness–creep rupture strength relationship for welds in P91 main steam pipe

Abstract

An experimental study on the impact toughness, hardness, fracture toughness and creep rupture strength of P91 steel weld metals under two conditions (R and D) has been carried out. The results show that weld metal having higher hardness, lower impact toughness and lower fracture toughness (sample R) has higher creep rupture strength. The creep rupture strength is closely related to creep life of component. Thus, assessment of the weld hardness of P91 steel is of value in assessing the creep life of pipes in service. The lower toughness of sample R is attributed mainly to the higher welding current and input line energy used. The lower hardness and strength of sample D, which received a second post-weld heat treatment, partially result from too high a secondary tempering temperature being employed.     

注汽锅炉高温高压炉管破裂诊断分析

油田注汽锅炉的炉管是高温高压设备，由于长期在恶劣工况下运行．容易发生材质老化和损伤积累，突发性事故也会频频发生，给企业职工的生命安全和国家财产带来严重威胁，因此时主蒸汽炉管进行诊断分析与寿命预测尤为迫切和重要。本文结合工程实例，对主蒸汽炉管的失效模式、影响运行寿命的主要因素以及寿命预测的方法进行了较为系统的分析，为油田注汽锅炉主蒸汽炉管的寿命管理提供了理论依据和分析方法，以达到预防和降低事故、减少损失的目的。     

Applicability of highly reflective aluminium coil for solar concentrators

Because of their manufacturing flexibility and their low costs, mirrors based on anodized or coated sheet aluminium are a promising alternative as primary or secondary concentrators in a number of solar energy applications. They offer solar weighted reflectances of 88–91%, good mechanical properties and are easy to recycle. However, problems occur due to their limited corrosion resistance. Therefore, prior to application, lifetime tests including outdoor and accelerated ageing tests are necessary to prove their optical durability in terms of achieving a 10-year service lifetime. In this study the optical properties of a number of different aluminized reflector materials after accelerated and outdoor exposure tests have been investigated. Optical testing has been performed by measuring the spectral hemispherical reflectance of exposed samples and calculating the solar weighted value. Additionally, specular reflectance has been measured with a simple mobile reflectometer. Materials involved are standard commercial anodized sheet aluminium with layers of different thicknesses and standard high specular aluminium with a metaloxide layer system plus an anti-oxidation polymer coating. Results show that optical degradation is strongly dependent on climatic conditions. Non-organic coatings involved are primarily attacked by humid climates with higher amounts of atmospheric pollution. Standard anodized materials withstand outdoor and accelerated weathering. However reflectance tends to become less specular, which limits their application in concentrating technologies. Finally, small scale application tests have been performed to demonstrate the applicability concerning handling and mechanical connection with support structures. By measuring power density in the focus of a test collector, minimum specular reflectance requirements for trough systems can be defined.     

Slow burst testing of samples as a method for quantification of composite cylinder degradation

The current practise to focus periodic retesting of composite cylinders primarily on the hydraulic pressure test has to be evaluated as critical. The test itself always causes a certain amount of micro damage to the cylinders but does not necessarily deliver sufficient and evaluable information. Thus BAM Federal Institute of Materials Research and Testing (Germany) moves the focal point to a new approach for validation of composite cylinders, based on destructive sample tests parallel to operation. Statistical assessment of results of these destructive tests is employed for the estimation of remaining safe service life, based on reliability demands. The estimated service life is also used for the determination of re-test periods of the examined population of composite cylinders.An essential aspect of this approach is the validation of current residual strength and its prediction at any point of service life. In cases of gas cylinders with very high cycle strength, residual strength cannot be quantified statistically by means of hydraulic load cycles. As a replacement, creep tests or burst tests may be employed. BAM suggests the “slow burst test SBT” as a combination of these two test procedures. This is a compromise between the practicability of the (conventional) burst test and the practical relevance of sustained loads during service, to be tested in creep rupture tests. In this paper, a variety of 99 burst results of a cylinder design type used for breathing apparatus (CFRP with PE-liner) is evaluated. The influence of test procedure parameters and nature and intensity of artificial ageing on the test sample strength are analysed statistically. This leads to an evaluation of different procedures of artificial ageing and the recommendation to substitute conventional burst tests by slow burst tests for the assessment of composite pressure receptacles.     

Creep rupture behaviour and recrystallisation in HR6W and Haynes alloy 230 cold bent boiler tubing for ultrasupercritical steam boiler applications

Abstract

Creep rupture experiments were conducted on tubes of HR6W and Haynes 230, candidate ultrasupercritical alloys, to evaluate the effects of cold work and recrystallisation during high temperature service. These creep tests were performed by internally pressurising cold bent boiler tubes at 775°C for times up to 8000 h. The bends were fabricated with cold work levels beyond the current ASME Boiler and Pressure Vessel Code section I limits for austenitic stainless steels. Destructive metallographic evaluation of the tube bends after testing was used to determine the effects of cold work and the degree of recrystallisation. The metallographic analyses, combined with an evaluation of the creep and rupture data, suggest that solid solution strengthened nickel based alloys can be fabricated for high temperature service at ultrasupercritical conditions utilising levels of cold work higher than the current allowed levels for austenitic stainless steels.     

Predicting weld creep strength reduction for 9% Cr steels

In design standards and in post-service life assessment, the cross-weld (CW) creep strength of ferritic steels is nearly universally assumed to be 80% of the corresponding value for the parent material (PH). However, CW data assessment of some 9% Cr steels such as E911 and P91 suggests that this would not hold at least at the high temperature end of the testing range. The resulting weld creep strength factor (WSF) is then attaining values well below 0.8 when extrapolated to typical design life of 100 000 h or more. Under such conditions the conventional value of 0.8 would result in non-conservative (too long) predicted life for structures subjected to CW loading in the creep regime.To accommodate the CW strength data for realistic values of WSF requires appropriate correction based on actual data. For this purpose, an alternative assessment approach, rigidity parameter correction (RPC), is proposed. This approach can be used to predict CW rupture strength from the PM master curves, with any PM rupture model optimized to correspond to the welded materials data.     

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.     

Electromagnetic‐Thermal Modeling of Induction Heating of Moving Wire

The strength in a high carbon wire is attributed to the pearlitic microstructure, which is required for ease of wire drawing. During cold drawing of high carbon steel wires, residual stress develops which has to be relieved in order to obtain the desired mechanical properties. To achieve this, the wire is passed through a closed loop online an induction furnace at a particular speed in order to heat it to a uniform temperature range. This research work presents the electromagnetic‐thermal modeling of the induction heating of a moving wire based on the finite element method using the software package, COMSOL MultiphysicsTM. The furnace had a complicated geometry for the coils and this is, perhaps, for the first time an exhaustive study which is being reported. A unique grid generation technique was developed considering the skin effect. This work is aimed at enabling modeling of the process and will in turn be useful when defining individual parameters affecting the temperature distribution in a component, subjected to induction heating. The temperature distribution in the work piece depends primarily on parameters like coil position, line speed, frequency of the current, thermal and magnetic properties of the work piece, and so on. The impact of power supply frequency and line speed were studied during the heating of the moving wire (workpiece). An in‐situ customized furnace of lower capacity was developed to carry out the validation experiments. The present modeling results are validated with online plant trial data and found to be in good agreement. Finally, the desired mechanical property achieved during trials was confirmed through tensile testing.     

“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.     

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.     

Reliability of commercially available hydrogen sensors for detection of hydrogen at critical concentrations: Part I – Testing facility and methodologies

A facility for testing the performance of hydrogen safety sensors under a wide range of ambient conditions is described. A specific test protocol was developed to test sensors under conditions which could reasonably be expected during the sensors' service life. The tests were based on those described in IEC 61779 and were adapted following consultation with car manufacturers and after careful consideration of the sensors expected service environmental conditions. The protocol was evaluated by using it to test a large number of commercially available sensors. Observations made and experience gained during the testing campaign allowed the test protocol to be fine-tuned bearing in mind the sensor performance and behaviour during tests. The result of this work is an experimentally evaluated methodology which may be used as a guideline for testing the suitability of hydrogen sensors for automotive applications.     

Creep life assessment by low strain rate tensile testing

A testing method, originally proposed by Rajakovics,⁵ has been investigated for its ability to extrapolate creep rates from the results of short-term tests. Low constant strain rate tensile tests have been performed on a number of high temperature materials in the virgin, service-aged and welded conditions in the temperature range from 525°C to 1050°C. A technique has been developed that permits one to determine the steady-state creep rate under a known stress or the creep stress for a given creep rate. A step towards a quantitative residual life assessment of service-aged materials has been made. The results of the proposed extrapolation technique are compared with results from creep tests.     

The effect of residual stress on the creep deformation of welded pipe

The finite element method has been used to study the creep deformation of pipe butt welds in the presence of an initial residual stress distribution. The study has used values for pipe dimensions, temperature (575°C) and internal pressure (70 bar) which correspond to the conditions being used in a pressure vessel testing programme which is investigating transverse weld metal cracking in CrMoV reheat pipe welds. Two sets of steady state creep data for the weld metal have been used, one to represent a weld weaker than the parent and the other one that was stronger. A residual stress distribution, consistent with experimental data, has been generated as àn initial condition for the analysis. The results are presented, discussing the effects of residual stress on both the total and creep strain accumulations for the two weld properties. The steady state creep laws used in the analysis do not allow for damage accumulation, but the possibility of such an effect is discussed using the Kachanov model. The results of the finite element analysis are compared with the experimental creep strain data from the pressure vessel testing programme.