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.     

梨果实蠕变基本流变特性研究

应用TA-XT2i质地分析仪对梨果实进行蠕变测试,结果表明,蠕变试验条件对蠕变变形量的影响是载荷大于蠕变时间,载荷越大,蠕变时间越长,蠕变变形量越大,越容易发生损伤.     

加热炉炉管损伤及寿命计算模型的研究

研究了不同应力函数准则、开停车对炉管蠕变损伤及寿命的影响。认为不同的应力函数准则对炉管的损伤或寿命预测将引起较大差异 ,开停车对炉管的损伤演化发展产生了巨大的作用。比较了不同损伤蠕变耦合模型的计算结果 ,为炉管损伤及寿命评估提供了有效的方法和手段。     

Compressive fatigue behaviour of refractories with carbonaceous binders

Furnace linings of magnesia-carbon and micro-porous carbon bricks experience cyclic compressive loads. An experimental programme has been carried out to assess the sensitivity of these materials to compressive fatigue failure. Next to room temperature tests, a number of high temperature tests have been performed. Results of the fatigue tests have been analysed together with the data of the monotonic stress–strain loading and creep tests. Compressive fatigue failure has been seen in both the materials. The less brittle material has shown lower fatigue life. The curve relating the fatigue strains with the amount of cycles has been of classical sigmoid shape with three phases. The strain rates of the secondary (linear) phase have shown good correlation with the number of cycles to failure. The grain–matrix interface has been found to play the critical role in the initiation and propagation of the fatigue cracks.     

Integrity assessment of an embrittled steam turbine casing

This work reviews a recent structural integrity assessment carried out on a high-pressure turbine inner casing that had suffered from temper embrittlement. The assessment was carried out to demonstrate that the casing can be safely returned to service based on revised operating conditions with particular emphasis on the control of temperature ramp rates during start-up and shutdown events. A retirement-for-cause philosophy was adopted to take into consideration that some operational flexibility will be required prior to the casing being replaced at the next planned outage.     

A CDM-based study of fatigue–creep interaction behavior

Under stress control mode, the damage evolution during fatigue, creep and their interaction behavior actually is a ductility exhaustion process in response to cyclic and static creep. Based on the ductility dissipation theory and effective stress concept of continuum damage mechanism (CDM), a new fatigue–creep interaction damage model has been developed in this paper, where the change of the inelastic strain energy density is used to define the damage variable. To assess this damage model, high temperature fatigue–creep interaction experiments have been carried out for 1.25Cr0.5Mo steel under stress control mode employing a trapezium waveform. The damage evolution laws of 1.25Cr0.5Mo steel under various combinatorial conditions with different maximum stresses and stress amplitudes are derived. Results indicate that the damage parameter and damage model presented in this paper are applicable to describe the damage evolution for fatigue–creep interaction.     

Overview of the HIDA Project

HIDA is a recently completed four-year-duration large research, development and validation project aimed at harmonising and expanding current high temperature crack assessment procedures and practices with a view to developing a unified European/International procedure. The project was started in 1996 and completed in early 2000. The work involved analysis of currently available published and unpublished data and generation of data from various standard fracture mechanics and validation type feature specimen tests on a range of steels used in high temperature plants. The project covered creep, high temperature fatigue and creep-fatigue interaction.This paper outlines the background issues and methodology applied to achieve the project objectives. Details are covered in other HIDA papers presented at the first HIDA Conference (held at CEA Saclay, France, in April 1998),¹ and also in the HIDA papers being presented at this Conference.The project Consortium now intends to expand this work through a new European Commission funded project ‘HIDA Applicability’ to incorporate sensitivity and probabilistic assessment.     

Creep crack growth in welded components — a numerical study and comparison with the R5 procedures

In the present study, creep crack growth (CCG) in a circumferentially welded low alloyed pipe is numerically investigated for a number of different combinations of weldment constituent material properties. A creep ductility based damage model describes the accumulation of creep damage ahead of the crack tip where a constraint parameter and the creep strain rate perpendicular to the crack plane are used as characterising parameters. It is assumed that a fully circumferential creep crack, located in the heat affected zone with a depth of one quarter of the pipe thickness, is growing at a constant rate from the outer surface towards the inside. The numerical results reveal that not only the properties of the zone containing the crack, but also the deformation properties of the surrounding material influence the CCG behaviour. This influence can be noted on the characterising parameters used for the CCG rate predictions as well as on the CCG rate itself. The mismatch influence on corresponding C^* values is, however, marginal. This indicates that determination of the CCG rate in weldments, based on the C^* value only, may result in uncertain estimates.The numerically investigated cases are also assessed by use of the R5 procedures for the sake of comparison. Considering the stress re-distribution, due to the mismatch effect, the CCG rate is determined for the different weldment configurations. The comparison shows that the assumption of plane strain or plane stress conditions in the R5 analysis is essential for the agreement of the results between R5 and the two-parameter approach. Assuming plane stress conditions at the crack tip results in a relatively good agreement for the axial stress dominated cases investigated. However, for the hoop stress dominated cases, the R5 procedures predict higher CCG rates by an order of magnitude.     

Creep crack growth in welds: a damage mechanics approach to predicting initiation and growth of circumferential cracks

The results of damage mechanics finite element analyses have been used to estimate the initiation and growth of type IV cracks in a series of internally pressurised circumferential pipe welds, in main steam pipelines made of 1/2CrMoV steel. The material properties used, for the various zones of new, service-aged and repaired welds, were produced from creep test data at 640°C. Damage distributions and accumulation with time within the HAZ are presented, from which the crack initiation times and positions for these welds, under a closed-end condition, and with additional axial (system) loading, were identified. By investigating the propagation of damage through the wall thickness, the remaining lives of the various weld types were estimated. The method provides a means for predicting the initiation and growth of type IV cracks in these CrMoV weldments, and for estimating the length of time a weld can safely be left in service, after damage, or type IV cracking, is identified during inspection.     

Creep crack growth of high temperature weldments

The experimental programme of the EC supported project (SMT 2070) SOTA aimed at addressing a technical and industrial need to provide guidelines for creep crack growth (CCG) testing and data analysis of weldments. Mechanical and creep properties were determined on two pressure vessel steels of P22 (2.25Cr1Mo) and P91 (9Cr1MoVNb). The specimens were taken from pipe welds for weld metal (WM) tests, and simulated heat affected zone (HAZ) material for the HAZ tests.The CCG tests were carried out on cross-weld compact tension (CT) specimens machined out from weldment of pipes. The tests were done at 550 and 600°C on P22 and P91 materials, respectively. The CT specimens were notched using electrical discharge method, for a sharp starter crack. This method of initiating sharp starter crack was chosen to make sure that all partners will test specimens with starter crack location as specified in the work programme to study crack initiation and growth in WM and HAZ (both in the centre and type IV region). The CCG tests were carried out following the ASTM E1457-92 [ASTM E1457-92, Standard test method for measurement of creep crack growth rates in metals, ASTM, Philadelphia, PA 19103, USA]. The partners assessed their data and sent both assessed and their raw data to be further assessed centrally. All the data from partners were analysed and compared with those of partners' own assessed data.The present paper reports on the analyses of CCG data obtained in the programme including six laboratories from six European countries. The programme addresses the differences and difficulties in testing and the assessment of weldments, and provides guidelines for harmonisation of testing procedures for reliable data production for remanent life assessment of plants with welded components.