Search Results for impression creep testing

This paper reports the results of a collaborative small scale creep testing exercise carried out by the UK generating companies Centrica, SSE, Engie and RWE as part of an investigation of an ex-service grade 91 bend.

The impression creep test method using a rectangular indenter has been well established and the applicability of the technique has been supported by the test data for a number of metallic materials at different temperatures and stresses. The technique has proved to be particularly useful in providing material data for on-site creep strength assessments of power plant components operating in the creep regime. Due to these reasons, “standard” assessment procedures using the impression testing method are needed in order for the technique to be more widely used. This paper will first address some key issues related to the use of the impression creep test method, involving the data conversion method, typical test types and validity of the test technique etc. Then some recommendations on a number of practical aspects, such as the basic requirements of test rigs, “standard” specimen geometry, indenter dimensions, sampling procedures for scoop samples, specimen preparation, temperature and loading control, and displacement measurement, are briefly addressed. Finally, applications of the test data to assist with the risk management and life assessment programme of power plant components, particularly those with service-exposed materials, using data obtained from scoop samples, are described. Proposals for future exploitation and for improvement of the technique are addressed.

The Creep Strength Enhanced Ferritic steel grade 91 is widely used for both retrofit applications and primary construction on high temperature power plant. Although to date most structural integrity issues with this material have been associated with welds, as the operating hours of these plants accumulate, there will be a growing need for remanent creep life assessment of the base material. Arguably this is already the case for aberrant grade 91 material entering service in an incorrectly heat treated condition. In these circumstances the strength may fall below the normally accepted lower bound of the creep strength range and some indication of actual strength may be required. One strategy to address potential base material failure is to use small scale sampling of individual components, followed by small scale creep testing, to investigate the current creep strength present. The data can be compared with the equivalent data produced for well characterised material known to be at the lower bound of the creep strength range. This paper describes a methodology for using the impression creep data obtained to provide both creep strength ranking and an estimate of absolute creep strength for individual grade 91 components. This will enable appropriate judgements to be made by plant operators on repair/run decisions. For those components remaining in service, it allows for the weakest items to be given priority for early re-inspection at future outages. The ultimate goal is to identify base material creep damage development at as early a stage as possible and well in advance of failure in service.

This paper investigates the effect of high temperature tensile strain on subsequent creep strength in grade 91 steel. Failed hot tensile specimens have been sectioned at various positions along the specimen axis, and therefore at different levels of hot tensile strain, to obtain material for creep strength evaluation. Because of the limited amount of material available for creep testing obtained in this way, creep testing has been carried out using the specialised small-scale impression creep testing technique. The grade 91 material has been tested in both the normal martensitic condition and in an aberrant mis-heat treated condition in which the microstructure is 100% Ferrite. The latter condition is of interest because of its widespread occurrence on operating power plant with grade 91 pipework systems.

This paper reports the results of a collaborative investigation of an ex-service grade 91 bend carried out by the UK generating companies Centrica, SSE, Engie and RWE. As part of the handover exercise for Centrica’s Langage power station in 2009 a number of routine checks were carried out on the main steam and hot reheat grade 91 steam pipework. In some cases low hardness readings were found with subsequent metallurgical replication showing the presence of an aberrant non martensitic microstructure. This led to a more extensive inspection programme on the steam lines and the discovery of other areas of suspect material. A review of the operating capability of the plant, including detailed pipework stress analysis and a pipework peaking assessment, along with the assumption that lower strength grade 91 material was present, led to the steam lines being down rated and returning to service under these revised conditions. At the first C inspection in December 2012, after the HRSG and associated pipework had operated for 18720 hours, a bend with a soft weld, along with a section of the straight pipe on either side, was removed from service. An investigation was undertaken to establish how long this component would have survived, had it been left in service, and to consider the implications for the future operation of the plant.

G115 is a novel ferritic heat resistant steel developed by CISRI in the past decade. It is an impressive candidate material to make tubes, pipes, and forgings for advanced ultra super critical (A-USC) fossil fired power plants used for the temperature scope from 600°C to 650°C. The successful development of G115 extends the upper application temperature limitation of martensitic steel from 600°C to about 650°C. This breakthrough is imperative for the design and construction of 610°C to 650°C A-USC fossil fired power plants, from the viewpoint of the material availability and economics of coal fired power plant designs. This paper introduces the development history and progress of G115 steel. The strengthening mechanism of the novel martensitic steel is briefly discussed, and the optimized chemical composition and mechanical properties of G115 steel are described. The details of industrial trials of G115 tube and pipe at BaoSteel in the past years are reviewed, with the emphasis on the microstructure evolution during aging and creep testing. These tests clearly show that the microstructure of G115 steel is very stable up to the temperature of 650°C. Correspondingly, the comprehensive mechanical properties of G115 steel are very good. The creep rupture time is longer than 17000 hours at the stress of 120MPa and at the temperature of 650°C and 25000+ hours at the stress of 100MPa and at the temperature of 650°C, which is about 1.5 times higher than that of P92 steel. At the same time, the oxidation resistance of G115 steel is a little bit better than that of P92 steel. If G115 steel is selected to replace P92 pipes at the temperature scope from 600°C to 650°C, the total weight of the pipe can be reduced by more than 50% and the wall thickness of the pipe can be reduced up to about 55%. In addition, the upper application temperature limitation of G115 steel is about 30°C higher than that of P92 steel. Thus, G115 steel is a strong candidate material for the manufacturing of 600+°C advanced ultra-super-critical (A-USC) fossil fuel power plants in China and elsewhere.

Preface for the 2016 Advances in Materials Technology for Fossil Power Plants conference.

Advanced martensitic 9% chromium steels have been identified as the most favored group of materials for high temperature applications in thermal power plants. To extend the temperature range of martensitic steels up to 650°C large effort was put on the development of new alloy concepts. The so-called MARBN concept (Martensitic steel with defined Boron/Nitrogen relation) provides increased creep rupture strength due to higher solid solution strengthening and improved microstructural stability. The major improvement is the reduction of type IV cracking in welded joints, which shifts the focus to the creep rupture strength of the weld metal. This paper illustrates the process experience of the steel foundry for production of heavy cast components in latest state of the art 9-12%Cr-MoCoVNbNB-alloyed cast steel grades and the newest state of development and prototype components in MARBN cast steel grades. Metallurgy, solidification, heat treatment and welding are main items to be considered for development of new, complex steel grades for foundry processing with the help of empiric processing in test programs and thermo-physical simulation. As welding is an essential processing step in the production of heavy steel cast components a good out-of-position weldability is required. Moreover a stress-relieve heat-treatment takes place subsequently after welding for several hours. This contribution also deals with the development of matching welding consumables for the production of heavy cast components and discusses the challenges of defining appropriate welding and heat treatment parameters to meet the requirements of sufficient strength and toughness at ambient temperature. Additionally, first results of creep rupture tests are presented.

This paper briefly introduces the state-of-the-art of the research and development of candidate heat resistant materials used for the manufacturing of 700°C advanced ultra-super-critical (AUSC) fossil fuel power plants (PP) in China, especially, focus on the impressive progress in the past three years. The detailed advancements (technical exploration and industrial investigation) of candidate materials spectra for the boiler system of A-USC PP will be presented in the current paper, including novel ferritic heat resistant steels, advanced austenitic heat resistant steels, Fe- Ni-based alloys and Ni-based alloys, which serve and cover the steam temperature scope from 600°C to 720°C. Some newly available data associated with above materials will be released.

There is an increased interest in miniature testing to determine material properties. The small punch test is one miniaturized test method that has received much interest and is now being applied to support the design and life assessment of components. This paper presents the results of a test program for a small punch creep test at 650°C of 316L stainless steel produced from additive manufacturing. A major finding is that the deflection rate curve versus time may have multiple minima as opposed to forged 316L with only one minimum. This is believed to be due to microcracking and has direct consequences on the determination of the creep properties that that are based on a single minimum value in the CEN Small Punch Standard. In the paper, aged and nonaged materials are compared, and small punch creep results are also compared with standard uniaxial creep tests. The multiple minima feature means that the approach to determine equivalent stress and strain rate from the minimum deflection rate needs to be modified. Some approaches for this are discussed in the paper. Under the assumption that the multiple minima represent cracking, it opens up opportunities to quantify reduced creep ductility by the small punch test.

A new martensitic steel for power generation applications was developed: Tenaris High Oxidation Resistance (Thor) is an evolution of the popular ASME grade 91, offering improved steam oxidation resistance and better long-term microstructural stability, with equal or better creep strength. Thanks to its design philosophy, based on consolidated metallurgical knowledge of microstructural evolution mechanisms, and an extensive development performed in the last decade, Thor was engineered to overcome limitations in the use of ASME grade 91, yet allow being processed in the same fashion, permitting the re-use of consolidated best practices for boiler fabrication. In order to evaluate the possibility to produce complete pressure part systems, various tests to manufacture components have been performed on Thor pipes and tubes (i.e. finning, bending, welding) and on Thor forged material (i.e. flanges). In all cases consolidated industrial best practices used on ASME grade 91 were applied, and resulting properties met ASME requirements.

The current research adopts a novel approach by integrating correlative microscopy and machine learning in order to study creep cavitation in an ex-service 9%Cr 1%Mo Grade 91 ferritic steel. This method allows for a detailed investigation of the early stages of the creep life, enabling identification of features most prone to damage such as precipitates and the ferritic crystal structure. The microscopy techniques encompass Scanning Electron Microscopy (SEM) imaging and Electron Back-scattered Diffraction (EBSD) imaging, providing insights into the two-dimensional distribution of cavitation. A methodology for acquiring and analysing serial sectioning data employing a Plasma Focused Ion Beam (PFIB) microscope is outlined, complemented by 3D reconstruction of backscattered electron (BSE) images. Subsequently, cavity and precipitate segmentation was performed with the use of the image recognition software, DragonFly and the results were combined with the 3D reconstruction of the material microstructure, elucidating the decoration of grain boundaries with precipitation, as well as the high correlation of precipitates and grain boundaries with the initiation of creep cavitation. Comparison between the 2D and 3D results is discussed.

Inconel alloy 740 was initially developed to enable the design of coal-fired boilers capable of operating at 700°C steam temperature and high pressure. The alloy successfully met the European program's targets, including 100,000-hour rupture life at 750°C and 100 MPa stress, and less than 2 mm metal loss in 200,000 hours of superheater service. However, thick section fabrication revealed weldability challenges, specifically grain boundary microfissuring in the heat affected zone (HAZ) of the base metal. This paper describes the development of a modified variant with significantly improved resistance to HAZ microfissuring and enhanced thermal stability, while maintaining desirable properties. The formulation process is detailed, and properties of materials produced within the new composition range are presented. Additionally, the microstructural stability of the original and modified alloy compositions is compared, demonstrating the advancements achieved in this critical material for next-generation power plants.