John Shingledecker

Small punch creep testing: review on modelling and data interpretation

Abstract In many situations where the characterisation of the mechanical behaviour of a specific material is required, source material for manufacture of conventional test specimens may be at a premium. Examples include the validation of new alloys for use in the power industry, the description of the heat affected zone (HAZ) of weldments1 or performing a remnant life study on an in service component (such as steam pipe work used extensively in the power generation industry). The potential for a limit in sample material has necessitated the development of small specimen designs and associated test methods, particularly for the determination of the creep behaviour of a sample material. The small punch creep test (SPCT) has the potential to characterise the full uniaxial creep curve (as the specimen is taken to fracture). It is for this reason that the small punch creep test has attracted much interest from the research community. Owing to the complex deformation mechanism interactions experienced in the small punch creep test, interpretation of the results has received attention from many authors since its application was proposed by Parker et al. in the 1990s2 (based on small punch plasticity test by Manahan et al. in the 1980s3–5). In this review paper, several methods for the interpretation of small punch creep test (SPCT) data are reported and compared, together with examples of their application. Considerations for finite element (FE) modelling of small punch creep tests are highlighted and critiqued. Recommendations for potential areas of future research are also presented based on the authors’ investigation into published literature and research.

Weldability and weld performance of candidate nickel base superalloys for advanced ultrasupercritical fossil power plants part I: fundamentals

Abstract Fossil fuel will continue to be the major source of energy for the foreseeable future. To meet the demand for clean and affordable energy, an increase in the operating efficiency of fossil fired power plants is necessary. There are several initiatives worldwide to achieve efficiencies >45% higher heating value (HHV) through an increase in steam temperature (700 to 760°C) and pressure (27.6 to 34.5 MPa). Realising this goal requires materials with excellent creep rupture properties and corrosion resistance at elevated temperatures. In order to accomplish this, three classes of materials have been identified: creep strength enhanced ferritic steels, austenitic stainless steels and nickel base superalloys. Although new alloys have been designed and developed to meet this need, welding can have a significant and often detrimental effect on the required mechanical and corrosion resistant properties. Two previous papers addressed the welding and weldability of ferritic and austenitic stainless steels. Welding and weldability of nickel base alloys will be discussed in a two part paper. In this paper, the primary focus will be on the fundamentals of welding and weldability of Ni base superalloys.

Weldability and weld performance of candidate nickel based superalloys for advanced ultrasupercritical fossil power plants Part II: weldability and cross-weld creep performance

Fossil fuel will continue to be the major source of energy for the foreseeable future. To meet the demand for clean and affordable energy, an increase in the operating efficiency of fossil fired power plants is necessary. There are several initiatives worldwide to achieve efficiencies >45%HHV (higher heating value) through an increase in steam temperature (700–760°C) and pressure (27.6–34.5 MPa). Realising this goal requires materials with excellent creep rupture properties and corrosion resistance at elevated temperatures. Two previous papers addressed the welding and weldability of ferritic and austenitic stainless steels. Welding and weldability of nickel based alloys will be discussed in a two-part paper. In this paper, the primary focus will be on the behaviour of candidate nickel based alloys that are being proposed in advanced ultrasupercritical power plants and with regard to weldability (Part I) and cross-weld creep performance (Part II).

Materials Considerations for Supercritical CO2 Turbine Cycles

Proposed open and closed Brayton-type cycle systems employing supercritical CO2 (sCO2) as the working fluid develop hot gas path environments that present challenging requirements of strength and environmental resistance for the materials of construction. The likely materials properties required by different cycle configurations are examined and compared with the capabilities of available materials. In some instances where peak operating conditions in the turbines in closed-cycle systems approach 500°–700°C at 200 bar, or 1150°C at 300 bar for open cycles, the range of available alloys with the requisite temperature-strength and capabilities is limited.The cycles are highly recuperated: hot, lower-pressure turbine exhaust is used to pre-heat high-pressure CO2 entering the external heater or combustor just upstream of the turbine. Obviously, there is a need to match alloy capabilities with the performance requirements of individual components. Where alloys are employed under conditions beyond current experience, or in unusual configurations (the unique properties of sCO2 may impact the design of some components), practical acceptance will depend on successful qualification testing involving close collaboration among equipment manufacturers, materials suppliers, and materials research and development groups.Examination of the range of scenarios published for sCO2 turbine systems suggests that approaches used in the U.S. Advanced, Ultra-Supercritical (A-USC) Steam Boiler program, and some of the data generated for fabrication, joining, mechanical properties, and performance in simulated service environments in that program, will be applicable to some of the components in these systems. An attempt to elaborate and prioritize the materials property and qualification activities required for the successful realization of these sCO2 cycles is presented as a guide to activities needed to facilitate materials selection.Copyright

Microstructural Evolution and Creep-Rupture Behavior of A-USC Alloy Fusion Welds

Characterization of the microstructural evolution of fusion welds in alloys slated for use in advanced ultrasupercritical (A-USC) boilers during creep has been performed. Creep-rupture specimens involving INCONEL® 740, NIMONIC® 263 (INCONEL and NIMONIC are registered trademarks of Special Metals Corporation), and Haynes® 282® (Haynes and 282 are registered trademarks of Haynes International) have been analyzed via light optical microscopy, scanning electron microscopy, X-ray diffraction, and thermodynamic and kinetic modeling. Focus has been given to the microstructures that develop along the grain boundaries in these alloys during creep at temperatures relevant to the A-USC process cycle, and particular attention has been paid to any evidence of the formation of local γ′-denuded or γ′-free zones. This work has been performed in an effort to understand the microstructural changes that lead to a weld strength reduction factor (WSRF) in these alloys as compared to solution annealed and aged alloy 740 base metal. γ′ precipitate-free zones have been identified in alloy 740 base metal, solution annealed alloy 740 weld metal, and alloy 263 weld metal after creep. Their development during long-term thermal exposure is correlated with the stabilization of phases that are rich in γ′-forming elements (e.g., η and G) and is suppressed by precipitation of phases that do not contain the γ′ formers (e.g., M23C6 and μ). The location of failure and creep performance in terms of rupture life and WSRF for each welded joint is presented and discussed.

Weldability and weld performance of candidate nickel based superalloys for advanced ultrasupercritical fossil power plants

Fossil fuel will continue to be the major source of energy for the foreseeable future. To meet the demand for clean and affordable energy, an increase in the operating efficiency of fossil fired power plants is necessary. There are several initiatives worldwide to achieve efficiencies >45%HHV through an increase in steam temperature (700–760°C) and pressure (27.6–34.5 MPa). Realising this goal requires materials with excellent creep rupture properties and corrosion resistance at elevated temperatures. Two previous papers addressed the welding and weldability of ferritic and austenitic stainless steels. Welding and weldability of nickel based alloys will be discussed in a two-part paper. In this paper, the primary focus will be on the behaviour of candidate nickel based alloys that are being proposed in advanced ultrasupercritical power plants and with regard to weldability (Part I) and cross-weld creep performance (Part II).

The effects of geometrical inaccuracies of the experimental set-up on small punch creep test results

The small punch creep testing technique is able to provide creep properties from a very small amount of material. However, a universal and robust technique, to convert small punch creep testing results to corresponding uniaxial creep test data, has still not been established. In addition, the experimental output can be affected by several sources of uncertainty, such as friction between the components of the test rig and the specimen, and inaccuracies in the geometry of the experimental set-up and the testing procedures. This article reports the results of three-dimensional elastic/creep finite element analyses of small punch creep testing, taking into account geometrical inaccuracies in the initial punch position and the loading direction. The results of the calculations show that the initial position of the punch and the loading direction can considerably affect the variation in the specimen’s central deflection with time and the final time to failure. The minimum displacement rate was found to decrease when the punch moves away from the centre of the specimen and when the angle between the loading direction and the axis of the test rig increases. The time to failure increases when the punch deviates from the perfect axi-symmetric configuration. The effects of the direction of the load increase as the initial distance of the punch from the centre of the specimen increases. Analytical correlations, corresponding to the inaccuracies investigated, are also proposed.

Microstructural evolution and mechanical properties of Grades 23 and 24 creep strength enhanced ferritic steels

Grades 23 and 24 creep strength enhanced ferritic (CSEF) steels are relatively new materials that have been designed with improved high-temperature strength compared to conventional 2.25Cr–1Mo alloys. These new steels share alloying strategies that were used in the past for CrMo derivatives such as the CrMoV steels. Grades 23 and 24 are often grouped with similar CSEF steels in terms of mechanical properties and fabrication characteristics. However, recent experience indicates that these alloys require more unique care to ensure adequate properties for the intended service in both the base metal and welds. The differences in behaviour between these new steels and more conventional CrMo steels can be attributed to relatively minor variations in composition that lead to rather significant differences in microstructure. This review summarises technical information and practical experience on the manufacture and use of Grades 23 and 24 CSEF steels for power generation applications. Factors that influence the microstructure and strength of base metal are discussed for each alloy along with influences from common fabrication processes such as welding and bending. The results of this review provides manufacturers, fabricators and end users with information that can be applied to help ensure optimal performance of these steels in high-temperature applications associated with power generation.

Concentrating solar power (CSP) power cycle improvements through application of advanced materials

Concentrating solar power (CSP) systems with thermal energy storage (TES) capability offer unique advantages to other renewable energy technologies in that solar radiation can be captured and stored for utilization when the sun is not shining. This makes the technology attractive as a dispatchable resource, and as such the Electric Power Research Institute (EPRI) has been engaged in research and development activities to understand and track the technology, identify key technical challenges, and enable improvements to meet future cost and performance targets to enable greater adoption of this carbon-free energy resource. EPRI is also involved with technically leading a consortium of manufacturers, government labs, and research organizations to enable the next generation of fossil fired power plants with advanced ultrasupercritical (A-USC) steam temperatures up to 760°C (1400°F). Materials are a key enabling technology for both of these seemingly opposed systems. This paper discusses how major strides in s...

Creep-Rupture Behavior of Ni-Based Alloy Tube Bends for A-USC Boilers

Advanced ultrasupercritical (A-USC) boiler designs will require the use of nickel-based alloys for superheaters and reheaters and thus tube bending will be required. The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code Section II PG-19 limits the amount of cold-strain for boiler tube bends for austenitic materials. In this summary and analysis of research conducted to date, a number of candidate nickel-based A-USC alloys were evaluated. These alloys include alloy 230, alloy 617, and Inconel 740/740H. Uniaxial creep and novel structural tests and corresponding post-test analysis, which included physical measurements, simplified analytical analysis, and detailed microscopy, showed that different damage mechanisms may operate based on test conditions, alloy, and cold-strain levels. Overall, creep strength and ductility were reduced in all the alloys, but the degree of degradation varied substantially. The results support the current cold-strain limits now incorporated in ASME for these alloys for long-term A-USC boiler service.