Regularised linear regression (RLR) and recurrent neural network (RNN) data-driven models for the prediction of the high-temperature (850 & DEG;C, 950 & DEG;C) fatigue life of Alloy 617 are developed, trained, and tested using available experimental datasets. The predictions of these data-driven models are compared with the semi-empirical Coffin-Manson and Goswami models. It is shown that the data-driven models can match, or in some cases even outperform, the semi-empirical models while providing the advantage that they are temperature independent. However, it is also shown that these data-driven models cannot extrapolate accurately beyond the experimental data used for their development and training.

The accurate prediction of elevated-temperature creep behaviour of alloys is important for preventing catastrophic failure of systems operating under prolonged elevated temperature-stress conditions. Here, we couple the Kachanov-Rabotnov (K-R) creep model with a multi-objective genetic algorithm (MOGA) to predict the creep behaviour of Alloy 617 at 800 degrees C, 900 degrees C, and 1000 degrees C, under various stress conditions. It is shown that the MOGAoptimised K-R creep model can capture the overall elevated-temperature behaviour of the alloy at 800 degrees C under a wide range of stress conditions. However, at 900 degrees C and 1000 degrees C, oxidation leads to the atypical accumulation of creep plasticity, which the K-R model cannot account for. Nevertheless, it is shown that the proposed methodology of optimising the K-R model with a MOGA can consistently provide accurate results within the limits of the K-R model.

Non-contacting laser ultrasound is used to measure Young's modulus, shear modulus, and Poisson's ratio as functions of temperature for Alloys 617 and 709. These alloys are of interest for structural applications in advanced nuclear reactors. Design rules for the construction of elevated-temperature nuclear components are presented in Section III, Division 5 of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel (B&PV) Code. The ASME B&PV Code for each material provides Young's modulus values tabulated as a function of temperature and a single, temperature-independent Poisson's ratio value. Elastic properties characterized by laser ultrasonics for Alloy 617 are in good agreement with data collected using an American Society for Testing and Materials (ASTM) standard resonant frequency technique and values reported from literature, demonstrating its accuracy. The shear modulus and Poisson's ratio as functions of temperature for Alloy 709 have not been reported previously. Laser ultrasonics is well suited for determining temperature-dependent elastic properties, and the method offers the ability to perform measurements on small irregularly shaped specimens.

Alloy 709 (20Cr-25Ni-1.5Mo-Nb steel), a candidate structural alloy for Gen IV nuclear reactors, is expected to undergo microstructural evolution during long-term service at elevated temperatures, which in turn affects the mechanical behavior through changing contributions from precipitate and solute strengthening. For safe reactor design, it is important to understand how microstructural evolution and mechanical behavior differ between long-term service and accelerated laboratory tests, such as low cycle fatigue (LCF) and creep-fatigue. In this study, a combined electron microscopy and small angle neutron scattering (SANS) analysis has been performed to quantify and compare the precipitation evolution in the alloy after a range of static aging treatments (at 550 to 750 degrees C for 1-2500 h) and laboratory LCF and creep-fatigue tests (at 550 and 650 degrees C). The experimental precipitation quantification results from the static-aged material are found to be deviated from the equilibrium thermodynamic predictions. In addition, dynamic precipitation that occurs during fatigue testing leads to finer and more uniform distribution of carbide precipitates with a higher volume fraction compared to static aging under similar time and temperature conditions. Thus, deformation during accelerated testing results in a significantly different microstructure compared to that expected during service conditions where the accumulation of deformation occurs over a much longer period.

The modular high temperature reactor is a leading candidate for near-term deployment of an advanced reactor concept. The US Department of Energy supports the development of high temperature reactors through qualification of fuels, materials, and analysis methods that exploit the capabilities of the national laboratories (mainly Idaho, Oak Ridge, and Argonne) to generate the fundamental fuel, material, and core behavior data needed to support design and licensing efforts by vendors. Industrial vendors of high temperature reactor concepts provide input to the R&D program and engage in the design of the systems, structures, and components that are unique to their designs. Although there is considerable variation in the plant designs being developed, they all exploit the properties and behavior of the TRISO fuel, material, and analysis methods being tested in the DOE qualification programs. This paper summarizes the goals and recent progress of the Department of Energy's Gas-Cooled Reactor research and development campaign and how it supports a wide variety of industrial efforts to deploy high temperature reactors to serve modern energy markets.

Alloy 709 is a leading candidate structural material for sodium-cooled fast spectrum reactors (FSR), where creepfatigue is a potential failure mode. Low cycle fatigue (LCF) and creep-fatigue tests with 30 min tensile hold times have been conducted to failure at 550 and 650 degrees C, corresponding to potential service and accelerated test temperatures, respectively. Creep-fatigue life is reduced relative to LCF at both temperatures, though significantly more at 550 degrees C compared to 650 degrees C. Differences in slip behavior, dynamic recovery, precipitate evolution, and strain-aging effects result in less stress relaxation and sustained high tensile stresses in creep-fatigue at 550 degrees C; thus, there is more intergranular damage compared to 650 degrees C.

The Ni-Cr-Mo-Co material Alloy 617 is the leading candidate for VHTR intermediate heat exchangers operating above 750°C. Time-dependent properties are an important consideration in qualifying the alloy for construction of nuclear components. Creep behavior of several different heats of Alloy 617 has been evaluated in the temperature range of 800–1000°C. Power law creep behavior was observed for the minimum creep rate, with a stress exponent of 5.6 and activation energy of approximately 400kcal/Mol. The Monkman-Grant approach relating minimum creep rate to time to rupture gave a reasonable representation of the data for all of the testing with a slope of −0.84. Similarly, a modified Monkman-Grant fit the strain to failure data reasonably well. A Larson-Miller analysis was carried out to compare rupture behavior determined in the current experiments and historical data with well-known provenance over a wide range of conditions. It appears that the properties of modern heats of material are near the lower bound of rupture behavior when all of the data are considered in the same analysis.

Alloy 617 (UNS N06617) 52Ni-22Cr-13Co-9Mo is the leading candidate material for the intermediate heat exchanger for the very high temperature reactor (VHTR). An ASME Task Group on Alloy 617 Qualification has drafted a Code Case for Alloy 617 to allow construction of components conforming to the requirements of Section III, Division 5, Subsection HB, Subpart B “Elevated Temperature Service” for service when Service Loading temperatures exceed the temperature limits established in Subsection HB, Subpart A. There are two categories of allowable stress levels in Division 5, Subsection HB, Subpart B. The first category, identified as So, applies to Design Loadings and the second category, identified as Smt, applies to Service Loadings. Both categories are based on the lesser of time dependent and time independent properties. The time dependent properties control at higher temperatures where creep effects are significant. The Design condition time-independent allowable stresses are determined per the criteria given in Appendix 5 of Section II, Part D and are based on extrapolated 100,000-hour creep properties. The time dependent allowable stress values for Service Loadings, St, as a function of time and temperature, up to the maximum design lifetime and use temperature, are required for the applications of the Service condition limits. The criteria for establishing St for base metal for each specified time, t, are based on the lesser of 1. 100% of the average stress required to obtain 1% total strain 2. 80% of the minimum stress to cause the initiation of tertiary creep, 67% of the minimum stress to cause rupture 3. Minimum stress-to-rupture curves are also required in the applications of the Service condition limits. In this paper, the technical background for time dependent allowable stress values for Alloy 617 is presented. These values are being recommended to the ASME Code committees for approval, but are subject to change upon feedback from Code committees.

Alloy 617 is the leading candidate material for an intermediate heat exchanger for the very high temperature reactor (VHTR). As part of evaluating the behavior of this material in the expected service conditions, creep–fatigue testing was performed. The cycles to failure decreased compared to fatigue values when a hold time was added at peak tensile strain. At 850°C, increasing the tensile hold duration continued to degrade the creep–fatigue resistance, at least to the investigated strain–controlled hold time of up to 60 minutes at the 0.3% strain range and 240 minutes at the 1.0% strain range. At 950°C, the creep–fatigue cycles to failure are not further reduced with increasing hold duration, indicating saturation occurs at relatively short hold times. The creep and fatigue damage fractions have been calculated and plotted on a creep–fatigue interaction D–diagram. Test data from creep–fatigue tests at 800 and 1000°C on an additional heat of Alloy 617 are also plotted on the D–diagram.

Alloy 709 (UNS S31025) has shown enhanced performance, particularly in creep strength, over current generation high temperature structural materials, specifically Type 316 stainless steel and Grade 91. Accurate knowledge of the thermal properties of the material is important to design efficient systems and to allow calculation of thermally induced stresses and stress gradients. Thermal diffusivity, thermal conductivity, specific heat capacity, and coefficients of thermal expansion have been determined for Alloy 709 as a function of temperature in the range of 20-850 degrees C. The results do not vary significantly for the three heats of Alloy 709 tested, and compare well with those reported for other high Cr, high Ni austenitic stainless steels. Deviations from monotonic behavior have been observed for the specific heat capacity and to a lesser degree for the thermal conductivity, while the thermal expansion and thermal diffusivity increase nearly linearly with increasing temperature. (C) 2017 Elsevier B.V. All rights reserved.

Abstract This article focuses on the mechanisms, models, prevention, correction, and effects associated with decarburization inherited from semi-finished product processing prior to induction heating. It discusses the diffusion of carbon in austenitic iron, which has a face-centered cubic crystal structure that provides an interstitial path for the migration of the relatively small carbon atoms. The article describes the evolution of steel microstructure with progressive decarburization (in air) to a steady-state carbon gradient using an iron-iron carbide phase diagram. It provides useful information on the impact of alloying on vulnerability to decarburization, and the impact of decarburization on the mechanical properties of steels and cast irons. The article also describes the technological operations that potentially cause decarburization and the practical implications for induction hardening.

The flow stress of many materials is a function of the applied strain rate at elevated temperature. The magnitude of this effect is captured by the strain rate sensitivity parameter “m”. The strain rate sensitivity of two face–center cubic solid solution alloys that are proposed for use in high temperature heat exchanger or steam generator applications, Alloys 800H and 617, has been determined as a function of temperature over that range of temperatures relevant for these applications. In addition to determining the strain rate sensitivity, it is important for nuclear design within Section III of the ASME Boiler and Pressure Vessel Code to determine temperature below which the flow stress is not affected by the strain rate. This temperature has been determined for both Alloy 800H and Alloy 617. At high temperature the strain rate sensitivity of the two alloys is significant and they have similar m values. For Alloy 617 the temperature limit below which little or no strain rate sensitivity is observed is approximately 700°C. For Alloy 800H this temperature is approximately 650°C.

Alloy 617 is the leading candidate material for an Intermediate Heat Exchanger (IHX) of the Very High Temperature Reactor (VHTR). To evaluate the behavior of this material in the expected service conditions, strain-controlled cyclic tests that include hold times up to 9000s at maximum tensile strain were conducted at 950°C. The fatigue resistance decreased when a hold time was added at peak tensile strain, owing to the mechanisms resulting in a change in fracture mode from transgranular in pure fatigue to intergranular in creep–fatigue. Increases in the tensile hold duration beyond an initial value were not detrimental to the creep–fatigue resistance. An analysis of the evolving failure modes was facilitated by interrupting tests during cycling for ex situ microstructural investigation.

Key thermophysical properties needed for the successful design and use of Alloy 617 in steam generator and heat exchanger applications have been measured experimentally, and results are compared with literature values and results obtained from some other commercial Ni-Cr alloys and model materials. Specifically, the thermal diffusivity, thermal expansion coefficient, and specific heat capacity have been measured for Alloy 617 over a range of temperatures, allowing calculation of thermal conductivity up to 1000 degrees C. It has been found that the thermal conductivity of Alloy 617 exhibits significant deviation from monotonic behavior in the temperature range from 600 degrees C to 850 degrees C. the temperatures of interest for most heat transfer applications. The non-linear behavior appears to result primarily from short-range order/disorder phenomena known to occur in the Ni-Cr system. Similar deviation from monotonic behavior was observed in the solid solution Ni-Cr-W Alloy 230, and lesser deviations were observed in iron based Alloy 800H and an austenitic stainless steel. Measured thermophysical property data are provided for four different heats of Alloy 617, and it is shown that property variations between the four different heats are not significant. Measurements were also obtained from Alloy 617 that was aged for up to 2000h at 750 degrees C, and it was found that this aging treatment does not significantly influence the thermophysical properties. (C) 2013 Elsevier B.V. All rights reserved.

Abstract This article describes the metallurgy and process specifics of subcritical annealing, which involves heating below the lower critical temperature such that austenite does not form during subcritical annealing. It provides information on the nominal subcritical annealing temperatures of plain carbon, low-alloy and high-alloy steels and temperature-time relations of subcritical annealing. Practical implications for induction annealing and induction normalizing are included. The article concludes by describing induction softening, which softens the threaded area on carburized components such as hypoid pinion gears, to prevent the occurrence of delayed fractures from occurring.

The low cycle fatigue behavior of Alloy 617 has been evaluated at 850 °C and 950 °C, the temperature range of particular interest for the intermediate heat exchanger on a proposed high-temperature gas-cooled nuclear reactor. Cycles to failure were measured as a function of total strain range and varying strain rate. Results of the current experiments compare well with previous work reported in the literature for a similar range of temperatures and strain rate. The combined data demonstrate a Coffin–Manson relationship, although the slope of the Coffin–Manson fit is close to −1 rather than the typically reported value of −0.5. At 850 °C and a strain rate of 10−3 /s Alloy 617 deforms by a plastic flow mechanism in low cycle fatigue and exhibits some cyclic hardening. At 950 °C for strain rates of 10−3–10−5 /s, Alloy 617 deforms by a solute drag creep mechanism during low cycle fatigue and does not show significant cyclic hardening or softening. At this temperature the strain rate has little influence on the cycles to failure for the strain ranges tested.

The term “drawing” is used to describe a number of metallurgical processing operations, and when searching titles in the metalworking or intellectual property literature. This chapter describes the concept of drawing, which involves pulling wire, rod, or bar through a die, or converging channel to decrease cross-sectional area and increase length. In the drawing process, a pulling force and a pressure force, from the die, combine to cause the wire to extend and reduce in cross-sectional area, while passing through the die. Because of this combined effect, the pulling force or drawing force can be less than the force that would cause the wire to stretch, neck, and break downstream from the die. The use of pulling or pushing forces, together with dies or rolls, is common to many deformation processes. In addition to the die, held in a die block, a basic drawing operation involves a payoff and a take-up. Also necessary is a system for applying lubricant to the wire before it enters the die. With liquid lubrication, the lubricant may be directed in a stream at the die entry, and the drawing system may even be submerged in lubricant. A drawing operation must have a method for pointing the wire. Pointing involves reducing the “front” end diameter of the wire sufficiently to allow it to be initially passed through the die and gripped en route to initial winding onto the take-up.

Any operation that plastically deforms the wire after final wire drawing is considered wire forming. Figure 18.1 illustrates the geometry basic to the plane of symmetry of an initially straight wire section subject to bending. Figure 18.2 illustrates the geometry basic to the twisting of an initially straight and untwisted wire section. Stretching in wire forming may be successfully undertaken to the extent of uniform elongation and no further. Upsetting is defined as compressive plastic deformation, on a section of rod or wire, in the axial direction. Swaging is defined as compressive plastic deformation, on a section of wire, perpendicular to the axial direction. Extrusion involves the pushing of a workpiece of a given cross-sectional area into a die or tool resulting in an extrudate of reduced cross section.

The drawing of shaped bar, rod, and wire stock with one-piece dies is an important near-net-shape technology, offering sizing, dimensional control, and surface quality not generally achievable with rolling and extrusion. However, unpowered two-roll and four-roll systems (called “turksheads”) involve minimal friction and offer much greater tooling flexibility, with a direct analogy to drawing. Driven roll systems (powered turksheads) avoid the necessity of pulling the rod and are directly analogous to basic rolling. In all rolling systems one has to consider roll gap geometry and the relative tendency for the workpiece to spread transversely, as opposed to its basic tendency to elongate.