Hot Isostatic Pressing (HIP) of type 316L austenitic stainless steel powder has been an established manufacturing practice for more than twenty five years within the oil and gas sectors and more recently in the naval defence industry. The successful ASME Code Case approval (N-834) has facilitated the manufacture of 316L components via Powder Metallurgy HIP (PM/HIP) for the civil nuclear sector. However, a number of issues have tended to hinder the uptake of PM/HIP as an alternative viable manufacturing route for both castings and forgings. Firstly, the powder specification for 316L and HIP processing parameters has typically been left to the discretion of the manufacturers. As such, the finer details of HIP product specification require greater clarity and definition for optimum performance/reproducibility. Secondly, comparison of historical data for 316L PM/HIP has shown variation in the Charpy impact toughness performance. These differences have been attributed to the oxygen content of the atomised powder, with greater oxygen contents yielding product with reduced impact properties. Based on these factors, a systematic study of the current state of the art of 316L commercial powder production, encapsulation/consolidation and selected HIP parameters was undertaken in collaboration with the Electric Power Research Institute (EPRI). A 316L powder specification was developed that primarily limited the oxygen content of the powder to under 130ppm. This lower oxygen limit reflects the improvements that commercial powder suppliers have been making over the past decade to ensure greater powder cleanliness. The test programme generated a significant body of test data based on 3 × 3 × 3 matrix of: powder supply, HIP service provider and HIP sustain times. The results were excellent across the full range of variables studied with all test billets passing the specification requirements of ASTM A988 and additional imposed requirements. Very consistent 316L material properties were produced for billets manufactured via differing HIP service providers across the comprehensive destructive test programme. This demonstrates the robustness and uniformity of the PM/HIP supply chain in producing 316L material of the requisite quality. In addition, no significant difference in material properties was noted for material pressed between 2–8 hours hold time, suggesting that the HIP process window is large with respect to hold time. Of significant note was that material produced with one powder yielded material with consistently the highest strengths and Charpy impact toughness. This has been attributed to chemical composition of the powder, which featured both a low oxygen and also a high nitrogen content.

Hot Isostatic Pressing (HIP) of type 316L stainless steel powder has been an established manufacturing practice for more than twenty-five years in the oil and gas sector and more recently in the naval defence sector. To demonstrate the capability of the powder metallurgy HIP (PM/HIP) for nuclear power applications a systematic study of 316L commercial powder production, encapsulation/consolidation providers and selected HIP parameters was undertaken by the Nuclear AMRC in collaboration with the Electric Power Research Institute (EPRI). In the study, the 316L powder specification limited the oxygen content of the powder to under 130 parts per million (ppm), which reflects the improvements that commercial powder suppliers have been making over the past decade to ensure greater powder cleanliness. The test programme assessed powder supply, HIP service provider and HIP sustain time. Excellent test results were achieved across the full range of variables studied with all billets meeting the specification requirements of ASTM A988 and additional requirements imposed based on nuclear manufacturing standards. Significantly, the study demonstrated the robustness of the PM/HIP supply chain, as material produced via differing HIP service providers resulted in very consistent material properties across the destructive test programme. Furthermore, no significant difference in material properties were noted for material HIP’ed between 2–8 hours hold time, suggesting that the HIP process window is large. Both these results are significant from an end-user standpoint as they highlight the uniformity of the process through the full manufacturing cycle from powder procurement to destructive testing. Despite all material passing specification requirements, some property variation was noted for differing powder suppliers. Considering the systematic approach, this was attributed to powder composition, with both low oxygen and high nitrogen contents contributing to improvements in Charpy impact strength and tensile strength respectively.

Dissimilar metal nozzle-to-safe end welds represent a complex manufacturing operation for nuclear reactor fabrication. Transitioning from a low alloy steel nozzle to corrosion-resistant reactor coolant system piping systems requires weld buttering, an intermediate heat treatment, and a dissimilar metal weld between the nozzle buttering and safe-end. The process can be both time consuming and costly, and often provides difficulties for inspection of welds in the field. Through the use of powder metallurgy and hot isostatic pressing (PM-HIP), a series of transition joint configurations have been evaluated that could reduce the complexity of nozzle-to-safe end fabrication by eliminating fusion welding from the process. Test coupons of SA508 low alloy steel have been joined via PM-HIP to either 316L SS or 347NG SS using various powder metal interlayer materials (Alloy 82, Nb-modified Alloy 600 (600M), Alloy 690, and 316L SS). The microstructural evolution and mechanical performance of the joints have been evaluated following a post-HIP solution anneal, quench and temper heat treatment. Particular attention was given to evaluating bond-line regions via Charpy V-notch impact testing, tensile testing, microhardness, optical microscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Results demonstrate that multiple transition layer configurations are capable of meeting the acceptable tensile and impact energy specifications of the two parent base material substrates. Interface impact toughness and ductility were dependent upon secondary phase precipitation within the diffusion region of the joints and presence of non-metallic inclusions. This assessment demonstrates that acceptable joint performance is achievable through proper material selection and should be considered for DMW applications in the future.

Low pressure electron beam welding offers the prospect of large increases in productivity for thick section welds in RPV steels. However, it is important to understand how this welding process affects the structural performance of the completed weld. This paper reviews and presents key results from a programme of weldment manufacture, materials characterisation, residual stress measurements, and finite element modelling of EB welds made in plate of three thicknesses, 30mm, 130mm, and 200mm, and in three steels: SA508 Gr 2, SA508 Gr 3 C1 1, and SA516 Gr 70.

For more than 60 years now, the nuclear power industry has relied on structural and pressure retaining materials generated via established manufacturing practices such as casting, plate rolling-and-welding, forging, drawing, and/or extrusion. During the past three years, EPRI has been leading the development and introduction of another established process, powder metallurgy and hot isostatic pressing (PM/HIP), for pressure retaining applications in the electric power industry. The research includes assessment of two primary alloys: 316L stainless steel and Grade 91 creep-strength enhanced ferritic steels, for introduction into the ASME Boiler and Pressure Vessel Code. Continuing DOE and EPRI research on other structural/pressure retaining alloys such as Alloy 690, SA 508 Class 1, Alloy 625, hard-facing materials, and others are also underway. This research will have a tremendous impact as we move forward over the next few decades on the selection of new alloys and components for advanced light water reactors and small modular reactors. Furthermore, fabrication of high alloy materials/components may require the use of new manufacturing processes to achieve acceptable properties for higher temperature applications such as those in Generation IV applications. Current research by EPRI and DOE will be reviewed and emphasis will be targeted at advanced applications where PM/HIP may be applied in the future.

The useful life of a steam turbine and the establishment of turbine outage schedules are often determined by corrosion fatigue to the low pressure (LP) blades in the phase transition zone (PTZ). Developing an effective corrosion damage prediction methodology is an important step to successfully reduce the number of unscheduled steam turbine outages. Tests with dual certified 403/410 12% Cr martensitic steel were performed to quantify the influence of corrosion pits on the fatigue life during testing in environments that are comparable to operational conditions. Threshold stress intensity factors ΔK th and fatigue limits Δσ 0 were determined in air and two aqueous solutions. Additionally, stress-life tests were performed with pre-pitted specimens in air and aqueous solutions. The data for transition from a pit-to-a-crack have been correlated using the Kitagawa Diagram. This presentation of the data relates the steady stress, cyclic stress and pit width to the prediction of fatigue failure. Ultrasonic fatigue testing was an essential aspect of this program. This testing technique makes it possible to accumulate cycles at a rate of approximately 20 kHz. At this rate one billion (10 9 ) cycles are accumulated in less than 14 hours. One billion cycles has been used as the definition for non-progressive crack or specimen run-out life. All of the data for the survival and failure stress intensity factor was well represented by the El Haddad refinement to the Kitagawa Diagram. Based on these test results a comprehensive methodology has been developed to quantify the risk of corrosion-fatigue failure at a pit.

Grade 92 steel is a class of the Creep Strength-Enhanced Ferritic (CSEF) steels developed for use in boilers and piping systems of ultra-supercritical steam fossil power plants. Although creep strength is a primary concern, consideration of the interaction of creep and fatigue damage is also important in evaluating the integrity of components as they will experience a range of cyclic loading. Although some studies have already been made on creep-fatigue behavior of this steel, test data under the conditions of creep damage dominance more relevant to plant evaluation, need to be supplemented. Girth welds often constitute critical locations dominating the integrity of piping systems and their creep-fatigue behavior is also of significant importance. Such a situation prompted the authors to initiate a study aiming at development of an extensive database on creep-fatigue behavior of base metal and welded joints of Grade 92 steel and establishment of an appropriate life estimation procedure. For the period of one and half year, a number of creep-fatigue data have been obtained on the base metal and cross-weld specimens at a wide range of loading conditions. Superiority of the energy-based approach to the conventional time fraction or ductility exhaustion approach for predicting creep-fatigue life was confirmed by their application to these creep-fatigue tests.

Failure under creep-fatigue interaction is receiving increasing interest due to an increased number of start-up and shut-down in fossil power generation plants as well as development of newer nuclear power plants employing low-pressure coolant. These situations have promoted the development of various approaches for evaluating its significance. However, most of them are fragment and rather limited in terms of materials and test conditions they covered. Therefore applicability of the proposed approaches to different materials or even different temperatures is uncertain in many cases. The present work was conducted in order to evaluate and compare the representative approaches used in the prediction of failure life under creep-fatigue conditions as well as their modifications, by systematically applying them to available test data on a wide range of materials which have been used or are planned to be used in various types of power generation plants. The following observations have been made from this exercise. (i) Time fraction model has a tendency to be unconservative in general, especially at low temperature and small strain range. Because of the large scatter of the total damage, this shortcoming would be difficult to cover by the consideration of creep-fatigue interaction in a fixed manner. (ii) Classical ductility exhaustion model showed a common tendency to be overly conservative in many situations, especially at small strain ranges. (iii) The modified ductility exhaustion model based on the re-definition of creep damage showed improved predictability with a slightly unconservative tendency. (iv) Energy-based ductility exhaustion model developed in this study seems to show the best predictability among the four procedures in an overall sense although some dependency on strain range and materials was observed.

Repairs of gas turbine buckets (blades) are currently limited to the upper tip region of the airfoil where operating stresses are commonly low. This limitation is predicated upon the use of low strength welding filler materials and the use of high energy welding processes such as gas tungsten arc welding (GTAW). When bucket damage is located within parts of the airfoil subjected to higher stress levels, buckets are often scrapped or replaced, costing utilities/independent power producers millions of dollars. Advanced repair techniques which allow repairs at higher stress regions of the bucket airfoil will significantly reduce the need and costs of scrapping buckets. “Structural” repairs to higher stress areas of the airfoil will require the use of welding fillers with similar composition and strength to that of the bucket base alloy and improvements in repair processing.