This study examines the corrosion resistance of additively manufactured 316L stainless steel (SS) for nuclear applications across three environments: pressurized water reactor primary water (PWR PW), hot concentrated nitric acid, and seawater. Wire-feed laser additive manufacturing (WLAM) specimens showed oxidation behavior similar to wrought 316L SS in PWR PW, though stress corrosion cracking (SCC) susceptibility varied with heat treatment. In nitric acid testing, laser powder bed fusion (L-PBF) specimens demonstrated superior corrosion resistance compared to conventional SS, primarily due to improved intergranular corrosion resistance resulting from cleaner feedstock powder and rapid solidification rates that minimized grain boundary segregation. Laser metal deposition (LMD) repair studies in seawater environments successfully produced dense, crack-free repairs with good metallurgical bonding that matched the substrate’s mechanical properties while maintaining corrosion resistance. These results emphasize the importance of corrosion testing for additively manufactured components and understanding how their unique microstructures affect performance.

This research compares creep crack growth behavior of two heats of creep strength enhanced ferritic (CSEF) steel, grade 91. These heats represent extremes of creep damage susceptibility, one heat exhibiting low creep ductility and the other high creep ductility. Creep crack growth tests were performed with compact tension specimens and were monitored with direct current potential drop and optical surface measurements. Load line displacement was measured throughout the duration of the tests. Specimens were sectioned, mounted, and analyzed using optical and scanning electron microscopy to assess the presence of oxidation, micro-cracking, creep damage, and void density. Tests were performed over a range of initial stress intensities on the low ductility material to investigate the impact of creep ductility. Metallurgical evidence and test data for each crack growth test was assessed to evaluate crack growth behavior linked to creep crack growth parameter (C*) and stress/creep damage distribution in the vicinity of the crack.

NAC International Inc. (NAC) is providing transportable storage canisters (TSCs) to Central Plateau Cleanup Company CPCCo) for long term dry storage of capsulized radioactive waste at the Hanford Site in Richland, WA. The TSC consists of 316/316L stainless-steel components welded to form a cylindrical canister that acts as a confinement boundary for the payload. The heat affected zones of the welded areas are most susceptible to Chloride Induced Stress Corrosion Cracking (CISCC), that may limit the life of the TSC. To mitigate CISCC during the anticipated 300-year storage period, an overcoating is applied to the heat affected zones of all external TSC fabrication welds, referred to as Cold Spray. This paper will discuss the purpose, development, and application of Cold Spray to the CPCCo TSCs. Cold Spray is a process whereby metal powder particles are deposited upon a substrate by means of ballistic impingement via a high-velocity stream of gas, resulting in a uniform deposition with minimal porosity and high bond strength. Temperatures are below the melting thresholds of many engineering materials enabling a large variety of application uses. NAC developed a process for Cold Spray application onto the 316/316L stainless-steel TSCs to serve as a CISCC protective/mitigative coating for its canister products. Testing during development arrived at nickel as the deposited coating material and nitrogen as the gas vehicle, along with a set of various application parameters. The qualified process was implemented onto the CPCCo TSCs. Prior to application, the equipment and process are validated via coupons that are sprayed and then tested to meet requirements for adhesion strength (ASTM C633) and porosity (ASTM E2109). After successful coupon testing, Cold Spray is performed on the external TSC fabrication welds, to include heat affected zones. Acceptance testing of the resulting deposition is performed via visual inspection.

For the safe life prediction of components under high cycle fatigue loading at high temperature, such as gas turbine blades and turbocharger components, the behavior of initial defects, which are physically short cracks below the long crack threshold ΔK is of crucial importance. The evolution of different crack closure mechanisms (such as plasticity, roughness and oxide induced crack closure) can lead to crack arrest by a reduction of the effective crack tip loading. To visualize the crack growth behavior of such cracks, cyclic crack resistance curves (cyclic R-curves) are used. The experimental determination of cyclic R-curves is challenging, especially under high temperature conditions due to a lack of optical accessibility. The formation of very short cracks in high strength materials makes it even more complicated to reliably determine these data. Within this study the crack growth behavior of physically short fatigue cracks in three different material states of the nickel alloy IN718 (wrought, cast and PBF-LB/M - processed) is experimentally determined at 650 °C. Based on a load increase procedure applied on Single Edge Notched (SEN) specimens with a compression pre-cracking procedure in advance, crack propagation of physically short cracks is measured with alternating current potential drop systems in air and under vacuum conditions. These examinations are carried out for three different load ratios (R = -1, 0 and 0.5) to investigate the amount of certain crack closure mechanisms active under different loading conditions. Moreover, the formation of a plastic wake along the crack flanks is determined by a finite element simulation. The results determined in air and under vacuum conditions are used to describe the impact of oxide induced crack closure on the behavior of physically short cracks. This allows the evaluation of the behavior of both near-surface and internal defects that are not accessible to the atmosphere.

High-performance Ferritic (HiperFer) steels represent a promising materials innovation for next-generation thermal energy conversion systems, particularly in cyclically operating applications like concentrating solar thermal plants and heat storage power plants (Carnot batteries), where current market adoption is hindered by the lack of cost-effective, high-performance materials. HiperFer steels demonstrate superior fatigue resistance, creep strength, and corrosion resistance compared to conventional ferritic-martensitic 9-12 Cr steels and some austenitic stainless steels, making them potentially transformative for future energy technologies. This paper examines the microstructural mechanisms underlying HiperFer’s enhanced fatigue resistance in both short and long crack propagation, while also presenting current findings on salt corrosion properties and exploring potential alloying improvements for fusion reactor applications, highlighting the broad technical relevance of these innovative materials.

The steam generation systems (SGS) of concentrated solar power (CSP) plants employ multiple heat exchangers arranged in series to convert thermal energy collected from the sun via a heat transfer fluid (HTF) to produce superheated steam in the Rankine cycle. Common CSP plant designs are based either on parabolic trough or central tower technology. The major Rankine cycle components consist of preheaters, evaporators, steam drums, superheaters, steam turbines, and water/air-cooled condensers, all connected through steel piping. For CSP plants capable of reheating the steam for improved efficiency, reheaters are also included in the Rankine cycle. In central tower design with directly heated water as the HTF, the receiver can also be considered part of the Rankine cycle. Operating experiences of CSP plants indicate that plant reliability is significantly impacted by failures in various components of the Rankine cycle. Many damage mechanisms have been identified, which include corrosion, thermal fatigue, creep, and stress corrosion cracking, among others. Much of the damage can be attributed to poor water/steam chemistry and inadequate temperature control. While damage in the Rankine cycle components is common, there is generally lack of comprehensive guidelines created specifically for the operation of these CSP components. Therefore, to improve CSP plant reliability and profitability, it is necessary to better understand the various damage mechanisms experienced by linking them to specific operating conditions, followed by developing a “theory and practice” guideline document for the CSP operators, so that failures in the Rankine cycle components can be minimized. In a major research project sponsored by the U.S. Department of Energy (DOE), effort is being undertaken by EPRI to develop such a guideline document exclusively for the CSP industry. This paper provides an overview of the ongoing DOE project along with a few examples of component failures experienced in the Rankine cycle.

Extended storage of spent nuclear fuel (SNF) in intermediate dry cask storage systems (DCSS) due to lack of permanent repositories is one of the key issues for sustainability of the current domestic Light Water Reactor (LWR) fleet. The stainless steel canisters used for storage in DCSS are potentially susceptible to chloride-induced stress corrosion cracking (CISCC) due to a combination of tensile stresses, susceptible microstructure, and a corrosive chloride salt environment. This research assesses the viability of the cold-spray process as a solution to CISCC in DCSS when sprayed with miniature tooling within a characteristic confinement in two different capacities: cleaning and coating. In general, the cold-spray process uses pressurized and preheated inert gas to propel powders at supersonic velocities, while remaining solid-state. Cold-spray cleaning is an economical, non-deposition process that leverages the mechanical force of the propelled powders to remove corrosive buildup on the canister, whereas the cold spray coating process uses augmented parameters to deposit a coating for CISCC repair and mitigation purposes. Moreover, both processes have the potential to induce a surface compressive residual stress that is known to impede the initiation of CISCC. Surface morphology, deposition analysis, and microstructural developments in the near-surface region were examined. Additionally, cyclic corrosion testing (CCT) was conducted to elucidate the influence of cold-spray cleaning and coating on corrosion performance.

In this paper, the dissimilar metal welds (DMWs) between 617B nickel-based alloy and 10%Cr martensitic heat-resistant steel filled by 617 filler metal was studied, focused on the high temperature creep rupture properties. The high temperature creep rupture properties of welded joints with different welding processes were tested, and the microstructure of welded joints before and after the creep rupture test was observed by OM and SEM. The results showed that, there were three failure modes: base metal failure, type W failure and interface failure, among which interface failure caused the most serious life reduction. The welded joints using ER NiCr-3 filler metal reduced the strain concentration at the interface, so the fracture location shifted from the interface to HAZ of 10%Cr martensitic heat-resistant steel under high temperature and low stress conditions, and creep rupture life was improved. Similarly, weld cap shifted the creep crack propagation path by changing the groove form, so as to altered the stress state of joint and prolong the creep rupture life.

The incore instrumentation system of a pressurized water reactor (PWR) facilitates neutron flux mapping and temperature measurements at specific core locations. A guide conduit, extending from the seal table to the lower reactor pressure vessel head, guides and protects each incore guide thimble between the table and the lower reactor vessel head. Each flux thimble houses a detector and drive cable. Once filled with reactor coolant, the conduit becomes an extension of the reactor coolant pressure boundary. This paper reports the examination results of cracking detected in a TP304 stainless steel guide conduit adjacent to a fillet weld at the upper surface of a TP304 seal table. The cracking resulted in reactor coolant leakage that was detected by the presence of boric acid deposits on the exterior of the conduit and table. Failure analysis including dimensional measurements, chemical analysis, stereomicroscopy, metallography, and scanning electron microscopy showed that extensive cracking of the conduit and seal table material occurred due to stress corrosion cracking (SCC). Assessment showed that chlorine-containing deposits were present on the exterior of the conduit and on the surfaces of the seal table and were due to the design and operation of HVAC systems at the coastal plant. Stainless steels are susceptible to SCC in environments with elevated temperatures, chloride contents, and increased tensile stress – particularly in non-post weld heat treated (PWHT) weld regions and the heat affected zone (HAZ). This was the apparent primary cause of the failure. However, chloride-induced SCC of such materials typically results in transgranular crack propagation, whereas the observed cracks were indicative of intergranular stress corrosion cracking (IGSCC). Microstructural analysis showed that the observed cracks initiated in sensitized areas of material adjacent to the weld. Sensitization of the material caused chromium depletion from adjacent areas and increased susceptibility of the depleted areas to IGSCC. In this case, the most probable source of sensitization was related to welding and the long-term growth of grain boundary carbides nucleated during welding. This was considered a contributing cause to the failure.

Supercritical carbon dioxide cooling during machining has been identified as an effective measure to mitigate the risk of stress corrosion cracking in materials utilized in the primary circuit of light water reactors, particularly in pressure vessel structural steels. This study aims to compare two different cooling methods, the novel supercritical carbon dioxide and conventional high pressure soluble oil, employed during both milling and turning processes for SA508 Grade 3 Class 2 and AISI 316L steels. As the surface conditions of materials are critical to fatigue properties, such as crack initiation and endurance life, the fatigue performance of both cooling methods for each process were then evaluated and the impact on properties determined. To compare the potential benefits of supercritical carbon dioxide cooling against conventional soluble oil cooled machining, fatigue specimens were machined using industry relevant CNC machine tools. Surface finish and machining methods were standardized to produce two different specimen types, possessing dog- bone (milled) and cylindrical (turned) geometries. Force-controlled constant amplitude axial fatigue testing at various stress amplitudes was undertaken on both specimen types in an air environment and at room temperature using a stress ratio of 0.1. The fatigue performance of the supercritical carbon dioxide cooled specimens revealed substantially greater endurance lives for both SA508 and 316L materials, when compared with specimens machined using high pressure soluble oil cooling.

Future, flexible thermal energy conversion systems require new, demand-optimized high-performance materials. In order to provide a basis for the targeted development of fatigue-resistant, cost-effective steel grades, the microstructural damage to materials and the failure of conventional and novel steels were investigated in thermo-mechanical fatigue and fatigue crack propagation experiments. Based on the results, improved, ferritic “HiperFer” (High performance Ferrite) steels were designed, produced and characterized. A brief description of the current state of development is given.

Cast nickel-based superalloys used as structural materials for gas turbine parts need to withstand high temperatures and dynamic mechanical loads. When in contact with ambient air, the formation of protective oxide scales causes a depletion of γ’-precipitates in the surface-near region and leaves a weakened microstructure. This environmentally based degradation of the material might be accelerated under cyclic thermal exposure. In this paper, the cyclic oxidation behavior of two cast nickel-based superalloys and one single crystalline variant are investigated: C1023, CM-247 LC and M-247 SX. Exposure tests were carried out under both isothermal and cyclic conditions in air at 850 °C, 950 °C and 1050 °C for times up to 120 h to investigate the impact of thermal cycling. The differences in oxidation mechanisms are analyzed phenomenologically via light and electron microscopy and brought in correlation with the oxidation kinetics, determined based on net mass change and depletion zone growth. An assessment of the impact of precipitation loss on local mechanical strength is attempted via nano-indentation method. The found relations can be transferred onto an acceleration of crack growth under creep-fatigue and thermo-mechanical fatigue conditions.

In this study, fatigue crack propagation behavior at lower temperature in single crystal nickel-base superalloys was investigated experimentally and analytically. Four types of compact specimens with different combinations of crystal orientations in loading and crack propagation directions were prepared, and fatigue crack propagation tests were conducted at room temperature and 450°C. It was revealed in the experiments that the crack propagated in the shearing mode at room temperature, while the cracking mode transitioned from the opening to shearing mode at 450°C. Both the crack propagation rate and the transition behavior were strongly influenced by the crystallographic orientations. To interpret these experimental results, crystal plasticity finite element analysis was carried out, taking account some critical factors such as elastic anisotropy, crystal orientations, 3-D geometry of the crack plane and the activities of all 12 slip systems in the FCC crystal. A damage parameter based on the slip plane activities derived from the crystal plasticity analysis could successfully rationalize the effect of primary and secondary orientations on the crystallographic cracking, including the crack propagation paths and crack propagation rates under room temperature. The proposed damage parameter could also explain the transition from the opening to crystallographic cracking observed in the experiment under 450°C.

Detailed knowledge of the creep and creep crack behavior is essential for a safe operation of thick-walled components in thermal power plants. High mechanical loads and temperatures of more than 700 °C often require the application of nickel-based alloys, e.g. alloy C-263. Unfortunately, manufacturing and non-destructive evaluation (NDE) of thick-walled components (> 50 mm) made of nickel-based alloys are quite challenging. Tolerable critical flaw sizes, experimentally validated for long service durations, play an important role in the quality assurance of such components. It is commonly accepted that manufacturing parameters, e.g. heat treatment procedures, have a significant influence on creep ductility and time-dependent crack behavior. By means of adjusting the process parameters, the ductility and the creep life of notched specimen can be significantly improved in the case of alloy C-263. Essential root cause is the decoration of grain boundaries with carbides which drastically influences creep crack initiation and growth. This results in significant differences for allowable critical flaw sizes and thus, the potential use of the candidate material. On a first generation of alloy C-263 “G1”, a dense population of carbides on the grain boundaries was found, which resulted in an inadmissible creep crack behavior. The resulting critical flaw sizes were only a few tenths of a millimeter. On a second generation “G2”, the grain boundary occupation was positively influenced, so that a satisfactory creep crack behavior could be found. The critical flaw sizes are in the order of one millimeter or more. A critical or impermissible material behavior under creep conditions can be demonstrated by testing smooth and notched round specimens. For example, the first generation “G1” notched round specimens fails earlier than the smooth round specimens, indicating notch sensitivity. On the second generation “G2”, however, a notch insensitivity was found. The critical defect sizes can be determined by a method that takes into account a simultaneous examination of the crack tip situation and the ligament situation.

Single crystal Ni-base superalloys are subjected to tension hold at high temperature in addition to cyclic loading during the operation of gas turbines. Various studies have investigated creep-fatigue crack propagation in superalloys under trapezoidal loadings and evaluated the life time based on parameters such as creep J-integral. However, it is still unclear how damage field and stress-strain condition change at the crack tip during hold time, and how it affects on fatigue crack propagation. In this study, the influence of the tension hold and accompanying creep at crack tip on subsequent fatigue crack propagation behavior was evaluated by introducing single tension holds into pure cyclic loadings. The series of the experiments revealed that because of the tension hold, material degradation and stress relaxation occurred simultaneously ahead of crack tip. In the region where material was degraded, the resistance against crack propagation was reduced, while in the region where stress was relaxed, the crack driving force was lowered.

Stress corrosion cracking (SCC) is a potential risk in structural steels used for steam boilers. To investigate the effect of dissolved oxygen (DO) on SCC susceptibility, three steels, T23, T24 and T91 were annealed at 1065°C and then quenched to create a susceptible microstructure and then exposed in a Jones test to stagnant and circulating water at 200°C with varying DO levels. The results indicated that among the tested steels, the SCC susceptibility was highest in T91 but lowest in T23 which did not exhibit crack initiation with 100 ppb DO. T24 showed no cracking with 50 ppb DO but cracked with 100 ppb DO under these conditions. Based on these results, the next planned step is to monitor crack growth in-situ and determine a critical DO content for each material.

There are main drivers for the design and assessment of steam turbine components of today such as demands for improved materials, higher plant cycling operation, and reduced life-cycle costs. New materials have been developed over the last decades resulting in advanced martensitic 9-10CrMoV steels already applied in different types of turbines successfully. Heavy cyclic loading getting more importance than in the past results in utilization of the fatigue capabilities at high and low temperatures which might lead to crack initiation and subsequent crack propagation. Fracture mechanics methods and evaluation concepts have demonstrated their applicability to assess the integrity of components with defects or crack-like outage findings. Based on realistic modelling of the failure mechanism, accurate prediction of crack sizes at failure state can be improved defining the appropriate damage criteria. Ductility is a main aspect for robust design but its value definition can depend on component type, design rules, real loading conditions, service experience, and material characteristics. The question which direct material parameter is able to serve as limit value in design and how it can be determined has to be solved. Examples of advanced analysis methods for creep crack growth and fatigue interaction involving the crack initiation time show that the reserves of new martensitic 9-10Cr steels in high temperature application can be well quantified. The creep rupture elongation A u and the loading conditions in the crack far field are main factors. If the A u value is sufficient high also after long-time service, the material remains robust against cracks. Investigations into the influence of stress gradients on life time under fatigue and creep fatigue conditions show that e.g. for 10CrMoWV rotor steel crack growth involvement offers further reserves. The consideration of constraint effect in fracture mechanics applied to suitable materials allows for further potentials to utilize margin resulting from classical design. The new gained knowledge enables a more precise determination of component life time via an adapted material exploitation and close interaction with advanced design rules.

Continuous and extensive works have been going to develop 700°C A-USC (Advanced Ultra Super Critical) power plants worldwide. Since Japanese national project launched in 2008, Ni based alloy HR6W (45Ni-24Fe-23Cr-7W-Ti, ASME Code Case 2684) was selected as one of the promising candidate materials of A-USC boiler tube and pipe for long-term creep strength evaluation and field exposure test. In the present study, to establish the creep damage and life assessment method for Ni based alloy component, long-term creep rupture properties, microstructural stability, and creep damage morphology of HR6W weldment were experimentally investigated. Creep tests of HR6W weldment were conducted at temperature range of 700 to 800°C for durations up to 70,000 hours. Failure behavior of creep void formation and creep crack growth was identified, and damage mechanism of weldment during creep were discussed and characterized. Furthermore, uniaxial interrupted creep tests were carried out, the creep damage evaluation was conducted and life assessment approach was proposed based on the metallographic quantification evaluation of creep void and microstructure evolution. It demonstrated the possibility and validity to evaluate creep damage of Ni based alloy component with creep void and microstructure parameters.

Alumina-forming austenitic stainless steels (AFAs) are potential materials for boiler/steam turbine applications in next generation fossil fuel power plants. They display a combination of good high temperature creep strength, excellent oxidation resistance and low cost. A recently-developed AFA alloy based on Fe-14Cr-32Ni-3Nb-3Al-2Ti (wt.%) shows better creep performance than a commercially-available Fe-based superalloy. In this paper we used scanning electron microscopy and transmission electron microscopy to study the fracture surfaces and cracking behavior in relation to the precipitates present in creep failure samples of this alloy tested at either 750°C/100 MPa or 700°C/170 MPa. It was found that most cracks are formed along the grain boundaries with precipitate-free zones beside the grain boundaries potentially providing the path for propagation of cracks.

The mechanisms of recent cracking failures of HR3C super heater pipes of a fossil power plant in the Netherlands were investigated. Initial failure investigations showed that pitting corrosion of the sensitized HR3C initiated subsequent stress corrosion cracking (SCC). It was concluded that magnesium chloride hydrates from condensed seawater had initiated pitting corrosion as well as SCC similar to the standard ASTM G36 SCC test. By experimental application of the ASTM G36 procedure, this tentative mechanism is reproduced and confirmed by a series of laboratory tests with pure magnesium chloride as well as with synthetic seawater. It included the effects of temperature, magnesium chloride concentrations of the evaporating water and applied bending moments on cracking. As a result for the 175h testing period in MgCl2*6H 2 O cracking increases significantly above 100°C up to 120°C but is reduced slightly at temperatures up to 155°C. With increasing bending moments, the U-shaped test pieces revealed increasing crack depths up to total fracture of the 5mm thick sections. Lower magnesium chloride concentrations as in concentrated seawater provided identical cracking, however, to a lower extent. It is therefore concluded that the operational failure of the sensitized HR3C super heater pipes was initiated in presence of condensed seawater and followed the same mechanism as found in the experimental investigation. As a conclusion, the presence of seawater saturated air at temperatures between 100° and 155°C should be avoided.