Thick-walled valves, steam chests, and casings suffer service damage from thermal stresses due to the significant through-thickness temperature gradients that occur during operating transients. Fatigue is the primary damage mechanism, but recent examination of turbine casings has revealed extensive sub-surface creep cavitation. The low primary stress levels for these components are unlikely to cause creep damage, so detailed inelastic analysis was performed to understand the complex stress state that evolves in these components. This illustrates that fatigue cycles can cause elevated stresses during steady operation that cause creep damage. This paper will explore a case study for a 1CrMoV turbine casing where the stress-strain history during operating transients will be related to damage in samples from the turbine casing. This will also highlight how service affects the mechanical properties of 1CrMoV, highlighting the need for service- exposed property data to perform mechanical integrity assessments. Finally, the consequences for repair of damage will be discussed, illustrating how analysis can guide volume of material for excavation and selection of weld filler metal to maximize the life of the repair. This, in turn, will identify opportunities for future weld repair research and material property data development.

In flexible operation with increased number of startup, shutdown, and load fluctuations, thermal fatigue damage is exacerbated along with existing creep damage in power plant pipe and pressure vessels. Recently, cracks were found in the start-up vent pipe branching from the reheat steam pipe within a heat recovery steam generator(HRSG) of J-class gas turbine, occurring in the P92 base material and repair welds. This pipe has been used at the power plant for about 10 years. Microstructural analysis of the cross-section indicated that the cracks were primarily due to thermal fatigue, growing within the grains without changing direction along the grain boundaries. To identify the damage mechanism and evaluate the remaining life, temperature and strain monitoring were taken from the damaged piping during startup and normal operation.

Main steam control valves are crucial components in power plants, as they are the final elements in the steam piping system before the steam enters the turbine. If any parts of these valves become damaged, they can severely harm the steam turbines. Recently, power plants have been required to operate under cyclical loading, which increases the risk of cracks in the control valve seats. This is due to the different rates of expansion between the Stellite surface and the underlying Grade 91 steel surface when exposed to high temperatures. To ensure a reliable power supply, power plants cannot afford long downtimes, making on-site service essential. This paper presents an on-site technique for post-weld heat treatment (PWHT) of Stellite seats. By using a heating pad arrangement and an induction heater, the required PWHT temperature of 740°C, as specified in the welding specification procedure (WPS), can be achieved. This method allows for on-site valve seat repair and can be applied to other power plants as well.

This paper reports on the latest in a series of projects aiming at the qualification of new and proven materials in components under a severe service environment. In the initial stages of the project (HWT I & HWT II), a test loop at Unit 6 of the GKM Power Plant in Mannheim was used to study the behavior of components for advanced ultra-supercritical (A-USC) plants made from nickel alloys at 725 °C under both static and fluctuating conditions. Due to recent changes in the operation modes of existing coal-fired power plants, the test loop was modified to continue operating the existing nickel components in the static section while applying thermal cycles in a different temperature range. HR6W pipes and valves were added to the bypass of the static section, and all components in the cyclic section were replaced with P92, P93, and HR6W components. The test loop achieved approximately 9000 hours of operation and around 800 cycles with holding times of 4 and 6 hours. After dismantling the loop, nondestructive and destructive examinations of selected components were conducted. The accompanying testing program includes results from thermal fatigue, fatigue, thermal shock, and long-term creep tests, focusing on the behavior of base materials and welds, particularly for HR6W, P92, P93, and other nickel-based alloys. Additionally, test results on dissimilar welds between martensitic steel P92 and nickel alloys A617 and HR6W are presented. Numerical assessments using standardized and numerical lifetime estimation methods complement the investigations. This paper provides insights into the test loop design and operational challenges, material behavior, and lifetime, including advanced numerical simulations and operational experiences with valves, armatures, piping, and welds.

This study aims to elucidate the chemical compositions and microstructural factors that affect longterm creep rupture strength and creep rupture ductility using multiple heats of Gr.92 steel. Evaluating the reduction behavior in long-term creep rupture strength, we propose a relative creep rupture strength value, which is expressed as the logarithmic ratio of the estimated creep strength for each rupture time exceeding 10,000 hours, with 10,000 hours as the reference. Higher initial hardness correlates with greater pronounced strength reduction in the long-term regime. While smaller prior austenite grain sizes lead to greater reductions in creep rupture strength, this effect diminishes above 30 μm. However, no clear correlation was observed between Cr content and creep strength reduction in this study. Brittle creep ruptures with smooth test specimens were observed just below the extensometer ridge in the parallel section of test specimen, indicating notch weakening. Even in heats with excellent creep ductility, the amount of inclusions tended to be higher than in heats with lower creep ductility. Factors other than inclusions also seem to influence long-term creep ductility.

Additive manufacturing is a groundbreaking manufacturing method that enables nearly lossless processing of high-value materials and produces complex components with a level of flexibility that traditional methods cannot achieve. Wire arc additive manufacturing (WAAM), utilizing a conventional welding process such as gas metal arc welding, is one of the most efficient additive manufacturing technologies. The WAAM process is fully automated and guided by CAD/CAM systems on robotic or CNC welding platforms. This paper explores the fundamental concepts and metallurgical characteristics of WAAM. It focuses primarily on the mechanical properties of printed sample structures made from P91, X20, and alloys 625 and 718 wire feedstock. The study particularly addresses the anisotropy of mechanical properties through both short-term and long-term testing, comparing these results to materials processed using conventional methods.

Grade 91 creep strength-enhanced ferritic steel is a critical material in power generation, widely used for high-temperature, high-pressure tubing and piping applications. Its superior elevated-temperature strength derives from a distinctive microstructure of tempered martensite with uniformly dispersed secondary phases (carbides and carbo-nitrides). This microstructure, crucial for reliable service performance, is achieved through precise control of the manufacturing process, including steelmaking, hot forming, and final heat treatment. This investigation builds upon earlier research into the relationship between manufacturing parameters and short-term creep-rupture properties in T91 tubes, and a recent update that included test results exceeding 30,000 hours. This study presents a comprehensive metallurgical analysis of ruptured test specimens. The investigation focuses on correlating manufacturing parameters with not only creep strength but also material ductility and microstructural evolution during long-term exposure, providing valuable insights into the material’s behavior under extended service conditions.

This study investigates a novel approach to addressing the persistent Type IV cracking issue in Grade 91 steel weldments, which has remained problematic despite decades of service history and various mitigation attempts through chemical composition and procedural modifications. Rather than further attempting to prevent heat-affected zone (HAZ) softening, we propose eliminating the vulnerable base metal entirely by replacing critical sections with additively manufactured (AM) weld metal deposits using ASME SFA “B91” consumables. The approach employs weld metal designed for stress-relieved conditions rather than traditional normalizing and tempering treatments. Our findings demonstrate that the reheat cycles during AM buildup do not produce the substantial softening characteristic of Type IV zones, thereby reducing the risk of premature creep failure. The study presents comprehensive properties of the AM-built weld metal after post-weld heat treatment (PWHT), examines factors influencing deposit quality and performance, and explores the practical benefits for procurement and field construction, supported by in-service data and application cases.

In this study, the creep strength of welded joints of Grade 91 Type 1 and Type 2 steels was evaluated. It was determined that impurity elements in the Type 1 steel reduced its creep strength. This reduction was attributed to an increase in the amount of residual carbides in the fine-grain heat-affected zone during welding.

Tenaris' High Oxidation Resistance (THOR) 115, or T115, is a creep strength-enhanced ferritic (CSEF) steel introduced in the past decade. It is widely used in constructing high-efficiency power plants and heat recovery steam generators (HRSGs) due to its superior steam oxidation resistance and long-term microstructural stability, making it a viable alternative to stainless steels at elevated steam temperatures. The creep damage tolerance of T115 has been recently validated under ASME BPVC CC 3048 guidelines, which address safety concerns related to creep damage in boiler components. Testing confirmed T115's consistent creep damage-tolerant behavior, with cross-weld creep behavior reassessed through extensive metallographic examination of specimens from a 1.5-inch thick pipe girth weld, providing insights into creep damage distribution and hardness, and its relative performance compared to Grade 91 CSEF steel.

Coke drums experience failures in through-wall cracking throughout their operating life, resulting from low cycle fatigue. Coke drums are typically fabricated from Chrome Moly (CrMo) steels. This study was performed on P4 (1.25Cr-0.5Mo) base material using ER70S-B2L and Alloy 625 (ERNiCrMo-3) filler materials. Specimens were welded with the temper-bead/controlled deposition welding technique. The weld processes used were HP-GTAW, GMAW and SMAW. The fatigue performance, HAZ hardness and toughness of the weld samples was evaluated. The HP-GTAW welds exhibited an order of magnitude improvement in fatigue performance when compared to the other weld processes using ER70S-B2L filler material. The HP-GTAW welds also exhibited improved HAZ hardness and toughness when compared to the other weld processes. This presentation will introduce the HP-GTAW process, its features, and benefits and where it is applied in Coke drum repair welding. Comparative test results of the different weld processes for fatigue performance, HAZ tempering, and toughness will also be presented.

The time-dependent behavior of 9Cr creep strength enhanced ferritic (CSEF) steels has long fixated on the creep life recorded in uniaxial constant load creep tests. This focus is a consequence of the need to develop stress allowable values for use in the design by formulae approach of rules for new construction. The use of simple Design by Formula rules is justified in part by the assumption that the alloys used will invariably demonstrate high creep ductility. There appears to be little awareness regarding the implication(s) that creep ductility has on structural performance when mechanical or metallurgical notches (e.g., welds) are present in the component design or fabricated component. This reduced awareness regarding the role of ductility is largely because low alloy CrMo steels used for very many years typically were creep ductile. This paper focuses on the structural response from selected tests that have been commissioned or executed by EPRI over the last decade. The results of these tests demonstrate unambiguously the importance that creep ductility has on long-term, time-dependent behavior. The metallurgical findings from the selected tests are the focus of the Part II paper. The association of performance with notch geometry, weld strength, and other potential contributing factors will be highlighted with a primary objective of informing the reader of the variability, and heat-specific behavior that is observed among this class of alloys widely used in modern thermal fleet components and systems.

Supercritical CO 2 (sCO 2 ) is of interest as a working fluid for several concepts including the direct- fired Allam cycle as a low-emission fossil energy power cycle. Over the past 10 years, laboratory exposures at 300 bar sCO 2 have found reasonably good compatibility for Ni-based alloys at <800°C, including an assessment of the sCO 2 impact on room temperature mechanical properties after 750°C exposures. However, initial screening tests at 1 and 20 bar CO 2 at 900°-1100°C showed poor compatibility for Ni-based alloys. In an open cycle, the introduction of 1%O 2 and 0.1- 0.25%H 2 O impurities at 300 bar increased the reaction rates ≥2X at 750°C. At lower temperatures, steels are susceptible to C ingress and embrittlement. Creep-strength enhanced ferritic steels may be limited to <550°C and conventional stainless steels to <600°C. Two strategies to increase those temperatures are higher Ni and Cr alloying additions and Al- or Cr-rich coatings. Alloy 709 (Fe- 20Cr-25Ni) shows some promising results at 650°C in sCO 2 but reaction rates were accelerated with the addition of O 2 and H 2 O impurities. Pack aluminized and chromized Gr.91 (Fe-9Cr-1Mo) and type 316H stainless steel show some promise at 600°-650°C but further coating optimization is needed.

This study evaluates various nondestructive testing methods for detecting creep damage and assessing residual life in Grade 91 steel welds. Three primary detection techniques were investigated: phased array ultrasonic testing (PAUT), eddy current testing with high-temperature superconductor direct current and superconducting quantum interference device (ECT•HTS-dc-SQUID), and replica observation. PAUT detected creep damage between 60-80% of creep life, while ECT•HTS-dc-SQUID showed detection capability between 80-90% of creep life. Replica observation revealed creep voids only in the final stages before rupture. Additionally, three strain measurement methods were evaluated: capacitive strain sensors (providing continuous monitoring during creep exposure), laser displacement meters (used during test interruptions), and SPICA strain measurement. Both capacitive sensors and laser meters produced results comparable to conventional extensometer measurements. The SPICA method proved particularly effective in measuring heat-affected zone (HAZ) strain after creep exposure, revealing higher strain values in the HAZ compared to base and weld metal, with a consistent increase during creep exposure.

The time-dependent behavior of 9Cr creep strength enhanced ferritic (CSEF) steels has long fixated on the creep life recorded in uniaxial constant load creep tests. This focus is a consequence of the need to develop stress allowable values for use in the design by formulae approach of rules for new construction. The use of these simple rules is justified in part by the assumption that the alloys used will invariably demonstrate high creep ductility. There appears to be little awareness regarding the implication(s) that creep ductility has on structural performance when mechanical or metallurgical notches (e.g., welds) are present in the component design or fabricated component. This reduced awareness regarding the role of ductility is largely because low alloy CrMo steels used for very many years typically were creep ductile. This paper focuses on the structural response from selected tests that have been commissioned or executed by EPRI over the last decade. The results of these tests demonstrate unambiguously the importance that creep ductility has on long-term, time-dependent behavior. This is the second part of a two-part paper; Part I reviewed the selected tests and discussed them from a mechanical perspective. The association of performance with specific microstructural features is briefly reviewed in this paper and the remaining gaps are highlighted for consideration among the international community.

This study addresses the welding challenges encountered when joining Haynes 282, a heat-resistant superalloy, to 3.5NiCrMoV high-strength low alloy steel (HSLA) for advanced power plant applications, particularly in thick-section components like rotors. The project demonstrated successful thick-section dissimilar metal welding up to 76 mm (3 in.) using two techniques: keyhole tungsten inert gas welding and conventional gas tungsten arc welding with Haynes 282 filler metal. Various groove weld geometries were evaluated, supported by computational weld modeling to predict and minimize weld distortion. The results validate these welding approaches for critical power plant components requiring both high-temperature performance and cost-effectiveness.

This paper presents three recent example cases of cracking in Grade 91 steel welds in longer-term service in high temperature steam piping systems: two girth butt welds and one trunnion attachment weld. All the cases were in larger diameter hot reheat piping, with the service exposure of the welds ranging from approximately 85,000 to 150,000 hours. Cracking in all cases occurred by creep damage (cavitation and microcracking) in the partially transformed heat-affected zone (PTZ, aka Type IV zone) in the base metal adjacent to the welds. The location and morphology of the cracking are presented for each case along with operating conditions and potential contributors to the cracking, such as system loading, base metal chemical composition, and base metal microstructure.

To operate future fusion power plants economically, the fusion community needs structural materials that can last longer and operate at higher temperatures than current materials, as well as better heat-dissipating components and a reliable supply chain for them. Additive manufacturing (AM) of existing reduced activation ferritic/martensitic steels (RAFMS) has the potential to solve these problems, which is why we developed a laser powder bed fusion (LPBF) process for Eurofer97 steel, demonstrating excellent mechanical properties (~30% improved yield strength, ~6300% improved creep rupture life, similar toughness and ductility compared to wrought Eurofer97) and low process scatter (relative standard error for yield, tensile strength and elongation on build plate <1%). The main remaining challenge is the consistent sourcing of quality Eurofer97 powder, but we have shown that this can already be done if the limits on impurity elements are slightly relaxed. Our work lays the groundwork for the manufacture of complex fusion components that can dissipate heat better and have a higher operating temperature thanks to the improved material properties. It also helps plug the current RAFMS supply chain gap, enabling easier component prototyping and small-scale manufacture that can smoothly scale in volume in the future.

High-pressure valves and fittings used in coal-fired 600/625 °C power plants are hardfaced for protection against wear and corrosion and to provide optimum sealing of the guides and seats. Stellite 6 and Stellite 21 are often used for hardfacing, which is carried out by build-up welding, usually in several layers. The valve materials are generally heat-resistant steels such as 10CrMo9-10 (1.7380), X20CrMoV1 (1.4922), or Grade 91 / Grade 92 (1.4903 / 1.4901). In recent years, cracks or delaminations have frequently occurred within the hardfaced layer. The influence of cycling operation is not well understood. Other essential factors are the chemical composition of the base material and of the filler metal; especially in terms of the resulting iron dilution during the deposition of the welding overlays. The research project was initiated to investigate the crack and delamination behavior and to understand the involved damage mechanisms. Thermostatic and cyclic exposure tests have shown that cracking is favored by the formation of brittle phases due to iron dilution from the substrate material during the manufacturing process. Recommendations for the welding process of hardfaced sealing surfaces of fittings were derived from the investigation results.

In this work, two unique heats of 9Cr creep strength enhanced ferritic (CSEF) steels extracted from a retired superheat outlet header after 141,000 hours of service were evaluated. These two CSEF steels were a forging manufactured to SA-182 F91 (F91) reducer and a seamless pipe produced to SA-335 P91 (P91) pipe. Their creep deformation and fracture behavior were assessed using a lever arm creep frame integrated with in-situ high-temperature digital image correlation (DIC) system. Critical metallurgical and microstructure factors, including composition, service damage, grain matrix degradation, precipitates, and inclusions were quantitatively characterized to link the performance of the two service aged F91 and P91 CSEF steels. The creep test results show the F91 and P91 steels exhibit a large variation in creep strength and creep ductility. The F91 steel fractured at 572 hours while P91 steel fractured at 1,901 hours when subjected to a test condition of 650 °C and 100 MPa. The nominal creep strains at fracture were 12.5% (F91) and 14.5% (P91), respectively. The high-resolution DIC strain measurements reveal the local creep strain in F91 was about 50% while the local creep strain in P91 was >80%. The characterization results show that the F91 steel possessed pre-existing creep damage from its time in service, a higher fraction of inclusions, and a faster matrix grain coarsening rate. These features contribute to the observed reduction in performance for the F91 steel. The context for these findings, and the importance of metallurgical risk in an integrated life management approach will be emphasized.