Advanced power systems that operate at temperatures higher than about 650°C will require nickel-base alloys in critical areas for pressure containment. Age-hardened alloys offer an additional advantage of reduced volume of material compared with lower strength solid solution-strengthened alloys if thinner tube wall can be specified. To date, the only age-hardened alloy that has been approved for service in the time dependent temperature regime in the ASME Boiler and Pressure Vessel Code is INCONEL alloy 740H. Extensive evaluation of seamless tube, pipe, and forged fittings in welded construction, including implant test loops and pilot plants, has shown the alloy to be fit for service in the 650-800°C (1202-1472°F) temperature range. Since, nickel-base alloys are much more expensive than steel, manufacturing methods that reduce the cost of material for advanced power plants are of great interest. One process that has been extensively used for stainless steels and solution-strengthened nickel-base alloys is continuous seam welding. This process has rarely been applied to age-hardened alloys and never for use as tube in the creep-limited temperature regime. This paper presents the initial results of a study to develop alloy 740H welded tube, pipe and fittings and to generate data to support establishment of ASME code maximum stress allowables.

Sufficient available energy in combination with lowest environmental pollution is a basic necessity for a high standard of living in every country. In order to guarantee power supply for future generations it is necessary to use fossil fuels as efficient as possible. This fact calls for the need of power plants with improved technologies to achieve higher efficiency combined with reduced environmental impact. In order to realize this goal it is not only a challenge for power station manufacturers, but also for manufacturers of special steels and forgings, who have to produce improved components with more advanced materials and more complex manufacturing processes. This paper reports about experiences in the fabrication of forged components for gas and steam turbines followed by achievable mechanical properties and ultrasonic detectability results. The materials are the creep resistant martensitic Cr steels developed in the frame of the European Cost research programme. Whereas Boron containing 10% Cr steels are suitable for steam temperatures of 625°C and slightly higher, Ni-based alloys shall be used for temperatures of 700°C and above. One pilot rotor forging, representing a HP-rotor for welded construction, has been manufactured out of alloy Inconel 625 within the frame of the European Thermie project AD700.

The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Codes (BPVC) and Code for Pressure Piping have been utilized extensively for the construction and maintenance of plants in the power generation industry. These codes consist of different relevant sections that are applicable to the various pressure retaining components and their service application. This paper presents a comparison of the welding requirements between the various ASME construction codes outside of the qualification requirements within Section IX. Topics of discussion include preheat temperature, interpass temperature, postweld heat treatment, toughness testing, filler material requirements, and use of standard welding procedure specifications. Individual paragraphs and subparagraphs specific to these topics are compared and contrasted to establish their similarities and differences.

The efficiency of power plants is depending on the steam temperature and/or the steam pressure. Efficiency increasing from 35% to 42-45% require increasing of the steam temperature over 600°C and the pressure over 26 MPa. According to the designer opinion it is not profitable to use classical low alloy creep resistant steels 16Mo3, 13CrMo4-5 or 10CrMo9-10 for membrane waterwall construction for these service condition. New modified low alloy creep resistance T23 and T24 (7CrMoVTiB10-10) steels were developed for membrane waterwalls. Welding of these steels with small thickness (around 6.3 mm) should be enabled without preheating and post weld heat treatment (PWHT) due to the lower carbon content below 0.1%. High creep rupture strength (CRS) values are achieved by Ti, N and B elements alloyed to T24 steel. The original expectation that the welding small thickness without preheating was early overcome and was wrong. According to the present experience the T24 steel is welded with preheating at 150-250°C depending on the wall thickness and welded joint toughness in order to achieve required hardness and impact toughness values. Opinions on the T24 welded joints post weld heat treatment (PWHT) requirements are still inconsistent. Especially the membrane waterwalls of the supercritical power plants are still produced without PWHT.

Fossil fuels continue to be the primary source of energy in the U.S and worldwide. In order to improve the efficiency of fossil power plants, advanced structural materials need to be developed and deployed to meet the need of high temperature creep resistance and corrosion resistance. Examples include creep strength enhanced ferritic (CSEF) steels, austenitic stainless steels, nickel-based superalloys, and oxide dispersion strengthened alloys. Welding is extensively used in construction of fossil power plants. The performance of the weld region can be critical to the safe and economical operation of fossil power plants. Degradations in performance such as reduced creep strength and premature failure in the weld region (e.g. Type IV failure in ferritic steels) are examples of longstanding welding and weldability problems for boiler and other components. In the past, extensive studies have been carried out to characterize the different microstructures in different regions of a weld, and to a certain extent, to establish the correlations between the microstructure and the creep strength. However, the metallurgical or microstructural induced local stress/strain variations have been seldom quantified. In addition, it has been long recognized that, due to the sharp microstructure and property gradients in the weld and HAZ, the standard creep testing procedure for the base metal can produce erroneous results when used for weld testing.

Xcel Energy’s Comanche Unit 3 experienced widespread cracking of T23 membrane wall tubes within the evaporator section, initially occurring during the boiler construction phase, primarily at shop and field tube butt welds. The majority of the tube cracking was attributed to stress-corrosion cracking (SCC), and a lesser number of fabrication-related hydrogen induced cracking (HIC), weld solidification cracking, and brittle cracking within tube swage sections were also experienced. Hundreds of tubes were replaced prior to Unit commissioning, due to both actual tube leaks and those replaced due to weldment cracking and other identified weld defects during radiographic testing. Elevated stress levels and material susceptibility (i.e. hardness in the as-welded condition) were considered the critical factors in the tube cracking.

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.

A new martensitic steel was developed for power generation applications. Tenaris High Oxidation Resistance (Thor) is an evolution of Grade 91, designed to have improved steam oxidation resistance and better long-term microstructural stability, with equal or better creep strength. Based on consolidated metallurgical knowledge of microstructural evolution mechanisms, and extensive development performed in the last decade, Thor was engineered to overcome temperature limitations of Grade 91, yet it can be processed in the same fashion, permitting the use of existing best practices for Grade 91 boiler fabrication. Welding trials were performed on Thor tubes and pipe using welding procedures that are routinely employed in the construction of Grade 91 steel components. A summary of relevant results is presented, demonstrating the applicability of long-established and tested welding procedures to components manufactured with Thor steel.

Construction of boilers that can take advantage of the higher efficiencies offered by thermodynamic cycles operating in the ultrasupercritical range will require materials having elevated temperature properties considerably superior to those of the alloys used in more conventional boilers. While many of the materials currently under consideration for ultrasupercritical boiler applications have seen use in other applications, few have been fully investigated using the product forms and section sizes required by high-temperature, high- pressure steam generators. Before any material can be considered truly applicable for use in these advanced plants, the requirements and effects of boiler industry fabrication processes must be explored in addition to determining the properties of the basic alloys. This need was recognized in a materials evaluation program sponsored by the U.S. Department of Energy and the Ohio Coal Development Office and a portion of this program has been devoted to studying the weldability of candidate ultrasupercritical boiler alloys. This paper describes the results of welding trials involving two of these alloys: Super 304H stainless steel and Controlled Chemistry Alloy 617, a variant of Inconel 617 that has been dubbed “CCA 617.” The CCA 617 was represented in both thick plate and tubular product forms, but the stainless steel was only available as tubing. Issues that might be encountered in fabricating advanced boiler headers and piping were addressed while welding the CCA 617 plate with shielded metal arc and submerged arc processes. Similarly, experience working with tubular product forms of both alloys was gained while making butt joints with an orbital gas tungsten arc process. The paper describes the problems presented, the procedures developed, and the basic characteristics of the welds produced.

T24 tube material (7CrMoVTiB10-10), with its combination of high creep strength and potential to be welded without using preheat, is regarded as a candidate waterwall material for Ultra Supercritical (USC) boilers. However, its reputed sensitivity to hydrogen and potential for secondary hardening may have adverse impacts on construction of waterwall panels. Doosan Babcock Ltd have investigated the response of welds made in T24 tubing to secondary hardening via changing hardness in a series of ageing heat treatment trials. Also, the response of the material to hydrogen infusion has been investigated

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.

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.

In advanced ultra-supercritical (A-USC) power plants, which operate at steam temperatures of 700 °C or higher, there is a need to replace 9 to 12Cr martensitic steels with high-strength nickel-base superalloys or austenitic steels for components exposed to the highest temperatures. However, due to the high cost of nickel-base superalloys, it is desirable to use 9 to 12% Cr martensitic steels for components exposed to slightly lower temperatures, ideally expanding their use up to 650 °C. Key challenges in developing ferritic steels for 650 °C USC boilers include enhancing oxidation resistance and long-term creep rupture strength, particularly in welded joints where resistance to Type IV cracking is critical for constructing thick-section boiler components. The current research aims to investigate the creep deformation behavior and microstructure evolution during creep for base metals and heat-affected-zone (HAZ) simulated specimens of tempered martensitic 9Cr steels, including 9Cr-boron steel and conventional steels like grade 91 and 92. The study discusses the creep strengthening mechanisms and factors influencing creep life. It proposes an alloy design strategy that combines boron strengthening and MX nitride strengthening, avoiding the formation of boron nitrides during normalizing heat treatment, to improve the creep strength of both base metal and welded joints.

Grade 91 steel, while increasingly popular in high-temperature power plants for both retrofit and new construction applications, faces significant challenges with Type IV cracking at the outer parent side edge of the weld heat affected zone. This structural integrity issue has led to extensive weld inspection requirements and, in severe cases, the premature replacement of grade 91 retrofit headers before their intended design life. This paper presents a method for estimating Type IV cracking timelines in operating grade 91 components by analyzing crossweld Type IV data to determine when Type IV life deviates from parent life. By combining test results from various temperatures, the method generates a generalized prediction of Type IV life that can be extrapolated to any temperature of interest, providing a practical lower bound estimate for service life of the weakest grade 91 material. This approach, which can be applied to service operating conditions to establish realistic inspection timelines for plant components, has already successfully identified early-stage Type IV cracking in two retrofit headers and is being expanded to additional grade 91 components.

Creep strength enhanced ferritic (CSEF) steels, particularly modified 9Cr steels Grade 91 and 92, are increasingly used in advanced coal-fired power plants for header and steam piping construction. While these materials typically enter service after receiving a standard high-temperature normalizing treatment followed by lower temperature tempering to achieve optimal microstructure, practical situations like welding operations may expose components to additional heat treatment exceeding the Ac 1 , and potentially the Ac 3 , temperature before returning to tempering temperature. This research examines the effects of simulated post weld heat treatments (PWHT) on Grade 91 and 92 materials using dilatometer-controlled heating and cooling rates, with peak temperatures below Ac 1 , between Ac 1 and Ac 3 , and above Ac 3 , followed by heat treatment at 750°C for 2 hours. Hardness measurements revealed significant reduction when exceeding the Ac 1 temperature, while advanced electron microscopy, including electron back scatter diffraction, was employed to analyze changes in martensite laths and grain structure, along with detailed carbide size distribution analysis using both scanning and transmission electron microscopy. The findings are discussed in terms of how such PWHT overshoots might affect mechanical properties during high-temperature service.

Components in ultrasupercritical steam (USC) boilers will operate under significantly more severe conditions than current subcritical and supercritical steam boilers. Existing construction rules for power boilers lack design guidance or criteria to assess the adequacy of designs for USC conditions. A Department of Energy (DOE) project addresses this by evaluating advanced materials under conditions similar to potential USC service environments. The project focuses on six tubing alloys and four thick-section alloys. Testing is underway for pressurized tube bends, notched thick-section bars, fatigue, and thermal shock on thick-section tubing made of materials like CCA617, Alloy 230, and Alloy 740. Further testing is planned for pressurized tubes, dissimilar metal welds, and thick-section weldments. This paper summarizes the status of this initial testing program aimed at enabling USC boiler material qualification.

The Creep Strength Enhanced Ferritic steel grade 91 is widely used for both retrofit applications and primary construction on high temperature power plant. Although to date most structural integrity issues with this material have been associated with welds, as the operating hours of these plants accumulate, there will be a growing need for remanent creep life assessment of the base material. Arguably this is already the case for aberrant grade 91 material entering service in an incorrectly heat treated condition. In these circumstances the strength may fall below the normally accepted lower bound of the creep strength range and some indication of actual strength may be required. One strategy to address potential base material failure is to use small scale sampling of individual components, followed by small scale creep testing, to investigate the current creep strength present. The data can be compared with the equivalent data produced for well characterised material known to be at the lower bound of the creep strength range. This paper describes a methodology for using the impression creep data obtained to provide both creep strength ranking and an estimate of absolute creep strength for individual grade 91 components. This will enable appropriate judgements to be made by plant operators on repair/run decisions. For those components remaining in service, it allows for the weakest items to be given priority for early re-inspection at future outages. The ultimate goal is to identify base material creep damage development at as early a stage as possible and well in advance of failure in service.

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.

In order to improve thermal efficiency of fossil-fired power plants through increasing steam temperature and pressure high strength martensitic 9-12%Cr steels have extensively been used, and some power plants have experienced creep failure in high temperature welds after several years operations. The creep failure and degradation in welds of longitudinally seam-welded Cr- Mo steel pipes and Cr-Mo steel tubes of dissimilar metal welded joint after long-term service are also well known. The creep degradation in welds initiates as creep cavity formation under the multi-axial stress conditions. For the safety use of high temperature welds in power plant components, the complete understanding of the creep degradation and establishment of creep life assessment for the welds is essential. In this paper creep degradation and initiation mechanism in welds of Cr-Mo steels and high strength martensitic 9-12%Cr steels are reviewed and compared. And also since the non-destructive creep life assessment techniques for the Type IV creep degradation and failure in high strength martensitic 9-12%Cr steel welds are not yet practically established and applied, a candidate way based on the hardness creep life model developed by the authors would be demonstrated as well as the investigation results on the creep cavity formation behavior in the welds. Additionally from the aspect of safety issues on welds design an experimental approach to consider the weld joint influence factors (WJIF) would also be presented based on the creep rupture data of the large size cross-weld specimens and component welds.

Early supercritical units such as American Electric Power (AEP) Philo U6, the world’s first supercritical power plant, and Eddystone U1 successfully operated at ultrasupercritical (USC) levels. However due to the unavailability of metals that could tolerate these extreme temperatures, operation at these levels could not be sustained and units were operated for many years at reduced steam (supercritical) conditions. Today, recently developed creep strength enhanced ferritic (CSEF) steels, advanced austenitic stainless steels, and nickel based alloys are used in the components of the steam generator, turbine and piping systems that are exposed to high temperature steam. These materials can perform under these prolonged high temperature operating conditions, rendering USC no longer a goal, but a practical design basis. This paper identifies the engineering challenges associated with designing, constructing and operating the first USC unit in the United States, AEP’s John W. Turk, Jr. Power Plant (AEP Turk), including fabrication and installation requirements of CSEF alloys, fabrication and operating requirements for stainless steels, and life management of high temperature components