Search Results for dimensional stability

The paper deals with microstructural evolution in the AISI 316LN + 0.1 wt.% Nb steel during long-term creep exposure at 600 and 625°C. The following minor phases formed: Z-phase (NbCrN), M 23 C 6 , M6X (Cr3Ni2SiX type), η-Laves (Fe2Mo type) and σ-phase. M6X gradually replaced M 23 C 6 carbides. Primary Z-phase particles were present in the matrix after solution annealing, while secondary Z-phase particles formed during creep. Precipitation of Z-phase was more intensive at 625°C. The dimensional stability of Z-phase particles was excellent and these particles had a positive effect on the minimum creep rate. However, niobium also accelerated the formation and coarsening of σ-phase, η-Laves and M6X. Coarse particles, especially of σ-phase, facilitated the development of creep damage, which resulted in poor long-term creep ductility.

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.

Research and development of 700°C A-USC steam turbine unit needs to be supported by materials with excellent overall performance. Waspaloy is a kind of γ′ phase precipitation hardening superalloy developed by the United States in the 1950s. In the 700°C R&D Plan of Shanghai Turbine Plant, it was selected as a candidate material for high temperature blades and bolts. The composition, microstructure, properties, blade die forging process and bolt rolling process of Waspaloy alloy were researched in this paper. Simultaneously, Shanghai Turbine Plant successfully manufactured Waspaloy alloy trial production for high temperature bolts and blades. The results show that Waspaloy not only has excellent processing performance, but also has good high temperature strength, long-term performance, stress relaxation resistance and long term aging performance stability at 700°C. It can fully meet the requirements of high-temperature blades and bolts of 700°C A-USC unit. It shows that the 700°C A-USC unit high temperature blades and bolts were successfully developed by Shanghai Turbine Plant.

This overview paper summarizes part of structure stability study results in China on advanced heat-resistant steels, nickel-iron and nickel base superalloys such as 12Cr2MoWVTiB(GY102) ferritic steel, Super 304H austenitic steel, GH2984, Nimonic 80A and INCONEL 740 superalloys for fossil power plant application. China had established first USC power plant with steam parameters of 650°C and 25 MPa in the year of 2006. Austenitic heat-resistant steel Super 304H is mainly used as boiler superheater and reheater material. Ni-Cr-Fe base superalloy GH2984 was used as tube material for marine power application. Ni-Cr-Co type INCONEL 740 has been studied in a joint project with Special Metals Corp., USA for European USC model power plant with the steam temperature of 700°C. Nimonic 80A has been used as several stage USC steam turbine bucket material at 600°C in China. Structure stability study of Nimonic 80A shows its possibility of 700°C application for USC steam turbine buckets.

A compositional modification has been proposed to validate an alloy design which potentially eliminates the requirement of post-weld heat treatment (PWHT) while preserving the advantage of mechanical properties in a reduced activation bainitic ferritic steel based on Fe-3Cr-3W-0.2V- 0.1Ta-Mn-Si-C, in weight percent, developed at Oak Ridge National Laboratory in 2007. The alloy design includes reducing the hardness in the as-welded condition for improving toughness, while increasing the hardenability for preserving the high-temperature mechanical performance such as creep-rupture resistance in the original steel. To achieve such a design, a composition range with a reduced C content combining with an increased Mn content has been proposed and investigated. Newly proposed “modified” steel successfully achieved an improved impact toughness in the as- welded condition, while the creep-rupture performance across the weldments without PWHT demonstrated ~50% improvement of the creep strength compared to that of the original steel weldment after PWHT. The obtained results strongly support the validity of the proposed alloy design.

This overview paper contains a part of structure stability study on advanced austenitic heat-resisting steels (TP347H, Super304H and HR3C) and Ni-base superalloys (Nimonic80A, Waspaloy and Inconel740/740H) for 600-700°C A-USC fossil power plant application from a long-term joint project among companies, research institutes and university in China. The long time structure stability of these advanced austenitic steel TP347H, Super304H, HR3C in the temperature range of 650-700 °C and Ni-base superalloys Nimonic80A, Waspaloy and Inconel740/740H in the temperature range of 600-800°C till 10,000h have been detailed studied in this paper.

INCONEL alloy 740H has been specified for tube and pipe for the boiler and heat exchanger sections of AUSC and sCO 2 pilot plants currently designed or under construction. These systems also require fittings and complex formed components such as flanges, saddles, elbows, tees, wyes, reducers, valve parts, return bends, thin-wall cylinders and tube sheets. The initial evaluation of alloy 740H properties, leading to ASME Code Case 2702, was done on relatively small cross-section tube and plate. The production of fittings involves the use of a wide variety of hot or cold forming operations. These components may have complex geometric shapes and varying wall thickness. The utility industry supply chain for fittings is largely unfamiliar with the processing of age-hardened nickel-base alloys. Special Metals has begun to address this capability gap by conducting a series of trials in collaboration with selected fittings manufacturers. This paper describes recent experiences in first article manufacture of several components. The resulting microstructure and properties are compared to the published data for tubular products. It is concluded that it will be possible to manufacture most fittings with properties meeting ASME Code minima using commercial manufacturing equipment and methods providing process procedures appropriate for this class of alloy are followed. INCONEL and 740H are registered trademarks of Special Metals Corporation.

Monoblock low-pressure (LP) turbine rotor shaft forgings for nuclear power plants have been produced from up to 600 ton ingots. However, ingots greater than 600 tons are necessary to increase the generator capacity. Segregation, non-metallic inclusions, and micro porosities inevitably increase with the increase in ingot size. Manufacturing such massive ingots with high soundness is quite difficult. Thus, the development of 650 ton ingot production was carried out in 2010. The 650 ton ingot was dissected and investigated to verify its internal quality. The internal quality of the 650 ton ingot was found to be equal to that of 600 ton ingots. Subsequently, in 2011, we produced a 670 ton ingot, the world’s largest, to produce a trial LP rotor shaft forging with a diameter of 3,200 mm. Results show that the internal quality, mechanical properties, and heat stability are the same as LP rotor shaft forgings made from 600 ton ingots.

Inconel 740H is one of the most promising candidate Ni-base superalloys for the main steam pipe of 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants. After processing and welding in manufacturing plant in solution-annealed state, large components was commonly suggested to have an extra aging treatment at 800 °C for 16 h, in order to obtain homogeneous γ′ precipitates. In this present work, creep tests and microstructure analyses were conducted on Inconel 740H pipe specimens under two different heat treatments to verify the necessity of aging process. Here we show that aging treatment has limited effect on the creep rupture life of Inconel 740H pipe. Both in grain interiors and along grain boundaries, crept specimens under two different heat treatments have the same precipitates. But the shape and distribution of γ′ in solution annealed sample is not as regular as the aged ones. Our results provide the underlying insight that aging treatment is not so necessary for the straight pipes if the on-site condition was hard to control. But for both groups of specimens, a small amount of h particles and some banded like M 23 C 6 were emerged during creep, which would be harmful to mechanical properties for the long run.

In order to meet the 2010-2020 DOE Fossil Energy goals for Advanced Power Systems, future oxy-fuel and hydrogen-fired turbines will need to be operated at higher temperatures for extended periods of time, in environments that contain substantially higher moisture concentrations in comparison to current commercial natural gas-fired turbines. Development of modified or advanced material systems, combined with aerothermal concepts are currently being addressed in order to achieve successful operation of these land-based engines. To support the advanced turbine technology development, the National Energy Technology Laboratory (NETL) has initiated a research program effort in collaboration with the University of Pittsburgh (UPitt), and West Virginia University (WVU), working in conjunction with commercial material and coating suppliers as Howmet International and Coatings for Industry (CFI), and test facilities as Westinghouse Plasma Corporation (WPC) and Praxair, to develop advanced material and aerothermal technologies for use in future oxy-fuel and hydrogen-fired turbine applications. Our program efforts and recent results are presented.

Microstructure change during creep at 650°C has been examined for a high-B 9%Cr steel by FIB-SEM serial sectioning 3D observation, Nano-SIMS, SEM, EBSD and TEM. The precipitates formed in the steel were M 23 C 6 , Laves phase, and a quite small amount of MX. For as-tempered steel, precipitation of M 23 C 6 on the prior austenite grain boundaries was clearly found, while precipitation of the Laves phase was not confirmed during tempering. The volume fraction of the Laves phase gradually increased with elapsed time, while M 23 C 6 appeared to increase once and decrease afterward, based on the comparison between the 2,754 h ruptured sample and the 15,426 h ruptured sample. Nano-SIMS measurements have revealed that B segregates on the prior austenite grain boundaries during normalizing, and it dissolves into M 23 C 6 .

A new martensitic steel for power generation applications was developed: Tenaris High Oxidation Resistance (Thor) is an evolution of the popular ASME grade 91, offering improved steam oxidation resistance and better long-term microstructural stability, with equal or better creep strength. Thanks to its design philosophy, based on consolidated metallurgical knowledge of microstructural evolution mechanisms, and an extensive development performed in the last decade, Thor was engineered to overcome limitations in the use of ASME grade 91, yet allow being processed in the same fashion, permitting the re-use of consolidated best practices for boiler fabrication. In order to evaluate the possibility to produce complete pressure part systems, various tests to manufacture components have been performed on Thor pipes and tubes (i.e. finning, bending, welding) and on Thor forged material (i.e. flanges). In all cases consolidated industrial best practices used on ASME grade 91 were applied, and resulting properties met ASME requirements.

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.

Boron and nitride additions are emerging as a promising design concept for stabilizing the microstructure of creep-resistant martensitic high-chromium steels. This approach, known as MarBN steel (martensitic steel strengthened by boron and nitrogen), combines the benefits of solid solution strengthening from boron with precipitation strengthening from nitrides. However, initial welding trials revealed challenges in achieving a uniform fine-grained region in the heat-affected zone (HAZ), which is crucial for mitigating Type IV cracking and ensuring creep strength. Despite these initial hurdles, preliminary creep test results for welded joints have been encouraging. This study presents an improved MarBN steel formulation and its investigation through uniaxial creep tests. Base material and welded joints were subjected to creep tests at 650°C for up to 25,000 hours under varying stress levels. The analysis focused not only on the creep strength of both the base material and welded joints but also on the evolution of damage. Advanced techniques like synchrotron micro-tomography and electron backscatter diffraction were employed to understand the underlying creep damage mechanisms. By combining long-term creep testing data with 3D damage investigation using synchrotron micro-tomography, this work offers a novel perspective on the fundamental failure mechanisms occurring at elevated temperatures within the HAZ of welded joints in these advanced steels.

The global transition toward high-efficiency steam power plants demands increasingly advanced steel rotor forgings capable of operating at temperatures of 600°C and above. The European Cost program has been instrumental in developing creep-resistant 10%-chromium steels for these critical applications, with Steel Cost E emerging as a prominent material now widely utilized in steam turbine shafts and experiencing significant market growth. Saarschmiede has pioneered a robust, fail-safe manufacturing procedure for Cost E rotors, establishing a comprehensive database of mechanical properties and long-term performance data that enhances turbine design reliability. The company has expanded its manufacturing capabilities to include Cost F rotor forgings for high-pressure and intermediate-pressure turbines, with component weights reaching up to 44 tonnes. Investigating methods to further increase application temperatures, researchers within the Cost program discovered the potential benefits of boron additions to 10%-chromium steels. Leveraging this insight, Saarschmiede has produced full-size trial rotors to develop and refine production procedures, with these prototype components currently undergoing extensive testing to validate their performance and potential for advanced high-temperature applications.

This paper presents the CEN WS064 Prospective Group 2, a project involving different European stakeholders from more than 20 organizations with the objective to identify the needs and propose code developments research for the nuclear design and construction code RCC-MRx for innovative reactors with more onerous operational conditions: i) reactor components are generally exposed to higher temperatures; ii) have innovative and more corrosive coolants such as liquid lead or molten salt; iii) materials and components are generally exposed to higher radiation levels than light-water reactors. The main outputs of the CEN WS064 are code evolution proposals and proposals for pre-normative research in support of code evolution. The code evolution is driven by further improving safety and cost reduction. Nuclear Design Codes are robust engineering tools but should incorporate new technologies and research. The paper describes the adopted methodology and the rationale for identifying code evolution needs. Code evolution and research proposals will be discussed. Examples of proposals that will be discussed include: Guideline for design of material/components with innovative coolants, extension of design life to 60 years; qualification of new materials and components with advanced manufacturing. A general requirement is that code evolution and associated material and component qualification and codification need to be significantly accelerated for which new approaches such as AI tools will play an important role.

The current study proposed a new method that utilizes digital image correlation (DIC) techniques to measure in-situ full field strain maps of creep resistant material welds. The stress-rupture test is performed in a Gleeble thermal mechanical simulator. This technique successfully captured a significant difference in the local creep deformation between two Grade 91 steel welds with different pre-welding conditions (standard and non-standard). Strain contour plots exhibited inhomogeneous deformation in the weldments, especially at the heat-affected zone (HAZ). Standard heat-treated specimens had significant creep deformation in the HAZ. On the other hand, non-standard heat treated specimens showed HAZ local strains to be 4.5 times less than that of the standard condition, after a 90-hour creep test at 650°C and 70 MPa. The present study measured the full field strain evolution in the weldments during creep deformation for the first time. The proposed method demonstrated a potential advantage to evaluate local creep deformation in the weldments of any creep resistant material within relatively short periods of time.

A comprehensive EPRI initiative launched in 2006 has addressed the critical need to better understand creep-fatigue interactions in power plants experiencing cyclic operation. This international collaboration of industry experts has focused on evaluating current test methods, analyzing crack initiation and growth methodologies, examining life prediction approaches for various applications, identifying deficiencies in creep-fatigue damage assessment, and determining future research requirements. This paper presents key findings from the project, with particular attention to the performance of creep-strengthened ferritic steels, specifically Grade 91 and 92 steels, providing essential insights for power plants facing increasingly demanding operational conditions.

Super 304H is a new generation of advanced austenitic stainless steels that is increasingly being used in superheater/ reheater (SH/RH) sections of thermal ultra-supercritical steam power plants due to its high creep strength combined with good oxidation resistance and microstructure stability. However, recent studies have shown significant microstructural changes and associated degradation in creep performance during long-term service exposure in this alloy. Microstructure evolution during service and its effect on the long-term creep performance has not been comprehensively assessed. In this work, variations in the microstructure of long-term service exposed Super 304H RH tubes (~99,600 hours at 596°C steam temperature) are documented. The results for the ex-service material are compared to well-documented laboratory studies to provide perspective on improved life management practices for this mainstay advanced stainless steel.

This paper presents results and analyses from long-term creep-rupture testing of alloy CCA617 (also known as alloy 617B) in wrought and welded forms at temperatures and stresses relevant to power generation under advanced steam conditions. The refined controlled chemical composition of CCA617 resulted in increased creep-rupture strength compared to the conventional alloy 617 chemistry at applied stress levels of ~150 MPa and above. Long-term creep rupture testing of weldments (in one case, over 100,000 h) showed that their creep-rupture lives were dependent on the welding process. Gas-tungsten-arc and shielded metal-arc weldments of CCA617 performed nearly equivalent to standard alloy 617 base metals in creep, but there was some debit in creep-rupture resistance when compared to CCA617 base metal. Submerged arc welding produced weldments that were notably weaker than both versions of alloy 617 base metal under creep conditions, possibly due to lack of optimization of filler wire composition and flux.