Search Results for thermal expansion coefficient

Advanced 700C class steam turbines require austenitic alloys instead of conventional ferritic heat-resistant steels which have poor creep strength and oxidation resistance above 650C. Austenitic alloys, however, possess a higher thermal expansion coefficient than ferritic 12Cr steels. Therefore, Ni-based superalloys were tailored to reduce their coefficients to the level of 12Cr steels. Regression analysis of commercial superalloys proves that Ti, Mo and Al decrease the coefficient quantitatively in this order, while Cr, used to secure oxidation resistance, increases it so significantly that Cr should be limited to 12wt%. The newly designed Ni-18Mo-12Cr-l.lTi-0.9Al alloy is strengthened by gamma-prime [Ni 3 (Al,Ti)] and also Laves [Ni 2 (Mo,Cr)] phase precipitates. It bears an RT/700C mean thermal expansion coefficient equivalent to that of 12Cr steels and far lower than that of low-alloyed heat resistant steels. Its creep rupture life at 700C and steam oxidation resistance are equivalent to those of a current turbine alloy, Refractaloy 26, and its tensile strength at RT to 700C surpasses that of Refractaloy 26. The new alloy was trial produced using the VIM-ESR melting process and one ton ingots were successfully forged into round bars for bolts without any defects. The bolts were tested in an actual steam turbine for one year. Dye penetrant tests detected no damage. The developed alloy will be suitable for 700C class USC power plants.

Austenitic heat resistant steels are one of the most promising materials to be applied around 650°C, due to its superior creep strength than conventional ferritic steels and lower material cost than Ni based superalloys. The problem of austenitic steels is its high thermal expansion coefficient (CTE), which leads to high deformation and stress when applied in rotors, casings, blades and bolts. To develop low CTE austenitic steels together with high temperature strength, we chose the gamma-prime strengthened austenitic steel, A-286, as the base composition, and decreased the CTE by introducing the invar effect. The developed alloy, Fe-40Ni-6Cr-Mo-V-Ti-Al-C-B, showed low CTE comparable to conventional ferritic steels. This is mainly due to its high Ni and low Cr composition, which the invar effect is prone even at high temperature region. This alloy showed higher yield strength, higher creep rupture strength and better oxidation resistance than conventional high Cr ferritic steels and austenitic steels. The 2 ton ESR ingot was forged or hot rolled without defects, and the blade trial manufacturing was successfully done. This alloy is one of the best candidates for USC and A-USC turbine components.

The use of creep strength-enhanced ferritic alloys like Grade 91 has become popular in fossil power plants for applications at temperatures above 566°C (1050°F). Compared to Grades 11 and 22, Grade 91 offers higher stress allowables, better ramp rate tolerance, weight reduction, and lower thermal expansion coefficients at operating temperatures. However, Grade 91's superior elevated temperature strength requires specific microstructure and metallurgical considerations. This paper highlights concerns that warrant further investigation. Initial operating stresses in Grade 91 piping systems may exceed 262 MPa (38 ksi), and lack of creep relaxation below 593°C (1050°F) could lead to weldment failures within years, especially above 159 MPa (23 ksi) after one year. While cold spring can reduce initial stresses for systems below 593°C (1050°F), creep relaxation rates up to 206 MPa (30 ksi) need study. Above 593°C (1050°F) and below 103 MPa (15 ksi), weldments may fail prematurely by Type IV creep mechanism. Long-term creep rupture studies on cross-weld and multiaxially loaded thick-walled specimens should evaluate deteriorated weldment properties, particularly below 103 MPa (15 ksi).

Advanced 700°C-class steam turbines demand austenitic alloys for superior creep strength and oxidation resistance beyond 650°C, exceeding the capabilities of conventional ferritic 12Cr steels. However, austenitic alloys come with a higher coefficient of thermal expansion (CTE) compared to 12Cr steels. To ensure reliability, operability, and performance, these advanced turbine alloys require low CTE properties. Additionally, for welded components, minimizing CTE mismatch between the new material and the welded 12Cr steel is crucial to manage residual stress. This research investigates the impact of alloying elements on CTE, high-temperature strength, phase stability, and manufacturability. As a result, a new material, “LTES700R,” was developed specifically for steam turbine rotors. LTES700R boasts a lower CTE than both 2.25Cr steel and conventional superalloys. Additionally, its room-temperature proof strength approaches that of advanced 12Cr steel rotor materials, while its creep rupture strength around 700°C significantly surpasses that of 12Cr steel due to the strengthening effect of gamma-prime phase precipitates. To assess the manufacturability and properties of LTES700R, a medium-sized forging was produced as a trial run for a turbine rotor. The vacuum arc remelting process was employed to minimize segregation risk, and a forging procedure was meticulously designed using finite element method simulations. This trial production resulted in a successfully manufactured rotor with satisfactory quality confirmed through destructive evaluation.

The operating temperature of ultrasupercritical (USC) power plants is increasing, with planned temperatures reaching up to 700°C. Austenitic superalloys are promising alternatives to ferritic heat-resistant steels due to their potential for high strength at temperatures around 650-700°C. While austenitic nickel-base superalloys generally exhibit higher creep rupture strength than ferritic heat-resistant steels, they also have drawbacks, including higher thermal expansion, lower creep rupture ductility, and increased costs. Initially, the researchers focused on developing a molybdenum-containing superalloy to achieve low thermal expansion. They systematically investigated the effects of molybdenum and cobalt content, gamma prime phase amount, and aluminum/titanium ratio on thermal expansion, tensile properties, and creep-rupture properties. These investigations were conducted using the conventional molybdenum-containing Alloy 252 as a reference. The developed superalloy, notably free of cobalt and combined with a modified heat treatment, demonstrated significantly improved creep rupture elongation compared to Alloy 252, while maintaining low thermal expansion and high creep rupture strength similar to the reference alloy. Additionally, the research evaluated creep-rupture properties at 700°C for up to approximately 20,000 hours to assess long-term applications. The study also examined the weldability and mechanical properties of weld joints at 750°C, focusing on potential boiler tube applications.

Advanced 700°C-class steam turbines require the use of austenitic alloys instead of conventional ferritic 12Cr steels, which are inadequate in creep strength and oxidation resistance above 650°C. While austenitic alloys offer improved performance, they traditionally possess a significantly higher coefficient of thermal expansion (CTE) compared to 12% Cr steels. Through extensive research, the authors systematically investigated the effects of various alloying elements on thermal expansion and high-temperature strength. As a result of these investigations, they developed "LTES700," an innovative nickel-based superalloy specifically designed for steam turbine bolts and blades. This novel alloy uniquely combines a coefficient of thermal expansion comparable to 12Cr steels with high-temperature strength equivalent to conventional superalloys like Refractaloy 26, effectively addressing the critical limitations of previous materials.

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.

In this study, creep rupture behaviors and rupture mechanisms of dissimilar welded joint between Inconel 617B and COST E martensitic steel were investigated. Creep tests were conducted at 600 ℃ in the stress range 140-240 MPa. Scanning electron microscopy (SEM) and micro-hardness were used to examine the creep rupture behaviors and microstructure characteristics of the joint. The results indicated that the rupture positions of crept joints shifted as stress changed. At higher stress level, the rupture position was located in the base metal (BM) of COST E martensitic steel with much plastic deformation and necking. At relatively lower stress level, the rupture positions were located in the fine-grained heat affected zone (FGHAZ) of COST E or at the interface between COST E and WM both identified to be brittle fracture. Rupture in the FGHAZ was caused by type Ⅳ crack due to matrix softening and lack of sufficient precipitates pinning at the grain boundaries (GBs). Rupture at the interface was related to oxide notch forming at the interface.

Austenitic stainless steels have been used for boiler tubes in power plants. Since austenitic stainless steels are superior to ferritic steels in high temperature strength and steam oxidation resistance, austenitic stainless steel tubes are used in high temperature parts in boilers. Dissimilar welded joints of austenitic steel and ferritic steel are found in the transition regions between high and low temperature parts. In dissimilar welded parts, there is a large difference in the coefficient of thermal expansion between austenitic and ferritic steel, and thus, thermal stress and strain will occur when the temperature changes. Therefore, the dissimilar welded parts require high durability against the repetition of the thermal stresses. SUPER304H (18Cr-9Ni-3Cu-Nb-N) is an austenitic stainless steel that recently has been used for boiler tubes in power plants. In this study, thermal fatigue properties of a dissimilar welded part of SUPER304H were investigated by conducting thermal fatigue tests and finite element analyses. The test sample was a dissimilar welded tube of SUPER304H and T91 (9Cr-1Mo-V-Nb), which is a typical ferritic heat resistant boiler steel.

The development of materials technologies for piping and tubing in advanced ultrasupercritical (A-USC) power plants operating at steam temperatures above 700°C represents a critical engineering challenge. The 23Cr-45Ni-7W alloy (HR6W), originally developed in Japan as a high-strength tubing material for 650°C ultra-supercritical (USC) boilers, was systematically investigated to evaluate its potential for A-USC plant applications. Comparative research with γ-strengthened Alloy 617 revealed that the tungsten content is intimately correlated with Laves phase precipitation and plays a crucial role in controlling creep strength. Extensive creep rupture tests conducted at temperatures between 650-800°C for up to 60,000 hours demonstrated the alloy's long-term stability, with 105-hour extrapolated creep rupture strengths estimated at 88 MPa at 700°C and 64 MPa at 750°C. Microstructural observations after creep tests and aging confirmed the material's microstructural stability, which is closely linked to long-term creep strength and toughness. While Alloy 617 exhibited higher creep rupture strength at 700 and 750°C, the materials showed comparable performance at 800°C. Thermodynamic calculations and microstructural analysis revealed that the Laves phase in HR6W gradually decreases with increasing temperature, whereas the γ' phase in Alloy 617 rapidly diminishes and almost completely dissolves at 800°C, potentially causing an abrupt drop in creep strength above 750°C. After comprehensive evaluation of creep properties, microstructural stability, and other reported mechanical characteristics, including creep-fatigue resistance, HR6W emerges as a promising candidate for piping and tubing in A-USC power plants.

Development activities initiated over a decade ago within the COST 522 program and continuing through the COST 536 Action have yielded significant progress in constructing a new generation of steam power plants capable of operating under advanced steam conditions. These innovative plants promise substantially improved thermal efficiency, with steam temperatures reaching up to 620°C (1150°F). Recent successful power plant orders in Europe and the United States stem from critical advancements, including the development, testing, and qualification of 10% Cr steels with enhanced long-term creep properties for high-temperature components such as turbine rotors and valve casings. Extensive in-house development efforts have focused on fabrication, weldability, mechanical integrity, and fracture mechanics evaluations of full-sized forged and cast components. These materials will be implemented in several new coal-fired power plants, notably the Hempstead plant in the USA, which will operate with live steam temperatures of 599°C (1111°F) and reheat steam temperatures of 607°C (1125°F). The improved creep properties enable the construction of casings with reduced wall thicknesses, offering greater thermal flexibility at lower component costs and facilitating welded turbine rotors for high-temperature applications without requiring cooling in the steam inlet region. Looking forward, further efficiency improvements are anticipated through the introduction of nickel alloys in steam turbine and boiler components, with the European AD700 project targeting reheat steam temperatures of 720°C (1328°F) and the US Department of Energy project aiming even higher at 760°C (1400°F). The AD700 project has already demonstrated the technical feasibility of such advanced steam power plants, with engineering tasks progressing toward the construction of a 550 MW demonstration plant, while DOE activities continue to address boiler concerns and focus on rotor welding, mechanical integrity, and steam oxidation resistance.

INCONEL 740H has been developed by Special Metals for use in Advanced Ultra Super Critical (A-USC) coal fired boilers. Its creep strength performance is currently amongst the ‘best in class’ of nickel based alloys, to meet the challenge of operating in typical A-USC steam temperatures of 700°C at 35 MPa pressure. Whilst the prime physical property of interest for INCONEL 740H has been creep strength, it exhibits other physical properties worthy of consideration in other applications. It has a thermal expansion co-efficient that lies between typical values for Creep Strength Enhanced Ferritic (CSEF) steels and austenitic stainless steels. This paper describes the validation work in support of the fabrication of a pipe transition joint that uses INCONEL 740H pipe, produced in accordance with ASME Boiler Code Case 2702, as a transition material to join P92 pipe to a 316H stainless steel header. The paper gives details of the material selection process, joint design and the verification process used for the joint.

In response to the need to reduce carbon dioxide gas emissions, Japan has been actively researching 700°C-class thermal power plants with a focus on improving overall plant efficiency. This technological advancement is fundamentally grounded in advanced materials development, encompassing the creation of high-strength alloys, fireside corrosion-resistant materials, and steamside oxidation-resistant alloys. A significant challenge emerged as some of the developed materials fell outside the scope of existing domestic technical standards. Moreover, the potential failure modes for advanced ultra-supercritical (A-USC) components operating at 700°C were anticipated to differ substantially from those observed in traditional ultra-supercritical (USC) components at 600°C. Consequently, researchers systematically examined and analyzed the potential failure modes specific to 700°C A-USC components, using these insights to establish comprehensive performance requirements. The research initiative, which commenced in June 2006, was strategically planned to develop a draft technical interpretation by March 2011. This paper provides a detailed overview of the investigative process, encompassing the comprehensive analysis of failure modes, the derivation of performance requirements, and the progression toward developing a new technical interpretation framework for high-temperature power plant components.

In order to enable safe long-term operation, metallic pipes operated in the creep range at high temperatures require considerable wall thicknesses at significant operating pressures, such as those required in thermal power plants of all kinds or in the chemical industry. This paper presents a concept that makes it possible to design such pipes with thinner wall thicknesses. This is achieved by adding a jacket made of a ceramic matrix composite material to the pipe. The high creep resistance of the jacket makes it possible to considerably extend the service life of thin- walled pipes in the creep range. This is demonstrated in the present paper using hollow cylinder specimens. These specimens are not only investigated experimentally but also numerically and are further analyzed after failure. The investigations of the specimen show that the modeling approaches taken are feasible to describe the long-term behavior of the specimen sufficiently. Furthermore, the paper also demonstrates the possibility of applying the concept to pipeline components of real size in a power plant and shows that the used modeling approaches are also feasible to describe their long-term behavior.

Qualifying welding procedures for repair of components in high temperature service requires careful consideration of factors including identification of the materials involved, existing mechanical properties and service operating parameters such as temperature, pressure and environment. Selection of weld metals to match, under match or overmatch base material as well as direct and indirect consequences on the heat-affected zone also require evaluation. Application of post weld heat treatment and ramifications where dissimilar base materials are involved are discussed plus the necessity of conducting tests at the operating temperatures and conditions where information is not available from the literature. Each of these factors is discussed and examples provided.

Recently, a γ’ precipitation strengthened Ni-base superalloy, USC141, was developed for 700°C class A-USC boiler tubes as well as turbine blades. In boiler tube application, the creep rupture strength of USC141 was much higher than that of Alloy617, and the 105 hours’ creep rupture strength of USC141 was estimated to be about 180MPa at 700°C. This is because fine γ’ particles precipitate in austenite grains and some kinds of intermetallic compounds and carbides precipitate along austenite grain boundaries during creep tests. Good coal ash corrosion resistance is also required for tubes at around 700°C. It is known that coal ash corrosion resistance depends on the contents of Cr and Mo in Ni-base superalloys. Therefore the effect of Cr and Mo contents in USC141 on coal ash corrosion resistance, tensile properties, and creep rupture strengths were investigated. As a result, the modified USC141 containing not less than 23% Cr and not more than 7% Mo showed better hot corrosion resistance than the original USC141. This modified alloy also showed almost the same mechanical properties as the original one. Furthermore the trial production of the modified USC141 tubes is now in progress.

Significant research efforts are underway in Europe, Japan, and the U.S. to develop the technology to increase the steam temperature in fossil power plants in order to achieve greater efficiency and reduce the amount of greenhouse gases emitted. The realization of these advanced steam power plants will require the use of nickel-based superalloys having the required combination of high-temperature creep strength, oxidation resistance, thermal fatigue resistance, thermal stability, and fabricability. Haynes 230 and 282 alloys are two materials that meet all of these criteria. The metallurgical characteristics of each alloy are described in detail, and the relevant high-temperature properties are presented and discussed in terms of potential use in advanced steam power plants.

The capacity of PC power plants in Japan rose to 35GW in 2004. The most current plants have a 600 deg-C class steam temperature and a net thermal efficiency of approximately 42% (HHV). Older plants, which were built in the ‘70s and early ‘80s, will reach the point where they will need to be rebuilt or refurbished in the near future. The steam temperatures of the older plants are 538 deg-C or 566 deg-C. We have done a case study on the refurbishment of one of these plants with the advanced USC technology that uses a 700 deg-C class steam temperature in order to increase the thermal efficiency and to reduce CO 2 emissions. The model plant studied for refurbishing has a 24.1MPa/538 deg-C /538 deg-C steam condition. We studied three possible systems for the refurbishing. The first was a double reheat system with 35MPa/700 deg-C /720 deg-C /720 deg-C steam conditions, the second one was a single reheat 25MPa/700 deg-C/720 deg-C system, the last one was a single reheat 24.1MPa/610 deg-C/720 deg-C system. In addition to these, the most current technology system with 600 deg-C main and reheat temperatures was studied for comparison. The study showed that the advanced USC Technology is suitable for refurbishing old plants. It is economical and environmentally-friendly because it can reuse many of the parts from the old plants and the thermal efficiency is much higher than the current 600 deg-C plants. Therefore, CO 2 reduction is achieved economically through refurbishment.

High Cr ferritic steels have been developed for the large components of fossil power plants due to their excellent creep resistance, low thermal expansion, and good oxidation resistance. Development works to improve the operating temperature of these steels mainly focused on the high mechanical properties such as solid solution strengthening and precipitation hardening. However, the knowledge of the correlation between Laves phase precipitation and oxidation behavior has not clarified yet on 9Cr ferritic steels. This research will be focused on the effect of precipitation of Laves phase on steam oxidation behavior of Fe-9Cr alloy at 923 K. Niobium was chosen as the third element to the Fe- 9Cr binary system. Steam oxidation test of Fe-9Cr (mass%) alloy and Fe-9Cr-2Nb (mass%) alloy were carried out at 923 K in Ar-15%H 2 O mixture for up to 172.8 ks. X-ray diffraction confirms the oxide mainly consist of wüstite on the Fe-9Cr in the initial stage while on Nb added samples magnetite was dominated. The results show that the Fe-9Cr- 2Nb alloy has a slower oxidation rate than the Fe-9Cr alloy after oxidized for 172.8 ks

We have reported on the effort being done to develop the A-USC technology in Japan, which features the 700 deg-C steam condition, since the 2007 EPRI conference. Our 9 year project began in 2008. There have been some major changes in the electricity power market in the world recently. At first, the earthquake changed the power system violently in Japan. Almost all nuclear power plants have been shut down and natural gas, oil and coal power plants are working fully to satisfy the market's demands. In the USA, the so called ‘Shale gas revolution’ is going on. In Europe, they are working toward the target of reducing CO 2 emissions by the significant use of renewables with the backup of the fossil fuel power systems and enhancing power grids. A very rapid increase in power generation by coal is being observed in some countries. Despite some major changes in the electric sector in the world and the CO 2 problem, the global need for coal power generation is still high. We can reconfirm that the improvement of the thermal efficiency of coal power plants should be the most fundamental and important measure for the issues we are confronting today, and that continuous effort should be put towards it. Based on the study we showed at the 2007 conference, we developed 700 deg-C class technology mainly focusing on the material and manufacturing technology development and verification tests for key components such as boilers, turbines and valves. Fundamental technology developments have been done during the first half of the project term. Long term material tests such as creep rupture of base materials and welds will be conducted for 100,000hrs continuing after the end of the project with the joint effort of each participating company. Today, we are preparing the plan for the second half of the project, which is made up of boiler components test and the turbine rotating tests. Some boiler superheater panels, large diameter pipes and valves will be tested in a commercially operating boiler from 2015 to 2017. The turbine rotor materials which have the same diameter as commercial rotors will be tested at 700 deg-C and at actual speed.