Development of 9 wt.% Cr steels for next generation power plant
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There is a need to increase the thermal efficiency of coal fired power plants. High chromium ferritic steels have historically been used to manufacture steam pipes, tube and headers. Over the last forty years there has been a continuous development of the 9 – 12 wt. % chromium martensitic alloys which has allowed the service temperature to be increased from 510°C – 550°C1. There is now a high demand for Ultra Super Critical coal fired power plants which operate at 650°C and hence have an increased thermal efficiency. The aim of this research project was to develop a MarBN steel (Martensitic Steel Strengthened with Boron and Nitrides) with an optimised composition in order to allow MarBN to be used at 650°C. In order for MarBN to be used within high temperature applications it is vital to understand how microstructural changes can have an impact on their creep strength. This is a key area that the present research has focused upon. The long term creep strength of 9 wt. % chromium ferritic steels is derived from the stability of the microstructure over its service life, which has a direct relationship to the stability of the precipitates and the lath structure within the microstructure.Keywords:
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Two major drivers for the use of newer steels in the automotive industry is fuel efficiency and increased safety performance.Fuel efficiency is mainly a function of weight of steel parts,which in turn,is controlled by gauge and design.Safety is determined by the energy absorbing capacity of the steel used to make the part.All of these factors are incentives for the U.S.automakers to use Advanced High Strength Steels (AHSS) to replace the conventional steels used to manufacture automotive parts in the past.AHSS is a general term used to describe various families of steels.The most common AHSS is the dual-phase steel that consists of a ferrite-martensite microstructure.These steels are characterized by high strength,good ductility,low tensile to yield strength ratio and high bake-hardenability.Another class of AHSS is the multi-phase steel which have a complex microstructure consisting of various phase constituents and a high yield to tensile strength ratio.Transformation Induced Plasticity (TRIP) steels is the latest class of AHSS steels finding interest among the U.S.automakers.These steels consist of a ferrite-bainite microstructure with significant amount of retained austenite phase and show the highest combination of strength and elongation,so far,among the AHSS in use.High level of energy absorbing capacity combined with a sustained level of high n value up to the limit of uniform elongation as well as high bake hardenability make these steels particularly attractive for safety critical parts and parts needing complex forming.Finally,martensitic steels with very high strengths are also in use for certain parts.The role of Niobium in all of the above families of advanced steels for the automotive industry will be discussed in this paper.
Hardenability
TRIP steel
High strength steel
Ductility (Earth science)
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The improvement of thermal efficiency of power plants has provided the incentive for the development of the martensitic–ferritic 9–12%Cr creep-resistant steels. Good progress has been made in developing such steels, which are being used particularly in the wrought form as tubes and pipes for fossil fuelled power stations. They are also finding use in high temperature process plant within the oil and gas sector, and are being considered for use in generation IV nuclear designs. The high temperature conditions that these steels operate under in fossil fuelled power stations induce type IV cracking. This type of cracking occurs in the intercritical or fine grain region of the heated affected zone via a creep mechanism, and results in fractures with relatively little total cross-weld strain. Despite the occurrence of type IV cracking experienced in lower alloy predecessors, successor alloys have been introduced and widely used with insufficient consideration given to the consequences of welding them. Unfortunately, the newer steels suffer from reduced cross-weld creep strength due to type IV cracking to a greater degree in the temperature range of operation expected of them, and thus many failures by this mechanism have occurred. The subject of type IV cracking has been an area of active research interest. This review aims to serve as an update, drawing selectively on some of the vast amount of literature that has been published over the last 30 years.
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The need for improved wear and abrasion resistant steels for components in advanced fossil energy conversion systems is described. Desirable combinations of mechanical properties for these components are enumerated. A critical component, coal feeders, in coal gasification plants requires adequate room temperature toughness and high strength at both room and moderately elevated temperatures. Through modification of both composition and heat treatment, it has been shown that commercial secondary hardening matrix steels are promising candidates for this application. It is further shown that improvements can be achieved by the synthesis of new secondary hardening steels. A key feature of the design of these steels is the suppression, by composition control, of solid-state tempering reactions that (in commercial secondary hardening steels) lead to inadequate toughness. In other components for advanced coal technology, hot strength is not required but hardness and impact strength are. Modified medium-alloy, ultra-high-strength steels are described with combinations of strength and toughness achievable only in the high-alloy (and expensive) maraging steels.
Tempering
Hardening (computing)
Precipitation hardening
Heat treating
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There have been concerted world wide efforts to develop steels suitable for use in efficient fossil fired power plants. Ferritic alloys containing between 9 and 12 wt-% chromium are seen as the most promising materials in this respect, especially for thick walled components such as headers and the main steam pipe in boilers. However, the performance of the improved steels has often not been realised in service, because premature failures occur in the heat affected zone of welded joints in a phenomenon referred to as type IV cracking. This review assesses the relationship between the composition and microstructure of 9–12 Cr steels, the welding and fabrication procedures and how these factors translate into a propensity for type IV failures.
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Status of Development of the VM12 Steel for Application at High Temperature in Advanced Power Plants
Improvement of boiler efficiency is reached by increasing the pressure and temperature of newly designed boilers. Recently developed steel grades such as T/P91 and T/P92 are used in the advanced power plants thanks to their high creep rupture strength resistance which enables maintaining acceptable thickness of the tubes and pipes. Nevertheless their operating temperature range is limited by their oxidation behavior which is lower than classical 12%Cr steels or austenitic steels. For these reasons we have developed a new steel grade which combines good creep resistance and high steam-side oxidation behavior. This new steel, based on chromium content of 12% and with other elements such as cobalt, tungsten and boron, is named VM12. Manufacturing of this grade has been proved by production of several laboratory and industrial heats and rolling of tubes and pipes in several dimensions by different rolling processes. In addition to base metal property investigations — including creep tests and high temperature oxidation behavior — welding, cold bending and hot induction bending qualifications took place. This paper summarizes the results of the investigations and presents the first findings for processing.
Base metal
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Advanced ferritic/martensitic steels are being used extensively in fossil energy applications. New steels such as 2 1/4Cr-W-V (T23, T24), 3Cr-W-V, 9Cr-Mo-V (T91), 7Cr-W-V, 9Cr-W-V (T92 and T911), and 12Cr-W-V (T122, SAVE 12, and NF12) are examples of tubing being used in boilers and heat recovery steam generators (1). Other products for these new steels include piping, plates, and forgings. There is concern about the high-temperature performance of the advanced steels for several reasons. First, they exhibit a higher sensitivity to temperature than the 300 series stainless steels that they often replace. Second, they tend to be metallurgically unstable and undergo significant degradation at service temperatures in the creep range. Third, the experience base is limited in regard to duration. Fourth, they will be used for thick-section, high-pressure components that require high levels of integrity. To better understand the potential limitations of these steels, damage models are being developed that consider metallurgical factors as well as mechanical performance factors. Grade 91 steel was chosen as representative of these steels for evaluation of cumulative damage models since laboratory and service exposures of grade 91 exceed 100,000 hours.
Structural material
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The challenge of growing continuously in a sustainable way is the main driver to improve efficiency in the use of natural resources. The increasing demand for energy has made thermal power based countries to set audacious programs to increase efficiency of thermal power generation. In China, coal-burning accounts nowadays for approximately 65% of the total primary energy supply being responsible for around 25% of the countries' CO2 emission, this coal-based energy supply scenario is believed to continue until 2020. Therefore, the country has invested strongly in the last years in the construction of more efficient power plants. To attend higher operating temperatures and steam pressures, the application of higher performance materials is mandatory, presenting improved mechanical resistance — to stand the higher pressures applied — and having sufficient high temperature and corrosion resistance with the best cost-benefit relation possible. The present work addresses some research developments made in niobium containing austenitic stainless steels for super heaters and re-heater tubes in the past years as a joint effort between industry and academia to understand mechanisms and optimize the steel chemical composition, improving its performance. Niobium role has been studied in detail in heat resistant stainless steels TP347H, Super 304 and HR3C, a summary of such studies is presented in this paper. Niobium improves high temperature properties as it precipitates as nano-size MX and NbCrN, well dispersed in the matrix, hindering dislocation movement, increasing precipitation strengthening and creep resistance.
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Increasing the operating temperatures of fossil power plants is fundamental to improving thermal efficiencies and reducing undesirable emissions such as CO{sub 2}. One group of alloys with the potential to satisfy the conditions required of higher operating temperatures is the advanced ferritic steels such as ASTM Grade 91, 9Cr-2W, and 12Cr-2W. These are Cr-Mo steels containing 9-12 wt% Cr that have martensitic microstructures. Research aimed at increasing the operating temperature limits of the 9-12 wt% Cr steels and optimizing them for specific power plant applications has been actively pursued since the 1970's. As with all of the high strength martensitic steels, specifying upper temperature limits for tempering the alloys and heat treating weldments is a critical issue. To support this aspect of development, thermodynamic analysis was used to estimate how this critical temperature, the A{sub 1} in steel terminology, varies with alloy composition. The results from the thermodynamic analysis were presented to the Strength of Weldments subgroup of the ASME Boiler & Pressure Vessel Code and are being considered in establishing maximum postweld heat treatment temperatures. Experiments are also being planned to verify predictions. This is part of a CRADA project being done with Alstom Power, Inc.
Tempering
Superheater
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AbstractNowadays, intense efforts are made to increase efficiency and thereby minimize harmful emissions of power plants. This can be achieved by increasing operating pressure and temperature to ultrasupercritical conditions. Presently martensitic 9% Cr-steels, e.g. P91, E911 and P92 are used for power plants with advanced steam parameters. Whilst these materials have the highest creep rupture strength values of ferritic steels, their oxidation resistance is lower than 12% Cr-steels, such as X20CrMoV12-1. With increasing steam temperature (target: 650°C) the lifetime of components made of 9% Cr-steel becomes limited not only by creep but also by oxidation. The present paper reports a new 12Cr martensitic steel developed by Vallourec & Mannesmann, which is designed for use at temperatures up to 650°C. It is the outcome of a normative research project of the Belgian Welding Institute in collaboration with Laborelec and with industrial partners (Carnoy Industrial Piping, Cockerill Mechanical Industries, Fabricom, Stork Mec, Böhler Thyssen Welding Germany, Vallourec & Mannesmann Tubes, AIB-Vinçotte, VCL, Tractebel and WTCM). Base metal properties (creep strength, toughness, reheat cracking susceptibility, oxidation behavior…), welding and high temperature behavior of the new 12% Cr-steel, and welds are addressed.Keywords: 12%Cr steelpower plantweldingcreep
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The project, 'Development of a New Class of Fe-Cr-W(V) Ferritic Steels for Industrial Process Applications', was a Cooperative Research and Development Agreement (CRADA) between Oak Ridge National Laboratory (ORNL) and Nooter Corporation. This project dealt with improving the materials performance and fabrication for the hydrotreating reactor vessels, heat recovery systems, and other components for the petroleum and chemical industries. The petroleum and chemical industries use reactor vessels that can approach the ship weights of approximately 300 tons with vessel wall thicknesses of 3 to 8 in. These vessels are typically fabricated from Fe-Cr-Mo steels with chromium ranging from 1.25 to 12% and molybdenum from 1 to 2%. Steels in this composition have great advantages of high thermal conductivity, low thermal expansion, low cost, and properties obtainable by heat treatment. With all of the advantages of Fe-Cr-Mo steels, several issues are faced in design and fabrication of vessels and related components. These issues include the following: (1) low strength properties of current alloys require thicker sections; (2) increased thickness causes heat-treatment issues related to nonuniformity across the thickness and thus not achieving the optimum properties; (3) fracture toughness (ductile-to-brittle transition ) is a critical safety issue for these vessels, and it is affected in thick sections due to nonuniformity of microstructure; (4) PWHT needed after welding and makes fabrication more time-consuming with increased cost; and (5) PWHT needed after welding also limits any modifications of the large vessels in service. The goal of this project was to reduce the weight of large-pressure vessel components (ranging from 100 to 300 tons) by approximately 25% and reduce fabrication cost and improve in-service modification feasibility through development of Fe-3Cr-W(V) steels with combination of nearly a 50% higher strength, a lower DBTT and a higher upper-shelf energy, ease of heat treating, and a strong potential for not requiring PWHT.
Brittleness
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