Search Results for alloy design

The addition of boron without the formation of any boron nitrides during normalizing heat treatment at high temperature minimizes the degradation in creep strength of both base metal and welded joints of 9Cr steel at 650 °C and long times. The enrichment of soluble boron near prior austenite grain boundaries (PAGBs) by the segregation is essential for the reduction of coarsening rate of M 23 C 6 carbides in the vicinity of PAGBs, enhancing boundary and subboundary hardening, and also for the production of same microstructure between the base metal and heat-affected-zone (HAZ) in welded joints, indicating no Type IV fracture in HAZ. Excess addition of boron and nitrogen promotes the formation of boron nitrides during normalizing, which reduces the soluble boron concentration and accelerates the degradation in creep rupture ductility at long times. 9Cr- 3W-3Co-VNb steel with 120 - 150 ppm boron and 60 - 90 ppm nitrogen (MARBN) exhibits not only much higher creep strength of base metal than Gr.92 but also substantially no degradation in creep strength due to Type IV fracture at 650 °C. The pre-oxidation treatment in Ar gas promotes the formation of protective Cr 2 O 3 scale on the surface of 9Cr steel, which significantly improves the oxidation resistance in steam at 650 °C.

To enhance long-term creep strength at 650°C, stabilization of the lath martensitic microstructure near prior austenite grain boundaries has been investigated for a 9Cr-3W-3Co-0.2V-0.05Nb steel. This was achieved by adding boron to stabilize M 23 C 6 carbides and dispersing fine MX nitrides. Creep tests were conducted at 650°C for up to approximately 3 × 10 4 hours. Adding a large amount of boron exceeding 0.01%, combined with minimized nitrogen, effectively stabilized the martensitic microstructure and improved long-term creep strength. The amount of available boron, free from boron nitrides and tungsten borides, is crucial for enhancing long-term creep strength. Reducing the carbon concentration below 0.02% led to a dispersion of nano-sized MX nitride particles along boundaries and in the matrix, resulting in excellent creep strength at 650°C. A critical issue for the 9Cr steel strengthened by MX nitrides is the formation of Z-phase, which degrades long-term creep strength. Excess nitrogen additions of 0.07 and 0.1% promoted Z-phase formation during creep. The formation of a protective Cr-rich oxide scale was achieved through a combination of Si addition and pre-oxidation treatment in argon, significantly improving the oxidation resistance in steam at 650°C.

A new Ni-base superalloy has been developed for Advanced Ultra Super Critical (A-USC) power plants operating above 750°C, targeting reduced CO 2 emissions through improved efficiency. While existing research focuses on 700°C-class materials, this study presents a novel alloy design for higher-temperature applications. Using the CALPHAD method, a prototype alloy (Ni-23Co-18Cr-8W-4Al-0.1C) was developed by eliminating Ti, Nb, and Ta to improve hot-workability while maintaining strength. The resulting alloy demonstrates twice the creep strength of Nimonic 263, with an estimated 10 5 h steam turbine creep resistance temperature of 780°C, marking a significant advancement in A-USC material capabilities.

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.

A new alloy design concept for creep- and corrosion-resistant, fully ferritic alloys was proposed for high-temperature structural applications in current/future fossil-fired power plants. The alloys, based on the Fe-30Cr-3Al (in weight percent) system with minor alloying additions of Nb, W, Si, Zr and/or Y, were designed for corrosion resistance though high Cr content, steam oxidation resistance through alumina-scale formation, and high-temperature creep performance through fine particle dispersion of Fe 2 (Nb,W)-type Laves phase in the BCC-Fe matrix. Theses alloys are targeted for use in harsh environments such as combustion and/or steam containing atmospheres at 700°C or greater. The alloys, consisting of Fe-30Cr-3Al-1Nb-6W with minor alloying additions, exhibited a successful combination of oxidation, corrosion, and creep resistances comparable or superior to those of commercially available heat resistant austenitic stainless steels. An optimized thermo-mechanical treatment combined with selected minor alloying additions resulted in a refined grain structure with high thermal stability even at 1200°C, which improved room-temperature ductility without sacrificing the creep performance. The mechanism of grain refinement in the alloy system is discussed.

By utilizing computational thermodynamics in a Design of Experiments approach, it was possible to design and manufacture nickel-base superalloys that are strengthened by the eta phase (Ni3Ti), and that contain no gamma prime (Ni3Al,Ti). The compositions are similar to NIMONIC 263, and should be cost-effective, and have more stable microstructures. By varying the aging temperature, the precipitates took on either cellular or Widmanstätten morphologies. The Widmanstätten-based microstructure is thermally stable at high temperatures, and was found to have superior ductility, so development efforts were focused on that microstructure. High temperature tensile test and creep test results indicated that the performance of the new alloys was competitive with NIMONIC 263. SEM and TEM microscopy were utilized to determine the deformation mechanisms during creep.

Effects of alloying additions of Ti or Mo to a simplified chemical composition of the γ′′-Ni 3 Nb strengthened type Ni-based alloy 718 on the precipitation mode of δ-Ni 3 Nb phase were investigated to aim at designing grain boundaries using the δ phase for raising temperature capability of the γ′′ strengthened Ni-based wrought alloys. In the base alloy of Ni-22Cr-16Fe-3.5Nb, the δ phase precipitated at the grain boundaries of the matrix phase in a platelet form by continuous precipitation mode at temperatures above 1273K (1000°C) but in a lamellar morphology by discontinuous precipitation mode below that temperature. The boundary temperature where the continuous/discontinuous precipitation mode changes was raised by addition of 1 % Ti and lowered by addition of 5% Mo. The increase in the boundary temperature by Ti addition can be considered to have occurred by an increase in the solvus temperature of γ′′ phase. The decrease in the boundary temperature by Mo addition can be interpreted by the reduction of the strain energy caused by the coherent γ′′ precipitates and/or the volume change by the formation of δ phase from the γ/γ′′ phases, which may promote the continuous precipitation with respect to the discontinuous precipitation.

The aspiration to deploy Nb-based alloys as viable upgrade for Ni-based superalloys is rooted in their potential for superior performance in high-temperature applications, such as rocket nozzles and next-generation turbines. However, realizing this goal requires overcoming formidable design hurdles, including achieving high specific strength, creep resistance, fatigue, and oxidation resistance at elevated temperatures, while preserving ductility at lower temperatures. Additionally, the requisite for alloy bond-coatings, to ensure compatibility with coating materials, further complicates the design process. QuesTek Innovations has its Integrated Computational Materials Engineering (ICME) technologies to design a superior performance high-temperature Nb-based superalloy based on solid solution and precipitation strengthening. Additionally, utilizing a statistical learning method from very limited available data, QuesTek engineers were able to establish physics-based material property models, enabling accurate predictions of equilibrium phase fraction, DBTT, and creep properties for multicomponent Nb alloys. With the proven Materials by Design methodology under the ICME framework, QuesTek successfully designed a novel Nb superalloy that met the stringent design requirements using its advanced ICMD materials modeling and design platform.

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.

In Europe, the development of boilers and steam turbines for operation above 700°C is part of the EU-supported AD700 project. This collaborative effort includes major European power plant manufacturers, utilities, and research institutes. The project began in 1998 and was extended to 2003, with a second phase running from 2002 to 2005, potentially extending further for long-term creep tests. The goal is to develop the necessary technology for constructing and operating such plants. This paper outlines the development of high-temperature materials crucial for the AD700 project. It covers factors influencing alloy design and selection, the scope and results of investigations on candidate alloys, and the ongoing program for full-scale prototype component manufacturing. These prototypes undergo extensive long-term testing. Additionally, the development of joining procedures for these materials is discussed.

The advancement of additive manufacturing (AM) technology has heightened interest in producing components from nickel-based superalloys for high-temperature applications; however, developing high gamma prime (γ’) strengthened alloys suitable for AM at temperatures of 1000°C or higher poses significant challenges due to their “non-weldable” nature. Traditional compositions intended for casting or wrought processes are often unsuitable for AM due to their rapid heating and cooling cycles, leading to performance compromises. This study introduces ABD-1000AM, a novel high gamma prime Ni-based superalloy designed using the Alloys-by-Design computational approach to excel in AM applications at elevated temperatures. Tailored for AM, particularly powder bed fusion, ABD-1000AM demonstrates exceptional processing capability and high-temperature mechanical and environmental performance at 1000°C. The study discusses the alloy design approach, highlighting the optimization of key performance parameters, composition, and process-microstructure-performance relationships to achieve ABD-1000AM’s unique combination of processability and creep resistance. Insights from ABD-1000AM’s development inform future directions for superalloy development in complex AM components.

New Fe-base ferritic alloys based on Fe-30Cr-3Al-Nb-Si (wt.%) were proposed with alloy design concepts and strategies targeted at improved performance of tensile and creep-rupture properties, environmental compatibilities, and weldability, compared to Grade 91/92 type ferritic-martensitic steels. The alloys were designed to incorporate corrosion and oxidation resistance from high Cr and Al additions and precipitate strengthening via second-phase intermetallic precipitates (Fe2Nb Laves phase), with guidance from computational thermodynamics. The effects of alloying additions, such as Nb, Zr, Mo, W, and Ti, on the properties were investigated. The alloys with more than 1 wt.% Nb addition showed improved tensile properties compared to Gr 91/92 steels in a temperature range from 600-800°C, and excellent steam oxidation at 800°C as well. Creep-rupture properties of the 2Nb-containing alloys at 700°C were comparable to Gr 92 steel. The alloy with a combined addition of Al and Nb exhibited improved ash-corrosion resistance at 700°C. Additions of W and Mo were found to refine the Laves phase particles, although they also promoted the coarsening of the particle size during aging. The Ti addition was found to reduce the precipitate denuded zone along the grain boundary and the precipitate coarsening kinetics.

Driven mainly by the environmental and economic concerns, there is an urgent need for increasing the thermal efficiency of fossil fuel power generation plants, which still languishes at around 32% under current practices. Several programs have been undertaken worldwide to address this issue. One of the immediate options is to increase the steam temperature and pressure (to the supercritical range). However, the current power plant materials appear to have inadequate creep resistance under these demanding conditions along with corrosion/oxidation problems. Hence, to meet these challenges a variety of new steels and stainless steels have been developed in the United States, Japan, and Europe. Alloy design and microstructural design approaches in developing these alloys (ferritic/martensitic, austenitic and oxide-dispersion- strengthened steels) will be briefly reviewed. Further, this paper presents creep data of these steels found in the literature in terms of Larson-Miller parameters (LMP). A detailed account of plausible creep micromechanisms in these advanced steels is also be summarized.

Recently advanced ultra-super critical (A-USC) pressure power plants with 700°C class steam parameters have been under development worldwide. Japanese material R&D program for A- USC beside the plant R&D program started in 2008, launched in 2007 under the METI/NEDO foundation includes not only alloy design explores and novel ideas for developing new steels and alloys that can fill critical needs in building 700°C class advanced power plants, but also fundamental studies on creep strength and degradation assessment, which are absolutely needed to assure the long-term safe use of newly developed steels and alloys at critical temperature conditions, for instance, 650°C for ferritic steels, 700°C for austenitic steels and 750°C for Ni- based alloys. This program concept has been based on the lessons from materials issues recently experienced in the creep strength enhanced ferritic steels used for 600°C class ultra-super critical power plants. Particular outputs from the program up to now are recognized as the ferritic steel having the creep strength of 100MPa at 650°C beyond 30,000h without any Type IV degradation and as the austenitic steel developed by means of inter-metallic compounds precipitation strengthening of grain boundary which should be strongest in creep ever found. Concurrently great progresses have been seen in the research works with positron annihilation life monitoring method applicable to various kinds of defects, structural free energy values, small punch creep test data for very limited interest area, crystallographic analyses, optimum time-temperature parameter regional creep rupture curve fitting method, hardness model, etc. which would highly contribute to find out and establish the structural parameters affecting to creep strength and degradation resulting in accurately estimating the 100,000h creep strength.

As conventional coal-fired power plants seek to reduce greenhouse gas emissions by increasing efficiency, the temperature limitations of traditional ferritic/martensitic steels used in high-temperature components present a significant challenge. With Advanced Ultra Supercritical (A-USC) power plants proposing steam temperatures of 760°C, attention has turned to nickel-based superalloys as potential replacements, since ferritic/martensitic steels cannot withstand such extreme conditions. However, the current absence of cast nickel-based superalloys combining high strength, creep-resistance, and weldability has led to the development of cast analogs of wrought nickel-based superalloys, including H263, H282, and N105. This paper examines the alloy design criteria, processing experiences, as-processed and heat-treated microstructures, and selected mechanical properties of these materials while also discussing their potential for full-scale development.

Materials are developed and improved by adjusting both the alloy chemistry and the processing conditions to achieve desired microstructures and properties. Traditionally, these improvements have been made by a slow and labor-intensive series of experiments. But it is now possible to replace this expensive trial and error process by carrying out only a few ‘key’ experiments in conjunction with thermodynamic calculations. These calculations are powerful tools for alloy design, enabling improvement in the selection of alloy chemistry and the parameters used for fabrication steps such as heat treatments. In order to have the utmost confidence in the results obtained from the calculations, it is essential to have high quality thermodynamic databases. Such databases can be used not only in phase equilibrium calculations but also as the critical input for further kinetic simulations. In the present paper, we present our work on the development of reliable thermodynamic databases for nickel-based superalloys and iron alloys. We first briefly describe the methodology of developing these databases and then discuss some specific examples using the databases. With the aid of these examples, the usefulness of thermodynamic databases in aiding the development of advanced materials is discussed.

Inconel alloy 740/740H (ASME Code Case 2702) is an age-hardenable nickel-based alloy designed for advanced ultrasupercritical (A-USC) steam boiler components (superheaters, reheaters, piping, etc.). In this work, creep testing, beyond 40,000 hours was conducted a series of alloy 740 heats of varying product form, chemistry, and grain size. Long-term creep-rupture strength was found to be weakly dependent on grain size. Analysis of the time-to-rupture data was conducted to ensure long-term strength projections and development of ASME stress allowables. Testing was also conducted on welded joints in alloy 740 with different filler metal and heat-treatment combinations. This analysis shows the current weld strength reduction factor of 30% (Weld Strength Factor of 0.70) mandated by ASME Code Case 2702 is appropriate for 740 filler metal but other options exist to improve strength. Based on these results, it was found that alloy 740 has the highest strength and temperature capability of all the potential A-USC alloys available today.

Development of the advanced USC (A-USC) boiler technology has been promoted in recent years, which targets 700°C steam condition. HR6W (Ni-23Cr-7W-Ti-Nb-25Fe) and HR35 (Ni-30Cr-6W-Ti-15Fe) have been developed for A-USC boiler tubes and pipes. The former alloy is mainly strengthened by Fe 2 W type Laves phase. The latter one employs precipitation strengthening of α-Cr phase in addition to Laves phase. Characteristic alloy design of both alloys, which does not use precipitation strengthening of γ′ phase (Ni 3 Al), leads to superior ductility and resistance to stress-relaxation cracking. Stability of creep strength and microstructure has been confirmed by long-term creep rupture tests. The 100,000h average creep rupture strength of HR6W is 85MPa at 700C. That of HR35 is 126MPa at 700°C which is comparable with conventional Alloy617. Tubes of both alloys have been evaluated by the component test in Japanese national A-USC project with γ′ hardened Alloy617 and Alloy263. Detailed creep strength, deformation behavior and microstructural evolution of these alloys are described from the viewpoint of the difference in strengthening mechanisms. Capability of these alloys for A-USC boiler materials has been demonstrated by the component test in the commercial coal fired boiler as the part of the A-USC project.

A 9Cr-3W-3Co-VNbBN steel, designated MARBN ( MAR tensitic 9Cr steel strengthened by B oron and N itrides), has been alloy-designed and subjected to long-term creep and oxidation tests for application to thick section boiler components in USC power plant at 650 o C. The stabilization of lath martensitic microstructure in the vicinity of prior austenite grain boundaries (PAGBs) is essential for the improvement of long-term creep strength. This can be achieved by the combined addition of 140ppm boron and 80ppm nitrogen without any formation of boron nitrides during normalizing at high temperature. The addition of small amount of boron reduces the rate of Ostwald ripening of M 23 C 6 carbides in the vicinity of PAGBs during creep, resulting in stabilization of martensitic microstructure. The stabilization of martensitic microstructure retards the onset of acceleration creep, resulting in a decrease in minimum creep rate and an increase in creep life. The addition of small amount of nitrogen causes the precipitation of fine MX, which further decreases the creep rates in the transient region. The addition of boron also suppresses the Type IV creep-fracture in welded joints by suppressing grain refinement in heat affected zone. The formation of protective Cr 2 O 3 scale is achieved on the surface of 9Cr steel by several methods, such as pre-oxidation treatment in Ar gas, Cr shot-peening and coating of thin layer of Ni-Cr alloy, which significantly improves the oxidation resistance of 9Cr steel in steam at 650 o C. Production of a large diameter and thick section pipe and also fabrication of welds of the pipe have successfully been performed from a 3 ton ingot of MARBN.

The increasing steam parameters in modern high-efficiency fossil fuel power plants demand advanced materials with enhanced creep strength for operation under extreme temperature and pressure conditions. Tenaris has focused on developing ferritic-martensitic and austenitic grades for tube and pipe applications. At TenarisDalmine, efforts on ferritic-martensitic steels include ASTM Grade 23, a low-alloyed alternative to Grade 22 with 1.5% W, offering good weldability, creep resistance up to 580°C, and cost competitiveness. Additionally, ASTM Grade 92, an improved version of Grade 91, provides high creep strength and long-term stability for components like superheaters and headers operating up to 620°C. At TenarisNKKT R&D, austenitic steel development includes TEMPALOY AA-1, an improved 18Cr-8NiNbTi alloy with 3% Cu for enhanced creep and corrosion resistance, and TEMPALOY A-3, a 20Cr-15Ni-Nb-N alloy with superior creep and corrosion properties due to its higher chromium content. This paper details the Tenaris product lineup, manufacturing processes, and key material properties, including the impact of shot blasting on the steam oxidation resistance of austenitic grades. It also covers ongoing R&D efforts in alloy design, creep testing, data assessment, microstructural analysis, and damage modeling, conducted in collaboration with Centro Sviluppo Materiali.