Search Results for time-temperature-transformation diagram

Due to their excellent high temperature oxidation resistance, utilities worldwide are adopting advanced austenitic stainless steels (A-ASS) for critical plant components, such as heat exchangers, as they aim to achieve higher operating conditions. However, challenges may be encountered in developing life assessment and life management strategies for such components. This is because conventional methods used for life assessment, such as measuring steam side oxide scale thickness in ferritic and conventional austenitic material to predict tube metal temperature, may not be successfully applied to A-ASS. In such instances, tracking the formation and evolution of microstructural features during service, may offer a possible method to predict the temperature of these steels. For such metallurgy based lifing strategy to be successful, it is essential to develop a good understanding of microstructure evolution in these steels. In this work one heat of Super 304H, that has been creep tested at 600°C, 650°C and 700°C, with applied stress ranging from 110 to 340 MPa, is characterized using a combination of advanced characterization tools and image analysis methods. The amount of sigma phase formed at the gauge and grip sections of the samples is quantified and the methodology used to quantify this phase is presented. From the results, a time-temperature-transformation diagram for sigma formation is developed.

This study demonstrates the Electro-Thermal Mechanical Testing (ETMT) system's capability to analyze the thermo-mechanical behavior of Inconel 718 (IN718) at a heating rate of 5 °C/s, achieving temperatures up to 950 °C. The temperature profile peaks at the sample's center and is approximately 25 °C at the extremes. Upon reaching 950 °C, the sample was aged for 30 hours before being rapidly quenched. This process froze the microstructure, preserving the phase transformations that occurred at various temperatures across the temperature parabolic gradient, which resulted in a complex gradient microstructure, providing a comprehensive map of phase transformations in IN718. The integration of thermal measurement, COMSOL modeling, scanning electron microscopy enabled a thorough characterization of the microstructural evolution in IN718, linking observed phases to the specific temperatures which provided a rapid screening of the effect of using different heating treatment routes.

The formation of periodically arrayed rows of very fine Fe 2 Hf Laves phase particles was recently found in 9 wt. % chromium ferritic matrix through interphase precipitation along a reaction path of δ-ferrite → γ-austenite + Fe 2 Hf with a subsequent phase transformation of the γ phase into the α-ferrite phase. One of the problems on the formation of the fine Laves phase dispersion is a poor heat treatability; the interphase precipitation (δ-Fe→γ-Fe+Fe 2 Hf) is competitive with the precipitation of Laves phase from the δ phase in the eutectoid-type reaction pathway (δ→δ+Fe 2 Hf). In the present work, the effect of supersaturation on the precipitation of Laves phase from δ phase (δ→δ+Fe 2 Hf) and the δ→γ transformation in the reaction pathway was investigated by changing the Hf and Cr contents. The results obtained suggest that it is effective to have a high supersaturation for the precipitation of Laves phase and an adequately high supersaturation for the δ→γ transformation at the same time in order to widen the window of the interphase precipitation

The rising global energy demand has led to a surge in the construction of high-efficiency power plants with advanced steam parameters. National and international projects indicate that fossil fuels will continue to be the primary source of power generation in the coming years, despite significant efforts and progress in utilizing alternative energy sources. Economic pressures and climate protection concerns necessitate more cost-efficient and environmentally sustainable energy production. Achieving this requires reducing specific fuel and heat consumption per kilowatt-hour, making it essential to improve the efficiency of new power plants beyond those commissioned in Germany between 1992 and 2002. While new construction and process innovations contribute to efficiency gains, the primary factors driving improvement are increased steam pressure and temperature. Current design parameters include steam temperatures of 605 °C (live steam) and 625 °C (hot reheat steam), along with pressures of 300 bar (live steam) and 80 bar (hot reheat steam), which have become critical for obtaining building and operating licenses in Germany. However, the European Creep Collaborative Committee’s (ECCC) 2005 reassessment of the creep strength of steel T/P92 (X10CrWMoVNb9-2) has placed limitations on further increasing steam temperatures beyond 625 °C.

In thermal power plants, weldments of all currently used martensitic 9% chromium steels are prone to Type IV cracking in the fine-grained region of the heat-affected zone (HAZ). Japanese researchers have introduced a new martensitic steel for ultra-supercritical (USC) steam conditions that demonstrates resistance to Type IV cracking. This study compares a modified version of this boron-nitrogen balanced advanced 9Cr-3W-3Co steel with CB2, the most promising 9% Cr steel developed through the European research initiative COST, in terms of weldability. The HAZ was analyzed using the "Heat-Affected Zone Simulation" technique with a Gleeble 1500 thermo-mechanical simulator. Basic optical microscopy was complemented by advanced electron microscopy techniques, including energy-filtered TEM (EFTEM), electron energy loss spectroscopy (EELS), electron backscatter diffraction (EBSD), and energy-dispersive X-ray analysis (EDX). Phase transformations in the HAZ were directly observed using in situ X-ray diffraction with synchrotron radiation at the Advanced Photon Source (APS) of Argonne National Laboratory, IL, USA. Although both steels exhibited similar transformation behavior, their resulting microstructures after the weld thermal cycle differed significantly. At peak temperatures above 1200°C, delta ferrite formed and remained stable down to room temperature due to rapid cooling in both steels. While CB2 exhibited conventional coarse-grained (CG), fine-grained (FG), and intercritical HAZ regions, the boron-nitrogen balanced 9Cr steel did not develop a fine-grained HAZ. Since Type IV cracking primarily occurs in the FGHAZ, this alloy shows strong potential for eliminating Type IV cracking as a major life-limiting factor in heat-resistant steel weldments.

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 objective of this study was to determine the typical range of weld metal cooling rates and phase transformations during multipass gas-tungsten arc (GTA) welding of Grade 23 (SA-213 T23) tubing, and to correlate these to the microstructure and hardness in the weld metal and heat affected zone (HAZ). The effect of microstructure and hardness on the potential susceptibility to cracking was evaluated. Multipass GTA girth welds in Grade 23 tubes with outside diameter of 2 in. and wall thicknesses of 0.185 in. and 0.331 in. were produced using Grade 23 filler wire and welding heat input between 18.5 and 38 kJ/in. The weld metal cooling histories were acquired by plunging type C thermocouples in the weld pool. The weld metal phase transformations were determined with the technique for single sensor differential thermal analysis (SS DTA). The microstructure in the as-welded and re-heated weld passes was characterized using light optical microscopy and hardness mapping. Microstructures with hardness between 416 and 350 HV 0.1 were found in the thick wall welds, which indicated potential susceptibility to hydrogen induced cracking (HIC) caused by hydrogen absorption during welding and to stress corrosion cracking (SSC) during acid cleaning and service.

The creep enhanced low alloy steel with 2.25Cr-1.6W-V-Nb (HCM2S; Gr.23, ASME CC2199) has been originally developed by Mitsubishi Heavy Industries, Ltd. and Sumitomo Metal Industries, Ltd. The steel tubes and pipe (T23/P23) are now widely used for fossil fired power plants all over the world. Recently, the chemical composition requirements for ASME Code of the steel have been changed and a new Code Case 2199-4 has been issued with the additional restriction regarding Ti, B, N and Ni, and the Ti/N ratio incorporated. In this study, the effects of additional elements of Ti, N and B on the mechanical properties and microstructure of T23/P23 steels have been evaluated. It is found that N decreases the hardenability of the steel by forming BN type nitride and thus consuming the effective B, which is a key element for hardening of the steel. The addition of Ti, on the other hand, enhances the hardenability of the steel by precipitating TiN and thus increasing the effective B. It is also found that too much addition of Ti degrades the Charpy impact property and creep ductility of the steel to a great extent. This phenomenon might affect the steel's long-term creep rupture properties, although a steel with the original chemical composition has demonstrated high creep strength at temperatures up to 600°C for more than 110,000 h.

In the present study, the precipitation kinetics of topologically close-packed (TCP) Fe 2 Nb Laves and geometrically close-packed (GCP) Ni 3 Nb phases is studied quantitatively in experimental alloys with different Ta / Nb+Ta ratio, to clarify the mec4hanism of the Ta effect. The microstructure of alloys is changed from Widmanstätten structure to lamellar structure due to discontinuous precipitation, with increasing Ta / Nb+Ta. It is confirmed that Ta partitions into both Fe 2 Nb Laves and Ni 3 Nb phases. However, two phases stability is changed by added Ta content. Ta accelerates the formation kinetics of the precipitates at grain boundaries, as well as γ“-GCP phase within grain interiors, due to increased supersaturation by Ta addition. Besides, Ta retards the transformation kinetics of metastable γ“-Ni 3 Nb to stable the δ-Ni 3 Nb phase. The results indicate that Ta decreases the driving force for the transformation of the δ-GCP phase.

The delivery state of austenitic heat resistant steel boiler tubes is paramagnetic, such as TP304H, TP347H and S30432, the material state, however, appears obviously magnetic after long-time high-temperature service. Vibrating Sample Magnetometer (VSM) has been employed to test the magnetism difference after high-temperature service, and XRD, SEM, TEM, SAED and EDS has been adopted to observe and analyze their microstructure, phase structure and composition. The research results show that compared with the delivery state, the lath α´-Martensite and sometimes the lamellar ε-Martensite will occur in areas adjacent to grain boundaries due to martensite transformation in the microstructure of austenitic heat resistant steel boiler tube after high temperature service. There are high density dislocations tangled together in the substructure of α´-Martensite, and lamellar stacking faults arrayed orderly by a large number of dislocations in the substructure of ε-Martensite. The magnetism of α´-Martensite, its internal stress and carbides is the reason why the austenitic heat resistant steel boiler tubes appear obviously magnetic after high temperature service, and the α´-Martensite plays a major role.

Alloy 718, widely used for its high-temperature performance in various applications, is being investigated for use in advanced power plants. Driven by the need for efficiency improvements, these plants demand higher temperatures and pressures, putting significant stress on critical components like boiler tubes and turbines. With existing steels and alloys struggling at such high temperatures, researchers are exploring alternatives. New generation plants target steam turbine inlet temperatures of 720°C and pressures of 350MPa, necessitating superalloys for high- and intermediate-pressure rotor sections. The Thermie Advanced project explored the potential of 718 for these applications. A trial rotor disk, forged using advanced processes, underwent a novel heat treatment to enhance microstructural stability and improve creep behavior. Ongoing creep tests exceeding 100,000 hours suggest a potential 50°C increase in the operational limit compared to standard 718. This 12-year research effort holds promise for utilizing 718 in forged components of advanced ultra-supercritical power plant steam turbines, potentially operating up to 700°C.

The precipitation behavior of various phases in austenitic heat-resistant model steels, including the Fe 2 Nb Laves phase (C14 structure) on grain boundaries (GB) and grain interiors (GI), and the Ni 3 Nb metastable γ“ phase and stable δ phase on GI, was investigated through experimental study at different temperatures and thermokinetic calculation. The steel samples were prepared by arc melting followed by 65% cold rolling. Subsequently, the samples were solution treated within the γ single-phase region to control the grain size to approximately 150 μm. Aging of the solution-treated samples was carried out at temperatures ranging from 973 K to 1473 K for up to 3600 hours. Microstructural observations were conducted using FE-SEM, and the chemical compositions of the γ matrix and precipitates of Laves and δ phases were analyzed using EPMA. The precipitation modeling was performed using MatCalc software, utilizing a thermodynamic database constructed by our research group to calculate the chemical potential of each phase. Classical nucleation theory was applied for nucleation, while the SFFK model was employed for the growth and coarsening stages. Distinct phases were defined for grain boundary and grain interior Laves phase, with all precipitates assumed to have spherical morphology in the calculations. The precipitation start time was defined as the time when the precipitate fraction reached 1%. Experimental results indicated that above 973 K, Laves phase nucleation primarily occurred on grain boundaries before extending into the grain interior, with the nose temperature located around 1273 K. To replicate the experimentally determined Time-Temperature-Precipitation (TTP) diagram, interaction parameters among elements were adjusted. Additionally, by introducing lower interfacial energy between the γ matrix and Laves phase, the TTP diagram was successfully reproduced via calculation, suggesting relative stability at the interface.

Approximately 75% of the worldwide energy supply is based on fossil energy but the discussions on CO 2 emission require improvements of the conventional power technologies and also an increase of renewable energy resources. Over the past 40 years, enormous efforts, especially in the development of new materials, were made to establish the technology for the ultra-supercritical power plants, which are the standard of today’s power generation. For decades voestalpine Boehler Special Steel has been a full package supplier of customized high quality special steels and forgings with close relationships to plant manufacturers to provide products ahead of their time. This paper reports on improvements and research activities of the currently best available martensitic 9% Cr steel FB2 and the latest generation, the so-called MarBN steels, raising the operating temperatures of the 9% Cr steel class from 620 °C to 650 °C. Increasing the operating temperature requires adaptations in processes and manufacturing methods to adjust optimized microstructures with improved toughness properties and increased creep rupture strength at the same time. The microstructure of two Boron containing 9% Cr steels, FB2-2 and NPM1, developed within the framework of COST / KMM-VIN, have been investigated comparatively after different heat treatments and discussed after creep rupture tests at 650°C. The results show a dependency of the creep rupture strength on the stability of precipitates and the creep rupture time of both steels was increased by more than 30 % without negatively affecting the creep rupture strain and impact values.

In this study the effect of Widmanstätten-type morphology α 2 plates on creep has been investigated by preparing nearly equiaxed γ (N γ ) and nearly equiaxed γ having Widmanstätten-type α 2 plates within grain (Wα 2 ). Creep tests were conducted at 1073 K under constant stresses, high stress and low stress, in air. At the high stress, Wα 2 shows creep rate smaller than N γ in transient stage, both specimens show similar minimum creep rate and the creep strain at minimum creep rate is 3 % for Wα 2 and 10 % for N γ, since N γ shows prolonged primary region. In acceleration stage, both show similar behavior with rupture time of about 50 h and rupture elongation of 60 %. At the low stress, on the other hand, reverse behavior occurs, that is, W α 2 shows creep rate higher than Nγ in transient stage. The regions near grain boundaries progressively deformed for both specimens at high stress level, whereas deformed region is extended within grain interiors. From these results it is suggested that α 2 plate act as the obstacle for dislocation motion in the γ matrix at high stress and that interfacial dislocation promote the creep deformation at low stress.

Precipitation of Z-phase, Cr(V,Nb)N, is known to negatively affect creep properties of 9-12%Cr steels for power plant applications as it dissolves finely distributed MX particles, (V,Nb)N, especially in high Cr steels. As the Z-phase precipitates slowly as large particles, this causes a net drop in precipitation strengthening. Two model alloys containing 9 and 12%Cr, but otherwise having similar composition, were produced in order to quantify the difference in Z-phase precipitation speed at different Cr levels. The nitride precipitation behavior was followed at different temperatures using TEM and XRD, allowing for a quantification of the Z-phase precipitation. The Z-phase was found to precipitate 20-50 times faster in the 12%Cr steel compared to 9%Cr steel in the temperature range 600- 650°C. The transformation of MX into Z-phase was followed in a Ta containing alloy without V or Nb. In this alloy the Z-phase precipitates very quickly, and thus appears as finely distributed particles which have the same strengthening effect as MX particles. Investigations using atomic resolution microscopy showed how Cr diffuses from the matrix into the TaN MX particles and gradually transforms them both chemically and crystallographically into Z-phase CrTaN particles.

This paper addresses the limitations of P92 steel used in ultra-supercritical power plants, particularly ferrite formation in thick components and its impact on short- and long-term properties. A guideline for determining ferritic content in P92 steel is presented. Furthermore, a novel 12% Cr boiler steel grade, VM12-SHC, is introduced. This new material offers good creep properties and oxidation resistance, overcoming the limitations of P92 steel. Finally, the development of matching filler metals for welding P92 and VM12-SHC steels is presented, ensuring optimal weld compatibility and performance in power plant applications.

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.

Due to a high degree of mixing between substrate and weld deposit, fusion welding of dissimilar metal joints functionally produce new, uncharacterized alloys. In the power generation industry, such mixing during the application of cobalt-based hardfacing has led to a disconcerting number of failures characterized by the hard overlay welds disbonding. Investigations into this failure mechanism point to the unknown alloy beneath the surface of the hardfacing layer transforming, hardening, and becoming brittle during service. This research describes a methodology for exploring a chemical space to identify alloy combinations that are expected to be safe from deleterious phase formation. Using thermodynamic modeling software and a stepped approach to potential chemistries, the entire phase stability space over the full extent of possible mixing between substrate and weld material can be studied. In this way diffusion effects – long term stability – can also be accounted for even in the case where mixing during application is controlled to a low level. Validation of predictions specific to the hardfacing system in the form of aged weld coupons is also included in this paper. Though the application of this methodology to the hardfacing problem is the focus of this paper, the method could be used in other weld- or diffusion- combinations that are expected to operate in a high temperature regime.

A new 9%Cr steel with high boron levels (boron steel) has been developed by optimization studies on steels and alloys that are applicable to advanced ultra-super critical power plants operated at steam conditions of 700°C and 30 MPa and above. The composition and heat treatment condition of boron steel was optimized by the initial hardness, tensile strength, yield strength, and Charpy impact values on the basis of the fundamental investigation with the stability of the long-term creep strength. Creep testing of boron steel was conducted at temperatures between 600 and 700°C. The creep rupture strength at 625°C and 105 h is estimated to be 122 MPa for the present 9% Cr steel with high boron by Larson-Miller parameter method. Furthermore, physical properties as a function of temperature, metallurgical properties, tensile properties, and toughness were examined to evaluate the applicability of the steel for a 625°C USC power plant boiler. It was also confirmed that the steel has good workability for such an application by the flaring and flattening tests with tube specimens having an outer diameter of approximately 55 mm.

The long-term performance of superheater super 304h tube during the normal service of an ultra-supercritical 1000mw thermal power unit was tracked and analyzed, and the metallographic structure and performance of the original tube sample and tubes after 23,400h, 56,000h, 64,000 h, 70,000 h and 80,000 h service were tested. The results show that the tensile strength, yield strength and post-break elongation meet the requirements of ASME SA213 S30432 after long-term service, but the impact toughness decreases significantly. The metallographic organization is composed of the original complete austenite structure and gradually changes to the austenite + twin + second phase precipitates. With the extension of time, the number of second phases of coarseness in the crystal and the crystal boundary increases, and the degree of chain distribution increases. The precipitation phase on the grain boundary is dominated by M 23 C 6 , and there are several mx phases dominated by NbC and densely distributed copper phases in the crystal. The service environment produces a high magnetic equivalent and magnetic induction of the material, the reason is that there are strips of martensite on both sides of the grain boundary, and the number of martensite increases with the length of service.