Search Results for isothermal aging

The morphological evolution of secondary γ′ precipitates under the coarsening process was investigated for commercial wrought Ni-based superalloys, which can be classified into two processes, i.e. “localization process” and “aggregation process”. The localization process was defined as a phenomenon in which cuboidal γ′ precipitates were arranged in the <100> direction for superalloys. In contrast, the aggregation process was defined as a phenomenon in which neighboring spherical γ′ precipitates coarsen while overlapping their interfaces for superalloys. All the wrought Ni-based superalloys could be classified into the above two processes based on their volume fraction and lattice misfit. The coarsening of γ′ precipitates follow the aggregation process when the misfit is smaller than 0.05%, and it follows the localization process otherwise.

In this paper we tried to model the creep-strength degradation of selected advanced creep resistant steels which occurs under operating conditions. In order to accelerate some microstructure changes and thus to simulate degradation processes in long-term service, isothermal ageing at 650°C for 10 000 h was applied to P91, P92 and P23 steels in their as- received states. The tensile creep tests were performed at temperature 600°C in argon atmosphere on all steels both in the as-received state and after isothermal ageing, in an effort to obtain a more complete description of the role of microstructure stability in high temperature creep of these steels. Creep tests were followed by microstructure investigations by means of transmission and scanning electron microscopy and by the thermodynamic calculations. The applicability of the creep tests was verified by the theoretical modelling of the phase equilibrium at different temperatures. It is suggested that under restricted oxidation due to argon atmosphere microstructure instability is the main detrimental process in the long-term degradation of the creep rupture strength of these steels.

In this study, the role of minor alloying additions in 347H stainless steels (UNS34709, ASTM A240/240M) on creep-rupture properties at 650-750°C and microstructure evolution during isothermal exposure at 750°C has been investigated, aiming to provide the experimental dataset as boundary conditions of physics-based modeling for material/component life prediction. Four different 347H heats containing various amounts of boron and nitrogen additions were prepared and evaluated. The combined additions of B and N are found to stabilize the strengthening secondary M 23 C 6 carbides and retarding the transition from M 23 C 6 to sigma phase precipitates during thermal exposure. The observed kinetics of microstructure evolution reasonably explains the improvement of creep-rupture properties of 347H stainless steels with the B and N additions.

The microstructural evolution has been investigated for an 18Cr-12Ni stainless steel (347HFG) that has been subject to a thermo-mechanical treatment to obtain a fine grain size (ASTM 7-10). In particular, sigma phase precipitation and growth has been evaluated. Samples of 347HFG stainless steel have been isothermally heat treated to reproduce and accelerate the ageing conditions experienced in-service at temperatures between 600 and 750 °C for up to 10,000 hours. Results have shown that sigma phase is precipitated at triple points and along grain boundaries after as little as 1000 hours which is contrary to thermodynamic predictions. In addition X-ray diffraction (XRD) and image analysis has been carried out to semi-quantitatively measure the amount of sigma phase present. The area fraction of sigma has been found to be 2.77 and 2.23 percent at 700 and 750 °C respectively. This is a higher volume fraction of sigma phase than has been previously observed in regular 347H at these conditions. It is thought that this is due to the reduced grain size that has provided an increase in nucleation sites and diffusion paths that can enhance the precipitation and growth of sigma phase. The results from this study are discussed with regards to the effect of precipitation on the service life of a 347HFG stainless steel tube operating in advanced supercritical boilers.

A detailed examination has been carried out of the microstructural evolution and mechanical properties of samples of T91 and T92 steels which have been subjected to both a ‘normal’ pre­service heat treatment and an extended stress relief heat treatment at 765°C for up to 16 hours. The samples have subsequently been creep tested to failure at different stresses ranging from 66 to 112 MPa. In each case, a reduction in rupture time was observed of 20-30% in the samples which had experienced the additional stress relief heat treatment compared to those which had not. It is shown that these data, when compared with the mean values expected from European Creep Collaborative Committee (ECCC) Datasheets, result in a reduction in stress of approximately 10% of the mean value predicted from the ECCC data, which is within the allowable scatter band.

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

Martensitic 9Cr steels have been developed which are strengthened by boron in order to stabilize the microstructure and improve their long-term creep strength. Boron plays a key role in these steels by stabilising the martensitic laths by decreasing the coarsening rate of M 23 C 6 carbides, which act as pinning points in the microstructure. In this work two modified FB2 steel forgings are compared. Both forgings have similar compositions but one underwent an additional remelting process during manufacture. Creep tests showed that this additional processing step resulted in a significant increase in time to failure. In order to investigate the effect of the processing route on microstructural evolution during aging and creep, a range of advanced electron microscopy techniques have been used including ion beam induced secondary electron imaging and High Angle Annular Dark Field (HAADF) imaging in the Scanning Transmission Electron Microscope. These techniques have enabled the particle population characteristics of all the second phase particles (M 23 C 6 , Laves phase, BN and MX) to be quantified for materials from both forging processes. These quantitative data have enabled a better understanding of how the processing route affects the microstructural evolution of FB2 steels.

In this study, 25Cr2Ni2Mo1V filler metal was deposited to weld low pressure steam turbine shafts, which are operated in fossil power plants. A comparison experiment was conducted on the weld metals (WMs) before and after varied various aging duration from 200 hours up to 5000 hours at 350 ℃. Microstructure was characterized by means of scanning electron microscopy (SEM) and electron back-scattered diffraction (EBSD) techniques. In addition, mechanical properties of corresponding specimens were evaluated, e.g. Vickers microhardness, Charpy V impact toughness and tensile strength. It is shown that the tensile strength remained stable while impact energy value decreased with increasing aging duration. Based on the experiment above, it was concluded that the variation of mechanical properties can be attributed to the redissolution of carbides and reduction of bainite lath substructure.

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

Ni-base superalloys used for hot section hardware of gas turbine systems experience thermomechanical fatigue (TMF), creep, and environmental degradation. The blades and vanes of industrial gas turbines (IGTs) are made from superalloys that are either directionally-solidified (DS) or cast as single crystals (SX). Consequently, designing and evaluating these alloys is complex since life depends on the crystallographic orientation in addition to the complexities related to the thermomechanical cycling and the extent of hold times at elevated temperature. Comparisons between the more complex TMF tests and simpler isothermal low cycle fatigue (LCF) tests with hold times as cyclic test methods for qualifying alternative repair, rejuvenation, and heat-treatment procedures are discussed. Using the extensive set of DS and SX data gathered from the open literature, a probabilistic physics-guided neural network is developed and trained to estimate life considering the influence of crystallographic orientation, temperature, and several other cycling and loading parameters.

The creep rupture properties of welded joints of advanced 9%Cr-Mo-Co-B steel used for 620°C USC steam turbine have been studied. The welded joints were prepared by means of shielded metal arc welding (SMAW). A lot of creep tests have been conducted and the results indicate that fracture usually occurs in the intercritical heat affected zone (ICHAZ) of the welded joint and is typical of Type IV cracking. The microstructure of the HAZ has been investigated by using optical microscopy, SEM and TEM. The degradation mechanism of welded joint of the 9%Cr-Mo-Co-B steel has been explored by analysing the phases of precipitates. Creep voids were observed in the vicinity of the coarse Laves phase particles, resulting in the degradation of the creep rupture properties.

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.

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

The creep strength and ductility of Grade P22 steel (2¼ Cr) was measured at 600°C under standard uniaxial tensile conditions at 150MPa. Test specimens were prepared by solution heat treatment at austenitization temperatures ranging from 900°C - 1200°C followed by normalization at 900°C before continuous air cooling to room temperature. In addition to specimens tested in the solution treated state, creep tests were also performed after tempering. The variable austenitization temperatures gave rise to different prior austenite grain (PAG) sizes, which in turn influenced the crystallographic packet and block boundary misorientation angle distribution. The latter parameters were measured using electron backscattered diffraction which also allowed partial reconstruction of the PAG boundaries. The time to creep failure at 600°C increased as function of PAG size up to approximately 70µm, but significantly decreased when the average prior austenite grain size measured approximately 108 µm. However, the minimum creep rate decreased even up to the largest PAG size with corresponding decrease in creep ductility. The stability of the crystallographic packet and block boundaries influences the high strength-low ductility for the large PAGs in comparison to the dominant effect of PAG boundaries at the smallest grain size where extensive recovery and recrystallization reduces creep strength.

Strengthening of Ni-based superalloys is in principle designed using GCP (Geometrically Close-packed phase) of Ni 3 Al-γ' (L1 2 ). However, game-changing microstructural design principle without relying on γ' phase will be needed for further development of the alloys. We are currently constructing a novel microstructure design principle, using thermodynamically stable TCP (Topologically Close-packed phase) for grain boundaries, together with GCP other than γ' phase for grain interiors, based on grain boundary precipitation strengthening (GBPS) mechanism. One of the promising systems is Ni-Cr-Mo ternary system, where TCP of NiMo (oP112) phases, μ (hR13) and P (oP56), together with GCP of Ni 3 Mo (oP8) and Ni 2 Cr (oP6) exists. In this study, thus, phase equilibria among A1 (fcc)/TCP/GCP phases in Ni-Cr-Mo and Ni-Cr-W systems have been examined at temperature range from 973 K to 1073 K, based on experiment and calculation. In Ni-Cr-Mo system, Ni 2 (Cr, Mo) with oP6 Pearson symbol, which is stable at about 873 K in Ni-Cr binary system, is formed to exist even at 1073 K. oP6 phase is coherently formed in A1 matrix with a crystallographic orientation of {110} A1 // (100) oP6 , <001>Α1 // [010]oP6, indicating GCP at composition range around Ni-15Cr-15Mo as island. In Mo-rich region there is Α1/NiMo/oP6 three-phase coexisting region, whereas another three-phase coexisting region of Α1/P/oP6 exists in Cr-rich region. Based on vertical section, it is possible to design microstructure with TCP at grain boundaries, together with oP6 phase within grain interiors by two-step heat treatment.

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 microstructural evolution of the Ni-based superalloy CMSX-4 including the change in gamma prime size and distribution and the degree of rafting has been examined in detail using field emission gun scanning electron microscopy (FEGSEM) and transmission electron microscopy (TEM) after high temperature degradation and rejuvenation heat treatments. The relationship between the microstructure, mechanical properties and the applied heat treatment procedures has been investigated. It is shown that there are significant differences in the rafting behaviour, the size of the ‘channels’ between the gamma prime particles, the degree of rafting and the size of the tertiary gamma prime particles in each of the different microstructural conditions studied. Chemical segregation investigations were carried out to establish the cause of reduced mechanical properties of the rejuvenated sample after high temperature degradation compared to an as-received sample after the same degradation procedure. The results indicate that although the microstructure of as-received and rejuvenated samples were similar, the chemical segregation was more pronounced in the rejuvenated samples, suggesting that chemical segregation from partitioning of the elements during rejuvenation was not completely eliminated. The aim of this research is to provide greater understanding of the suitability of rejuvenation heat treatments and their role in the extension of component life in power plant applications.

This study investigates the influences of product chemistry and grain size on the high-temperature creep properties of 316 stainless steels by analyzing an extensive range of historical and modern literature data. The investigated 316 stainless steel creep property dataset, including more than 160 heats and 2,400 creep testing data, covers a wide spectrum of elemental compositions and product forms. To perform a prudent analysis of the creep property dataset, a statistical overview was first implemented to understand the data distribution relevant to data sources, chemistries, product forms, testing temperatures, and grain sizes. The creep data of 550°C, 600°C, 650°C, 700°C, and 750°C with ±10°C were grouped together, and the analytical study was performed on each sub dataset to investigate the temperature-specific creep performance. The creep strength was evaluated using the average stress ratio (ASR) between the experimental and predicted creep data of tested 316SS heats. The influence of composition and grain size on the creep strength ratio were evaluated using linear correlation analysis. Effects of specified and non-specified elements including C, N, and B were specifically investigated to understand their impacts on the creep strength with regards to the variation of creep temperature. In addition to the literature data, the most recent EPRI creep data of three commercial heats were used to validate the correlations from the historical creep property dataset.

A new ferritic steel branded as Thor 115 has been developed to enhance high-temperature resistance. The steel design combines an improved oxidation resistance with long-term microstructural stability. The new alloy was extensively tested to assess the high-temperature time- dependent mechanical behavior (creep). The main strengthening mechanism is precipitation hardening by finely dispersed carbide (M 23 C 6 ) and nitride phases (MX). Information on the evolution of secondary phases and time-temperature-precipitation behavior of the alloy, essential to ensure long-term stability, was obtained by scanning transmission electron microscopy with energy dispersive spectroscopy, and by X-ray powder diffraction on specimens aged up to 50,000 hours. The material behavior was also tested in service conditions, to validate the laboratory results: Thor 115 tubing was installed in a HRSG power plant, directly exposed to turbine flue gasses. Tubing samples were progressively extracted, analyzed and compared with laboratory specimens in similar condition. This research shows the performance of Thor 115 regarding steam oxidation and microstructure evolution up to 25,000 exposure hours in the field. So far, no oxide microstructure difference is found between the laboratory and on field tubing: in both cases, the oxide structure is magnetite/hematite and Cr-spinel layers and the oxide thickness values lay within the same scatter band. The evolution of precipitates in the new alloy confirms the retention of the strengthening by secondary phases, even after long-term exposure at high temperature. The deleterious conversion of nitrides into Z phase is shown to be in line with, or even slower than that of the comparable ASME grade 91 steel.

Creep properties of 9Cr heat resistant steels can be improved by the addition of boron and nitrogen to produce martensitic boron-nitrogen strengthened steels (MarBN). The joining of this material is a crucial consideration in the material design since welds can introduce relatively weak points in the structural material. In the present study, creep tests of a number of MarBN weld filler metals have been carried out to determine the effect of chemistry on the creep life of weld metal. The creep life of the weld metals was analysed, and the evolution of creep damage was investigated. Significant differences in the rupture life during creep have been observed as a function of boron, nitrogen and molybdenum concentrations in the weld consumable composition. Although the creep lives differed, the particle size and number in the failed creep tested specimens were similar, which indicates that there is a possible critical point for MarBN weld filler metal creep failure.