Creep brittle behaviour in tempered martensitic, creep strength enhanced ferritic (CSEF) steels is linked to the formation of micro voids. Details of the number of voids formed, and the tendency for reductions in creep strain to fracture are different for the different CSEF steels. However, it appears that the susceptibility for void nucleation is related to the presence of trace elements and hard non-metallic inclusions in the base steel. A key factor in determining whether the inclusions present will nucleate voids is the particle size. Thus, only inclusions of a sufficient size (the critical inclusion size is directly linked to the creep stress) will act directly as nucleation sites. This paper compares results from traditional uniaxial laboratory creep testing with data obtained under multiaxial conditions. The need to understand and quantify how metallurgical and structural factors interact to influence creep damage and cracking is discussed and the significant benefits available through the use of high quality steel making and fabrication procedures are highlighted. Details of component behaviour are considered as part of well-engineered, Damage Tolerant, design methods.

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.

Diffusion bonding is a key manufacturing process for nucleation applications including compact heat exchangers. Accurately predicting the alloy's behavior during the diffusion bonding process presents challenges, primarily due to the intricate interplay of microstructural evolution and physical processes such as compressive loading, temperature history, and component migration. The current study develops a phase-field model designed to simulate the diffusion bonding in 316H stainless steel, a material with exceptional high-temperature strength, corrosion resistance and suitability to high-pressure conditions. Our model incorporates a multi-phase, multi-component framework that aligns the experimental observations with the grain growth and heterogeneous nucleation, where arbitrary external compressive load and temperature history are considered. The simulations focus on grain nucleation, growth, and microstructure evolutions across diffusion bonding line under a variety of temperature profiles, mechanical loads, and surface roughness conditions, mirroring experimental setups. Our model predicts consistent simulation results with experiments in terms of the grain size and distribution near the bonding area, offering a better understanding of the diffusion bonding mechanism and the manufacturing process for building reliable compact heat exchangers.

Damage in the grade 91 steel partially transformed zone of weld heat affected zones has historically been associated with many different types of microstructural features. Features described as being responsible for the nucleation of creep damage include particles such as laves phase, coarse M 23 C 6 , inclusions, nitrides, or interactions between creep strong and creep week grains, grain boundaries and potentially other sources. Few studies have attempted to link the observations of damage on scales of increasing detail from macro, to micro, to nano. Similarly, assessments are not made on a statistically relevant basis using 2D or 3D microscopy techniques. In the present paper, 2D assessment using scanning electron microscopy (SEM) and quantification techniques such as energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) are utilized in combination with 3D serial sectioning of large volumes using plasma focused ion beam milling (P-FIB) and simultaneous EDS to evaluate an interrupted cross-weld creep test. Moreover, the sample selected for examination was from a feature cross-weld creep test made using a parent material susceptible to the evolution of creep damage. The test conditions were selected to give creep brittle behaviour and the sample was from a test interrupted at an estimated life fraction of 60%. The findings from these evaluations provide perspective on the features in the microstructure responsible for the nucleation and subsequent growth of the observed damage.

In wrought nickel-base alloys used at elevated temperatures for extended periods of time, it is commonly observed that unwanted phases may nucleate and grow. One such phase is the eta phase, based on Ni 3 Ti, which is a plate-shaped precipitate that nucleates at the grain boundaries and grows at the expense of the strengthening gamma prime phase. In order to study the effects of eta phase on creep performance, Alloy 263 was modified to contain 3 different microstructures: standard (contains gamma prime); aged (contains gamma prime and eta); and modified (contains only eta and no gamma prime). These microstructures were then creep tested in the range of 973-1123 K (700-850°C). An extensive test matrix revealed that the eta-only modified alloy had creep rupture strengths within 10% of the standard alloy even though this alloy had no strengthening gamma prime precipitates. It also exhibited superior creep ductility. A preliminary test matrix on the aged material containing eta and gamma prime prior to the creep tests revealed that the performance of this microstructure was generally between that of the standard alloy (best) and the eta-only alloy (worst). The aged material exhibited far superior creep ductility. These results suggest that the presence of the eta phase may not be deleterious to creep ductility, and in fact, may enhance it.

Contrary to expectations, long-term performance of creep stress enhanced ferritic steels (CSEF) falls short of predictions based on short-term data. This discrepancy is attributed to the formation and growth of creep voids, leading to reduced ductility. This study investigates cavities in creep-tested P92 steel, revealing an association with large ceramic particles (1-2 μm) in standard samples. Three distinct particle compositions were identified: boron nitride (BN), manganese sulfide, and γ-Al 2 O 3 . Statistical analysis showed a strong correlation between BN particles and cavity formation. Using a 3D “slice and view” technique with a focused ion beam/field emission gun scanning electron microscope (FIB-FEGSEM), the study revealed irregular shapes for both cavities and associated particles. Furthermore, analysis of the head-gauge transition area (lower stress exposure) showed small cavities near BN particles, suggesting preferential nucleation on these hard, irregular features. These findings strongly support the hypothesis that BN particles play a key role in cavity nucleation, impacting the long-term performance of P92 steel.

As long term laboratory creep data became available the original estimates of the allowable stresses for creep strength enhanced ferritic steels (CSEF) had to be reduced. Thus, even in properly processed steel, the long term performance and creep rupture strength is below that originally predicted from a simple extrapolation of short term data. One of the microstructural degradation mechanisms responsible for the reduction in strength is the development of creep voids. Nucleation, growth and inter linkage of voids also result in a significant loss of creep ductility. Indeed, elongations to rupture of around 5% in 100,000 hours are now considered normal for long term creep tests on many CSEF steels. This relatively brittle behaviour, and the associated creep void development, promotes burst rather than leak type fracture in components. Moreover, the existence of significant densities of voids further complicates in-service assessment of condition and weld repair of these steels. The present paper examines background on the nucleation and development of creep voids in 9 to 12%Cr martensitic steels and discusses factors affecting brittle behavior.

The size and distribution of the Laves phase particles in a 9.85Cr-3Co-3W-0.13Mo-0.17Re- 0.03Ni-0.23V-0.07Nb-0.1C-0.002N-0.008B steel subjected to creep rupture test at 650°C under an applied stresses of 160-200 MPa with a step of 20 MPa were studied. After heat treatment consisting of normalizing of 1050°C and tempering of 770°C, M 23 C 6 and Fe 3 W 3 C carbides with the mean sizes of 67±7 and 40±5 nm, respectively, were revealed along the boundaries of prior austenite grains and martensitic laths whereas round NbX carbonitrides were found within martensitic laths. During creep metastable Fe 3 W 3 C carbides dissolved and the stable Laves phase particles precipitated; volume fraction of Laves phase increases with time. The Laves phase particles nucleated on the interfacial boundaries Fe 3 W 3 C/ferrite during first 100 h of creep and provided effective stabilization of tempered martensitic lath structure until their mean size less than 150 nm.

For VM12-SHC 11-12 wt. % Cr steel, there have been no systematic investigations to define the regions or characterise the microstructures within the heat-affected zone (HAZ) of weldments. In similar steels, these regions relate to the Ac 1 and Ac 3 transformation temperatures and can affect weldment performance. In this study, controlled thermal cycles were applied to VM12-SHC parent metal using a dilatometer and the Ac 1 and Ac 3 temperatures were measured for various heating rates. The Ae 1 and Ae 3 temperatures were also calculated by thermodynamic equilibrium modeling. Through dilatometry, thermal cycles were then applied to simulate the microstructures of the classically defined HAZ regions. The microstructural properties of each simulated material were investigated using advanced electron microscopy techniques and micro-hardness testing. It was found that the simulated HAZ regions could be classified as; (1) the completely transformed (CT) region, with complete dissolution of pre-existing precipitates and complete reaustenitisation; (2) the partially transformed (PT) region, exhibiting co-existing original martensite with nucleating austenite microstructures with partial dissolution of precipitates; and (3) the over tempered (OT) region, with no phase transformation but precipitate coarsening and decreased hardness.

In order to understand the microstructural evolution during service that 9Cr steels experience it is important to be able to quantify key microstructural parameters that define the characteristics of the secondary phases (e.g. precipitated phases and inclusions) and the steel matrix. The average size of M 23 C 6 , Laves phase and MX particles in these materials have been reported in many studies, however comparability between these studies is compromised by variations in technique and different/incomplete reporting of procedure. This paper provides guidelines on what is required to accurately measure these parameters in a reproducible way, taking into account macro-scale chemical heterogeneities and the statistical number of particles required to make meaningful measurements. Although international standards do exist for inclusion analysis, these standards were not developed to measure the number per unit area of hard particles that can act as creep cavity nucleation sites. In this work a standardized approach for measuring inclusions from this perspective is proposed. In addition the associated need to understand the segregation characteristics of the material are described, which in addition to defining the area that needs to be analysed to measure the average number of inclusions per unit area, also allows the maximum number of inclusions per unit area to be determined, a parameter which is more likely to define the damage tolerance of the material.

The behavior of strain-induced abnormal grain growth (AGG) in superalloy 718 has been investigated using compression testing and subsequent heat treatment below the d-phase solvus temperature of 980 °C. The nuclei of AGG grains were slightly newly recrystallized grains by a nucleation because small grains without dislocation was observed in the as- deformed microstructure. AGG was caused by the difference in intragranular misorientation (related to the stored strain energy in a grain) between dynamic recrystallized grains and deformed matrix. The initiation of AGG was retarded with decreasing plastic strain and produced microstructures consisted of larger grains having more complex morphology. It was observed that grain boundary migrated locally in the direction perpendicular to, or mainly in the direction parallel to the S3 {111} twin boundaries along with the formation of high-order twins. As a result of multiple twinning, AGG grains seemed to evolve with the growing directions changed.

The powder metallurgy (P/M) process has been applied to a high strength turbine disk alloy. It is known that P/M alloys show characteristic microstructures such as prior powder boundaries (PPB) compared to microstructures of conventional cast and wrought (CW) alloys. High temperature tensile tests were conducted on CW and P/M processed alloy720Li in order to reveal the effect of temperature and strain rate on deformation behavior and to demonstrate the effect of microstructure derived from P/M process on deformability. The fracture mode of the P/M material changed from grain interior fracture to fracture around large PPB with an increment of strain rate. In addition, samples ruptured at higher temperature showed grain boundary fracture regardless of strain rate. On the other hand, the CW material showed good deformability with chisel point fracture in the entire temperature and strain rate condition range. In the P/M material, melting of grain boundaries occurred at super solvus temperature conditions. Large PPB acts as nucleation site of voids at higher strain rate conditions. Precipitation strengthening by γ’ phase degrades deformability at sub solvus temperature conditions. However, deformability near the solvus temperature and low strain rate condition in as HIPed P/M material increased with fine grain size distribution in spite of the presence of large grains resulting from PPB.

The nucleation and growth of precipitates such as Laves phases, carbides and nitrides reduce fracture toughness and high-temperature strength of high chromium steels used in thermal power plants. For this reason, to ensure a long-term plant reliability, it is important to estimate material deterioration by aging. The study presented in this paper involves micro structural evolution by thermal aging of COST-E, F, and FB2 steels, all turbine materials. The results indicate that the Laves phases and other precipitates can be separately detected and quantified by the electrochemical technique. The results also clarify the correlation between the amount of Laves phases precipitated and electrochemical polarization parameters.

Recovery of microstructure and void formation were investigated in creep-ruptured specimens of ASME Gr. T91 steels to understand the cause of loss of creep rupture ductility in the long-term creep condition and its heat-to-heat variation. The specimens studied were two heats (MGA, MGC) of Gr. T91 steels creep-ruptured at 600 °C under the stress conditions of 160-80 MPa. The reduction of area at rupture (RA) was 55% for MGA, but 83% for MGC in the long-term condition (under the creep stress of 80 MPa), while RA was higher than 80 % for the two heats in the short-term conditions (under the creep stresses above 100 MPa). In both heats, equiaxed grains were observed in the vicinity of ruptured surface in the long-term condition, indicating that recovery and recrystallization occurred extensively in the creep condition, while grains were elongated in the short-term conditions. In the uniformly deformed regions with a small area reduction in the long-term crept specimens, recovered and recrystallized grains were observed in the limited region close to high angle grain boundaries in MGA, while they were extended into grain interiors in MGC. In the long-term creep conditions two types of voids were observed: fine ones with a diameter below 1 μm and coarse ones with a diameter from 2 μm up to 50 μm. Fine creep voids were found to grow with necking in MGA while they neither nucleated nor grew with necking in MGC. Coarse creep voids increased in size and in number with necking in both heats and were larger and denser in MGA than in MGC.

Two Z-phase strengthened test steels with similar chemical composition were studied. The main difference in composition is the addition of 1 wt% Cu into one of the steels (referred to as “ZCu”). Mechanical testing was performed. The impact strength is very different: 3 J vs. 46.3 J, for the original and the Z-Cu steel, respectively. In the original steel that contains no Cu, much more Laves-phase (Fe 2 (W,Mo)) precipitates had formed along the prior austenite grain boundaries than in the steel with Cu addition. This is believed to be the reason for the difference in impact strength. Furthermore, the Cu addition also influenced the morphology of Laves-phase precipitates; fine rod-shaped instead of coarse equiaxed Laves-phase particles were observed in Z-Cu steel in comparison to the original steel. No partitioning of Cu into the Laves-phase particles was detected by using atom probe tomography (APT). The main function of Cu seems to be the formation of Cu precipitates that act as nucleation site for Laves-phase.

A novel multi-component, multi-particle, multi-phase precipitation model is used to predict the precipitation kinetics in complex 9-12% Cr steels investigated within the European COST project. These steels are used for tubes, pipes, casings and rotors in USC (ultra super critical) steam power plants for the 21 st century. In the computer simulations, the evolution of the precipitate microstructure is monitored during the entire fabrication heat treatment including casting, austenitizing, several annealing treatments. The main interest lies on the concurrent nucleation, growth, coarsening and dissolution of different types of precipitates.

This study explores methods to enhance the creep strength of 12%Cr martensitic/ferritic steels. The approach focuses on utilizing various precipitates to hinder microstructure coarsening and dislocation movement. A combination of Laves phase (slow precipitation) and MX carbonitrides (dislocation pinning) is used for sustained strengthening. Different MX-forming elements (V, Ta, Ti) are investigated to identify the optimal combination for high quantities of finely distributed strengthening particles. Additionally, cobalt and copper are employed to promote a fully martensitic microstructure and potentially slow down diffusion or provide nucleation sites for Laves phase precipitation. Long-term creep tests confirm the effectiveness of Laves phase precipitation, particularly with tungsten present. Tantalum's influence on both MX precipitation and the Laves phase is also observed. Combining multiple MX-forming elements (V/Ta, V/Ti, Ta/Ti) further improves creep strength, supported by predictions of high MX carbonitride formation from Thermo-Calc calculations. Partially replacing cobalt with copper (1%) also demonstrates positive effects on creep properties.

This study examines the influence of carbon and austenite stabilizing elements (Ni, Mn, Co, Cu) on Laves phase precipitation, Fe 2 W formation, and creep rupture strength (CRS) in 9-12% Cr steels at 600-700°C. Nickel and manganese had minimal impact on Laves phase and coarse carbide formation up to 1% content. While cobalt increased Laves phase fraction at 650°C, it did not improve long-term CRS and even caused a rapid decrease in short-term CRS. Copper, on the other hand, promoted the precipitation of fine Cu-rich particles that acted as nucleation sites for Laves phase and M 23 C 6 carbide. This resulted in a different needle-like Laves phase morphology compared to the globular type observed in nickel and cobalt alloys, leading to improved CRS in the copper alloy. Increasing carbon content from 0.1% to 0.2% effectively suppressed Laves phase formation, as confirmed by Thermo-Calc calculations. Notably, for cobalt alloys with higher tungsten content, higher carbon content (0.09% to 0.19%) improved CRS at 650°C, whereas the opposite effect was observed in nickel and nickel-manganese alloys. Copper alloys maintained improving CRS trends even with increased carbon, leading to the overall best CRS performance among the tested alloys with 0.2% carbon.

Diffusion aluminide coatings have been evaluated as a strategy for improving the oxidation resistance of austenitic and ferritic-martensitic (FM) steels, particularly in the presence of steam or water vapor. The objective was to evaluate the strengths and weaknesses of these coatings and quantify their performance and lifetime. Long-term diffusion and oxidation experiments were conducted to study the behavior of various model diffusion coatings and produce a better data set for lifetime predictions. The key findings are that (1) thin coatings (<50μm) with relatively low Al contents appear to be more effective because they avoid high thermal expansion intermetallic phases and have less strain energy to nucleate a crack; and (2) the low Al reservoir in a thin coating and the loss of Al due to interdiffusion are not problematic because the low service temperatures of FM steels (<600°C) and, for austenitic steels at higher temperatures, the phase boundary between the ferritic coating-austenitic substrate inhibits Al interdiffusion. Unresolved issues center on the effect of the coating on the mechanical properties of the substrate including the reaction of N in the alloy with Al.

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.