Search Results for coarsening

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.

A newly developed ferritic heat-resistant steel; 9Cr-3W-3Co-Nd-B steel has higher creep rupture strength both in the base metal and welded joints than the conventional high-Cr ferritic heat-resistant steels. The creep rupture strengths of 9Cr-3W-3Co-Nd-B steel welded joints were below the lower limit of the base metal in long-term creep stage more than 20,000 hours. The creep rupture position was heat-affected zone (HAZ) from 1.0 to 1.5 mm apart from the fusion line on the welded joint specimen ruptured at 34,966 hours. The equiaxed subgrains and coarsened precipitates were observed in HAZ of the ruptured specimen. In order to clarify the creep fracture mechanism of the welded joints, the microstructures of HAZ were simulated by heat cycle of weld, then observed by EBSD analysis. Fine austenite grains formed along the prior austenite grain boundaries in the material heated just above A C3 transformation temperature, however there were no fine grains such as conventional steel welded joints. The prior austenite grain boundaries were unclear in the material heated at 1050 °C. The creep rupture life of the material heated at just above A C3 transformation temperature exceeded the lower limit of base metal and there was no remarkable degradation, although it was shorter than the other simulated materials. It is, therefore, concluded that the creep fracture of 9Cr-3W-3Co-Nd-B steel welded joint in long-term stage occurred at HAZ heated at from just above A C3 transformation temperature to 1050 °C. It is speculated that the fine austenite grains formed along the prior austenite grain boundaries and inhomogeneous microstructures cause the coarsening precipitates and recovery of lath structure during long-term creep deformation.

This study presents a characterization of the microstructural evolutions taking place in a 9%Cr martensitic cast steel subjected to fatigue and creep-fatigue loading. Basis for this study of investigation is an extensive testing program performed on a sample heat of this type of steel by conducting a series of service-like high temperature creep-fatigue tests. The major goal here was to systematically vary specific effects in order to isolate and describe relevant damage contributing mechanisms. Furthermore, some of the tests have been interrupted at several percentages of damage to investigate not only the final microstructure but also their evolution. After performing those tests, the samples were examined using transmission electron microscopy (TEM) to characterize and quantify the microstructural evolutions. The size distribution of subgrains and the dislocation density were determined by using thin metal foils in TEM. A recovery process consisting of the coarsening of the subgrains and a decrease of the dislocation density was observed in different form. This coarsening is heterogeneous and depends on the applied temperature, strain amplitude and hold time. These microstructural observations are consistent with the very fast deterioration of creep properties due to cyclic loading.

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.

A 10%Cr martensitic steel with 3%Co and 0.008%B exhibits extremely long creep rupture time of approximately 40000 h under an applied stress of 120 MPa at a temperature of 650°C. The steel’s microstructure after creep tests interrupted at different creep stages was examined by transmission and scanning electron microscopy. It was shown that superior creep resistance of this steel was attributed to slow increase in creep rate at the first stage of tertiary creep whereas the rapid acceleration of creep rate took place only at the short second stage of tertiary creep. Transition from minimum creep rate stage to tertiary creep was found to be accompanied by coarsening of Laves phase particles, whereas M 23 C 6 – type carbides demonstrated high coarsening resistance under creep condition. Strain-induced formation of Z-phase does not affect the creep strength under applied stress of 120 MPa due to nanoscale size of Z-phase particles.

Three Ni-base wrought alloys with different hardening mechanisms (INCONEL 706, Waspaloy and INCONEL 617) were investigated as candidates for steam turbine rotor applications at temperatures up to 700 °C in respect to their microstructure and microstructural stability. The Nb containing alloy Inconel 706 had a complex microstructure with γ', γ" and η phases which are stable in long term service up to 620 °C. At higher temperatures significant particle coarsening and phase transformation were observed. Waspaloy is hardened by γ' particles and after ageing at 700 °C and higher, it tended to a stable microstructure. Inconel 617 is a solid solution hardened material additionally hardened by homogeneously distributed fine M 23 C 6 carbides. After long term ageing at temperatures of 650 °C to 750 °C the carbides tended to form carbide films along the grain boundaries and at 700 °C to 750 °C γ' precipitated as homogeneously distributed particles with low coarsening during long term service. In order to optimize the candidate alloys Inconel 706 and Waspaloy were modified to the new alloys DT 706 and DT 750. The aspects of modification and first experimental results are reported.

The morphology of γ/γ' microstructures in single crystal superalloys is known to evolve during service conditions according to established materials science principles, potentially offering a novel approach for failure analysis. This study investigated the morphological changes in γ/γ' microstructures of CMSX-4, a single crystal Ni-base superalloy, under various loading conditions. The experimental parameters included tensile and compressive stress levels, loading temperature, loading rate, monotonic versus cyclic loading, and multi-axial stress states. Results demonstrated that the γ/γ' microstructures exhibited highly sensitive responses to these loading conditions. A newly developed quantitative image analysis method was used to characterize these morphological changes, and the findings were compiled into a two-dimensional map to facilitate failure analysis and other engineering applications.

9-10%Cr-3%Co martensitic steels are the prospective materials for elements of boilers, tubes and pipes for fossil power plants which are able to work at ultra-supercritical parameters of steam (T=620-650°C, P=25-30 MPa). The effect of creep on the microstructure of the 10 wt.%Cr-3Co- 3W-0.2Re martensitic steel was investigated in the condition of 650°C and an applied stress of 140 MPa, time to rupture was more than 8500 h. Previously, this steel was subjected to the normalizing at 1050°C and tempering at 770°C. This heat treatment provided the hierarchical tempered martensite lath structure with the mean size of prior austenite grains of 59 μm and with high dislocation density (2×10 14 m -2 ) within martensitic laths. Boundary M 23 C 6 and M 6 C carbides and randomly distributed within matrix Nb-rich MX carbonitrides were detected after final heat treatment. The addition of Re in the steel studied positively affected creep at 650°C/140 MPa and stabilized the tempered martensite lath structure formed during 770°C-tempering. The formation of the subgrains in the gage section was accompanied by the coarsening of M 23 C 6 carbides and precipitations of Laves phase with fine sizes during creep. No depletion of Re and Co from the solid solution during creep was revealed whereas W content decreased from 3 to 1 wt.% for first 500 h of creep. Reasons of improved creep as well as mechanisms of grain boundary pinning by precipitates are discussed.

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.

Directional coarsening of the γ' phase (rafting) in Ni-based single crystal superalloys during creep at 1273 K was simulated by the phase-field method. The inelastic strain introduced in the γ phase was assumed to be composed of plastic strain (ε p ) and creep strain (ε c ). The simulations were performed with various sets of values of material parameters and the magnitude of external tensile stress. We let a feed-forward neural network learn the simulation data in order to enable fast and exhaustive prediction of the time to rafting, t raft . From the analysis based on the trained neural network, it has been shown that t raft becomes longer with increasing magnitude of γ/γ' lattice misfit, with decreasing creep coefficient, and with increasing yield stress of the γ phase (σγ ys ). The sensitivity of t raft to σ γ ys is high when the ratio of ε p to the total inelastic strain (ε p + ε c ) is high.

A new nickel-base superalloy GH750 has been developed as boiler tube of advanced ultrasupercritical (A-USC) power plants at temperatures about and above 750°C in China. This paper researched the weld solidification of GH750 filler metal, microstructure development and property of GH750 welded joint by gas tungsten arc weld. Liquid fraction and liquid composition variation under non-equilibrium state were calculated by thermo-dynamic calculation. The weld microstructure and the composition in the dendrite core and interdendritic region were analyzed by SEM(EDX) in detail. The investigated results show that there is an obvious segregation of precipitation-strengthening elements during the weld solidification. Titanium and Niobium are the major segregation elements and segregates in the interdendritic region. It was found that the changing tendency of the elements’ segregation distribution during the solidification of GH750 deposit metal is agree with the thermodynamic calculation results. Till to 3,000hrs’ long exposure at 750°C and 800°C, in comparison with the region of dendrite core of solidification microstructure, not only the coarsening and the accumulation of γʹ particles are remarkable in the interdendritic region, but also the small quantity of the blocky and needle like η phases from. The preliminary experimental results indicate that the weakening effect of creep-rupture property of the welded joint is not serious compared with GH750 itself.

Metallographic tests, micro-hardness tests, mechanics performance tests and Energy Dispersion Spectrum (EDS) were conducted for a 2.25Cr-1Mo main steam pipe weldment served for more than 32 years. Microstructural evolution of the 2.25Cr-1Mo base metal and weld metal was investigated. Degradation in micro-hardness and tensile properties were also studied. In addition, the tensile properties of subzones in the ex-service weldment were characterized by using miniature specimens. The results show that obvious microstructural changes including carbide coarsening, increasing inter lamella spacing and grain boundary precipitates occurred after long-term service. Degradation in micro-hardness is not obvious. However, the effects of long term service on tensile deformation behavior, ultimate tensile strength and yield stress are remarkable. Based on the yield stress of micro-specimens, the order of different subzones is: WM>HAZ>BM, which is consistent with the order of different subzones based on micro-hardness. However, the ultimate tensile strength and fracture strain of HAZ are lower than BM.

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.

The paper deals with microstructural evolution in the AISI 316LN + 0.1 wt.% Nb steel during long-term creep exposure at 600 and 625°C. The following minor phases formed: Z-phase (NbCrN), M 23 C 6 , M6X (Cr3Ni2SiX type), η-Laves (Fe2Mo type) and σ-phase. M6X gradually replaced M 23 C 6 carbides. Primary Z-phase particles were present in the matrix after solution annealing, while secondary Z-phase particles formed during creep. Precipitation of Z-phase was more intensive at 625°C. The dimensional stability of Z-phase particles was excellent and these particles had a positive effect on the minimum creep rate. However, niobium also accelerated the formation and coarsening of σ-phase, η-Laves and M6X. Coarse particles, especially of σ-phase, facilitated the development of creep damage, which resulted in poor long-term creep ductility.

Structural changes in P92-type steel after creep at temperature of 600°C under a stress of 140 MPa were investigated. The steel was solution treated at 1050°C and tempered at 780°C. The structure in the grip portion of the creep specimen changed scarcely after creep exposure for 6876 h. In contrast, the structural changes in the gage and neck sections were characterized by transformation of the tempered martensite lath structure into relatively coarse subgrain structure. The formation of a well-defined subgrain structure in the gage and neck sections was accompanied by the coarsening of M 23 C 6 carbides and precipitations of Laves phase during creep. Mechanisms of grain boundary pinning by precipitates are discussed.

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.

The Institute of Materials Science, Welding and Forming (IWS) conducts research activities on ferritic/martensitic 9-12% Cr steels through an interconnected network of projects. These projects focus on mechanical properties of base and weld metals, microstructural characterization of creep and damage kinetics, weldability, microstructure analysis during creep, modeling of precipitation and coarsening kinetics, and deformation behavior under creep loading. The individual projects are briefly described, outlining the conceptual approach towards quantitatively describing the creep behavior of 9-12% Cr steels. The research efforts aim to comprehensively understand and model the creep performance of these advanced steel grades by investigating their microstructural evolution, damage mechanisms, precipitation kinetics, and deformation characteristics under creep conditions. The integrated projects examine both base metals and welded joints, providing insights into material properties, weldability, and microstructure-property relationships critical for their application in high-temperature components.

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.

The effect of Cr content on the creep strength at 650°C was examined with high Cr heat resistant steels for the USC high-temperature rotor shafts. The amount of Cr was varied from 8.5% to 11.5%, and then, the alloying effect of Cr was investigated on the stability of the precipitates at 650°C. Within the present range of the Cr content, the short-term creep rupture life under the higher applied stress increased with the Cr content in the steels, whereas the long-term creep rupture life under the lower applied stress decreased with the Cr content in the steels. For example, under the applied stress of 98MPa, the 9%Cr steel exhibited the longest creep rupture life among the experimental steels. Also, it was found from the experiment using the extracted residues that the degree of solution strengthening and the sorts of precipitates scarcely changed regardless of the Cr content in the steels. The Laves phase precipitated finely in the lath was enlarged in the 11.5%Cr steel even after a short-term creep. This result indicates that the coarsening of precipitates such as the Laves phase promotes the recovery of the lath in the early stage of creep deformation. It was suggested that 9%Cr is desirable content in the ferritic steel for suppressing the degradation of creep strength in 98MPa at 650°C.

The development of 9-12% chromium steels during the last twenty years is reviewed. The significant increases in creep strength that have been achieved by minor alloying additions of V, Nb, W, Mo, N and B are discussed and the mechanisms by which the individual elements contribute to the long-term creep strength are evaluated. The basic strengthening is provided by the martensitic transformation that allows the formation of a sub-grain structure from the martensite laths. The sub-grain boundaries are stabilized by precipitates, mainly M 23 C 6 ; within the sub-grains, fine nitride and carbonitride precipitates interact with dislocations, thereby enhancing the strength. The relative contributions of the martensitic transformation and the various precipitates to the overall creep strength of the steels are assessed. Of particular importance for the long-term creep strength is the stability of the microstructure, especially the time dependent coarsening of the various precipitates and the possible formation of additional phases, such as Laves phase (Fe 2 (W,Mo) and the Z phase (CrNbN). It is shown that microstructural changes that occur during exposure at anticipated service temperatures have a large impact on the strength and these changes must be taken into account in the derivation of long-term design stresses. Finally, the potential for achieving further increases in the creep strength of 9-12% chromium steels is discussed, especially in view of the need for higher chromium contents to ensure adequate steam oxidation resistance.