Martensite Lath Boundaries Research Articles

Achieving High Plasticity and High Toughness of Low-Carbon Low-Alloy Steel through Intercritical Heat Treatment

Medium manganese steel has excellent comprehensive properties due to the TRIP effect of retained austenite, but its welding performance is unsatisfactory for its high alloy content. This study obtained retained austenite in low-carbon low-alloy steel with low contents of silicon and manganese elements through intercritical heat treatment. The influence of intercritical quenching temperature on the content and characteristics of the retained austenite, as well as the functional mechanism of the retained austenite during low-temperature impact, was studied. The results showed that the content of the retained austenite increased from 12% to 17%, and its distribution extended from grain boundaries to martensite lath boundaries, with increasing intercritical quenching temperature. The retained austenite on the grain boundaries was in blocks, and that on the martensitic lath boundaries formed slender domains. The stability of the retained austenite was achieved through the enrichment of C and Mn during intercritical heat treatment. The contribution of retained austenite to low-temperature mechanical properties was closely related to its stability. The retained austenite with poor stability underwent martensite transformation at low temperatures, and the high-carbon martensite was a brittle phase that became the nucleation site of cracks or the path of crack growth during impact. Stable retained austenite passivated crack tips and hindered crack propagation during impacts, which improved the impact performance of the steel.

Effect of heat treatment on precipitation behavior of second phase and property evolution of martensitic stainless steel

The microstructural optimization for 15–5PH stainless steel by heat treatment to enhance toughness and corrosion resistance were investigated. The results indicate that Cu-rich nanoparticles with a size of less than 30 nm are homogeneously dispersed within the matrix, exhibiting a K-S orientation relationship with the martensite matrix during the aging treatment. Additionally, reversed austenite is found to be distributed at the martensite lath boundaries, with the maximum content of 1.62% being observed at 550 °C. With increasing aging temperature, the dispersion strengthening effect caused by the precipitation of Cu-rich phases gradually weakens. After immersion in a 6% FeCl3 solution for 12 h, the greatest weight loss is observed in the 600 °C treated samples, followed by those treated at 450 °C, 550 °C, and 1050 °C.

A multi-method approach to the study of hydrogen trapping in a maraging stainless steel: the impact of B2-NiAl precipitates and austenite

The trapping behavior of hydrogen in a Fe-Cr-Ni-Al-Mo alloy (PH13–8Mo martensitic stainless steel) was investigated. Three different metallurgical states of the same alloy were obtained by heat treatment in an attempt to separate the impact of different microstructural features on the hydrogen trapping behavior. Microstructure analysis including dislocation density measurements and characterization of B2-NiAl precipitates and reverted austenite was conducted, using mainly X-ray Diffraction and Transmission Electron Microscopy. From electrochemical permeation conducted under varying hydrogen fugacity, the trapping characteristics (binding energy, trap density) of the three metallurgical states studied were first determined using the analytical one trap model of Kumnick and Johnson. Another more sophisticated numerical model was then developed in order to introduce two different types of traps. This model was used to simulate the permeation curves and the trapping characteristics were identified using an inverse approach. In a separate approach, Thermal Desorption Spectroscopy was also used to determine the trapping energies and the amount of hydrogen stored in different traps. Combination of these different experimental and modelling approaches have produced consistent results. It is shown that low-misfit coherent B2-NiAl precipitates have a very limited trapping capability. The high dislocation density, including dislocation walls (martensite lath boundaries), significantly traps hydrogen, at an intermediate energy of about 35 kJ/mol. Filmy reverted austenite has a double impact: hydrogen is trapped both at the martensite-austenite interfaces (∼35 kJ/mol) and in the austenite bulk (∼20 kJ/mol).

Local microstructure evolution of a V-containing Fe–Cr–Ni–Mo weld metal subjected to post-weld heat treatment

The high-strength steels are widely used in modern shipbuilding and marine facilities. However, welding of >900 MPa grade steels is particularly challenging owing to the lack of understanding of fine microstructure for the weld metal (WM). The present investigation is mainly focused on a V-containing Fe–Cr–Ni–Mo WM fabricated by gas metal arc welding (GMAW) in aspects of localized microstructure evolution during post-weld heat treatment (PWHT). Microanalysis of the localized microstructure of inter-dendritic regions (IDRs) and dendrite core region (DCRs) for WMs was conducted by transmission electron microscopy (TEM), transmission kikuchi diffraction (TKD) and atom probe tomography (APT), complimented by nanoindentation hardness testing. It is found that the microstructure of WM in as-welded (AW) and PWHT condition was characterized by elongated martensitic laths with high-density dislocations in IDRs, while that in DCRs was coarse bainitic microstructure with relatively fewer dislocations. Most of retained austenite (RA) formed during cooling after welding was decorated martensite lath boundaries within IDRs owing to the segregation of austenite stabilizing elements after solidification. For PWHT condition, more numerous and smaller nano-scale MC carbides were observed in IDRs compared to DCRs. Furthermore, V atoms were mainly responsible for the nucleation of nano-scale MC carbides, and the growth and coarsening process of them were mainly controlled by diffusion of Mo atoms. Besides, the RA could also decompose into nano-scale MC carbides during PWHT. The IDRs of the PWHT specimens showed the average nanohardness value of 7.4 ± 2.3 GPa, which was higher than that of the DCRs (4.0 ± 0.2 GPa).

Characterisation of corrosion damage in T91/F91 steel exposed to static liquid lead-bismuth eutectic at 700–715 °C

T91 samples were exposed to static liquid lead-bismuth eutectic (LBE) at 700–715 °C for 250–500 h in either an oxidising or reducing environment. Corrosion damage was characterised using electron microscopy techniques, which identified networks of LBE intrusion beneath LBE-wetted surfaces. Under reducing conditions these networks are uniformly distributed, while they appear patchier and deeper under oxidising conditions. The individual intrusions preferentially follow microstructural features, initially along prior-austenite grain boundaries, followed by penetration down martensite lath boundaries. Local depletion of Cr was observed within 4 μm of LBE intrusions and along intersecting boundaries, suggesting local Cr dissolution as the main corrosion mechanism.

Structural Evolution of 10% Cr–3% Co Steel Microalloyed with Re and Cu during Creep Au 923 К

Structural evolution of the tempered lath martensite of the 10% Cr--3% Co steel microalloyed with rhenium and copper with a low nitrogen content and a high boron content during creep at 923 K was investigated for the purpose of establishment of the rison of decrease in creep resistance of this steel under the low applied stress. The tempered martensite lath structure of 10%Cr-3%Co steel with an average lath size of 370 nm and a high dislocation number density of 2 ×1014 m–2 was observed after normalizing at 1323 K with the following tempering at 1043 K for 3 h. The structure was stabilized by M23C6 carbides, M6C carbides, and NbX carbonitrides. During long-term creep, the lath structure strongly experienced an evolution: the width of the martensitic laths increased significantly, dislocation density decreased, the Laves phase and Cu-enriched particles remarkably coarsen. Such structural evolution correlates with an appearance of creep strength breakdown on curves “Applied stress vs. Time to failure” and “Minimum creep rate vs. Applied stress”. Significant coarsening of the Laves phase particles and Cuenriched particles via formation of the large particles with sizes more than 250 nm along high-angle boundaries and full dissolution of the fine particles with sizes less than 50 nm along low-angle boundaries of martensite laths is considered to be the main cause of degradation of the creep resistance of the steel studied.

Microstructural evolution and mechanical properties of 60Si2CrVNb spring steel under quenching-tempering heat treatment process

This study investigated the evolution of microstructure and mechanical properties of 60Si2CrVNb spring steel under different quenching and tempering processes. The morphology of martensite, precipitate phases, and carbides, as well as the analysis of mechanical properties, were examined. The results revealed that the highest ultimate tensile strength (2260 MPa) and yield strength (2091 MPa) of 60Si2CrVNb spring steel were achieved through austenitization at 900 °C and subsequent tempering at 350 °C, which is related to the maximum dislocation strengthening effect. The optimal combination of mechanical properties for 60Si2CrVNb spring steel was obtained by quenching at 900 °C for 30 min and tempering at 400 °C for 90 min. Under these conditions, the steel exhibited an ultimate tensile strength of 2084 MPa, yield strength of 1846 MPa, elongation of 10.8%, and area reduction of 41.5%. Increasing the quenching temperature resulted in the gradual dissolution of undissolved carbides, an increase in the average grain size of austenite, coarsening of martensite, and an initial increase and subsequent decrease in tensile strength and plasticity. Furthermore, the fracture type transformed from ductile fracture to mixed fracture of ductile and brittle. On the other hand, the increasing tempering temperature led to a decrease in dislocation density and gradual decomposition of martensite lath boundaries. Additionally, the type of carbides changed from small-sized M2C and MC carbides to large-sized M3C, M7C3, and M23C6 carbides. As a result, the tensile strength of the steel decreased significantly, while its plasticity steadily increased. Moreover, the fracture behavior transformed from brittle fracture to ductile fracture.

Influence of intercritical annealing temperature on microstructure, microtexture, and tensile behavior of TRIP-assisted medium manganese steel

The present work investigates, the effect of intercritical annealing temperature on the microstructure, microtexture, tensile behavior, and mechanical stability of austenite in thermomechanically processed Fe-0.18C-4Al-7Mn (wt.%) medium manganese steel. The steel is subjected to thermomechanical processing (hot forging and hot rolling) followed by intercritical annealing at various temperatures (700, 750 and 800 °C) for 1 hour. The development of a complex microstructure with blocky and lath-type reverted austenite (RA), fine polygonal and blocky type intercritical ferrite (IF), coarse elongated ferrite (EF), and lath martensite (M) is revealed by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses. The steel annealed at 750 °C exhibits an excellent ultimate tensile strength (UTS) of 802 MPa and total elongation (TE) of 61% owing to its high RA content, high transformation-induced plasticity (TRIP) effect and low mechanical stability of RA. Further, to understand the dislocation phenomena during tensile deformation, the tensile samples are analyzed using TEM, which reveals dislocation interactions, dislocation tangles, and dislocation accumulations at the IF grain boundaries and martensite lath boundaries. The manifestation of the TRIP effect is elucidated by XRD and EBSD studies. In addition, to study the texture evolution and the role of texture components on the TRIP effect, the orientation distribution function is employed. The major texture component, i.e., Ms Brass {011} 〈211〉 indicates the TRIP effect resulting in the deformation-induced martensitic transformation under tensile deformation. Additionally, Ms Brass component shifts towards G/B (Goss/Brass) twin {011} 〈111〉 component by grain rotation to accommodate the deformation.

Austenite Nucleation and Growth as a Function of Starting Microstructure for a Fe–0.15C–5.56Mn–1.1Si–1.89Al Medium‐Mn Steel

The effects of starting microstructure and intercritical annealing temperature on the phase‐transformation kinetics and microstructural evolution of a prototype Fe–0.15C–5.56Mn–1.1Si–1.89Al medium‐Mn third‐generation advanced high‐strength steel are examined. The starting microstructures comprise 1) an as‐received cold‐rolled (CR) microstructure containing a significant fraction of ferrite and tempered martensite and 2) an austenitized and quenched martensite–ferrite (MF) microstructure. Based on the microstructural observations, two different scenarios for austenite formation during intercritical annealing are proposed. For the CR starting microstructure, austenite can nucleate at ferrite/cementite interfaces, and at ferrite grain boundaries. In the case of the MF starting microstructure, which contains thin films of interlath retained austenite (RA), austenite forms on the martensite lath boundaries or grows directly from the existing interlath RA. The studies are interpreted using the DICTRA module of Thermo‐Calc. At 665 °C, the austenite reversion kinetics of the MF samples are faster than the transformation kinetics in the CR structure. At higher temperatures where most of the cementite has dissolved, the austenite fraction in both starting microstructures is very similar.

Effect of Si content on the microstructure and mechanical properties of 9Cr-ferritic/martensitic steels

9Cr-F/M−xSi (x = 0–1.0 wt%) steels were fabricated through vacuum induction melting technique and processed by hot forging, hot rolling, normalizing and tempering, subsequently. Their microstructure and mechanical properties were characterized using OM, SEM, TEM, Vickers hardness tester and tensile tester. The steel has a typical ferrite/martensitic structure, with M23C6 and MX phases precipitated at the martensite lath boundary or in the lath. With Si content increasing, the average ultimate tensile strength (UTS) and hardness of 9Cr-F/M−xSi increase simultaneously from 678 MPa and 235 HV for 9Cr-F/M−0Si steel to 788 MPa and 265 HV for 9Cr-F/M−1.0Si steel, respectively, while the total elongation kept almost constant at 22.5%. The main strengthening mechanism is the solid solution strengthening due to silicon addition and the change in the carbide precipitation behavior caused no remarkable hardening contribution. These results can provide a reference for the composition design of structural materials for nuclear reactors.

Compelling Evidence for the Role of Retained Austenite in the Formation of Low Cycle Fatigue Extrusions in a 9Ni Steel

The 9Ni martensitic steels have a martensitic microstructure which contains retained austenite after solution heat treatment and water quenching. Under low cycle fatigue, extrusions formed at the surface of the material and were very close to martensite lath boundaries. The presence of retained austenite at martensite laths has been highly suspected to impact the cyclic plasticity. However, the nano-size of the austenitic phase makes it difficult to obtain clear evidence of its role. The paper focuses on the precise determination of these extrusions and the link with the retained austenite. The paper also emphasizes the innovative and promising use of magnetic force microscopy (MFM) to document cyclic plasticity of a 9Ni steel. It is shown that electron microscopies, even the most advanced ones, may be unsuccessful in reaching this goal, while magnetic force microscopy (MFM) overcame the difficulty. This technique has allowed imaging of both the extrusion and the retained austenite. These analyses confirm that the fatigue extrusions originated from a local displacement of martensite lath. The proposed mechanism, in which the retained austenitic film acts as a lubricant film or greasy film promoting a flowing of martensite along the interfaces, is unambiguously demonstrated.

Nondestructive Ultrasonic Evaluation of Ferritic CrW Steel upon Long-term Aging

The microstructural evolution of heat-resistant ferritic CrW steel during long-term aging was characterized by ultrasonic wave velocity measurement. Long-term aging was conducted at 700°C for 4000 h. The ultrasonic wave velocity increased monotonically upon long-term aging. The ultrasonic wave velocity increased rapidly during the initial 1000 h of aging and moderately thereafter. The MSUB23/SUBCSUB6/SUB carbide and Fe2W Laves phases underwent coarsening through the Ostwald ripening mechanism, causing a decrease in Cr and W solute atom concentration of the iron matrix during 4000-h aging. The dislocation density also decreased because of the heat treatment. The width of martensite lath increased owing to dislocation recovery at the lath boundaries. The ultrasonic wave velocity depends on the Young’s modulus of the material, which may influence microstructural features such as dislocation substructure, precipitates, solute atom concentration, and martensite lath boundaries. This study utilizes ultrasonic wave velocity to quantify the damage level and microstructural changes during the long-term aging of ferritic CrW steel.br/

Effect of Si on the dislocation state within martensite of ultra-high strength hot-rolled medium Mn steel with good ductility

Medium Mn steels are promising candidates for the third-generation advanced high-strength steels due to their good combination of ultimate tensile strength (UTS) and total elongation after fracture (TEL). Most investigations of medium Mn steels focused on the volume fraction of retained austenite (RA) and stability of RA; however, the strengthening phase martensite was paid little attention to. The present study systematically investigated the effect of Si on the deformation behavior of martensite in 0.2C–6.8Mn-(0, 2, 4)Si (wt. %) hot-rolled medium Mn steels without any heat treatment. The UTS and TEL of the 4Si hot-rolled medium Mn steel are 2163 MPa and 11.7%, respectively, which is rare in the existing hot-rolled medium Mn steels. All three hot-rolled steels had practically equal fraction and mechanical stability of RA, so that the influence of Si on the evolution of martensite fine structure and the nature of the ultra-high UTS and good ductility during deformation could be revealed. By combining the X-ray and neutron diffraction techniques, it was found that the addition of Si increased the dislocation density, especially screw dislocations, and decreased the crystallite size within martensite. The ultra-high UTS originates from the enhanced strain hardening rate caused by the Si alloying, high dislocation density and possibly increased number of martensite lath boundaries and dislocation cell formation. The good ductility mainly stems from the formation of dislocation cell structure due to the addition of Si causing dislocations to be mostly of screw type. The dislocation cells can also improve the strain hardening rate. This work opens a new pathway towards tailoring the dislocation state within martensite for substantially enhanced the mechanical properties of hot-rolled medium Mn steels.

A systematic study on microstructure evolution, mechanical stability and the micro-mechanical response of tensile deformed medium manganese steel through interrupted tensile test

In the current work, the microstructure evolution, mechanical stability of austenite and the micro-mechanical response of tensile deformed Fe-0.18C-4Al-7Mn (Wt%) steel are investigated. Interrupted tensile tests on the annealed samples (750 °C, 1 h) were performed at a tensile elongation of 5, 10, 20, 30, 40 and 60% to understand the deformation behavior and transformation-induced plasticity (TRIP) effect. Strain-induced martensitic transformation (SIMT) of reverted austenite is explored using X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) analysis. The coefficient of mechanical stability of austenite (k-parameter) is quantified through the interrupted tensile tests using XRD analysis. Instrumented nanoindentation test was conducted on individual phases of the interrupted tensile-tested samples to determine their hardness and modulus values. The scanning electron microscopy (SEM) analysis reveals the gradual decrease in width and increase in length of austenite-martensite islands with an increase in the percentage of elongation. Evolution of dislocation lines, dislocation tangles and dislocation accumulations occur at ferrite grain boundaries and martensite lath boundaries during tensile deformation. The volume fraction of austenite decreases, and the transformation of austenite to martensite and dislocation density of austenite increase with the increase in the tensile elongation. Austenite is found to be mechanically more stable at 30% of tensile elongation due to the low austenite transformation ratio. Nanoindentation results show that the hardness increases with an increasing percentage of deformation. Multiple pop-ins are observed in load (P)–indentation depth (h) curves due to dislocation interaction at the interphase boundary.

Effects of thermal treatment on the mechanical and electromagnetic properties of 9Cr-1Mo steel

This paper presents the effects of thermal treatment conditions on the mechanical properties of 9Cr-1Mo steel. Each sample was normalized at 1,040 °C for 1 h prior to tempering at different temperatures for 2 hours. Three levels of tempering conditions, 600, 650, and 700 °C, were tested in this study. After treatment, the metallurgical structures of the samples were observed and characterized by optical microscope (OM), scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDS) and X-ray diffraction (XRD). Hardness and static tensile of the samples at room and elevated temperatures were also measured and analyzed. Non-destructive testing (NDT) associated with the eddy current method was employed to detect the microstructural changes of samples. The results revealed that the lath martensitic structure, lath boundaries, coarse precipitates, and prior austenite grain boundaries were found at grain boundaries and inside the matrix region. The average size of the particle and area fraction of carbide precipitates decreased with tempering temperature increases from 600 − 650 °C. The true yield stress and true ultimate tensile stress decreased while true strain increased when the tempering process was applied after the normalization. In addition, the use of the eddy current method at 100 kHz with proper conductivity could more accurately detect the microstructural changes of 9Cr-1Mo steel specimens than using the phase angle approach. The correlation coefficient of conductivity and true ultimate tensile strength was found to be 0.9459, indicating a high correlation between them.

EFFECT OF TANTALUM ON THE IMPACT RESISTANCE OF 12% CR STEELS SUBJECTED TO THERMOMECHANICAL TREATMENT

The effect of tantalum on the impact resistance of 12% Cr steels with different tantalum content (12CrTaNb and 12CrNb) was investigated in impact tests in the temperature range from -40 to +120°C with determining the brittle-ductile transition temperature as the temperature in the middle between the upper and lower shelf energies. Both 12% Cr steels were thermomechanically treated by alternating 1050°C annealing and forging in addition to standard normalizing treatment with high-temperature tempering. It was found that for Ta-alloyed 12% Cr steel, the entire fracture toughness versus temperature curve is 30-50 J/cm2 higher over the entire temperature range, including the upper and lower shelf energies, and the brittle-ductile transition temperature is 10°С lower. The main structural parameters of Ta-alloyed 12% Cr steel which can have a beneficial effect on toughness are a smaller initial austenite grain size, a larger average size but lower density of M23C6 carbide particles along low-angle martensite lath boundaries, and a larger volume fraction of VX carbonitrides.

Effects of Cu-rich phases on microstructure evolution and creep deformation behavior of a novel martensitic heat-resistant steel G115

In this work, effects of Cu-rich phases on the microstructure evolution and creep deformation behavior of G115 steels with 1 (1Cu steel) and 2 wt% Cu (2Cu steel) were systematically investigated under 160–220 MPa at 650 °C. The results showed that dispersedly distributed Cu-rich phases played a vital role in retarding the microstructural degeneration and improving the creep strength. The Cu-rich phases inside grains could induce dislocation tangle and prohibit dislocation annihilation during creep, but they could be dissolved into the matrix due to the cutting effect of mobile dislocation lines. The Cu-rich phases at grain boundaries could hinder the migration of martensitic lath boundaries and thus delayed the creep failure behavior. Moreover, the independent Cu-rich phases at grain boundaries could provide heterogeneous nucleation sites for Laves phases, which strengthening pinning effect. As a result, compared with the 1Cu steel, the creep threshold stress of the 2Cu steel was enhanced from 103.36 to 117.93 MPa, and the creep lifetime was also prolonged markedly.

Si segregation and relevant effect on the corrosion propagation in F/M steel exposed to liquid lead-bismuth eutectic

ABSTRACT Clarification of the relationship between oxygen diffusion and microstructural evolution in liquid lead-bismuth eutectic (LBE) is of great importance for understanding and consequently tailoring the oxidation process. In the present study, a Si-modified ferritic martensitic steel was exposed to oxygen-saturated LBE at 500°C at various times and the interface between the internal oxidation zone and matrix was investigated. Microstructural characterisation reveals that grain boundaries and martensitic lath boundaries provide a fast pathway for oxygen diffusion, resulting in the high amount of oxidation products, as well as cracks. The internal stress caused by the formation of oxidation products along grain boundary results in the proliferation of dislocations, which subsequently play as the new diffusion channel for oxygen. The chemical analysis demonstrates that Si preferably concentrates along the dislocations in a form of Si-rich oxide, suggesting the oxidation of Si is prior to the Cr-rich oxide.

Microstructural evolution and the effect on strength of G115 steel after long-term creep at 923 K

The microstructural evolution and the effect on the strength of G115 steel after creep under 923 K and 130 MPa for up to 7288 h were systematically investigated using OM, SEM and TEM. The results show that the variation tendency of total calculated strengthening contribution is in a good agreement with the measured hardness of G115 steel after creep at 923 K. A new nucleation mechanism of Laves phase nucleated adjacent to Cu-rich phase in the grain interiors is found in G115 steel during creep at 923 K. Compared with Laves phase at grain boundaries and martensitic lath boundaries, the growth velocity of Laves phase nucleated adjacent to Cu-rich phase is lower. Laves phase and Cu-rich phase play a vital role in the strength of G115 steel during creep at 923 K in addition to M23C6 carbides. Not only Laves phase and Cu-rich phase in the grain interiors help to increase the strength by enhancing the dislocation strengthening and precipitation strengthening, but also Laves phase and Cu-rich phase affect the strength variation by influencing the precipitation strengthening obviously. In contrast, Laves phase at martensitic lath boundaries is easy to coarsen, which may be unfavorable to the strength of G115 steel due to the weakened martensitic lath strengthening. In order to increase the beneficial effect of Laves phase and Cu-rich phase, it is necessary to increase the volume fraction of Cu-rich phase and Laves phase in the grain interiors, and control the growth of Laves phase. Increasing the amount of Cu-rich phase can promote the precipitation of Laves phase with low growth rate by providing more nucleation sites for Laves phase in the grain interiors.

Effect of Long-Term Thermal Aging on Microstructure Evolution and Creep Deformation Behavior of a Novel 11Cr-3W-3Co Martensite Ferritic Steel.

This paper focused on the microstructure evolution under different thermal aging times at 650 °C and its effect on creep behavior in 11Cr-3W-3Co heat-resistant steel. After short-term thermal aging at 650 °C (>750 h), a Laves phase was found in the regions adjacent to the PAG boundaries, martensitic lath boundaries, and M23C6 carbides, and gradually swallowed adjacent M23C6 carbides with the aging time increased. Higher contents of Si and P are good promoters of the nucleation of the Laves phase during long-term aging. In addition, the coarsening behavior of the Laves phase, M23C6, and MX were investigated. As the aging time increases, the coarsening behavior among precipitated phases in the above-mentioned example exhibits remarkable variability, which is discussed in detail in this paper, and the evolution of the subgrain size was also analyzed in detail. The increasing rate of subgrain size is, in general, consistent with that of the M23C6 carbide size. The evolution of dislocation density in different aging times shows an obvious difference, and the decreasing rate of dislocation density is significantly affected by the precipitated phase after long-term aging time. The creep performance of the material decreases significantly as the aging time increases, which is closely related to the coarsening of the precipitates such as M23C6 carbides and subgrain during long-term aging.