Carbon Extraction Replicas Research Articles

Metallographic and Extraction Replica Methods for Characterization of Tungsten Heavy Alloy: H13 Clads

Tungsten heavy alloys are used in demanding high pressure die casting applications due to their high temperature strength, high thermal conductivity, and low thermal expansion. High cost limits applications to small sintered die inserts and manual gas tungsten arc weld repairs. A new tungsten heavy alloy consumable, Anviloy wire, was developed for automated cladding of hot work tool steel dies. Literature regarding characterization of tungsten heavy alloy die steel clads was lacking. Understanding base metal dilution effect on clad microstructure is critical but required new sample preparation methods. An Anviloy wire-H13 clad was made using hot wire gas tungsten arc cladding and analyzed with metallography. Samples were found to have grain boundary M6C carbide phase as-welded with the help of an alkaline sodium picrate etchant. An isothermally aged arc crucible melted sample of the same composition was characterized using metallography, scanning electron microscopy, and transmission electron diffraction. The clad representative arc crucible melted sample was subjected to isothermal aging at 725°C for 100 hours. Isothermal aging resulted in precipitation of a high volume fraction of intermetallic platelets. Using a new carbon extraction replica sample preparation method involving two chemical polishing steps, transmission electron diffraction of precipitates indicated they were mu phase intermetallic.

Optimized carbon extraction replicas for transmission electron microscopy of nanoprecipitates in microalloyed low-carbon steels

Nanoprecipitates play a decisive role in strengthening of microalloyed low-carbon steels. The direct carbon extraction replica (CER) method is a promising way to extract the nanoprecipitates from the surrounding iron matrix for a detailed characterization of their structure and chemistry using electron microscopy. Better results can generally be obtained by electron microscopy when the magnetic matrix is removed. However, there are challenges in the preparation of relatively thin CERs, especially during stripping and cleaning stages of the procedure. In this contribution, a systematic study of different parameters associated with the direct CER preparation method was performed on a vanadium-microalloyed low-carbon model steel to improve the replication procedure. Relatively low nanoscale surface kurtosis of the iron matrix with respect to the thickness of the deposited carbon layer was found to be important for successful replications from different crystallographic planes of the matrix during the stripping stage. Furthermore, modified water-based stripping and cleaning solutions were introduced that minimized wrinkling, coalescence, and disintegration of ∼8 nm thick CERs. These results are supported by (scanning) transmission electron microscopy observations, which showed successful extraction of V-rich nanoprecipitates in a size range of 1–50 nm from both the ferritic and bainitic microstructures. The modified method improves the common direct CER preparation procedure and can be used to study precipitates in a wide range of microalloyed low-carbon steels.

Thermomechanical Control of Microstructure and Precipitation in Vanadium Microalloyed Steel: Influence of Finish Rolling and Coiling Temperatures

Six hot compression tests are conducted using the Gleeble3500 thermomechanical simulator to investigate the microstructural evolution and precipitation behavior in low‐C–Mn V‐microalloyed steel. The specimens are subjected to hot isothermal compression deformation of 87%. The optical microscopy and transmission electron microscopy using carbon extraction replica method are used to characterize the microstructures and precipitation after the simulated thermomechanical controlled process and coiling. The results indicate that increasing the finish rolling temperature benefits the refinement of ferrite grains but has little influence on the refinement of the precipitates. It is also observed that lower coiling temperatures (CTs) promote the formation of fine precipitates. When the CT is 500 °C, the average precipitate size is found to be 86 nm. Furthermore, it is found that the CT significantly influences the nucleation sites of the precipitates inter alia, the matrix, interphase, grain boundaries, and dislocations. As expected, at higher CTs, nucleation is predominantly on the defects rather than the matrix.

Scanning precession electron diffraction study of carbide precipitation sequence in low alloy martensitic Cr-Mo-V tool steel

Precipitation of carbides after tempering of a medium carbon low alloyed Cr-Mo-V tool steel at 600°C was studied with scanning precession electron diffraction (SPED) in a transmission electron microscope (TEM). The precipitation sequence was evaluated by mapping the carbide distribution on carbon extraction replicas prepared from samples tempered for different durations of up to 24 h. The SPED results were supplemented by equilibrium calculations and energy dispersive x-ray spectroscopy (EDX) measurements in a TEM. It was found that ε-Fe2C precipitates within martensite laths during quenching via auto-tempering. During reheating to tempering temperature ε-Fe2C was dissolved and replaced by cementite, θ-M3C, which predominately form on martensite boundaries. It was further found that the small carbides in the early stage of tempering have predominantly a cubic MC structure, even if the V/Mo-ratio of the studied steel was only 0.12, and it is known from literature that V and Mo form cubic MC or hexagonal M2C carbides, respectively. Later during tempering more stable carbides, such as M7C3 and M23C6, also form, and it was concluded that the M7C3 form both by separate nucleation and precipitation on the cementite/matrix interface. The latter phenomenon was seen as particles with a core of cementite and a shell of M7C3 after 24 h of tempering.

Precipitate number density determination in microalloyed steels by complementary atom probe tomography and matrix dissolution

Particle number densities are a crucial parameter in the microstructure engineering of microalloyed steels. We introduce a new method to determine nanoscale precipitate number densities of macroscopic samples that is based on the matrix dissolution technique (MDT) and combine it with atom probe tomography (APT). APT counts precipitates in microscopic samples of niobium and niobium-titanium microalloyed steels. The new method uses MDT combined with analytical ultracentrifugation (AUC) of extracted precipitates, inductively coupled plasma–optical emission spectrometry, and APT. We compare the precipitate number density ranges from APT of 137.81 to 193.56 × 1021 m−3 for the niobium steel and 104.90 to 129.62 × 1021 m−3 for the niobium-titanium steel to the values from MDT of 2.08 × 1021 m−3 and 2.48 × 1021 m−3. We find that systematic errors due to undesired particle loss during extraction and statistical uncertainties due to the small APT volumes explain the differences. The size ranges of precipitates that can be detected via APT and AUC are investigated by comparison of the obtained precipitate size distributions with transmission electron microscopy analyses of carbon extraction replicas. The methods provide overlapping resulting ranges. MDT probes very large numbers of small particles but is limited by errors due to particle etching, while APT can detect particles with diameters below 10 nm but is limited by small-number statistics. The combination of APT and MDT provides comprehensive data which allows for an improved understanding of the interrelation between thermo-mechanical controlled processing parameters, precipitate number densities, and resulting mechanical-technological material properties.Graphical abstract

On the variety and formation sequence of second-phase particles in nickel-based superalloys fabricated by laser powder bed fusion

We examine by transmission electron microscopy the precipitation state in the as-built microstructure of a hot-cracking sensitive nickel-based superalloy fabricated by laser powder bed fusion. Most observations are carried out on carbon extraction replica to isolate the precipitate signals from those of the matrix. Chemical maps measured by energy dispersive X-ray spectrometry are compared to phase and orientation maps obtained on the same precipitates by automated crystal orientation mapping. The chemical analyses are completed by electron energy-loss spectrometry measurements made on thin foils. The large fields of view investigated allow for achieving a representative picture of the second-phase particles in the as-built microstructure. A large variety of nano-particles is found: (Ti,Nb)(C,N) carbonitrides, (δ) Al2O3 alumina, (Cr,Mo)3B2 and (Cr,Mo)5B3 borides, and the intermetallic phase Ni7Zr2. The precipitates themselves are also complex in terms of their inner structure: carbonitrides can present a core-shell structure of composition and are frequently twinned, alumina particles serve as nucleation sites for carbonitride aggregates, and several crystallographic forms of borides may coexist next to each other. These observations provide the necessary information to retrace the solidification path of the microstructure during fabrication and to discuss the precipitation mechanisms under the extreme processing conditions associated with laser powder bed fusion.

Effect of Niobium and Titanium Addition on Formation of Second Phase Particles in CHQ Steel Using Transmission Electron Microscope

It is common practice that formation of second phase particles such as nitrides or carbides in the steel matrix has significant role to control the grain size of steel. An attempt is made in the present research work to find out the role of nitrogen to form the nitride particles either with Al, Ti, B, Cr or Si. Two steel samples Steel-A and Steel-B with same titanium and aluminum weight percent in the chemical composition were obtained in hot rolled conditions from international market with only the difference of presence of Niobium in Steel-A. Solution heat treatment was performed at 1350°C with 60 minutes holding time in protherm heat treatment furnace available locally was used to dissolve the particles and then steel samples were reheat treated at 800°C with holding time of 60 minutes and water quenched and microstructure was revealed. Transmission electron microscope connected with Ehlers-Danlos Syndrome (EDS) was used to reveal the morphology of second phase particles. Both samples for a high resolution power Transmission Electron Microscopy (TEM) (Jeol JEM 3010) analysis were prepared by using carbon extraction replica method in 5% Nital solution as an etching technique. Both samples were then caught in copper grid of 3mm for using TEM analysis. TEM micrographs clearly revealed the second phase particles in the matrix of steel. The EDS peaks were studied and it was found that the peaks showed the titanium peaks in both the samples A and B and surprisingly there was no any peak found for aluminum. Stoichiometric calculations were carried out and it was found that weight percent nitrogen required for forming TiN is 0.0073, however the total nitrogen present in both the steels A and B is 0.0058 and 0.0061 respectively. That means that all the nitrogen present in the steel matrix was consumed by titanium to form the Titanium Nitride (TiN) so there was no nitrogen remain to fulfil the requirement of aluminum to form the Aluminum Nitride (AlN) particles.

Characterization of the Microstructure and Precipitates Formed During the Thermomechanical Processing of a CrNiMoWMnV Ultrahigh‐Strength Steel

The effect of total applied strain (TAS), finish forging temperature (FFT) on the microstructure, and precipitation kinetics of a newly developed low‐cost, low‐alloy CrNiMoWMnV ultrahigh‐strength steel has been investigated. A Gleeble 3800 thermomechanical simulator is used to simulate the hot forging process and its influence on the precipitation kinetics. Field‐emission scanning electron microscopy combined with electron‐backscattered diffraction is used to characterize the final overall microstructures, whereas transmission electron microscopy on carbon extraction replicas is used to characterize the precipitates in terms of morphology, size distribution, mean equivalent circle diameter, the 90th percentile in the cumulative diameter distribution (D90%ppt), chemical composition, and crystallography. Thermo‐Calc software is used to predict the precipitates expected in austenite at equilibrium. The final microstructure consists of lath martensite and a small fraction of the precipitates AlN, TiN, and composite TiN–AlN. Differences in the degree of strain‐induced precipitation caused by variations in TAS and FFT have been shown to greatly influence precipitate size distributions. Variations in the degree of precipitate dissolution and coarsening cause variations in the prior austenite grain size, which subsequently cause variations in the effective grain size of the final microstructure, i.e., that are defined by high‐angle grain boundaries.

Deformation behaviour and microstructural evolution of EUROFER97-2 under low cycle fatigue conditions

Low cycle fatigue (LCF) behaviour of EUROFER97 version 2 was studied in normalized plus tempered conditions within a wide total strain range of 0.4–1.8% at room temperature (RT) and 550 °C. At the lowest strain (0.4%) EUROFER97-2 shows very low softening behaviour at both temperatures. However, by increasing the strain amplitude, the material showed an instant hardening followed by a sharp softening up to rupture and fatigue life was reduced. In-depth microstructural characterization by STEM in thin foils and also in a carbon extraction replica was performed on selected samples strained at 0.4, 0.8, 1 and 1.8% at both RT and 550 °C. The microstructural development is based on the rearrangement and/or annihilation of dislocations, which lead to a sub-grain formation and eventually growth, specially enhanced at 550 °C. The microstructural modifications, in regard to dislocations and sub-grain structures, are more intense as the strain amplitude applied is increased. No significant changes in composition, size, and distribution of M23C6 and MX precipitates were observed. Consequently, it is demonstrated that the fatigue life of EUROFER97-2 at room temperature and at 550 °C is strongly dependent on dislocation evolution.

The effect of double austenitization and quenching on the microstructure and mechanical properties of CrNiMoWMnV ultrahigh-strength steels after low-temperature tempering

With the aim of improving the strength and impact toughness combination of two ultrahigh-strength quenched and tempered steels, the effect of high-temperature austenitization and quenching prior to conventional austenitization, quenching and tempering at 200 °C has been investigated. The CrNiMoWMnV steels concerned had carbon contents of 0.18 and 0.32 wt% C and tensile strengths 1370 and 1840 MPa. Microstructures were characterized using laser scanning confocal microscopy, field emission scanning electron microscopy combined with electron back scattering diffraction and X-ray diffraction. Carbide characteristics were studied using transmission electron microscopy on carbon extraction replicas. Mechanical properties were characterized in terms of hardness, tensile and Charpy-V impact testing. The microstructure of the investigated steels after all treatments consisted of tempered martensite with small fractions of precipitates and retained austenite. The volume fraction of retained austenite was increased through the use of the double austenitization and quenching plus tempering route as compared to the conventional quenching and tempering route. Three main precipitate types were observed in all the investigated steels: complex carbides MxCy, aluminium nitride AlN, titanium-vanadium carbonitride (TiV)(CN) and complex AlN precipitates with a core of (TiV)(CN). The size of the largest precipitates was reduced as a result of the double austenitization treatment. In both investigated steels, the energy absorbed and the percentage ductile fracture in the CVN impact test were improved as a result of the extra austenitization and quenching: in the case of the 0.18C steel this was achieved without a loss of hardness or tensile properties, but with the 0.32C steel there was a slight decrease in the hardness and tensile properties.

Effect of continuous annealing temperature on microstructure and properties of ferritic rolled interstitial-free steel

In this report, the microstructure, mechanical properties, and textures of warm rolled interstitial-free steel annealed at four different temperatures (730, 760, 790, and 820°C) were studied. The overall structural features of specimens were investigated by optical microscopy, and the textures were measured by X-ray diffraction (XRD). Nano-sized precipitates were then observed by a transmission electron microscope (TEM) on carbon extraction replicas. According to the results, with increased annealing temperatures, the ferrite grains grew; in addition, the sizes of Ti4C2S2 and TiC precipitates also increased. Additionally, the sizes of TiN and TiS precipitates slightly changed. When the annealing temperature increased from 730 to 820°C, the yield strength (YS) and the ultimate tensile strength (UTS) showed a decreasing trend. Meanwhile, elongation and the strain harden exponent (n value) increased to 49.6% and 0.34, respectively. By comparing textures annealed at different temperatures, the intensity of {111} texture annealed at 820°C was the largest, while the difference between the intensity of {111} and {111} was the smallest when the annealing temperature was 820°C. Therefore, the plastic strain ratio (r value) annealed at 820°C was the highest.

Evolution of Al-containing phases in ODS steel by hot pressing and annealing

The crystal structures of Al2O3 and Y-Al-O particles in the as-hot pressed (as-hped) and as-heat treated oxide dispersion strengthened (ODS) steel were identified in this paper using high resolution transmission electron microscopy (HRTEM) techniques. The carbon extraction replicas were utilized to investigate the composition and crystal structure of the oxide particles in order to exclude the influence of the matrix. In the as-hped ODS steel, both of the γ-Al2O3 and α-Al2O3 particles with different size were identified, while, only the large α-Al2O3 particles were found in the as-heat treated ODS steel. In addition, YAlO3 and Y3Al5O12 with body-centred cubic structure were the mainly observed Y-Al-O particles in the as-hped ODS steel, and large amounts of Y4Al2O9 particles with a monoclinic structure generated in the ODS steel during annealing.

Effect of zirconium addition on the microstructure and mechanical properties of 15Cr-ODS ferritic Steels consolidated by hot isostatic pressing

The influence of Zr addition on the microstructure and mechanical properties of mechanically alloyed (MA) ODS ferritic steels were studied in this work. The microstructure characteristics included the grain size, oxide particles number densities, size distributions, crystal structures and compositions. TEM foils measurements were complemented by studies of alloys on carbon extraction replica and focus ion beam (FIB) foils. The tensile properties were carried out at different temperatures. The microstructure and mechanical properties were analyzed and compared with nominal compositions (wt.%): Fe-15Cr-2W-0.3Y2O3 and Fe-15Cr −2W-0.3Zr-0.3Y2O3. The experimental revealed that the addition of Zr increased the volume fraction of the smallest and equiaxed ferritic grains, number density of nano-oxide particles and decreased the average size of oxide particles within the ferritic matrix, promoting the formation of fine trigonal δ-phase Y4Zr3O12 nano-oxides and leading to the enhancement of the mechanical properties of the ODS steels.

Effects of annealing cooling rates on mechanical properties, microstructure and texture in continuous annealed IF steel

The mechanical properties, microstructure and texture of interstitial-free (IF) steel processed through continuous annealing routes under five cooling rates (5 °C/s, 50 °C/s, 200 °C/s, 500 °C/s and 1000 °C/s) have been studied. The cooling rates of 200 °C/s, 500 °C/s and 1000 °C/s are defined as ultra-fast cooling rates. This paper focuses on the differences of mechanical properties, microstructure and texture evolution between ultra-fast and common cooling rates (5 °C/s, 50 °C/s) for the first time. The overall structural features of specimens were observed by optical microscopy and nano-sized precipitates were observed by transmission electron microscope (TEM) on carbon extraction replicas. Texture measurement was carried out using D8 advance X-Ray Diffraction (XRD) and the (110), (200) and (112) pole figures were determined. The yield strength increases with increasing cooling rates, from 96 MPa under the cooling rate of 5 °C/s to 136 MPa under the cooling rate of 1000 °C/s, while the ultimate tensile strength changes very little (around 280 MPa). The samples annealed under the cooling rates of 5 °C/s and 500 °C/s have low rave values (2.13 and 2.21 respectively). By contrast, the samples in cases of 50 °C/s, 200 °C/s and 1000 °C/s possess higher rave values (2.32, 2.40 and 2.51 respectively). Three morphologies of TiN particles can be observed in all samples. TiN, with CaO and SiO2 acting as heterogeneous nuclei, is prone to have big size. The size of pure TiN precipitates deceases as increasing of cooling rates. FeTiP is found near TiC precipitates under the cooling rate of 5 °C/s. The sample annealed under the cooling rate of 5 °C/s has the weakest {111} <112> and {111} <110> orientations. The intensity of {111} <110> component increases as the cooling rate increases from common cooling rates of 5 °C/s and 50 °C/s to ultra-fast cooling rates of 200 °C/s, 500 °C/s and 1000 °C/s. The intensity of {011} <100> orientation decreases as the cooling rate increases from 5 °C/s to 500 °C/s. The relationship between r value, microstructure and texture components has been studied in this paper.

Quantitative Analysis on Carbide Precipitation in V-Ti Microalloyed TRIP Steel Containing Aluminum

Introducing fine precipitates is an important way to enhance the properties of transformation-induced plasticity (TRIP) steels. In present work, two V-Ti microalloyed TRIP steels containing aluminum with different content were compared. The average size, size distribution and numbers of vanadium-titanium carbides in samples cold rolled, quenched after being held at 800°C and quenched after intercritical annealing at 800°C and being held at bainitic isothermal transformation temperature of 400°C were investigated by using the technique of carbon extraction replica, twin jet chemical polishing thinning and transmission electron microscopy. The carbides were identified to be (Ti,V)C precipitates in steel A and VC in steel B respectively, precipitated mainly from ferrites grains. The average equivalent radius was 3~6nm. Comparison of the experimental results in A and B steel revealed low carbon diffusion rate caused by aluminum inhibited the coarsening of vanadium-titanium carbides. The experimental results also showed that VC carbides dissolution occurred during the intercritical annealing at 800°C.

Structure of the surface layer of a wear-resistant coating after treatment with a high-intensity electron beam

Transmission electron microscopy is used to investigate the carbon extraction replicas of a wearresistant surface layer formed on Hardox 400 steel and its phase composition. The formation of a multiphase state is revealed, which includes a-iron grains and inclusions of carbide phases based on iron, chromium, and niobium. The surface layer is additionally treated by a high-intensity pulsed electron beam. The relative position of α-iron grains and particles of carbide phases is examined by the electron-diffraction microscopy of thin foils. The main carbide phase is iron carbide located as extended interlayers which separate a-iron grains. Nanoscale particles of chromium and niobium carbides are located at the interfaces of the a-iron–iron-carbide system and in the body of the a-iron grains. It is established that the surface layer is in the elastic-stress state during pulsed electron-beam treatment due to ultrahigh heating and cooling rates. It is shown that stress concentrators are the interphase boundaries between the carbide and the a-phase of iron.

Influence of Different Levels of Cu Equivalent on the Hot Ductility of 20CrMnTi Steel

AbstractThe hot ductility of 20CrMnTi steel with different levels of Cu equivalent was investigated. The results show that Sn and Cu in 20CrMnTi steel are detrimental to its hot ductility. Sn was found to segregate to the boundaries tested by EPMA, moreover, Cu was not found to segregate to boundaries, however, the fracture morphology was examined with SEM and showed many small and shallow dimples on the fracture of steels with large Cu equivalent (&gt;0.15) and fine copper sulfide was found from carbon extraction replicas using TEM. The adverse effect of large Cu equivalent (&gt;0.15) on the hot ductility was due to Sn segregation and fine copper sulfide in the steel as well as their retarding the occurrence of dynamic recrystallization (DRX). The proeutectoid ferrite film precipitating along the austenite grain boundary causes the ductility trough of the five examined steels. Moreover, in this case, the level of Cu equivalent should be controlled below 0.15, which would not deteriorate the hot ductility significantly.

Effect of double quenching and tempering heat treatment on the microstructure and mechanical properties of a novel 5Cr steel processed by electro-slag casting

The effect of double quenching and tempering (DQT) treatment as well as conventional high temperature quenching and tempering (CQT) treatment on the microstructures and mechanical properties of low carbon 5Cr martensitic as cast steel produced by electroslag casting was investigated. The microstructure changes were characterized by optical microscope (OM), scanning electron microscope (SEM), electron back scatter diffraction (EBSD) and transmission electron microscopy (TEM). The characteristics of carbides precipitated during tempering were analyzed on both carbon extraction replica and thin foil samples by TEM. The mechanical performance was evaluated by Vickers hardness test, tensile test, and Charpy V-notch impact test at ambient temperature. The results of microstructure study indicated that DQT treatment led to a finer microstructure than that of CQT. The carbides of the tempered samples were identified as M7C3. The carbides along the prior austenite grain boundaries nucleated directly while those within the laths should be transformed from cementite which formed at the early tempering stage. Compared with CQT condition, yield strength slightly increased after DQT treatment, and impact toughness improved a lot. The strengthening mechanisms were analyzed and it was found that grain refining and precipitation strengthening were mainly responsible for the increase of strength. The superior toughness of DQT condition was attributed to the finer microstructure resulting in more frequent deflections of the cleavage crack and the smaller size of carbides along the prior austenite boundaries. EBSD analysis showed that both martensitic block and packet of low carbon 5Cr tempered martensitic steel could hinder crack propagation, while the latter was more effective.

Analysis of Recrystallization and Strain-Induced Precipitation on High Nb- and N-Bearing Austenitic Stainless Steel

The mechanical properties and corrosion resistance of stainless steels are due to the combined effect of chemical composition and thermomechanical processing. The objective of this study was to investigate the interaction precipitation-recrystallization of an austenitic steel with high additions of nitrogen and niobium through continuous-cooling multiple deformation hot-torsion tests. Samples were heated up to a soaking temperature of 1250 oC and kept at this temperature for 5 minutes, and then deformed during cooling. The deformation pass was 0.3 with a strain rate of 1 s-1 and interpass times of 20 or 50 s. The evolution of the microstructure was investigated by optical, EBSD and transmission electron microscopy, using thin foils and carbon extraction replica samples. The results showed that some precipitates were not dissolved after reheating and the presence of niobium-and chromium-rich particles after processing was confirmed. The strain accumulation with the interpass time of 20 s yielded finer precipitation and improved grain refinement than observed after 50 s. Some interaction of the precipitates with dislocations and grain boundary could be evidenced.

Investigation on Dissolving Process of Secondary Phases and Austenitic Grain Growth in a Microalloyed Steel

The secondary phases such as carbonitrides in the microalloyed steel play very important roles in retarding the austenitic grains growth during reheating. The dissolving process of carbonitrides containing Nb, Ti, Mo will affect the austenitic grain sizes directly. In the present work, the dissolving behaviors of secondary phases in low carbon microalloyed steel during isothermal holding at different temperatures were investigated by electrolytic experiment, carbon extraction replicas, TEM and EDX analysis. Meanwhile, the austenitic grain sizes were measured corresponding to the temperatures. The experimental results indicate that there are two types of carbonitrides in as-forged steel. One is the coarsened Ti-rich precipitates originated from solidification, and the other is the finer Nb-rich particles attributed to strain-induced process. The strain-induced precipitates disappear after being held for 1 h at 1000°C. At 1000~1220 °C, the austenitic grains grow obviously due to rapid dissolving of the carbonitrides containing Nb and Mo. However, some undissolved Nb, Ti carbidenitrides still hinder the grain boundary migration. When the reheating temperature rises to 1270°C, the grain size grows abnormally after being held for 2h. At the holding temperature, few Nb-bearing TiN precipitates can be stable while the pinning effect weakens markedly.