Isothermal Treatment Research Articles

Thermal stability of microstructure and corresponding hardness variations in two high-carbon nanostructured bainitic steels

The autotempering phenomenon and carbon partitioning from bainitic subunits during the bainite transformation suggest that the tempering behavior of the nanostructured bainite may be different compared to that of martensite. This article aims to investigate the thermal stability of the microstructure in two low alloy nanostructured bainitic steels containing 0.89 wt% carbon content. Nanostructured bainite was obtained after austempering at 250 °C for optimum heat treatment time. Nanostructured bainitic steels were tempered at the temperature range of 300–700 °C for 30 min. Additionally, the effect of prolonged austempering beyond the optimum heat treatment condition at austempering temperature was studied. Microstructural characteristics were evaluated using optical and field emission gun scanning electron microscopes and X-ray diffraction methods. Hardness variations during tempering of nanostructured bainite compared with that of martensite microstructure obtained in the same primary steels after austenitizing and quenching. Results indicated that microstructural characteristics of nanostructure bainite didn’t change significantly at heat treatment times longer than that needed for bainite transformation completion at isothermal bainite heat treatment temperature. Moreover, results approved the higher tempering resistance of nanostructured bainite compared to that of martensite in the same steel. It has been shown that tempering the nanostructured bainite up to 400 °C didn’t change the microstructure and hardness, while the changes were pronounced at higher tempering temperatures. It has been suggested that cementite precipitation was more significant when a higher number of dislocations were present in nanostructured bainite before starting the tempering process.

Thermoplastic elastomers based on lactide and caprolactone: The influence of chain microstructure on surface topography and subsequent interaction with cells

Surface biophysical properties of biomaterials, including surface topography and roughness, determine the interaction of a biomaterial with the surrounding cells, tissues and organs once implanted in the human body. Herein, the surface topography of thermoplastic copolymers based on lactide and caprolactone showing elastomeric behaviour was modulated by precisely controlling the chain microstructure (i.e., distribution and length of the repetitive units within the polymeric chain). The synthesized copolymers were subjected to different thermal treatments from the melt, leading to polymeric films with various surface textures and roughness values. Copolymers synthesized with triphenyl bismuth as a catalyst, with a more random distribution of the repetitive units, showed limited crystallization capability. Accordingly, only the copolymer with higher amount of l-lactide (i.e., 80 wt%) subjected to an isothermal treatment from the melt at 70 °C was able to crystallize, and spherulites of around 7 μm were discernible by atomic force microscopy. In contrast, the copolymers synthesized with stannous octoate, which had a more blocky nature, showed axialitic crystalline domains at the submicron-to nanoscale when subjected to an isothermal treatment from the melt at 50 °C, whereas well-defined spherulites of sizes up to 14 μm were obtained at 70 °C. Human fibroblast showed a more elongated morphology when seeded on those samples having higher roughness values and larger spherulites, whereas they had a more spread morphology when seeded on the amorphous, smooth surfaces. As concluded from the present study, by precisely controlling the chain microstructure of the synthesized copolymers, a wide variety of surface topographies can be obtained, which has a clear impact on the way the biomaterial interacts with cells. This opens the possibility to study the influence of surface biophysical properties on more complex cell processes in the future, including inflammatory or foreign body response processes.

Mechanical properties and micro-structural of Al-Zn-Mg-Cu alloy by semi-solid SIMA processed

This paper investigates the microstructure and mechanical behavior of 7050 aluminum alloy during semi-solid process. The semi-solid process involves multi-directional forging at 300 °C, and followed by semi-solid treatment at 590–630 °C for 10–30 min. The microstructure was characterized by scanning electron microscopy, X-ray diffraction, electron back scattered diffraction and transmission electron microscopy. There are many copper enriched at grain boundaries during isothermal treatment. It also can be seen that the reduce of Mg, Zn and Cu are inside the grain clearly. The mechanical properties of different treated samples in semi-solid state were studied by tensile test. The results show that the tensile strength and yield strength of the material decrease with the increase of holding time and due to its internal solid-liquid changes.

Decomposition Mechanism of Prior Austenite in Low‐Temperature Zone and Its Relationship to Fracture Behavior of High‐Strength Medium‐Mn Steel

Herein, the isothermal and continuous cooling transformation mechanisms of prior austenite and its relationship to fracture behavior of high‐strength medium‐Mn steel are studied. Furthermore, the precipitation behavior of cementite is also analyzed. The results indicate that the short rod‐like cementite forms in the martensite matrix during the air‐cooling process, the solubility difference of supersaturated elements provides a driving force for the element diffusion. Accordingly, it is proved that no isothermal transformation of prior austenite occurs during the 360 °C × 7 days isothermal treatment. The lath‐like microstructure is the martensite, which is produced during the subsequent continuous cooling process. The prior austenite is “divided” by the preferential martensite (transformed via heterogeneous nucleation), and the interfaces between preferential martensite and untransformed austenite provide nucleation sites for the subsequent martensite. The introduction of initial martensite before the isothermal treatment leads to the reversible brittleness of medium‐Mn steel. The fracture strength decreases with the increase of the initial martensitic fraction and isothermal time. The grain boundary binding force of prior austenite grains (PAGs) increases in the experimental steels subjected to annealing, and the plastic deformation of the martensite matrix is prior to grain boundary cracking, which inhibits the intergranular brittle fracture.

Coarsening of needle-shaped precipitates in CoNiCrAl alloys during isothermal heat treatment: Kinetics in length and thickness

This paper investigated the coarsening kinetics of needle-shaped γ precipitates in CoNiCrAl alloys after isothermal heat treatments at 900 °C and 1100 °C. The CoNiCrAl alloys were prepared by vacuum induction melting and casting process. They were characterised to exhibit a two-phase γ + β structure and some needle-shaped secondary γ precipitates were formed within the primary β phase during isothermal heat treatments. The coarsening of the needle-shaped secondary γ precipitates occurred with heat treatment time and became more rapid at elevated temperatures. In particular, the coarsening kinetics of needle-shaped γ precipitates in length and thickness were specifically studied. It was found that the coarsening kinetics in length and thickness was a volume diffusion governed process and generally followed the classic Lifshitz-Slyozov-Wagner (LSW) theory. An analytical model was proposed to estimate the lengthening and thickening of the precipitates, which considered the aspect ratio and volume fraction of the precipitates. The calculated coarsening rate in thickness agreed well with the experimentally determined value. But some discrepancies were found between the predicted and measured coarsening rate in length. This might be due to the termination migration effects of the needle-shaped precipitates which restricted the coarsening rate in length.

Fatigue Properties of Thixoformed Al-6wt%Si-2.5wt%Cu

One of the main functions of semisolid metal alloy forming processes, notably rheocasting and thixoforming, is the manufacture of parts, casings and frames for mechanical assemblies and systems. These parts not only must have the required minimum mechanical properties in terms of yield strength and elongation, but also must be able to withstand cyclic tensile and compressive forces. However, there is little fatigue strength data for the materials used for these parts. The present work seeks to fill this gap by determining the fatigue limit at 107 cycles of Al-6.0wt%Si-2.5wt%Cu alloy, or simply Al6Si2.5Cu, thixoformed in a pneumatic press at 585 °C (the temperature corresponding to 40 % solid fraction) with isothermal treatment times of 30 and 60 seconds. The parts were also subjected to T6 heat treatment, for which they were solution heat treated at a temperature of 520 °C for 4 hours followed by aging at 180 °C for 10 hours. For all the conditions tested, the microstructures were characterized to determine the grain size, appearance and shape of the silicon particles, in addition to the residual porosity. For the best conditions observed, 30 s holding time and T6 heat treatment, the grain size varied between 100 µm and 130 µm; the shape factor was around 0.60, indicating an excellent degree of roundness; and there was low residual porosity of around 0.3 %, resulting in a yield strength of up to 240 MPa with 4.5 % elongation. The average fatigue strength was estimated by the staircase method and was between 95 MPa and 98 MPa for 107 cycles.

Disentangling the contributions of initial heterogeneities and dynamic stress adaptation to nonlinearities in bacterial survival curves

The deviations from log-linearity that are often observed in bacterial survivor curves can be explained using different arguments, both biological and experimental. In this study, we used Bacillus subtilis as a model organism to demonstrate that the generally accepted vitalistic arguments (initial heterogeneities in the stress resistance of the cells in the population) may fail to describe microbial inactivation in some situations. In this sense, we showed how dynamic stress acclimation during an isothermal treatment provides an alternative explanation for survivor curves with an upwards curvature. We also provided an innovative experimental approach based on preadaptation experiments to evaluate which hypothesis is more suitable for the bacterial response. Furthermore, we used our experimental results to define bounds for the possible stress acclimation that may take place during dynamic treatments, concluding that the magnitude of stress acclimation may be larger for dynamic treatments than for isothermal experiments. We also evaluated the contribution of the SigB general stress response system to heat resistance by comparing the heat survival of wt and the ΔsigB mutant. Both strains survived better in 51, 52.5 and 55 °C when cells were pre-adapted at 48 °C than non-pre-adapted cells. However, ΔsigB was less resistant to heat than wt due to the missing SigB general stress system. Although these conclusions were based on B. subtilis as a model organism, this study can be the first step towards the development of a novel methodology able to estimate dynamic effects using only isothermal experiments. This would improve the models developed within the predictive microbiology community, improving our ability to predict microbial inactivation during industrial treatments, which are most often dynamic.

Study of structure evolution of ultra-rapidly annealed Fe75.3Ni10B14Cu0.7 alloys

The article concerns modern soft magnetic materials made by rapid solidification and subsequent ultra-rapid annealing. Due to the lower amount of non-magnetic elements, higher magnetisation is possible. Unfortunately, the lack of those elements causes difficulty during isothermal nanocrystallisation operation, being crucial from the final product properties point of view. Instead of isothermal heat treatment, ultra-rapid annealing (URA) is implemented. The seconds-long annealing at a higher temperature than the temperature of conventional nanocrystallisation enables material with a higher amount of ferromagnetic elements and optimal structure. It reflects in materials with good magnetic softness and higher possible magnetisation values. The amorphous alloy with Fe75.3Ni10B14Cu0.7 chemical composition in the shape of a ribbon was heat treated between cooper blocks at 773 K (500 °C) for different times in the 1–60 s range. To study structure evolution, we brought XRD, DSC, Flash DSC, Mössbauer spectroscopy, TEM, and HRTEM techniques to play. Based on the DSC results, we estimated the minimum heating speed to obtain optimal structure at 6000 K/min. The Mössbauer results allowed us to study annealing time-dependent local atomic ordering in the amorphous state. A comparison of X-Ray and synchrotron diffraction results drove us to the conclusion that X-Ray measurements are sufficient to determine the material structure. TEM shows the formation of α-Fe crystallites even after an annealing time of 0.5 s. The increase in annealing time leads to an increase in the crystalline phase volume fraction and growth of α-Fe crystallite size.

Effect of sub-solvus treatment on discontinuous precipitation behavior and mechanical properties of AlCrFeNiV multi-principal element alloy

The discontinuous precipitation (DP) behavior of AlCrFeNiV multi-principal element alloys (MPEAs) based on initial face-centered-cubic (FCC) single-phase structure is investigated in this work. The investigation focuses on the impact of a novel sub-solvus (SS) treatment, which accelerates the DP transformation kinetics by refining the microstructure, as compared to the conventional total-solution (TS) treatment. This acceleration is attributed to an increase in nucleation sites or localized changes in chemical composition. The presence of NiAl phases with a B2 structure, which precipitate along prior grain boundaries, is believed to play a dominant role in grain refinement. Additionally, highly dispersed L12 nano-precipitates are observed within the FCC matrix in both TS- and SS-treated samples. Building upon the strengthening effect of L12 nanoprecipitates, further optimization strategies are explored through variations in isothermal aging treatments. This leads to the preparation of DP structures with various degrees and morphologies, consisting of BCC phases and a coherent FCC/L12 microstructure. The additional HDI hardening stemming from the resulting DP/non-DP heterogeneous structure and the spheroidization of the ultrafine DP lamellar structure allowed the alloy to exhibit excellent mechanical properties, the latter of which yielded an interesting combination of strength (1907 MPa tensile strength, 1694 MPa yield strength) and ductility (10.1% elongation). These findings highlight the effect of the prior grain refinement on the DP transition kinetics, BCC lamellae stability, and the overall mechanical performance of AlCrFeNiV multi-principal element alloys (MPEAs).

Microstructural engineering by heat treatments of multi-principal element alloys via spinodal mediated phase transformation pathways

Nanoscale multi-phase microstructures observed in multi-principal element alloys (MPEAs) such as AlMo0.5NbTa0.5TiZr, Al0.5NbTa0.8Ti1.5V0.2Zr, TiZrNbTa, AlCoCrFeNi and Fe15Co15Ni20Mn20Cu30 that exhibit promising mechanical or functional properties may have evolved through spinodal-mediated phase transformation pathways (PTPs). The microstructures in such MPEA systems could be further engineered for targeted applications by appropriately designing the alloy composition and heat-treatment schedule. In this study, we investigate systematically how different heat treatment schedules such as single-step isothermal aging, two-step isothermal aging and continuous cooling alter the interplay among the various factors associated with alloy composition, such as volume fraction of individual phases, lattice misfit and modulus mismatch between the co-existing phases. We have determined the degree to which these factors influence significantly the spinodal-mediated PTPs and the corresponding microstructures by use of high-throughput phase-field simulations. In particular, we demonstrate that the microstructural topology (i.e., which phase forms the continuous matrix and which phase forms discrete precipitates) in the same MPEA having an asymmetric miscibility gap could be inverted simply by a continuous cooling heat treatment. Further, we reveal a rich variety of novel hierarchical microstructures that could be designed using two-step isothermal aging heat treatments in MPEA systems with symmetric or asymmetric miscibility gaps. These simulation results may shed light on novel microstructure design and engineering for the above-mentioned MPEA systems.

Super cold-crystallization of sodium iodine in selenium based chalco-halide glasses

Mechanical properties of a chalco-halide glass of the composition (0.72 GeSe2–0.28 Ga2Se3)75 NaI25 are studied during isothermal treatment below Tg (Tg-60 °C). The evolution of the Young's Modulus (E), the damping (Q−1) and the apparent viscosity (J−1, reverse of the creep rate) during heat treatment show that a structural rearrangement occurs in the glass below Tg. X-Ray diffraction and transmission measurements of the sample after heat treatment highlight the beginning of a super cold crystallization of cubic NaI within the glass after 1 month at Tg-60 °C. After 3 months of heat treatment, NaI crystals of about 31 nm size have been observed in the glass matrix by transmission electron microscopy.

Accelerated coarsening behavior of the γ´-phase in CMSX-4 during non-isothermal heat treatment

In literature, microstructural investigations can be found comparing the evolution of the well-known γ/γ ´ -phase morphology of nickel-based superalloys for long-term heat treatments with and without mechanical load, as well as for isothermal and non-isothermal creep tests. However, data of experiments conducted under non-isothermal conditions without an applied load are hard to find. To close this gap, isothermal and non-isothermal heat treatments without a mechanical load were conducted. Conductive heating was applied to samples that consisted of segments with different cross sections and allowed the simultaneous testing at different temperatures. The microstructural changes were examined by scanning electron microscopy after 500 h and 1000 h. Several established methods for the quantification of the γ ´ -phase coarsening were evaluated and employed for comparison of the two heating conditions studied. The coarsening was more distinct in samples heat-treated under non-isothermal conditions at all temperatures investigated. This was also verified by calculating the connectivity number NA, which was found to be the most useful parameter for quantification of microstructural changes observed. The γ ´ -precipitates became more connected during the heat-treatment, but no preferred direction of coarsening was observed like during rafting under an applied load. Therefore a more general particle agglomeration mechanism is assumed to be responsible for coarsening of the γ ´ -phase.

Experimental study on the microstructure and mechanical properties of the Mg-Y-Zn alloys with LPSO phases tailored by CALPHAD method

Long-Period Stacking Ordered (LPSO) phases in the Mg-Y-Zn alloys can significantly improve comprehensive mechanical properties of the Mg-based alloys. The nominal compositions of the Mg95Y3Zn2 and Mg86Y8Zn6 (at%) alloys with different contents of LPSO phases were tailored by CALPHAD method, and the tested compositions of these two alloys were Mg95.2Y2.6Zn2.2 and Mg86.2Y7.7Zn6.1, respectively. The microstructure evolutions of the studied alloys in the states of as-cast and isothermal treatments at different times were observed, and the mechanical properties of the as-extruded alloys after homogenization (short term, 12 h) and the isothermal treatment (long term, 240 h) at 500 ℃ were measured. After the isothermal treatment at 500 °C for 240 h, the phase constituent of the Mg95.2Y2.6Zn2.2 alloy changed from the as-cast three phases (18 R + α-Mg + W-Mg3Y2Zn3) into the equilibrium three phases (18 R + α-Mg + 14 H), while that of the Mg86.2Y7.7Zn6.1 alloy completely changed from the as-cast four phases (18 R + α-Mg + W-Mg3Y2Zn3 + 14 H) into the equilibrium two LPSO phases (18 R + 14 H), where the 14 H-type LPSO phase is formed by the four-phase invariant reaction of “18 R + α-Mg + W-Mg3Y2Zn3 → 14 H” at 546 ℃ and the three-phase univariant reaction of “18 R + α-Mg → 14 H” below 546 ℃. The results of mechanical properties show that the yield strength of the Mg86.2Y7.7Zn6.1 alloy composed entirely of LPSO phases is better than that of the Mg95.2Y2.6Zn2.2 alloy composed of both LPSO phases and some α-Mg matrix, while the plasticity of the Mg86.2Y7.7Zn6.1 alloy is not as good as that of the Mg95.2Y2.6Zn2.2 alloy. Therefore, the amount of LPSO phases can be increased for pursuing high strength of Mg alloys, but a certain amount of α-Mg phase needs to be kept for retaining good plasticity. For this purpose, the alloy composition with a specific amount of the LPSO phases and α-Mg matrix in the Mg-Y-Zn system can be tailored by CALPHAD method.

Phase-Field Modeling of Spinodal Decomposition in Fe–Cr–Co Alloy under Continuous Temperature-changing Conditions

Fe-Cr-Co alloys are becoming important as a half-hard magnet for their novel applications, including non-contact electromagnetic brakes, because of the controllability of its magnetic hardness depending on the modulated structure formed by spinodal decomposition. However, the experimental optimization of the complicated heat-treatment process to control the microstructure significantly increases the development cost, and microstructure prediction by computational simulation is desired. In this study, we first developed the method of phase-field simulation for spinodal decomposition in Fe-Cr-Co alloy during various heat treatments, including isothermal heat treatment, multistep continuous fast and slow cooling, which allows us to conduct a simulation of spinodal decomposition under conditions close to the condition of practical heat treatment. The simulation results revealed that the morphology of the modulated structure is predominantly determined by the cooling rate and does not change significantly during the subsequent isothermal annealing process, while the difference between the concentrations of the FeCo-rich magnetic phase and the Cr-rich non-magnetic phase increases. Continuous cooling at rates higher than 140 K/h demonstrates the maximum number densities of the ferromagnetic particles of α1-phase seemingly almost reaching saturation, which is expected to give rise to exhibiting the largest coercive force of the Fe-Cr-Co magnet. Moreover, this method can be extended to other materials for designing a modulated structure to show a desired property.

Microstructure evolution of as-cast CoNiCrAl bond coat alloys after isothermal heat treatments

In this paper, the microstructure evolution of two as-cast CoNiCrAl bond coat alloys after various isothermal heat treatments were investigated. The CoNiCrAl bond coat alloys were prepared by vacuum induction melting and casting process. The as-cast alloys were subjected to heat treatments from 550 °C to 1300 °C for periods up to 100 h to study the effects of temperature on the microstructure and phase distribution. The microstructure of heat treated CoNiCrAl bond coat alloys was characterised by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). The CoNiCrAl alloys exhibited a two phase γ + β structure, with the β phase acting as the primary phase. It was shown that the precipitation of needle-like secondary γ phase was slow at temperatures below 600 °C but became rapid above 900 °C. It was found that the morphology of the secondary γ phase changed from needle-like to rod-like at 1100 °C. A homogeneous γ + β two phase structure was obtained at 1300 °C even after 2 h heat treatment. It was revealed that the heat treatment temperature had a significant influence on the formation and distribution of the secondary γ phase. In addition, the precipitation of sub-micron Cr-rich particles occurred at 1300 °C in both CoNiCrAl alloys, which was due to the low solubility of Cr in the β phase and rapid coalescence of γ phase at elevated temperatures.

Modeling survival curves of Anisakis L3 after isothermal heat treatments at lethal temperatures

Even though anisakiasis is considered, nowadays, a significant threat to public health all over the world, no attempt has been made up to date to mathematically describe the thermal susceptibility of Anisakis larvae in the third stage (L3). To fill this gap, in this paper, more than 10,000 free (non-encysted) Anisakis L3 were individually heat treated in a thermal cycler at temperatures between 44 °C and 61 °C for different exposure times. After heat exposition, viability was assessed in each larva, survival curves at isothermal conditions were derived, and the effectiveness of four kinetic models (fundamental kinetic model, Mafart model, and probit and logit models) in describing these curves was tested. Evaluation of larvae viability after heat exposition revealed sigmoidal survival curves that increased their steepness with temperature. Of the four models tested, the Mafart model was the one that best fitted the data only differing from the observed survival ratios by 0.12 units on average. Validation experiments performed at temperatures different to those used to create the model corroborated its predictive capacity. Future efforts should be focused in predicting larvae viability at non-isothermal conditions as those occurring during fish cooking.

Influence of Austempering Temperature on Microstructure and Mechanical Properties of High‐Silicon Carbide‐Free Bainitic Steel

The microstructural evolution of a novel high‐silicon carbide‐free bainitic steel at different austempering temperatures is investigated. The microstructure is evaluated by means of optical and electron microscopy, X‐ray diffraction, microhardness, and nanohardness. Results show a variation in the amount of stabilized retained austenite changing the temperature of the isothermal treatment. In particular, it is observed an increase in the retained austenite volume fraction increasing the temperature up to 350 °C, while further increase leads to a reduction. Moreover, increasing the isothermal holding temperature from 250 °C, through 300, 350, and 370 °C, a progressive bainite coarsening and an increase in the amount of stabilized carbon‐enriched retained austenite are observed. Tensile tests reveal an excellent combination of mechanical properties: mechanical strength in the range 1276–1988 MPa and total elongation 0.18–0.44.

In situ growth of AuNPs with a nanocavity on the surface of optical fibre for development of SPR sensor

Gold nanoparticles (AuNPs) integrated within optical fibres show great potential as sensor materials in the field of sensing, utilizing the Surface Plasmon Resonance (SPR) principle. This study presents a straightforward, cost-effective, and replicable approach for producing permanent in-situ embedding of AuNPs with nanocavities on the surface of optical fibres. The process involves chemical vapour etching (CVE), followed by gold sputtering and isothermal heat treatment. The analysis conducted using High-Resolution Transmission Electron Microscopy (HRTEM) and Energy-Dispersive X-ray Spectroscopy (EDX), confirmed the presence of nearly spherical AuNPs embedded on the surface of the optical fibre, with an average diameter of 60 ± 5 nm. Additionally, the detection of two distinct plasmon absorption peaks at 567 ± 5 nm and 620 ± 5 nm using a Fibre Optic Spectrometer further validated the presence of AuNPs. To verify the existence of nanocavities surrounding the AuNPs, Field Emission Transmission Microscopy (FESEM) was employed, along with subjecting the AuNPs embedded fibre to three different NaCl solutions of varying concentrations. The results supported the presence of a nanocavity encapsulating the AuNPs. A proposed mechanism was put forth to explain the spectral behaviour resulting from the creation of the nanocavity. This specialty optical fibre is expected to have broad applications in the development of SPR-based chemicals and biosensors, as it facilitates close interaction between the AuNPs and analytes through the nanocavity surrounding the AuNPs on the fibre surface. The fabrication process demonstrated excellent repeatability, with strong adhesion of AuNPs to the optical fibre surface, and no leaching even after repeated use. Thus, we believe that this fabrication method is an exceptional technique for permanently incorporating AuNPs into various glass-based optical waveguide materials for SPR-based sensing applications.

Study on the Semi-Solid Thixotropic Forging Forming Process for the Low-Carbon Steel Claw Pole.

Low-carbon steel has been popularly applied in numerous applications because of its unique features, such as good plasticity, high strength, great hardness, and excellent toughness. Additionally, the semi-solid thixotropic forging forming method has been widely used in light alloys, due to its advantages of low forming force and high forming quality, whereas its application in ferrous materials is still limited. In this study, the semi-solid thixotropic forging forming process is proposed for producing the low-carbon steel claw pole, with the main stages being radial forging deformation, isothermal treatment, and forging forming. The effect of the area reduction rate on the effective strain from the cross sections of the radial-forged metal bar was studied using numerical simulations. The effect of the isothermal holding process on the microstructures of radial-forged billets was investigated, to obtain the ideal semi-solid microstructures. The microstructure and mechanical properties of low-carbon steel claw poles from the thixotropic forging experiment are presented and discussed. It was found that when the area reduction rate was 67%, the effective strain at the edge of the metal bar exceeded 5.0, while the effective strain at the center was above 1.2, indicating an excellent quality of forging for the bar. The optimization of the process parameters for preparing low-carbon steel semi-solid billets with fine and globular microstructures was achieved with an area reduction rate of 67%, an isothermal temperature of 1500 °C, and a duration time of 15 min. Moreover, the low-carbon steel claw pole fabricated with the optimized operating parameters was found fully filled, with a sharp profile and a flat surface, where the yield strength and tensile strength increased by 88.5% and 79.8%, respectively, compared to the starting materials.

Investigation of thermal phase evolution in Cr2(C,N) inhibitor for optimizing high-performance cemented carbide sintering process

The introduction of inhibitors containing nitrogen has demonstrated its efficacy in the fabrication of high-performance cemented carbides. Nevertheless, the thermal instability of nitrogen-doped compounds also presents a heightened risk of nitrogen removal, which can adversely affect the density of the carbides. Hence, this study aimed to explore the thermal stability of a widely employed nitrogen-containing Cr2(C,N) inhibitor. The findings reveal that a well-established denitrification process commences at 950 °C and intensifies above 1200 °C, accompanied by a Cr2(C,N) → Cr7C3 → Cr23C6 phase evolution. Utilizing the aforementioned insights, the sintering process was optimized by adding an isothermal treatment to obtain dense and refined cemented carbide exhibiting high performance. As a result, the sintered ultrafine WC-10Co alloy maintains a series of excellent comprehensive performances combined with high transverse rupture strength (4850 MPa), high fracture toughness (10.6 MPa·m1/2), and high hardness (1870 kg/mm2). This work provides basic guidance on sintering strategy design for preparing high quality cemented carbides with nitrogen-contained inhibitors.