Isothermal Transformation Research Articles

Application of the Nelder-Mead method to optimize the selecting of the Likhachev-Volkov model constants

A new method was proposed for selecting constants of the modified Likhachev - Volkov microstructural model to describe the reversible strain variation during isothermal martensitic transformation in Ti49Ni51 alloy. The experimental data were approximated by solving the optimization problem using the Nelder - Mead method. A set of model parameters was found that provided the best approximation. Using the selected constants set, model qualitatively and quantitatively describe the strain variation during cooling and heating of the model sample, as well as during isothermal holding under constant stress. The proposed method of selecting constants allowed performing the smallest number of numerical experiments without requiring additional real experiments.

Isothermal Martensitic Transformation in Ti-Ni-Cu-Co Shape Memory Alloy: Insight from a Thermally Activated Kinetic Model

Isothermal Martensitic Transformation in Ti-Ni-Cu-Co Shape Memory Alloy: Insight from a Thermally Activated Kinetic Model

Strain variation in the Ti40,7Hf9,5Ni41,8Cu8 alloy during isothermal martensitic transformation under a constant stress

Strain variation on holding under a constant stress was studied in the Ti40,7Hf9,5Ni41,8Cu8 alloy. It was found, that on holding under stress, the isothermal strain rose up to saturation, which value depended on holding temperature and stress. It was found that the dependencies of the isothermal strain on the holding temperature and stress were non-monotonic. This allowed to find the optimal value of stress and time at which the isothermal strain attained the maximum value of 3.2 %. It was found that the maximum isothermal strain in the Ti40,7Hf9,5Ni41,8Cu8 alloy was less than in the Ti40,7Hf9,5Ni44,8Cu5 alloy.

Current Status of Carbidic Austempered Ductile Iron

Grinding balls in wet ball mill are important consumables in mine grinding equipment, which have poor wear resistance and large consumption. It is imperative to find excellent wear-resistant materials for the grinding balls. Carbidic Austempered Ductile Iron (referred to as CADI) was used as small and medium-sized wet ball mills. This grinding ball has the advantages of less wear, low crushing rate, power saving and low noise. However, the CADI grain boundaries are distributed with net-like eutectic carbides, which seriously damage the continuity of the matrix. In addition, the mechanism of corrosion wear and impact fatigue is lack of research due to complex phase composition and unclear mechanism of phase properties on improving performance. So CADI can’t be applied to the grinding balls in large wet ball mill. Based on the above problems, this paper first analyzed the heteronucleation mechanism and adsorption mechanism of M3C type carbides by using the first principle of microalloying elements, and then verified it by combining with experimental results. Then the thermodynamics and kinetics of austenite homogenization and isothermal transformation of ductile iron containing carbides were analyzed by means of modeling calculation and experiment. On this basis, a new type heat treatment process comprising super-high temperature pretreatment and austempering treatment (S&A treatment) was used to process CADI, which provides a new idea for further improving toughness of CADI. Finally, the CADI corrosion wear and impact fatigue failure mechanism were revealed by analyzing the change rule of the sample surface and cross section after corrosion wear and impact fatigue.

Nanobainite Generated in Low‐ and Medium‐Carbon Steels via an Economical Alloying Strategy

A low‐cost strategy for generating fast transforming nanobainitic (nB) steels in low‐ and medium‐carbon alloys is investigated. Accelerating the bainite transformation relies on adding 0.7–3 wt% Al. Alloys and heat treatments are designed via thermodynamic calculations and dilatometry. nB microstructures are generated via isothermal holding and continuous cooling. The microstructures generated are investigated via microscopy and mechanical characterization. Incubation periods less than 150 s as well as isothermal transformation times ranging from 2000 to 4000 s are recorded for all conditions. Increasing the Al content from 0.7 to 2.8 wt% lowers the incubation and transformation times from 150 to 15 s and 3000 to 2000 s, respectively, at a cost of a reduction in tensile strength and elongation % (EL%) from 1330 to 1270 MPa and from 13.5 to 7.7%, respectively. The introduction of δ‐ferrite to the microstructure of the high Al alloy increased EL% up to 16% and reduced the tensile strength to 1105 MPa. Continuous cooling at a rate of 0.03 K s−1 increased the tensile strength by 100 MPa at similar EL%. Lowering the cooling rate to 0.003 K s−1 yielded similar properties as isothermal treatment because most of the transformation is concluded near the starting temperature.

Investigation of α phase evolution and tensile fracture behavior of Ti5Al2Sn2Zr4Mo4Cr alloy fabricated by directed energy deposition

α phase evolution and tensile fracture behavior of Ti5Al2Sn2Zr4Mo4Cr alloy fabricated by directed energy deposition (DED) were studied. Samples with a different number of layers were fabricated by the same processing parameters. Microstructure evolution was analyzed based on the thermal history calculated through finite element analysis and isothermal transformation kinetics. The results suggested that the samples exhibit various α phase characteristics. Two kinds of starting microstructures at different layers, single β phase and mixed α/β phases can be obtained depending on the deposited layer’s temperature and cooling rate. Further evolution of α phase is closely related to the consecutive thermal cycles with different durations and different temperature amplitudes. The in-situ scanning electron microscopy tensile tests were performed in the horizontal (perpendicular to the build direction) and vertical (parallel to the build direction) directions under the as-deposited and heat treatment conditions. The results showed that the cracks and micro-voids usually initiate on the grain boundaries or α/β interface. α phase characteristic plays an important role in deformation behavior. The sample containing fine α laths usually fractured quickly after the first surface crack was observed. The coarse α laths resulted in a larger crack tip plastic zone and delayed the crack propagation.

Kinetic Prediction for Isothermal Transformation of Inclusions in a Bearing Steel

Kinetic Prediction for Isothermal Transformation of Inclusions in a Bearing Steel

Non-classical nucleation in vapor\u2013liquid\u2013solid growth of monolayer WS2 revealed by in-situ monitoring chemical vapor deposition

The very early nucleation stage of a transition metal dichalcogenide (TMD) was directly observed with in-situ monitoring of chemical vapor deposition and automated image analysis. Unique nucleation dynamics, such as very large critical nuclei and slow to rapid growth transitions, were observed during the vapor–liquid–solid (VLS) growth of monolayer tungsten disulfide (WS2). This can be explained by two-step nucleation, also known as non-classical nucleation, in which metastable clusters are formed through the aggregation of droplets. Subsequently, nucleation of solid WS2 takes place inside the metastable cluster. Furthermore, the detailed nucleation dynamics was systematically investigated from a thermodynamic point of view, revealing that the incubation time of metastable cluster formation follows the traditional time–temperature transformation diagram. Quantitative phase field simulation, combined with Bayesian inference, was conducted to extract quantitative information on the growth dynamics and crystal anisotropy from in-situ images. A clear transition in growth dynamics and crystal anisotropy between the slow and rapid growth phases was quantitatively verified. This observation supports the existence of two-step nucleation in the VLS growth of WS2. Such detailed understanding of TMD nucleation dynamics can be useful for achieving perfect structure control of TMDs.

Accelerated Isothermal Phase Transformation and Enhanced Mechanical Properties of Railway Wheel Steel: The Significant Role of Pre‐Existing Bainite

Exploring possible ways to accelerate bainite transformation kinetics and enhance mechanical properties, especially for large‐scale parts, attracted much attention both in industrial manufacturing and fundamental research. Herein, prior continuous cooling transformed bainite is introduced to promote isothermal transformation and improve strength toughness matching of bainite railway wheel steel. Phase transformation study by dilatometer indicates that the time–temperature–transformation curves of tested steel swing back at lower isothermal temperature. This acceleration effect is due to the formation of leaf‐shaped bainite at initial isothermal transformation stage. By tailoring the cooling rate, leaf‐shaped bainite can also be obtained during continuous cooling. It is confirmed that the formation of leaf‐shaped bainite during continuous cooling stage accelerates the subsequently isothermal bainite transformation. In situ microstructure evolution observation demonstrates that prior bainite/ austenite interface provides favorable nucleation site for initial isothermal bainite transformation. Moreover, pre‐existing bainite formed during continuous cooling stage can effectively divide the original austenite grain size. Concurrent enhancement of tensile strength and impact toughness in bainite wheel steel can be achieved by controlling the cooling rate.

Effect of Carbon Partitioning and Residual Compressive Stresses on the Lattice Strains of Retained Austenite During Quenching and Isothermal Bainitic Holding in a High‐Silicon Medium‐Carbon Steel

The residual compressive stresses and dimensional changes related to the lattice strains of retained austenite (RA) phase in a high‐Si, medium‐carbon steel (Fe‐0.53C‐1.67Si‐0.72Mn‐0.12Cr) are investigated for samples austenitized and quenched for isothermal bainitic transformation (Q&B) in the range 5 s to 1 h at 350 °C. Also, samples are directly quenched in water (DWQ) from the austenitization temperature for comparison with Q&B samples. Field emission scanning electron microscopy (FE‐SEM) combined with electron backscatter diffraction (EBSD) analyses, and X‐ray diffraction are used to investigate the microstructural evolution, phase distribution, and lattice parameters of RA phase. While the Q&B samples showed formation of bainite and high‐carbon fresh martensite in conjunction with stabilization of various fractions of RA, the DWQ samples displayed nearly complete martensitic microstructure. For short holding durations (≪200 s), there was limited formation of bainite and the inadequate carbon partitioning to the adjacent untransformed austenite areas resulted in significant martensite formation and the associated c/a ratio of martensite resulted in high compressive residual stresses within the RA phase. While, at long isothermal holding times (≫ 200 s), there was a significant formation of bainite. The DWQ samples displayed maximum lattice strain in a small fraction of untransformed RA phase.

Formation of nodular bainite in an Fe-9.10Ni-0.06C (wt. %) alloy: A new microstructure for cryogenic steels

Quenched and tempered Fe-9Ni (wt. %) steels have been widely used in cryogenic applications. For the first time, the formation of nodular bainite in an Fe-9.10Ni-0.06C (wt. %) steel during isothermal transformation at 550 and 500°C for 24 h is reported. Electron microscopy revealed that proeutectoid ferrite forms predominantly by a massive transformation at prior austenite grain boundaries or at intragranular inclusions as the first stage of austenite decomposition. Then, nodular bainite composed of a ferritic matrix and rows of fine cementite precipitates, forms by interphase boundary precipitation during decomposition of adjacent C-enriched austenite. Both the Bagarayatskii and Isaichev orientation relationships were observed between ferrite and cementite in the nodular bainite. Microhardness of nodular bainite in the sample transformed at 500°C (214 ± 4 VHN) was higher than that in the sample transformed at 550°C (195 ± 5 VHN), due to smaller sizes of cementite precipitates in the former.

Microstructure and mechanical properties of a medium-carbon bainitic steel treated by quenching and two-step bainitic isothermal transformation below Ms

ABSTRACT A quenching and two-step bainitic isothermal transformation below Ms (Q-2BT) process was proposed for a medium-carbon steel. The evolution of microstructure and its correlation with mechanical properties were investigated and compared with the samples subjected to a quenching and one-step bainitic isothermal transformation below Ms (Q-1BT) process. Results showed that the Q-2BT sample has a finer martensite/bainite/retained austenite multiphase microstructure than the Q-1BT ones because of the formation of smaller bainite and the decomposition of large untransformed austenite blocks. Meanwhile, an optimal strength (1582 MPa), elongation (20.2%) and toughness (110 J) combination can be obtained after the Q-2BT process. The significantly improved impact toughness can be attributed to refined microstructure, increased filmy retained austenite as well as a high proportion of high-angle boundaries (70.7%). In addition, the Q-2BT sample exhibits a longer hardening plateau and a higher hardening exponent (0.084) because of the larger amount of retained austenite (11.3%) and more filmy austenite with high stability.

Construction of a TTT-η Diagram of High-Refractive Polyurethane Based on Curing Kinetics.

A time–temperature–transformation–viscosity (TTT-η) diagram can reflect changes in the physical states of a resin, which take on significance for the study of the curing process of polyurethane resin lenses. Coupling the differential scanning calorimetry (DSC) test, the curing kinetic parameters of 1,4-bis(isocyanatomethyl)cyclohexane (H6XDI)/2,3-bis((2-mercaptoethyl)thio)-1-propanethiol (BES) polyurethane system were obtained. By phenomenological modeling, the relationships between degree, temperature, and time were obtained. An isothermal DSC test was carried out at 423 K. Based on the DiBenedetto equation, the relationships between glass transition temperature, degree of cure, and time were obtained, and the glass transition temperature was thus correlated with temperature and time. The gelation time at different temperatures was measured by rotary rheometry, and the relationship between gelation time and gelation temperature was established. The time–temperature–transformation (TTT) diagram of H6XDI/BES system was constructed accordingly. Subsequently, a six-parameter double Arrhenius equation was used as the basis for the rheological study. The viscosity was examined during the curing process. The TTT-η diagram was obtained, which laid the theoretical foundation for the optimization and setting of processing parameters.

Assessing Fast Structure Formation Processes in Isotactic Polypropylene with a Combination of Nanofocus X-ray Diffraction and In Situ Nanocalorimetry.

A combination of in situ nanocalorimetry with simultaneous nanofocus 2D Wide-Angle X-ray Scattering (WAXS) was used to study polymorphic behaviour and structure formation in a single micro-drop of isotactic polypropylene (iPP) with defined thermal history. We were able to generate, detect, and characterize a number of different iPP morphologies using our custom-built ultrafast chip-based nanocalorimetry instrument designed for use with the European Synchrotron Radiation Facility (ESRF) high intensity nanofocus X-ray beamline facility. The detected iPP morphologies included monoclinic alpha-phase crystals, mesophase, and mixed morphologies with different mesophase/crystalline compositional ratios. Monoclinic crystals formed from the mesophase became unstable at heating rates above 40 K s−1 and showed melting temperatures as low as ~30 K below those measured for iPP crystals formed by slow cooling. We also studied the real-time melt crystallization of nanogram-sized iPP samples. Our analysis revealed a mesophase nucleation time of around 1 s and the co-existence of mesophase and growing disordered crystals at high supercooling ≤328 K. The further increase of the iPP crystallization temperature to 338 K changed nucleation from homogeneous to heterogeneous. No mesophase was detected above 348 K. Low supercooling (≥378 K) led to the continuous growth of the alpha-phase crystals. In conclusion, we have, for the first time, measured the mesophase nucleation time of supercooled iPP melted under isothermal crystallization conditions using a dedicated experimental setup designed to allow simultaneous ultrafast chip-based nanocalorimetry and nanofocus X-ray diffraction analyses. We also provided experimental evidence that upon heating, the mesophase converts directly into thermodynamically stable monoclinic alpha-phase crystals via perfection and reorganization and not via partial melting. The complex phase behaviour of iPP and its dependence on both crystallization temperature and time is presented here using a time–temperature–transformation (TTT) diagram.

JMAK model applied on the κ-carbide precipitation in FeMnAlC steels

Different lightweight steels alloys have been treated by different isothermal treatments in order to understand the basics of precipitation kinetics of κ-carbide for such class of new steel grades featured by high concentration of Mn and Al. The κ-carbide precipitation plays a significant role to induce the strengthening of these steel grades that maintain a significant ductility up to the fracture as a consequence of their duplex ferrite-austenite structure that exploits the twinning mechanism of austenite during the plastic deformation. In this paper the results about isothermal transformations involving the κ-carbide precipitation have been discussed. In order to deepen the comprehension of the mechanisms involved in precipitation of κ-carbides, the study has been performed by isothermal (Temperature Transformation Treatment) experiments applying different temperatures and holding times. The observed transformations have been measured and it has been possible to apply the JMAK model to fit such results. In addition, it has been observed a relationship between the steel composition and the activation energy for the transformation.

Effects of alloying elements on isothermal transformation behaviour of Ti/Mo microalloyed steel

Isothermal transformation is the key stage to carbide precipitation in microalloyed steel. Detailed phase transformation data are needed in order to control and study the precipitation process of carbides accurately. However, alloying elements will change the transformation process in this microalloyed low carbon steel. Therefore, the effects of titanium and molybdenum elements on isothermal transformation behaviour were studied by a Gleeble 3800 thermal-mechanical simulator. Microscopic test and dilatometry were applied to analyse the isothermal transformation of steels. The results show that the ferrite in steel changes from polygonal ferrite to quasi-polygonal ferrite to acicular ferrite with the decrease of temperature from 700°C to 550°C, and the addition of titanium and molybdenum elements retards the isothermal transformation and result in prolonged transformation time. Besides, alloying elements also tend to inhibit the phase transformation in specific temperatures and lead to more untransformed austenite.

Heat treatment optimization of an automotive stamping die steel with low energy consumption

It has innovatively teat-treated an ultra-high carbon steel via a novel technique with low energy consumption for automotive stamping die application in this study. Formation mechanism of isothermal transformation product at low temperatures in an ultra-high carbon steel has been investigated by the means of X-ray diffraction (XRD) analysis and metallographic examination on the resulting microstructure, which is optimally designed to achieve an excellent combination of high impact toughness and high hardness. It is indicated that microstructure morphology of resulting bainite isothermally formed at temperatures below 200°C is observably different from water-quenched shear-transformed martensite; in the case of isothermal transformation above 200°C for a long time, it obtains bainite sheaf which is composed of slender bainitic ferrite plates separated by parallel parent phase films; bainite sheaf is achieved by nucleation and growth of new parallel bainite plates in the vicinity of both body sides of the already formed bainite plate.

Effect of boron segregation on bainite nucleation during isothermal transformation

The effects of boron segregation and austenite grain size on the bainitic isothermal transformation of a high-strength steel were studied independently. Dilatometry, microstructural observations and previous atom probe tomography analyses showed that bainitic transformation rate decreases with increasing boron excess at austenite grain boundaries. We also found that boron segregation causes an unexpected grain size effect: large austenite grains transform faster than small ones. A kinetic model assuming slow nucleation at austenite triple junctions and rapid growth of bainitic nuclei successfully described our experimental data. These results confirm that bainite nucleation is inhibited by segregated boron at grain boundaries. On this basis, austenitic grain size could be optimized to make the most of boron addition in advanced high-strength steels.

Effects of Isothermal Transformation at the Quenching Temperature on the Microstructure and Mechanical Properties of a Medium-Carbon Steel

Effects of Isothermal Transformation at the Quenching Temperature on the Microstructure and Mechanical Properties of a Medium-Carbon Steel

Microstructural evolution during tempering of an ausformed carbide-free low temperature bainitic steel

• Limited effect of temerping at 400 and 500°C on non-ausformed crabide-free bainite. • Dilatometric contraction occurred for ausformed bainite sample due to cementite precipitation. • Carbon diffusion was enhanced by dislocations generated during ausforming processes. The influence of tempering at 400 and 500 °C on two bainitic ferrite microstructures obtained via pure isothermal transformation and ausforming followed by the isothermal transformation was studied in a medium C-Si rich bainitic steel. To understand the dilatometric behaviour and mechanisms of carbide-free bainite decomposition, extensive microstructural characterization using scanning electron microscopy, X-ray diffraction, and microhardness measurements were conducted. The softening due to tempering at 400 °C was negligible for both microstructures. Tempering at 500 °C led to a remarkable contraction in dilatometric signal in an ausformed sample, while a pure isothermal sample showed slight contraction. The main mechanism governing microstructural changes during tempering at 500 °C was the decomposition of as-ausformed highly dislocated bainitic ferrite plates and/or film-like retained austenite into cementite and ferrite as a result of enhanced carbon diffusion. While tempering at 400 °C did not result in changes in the size of the bainitic plates, the higher tempering temperature of 500 °C led to a thickening of the bainitic ferrite plates for both testing conditions. However, the thickening was restricted in the ausformed bainite, which was related to the pinning effect of very small cementite precipitates, which moderate the tempering effects on hardness reduction.