Austenite Transformation Temperature Research Articles

Study of Phase Transformations and Interface Evolution in Carbon Steel under Temperatures and Loads Using Molecular Dynamics Simulation

Carbon steel materials are widely used in mechanical transmission. Under different temperature and pressure service conditions, the microscopic changes of stress and strain that are difficult to detect and analyze by experimental means will lead to failure deformation, thus affecting their operational stability and life. In this study, the molecular dynamics method is used to simulate the heating–cooling phase transition process of common carbon steel materials. Austenite transformation temperatures of 980 K (0.2 wt.%) and 1095 K (0.5 wt.%) are acquired which is determined by the volume hysteresis before and after transformation, which is consistent with the results of JMatPro phase diagram analysis. The internal stress state of the material varies between compressive stress and tensile stress due to the change of phase structure, and the dislocation characteristics during the phase transition period are observed to change significantly. Then, an α/γ two-phase interface model is constructed to study the migration of the phase interface and the change of the phase structure by applying a continuously changing external load. At the same time, the transition pressure of α→ϵ is obtained with a value of 37 GPa under three different initial loads showing the independence of the initial load and the historical path. Based on the molecular dynamics simulation and the phase diagram calculation of the carbon steel, the analysis method for the microstructure transformation and the stress–strain behavior of the phase interface under the external load can provide a reference for the design of microstructure and mechanical properties of alloy steel in the future.

Microstructure and properties of 3D-printed ZrCu–Ti6Al4V laminated composite

The microstructures, shape memory properties, and mechanical properties of 3D-printed ZrCu–Ti6Al4V laminated composites were investigated in this study. The microstructure of the laminated composite effectively combines the characteristics of two alloys. The shape memory effect and mechanical properties of the laminated composite are affected by the thickness of the layer. The results reveal that higher austenite transformation temperature than ZrCu with total recovery strain close to ZrCu, and a compression strain exceeding 21% can be achieved by 3D-printed ZrCu–Ti6Al4V laminated composite with thickness of 0.25 mm. Additionally, the composite with thickness of 0.5 mm demonstrates excellent fracture toughness values of 44.30 MPa⋅m1/2, as well as high yield strength and flow stress values reaching 1818 MPa and 2194 MPa respectively at a strain rate of 3500 s-1. This study highlights that incorporating Ti6Al4V layers into ZrCu through 3D-printing can significantly enhance mechanical properties while maintaining favorable shape memory properties, thus combining the advantages of functional and structural materials.

The effect of Hf on the microstructure, transformation temperature, and the shape memory properties of Zr–Cu-based high-entropy shape memory alloys

The microstructures, phase transformation temperatures, and shape memory properties of Hf-modified Zr50−xHfxCu25Ni7.5Co17.5 and Zr50−xHfxCu25Ni10Co15 high-entropy shape memory alloys (SMAs) were investigated. Hf additions can alter the valence electron concentration, strengthen the alloy matrix, and introduce more secondary phase particles, which significantly influence the properties of the alloys. The results reveal that high austenitic transformation temperature (over 600 K) with very small thermal hysteresis (70–80 K) can be achieved by tailoring the Hf contents, and the best shape memory recovery strain (up to 2.44%) compared with other Zr–Cu-based SMAs could be obtained in Zr42Hf8Cu25Ni10Co15 alloy. This study demonstrated that high-performance Zr–Cu-based high-entropy SMAs can be realized by incorporating Hf additions.

Carbide precipitation during tempering of hybrid steel 60

The effects of carbide precipitation on mechanical performance of Hybrid Steel 60, known as a novel bearing steel, have not been investigated. In this study, the austenite transformation temperatures of Hybrid Steel 60 during heating were revealed by the thermal expansion curve. The temperature and effective activation energy of the second phase precipitation were determined by the differential scanning calorimetry (DSC) curve. Different solid solution structures after austenitization were detected using various cooling rates. The solubility temperature was determined based on hardness and residual austenite content. The carbides precipitated at the peak temperature were qualitatively identified using XRD. It was discovered that the temperature points Ac1 and Ac3 of the steel were 786 °C and 864 °C, respectively. In addition, the effect of solid solution temperature on quenching hardness is minimal, while the cooling rate has a greater impact on hardness, reaching a peak at 5 °C s−1. The primary carbide phase in Hybrid Steel 60 is the M7C3 and VC. When the temperature ranges from 500 °C to 550 °C, M23C6 begins to precipitate. As a result, after tempering at 525 °C, the hardness peak value reached 566 HV.

EFFECT OF STEEL’S THERMAL CONDITION ON THE TRANSFORMATION TEMPERATURES OF TWO HOT-WORK TOOL STEELS WITH INCREASED THERMAL CONDUCTIVITY

The aim of our study was to investigate how different thermal conditions affect the transformation temperatures of two hot-work steels with high thermal conductivity. We focused on two conditions: soft annealing, and quenching and tempering. Soft annealing results in a ferritic steel matrix with spherical carbides, while quenching and tempering result in a fully hardened and tempered martensitic matrix with secondary and tempering carbides. We analysed samples using a simultaneous thermal analysis (STA) and differential scanning calorimetry (DSC) to determine the transformation temperatures. Controlled heating and cooling allowed us to observe the energy changes associated with the phase transformations. The equilibrium temperatures were calculated using the CALPHAD method. Our study investigated the influence of thermal conditions on different transformation temperatures, including solidus/liquidus temperatures, austenite solid transformation temperatures (A1 and A3), austenite solidification temperatures and bainite and/or martensite transformation temperatures. A DSC analysis was used to quantitatively measure the transformation temperatures and energy absorption during the endothermic processes (austenite solid transformation and melting) and to study the energy release during the exothermic processes (solidification and transformation). The results showed that soft annealing reduced the solidification interval and the solidus temperature, while energy absorption increased during melting. Conversely, quenching and tempering reduced the austenite solid transformation temperatures and energy release during solidification, including δ-ferrite solidification.

MOGA and TOPSIS-based multi-objective optimization of wire EDM process parameters for Ni50.3-Ti29.7-Hf20 alloy

Conventional machining techniques face challenges in processing Ni-Ti-Hf alloys, which exhibit superior properties and are increasingly considered promising materials for high-temperature shape memory actuator applications. Thus, this article focuses on investigating the effect of Wire Electric Discharge Machining (WEDM) input parameters, namely discharge time (TON), pause time (TOFF), gap voltage (SV), and wire travel speed (WF), on the surface quality and shape memory properties of these alloys. These parameters were optimized to obtain a better removal rate (MRR) and surface finish quality (Ra) by employing a hybrid approach of Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) and Multi-Objective Genetic Algorithm (MOGA). TON emerged as the most influencing parameter for both MRR and Ra, and the sample machined using optimal parameter setting, which had a MRR of 5.287 mm3/min and Ra of 2.335 µm, showed better surface quality with fewer surface defects and irregularities, lower recast layer thickness of 10.057 µm, and better shape memory properties with less than 15 % deviation in their latent heat of transformation values and a less than 5ºC change in their austenite and martensite transformation temperature values, which indicates MOGA was successful in finding a trade-off between the two responses.

Effect of Heat Treatment Time and Temperature on the Microstructure and Shape Memory Properties of Nitinol Wires.

In this study, the effect of heat treatment parameters on the optimized performance of Ni-rich nickel-titanium wires (NiTi/Nitinol) were investigated that were intended for application as actuators across various industries. In this instance, the maximum recovery strain and actuation angle achievable by a nitinol wire were employed as indicators of optimal performance. Nitinol wires were heat treated at different temperatures, 400-500 °C, and times, 30-120 min, to study the effects of these heat treatment parameters on the actuation performance and properties of the nitinol wires. Assessment covered changes in density, hardness, phase transition temperatures, microstructure, and alloy composition resulting from these heat treatments. DSC analysis revealed a decrease in the austenite transformation temperature, which transitioned from 42.8 °C to 24.39 °C with an increase in heat treatment temperature from 400 °C to 500 °C and was attributed to the formation of Ni4Ti3 precipitates. Increasing the heat treatment time led to an increase in the austenite transformation temperature. A negative correlation between the hardness of the heat-treated samples and the heat treatment temperature was found. This trend can be attributed to the formation and growth of Ni4Ti3 precipitates, which in turn affect the matrix properties. A novel approach involving image analysis was utilized as a simple yet robust analysis method for measurement of recovery strain for the wires as they underwent actuation. It was found that increasing heat treatment temperature from 400 °C to 500 °C above 30 min raised recovery strain from 0.001 to 0.01, thereby maximizing the shape memory effect.

The effect of high-temperature ECAP on dynamic recrystallization behavior and material strength of 42CrMo steel

With the development of mechanical equipment towards long life and high reliability, the traditional heat treatment method is challenging to meet the high strength requirements of 42CrMo steel for mechanical equipment. High-temperature equal channel angular pressing (ECAP) of 42CrMo steel was carried out above austenite transformation temperature, and the microstructure and material strength of the specimen at different radial positions were characterized and tested. The dynamic recrystallization (DRX) kinetic model of 42CrMo steel was established. The distribution of strain, strain rate, temperature and DRX volume fraction in high-temperature ECAP was studied by coupling finite element software and DRX kinetic model, and the formation mechanism of microstructure and material strength difference was revealed. The results show that the different degrees of DRX caused by the uneven distribution of strain and strain rate on the specimen is the main reason for the gradient distribution of microstructure and material strength in the radial direction of the specimen. From the top to the bottom of the specimen, with the decrease of strain and strain rate, the degree of DRX increases continuously, the microstructure inhomogeneity decreases, the size of the martensite packets and blocks increases gradually, and the strength decreases gradually. The grain boundary strengthening caused by the decreased size of martensite packets and blocks is the main reason for the radial difference in material yield strength. Grain boundary strengthening and the coordinated deformation of geometrically necessary dislocations at the soft-hard grain interface of heterogeneous materials are the main mechanisms leading to the tensile strength gradient distribution. This study reveals the dynamic recrystallization behavior and material strength variation of high-temperature ECAP, which contributes to the qualification of the ECAP process and promotes the development of severe plastic deformation on the further strength enhancement of high-strength steel.

To flat or not to flat? Exploring the impact of flat-side design on rotary instruments using a comprehensive multimethod investigation.

To assess the influence of a flat-side design on the geometry, metallurgy, mechanical performance and shaping ability of a novel nickel-titanium rotary instrument. Sixty-five new 25-mm flat-side rotary instruments (size 25, taper 0.04) and their nonflat-side prototypes (n = 65) were assessed for major deformations and examined regarding macroscopic and microscopic design, determination of nickel and titanium elements ratio, measurement of phase transformation temperature and evaluation of mechanical performance parameters including time/cycles to fracture, maximum torque, angle of rotation, maximum bending and buckling strengths and cutting ability. Additionally, unprepared canal areas, volume of hard tissue debris and percentage reduction of dentine thickness were calculated for each tested instrument after preparing mesial canals of mandibular molars (n = 12), using micro-CT imaging. Statistical analyses were performed using the U-Mann-Whitney test and independent Student t-test (α = 5%). The number of spirals (n = 8) and blade direction (clockwise) were similar between both flat and nonflat instruments, whilst the helical angles were equivalent (⁓25°). Flat-instruments showed inconsistencies in the homogeneity of the gold colour on the flat-side surface, blade discontinuity, and incomplete and variable S-shaped cross-sections. The titanium-to-nickel ratios were equivalent, but significant differences in the R-phase finish and austenitic start phase transformation temperatures were observed between the flat and nonflat-side instruments. The flat-side instruments demonstrated superior cutting ability compared to the nonflat instruments, as well as, significantly lower values for time to fracture, rotation to fracture and maximum torque to fracture (p < .001). No statistical difference was observed between tested instruments regarding angle of rotation (p = .437), maximum bending (p = .152) and buckling load (p = .411). Preparation protocols using flat and nonflat instruments did not show any statistically significant differences (p > .05). All flat-side instruments exhibited deformation after shaping procedures. The flat-side instrument showcased enhanced cutting ability compared to its nonflat counterpart. However, it exhibited inferior performance in terms of time, rotation and maximum torque to fracture, along with distinct phase transformation temperatures. No differences were observed in the titanium-to-nickel ratios, angle of rotation, maximum bending, buckling load, preparation time, percentage of untouched canal walls, volume of hard tissue debris and percentage reduction of dentine thickness.

Drawing fracture behavior and mechanism analysis of ER50-6G welding wire steel

In this paper, we investigated the drawing fracture behavior of ER50-6G welding wire steel and analyzed the factors affecting the fracture, such as continuous casting slag and inclusion, billet structure segregation, Ti inclusion, surface defect inheritance, metallographic structure type, fracture morphology, and so on. OLYMPUS GX53 optical microscope was used to observe and analyze the macrostructure of the cross-section and longitudinal section of the continuous casting billet and the original quality state of the wire rod surface. The continuous cooling transformation curve (CCT curve) of the steel was measured using a DIL805A thermal dilatometer and a Gleeble-3500 thermal simulation test machine. The microstructures of the non-fracture area and drawing fracture of wire rod samples were observed through a ZEISS-ULTRA scanning electron microscope. The results show that the austenite starting transformation temperature (Ac1) and the austenite finishing transformation temperature (Ac3) of ER50-6G welding wire steel are 732 °C and 865 °C, respectively. Under the actual cooling conditions of online production, the strip structure is composed of ferrite (F), pearlite (P), and a small amount of bainite (B). There is a small amount of slag and inclusion in the continuous casting billet; in addition, Al2O3 inclusions and TiC (or Ti(C,N)) inclusions can be observed in the wire rod. The scratch depth of the wire rod surface is 24∼62 μm and the width is 31∼50 μm. With the increase in drawing cumulative deformation, the Al2O3 and TiC (or Ti(C,N)) inclusions that are not easy to deform lead to drawing fracture and brittle fracture. In addition, a small amount of bainite in the wire rod microstructure causes the fracture of the wire with a fine diameter, whereas, the limited-depth scratches on the steel surface do not have an obvious effect on the drawing fracture.

Impact Energy Absorption Analysis of Shape Memory Hybrid Composites

Shape memory hybrid composites are hybrid structures with fiber-reinforced-polymer matrix materials. Shape memory wires due to shape memory/super-elastic properties exhibit a pseudo-elastic response with good damping/energy absorption capability. It is expected that the addition of shape memory wires in the glass-fiber-reinforced-polymer matrix composite (GFRP) will improve their mechanical and impact resistant properties. Stainless-steel wires are also expected to improve the impact resistance properties of GFRPs. In this research work, we investigated the effect of addition of shape memory wires and stainless-steel wires on the impact resistance properties of the GFRP and compared our results with conventional GFRPs. Super-elastic shape memory alloy wires and stainless-steel wires were fabricated as meshes and composites were fabricated by the hand-layup process followed by vacuum bagging and the compression molding setup. The shape-memory-alloy-wires-reinforced GFRP showed maximum impact strength followed by stainless-steel-wires-reinforced GFRPs and then conventional GFRPs. The effect of the energy absorption capability of super-elastic NiTi wires owing to their energy hysteresis was attributed to stress-induced martensitic transformation in the isothermal regime above the austenite transformation temperature. The smart shape memory wires and stainless-steel-wires-based hybrid composites were found to improve the impact strength by 13% and 4%, respectively, as compared to the unreinforced GFRPs. The shape-memory-reinforced hybrid composite also dominated in specific strength as compared to stainless-steel-wires-reinforced GFRPs and conventional GFRPs.

Resultant physical properties of as-built nitinol processed at specific volumetric energy densities and correlation with in-situ melt pool temperatures

In this study, direct comparisons between the properties of three groups of as-built nitinol (NiTi) samples fabricated using three pre-determined volumetric energy densities (VED) as calculated based on hatch spacings (VEDH = 40, 80, and 120 J/mm3), were examined. The additive manufacturing technique adopted is the powder bed fusion using a laser beam (PBF-LB), and the powder feedstock is Ni50Ti50. Although the three VEDH's increased by 100% for each sample group, each VEDH value was within the range of those recommended in the literature, as well as equivalent to 100 J/mm3 (VEDf), VED as calculated based on laser spot size. Significant differences were found between the thermal profiles and normalised thermal gradients, resulting in compositional differences, defects, phase, and phase transformation behaviour, Young's modulus, hardness, and microstructure. Thermal gradients and full-wave amplitude of the in-situ melt pool thermal radiation waveforms increased as VEDH increased. Higher VEDH's resulted in more martensitic NiTi samples with larger melt pool volume and >98% sample relative density. Austenite transformation temperatures increased with an increase in at.% Ti for each increase in VEDH, consequent on the higher normalised average melt pool temperatures. The calculated thermal gradient (normalised) was correlated to higher thermal stresses as VEDH increased, resulting in residual stress-induced weld-metal liquation microcracks. This work has demonstrated the potential of post-processed in-situ melt pool thermal radiation data, in providing insights into the physical property variations of as-built nitinol samples fabricated via PBF-LB.

Effect of Ta Content on Microstructure and Phase Transformation Temperatures of Ti75.5-Nb25.5 (%at.) Alloy

β-titanium shape memory alloys are used as shape memory-based biomedical applications and superelastic materials because they are made up of non-toxic components. Also, their behaviors are more semilar with that of human bone and dental biological tissues, such as biocompatibility, superior corrosion resistance, low density, and low modulus of elasticity. In the current study, the effect of Ta additive on the physical properties of Ti-Nb alloy has been investigated. Ti-Nb-Ta alloy samples were produced such that the Tantalum (Ta) element was substituted instead of Niobium (Nb), and the induction arc melting was used to melting the mixed powders. To investigate the crystal structure, microstructure, and phase transformation temperatures of all samples, the X-Ray Diffraction (XRD), Scanning Electron Microscopic (SEM), and Diffraction Scanning Calorimetry (DSC) measurements have been used. Both β-rich and α peaks were observed in the XRD and SEM measurements. Also, the DSC results revealed that the Ti-Nb-Ta alloys have only austenite transformation temperatures and their phase austenite phase transformation were decreased by the effect of Ta additive, additionally, the enthalpy change in〖 E〗_3 sample ( Ti_75.5 Nb_22.5 Ta_2 (%at)) has the highest value, while E_4 sample ( Ti_75.5 Nb_21.5 Ta_3 (%at)) has the lowest respective value.

Optimizing mechanical properties of 15Cr supermartensitic stainless steel by enhanced stability of retained austenite

The effect of annealing temperature below austenite start transformation temperature on the mechanical properties of martensitic stainless steel was investigated. The annealing treatment can transform the anisotropic compressive stress of retained austenite into hydrostatic pressure, which stabilizes the retained austenite and increases the elongation of the martensitic stainless steel.

Experimental studies of necking and fracture limits of boron steel sheet under hot stamping conditions

Boron steel is the most widely used material in hot stamping applications for forming automotive body panels with complex shapes and ultra-high strength. Due to the high austenitic transformation temperatures and the complex thermal cycle required for hot stamping, however, it is difficult to evaluate the formability of the material using standard punch test methods developed for room-temperature testing. In this study, a high-temperature modification of a recently developed biaxial test method has been used to determine, in a single test procedure and for the first time, forming limit curves (FLCs) and fracture forming limit curves (FFLCs) for 22MnB5 boron steel sheet with a thickness of 1.5 mm under thermal conditions that are representative of industrial hot stamping processes. A direct resistance heating strategy has been developed, and a recently proposed cruciform specimen design has been modified for high-temperature use. For tests with target temperatures in the range of 750 to 925 °C, the resulting test specimens had the highest temperature at the specimen centre and a temperature difference of less than 45 °C in the gauge area and fracture occurred close to the centre of this area under all test conditions investigated. Limit strains at the onset of necking and at fracture for the material have been determined by applying digital image correlation (DIC) to obtain full-field strain measurements, providing an experimental foundation for constructing both FLCs and FFLCs for industrial applications.

Modeling and design optimization of shape memory alloy-enabled building skins for adaptive ventilation

This paper presents a modeling and design study of shape memory alloy (SMA)-based skins that enable adaptive ventilation in buildings. The skins are formed by panels with attached SMA wires and can be periodically arranged to form a building envelope. When the environmental temperature is below the SMA austenitic transformation temperatures, the SMA wires are relaxed and the panel forms a closed surface, preventing circulation of exterior airflow into the building. When the environmental temperature exceeds such SMA transformation temperatures, the SMA wires contract and bend the panel to create an opening that enables circulation of airflow into the building. The response of the adaptive skins is evaluated through a thermomechanically coupled finite element model. The applied thermal loads are determined from published field measurements. Metallic materials and fiber-reinforced composites are explored as candidates for the panel. Genetic algorithms are employed for optimization, where the objective is that the panel exhibits a large opening when the environmental temperature exceeds a prescribed ideal temperature and stays closed otherwise. Optimization results show that two-layered composite panels with ply angles of 45°/−45° and thicknesses of 0.1/0.25 mm (layers ordered from the interior to the exterior side) display the most favorable performance.

A modified Johnson-Cook constitutive model for improved thermal softening prediction of machining simulations in C45 steel

A new modified Johnson-Cook model is developed to capture the non-linear thermal softening behaviour below the austenite transformation temperature (723°C) in C45 ferritic-pearlitic steel, which is not captured by the original Johnson-Cook (JC) model. The physical mechanisms responsible for this behaviour are also studied. The model is developed by experimental quasi-static and high strain rate axisymmetric compression testing from 20°C to 720°C and implemented into 3D finite element drilling simulations. The improvement over the original JC model is demonstrated by comparison to experimentally measured torque and thrust force.

The effect of vanadium and nickel on the microstructure and transformation temperatures of Ti50Pt50 alloy

Significant research has been done to produce shape memory alloys that have good shape memory properties and high martensitic transformation temperatures. The Ti50Pt50 alloys have been found to have high transformation temperature of around 1050℃ however, they exhibit negligible shape memory properties. The solid solution strengthening, and improved shape memory properties could be enhanced by ternary alloying. Therefore, this work investigates the effect of varying V and Ni contents, in the range of 6.25 to 12.5at%, on the austenitic and martensitic transformation temperatures, and hardness of the equi-atomic Ti50Pt50 alloy. Arc melting followed by casting and solution heat treatment was carried out to produce the alloys. As-produced alloys were characterized by using scanning electron microscopy, differential scanning calorimetry and hardness testing. The microstructures showed high volume fraction of second phases formed in the TiPtV alloy compared with Ti50Pt50 and TiPtNi alloys. The multiple phases formed in the TiPtV alloys could be the cause of high hardness values observed in these alloys as compared withTi50Pt50 and TiPtNi alloys. Thermal transformation studies revealed that TiPtV alloys exhibit transformation temperature close to Ti50Pt50 alloy, in contrast with TiPtNi alloys. TiPtNi alloys thermal behaviour was improved by solution heat treatment.

Investigations of mechanical properties and deformation behaviors of the Cr modified Ti–Au shape memory alloys

In view of the population aging issues, the functional materials, Ti–Au–Cr shape memory alloys (SMAs), have been explored in this study for the crucial biomaterial applications. This ternary system was chosen for solving the hypersensitivity problem in the conventional SMAs containing Ni and also for its high X–ray contrast, which is an essential requirement in medical applications. Physical metallurgy methods were utilized for the specimen preparation and the fundamental properties and functional performances were also evaluated. Cold–workability was greatly improved with the Cr addition. The parent β–phase was stabilized by 4.5 at.% Cr alloying concentration, while the discrimination of the α′–martensite phase and the α′′–martensite phase at low Cr addition concentrations was clarified. Lattice parameters and the deformation strains were further calculated based on the results of the X–ray diffraction analysis. Most of the specimens possessed shape recovery upon heating, while the Ti–4.0Au–5.5Cr alloy showed an almost perfect shape recovery rate of 96.7%. Pseudoelastic behavior was observed in most of the alloys. The austenite transformation temperatures were determined by the step–by–step heating processes.

Microstructures and Phase Transformations of Melt-Spun Ti-V-Al High Temperature Shape Memory Alloys with Addition of Zr

In this study, the effects of Zr addition on phase transformation temperatures, microstructure of Ti-12V-4Al (wt. %) high temperature shape memory alloys (HTSMAs) manufactured using melt-spinning technique were investigated. During heating, differential scanning calorimetry (DSC) curves showed that austenite transformation temperature of Ti-12V-4Al (wt. %) melt-spun ribbon was single-stage transformation and Ti-12V-4Al-0.5Zr (wt. %) melt-spun ribbon was two-stage transformation. In the scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyzes, unveiled that the melt-spun ribbons consisted of martensite, austenite and R phases. Transmission electron microscopy (TEM) analysis showed that the thickness of martensite plates in ribbons was thinned by the addition of Zr.