Heat Treatment Cycles Research Articles

Enhanced mechanical properties in an Al-Mg-Cu alloy processed by the combination of cyclic deformation and aging heat treatment

Aging is a traditional strengthening method used for heat-treatable Al alloys. Cyclic strengthening has recently received widespread attention as a novel method for alloy strengthening. This study applied cyclic deformation to introduce dislocations and promote cluster formation in an Al-Mg-Cu alloy. Cyclically-strengthened (CS) samples have higher strength and ductility than peak-aged samples. The aging behaviors of the CS samples and the effect of pre-aging (artificial aging prior to cyclic deformation) on cyclic strengthening were also studied. Recovery and precipitation promotion were found during the aging of the CS samples regardless of pre-aging for 20 min. The pre-aged CS samples show higher strength than the CS samples without pre-aging. Interestingly, after subsequent aging, the mechanical properties of the CS samples with and without 20 min of pre-aging became the same. In summary, aging and cyclic deformation methods were combined to maximize the effects of precipitation strengthening and work-hardening. The strength limit obtained by individual aging or cyclic deformation methods was successfully overcome. This study provides new insights into alloy strengthening. • Cyclically-strengthened sample had better strength and ductility than the peak-aged sample. • Cyclic deformation promotes precipitation and leads to cluster formation. • Pre-aged samples can be cyclically strengthened due to work-hardening effects. • Enhanced mechanical properties were obtained by combining cyclic deformation and aging.

Development of Micro Laser Powder Bed Fusion for Additive Manufacturing of Inconel 718.

The development of laser powder bed fusion (LPBF) additive manufacturing techniques for microfabrication raises the need for the employment of new process configurations and parameters. In this study, micro-LPBF of Ni-based superalloy Inconel 718 using a spot laser of 30 µm was examined. The response surface method with a central composite design was employed to determine the optimum process parameter. A wide range of heat treatment cycles was applied to additively manufacture Inconel samples. The mechanical behavior of heat-treated Inconel 718 parts fabricated via micro-LPBF was investigated and correlated to the microstructural characteristics. The result showed that using optimum input energy density led to a homogenous distribution of nanosized (<10 nm) circular γ′ and plate-like γ″ particles in the γ matrix. Uniaxial tensile tests on heat-treated samples showed that ageing temperature is the most determinant factor in the mechanical strength of additively manufactured Inconel 718.

The influence of DED process parameters and heat-treatment cycle on the microstructure and hardness of AISI D2 tool steel

The impact of both the laser directed energy deposition (DED) process parameters and post-deposition heat-treatment cycles on the microstructural characteristics and hardness are evaluated for cladding AISI D2 tool steel onto annealed AISI D2 substrates. The influences of powder feed rate, and laser scanning speed were assessed. After laser deposition, the DED samples were subjected to various tempering heat-treatment cycles. The microstructural and compositional characteristics of both the ‘as-printed’ and heat-treated DED processed samples were investigated using scanning electron microscopy with energy dispersive X-ray spectroscopy, confocal laser scanning microscopy, and X-ray diffraction. The indentation and scratch hardness responses were assessed to evaluate the effects of various heat-treatment schedules, with comparison made to wrought D2. It was demonstrated that the microstructure of the DED processed samples had a dendritic morphology with columnar grains. Moreover, the hardness values of the DED fabricated parts were found to be higher than an annealed wrought D2, but lower than the quenched D2. Subsequent tempering of the DED printed parts resulted in a final hardness, essentially equivalent to the as-quenched level.

Effect of the Tempering Process on the Corrosion Performance of Wire Arc Additively Manufactured 420 Martensitic Stainless Steel

With the aim of modifying the microstructure and improving the corrosion performance of a wire arc additive manufactured 420 martensitic stainless steel, heat treatment cycles consisting of austenitizing at 1,150°C followed by air cooling and subsequent tempering at different temperatures (300°C, 400°C, 500°C, and 600°C) were applied to the as-printed alloy. Microstructural analysis revealed that the austenitization and subsequent air-cooling treatment led to the removal of retained austenite and delta ferrite from the as-printed structure, while the tempering process resulted in the precipitation of a variety of carbide particles at different tempering temperatures. Electrochemical tests performed in an aerated 3.5 wt% NaCl solution showed that tempering at 400°C led to the highest corrosion resistance, while tempering at 500°C deteriorated the alloy’s resistance against localized corrosion. The most stable passive layer was found to form on the 400°C tempered sample due to the uniformity of Cr-concentration in the formed carbide precipitates and their surrounding matrix. However, Cr-rich carbide precipitates formed in the 500°C tempered sample were found to deteriorate the passive film stability throughout the immersion time in the electrolyte.

On the bending of MS1-P20 hybrid steels additively manufactured using laser powder bed fusion

Maraging steel (MS1)-tool steel (P20) bimetals additively manufactured using the laser powder bed fusion technique were studied under different heat treatment cycles and loading conditions. The hardening of P20 and aging of MS1 were performed sequentially on the hybrid samples. The interfacial characteristics and microstructural evolution of the bulk materials were studied using various advanced electron microscopy techniques. The post-processing procedures successfully produced a uniform martensitic structure throughout the MS1-P20 hybrid steels, leading to a less detectable interface under electron backscatter diffraction (EBSD) imaging. The mechanical performance of heat-treated hybrid steels was evaluated using complex loading conditions. 3-point and 4-point bending tests were performed to assess the impact of heat treatments on the mechanical performance of the hybrid steels. The heat-treated samples exhibited higher strength with relatively homogeneous hardness variations and deformed more uniformly in bending conditions.

Fatigue fracture behavior of high-strength low-alloy steel for flexible marine riser in the high cycle fatigue regime

The relationship between microstructure, stress ratio, and fatigue fracture behavior of high-strength steel for flexible marine risers was investigated by microstructure characterization and fatigue test. The microstructure characteristic, S-N curve, fatigue fracture morphology, and fatigue crack morphology after quenching and tempering treatment and cyclic heat treatment, respectively, were evaluated. The results revealed that after cyclic heat treatment, the microstructure was refined considerably, the grain boundary density and the content of high-angle grain boundary increased, which inhibited the fatigue crack propagation and improved the fatigue strength. The average stress increased by increasing the stress ratio, which promoted the crack initiation and propagation; consequently, the fatigue strength and fatigue life of the tested steel decreased.

3D In-situ Stop Action Study of Recrystallization in Additively Manufactured 316L Stainless Steel: Reconstruction Optimization and Observations

A volume of an additively manufactured 316L stainless steel sample has been tracked during its recrystallization using near- and far-field High Energy Diffraction Microscopy (HEDM) and absorption tomography at Advanced Photon Source beamline 1-ID. A near-field compatible in situ furnace allows monitoring of Bragg diffraction signals as they evolve out of a weak and diffuse background while the sample temperature is ≈ 1250° C. The sample is rapidly cooled to room temperature after observation of significant signal evolution and ∼ 0.035 mm3 is mapped by the near-field method. Four cycles of heat treatment follow the structure from a state of small, isolated grains through impingement of domains to near completion of recyrstallization. Here, the experiment and reconstructions are described, and recrystallized fractions, twin domains, and distributions of grain boundary types are discussed.

Tempering of an additively manufactured microsegregated hot-work tool steel: A high-temperature synchrotron X-ray diffraction study

Rapidly solidified microstructures, produced by concentrated heat sources, have gained increasing attention due to the widespread expansion of additive manufacturing techniques. In laser powder bed fusion of tool steels, the as-built microstructure consists of a microsegregated network of cell-like walls decorated with retained austenite (RA). This inhomogeneous elemental distribution leads to phase transformation sequences dictated by local – instead of global – equilibrium. This work uses synchrotron X-ray diffraction to study the kinetics of retained austenite decomposition during tempering of a microsegregated as-built AISI H13 tool steel. The RA decomposition products are also characterized by hardness tests, scanning electron microscopy, and atom probe tomography. Present results evidence the formation of high temperature para-equilibrium (T < 650 °C) or equilibrium (T ≥ 650 °C) microstructures. The para-equilibrium microstructural path evidences isothermally stable RA, which transforms into fresh martensite upon cooling due to carbon depletion. However, the cell-like microsegregation arrangement is retained due to insufficient homogenization of substitutional elements. The equilibrium microstructural path results in isothermal transformation of RA into ferrite + carbides, as well as dissolution of the cell-like microsegregation structure. These results provide insights towards the understanding and development of optimized heat treatment cycles appropriate for such increasingly common inhomogeneous microstructures. The results are relevant not only to laser powder bed fusion, but also to other localized melting/remelting processes with associated high cooling rates.

Influence of Free Ferrite on the Mechanical Properties of High Strength Intercritical Austempered Ductile Iron

The first stage of this study investigates the precipitation of free ferrite from the austenite on fully austenitized ductile iron. Several sets of samples of low alloy ductile iron are fully austenitized and then cooled down to different temperatures and different times within the intercritical austenite-ferrite-graphite phase field. Based on these results, heat treatment cycles aimed at obtaining microstructures composed of free ferrite and ausferrite are carried out. Tensile, impact and toughness tests are performed to characterize the mechanical properties. The results show that, related to the high strength austempered ductile iron grades, the best combinations of properties were obtained from the mixed structures composed of 5% free ferrite and 95% ausferrite, resulting from the austempering at 280°C. These amounts of free ferrite allow obtaining an increase of the elongation (about 50%) and impact toughness (about 10%) while the tensile strength and fracture toughness decrease by about 1.5 and 15% respectively.

The influence of holding time on the characteristics of friction stir welded dissimilar magnesium alloy joints during post welding heat treatment

Magnesium (Mg) alloys have been extensively researched to widen their uses due to their outstanding particular features and the fact that they are one of the lightest metals. Mg alloys could be used in a variety of industries, including aircraft, shipbuilding, automobiles, railways, and many more. Mg alloy welding is recognized as one of the most difficult processes in a variety of industries. Because of its ability to generate a high-quality joint, friction stir welding (FSW) of Mg alloys has sparked a lot of research and industry interest. Heat treatment after welding is an effective way to improve the characteristics and performance of welded joints. Therefore, the FSW process was used to prepare Mg alloy weld joints and diverse heat treatment conditions were applied. The microstructure and mechanical properties of as-welded and heat-treated FSW butt-joints made of dissimilar AZ31 and AZ91 plates were studied. It was observed that holding times of 30 min and 60 min were prosperous in recrystallizing the grain and enhancing the microstructure of the weld zones. The holding time of the heat treatment cycle was found to smooth the microhardness profile in this order: 60 min&gt;30 min&gt;15 min. The post-weld heat treatment (PWHT) for 60 min has been found to improve both tensile strength and percentage elongation. Heat treatment enhanced the ductility of joints. As a result, the tensile failure mode was altered from brittle to ductile. PWHT joints had the highest impact energy of 4.2 J for 30 min, which was around 93% that of welded joints.

Selective Oxidation of a Medium-Mn Third Generation Advanced High Strength Steel during Austenitizing and Intercritical Annealing

The effect of the simulated continuous galvanizing line N2−5 vol% H2 process atmosphere oxygen partial pressure (pO2) on the external and internal selective oxidation of a prototype medium-Mn third generation (3G) advanced high strength steel was determined during a two-stage heat treatment cycle (i.e., austenitizing and intercritical annealing) which had previously yielded 3G properties. Thick external oxides (∼200 nm) were observed after the austenitizing heat treatment, regardless of the process atmosphere pO2 employed. An intermediate flash pickling step was successful in reducing the external oxide thickness significantly (to ∼30 nm) along with revealing some extruded metallic Fe nodules on the surface. The austenitizing heat treatment also resulted in a solute-depleted surface layer with a minimum thickness of 2 μm. This solute-depleted layer inhibited the formation of external oxides during intercritical annealing, resulting in a surface similar to that observed after flash pickling comprising a near-pure Fe surface with isolated, nodular external oxides. These surfaces are promising in terms of successful reactive wetting of this prototype medium-Mn steel during subsequent continuous hot-dip galvanizing.

Applied Machine Learning in Industry 4.0: Case-Study Research in Predictive Models for Black Carbon Emissions

Industry 4.0 constitutes a major application domain for sensor data analytics. Industrial furnaces (IFs) are complex machines made with special thermodynamic materials and technologies used in industrial production applications that require special heat treatment cycles. One of the most critical issues while operating IFs is the emission of black carbon (EoBC), which is due to a large number of factors such as the quality and amount of fuel, furnace efficiency, technology used for the process, operation practices, type of loads and other aspects related to the process conditions or mechanical properties of fluids at furnace operation. This paper presents a methodological approach to predict EoBC during the operation of IFs with the use of predictive models of machine learning (ML). We make use of a real data set with historical operation to train ML models, and through evaluation with real data we identify the most suitable approach that best fits the characteristics of the data set and implementation constraints in real production environments. The evaluation results confirm that it is possible to predict the undesirable EoBC well in advance, by means of a predictive model. To the best of our knowledge, this paper is the first approach to detail machine-learning concepts for predicting EoBC in the IF industry.

Dependence of energy consumption in terms of 1 m 3 of concrete when using cements of different specific surface

Objective. The aim of the study is to find rational ways to change the total energy consumption spent on cement production in terms of 1 m3 of concrete.Method. To obtain reliable data, experimental and laboratory methods for testing cement were used.Result. Tests have shown that at constant concrete strength, the consumption of cement per 1 m3 of concrete decreases with an increase in the specific surface of the binder, especially significantly in the case of Portland slag cement. The calculation results show that with an increase in the specific surface area of cements, the total energy consumption for their production, referred to 1 m3 of concrete of the same strength, decreases, and most significantly with an increase in the specific surface area of cements from 300 to 400 m2 /kg and less from 400 to 500 m2 /kg.Conclusion. To reduce the energy intensity of concrete, it is necessary to use cements ground much thinner than they are ground at cement plants in open-type mills, despite the increase in energy consumption. Cements with high fineness of grinding can reduce the cycle of heat and moisture treatment of concrete products. Reducing the cycle allows to increase the turnover of molds and labor productivity at precast concrete plants, as well as further reduce energy consumption for the production of prefabricated reinforced concrete structures.

Phase transformation pathway and microstructural refinement by feathery transformation of Ru-containing γ-TiAl alloy

The phase transformation sequence and transition temperatures of Ti–48Al–4Nb–2Cr-0.3Ru alloy are studied by using in-situ synchrotron radiation high energy X-ray diffraction, which was confirmed as α → α+γ → α/α2+β/B2+γ. The α transus and β/B2 solvus temperatures are located at 1390 °C and 1300 °C, respectively. Significantly, the acceleration of Ru element on the precipitation of metastable massive and feathery microstructures was found for the first time. Based on that, a multi-step heat treatment consisting of homogenization treatment, cyclic air-cooling heat treatment, γ equiaxed treatment and lamella reprecipitation treatment was designed to optimize the Ti–48Al–4Nb–2Cr-0.3Ru alloy. The coarse microstructure after homogenization was refined by feathery microstructure during cyclic air-cooling heat treatment. Finally, a fine duplex microstructure with γ grain and lamellar colony sized approximately 25 μm and 40 μm, respectively, was achieved. First-principles calculation revealed that the interfacial energies can be decreased when Ru atom occupies the Al sublattice position of true twin and Σ5 coincidence site lattice boundaries, which promote the occurrence of metastable transformations. Comparing with the initial microstructure, the tensile properties of the duplex microstructure were highly improved both at room temperature and 800 °C.

Studies on Mechanical Attrition and Surface Analysis on Heat-Treated Nickel Alloy Developed through Additive Manufacturing

In this paper, the nickel-based superalloy SU718 is developed through the Direct Metal Laser Sintering (DMLS), an additive manufacturing process. Further, the material has been focused to study the effect of heat treatment and abrasive particle erosion. Two different heat treatment (HT) cycles are planed with ageing and annealing to enrich the metallurgical quality of the DMLS processed SU718 alloy. The heat treatment is performed with two different combinations of temperatures for annealing/solutionizing followed by ageing to improve the metallurgical properties. The influence of heat treatment on additively manufactured IN718 is imparting variations in the hardness, microstructure, and erosion resistance. Vickers hardness for as built, HT 1, and HT 2 of DMLS alloy is 264.15, 385.55, and 352.43 Hv; which has been increased for 45% for HT 1 and 33% for HT 2 from the as built DMLS alloy. After solutionizing, the grains are refined within the track boundary and the majority of the grains are homogenized. The air jet erosion test arrangement is used to conduct the study at a velocity of 250 m/s and impact angle of 90° at room temperature. The hardness of the treated samples has taken vital role to resist the erosion. The rate of erosion is higher for bare DMLS alloy whereas HT 1 has low erosion rate when compared with HT 2 and bare DMLS alloy. The erosion morphology of the samples was carried out by SEM images, and erosion mechanism is discussed. The ploughing and microcutting were found in all the impact angles, whereas erodent impingement is found in the bare DMLS alloy in additional. The good erosion resistance is observed for HT 1 DMLS alloy in all the impact angles.

High infrared emissivity energy-saving coatings based on LaMnO3 perovskite ceramics

In this preliminary work, a series of LaCrxMn1-xO3 (x = 0, 0.25, 0.5, 0.75, 1) ceramic powders were initially synthesized by a facile solid-state reaction method. In the next step, high emissivity coatings were successfully papered by air-spraying using LaMnO3 powder as the filler material (with the highest emissivity ε3–5μm, 600 °C = 0.922) and phosphate as the binder phase. The morphology, structure, and infra-red emissivity of the samples were studied using scanning electron microscopy (SEM), X-ray diffraction (XRD), and infra-red emissometery. High-temperature cyclic heat treatment results showed that the prepared coating has very good adhesion and excellent high-temperature resistance. A simulated furnace heating test revealed that the prepared coating can save input energy as high as 13.2%.

Improvement of osseointegration of Ti\u20136Al\u20134V ELI alloy orthodontic mini-screws through anodization, cyclic pre-calcification, and heat treatments

BackgroundMini-screws are widely used as temporary anchorages in orthodontic treatment, but have the disadvantage of showing a high failure rate of about 10%. Therefore, orthodontic mini-screws should have high biocompatibility and retention. Previous studies have demonstrated that the retention of mini-screws can be improved by imparting bioactivity to the surface. The method for imparting bioactivity proposed in this paper is to sequentially perform anodization, periodic pre-calcification, and heat treatments with a Ti–6Al–4V ELI alloy mini-screw.Materials and methodsA TiO2 nanotube-structured layer was formed on the surface of the Ti–6Al–4V ELI alloy mini-screw through anodization in which a voltage of 20 V was applied to a glycerol solution containing 20 wt% H2O and 1.4 wt% NH4F for 60 min. Fine granular calcium phosphate precipitates of HA and octacalcium phosphate were generated as clusters on the surface through the cyclic pre-calcification and heat treatments. The cyclic pre-calcification treatment is a process of immersion in a 0.05 M NaH2PO4 solution and a saturated Ca(OH)2 solution at 90 °C for 1 min each.ResultsIt was confirmed that the densely structured protrusions were precipitated, and Ca and P concentrations, which bind and concentrate endogenous bone morphogenetic proteins, increased on the surface after simulated body fluid (SBF) immersion test. In addition, the removal torque of the mini-screw fixed into rabbit tibias for 4 weeks was measured to be 8.70 ± 2.60 N cm.ConclusionsA noteworthy point in this paper is that the Ca and P concentrations, which provide a scaffold suitable for endogenous bone formation, further increased over time after SBF immersion of the APH group specimens. The other point is that our mini-screws have a significantly higher removal torque compared to untreated mini-screws. These results represent that the mini-screw proposed in this paper can be used as a mini-screw for orthodontics.

Enhanced superelasticity and two-way shape memory properties of bamboo-grained Au7Cu5Al4 microwires

A bamboo-like grain structure was obtained in melt-extraction Au7Cu5Al4 microwires via cyclic heat treatment. The superelasticity and two-way shape memory properties were investigated by performing uniaxial cyclic tensile tests and mechanically constrained thermal cycling, respectively. The superelasticity of bamboo-grained microwires was improved significantly duo to a reduction in the constraints caused by triple junctions. A recoverable strain of ∼3% was achieved over a wide temperature range of 358–423 K for the bamboo-grained microwire, which exhibited excellent cyclic stability under a stress of up to 470 MPa. The two-way shape memory properties were determined by conducting load-biased thermal cycling tests, and a recoverable strain of 1.2% with a maximum work output of 1.2 J cm−3 was achieved under a stress of 100 MPa. The results indicate that the Au–Cu–Al shape memory alloy is a potential candidate for small-sized actuators operating in harsh environments.

Microstructural evolution during multiple scans in laser polishing of Ti6Al4V

Consecutive thermal cycles during laser polishing lead the complex microstructural changes. It is more difficult for modeling microstructural evolution in multiple thermal cycles compared to the single thermal cycle of heat treatment. In the present work, a thermo-microstructural model is established to investigate the temperature history and microstructural evolution during multiple scans in laser polishing of Ti6Al4V (TC4). The temperature history of the present model is validated the temperature data recorded by the thermal camera. The phase transformation involved α phase and β phase, martensitic transformation and decomposition, as well as the reservation of β phase is considered in the microstructural model. The numerical results show that the non-uniform phase distribution is attributed to the reservation of β phase and the multiple thermal cycles.

Impact Toughness of Spring Steel after Bainite and Martensite Transformation

It has been reported that a multiphase microstructure with bainite, martensite, and retained austenite obtained by austempering, or quenching and tempering of spring steel containing Si, Mn, and Cr exhibits high strength and ductility. However, little research has been conducted on the bainite formation and impact fracture behavior of next-generation spring steel from the perspective of engineering and industrial applications. The microstructural transformation characteristics of bainite and martensite related to the heat treatment cycle on the maker side were quantitatively analyzed using electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM) analyses. Moreover, the effects and mechanical properties of bainite and martensite formation in response to changes in lath length and width were studied and analyzed. That is, to obtain the mechanical properties of spring steel with the highest quality, tensile and impact specimens, whose microstructure and notch shape change according to the heat-treatment cycle, were prepared and studied.