Double Heat Treatment Research Articles

Effect of heat treatment on microstructure and mechanical anisotropy of direct energy deposited Ti-6.0Al-2.5Nb-2.2Zr-1.2Mo alloy

Ti-6.0Al-2.5Nb-2.2Zr-1.2Mo alloy (Ti-6321) exhibits excellent corrosion resistance and impact toughness, which can be used as a deep-sea pressure-resistant hull. However, the alloy tends to have high strength but low plasticity, poor impact toughness, and anisotropic mechanical properties by fabricating by direct energy deposition (DED) technology. In order to improve the impact toughness of DEDed Ti-6321 alloy while reducing its anisotropy of mechanical properties, this study manipulates the microstructure and mechanical properties of the alloy through heat treatment. The research finds that with the increase of the single-stage annealing temperature within the (α+β) two-phase region, the width of the α phase increases, the Ultimate Tensile Strength (UTS) remains essentially unchanged, the Yield Strength (YS) decreases, and the impact absorption energy significantly improves. For double annealing heat treatment at two-phase region, temperature increase at the first-stage annealing enlarges the width and decreases the aspect ratio of the primary α phase. Leading to the decrease of the UTS & YS and increase of the impact absorption energy, with a conversely tendency at 1000 °C. The increase in the second-stage annealing temperature raises the size and volume fraction of the secondary α phase, causing the alloy's UTS and YS to initially rise then fall, with a slight increase in impact absorption energy. The anisotropy in the mechanical properties of DEDed Ti-6321 alloy mainly addressed from the different distribution of α phase orientations at different loading directions, and can be eliminated through sufficient β single-phase zone annealing.

Austenite formation and solute redistribution during double soaking of a 7Mn-steel

In this work, the influence of secondary soaking parameters on microstructural evolution in medium manganese steels is evaluated with a specific focus on the mechanisms driving austenite formation and solute partitioning at an intermediate secondary soaking temperature of 750 °C. The secondary soaking heat treatment represents the latter step of the double soaking heat treatment where it is used, after an initial intercritical annealing heat treatment, to form additional, solute-lean austenite. Here, data are obtained on a 0.14C-7.17 Mn steel for secondary soaking temperatures between 700 °C and 800 °C, and for times up to 1200 s. Austenite formation during secondary soaking is characterized using dilatometry and XRD, while Mn redistribution is experimentally observed using STEM-EDS. Manganese partitioning during secondary soaking, as well as calculated changes in carbon partitioning, are further explored through a DICTRATM model. Experimental and simulation results suggest that the ferrite-to-austenite transformation during the secondary soaking step may proceed under a partitioning, diffusional transformation at the 750 °C secondary soaking temperature. Manganese partitioning was demonstrated to primarily occur from ferrite to newly formed secondary austenite, while carbon diffusion principally occurred from primary to secondary austenite. Transformation of secondary austenite to martensite upon quenching along with some fraction of primary austenite, whose transformation was attributed to a reduction in C concentration, was shown demonstrated. The results of this study are expected to be of interest to those seeking to design multi-step heat treatments which produce austenite of variable solute concentrations and stabilities.

Effect of post-deposition heat treatments on the microstructure and tensile properties of wire-fed electron beam directed energy deposition GH4169 Ni-based alloy

The microstructure and tensile properties of GH4169 alloy fabricated by wire-fed electron beam directed energy deposition (EB-DED) were investigated in both as-deposited (As-Dep) and homogenization plus double aging heat treatment (HA) condition. The results show that thermal cycling caused an aging effect in the As-Dep samples, resulting in the precipitation of the γ’/γ” phases in close proximity to the Laves phase. The dissolution behavior of Laves phase in GH4169 alloy at various homogenization heat treatment temperatures and holding times was studied. Based on experimental results and Johnson-Mehl-Avramie-Kolmogorov (JMAK) analysis, the dissolution activation energy of Laves phase was calculated as 218.2 kJ/mol. The heat treatment regime utilized in HA treatment consisted of 1150 °C × 60 min/WC + 720 °C × 8 h/FC to 620 °C × 8 h/WC. After HA treatment, most of the Laves phases were dissolved and the γ’/γ” phases were precipitated. The mechanical properties of HA samples are superior to those of As-Dep samples. The morphology, distribution and evolutionary mechanism of the Laves phase, γ’/γ” phases are discussed and analyzed.

Microstructure formation mechanism and mechanical properties of super-thickness TC11 titanium alloy joint by electron beam welding and laser additive manufacturing hybrid connection technology

Advanced connection technology for extra-large/super-thickness titanium wrought components is urgent demand in aerospace manufacturing industries. For achieving high-performance titanium alloy super-thickness joint with a high connecting efficiency, a novelty concept of hybrid connection technology with electron-beam welding and subsequent laser additive connection is proposed. A super-thickness (200 mm) TC11 titanium alloy joint has been successfully prepared, and microstructure, tensile properties and fracture behavior of different regions in which are investigated. The results suggest that in stress-relieve heat treatment (SRT) condition, deformation and fracture of the joint are prone to occur in its low-strength wrought base material zone; after double annealing heat treatment (DAT), lamellar α phases in different zones of the joint become similar, associating with a higher fraction of fine secondary α phases in wrought base material zone. Therefore, the tensile properties of the joint are comparable to wrought properties. Although strength in different zones of hybrid connection joint is similar, ductility is not uniform and apparently lower in middle fusion zone due to grain heterogeneity. Overall, these findings indicate that hybrid connection can be one potential technology for fabricating high performance super-thickness titanium alloy joint with a high connection efficiency in engineering application fields.

Effects of heat treatments on the microstructure and mechanical properties of Inconel 718 alloy fabricated by electron beam freeform fabrication

The present study investigated the effects of different heat treatments on the microstructure and mechanical properties of Inconel 718 alloy fabricated by electron beam freeform fabrication. The Laves phases, γʹ phases, and (Nb,Ti)C particles were precipitated in the as-deposited specimen. Solution at 960 °C promoted the partial dissolution of Laves phases and the precipitation of δ phases. Further increasing the solution temperature to 1060 °C dissolved the Laves phases effectively, facilitating the adequate precipitation and uniform distribution of γʹ and γʹʹ phases during the subsequent double aging heat treatment. The ultimate tensile strength (UTS) and elongation of the as-deposited specimen were 770 MPa and 12.2 % respectively. The solution heat treatment at 1060 °C increased the elongation to 38.1 % but decreased the UTS to 734 MPa. The microhardness and UTS of specimens that underwent double aging heat treatment were significantly increased, with the maximum average microhardness and UTS reaching 491 HV0.5 and 1285 MPa, respectively. The undissolved Laves phases and the precipitated δ phases at grain boundaries impaired the ductility. The transformation from single solution or double aging heat treatment to solution plus double aging heat treatment led to a shift from a ductile fracture mode to a mixed ductile and brittle fracture mode.

Effect of heat treatment on microstructure and wear properties of laser additively manufactured 316 L stainless steel/Co-Cr-W alloy part using directed energy deposition method

The directed energy deposition additive manufacturing was applied to fabricate hard Co-Cr-W layers on the 316 L stainless steel substrate. The effect of different heat treatment cycles was investigated. The primary microstructure of deposited layers contained a cobalt matrix with interdendritic eutectic carbides mainly M7C3, M23C6, and M6C. Most of the primary carbides were dissolved in the matrix after the solution treatment at 1250 °C for 1 h. The aging treatment promoted the precipitation of (W/Co)6C carbides. The hard carbides caused the third-body wear and increased the wear volume loss of specimens. The solution & double aging treatment provided the optimum combination of hardness and wear resistance due to the best distribution of W/Co carbide. The wear volume loss after heat treatment was decreased from about 6.6 mm3 to 2.8 mm3 compared to the as-deposited specimen. The proposed solution & double aging heat treatment improved wear resistance by 50%.

Enhanced Carrier Collection in Cd/In-Based Dual Buffers in Kesterite Thin-Film Solar Cells from Nanoparticle Inks.

Increasing the power conversion efficiency (PCE) of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells has remained challenging over the past decade, in part due to open-circuit voltage (VOC)-limiting defect states at the absorber/buffer interface. Previously, we found that substituting the conventional CdS buffer layer with In2S3 in CZTSSe devices fabricated from nanoparticle inks produced an increase in the apparent doping density of the CZTSSe film and a higher built-in voltage arising from a more favorable energy-band alignment at the absorber/buffer interface. However, any associated gain in VOC was negated by the introduction of photoactive defects at the interface. This present study incorporates a hybrid Cd/In dual buffer in CZTSSe devices that demonstrate an average relative increase of 11.5% in PCE compared to CZTSSe devices with a standard CdS buffer. Current density-voltage analysis using a double-diode model revealed the presence of (i) a large recombination current in the quasi-neutral region (QNR) of the CZTSSe absorber in the standard CdS-based device, (ii) a large recombination current in the space-charge region (SCR) of the hybrid buffer CZTSSe-In2S3-CdS device, and (iii) reduced recombination currents in both the QNR and SCR of the CZTSSe-CdS-In2S3 device. This accounts for a notable 9.0% average increase in the short-circuit current density (JSC) observed in CZTSSe-CdS-In2S3 in comparison to the CdS-only CZTSSe solar cells. Energy-dispersive X-ray, secondary-ion mass spectroscopy, and grazing-incidence X-ray diffraction compositional analysis of the CZTSSe layer in the three types of kesterite solar cells suggest that there is diffusion of elemental In and Cd into the absorbers with a hybrid buffer. Enhanced Cd diffusion concomitant with a double postdeposition heat treatment of the hybrid buffer layers in the CZTSSe-CdS-In2S3 device increases carrier collection and extraction and boosts JSC. This is evidenced by electron-beam-induced current measurements, where higher current generation and collection near to the p-n junction is observed, accounting for the increase in JSC in this device. It is expected that optimization of the heat treatment of the hybrid buffer layers will lead to further improvements in the device performance.

Influence of a Novel Double Tempering Process on the Microstructure and Mechanical Properties of Cu-Alloyed Austempered Ductile Iron with Possible Nano (Micro)-Characterization Using Neutron Beam Techniques

In this paper, a novel method for the double heat treatment of ductile iron was applied. Ten sets of specimens (three specimens in each set) of ductile cast iron (DCI) containing 0.51% wt. Cu were prepared and converted to austenitic ductile iron. All specimens were austenitized at 850 °C for 60 min and annealed at 420 °C, 331 °C and 250 °C for 120, 68 and 30 min, respectively. Five sets of samples were then annealed at 500 °C for 60 min, creating a novel double heat treatment process for annealing. Finally, all specimens were slowly cooled in air at ambient temperature. Tensile strength, hardness and elongation were measured in all specimens to compare the specimens with and without subsequent tempering. A microstructural analysis was also performed, which showed that the microstructure changed for the specimens that were subsequently tempered with. The results show that specimens with subsequent tempering have slightly higher hardness, a small decrease in tensile strength and significantly higher elongation. In addition, specimens with subsequent tempering exhibit more uniform mechanical properties compared to specimens without subsequent tempering. The use of neutron beam techniques was proposed to further characterize the newly formed microstructure after subsequent tempering.

Effect of building direction on anisotropy of mechanical properties of GH4169 alloy fabricated by laser powder bed fusion

Ni Base superalloy samples were fabricated by laser powder bed fusion in vertical, oblique, and horizontal building directions. The microstructures, tensile properties, and fracture morphologies of the three specimens were studied. The precipitated strengthening phase structure, grain characteristics, and geometric necessary dislocation density of the samples were characterized. The reason for the anisotropy of yield strength for the three specimens was discussed based on the strengthening theory. The results show that the microstructures of the as-built alloys are composed of coarse columnar grains, fine dendritic structures, and high-density dislocations. Partial recrystallization occurs with the precipitation of the ellipsoidal γ″ strengthening phase in the alloys after solution and double aging heat treatment. Compared with the vertical and horizontal specimens, the oblique sample has the highest yield and tensile strength and has a good elongation of 19%. The effective grain sizes of the horizontal, vertical, and oblique samples are 11.92 μm, 15.15 μm, and 11.67 μm, respectively. There are no significant differences in the average Taylor factors of the three samples. The geometrically necessary dislocation densities of the three samples are 4.021 × 1014 m−2, 3.078 × 1014 m−2, and 4.362 × 1014 m−2, respectively. The anisotropy of yield strength for the samples with different building directions is explained by the effect of dislocation strengthening, followed by precipitation strengthening and grain boundary strengthening which is essentially related to the dislocation density, Taylor factor and effective grain size.

Determining the Behavior of Water in Buttermilk Cheese with Polymerized Whey Protein Using Differential Scanning Calorimetry and Nuclear Magnetic Resonance Analysis

In this research, the behavior of water in buttermilk cheese with the addition of polymerized whey proteins was determined. Various parameters of the produced cheese, such as texture, color, water activity, and unbound protein fraction, were examined. Four different samples of buttermilk cheese were prepared, including no addition of whey protein concentrate (BMC); addition of whey protein concentrate (BMC/WPC; 5.62%, w/v), single-heated polymerized whey protein (BMC/SPWP; 28%, w/v), and double-heated polymerized whey protein (BMC/DPWP; 28%, w/v). Differential Scanning Calorimetry (DSC) analysis showed that the highest percentage of freezable water in the water fraction and the lowest of unfreezable water was found in buttermilk cheese with WPC and buttermilk cheese with DPWP. Nuclear magnetic resonance (NMR) analysis showed that the relaxation times were longer in buttermilk cheese with WPC, compared to buttermilk cheese with SPWP and DPWP. Single heat treatment of whey proteins increased stickiness almost 3-fold, and double heat treatment had almost a 2-fold increase in work of shear of cheese samples. The calculated total color difference (ΔE) of the cheese samples suggested that those with polymerized whey protein may increase consumer acceptability.

Results of ITER TF Coil Double Pancake Heat Treatment in Japan

In Japan, 9 ITER TF coils were manufactured for the ITER project. Each TF coil consists of seven Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductor double pancakes (DPs). In DP manufacturing, heat treatment is one of the most important processes, because the insertion of a heat treated conductor into the radial plate groove (i.e., transfer) is complicated owing to the elongation of the conductor under heat treatment. A critical current distribution over the DP may also occur owing to the temperature distribution and disturbance in the heat treatment process. To address these issues, this study developed a procedure wherein the conductor length elongation caused by heat treatment is precisely considered, and transfer can be successfully achieved for all the DPs. In addition, strand witness samples were simultaneously heat treated with the DPs, and their critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) was measured. Subsequently, it was confirmed that the average I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> is equivalent to those in the strand acceptance test while the standard deviation of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> for the witness samples is approximately 4% larger than that for the strand acceptance test. It was also confirmed that the disturbance of the heat treatment was negligible. Related results are described herein.

EXPERIMENTAL INVESTIGATION of DOUBLE AGING HEAT TREATMENT of INCONEL 718 MONOBLOCK GAS TURBINE DISC IN A SALT BATH

The heat treatment of Inconel 718 (In718) monoblock gas turbine discs are mainly done by vacuum furnaces. Despite the vacuum furnaces allow to make a clean and reliable heat treatment, its operation costs affects the final product’s cost. In this work, as an alternative to the vacuum furnaces, the double aging heat treatment of In718 disc in a salt bath was investigated. The appropriate temperature for high and low temperature salt bath was identified in order to achieve necessary cooling rates of 125°C/min and 120°C/min for a 400 kg workpiece. It is shown that the heat treatment could be done when the high and low temperature salt baths temperatures are equal to 300°C. It is also found that the heat transfer coefficient for simulation could be taken as 650 W/m2.K regardless to the salt bath temperature.

Effect of heat treatment on microstructure and small punch creep property of selective laser melted Inconel 718 alloy

In present study, the Inconel 718 alloys fabricated by selective laser melting (SLM) were subjected to traditional homogenization plus double aging heat treatment (HT1), and a novel heat treatment (HT2). The effect of heat treatment on the anisotropic microstructure and the small punch creep (SPC) property of Inconel 718 samples was investigated. The experimental results show that the general high-angle grain boundaries with stress concentration, and the equiaxed and columnar grains are observed in the HT1 samples. Compared with the traditional solution treatment at 1065 °C for the HT1 samples, a higher solution performed at 1150 °C can efficiently eliminate the stress concentration and lead to the transformation of columnar grains to equiaxed grains in the HT2 samples by introducing complete recrystallization. The recrystallization is accompanied by the formation of annealing twins with high proportion Σ3n (n = 1, 2, 3) twin boundaries. The larger γ″ phases, less grain boundaries perpendicular to the load direction and smaller Laves phases in the HT1-Z (Z represents parallel to the building direction) sample result in a longer creep life. Microcracks distributed along the general high-angle grain boundaries rather than twin boundaries promote the final intergranular fracture. The anisotropy in creep rupture time of SLMed Inconel 718 alloy can be obviously alleviated by applying higher temperature solution treatment.

Enhancement of impact toughness properties of Al 7075 alloy via double aging heat treatment

In this study, Charpy impact test specimens were prepared by cutting Al 7075 alloy according to ASTM-E23 standards. The prepared test specimens were dissolved in a single-phase region for 1 hour at 480°C in vacuum atmosphere and artificially aged at 120°C for 24 hours after quenching. After the first step aging heat treatment applied, the specimens were artificially aged for 5-50 hours at 180°C for the second time. Microstructural characterization and fracture surface analyzes of the specimens were determined using SEM (Scanning Electron Microscope) device, crystallographic analysis of the precipitated phases were determined using XRD (X-ray diffraction) device. The increase in the amount of ƞı phase (MgZn2) with increasing double aging heat treatment time was effective in increasing the strength. After 10 hours of double aging heat treatment, the impact toughness value is improved by 300% compared to the single aged condition.

A Comparative Investigation on the Wear Performance of Compacted Graphite Iron (CGI) Treated with Single and Double Tempering

Compacted graphite iron (CGI) is a critical material in today's automotive and manufacturing industries. Heat treatment processes can improve CGI wear properties, related primarily to microstructural changes. In this study, single and double tempering heat treatments were used to improve the wear properties of CGI. Oil quenching was performed after 90 minutes of austenitization at 900°C, followed by 60 minutes of single and double tempering at three different temperatures (315, 350, and 375°C). The wear performance of the samples was compared using a pin-on disc test and hardness measurements. The volume loss and friction coefficient were evaluated, and wear maps were constructed to determine the samples' wear behavior. SEM and EDS analyses were carried out to worn surfaces to interpret the relationship between wear mechanism and microstructure. According to the study's findings, double tempering heat treatment may optimize wear performance better than traditional single tempering, and structures with high toughness-wear resistance combinations can be obtained.

Farklı ısıl işlemlerin takım çeliklerinin yorulma mukavemeti üzerine etkileri

Various heat treatments are applied to improve the mechanical properties of steels depending on the intended use. In this study, the variation of fatigue strength of H13 hot work tool steel with heat treatment in a GS 550 bath was investigated. It was investigated by applying four different heat treatments as first preheating, second preheating, hardening and second hardening. Each batch of samples was subjected to rotating bending fatigue test at room temperature after heat treatment. The fatigue strength of the untreated samples was determined as 470 MPa. The fatigue strength of the second hardened untempered samples (Group 1) increased to 610 MPa. The fatigue strength of the samples (Group 2), which were annealed for two hours at 550ºC as a second heat treatment after hardening, was measured as 630 MPa. The fatigue strength of the samples (Group 3) was found to be 720 MPa by applying the third heat treatment, which was annealing at 550ºC for two hours and at 610ºC for two hours. The fatigue strength of the samples (Group 4), which were subjected to the fourth heat treatment and tempered for two hours at 550ºC and then at 635ºC for two hours after hardening, was determined as 710 MPa. It has been observed that all applied heat treatments positively affect the fatigue strength of H13 hot work tool steel. While all heat treatments resulted in increased fatigue strength, the highest fatigue strength was obtained with double annealing heat treatment (550 °C and 610 °C two hours) after initial preheating and quenching.

Study on the NbTi/Cu30Ni Superconducting Switch Wire Recently in WST

The superconducting switch wire is a key component in the superconducting magnet. If the superconducting magnet quenches during the operation, the switch will disconnect automatically and avoid the magnet to be damaged. The Cu-30 wt.% Ni alloy was used as matrix to manufacture superconducting switch wires because of its high resistance characteristic. However, because of its high hardness and work hardening characteristics, the risk of wire breakage occurred easily in the cold drawing process. Therefore, the way to solve this problem is one of the difficulties in the preparation of the superconducting switch wire. NbTi/Cu30Ni superconducting switch wires with filaments number of 55 were designed and fabricated by double stacking method, hot extrusion, cold drawing and heat treatment. Then, the NbTi/Cu30Ni superconducting switch wires with the diameter of 0.500 mm, the Cu30Ni/SC of 1.1, and the maximum length of 40.68 km were obtained. Finally, the six insulated NbTi/Cu30Ni wires and one insulated Cu core wire were cabled to the final cable with the diameter of 1.700 mm and the twist pitch of 36 mm. WST has already achieved mass production of NbTi/Cu30Ni superconducting switch wire. The <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> at 5 T & 4.2 K of the wire is 280.7 A and the line resistance is 2352 mΩ/m. At present, the Western Superconducting Technologies Co., Ltd (WST) superconducting switch wires and cables have been successfully delivered to many different Chinese superconducting magnet manufactures and received good reviews and feedback.

HFC electric-welded pipes made of structural steel with resistance to hydrogen cracking

The results of studies carried out in laboratory and industrial conditions, the influence of an integrated approach consisting in the modification of liquid steel alkaline earth metals and rare earth metal, as well as double heat treatment (hardening from the austenitic region at 920 &ordm;C and subsequent high tempering at 650 &ordm;C) of pipes welded with high frequency currents from structural steel with a content of 0.18&ndash;0.22 % carbon on the resistance against cracking in H2S-containing media according to the NACE TM 0284 standard are presented. It is shown thatthe adjustment of the integrated approach in terms of limiting the casting rate made it possible to completely eliminate the formation of cracks of hydrogen origin in the metal (CLR = 0 %, CTR = 0 %). A technology for the production of rolled products and pipes welded with high frequency currents with requirements for cold resistance and corrosion resistance in environments containing hydrogen sulfide has been developed. The cost of pipe products made of structural steel with 0.18&ndash;0.22 % C, produced according to the developed technology, is less than 2000 rubles/ton compared to similar pipe products made of steel grades 05HGB, 13HFA, 09GSF.

Research on the microstructure and mechanical properties of doubled annealed laser melting deposition TC11 titanium alloy

TC11 titanium alloy has the advantages of light weight and high strength, and it has great potential to replace high-strength steel as the material of aerospace main bearing to achieve lightweight application. In this study, TC11 titanium alloy was prepared by laser melting deposition, and the deposited TC11 titanium alloy was subjected to double annealing heat treatment, so as to study the effects of different annealing temperatures and time on the microstructure and mechanical properties. The results indicated that the microstructure of TC11 titanium alloy grew alternately by the turns of columnar crystal / equiaxed crystal along the deposition direction. The deposited microstructure was mainly composed of the Widmanstatten structure and mesh basketweave structure. After double annealing, the α phase was coarsened and the aspect ratio decreased, and the continuous grain boundary α phase appeared discontinuous; however, when the annealing temperature was 1025 °C, the intragranular microstructure changed into a fine basketweave structure, and the continuous α phase was reformed at the grain boundary. With the extension of high time, the coarsening phenomenon of the αs phase was obvious. The microhardness increased firstly and then decreased with the annealing temperature, and reached the maximum at 995 °C. After double annealing, the impact toughness of TC11 titanium alloy was significantly improved, which was better than the forging standard. The strength and plasticity of TC11 titanium alloy were also significantly improved, meeting the forging standard, but when the annealing time was extended to 2 h, the strength was significantly reduced. The strength anisotropy of TC11 titanium alloy was less than 3% during the annealing in the two-phase region; the anisotropic plasticity increased firstly and then decreased with the increase of annealing temperature; with the extension of annealing time, both the anisotropy of strength and plasticity showed an increasing trend.

Ultrasonic fatigue endurance of Inconel 718 after the heat treatments: solution annealing and double aging

The present work deals with the effects of consecutive heat treatments: solution annealing and aging, on the ultrasonic fatigue endurance of the Inconel 718 (I-718). The solution annealing heat treatment was carried out at 1078° C/1hr; then, quenched in water. Afterwards, double aging heat treatment was carried out: firstly, at 720ºC/8hrs and cooling inside furnace for 1hr; then, maintained at 620ºC/8hrs. After the last process, the I-718 specimens were cooled with air to attain room temperature. The results of ultrasonic fatigue endurance show that at higher levels of stresses, up to 746 MPa, the ultrasonic fatigue endurance sensibly increases by a factor of 30 approximately for the heat-treated specimens, compared to the untreated specimens, obtained by others authors. Furthermore, the fracture surfaces were analyzed using Scanning Electron Microscopy (SEM), and the main trends of crack initiation and propagation were studied.