Double Heat Treatment Research Articles

Microstructure evolution of laser powder bed fusion ZK60 Mg alloy after different heat treatment

The laser powder bed fusion (LPBF) was employed to process the ZK60 Mg alloy. The effect of heat treatment on the microstructure and hardness of LPBF ZK60 Mg alloy was investigated. The results showed that the grain boundary of LPBF ZK60 Mg alloy was composed of a network Mg7Zn3 phase. Three types of precipitated phases were distributed inside the grains: the type-Ⅰ is Zn-rich precipitated phase with Zr-rich core, the type-Ⅱ is Zn-rich precipitated phase without Zr, the type-Ⅲ is Zr-rich particle. When the solution treatment was used, the Mg7Zn3 phase at the grain boundary melted and merged into the matrix. The precipitated phase in the grain mainly evolved into Zn-rich phase with Zn2Zr core. Continue with the aging heat treatment, the rod-shaped β1′-MgZn precipitated phase separated out from the α-Mg matrix. The length of the rod-shaped precipitated phase undergone double aging is longer and the quantity is more than that of the single aging. The rod-shaped β1′-MgZn precipitated phase significantly improved the mechanical properties of LPBF ZK60 Mg alloy. The average hardness increased from 0.88 ± 0.025 GPa to 1.01 ± 0.010 GPa and the ultimate tensile strength increased to 287 ± 4 MPa after the solution treatment plus double aging heat treatment.

Theoretical Substantiation of the Practical Need for the Sanitation of Spore Forms of Microorganisms in Raw Milk before Its Processing

In the production of sterilized dairy products, a relevant problem is the control of vegetative and spore forms of thermophilic bacteria, whose active growth by the end of the technological process in milk can reach a high number and form a biofilm on the equipment parts which have elevated temperatures. The results of the experiments showed the possibility of cleaning raw milk from spore forms of bacteria by provoking their germination by preliminary low-temperature heat treatment with their further destruction by pasteurization. Spores of Bacillus cereus, after primary heat treatment (65 or 72 °C), do not germinate both at low storage temperatures (at + 5 °C) and when stored in uncooled milk (51–65 °C), at least for six hours. To provoke the germination of spores of Bacillus cereus, it is necessary to lower the temperature to +37 °C and hold for 5-6 hours; at the end of the sixth hour, the proportion of spores germinated into the vegetative state is about 35%. Primary heat treatment of milk reduces the germination time of spores of Bacillus cereus by half: from 10-12 hours to six. With the help of further pasteurization at 75 °C, it is possible to reduce the concentration of spore forms of microorganisms in milk to 3.102 CFU/cm3. The technology of double heat treatment of milk with its intermediate exposure for an hour and a half prevents the accumulation of metabolic products of a large group of bacteria: spore and vegetative, psychrophilic and thermophilic ones.

Microstructural parameters as indicators of reliability of car castings made of 20GL steel

The complex influence of grain number, average area and grain hardness on the section of low-alloy cast steel 20GL of car castings “Side frame” and “Truck bolster” on the basis of statistical and microstructural analysis of samples obtained after open-hearth and electric arc melting and single and double heat treatment (HT), on increase of the toughness KCV-60 > 2.0. The frequency diagram of the relationship between the number of open-hearth and electric arc melts and the same level of impact toughness in 2018 with the imposition of grain microstructure and different levels of impact toughness (KCV-60 <2.0, KCV-60 ≥ 2.0 and above KCV-60 > 2.0∙102 kJ/m2). The relationship between the number of samples obtained by open-hearth melting and passed the 2nd HT, and the same level of impact strength in 2019 with the imposition of the grain microstructure from samples after the 1st and 2nd HT and different levels of impact toughness (KCV-60 < 2.0 and KCV-60 > 2.0∙102 kJ/m2). Microstructure of samples of low-alloy cast steel 20GL of car castings “Side frame” and “Truck bolster” types was studied by optical microscopy in the search for causes of falling toughness (KCV-60 <2.0). Statistical and microstructural analysis of the array of data on the size and average area of grains, their number and hardness in 20GL steel samples after single and double HT indicated different viscoplastic properties of these castings. The authors concluded that the grain size and hardness are the most important indicators of the microstructure of low-alloy steel 20GL and are unique in their effect on toughness.

Influence of Double Aging Heat Treatment on Phase Transformation and Dimensional Accuracy of Inconel 718 Alloy made Through Laser-Based Additive Manufacturing

Influence of Double Aging Heat Treatment on Phase Transformation and Dimensional Accuracy of Inconel 718 Alloy made Through Laser-Based Additive Manufacturing

Effect of increased length fraction of Ʃ3n special boundaries on OAIC response of cold rolled Ni-based alloy 718 thin sheets

Among the well-established Ni-based superalloys, alloy 718 is one of the most widely used in the industry. However, its application is limited up to 650°C due to the occurrence of the oxidation assisted intergranular cracking (OAIC) phenomenon. As a general rule, an increased fraction of special boundaries (3 < Ʃ<29) along the grain boundary network, in replacement of the high energy random high angle boundaries (RHABs), is usually associated with enhanced resistance to several intergranular failure mechanisms. In the present investigation, thin sheets of alloy 718 were subjected to three different thermomechanical processing (TMP) routes, in order to manipulate the grain boundary character distribution (GBCD), aiming to achieve an increased fraction of low-Ʃ special boundaries and, consequently, to reduce the alloy's OAIC susceptibility. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were used to characterize the different microstructures and GBCD resulting from TMP. Thin sheet specimens underwent hot tensile tests at 650 °C under secondary vacuum at a strain rate of 3.2 × 10−4 s−1. Fractography analyses were performed to quantify the fraction of brittle areas on the fracture surface. The results indicate that the sample processed through iterative steps of cold rolling and solution annealing, followed by a double aging heat treatment, resulted in a microstructure resistant to OAIC, with 100% of ductile fracture. Such good resistance to OAIC was attributed to the higher percentage of special Ʃ3n boundaries, as well as to the best configuration of grain boundaries network, presenting increased fraction of special triple junctions 3CSL and 2CSL as well as a favorable triple junction ratio (TJR).

Quality changes in fresh noodles prepared by different heat treatments during storage

The effects of single heat treatment (98°C-4 min, before and after packaging) and double heat treatment (98°C-4 min +90°C-15 min) on the quality of fresh noodles during storage (25°C and 4°C) were investigated. The safe storage period was determined based on the microbial content. Heat treatment significantly reduced the bacterial content, water activity, polyphenol oxidase activity, total polyphenols content, acidity, and fat acidity value of fresh noodles during storage, and improved the texture and cooking properties of fresh noodles, especially the double heat treatment. Additionally, double heat treatment combined with low-temperature storage could effectively prolong the noodle shelf-life. Practical applications Fresh noodles are prone to spoilage and enzymatic browning during storage. Heat treatment, as one of the popular physical preservation technologies, can solve these problems effectively. At present, heat treatment technology is mainly used in the processing of fresh pasta, and its application in fresh noodles especially the influence of heat treatment on the quality of fresh noodles during storage has not been reported. In order to improve the quality and shelf life of fresh noodles, different heat treatments combined with packaging technology and low-temperature storage were investigated in this study. This study could provide useful information for applying heat treatment on extending the shelf life of fresh noodles.

Ensuring the resistance against hydrogen induced cracking of pipes welded from structural steel using high-frequency currents

The results of a study of the resistance to hydrogen induced cracking of pipes welded by high-frequency currents, made of structural steel containing 0.16-0.22 % carbon according to the NACE TM 0284 standard, produced in the conditions of &ldquo;Vyksa Steel Works&rdquo; are presented. It is found that in the presence of non-metallic inclusions of elongated (extended) morphology in the metal, cold deformation during the forming of rolled products into a pipe reduces the corrosion resistance in hydrogen sulfide-containing media, even with subsequent double heat treatment in the form of quenching from the austenitic region and high tempering. It is clearly shown in the laboratory and confirmed in industrial conditions, the combined effect on the formation of nonmetallic inclusions by modifying them in the production of steel and the formation of the structure by applying double heat treatment provides resistance against hydrogen induced cracking of pipes welded by high-frequency currents made of structural steel containing 0.16-0.22 % carbon.

Tempering and Austempering of Double Soaked Medium Manganese Steels

The addition of a tempering or austempering step to the double soaking of a 0.14C–7.17Mn (wt pct) steel was investigated in the present contribution. The double soaking heat treatment is a two-step intercritical annealing heat treatment, which generates microstructures of athermal martensite, retained austenite and ferrite when applied to medium manganese steels. Microstructures following double soaking and (aus)tempering contained a combination of retained austenite, athermal or tempered martensite, and blocky or bainitic ferrite. X-ray diffraction, dilatometry and transmission Kikuchi diffraction were utilized to investigate microstructural changes which occurred during tempering or austempering. The resulting mechanical properties were measured using uniaxial tensile testing. The double soaking plus tempering heat treatment was shown to generate an ultimate tensile strength of 1,340 MPa in combination with 28 pct total elongation while the double soaking plus austempering heat treatment resulted in an ultimate tensile strength of 1,675 MPa and total elongation of 22 pct. Overall, both novel heat treatments produced a combination of strength and ductility desired for the third generation of advanced high strength steels.

Martensitic transformation in a α + β–Ti alloy after cryogenic treatment and its influencing to low elastic modulus biomaterials

The high strength titanium alloy is one of the most popular α + β-Ti type alloy and well responsive to the microstructure mainly depends on the processing history, heat treatment and selective faster cooling treatments. It has many engineering applications like military application, biomedical implant and aerospace application. The cryogenic treatment (CT) was typically done for faster cooling at low temperature (−20 to −100 °C) after solution treatment at 860 °C for 1 h then immediately quenched into liquid nitrogen bath with control manner. Subsequently, another identical sample quenched into the water to cool room temperature. After successfully solution treated both samples again preferred for ageing treatment at 500 °C for 5 h then air cooling. The examination of microstructures by SEM analysis revealed that after double steps heat treatment shows very fine α and β phases, with needle shape α are decomposed from α′ martensite tend to speed up by TiN or VN precipitates enhanced strength in the matrix. As well as, vanadium content is one of the important factors for martensite formation after solution treatment in the α + β phase field. The results show that after double steps heat treatment in CT obtained optimum mechanical properties such as 0.2% proof strength (PS) of 790 MPa, ultimate tensile strength (UTS) of 1200 MPa, strain (ε) of 70%, reduction area (RA) of 16.76%, and PS to UTS ratio of 0.66, respectively. To intuitive study estimate the effect in water quenching (WQ) as well as cryogenic treatment on microstructures and mechanical properties of high strength Ti-Al-V ternary alloy, which is potentially used in aerospace and structural applications owing to a high specific strength and high corrosion resistance. Meanwhile, after faster cooling in CT that martensite transformation tends to exhibit low elastic modulus for beneficial to orthopaedic implants in the human body.

Enhanced Performance of All-Inorganic QLED using Double Heat Treatment of NiO HTL

Enhanced Performance of All-Inorganic QLED using Double Heat Treatment of NiO HTL

Double heat treatment of welded butt joints of railway rails

Double heat treatment of welded butt joints of railway rails

Подвійна термічна обробка зварних стиків залізничних рейок

Подвійна термічна обробка зварних стиків залізничних рейок

Effects of double heat treatment of NiO hole transport layer on the performance of QLEDs

Parameters affecting the performance of QLED devices using NiO hole transport layer (HTL) thin film via the sol–gel process route were investigated. In the preparation of the NiO HTL thin film, a double heat treatment scheme, crystallization heat treatment at elevated temperatures followed by vacuum annealing treatment at lower temperatures, was introduced to form a crystalline NiO single phase as well as surface phases with reduced defect concentration. The decoupling of crystallization and vacuum annealing treatment of NiO HTL thin film was effective in enhancing the performance of the QLEDs. Also, the insertion of an interlayer between QD EML and NiO HTL was very effective in enhancing the performance of the device. With the modification of processing conditions, the luminance of QLED with the NiO HTL was improved from 30 to 11500 cd/m2 and the current efficiency from 0.25 to 23 cd/A, respectively.

The simultaneous improvements of strength and ductility in additive manufactured Ni-based superalloy via controlling cellular subgrain microstructure

Fine cellular subgrain structure was formed in the Selective Laser Melting (SLM) manufactured IN718 alloy via optimizing the processing parameters. During the subsequent homogenization heat treatment process, the Laves phase dispersed at the subgrain boundaries can be eliminated while the cellular subgrain structure is reserved in the printed samples after holding at 1080 ℃ for 50 min. With the prolongation of the holding time, the subgrain boundaries undergo low angle rotation via the motion of dislocation, which leads to the annihilation of the cellular subgrain structure. Moreover, during the subsequent double aging heat treatment process, the reserved cellular subgrain structure in the homogenized samples promotes the precipitation of γ” second phase nanoparticles, and these precipitated γ” phase nanoparticles prefer to distribute at subgrain boundaries. It was found that these unique subgrain boundaries with γ” phase precipitates can hinder but not fully terminate the motion of dislocation during the plastic deformation process, which contributes to increasing the strength as well as holding the stable plastic flow. Hence, the strength and ductility of final prepared IN718 alloy with cellular subgrain microstructure were improved simultaneously compared to the prepared alloy without cellular subgrain structure, which even exceed the mechanical properties standards (AMS 5662) of wrought IN718 alloy. These results in our work suggest that controlling the subgrain structure is a promising effective strategy to improve the mechanical properties of SLM manufactured nickel-based superalloy.

Effects of post-processing route on fatigue performance of laser powder bed fusion Inconel 718

Fatigue performance of Inconel 718 (IN718) parts built using laser powder bed fusion (L-PBF) is often poorer than that of the cast and wrought alloy parts. This is because of detrimental effects arising from the microstructures such as brittle phases and anisotropy as well as porosities. To improve fatigue performance of L-PBF IN718, two types of post processes namely hot isostatic pressing (HIP) (performed at 980 °C, 100 MPa and 4 h) and shot peening (performed at 0.45 mmA intensity and 200 % coverage) were employed individually as well as sequentially on different specimens in addition to a standard processing route consisting of L-PBF build followed by solution annealing and double aging heat treatment. Porosities were considerably reduced by HIP-ing (from 0.39 % to 0.08 % porosity volume ratio) but this did not translate to improvements in fatigue performance. This can be attributed to the presence of inclusions and yield strength reduction resulting from γ” solutionisation. High compressive residual stresses of close to 1 GPa were recorded following shot peening regardless of whether HIP-ing was carried out prior to the shot peening. Improvement in fatigue lives particularly at high stress levels was observed on shot peened specimens without HIP-ing (H + SP) which can be explained by the compressive residual stresses that delay the onset of fatigue crack initiation. Shot peening applied on the HIP-ed specimens (H + HIP + SP) however, resulted in fatigue performance deterioration. This is related to the harmful tensile stresses that must exist alongside the compressive residual stresses in a shot peened surface, as evident from the location of the sub-surface crack initiations near the residual stress transition region approximately 300 μm from the free surface. In this case, detrimental effects arising from tensile stresses appear to have outweighed the benefits from compressive stresses on fatigue performance.

The impact of heat-treatment protocol on the grain size and ionic conductivity of NASICON glass-ceramics

Na2AlTi(PO4)3 (NATP) and Na1.8Al0.8Ge1.2(PO4)3 (NAGP) NASICON (Na-Superionic Conductor) glass-ceramics are obtained by applying different single (SHT) and double (DHT) heat treatments on the respective precursor glass to evaluate its effect on the microstructure and in turn, on the total ionic conductivity. The grain and grain boundary contributions are also analyzed in the NATP composition. SHT with longer crystallization times favors the development of well-defined grains in both compositions and also promotes the grain growth in NAGP samples. This behavior causes a decrease in the activation energy of the grain boundary, which enhances the total ionic conductivity. Regarding DHT samples, microstructure with larger grains and higher ionic conductivity were obtained with shorter nucleation times for NATP and NAGP compositions. Finally, the microstructural variation generated by the different thermal treatments causes the total conductivity to increase up to two times.

Effect of post-processing annealing treatments on microstructure development and hydrogen embrittlement in API 5L X70 pipeline steel

The mechanical properties of steels are strongly influenced by microstructure and texture developed during thermomechanical processing. Here, different annealing treatments have been performed on pipeline steel at inter-critical temperatures. The aim of this study is to relate hydrogen embrittlement behavior to the microstructural characteristics obtained through one-step, and two-step annealing treatments. Similar grain orientation distributions were observed after the initial hot rolling and the two-step annealing procedures. However, in the later process a dual-phase (i.e. ferrite-martensite) structure was formed; unlike in the starting material where only segregated patches of martensite were observed within a ferritic microstructure. The one-step treated steel was comprised of relatively large and primarily ferrite grains. As a result, the preliminary processing revealed the highest strength, with reduced ductility. The one-step treatment lowered strength and ductility. Although the two-step treated steel showed the lowest strength, it was the most ductile with improved resistance to hydrogen embrittlement. The tempering operation introduced in the two-step process created minimal strain and tempered martensite inside the steel. This reduced the tendency for hydrogen damage. A direct relationship was established between high tensile strength and increased embrittlement risks. Lowering the strength and hardness through double heat treatment cycles delayed the onset of cracking after pre-hydrogen charging.

Features of Diffusion Interaction in Steel-Aluminum Composite After Explosive Welding and Aluminizing by Melt Immersion

Diffusion interaction at an interlayer boundary of St3 steel–AD1 aluminum after explosive welding and aluminizing by melt immersion is investigated. Test heat treatment regimes are revealed providing formation of coatings based on iron aluminides on a steel surface. It is shown that in order to form Fe2Al5 intermetallic on a steel surface with the hardness of 10 GPa, an explosion welded steel-aluminum composite must be given double heat treatment. After the first (660 °С, 3 h) a two-layer (Fe2Al5 and FeAl3) diffusion zone is formed, and after the second (640 °С, 3 h) it is destroyed along a main crack between the Fe2Al5 and FeAl3 interlayers. In order to form an Fe2Al5 intermetallic coating on a steel surface an aluminized layer obtained by melt immersion must be heat treated at 800 °C. Doping the diffusion zone with Si leads to the appearance of additional phases Al7Fe2Si and (Al, Si)5Fe3 within the composition, and an increase heat treatment duration leads to a reduction in coating hardness to 7.5–8 GPa.

Substantial improvement in sub-zero impact toughness in 12Cr stainless grade welding joints by applying double PWHT

Abstract A novel means to improve the sub-zero impact toughness in the welding joints of 12Cr stainless steels was developed. The results indicated that the optimal conditions for performing the double post-welding heat treatment (PWHT) procedures were as follows: heating at 1000 °C for 15 min, followed by air cooling and then heating at 780 °C for 15 min, and finally air cooling. The welding joints subjected to double PWHT exhibited excellent mechanical and enhanced corrosion-resistance properties. Furthermore, the transition of the substructure in the martensitic grains was identified as the decisive factor for the improvement in the sub-zero toughness.

Improved plasticity of Inconel 718 superalloy fabricated by selective laser melting through a novel heat treatment process

When Inconel 718 alloy is fabricated by selective laser melting and treated by traditional homogenization plus double aging heat treatment (HA), its strength improves greatly, but ductility decreases. A novel heat treatment (NHT) including higher temperature homogenization at 1150 °C for 2 h and one-time lower temperature aging treatment at 700 °C for 12 h is developed, which can overcome the strength-plasticity trade-off of Inconel 718 alloy. The results show that recrystallized grains with annealing twins and ultrafine strengthening phases form in the specimen subjected to the NHT. These microstructures differ from those in as-built and conventional heat-treated samples. Especially the morphology of strengthening phase γ″-Ni3Nb precipitated in the NHT specimen has changed a lot, it seems to be spherical rather than disc-like shape occurred in the traditional heat-treated samples. Consequently, the NHT process increases plasticity by 41%, while maintaining ultimate strength at the same level achieved by the traditional heat treatment. The enhanced ductility is attributed to the annealing twins and recrystallized grains without local strains, while the strength is provided by the smaller precipitates formed in the NHT one-time aging treatment.