Isothermal Treatment Temperature Research Articles

Strong and tough semi-crystalline poly(aryl ether nitrile) with low coefficient of thermal expansion

The crystallization of poly (aryl ether nitrile) (PEN) has an essential impact on improving heat resistance, mechanical properties, chemical resistance, and dielectric properties. However, during the synthesis process, high crystallinity PEN is prone to precipitate out, which makes it difficult to carry out the synthesis reaction, which poses a significant challenge to synthesizing high crystallinity and high molecular weight PEN. In this work, hydroquinone (HQ) as the main phenolic monomer and 4,4′-biphenol (BP) as the second phenolic monomer were used to solve the technical difficulties in synthesis by copolymerization. The crystalline PEN with high molecular weight and significant crystallinity was obtained by regulating the molar ratio of BP to HQ. The effects of isothermal heat treatment on its crystalline, mechanical, and thermal properties were investigated. The influence laws of molecular chain structure and heat treatment process on the properties of PEN films were revealed. Among them, when the isothermal treatment temperature was 270 °C and the HQ: BP was 85:15, the mechanical strength reached 130 MPa, and the elongation at break could be up to an astonishing 50 %. Besides, the CTE value from 50 °C–150 °C is 40.84 ppm/°C, which is lower than most thermoplastics. Therefore, PEN with the HQ to BP molar ratio of 85:15 is suitable for large-scale production, providing new crystalline plastic specifications for high-performance special engineering plastics. A crystal bridge-tie chain model is summarized by the properties of HQ/BP-PEN films under different crystallization conditions, which provides an idea for preparing stiff and tough films.

A new process for efficient purification of 6061 aluminum alloy scrap under semi-solid and super-gravity conditions

A new method for efficient and sustainable recovery of pure Al from 6061 aluminum alloy scrap based on semi-solid processing and supergravity separation was proposed in this study. Firstly, the microstructural variations of the alloy scraps after semi-solid isothermal treatment (SSIT) were investigated, and the liquid phase accumulated with a large amount of impurity elements would gradually aggregate at the grain boundaries and eventually form a dendritic network, while the spherical Al particles within the grain had high purity. Subsequently, the inter-dendritic liquid was successfully extruded from the intergranular by super-gravity separation technology and the pure Al was efficiently recovered. In this process, the effects of separation time (i.e., isothermal treatment time, t), separation temperature (i.e., isothermal treatment temperature, T) and gravity coefficient (G) on the purification effect were evaluated. The recovery rate of pure aluminum (Al ≈ 99.3 wt%) was 81.98% at the optimum conditions of t = 15 min, T = 640 °C, G = 500. In addition, it was found that increasing the separation time and temperature was beneficial to the purification effect of the alloy scrap but had a negative impact on the recovery rate. Therefore, this study has designed a new process for super-gravity purification and recovery, which realized cyclic recycling and significantly improves the recovery efficiency of aluminum scrap.

Al–Sm and Al–Ca phases thermal stability effect on mechanical properties of Mg–3Al–2Ca–2Sm alloy

The Al–Sm and Al–Ca phases thermal stability effect on mechanical properties of Mg–3Al–2Ca–2Sm alloy was studied. When the isothermal treatment temperature is 200°C and 300°C, the second phase formed in as-cast Mg–3Al–2Ca–2Sm alloy does not change significantly. The Al2Ca and Mg2Ca precipitated phases were precipitated during isothermal treatment. The alloy exhibits favourable tensile strength, elongation and fracture toughness. By increasing the isothermal treatment temperature to 400°C or even higher, the second phase began to decompose, except for the original particle Al2Sm phase. The acicular Al11Sm3 phase breaks down into the particle Al2Sm phase. Furthermore, the fish-bone Al2Ca phase and lamellar (Mg, Al)2Ca phase decompose into particle Al2Ca phase. The mechanical properties and fracture toughness of the alloy decrease gradually.

Effects of Isothermal Temperature on Microstructure and Properties for N80 Oil Pipe Steel

Some key temperature parameters in the process of controlled rolling and controlled cooling of N80 grade oil casing steel were simulated by heat treatment method, and the effects of different isothermal temperatures on the microstructure and mechanical properties of N80 grade oil casing steel were studied. The results show that when the isothermal temperature increases from 500 °C to 550 °C, the influence of different microstructure volume and grain size is not obvious; when the temperature reaches 600 °C, the ferrite grain coarsens obviously, the volume fraction of ferrite increases obviously, and the volume fraction of pearlite and bainite decreases obviously. In addition, when the isothermal temperature increases from 500 °C to 550 °C, the yield strength and tensile strength of the test steel increase slightly, and the plastic toughness increases obviously; but when the temperature increases to 600 °C, the yield strength and tensile strength of the test steel decrease obviously, and the plastic toughness continues to increase. The optimum isothermal treatment temperature of test steel is 550 °C.

Effect of isothermal process parameters on semi-solid microstructure of chip-based Al–Cu–Mn–Ti alloy prepared by SIMA method

A typical Al–Cu–Mn–Ti aluminum alloy chip was adopted to prepare semi-solid billets by a Strain-Induced Melt Activation (SIMA) method, and the effects of isothermal process parameters on the semi-solid microstructure evolution of the alloy were investigated in this work. The result showed that semi-solid billets with highly spheroidal and homogeneous fine grains could be prepared from chips by the SIMA method. With the increase of isothermal temperature, the finer and near-spherical grains are obtained, the grains coarsen and became ellipse at 903 K because of the coarsening mechanisms of coalescence and Ostwald ripening. The relationship of isothermal holding time and grains size followed the LSW theory well, and more spherical microstructure can be brought by prolonging the holding time until 3000 s. Thus, the optimal isothermal treatment temperature is 893 K and holding time is 3000 s, the corresponding average size and roundness of grains are 137 [Formula: see text]m and 1.108, respectively.

Evaluation of the Possibility to Obtain Nanostructured Bainite in High-Carbon and High-Silicon 9XC Bearing Steel

The main aim of this research is the evaluation of the possibility of obtaining a nanostructured bainite after isothermal heat treatment of 9XC (according to GOST standard) bearing steel. It was found that the annealing at low temperature (250 °C) allows to obtain the microstructure of nanobainite consisting of nanometric-scale bainitic ferrite laths and austenite with a dominant film-like morphology. In the case of higher isothermal treatment temperature (300 °C), a microstructure was slightly coarser and the proportion of austenite with blocky morphology increased. The identification of a highly refined microstructure was performed using TEM and SAED methods. Besides, isothermal annealing temperature significantly affects hardness. The hardness difference between annealing at 250 °C (630.0 ± 4.4 HV1) and 300° C (517.3 ± 1.1 HV1) was over 100 HV. Such a substantial decrease in hardness was caused by the fraction and morphology of the occurred phases. Due to the high silicon content in the tested steel (1.54%), the carbide precipitation was delayed, but locally the cementite precipitates were confirmed. Based on the research, it was found that the commercial 9XC steel is appropriate for nanostructurization in the low ranges of bainitic transformation. Designing of further heat treatment should be focused on the lowest possible temperature of isothermal annealing (even below 250 °C).

Effect of preheating/post-isothermal treatment temperature on microstructures and properties of cladding on U75V rail prepared by plasma cladding method

Rail cladding is a cost-effective approach for track maintenance and protection from rapid growth of mileage and Rolling Contact Fatigue (RCF) phenomenon. In this study, cladding layers are prepared on the surface of a U75V rail substrate using a plasma cladding approach with different substrate preheating temperatures. Preheating of the substrates creates an isothermal treatment for the cladding material after solidification of the molten pools. The effects of these post-solidification isothermal treatment temperatures (480°C, 340°C, and 200°C) on microstructures, mechanical properties and corrosion resistance are studied. The results indicate that the post-isothermal treating temperature affects the fractions of granular bainite, upper bainite, lower bainite, martensite and M/A island in the microstructure. The hardness of cladding layers is higher than that of the U75V substrate, and slightly increases as the pretreating temperature decreases. The specimen prepared at 340 °C has the highest tensile strength, which is 14.5% higher than the U75V rail. The interface between the cladding layer and the substrate also processes a strong bonding with the bonding strength of 789 MPa. Based on the comprehensive performance, the post-isothermal treating temperature of 340 °C is most suitable for cladding on the U75V rail. The plasma cladding process followed by post-isothermal treatment proves to be a flexible and effective approach to manipulate the microstructure of the cladding and restore the properties of the rail substrate.

Effect of water activity on the thermal inactivation kinetics of Salmonella in milk powders

Persistence of Salmonella in milk powders has caused several foodborne outbreaks. The determination of proper pasteurization processing conditions requires an understanding of the thermal inactivation kinetics of Salmonella in milk powders. However, there is a lack of knowledge related to the effects of water activity (aw) and fat content on Salmonella inactivation in milk powder during thermal processing. Two types of milk powders, nonfat dry milk and whole milk powder, with different fat contents (0.62 and 29.46% wt/wt, respectively) were inoculated with a 5-strain cocktail of Salmonella and equilibrated to 3 aw levels (0.10, 0.20, and 0.30) for isothermal treatments at 75, 80, and 85°C to obtain D-values (the time required to achieve a 10-fold reduction of the bacteria at the isothermal treatment temperature) and z-values (the increase in temperature required to achieve a 90% reduction of the decimal reduction time D). Stability tests showed that the inoculation method used in this study provided a high and stable population of Salmonella for thermal inactivation studies. A moisture sorption isotherm was measured to understand the relationship between aw and moisture content of milk powders. The thermal resistance of Salmonella was found to significantly increase as aw decreased, which suggested that a higher temperature or longer processing time would be required at low aw to achieve the desired inactivation of Salmonella. The microbial inactivation kinetics were not significantly different for the 2 milk powders; therefore, data were combined to develop a universal model. A response surface model was compared with a modified Bigelow model. The modified Bigelow model performed well to predict D-values [root mean square error (RMSE) = 1.47 min] and log reductions (RMSE = 0.48 log cfu/g). The modified Bigelow model developed here could be used to estimate D-value as a function of water activity and temperature to design a thermal pasteurization system for milk powders.

The Impact of Isothermal Treatment on the Microstructural Evolution and the Precipitation Behavior in High Strength Linepipe Steel.

Isothermal treatment affects the microstructural evolution and the precipitation behavior of high-strength low alloy (HSLA) steels. In this regard, thermal simulation of different isothermal treatment temperatures was adopted by using a thermomechanical simulator. The results showed that hardness reached the maximum value at 600 °C holding temperature, which was related to a finer grain structure and granular bainite. The strengthening effect of precipitates was remarkable due to the combination of small particle size and small interparticle spacing. It is presumed that the precipitation started after 600 s at 600 °C. Precipitation strengthening continued to exist, even though coarsening of ferrite grains led to softening phenomena when the specimen was isothermally held at 750 °C, which led to relatively high hardness. The precipitates were fcc (Ti, Nb) (N, C) particles, and belonged to MX-type precipitates. Average size of precipitates increased from 3.14 to 4.83 nm when the specimens were isothermally held between 600 °C and 800 °C. Interparticle spacing of precipitates also increased with increasing isothermal treatment temperatures. These led to a reduction in precipitation strengthening. At the same time the polygonal ferrite content increased and ferrite grain size got larger, such that the hardness decreased continuously.

Influence of Carbon Content and Isothermal Heat Treatment Temperature on the Microstructure and Mechanical Properties of Ultra-High Strength Bainitic Steels

The effect of carbon content and isothermal heat treatment conditions on the microstructure evolution and mechanical properties of ultra-high strength bainitic steels was investigated. A reduction in carbon content from 0.8 wt% to 0.6 wt% in super-bainite steel with typical chemistry effectively improved not only the Charpy impact toughness but also the strength level. This suggests that reducing the carbon content is a very promising way to obtain better mechanical balance between strength and impact toughness. The higher Charpy impact toughness at a lower carbon content of 0.6 wt% is thought to result from a reduction in austenite fraction, and refinement of the austenite grain. The coarse austenite grains have a detrimental effect on impact toughness, by prematurely transforming to deformation-induced martensite during crack propagation. Mechanical properties were also affected by the isothermal treatment temperature. The lower isothermal temperature enhanced the formation of bainitic ferrite with a refined microstructure, which has a beneficial influence on strength, but reduces impact toughness. The lower impact toughness at lower isothermal temperature is attributed to the sluggish redistribution of carbon from the bainitic ferrite into the surrounding austenite. Higher solute carbon in the bainitic ferrite contributes to an increase of strength, but at the same time, encourages a propensity to cleavage fracture. Key words: high strength, bainitic ferrite, heat treatment, austenite, impact toughness

Characterization of the Microstructure Evolution of Ni-Based Superalloy at Liquidus Temperature by Electromagnetic Field

The effect of isothermal treatment temperatures and isothermal treatment time on the microstructure was studied. The results showed that the globular and equiaxed grains with the average grain size of 60 μm and the shape factor of circle of 0.95 can be obtained when the melt of Ni-Cr-W superalloy was subjected to the heat treatment of 10 min at 1400 °C. The quenching results showed the volume fraction of the eutectic phase was the largest and the volume fraction of primary γ phase was the smallest after the isothermal treatment of 1400 °C. The optimal melt treatment temperature and time were 1400 °C and 10 min, respectively. Moreover, the effect of electromagnetic field on the solidification was also investigated. It was demonstrated that applying electromagnetic field was beneficial to the uniform temperature, solute field and the high density of the secondary nuclei, which contributed to grain refinement.

Crystallinity of poly(arylene ether nitrile) copolymers containing hydroquinone and bisphenol A segments

ABSTRACTThe hydroquinone (HQ) and bisphenol A (BPA) type poly(arylene ether nitrile) (PEN) (HQ/BPA‐PEN) were synthesized through nucleophilic aromatic substitution polymerization from HQ, BPA, and 2,6‐dichlorobenzonitrile (DCBN). The prepared copolymers were characterized by intrinsic viscosity, Fourier transform infrared (FTIR), and dynamic rheological analysis. The properties of resultant copolymers were studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and mechanical testing. The results showed that the PEN copolymers exhibited crystallization performance with excellent thermal and mechanical properties. HQ/BPA‐PEN10 was made into films by solution‐casting process and then were treated at different temperatures (200, 260, 280, 300, 310, and 320 °C) for different times (1, 2, 3, 4, and 5 h) to investigate the crystallinity. Results showed that when isothermal treatment temperature is 310 °C and isothermal treating time is 4 h, HQ/BPA‐PEN10 showed best properties. At this condition, the melting enthalpy, crystallinity, tensile strength, and elongation at break of the sample is 17.7 J/g, 14.11%, 132.9 MPa, and 6.1%, respectively. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2018,135, 46412.

Effect of silicon on stability of austenite during isothermal annealing of low-alloy steel with medium carbon content in the transition region between pearlitic and bainitic transformation

In a vast majority of steels, a prerequisite to successful heat treatment is the phase transformation of initial austenite to the desired type of microstructure which may consist of ferrite, pearlite, bainite, martensite or their combinations. Diffusion plays an important role in this phase transformation. Together with enthalpy and entropy, two thermodynamic quantities, diffusion represents the decisive mechanism for the formation of the particular phase. The basis of diffusion is the thermally-activated movement of ions of alloying and residual elements. It is generally known that austenite becomes more stable during isothermal treatment in the transitional region between pearlitic and bainitic transformation. This is due to thermodynamic processes which arise from the chemical composition of the steel. The transformation of austenite to pearlite or bainite is generally accompanied by formation of cementite. The latter can be suppressed by adding silicon to the steel because this element does not dissolve in cementite, and therefore prevents its formation. The strength of this effect of silicon depends mainly on the temperature of isothermal treatment. If a steel with a sufficient silicon content is annealed at a temperature, at which silicon cannot migrate by diffusion, cementite cannot form and austenite becomes stable for hours.

Formation of Structure and Properties of Carbide-Free Bainite in Steel 30KhGSA

The process of formation of carbide-free bainite, its structure and properties in steel 30KhGSA are studied. The results of a dilatometric analysis are used to determine the temperature and time boundaries of formation of upper and lower carbide-free bainite. The methods of scanning and transmission electron microscopy are used to determine the special features of the structure of bainite as a function of the temperature of isothermal treatment.

Effect of the thermal stabilization temperature on the change in the texture of polyacrylonitrile fiber

The effect of temperature of isothermal treatment on the change in sizes L010 of coherent scattering regions and texture of a polyacrylonitrile fiber during its transition to the structure of a thermally stabilized fiber is analyzed using X-ray structure analysis. An increase in the thermostabilization temperature at a constant stretching load stimulates simultaneously a more active increase in size L010 and texturing of polyacrylonitrile fibers at the initial stage. Active evolution of the phase transformation at temperatures 275–290°C during further thermostabilization is accompanied by a substantial decrease in the texture of the polymer that has not experienced the phase transformation by this instant.

Feasibility study for thixoforming nanostructured bainitic steels

Super Bainite Steel (0.74% C, 1.64% Si, 1.82% Mn, 1% Ni, 0.36% Mo, 0. 21% Cr, 0.047% Al bal. Fe) after cold plastic deformation was used as the starting material for bainitic treatment preceded by controlled cooling from the thixoforming temperature range. Differential Scanning Calorimetry was used to determine the amount of liquid phase fraction vs temperature in the solidus–liquidus range. The steel was heated up to 1430°C, which corresponded to about 30% of liquid fraction. It was then rapidly cooled in oil at three different temperatures: 240°C, 270°C and 300°C. The samples were then held at those temperatures for 5h. The microstructure of the samples after treatment at 240°C showed globular grains (average size of 115µm), containing carbide-free bainite as a mixture of ferrite plates with average thickness of 63nm, and retained austenite plates of 40nm in thickness. Plates of martensite and blocks of austenite were visible in some grains. X-ray studies confirmed the presence of 20.1% of austenite and 79.9% of ferrite with martensite. The sample revealed the highest compression strength of 6651MPa, at yield strength of 1780MPa and compression strain ε=38%. With the increase of isothermal treatment temperature to 270°C and 300°C, yield strength decreased to 1370MPa, 1375MPa and compression strength was 5243MPa, 4138MPa, respectively, while plastic strain reached 38.5% and 25%, respectively. Higher temperature of bainitic treatment led to coarsening of super bainite plates and as a consequence, a decrease in mechanical properties. Initiation of cracks was observed inside grains. They propagate through the globules and eutectic, unlike in thixoformed tool steels, in which the crack path runs along the eutectic mixture between the globular grains. Preliminary studies of the corrosion resistance of Super Bainite steels containing a carbide-free bainite structure were carried out. Results indicate slightly a higher resistance in chlorides environment of the sample isothermally treated at 240°C/5h preceded by controlled cooling from thixoforming temperature range than the sample treated in the conventional way (1000°C/15min/240°C/20h).

Ni-YSZ cermets for solid oxide fuel cell anodes via two-step firing

The electrochemical performance and dimensional stability of Ni-YSZ cermets, conventionally used as solid oxide fuel cell anodes, depend strongly on their microstructure and therefore fabrication conditions. This work was focused on the assessment of a less common two-step firing procedure for fabrication of Ni-YSZ cermets with comparatively low nickel fraction of 30 vol.%. The impact of different firing parameters including peak temperature (1623–1723 K), heating/cooling rate (4–10 K/min), and isothermal treatment temperature (1473–1573 K) and time (2–8 h), on the porosity and electrical conductivity of cermets was assessed employing Taguchi experimental planning. The applied procedure yielded Ni-YSZ composites with porosity 26–35% and electrical conductivity ranging from 170 to 420 S/cm at 873–1173 K in 10%H2–N2 atmosphere. Microstructural studies indicated that the conductivity is determined mainly by Ni particle size distribution. Analysis of results suggests that, for the studied range of sintering parameters, a higher peak temperature and ramp rate are favorable for the improvement of conductivity, whereas isothermal dwell temperature and time have a rather minor effect on the conductivity level.

Diffusion mechanisms and microstructure development in pack aluminizing of Ni-based alloys

Despite the large industrial use of pack aluminizing processes for the protection of parts (e.g. cooling channels) of gas turbine blades, systematic studies relating the formation mechanisms and the chemical composition of pack aluminized layers to important process parameters, including the nature of the base alloy and the temperature, are scarce. In this study, 4 different alloys (pure Ni, Ni–20Cr, Inconel 738 and directionally solidified CM247LC) were subjected to a pack aluminizing process at three different temperatures (950°C, 1000°C, 1040°C), using a pack mix containing a fluorine-based activator; the results were compared to those obtained with a chemical vapor deposition (CVD) aluminizing process performed at 1040°C with the same fluorine-based gaseous precursor.The microstructural characterization, performed by SEM+quantitative EDX analysis, XRD and nanoindentation testing, shows that, during the heating stage of the pack aluminizing process, Al is transported to the sample surface at temperatures too low to allow significant simultaneous diffusion of Ni; therefore, a δ-Ni2Al3 outer layer is formed by inward Al diffusion below the alloy surface, and its growth then continues during the isothermal stage as well. As a result, the chosen isothermal treatment temperature does not affect growth mechanisms, although it modifies the overall thickness of the aluminized layer. δ-Ni2Al3 is converted to β-NiAl after a subsequent vacuum heat treatment at 1120°C. In a CVD process, where gaseous precursor are introduced only after attaining the isothermal treatment stage, Al and Ni diffuse simultaneously from the very beginning of the aluminizing process and β-NiAl is directly developed.Less mobile species (heavy atoms, such as W) in the alloy composition hinder all diffusion phenomena, both during pack aluminizing and during subsequent vacuum heat treatment: after aluminizing, precipitates are developed within the δ-Ni2Al3 outer layer and, after vacuum heat treatment, the resulting β-NiAl layer exhibits a compositional gradient.

Estimation of phase fraction in dual phase steel using microscopic characterizations and dilatometric analysis

We made a quantitative comparison of three different methods, optical microscopy by tint etchant, EBSD phase mapping and dilatometry for analysis of phase fraction in steel subjected to intercritical annealing and isothermal treatment. While the results from optical microscopy and EBSD technique showed quantitative agreement, the dilatometry gave rather higher martensite fraction compared to the microscopic analyses. Nevertheless, all three methods showed qualitative agreement in the variation of martensitic fraction depending on the processing conditions. The analyses revealed that the martensitic fraction in final microstructure decreased as intercritical annealing temperature increased because it deteriorated the hardenability of austenite. Raising the isothermal treatment temperature increases the martensite fraction due to the increases of austenite fraction, which transformed into martensite afterward.

The Effect of Isothermal Treatment on the Microstructure and Properties of High Performance Steels

The microstructure and properties of 600MPa grade steels produced by isothermal treatment has been studied. The influence of isothermal treatment temperature on the microstructure of steels has been investigated by the aid of Optical microscopy and TEM. The results show that after various isothermal treatments the microstructure of steels varies with the isothermal temperature. When the temperature is lower than 530°C the microstructure is mainly composed of granular bainite and lath bainite. Whereas when the temperature is higher than 580°C the polygonal ferrite and quasi polygonal ferrite appears and the microstructure consists of ferrite and fine lath bainite. Since the microstructures are complex and the yield ratio of steels under these conditions are all lower than 0.8. The strength of sample for 630°C isothermal treatment is the highest. The yield strength is over than 800MPa and the tensile strength is over than 1000MPa due to the fine precipitates containing copper. These copper containing precipitates formed during the isothermal transformation by the way of inter-phase precipitation.