Reheating Treatment Research Articles

Anisotropy reduction and mechanical property improvement of additively manufactured stainless steel based on cyclic phase transformation

Owing to the special layer-by-layer deposition process of directed energy deposition (DED), columnar coarse grains, produced by cyclic reheating with intrinsic directional heat flow along the building direction, are difficult to avoid. These grains result in strong anisotropic characteristics with poor mechanical properties, which restrict the application of DED products. This work proposes a novel fabrication strategy based on the cyclic-phase-transformation behavior, which can reduce the anisotropy and improve the mechanical properties of DED-printed stainless steel. Using this fabrication strategy, 316 L powder (austenitic stainless steel) and 17–4PH powder (martensitic stainless steel) were mixed in different mass ratios to fabricate five types of DED-printed stainless steels. Among the five samples, P70 (mixture of 70 wt% 17–4PH powder and 30 wt% 316 L powder) showed the weakest anisotropy and the best mechanical properties, which can be attributed to the cyclic phase transformation under cyclic reheating treatment and the transformation-induced plasticity (TRIP) effect over a wide range of strains, respectively. Compared with the pure 316 L printed material, the tensile test results of P70 showed that the yield strength (YS) and ultimate tensile strength (UTS) increased by 35.4% and 34.5%, respectively, whereas the uniform elongation (UE) and total elongation (TE) were improved by 63.9% and 31.4%, respectively. In addition, the strength–ductility balance (UTS × UE) increased by as much as 120.4%. The proposed fabrication strategy is expected to reduce the anisotropy in other materials that undergo cyclic-phase-transformation phenomena during additive manufacturing.

Evolution of centerline carbides in a martensitic stainless steel slab during reheating treatment

The formation and evolution of centerline carbides in a martensitic stainless steel slab are investigated using various characterization techniques. The intensive formation of carbides was observed close to the shrinkage cavity at the centerline of the slab. Carbide structures include a complex eutectic-like mixture of δ-ferrite and M7C3 phases with M6C carbides at the δ/M7C3 interface. Nano-sized M7C3 particles also precipitate in the δ holding the orientation relationship of1¯10δ∥100M7C3;111δ∥100M7C3.This complex structure was surrounded by the retained γ. The decomposition of the coarse M7C3 was investigated after the reheating treatment to 1200, 1250, and 1300 °C. It was found that by increasing the reheating temperature, coarse M7C3 carbide completely decomposed, whereas δ-ferrite islands enlarged due to the thermodynamic stability of δ-ferrite at high temperatures. At the same time, the retained austenite reduces because of homogenizing of the segregated solutes. The new formation of M23C6 and the growth of M6C were identified after reheating to 1200 °C, while all carbides were fully decomposed at 1250 and 1300 °C. The effect of reheating temperature on the decomposition process of coarse carbide was further discussed.

Facile preparation of air-stable n-type thermoelectric single-wall carbon nanotube films with anionic surfactants

Thermoelectric generators based on single-wall carbon nanotubes (SWCNTs) have great potential for use in wearable and skin electronics because of their lightweight and mechanically soft structure. However, the fabrication of air-stable n-type thermoelectric SWCNTs using conventional processes is challenging. Herein, we propose a facile process for fabricating air-stable n-type SWCNT films with anionic surfactants via drop casting followed by heat treatment. We examined different surfactants (Sodium Dodecyl Sulfate, Sodium Dodecylbenzene Sulfonate, and Sodium Cholate) and heat-treatment temperatures. The optimal SWCNT film maintained the n-type Seebeck coefficient for 35 days. Moreover, to further extend the n-type Seebeck coefficient maintenance, we periodically reheated the SWCNT film with a surfactant that had returned to the p-type Seebeck coefficient. The reheated film recovered the n-type Seebeck coefficient, and the effect of the reheating treatment lasted for several reheating cycles. Finally, we elucidated a simple mechanism for realizing an air-stable n-type Seebeck coefficient based on spectroscopic analyses of the SWCNT films.

Morphological Evolution of Al-2wt%Si-2.5wt%Cu Alloy Produced by EMS and EMS Enhanced by ECAP

This work evaluates the morphological evolution at the semisolid state of the Al-2wt%Si-2.5wt%Cu alloy produced by direct chill casting under electromagnetic stirring (EMS) and this same raw material enhanced by one equal channel angular pressing pass (ECAP), specially designed to produce raw material for thixoforming processes. The traditional EMS production method is compared to ECAP in order to determine if a second procedure is necessary for this particular alloy. The ECAP process emerged as a promising technique capable of imposing severe homogeneous deformation in metals, which could improve thixoforming processes. The ECAP deformation occurs in a matrix that contains two channels of the same cross-sectional area and form an angle of Φ = 90° and Φ = 120°, the process can be repeated many times allowing to control the microstructure and properties of the materials. Both raw materials, EMS and EMS + ECAP were submitted to re-heating treatment in two conditions of solid fractions, 45% and 60% at soaking times of 0s, 30s and 90s. With this procedure primary particle sizes of about 60 μm were obtained, which exhibit favourable characteristics for the thixoforming process.

Morphological Characterization of Al-2.0wt%Si-2.5wt%Cu Alloy Produced by Direct Chill Casting and Electromagnetic Stirring for Thixoforming Process

This work evaluates, by means of its morphological characterization, a new Al-2.0wt%Si-2.5wt%Cu alloy produced by direct casting under electromagnetic stirring for thixoforming process. The alloy was submitted to re-heating treatment in two conditions of solid fractions, 45% and 60%, for 0, 30, 90 and 210 s. Re-heating treatment times promote solid phase particles globularization, presenting average grain sizes of approximately 90 μm for both solid fraction. These results show that the alloy is viable for thixoforming process, especially in the condition of 90 s of treatment time at 45% and 60% of solid fraction, that presents grain size about 90 μm and shape factor higher than 0.60. Another important fact about the small grain/primary particle size is related to the use of electromagnetic stirring in its production, that promotes the breakdown of the material structure and consequently stimulates globularization.

Microstructural characterization of laser melting deposited Ti–5Al-5Mo–5V–1Cr–1Fe near β titanium alloy

Laser melting deposition (LMD) is characterized by high temperature gradients, high cooling rates and reheating treatment effects. These features of LMD generally lead to an unique microstructure. In this study, the macrostructure, microstructure and hardness of laser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe near β titanium alloy have been investigated. The macrostructure is characteristic of novel β grain morphology and layer bands. The β grains exhibit successive alternation of large columnar and bamboo-like grain morphology, which is associated with the LMD processing parameters. Layer bands exhibit duplex microstructure and low hardness, while ultra-fine basket-weave microstructure, microsegregation and high hardness are observed between layer bands. Microstructure and hardness of the alloy are non-uniform due to the existence of the layer bands. Thus, special attention is given to the formation mechanism of layer bands. It is found that layer bands are heat-affected zones which resulting from the sub-melting-point reheating treatment when a new layer deposits.

Characterization of the Microstructure and Rheological Behavior of Al-4wt%Si-2.5wt%Cu Alloy Produced by Direct Chill Casting and Electromagnetic Stirring

This work involved an evaluation of a Al-4wt%Si-2.5wtCu alloy to be used as raw materials in the thixoforming process. The alloy was produced by direct casting under electromagnetic stirring to obtain ingots of 250 mm length and 30 mm diameter. The alloy was analyzed to determine their morphology and rheological behavior in the semi-solid range. The tests included characterization of the microstructural evolution by subjecting them to re-heating treatment in two conditions of solid fractions, 45% and 60%, for 0, 30, 90 and 210 s. A compression testing device designed specifically to evaluate semi-solid materials was used to determine the rheological behavior, and the tests were performed at the same heating rate and hold times to determine the apparent viscosity. The use of electromagnetic stirring to produce the raw material was effective in producing alloy with very small grain/primary particle sizes (80 up to 120 µm). Based on the morphological evolution in the semi-solid state, the alloy showed only minor variations in grain/primary particle size and surface factor (SF) as a function of the different globularization heat treatment times, suggesting that all these alloys are suitable for production. The Al-4.0wt%Si-2.5wt%Cu alloy with 45% solid fraction presented apparent viscosity about 1.5 x106 Pa.s.

Microstructure and mechanical properties of laser melting deposited 1Cr12Ni2WMoVNb steel

1Cr12Ni2WMoVNb martensitic stainless steel was fabricated by laser melting deposition (LMD) process. Microstructure was characterized by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Vickers microhardness and room-temperature tensile properties were evaluated as well. Results indicate that the laser deposited steel has a fine well-aligned dendritic structure with a primary dendrite arm spacing of approximately 13μm. In the interdendritic regions there are fine phases which have a core–shell structure consisting of ferrite and many carbide particles. During the LMD process, the depositing layer has a reheating treatment on the top un-melted region of the previous layer, resulting in the formation of interlayer heat-affected zone (ILHAZ). Due to the existence of the ILHAZ, microstructure and microhardness of the laser deposited steel are non-uniform. The adjacent layers have the same crystallographic orientation, revealing that epitaxial growth of the depositing layer on the previous layer is not interrupted by the ILHAZ. Room-temperature ultimate tensile strength of the laser deposited steel reaches 1223MPa, which is comparable to the wrought bar.

Combined effect of cooling slope angle and mould vibration on semisolid microstructures of wrought aluminium alloy 7075

Semisolid casting and forming have attracted growing attention of both research and industry. A suitable semisolid microstructure consists of globular grains in a matrix of lower melting point phase and may be produced by various methods, including the cooling slope technique. The combined effect of cooling slope angle and mould vibration on the semisolid microstructures of Al7075 is reported. The results indicate that mould vibration decreases average grain size significantly and that the effect increases with increasing size of primary crystals entering the mould through the cooling slope. Mould vibration was also found to improve the shape factor of alloy grains after appropriate reheating treatment.

Cook-chill 파전의 관능성 및 안전성 평가

The purposes of this study were to compare the effect of several reheating treatments (heating in the frying pan, convection oven and microwave oven) on sensory characteristics and to evaluate the safety during storage period of cook/chill Pajeon. The sensory evaluations were made on 5 sensory attributes by a 9-member panel using quantitative descriptive analysis (QDA). The fresh cooked Pajeon and the Pajeon reheated in the frying pan obtained a significantly (p for 1 day were not different from that of fresh cooked one. However, the scores of the reheated ones in a convection oven and in a microwave oven decreased with storage time up to 5 days at . On the other hand, the Pajeon reheated in the frying pan, even after 3 days` storage at , was not found to be inferior to the freshed cook one in every quality attributes except flavor. Therefore, the reheating treatment in frying pan may be superior to those in a convection oven and a microwave oven. The safety of Pajeon was also evaluated by measuring total count, coliform count, psychrotrophic count, acid value and peroxide value during 5 days of storage periods at . Total counts of Pajeon was ranged from not detectable to CFU/g. The coliform and psychrotroph were not detected at all experiments. The acid values were ranged from 1.90 to 4.03 mg of KOH/g of fat until 5 days at . And the peroxide values were ranged from 3.63 to 12.50 meq of peroxide/kg of fat until 5 days of storage period. Therefore, these results demonstrated that Pajeon is microbiologically and chemically safe during 5 days of storage period at refligeration temperature.

Effect of macrostructure and microstructure on the viscosity of the A356 alloy in the semi-solid state

An A356 alloy was processed from liquid state in different conditions to obtain globular as-cast structures instead of the conventional dendritic ones. These structures were produced in a semi-continuous casting system coupled to an electromagnetic power source. This device allows the variation of the intensity of the stirring by controlling the electromagnetic field, as well as of the cooling rate by controlling the casting velocity. Different cast conditions were used: the alloy was poured at 600°C and left to solidify under the action of a 600, 900 and 1200W electromagnetic field. Cast materials produced in these conditions are suitable for thixo-forming processes after a short re-heating treatment to complete spheroidisation of the primary phase. The generated structures were characterised by macrostructure (grain size) and microstructure (globule size and shape factor). The smaller the grain size, the smaller the globule size and shape factor. The Rheocast Quality Index ((RQI=globule size/grain size×shape factor)) was used to evaluate the morphological evolution during the partial melting treatment necessary to complete spheroidisation of the primary phase. Viscosity of thixotropic slurries obtained by re-heating the cast materials at 580°C for times of 0, 30, 90, 210 and 600s, was also determined by hot compress test. A comparison with a commercial ALTHIX®67S is described.

Strain-induced precipitation of NbC in Nb and Nb–Ti microalloyed HSLA steels

The precipitation start time ( P s) of strain-induced NbC carbides is delayed in Nb–Ti steel in comparison to the case of Nb steel. The delay of precipitation of strain-induced NbC carbides is attributed to both the insufficient solution of Nb during a reheating treatment, and the heterogeneous nucleation of (Nb,Ti)C carbides.

Chemical twinning in the Ti4C2S2-TiS system

Titanium carbosulfide Ti4C2S2 (H phase, P63/mmc, a=0.321 nm and c= 1.120) are TiS (Rm, a=0.3417 nm and c = 2.65) are frequently found to coexist within composite particle in ultra-low carbon interstitial-free (IF) steels. This more suggest that a direct transformation from TiS to Ti4C2S2 may be a major stabilization of C at higher temperatures in these steels. The deduced orient between Ti4C2S2 and TiS is in agreement with a reported crystal structure mode transformation mechanism between these two phases appears to be strongly related structures.At the as-cast condition, most of the S- and C-rich particles in the studied IF step as pure M4C2S2 (M=Ti, Nb). After a re-heating treatment at 1300°C, M4C2S2 discharge and a new phase were observed. This new phase can be indexed as a trigonated with a=0.33 nm and c=5.3 nm (Figure 1). After a second re-heating treatment appeared at the center layer of the multi-phase particles (Figure 2). It should be noted that (i) some planar defects were observed inside the new phase and Ti4C2S2 carbon and 2a).

Dependence of strength–ductility characteristics on thermal history in lowcarbon, 5 wt-%Mn steels

As a result of investigations into the improvement of the strength–ductility characteristics of some C–Mn steels, it has been found that the strength–ductility combination (expressed numerically as tensile strength × uniform elongation) of as hot rolled 0·1C–5Mn steels, with or without 2%Si, is greatly improved if an optimum reheating treatment in the range 650–675°C is applied to the steel. Since there is a direct connection with a large increase in the work hardening rate, it is very likely that this improvement is the result of the transformation induced plasticity of well stabilised austenite which has been formed by the reheating treatment.MST/798

Proceedings of the Seminar on “Refractories for Reheating and Heat Treatment Furnaces”

Proceedings of the Seminar on “Refractories for Reheating and Heat Treatment Furnaces”