Search

Abstract Comprehensive understanding of continuous cooling transformation behaviors is important in heat treatment of steels, but there is still a lack of research in the phase transition behaviors of 300M steel, which would greatly hinder the microstructure controlling and finally affect the application of this material. In this work, in-situ observations by high temperature confocal laser scanning microscope (HTCLSM) was introduced by considering its superiorities in clarity, accuracy, and directness, to systemically investigate the microstructure evolutions of 300M steel under various cooling rates (0.01–100 °C/s). Pearlite, bainite, and martensite were observed to form at the cooling rate range of 0.01–0.15, 0.03–1, and 0.3–100 °C/s, respectively. A continuous cooling transformation diagram was constructed based on the in-situ observations, verified by metallography, and compared with dilatometry. Results showed that the transition temperatures by in-situ observations agreed well with the results of dilatometry, while the transition starting temperatures by in situ observation were lower and the phase transition termination temperatures were higher. Finally, models accurately describing the relationships between Vickers hardness, retained austenite content, and cooling rates were established, and the phase transformation mechanisms and kinetics were analyzed. Our finding not only understands fundamentally the transformation mechanisms of different microstructures, but also provides a useful reference for practical microstructure control in heat treatment of 300M steel. Highlights • Phase transitions at various cooling rates are in-situ observed. • The continuous cooling transformation diagram of 300M steel is constructed. • The result by in-situ observation is verified and compared with traditional methods. • A new transformation mechanism of pearlite is proposed. [ABSTRACT FROM AUTHOR]

Abstract: The synthesis and antibacterial activity of oxazolidinones containing dihydro-1,2-oxazine and 2-pyrazoline ring systems are described. Linezolid analogs utilizing dihydro-1,2-oxazines as morpholine mimics were prepared utilizing a nitrosoamine/diene 4+2 cycloaddition strategy. Pyrazolidine, hexahydro-pyridazine, and 2-pyrazoline analogs more closely related to eperezolid were also prepared. The most active of these new oxazolidinones were the dihydro-1,2-oxazine 6 and the 2-pyrazoline 20 both of which had potency similar to linezolid against a panel of Gram-positive bacteria. [Copyright &y& Elsevier]

In this study, the influences of heat treatment on microstructure evolution and mechanical properties of directed energy deposition-Arc (DED-Arc) 300M steel are investigated. The results show that the original columnar grains are transformed into the equiaxed grains with the increased austenitizing temperature. The spherical cementite and rod-shaped ε -carbides in the as-deposited microstructure are completely dissolved into the matrix above 970 °C, which increases the carbon supersaturation in the martensite and facilitates the dispersive precipitation of fine ε -carbides. Meanwhile, the anisotropy of tensile properties resulting from the DED-Arc process is significantly weakened with the increased austenitizing temperature. The high microhardness and excellent tensile properties are obtained using the austenitizing temperature of 970 °C with the microhardness of 563 ± 12 HV, ultimate tensile strength of 1942 MPa, yield strength of 1707 MPa and elongation of 12.6%, which exceeds AMS6417K standard of 300M steel forgings. The main strengthening mechanism is ascribed to the reinforced solid solution strengthening effect caused by the carbon supersaturation and the coherent dispersion strengthening effect of fine ε -carbides. Based on the findings, the improved heat treatment procedure is proposed for DED-Arc 300M steel. [Display omitted] • The effects of heat treatment on microstructure and mechanical properties of DED-Arc 300M steel are studied. • An improved heat treatment procedure is proposed for DED-Arc 300M steel. • The mechanical properties of DED-Arc 300M steel under the improved heat treatment exceed AMS6417K forging standards. • The microstructure-property relationships and strengthening mechanisms of DED-Arc 300M steel are clarified. [ABSTRACT FROM AUTHOR]

Abstract: Vibrational loading was introduced as an innovative method to improve the thermoplastic formability of Zr35Ti30Be26.75Cu8.25 bulk metallic glass in a supercooled liquid state. The tensile strain, as a measure to characterize the formability, increased with increasing loading frequency, indicating a vibrational loading facilitated formability. The physical mechanism of this phenomenon was rationalized on the basis of both theoretical analysis and finite-element-method simulation. The theoretical analysis revealed that a more homogeneous distribution of flowing units with a smaller volume, together with a larger free volume concentration, existed in the specimen under relatively higher loading frequencies. The finite-element-method simulation combined with the free volume constitutive relation exhibited an increase in free volume concentration with increasing loading frequency, in agreement with theoretical analysis. Finally, compressive and hot-embossing tests under vibrational loading were carried out to further verify the applicability of this technique. The present results not only provide an effective method to facilitate the thermoplastic formability of bulk metallic glasses, especially in micro/nanoscale forming, but also offer a better understanding of the structural evolution of the metallic supercooled liquid under vibrational loading. [Copyright &y& Elsevier]

[Display omitted] • Methods to control the surface roughness during polishing process are analyzed. • Autocorrelation function of the polished surface is investigated. • Areal roughness S a can be reduced to 0.81 ± 0.01 μm from 8.67 ± 0.12 μm. • Possible changes of chemical composition and nano-hardness are discussed. Industrial products used in medical equipment and food manufacture are usually made from AISI 304 and place great demands on surface polishing. The application of plasma beam in the field of energy beam polishing is a new technology to obtain a desired level of surface roughness. Therefore, this paper details an investigation conducted to focus on the effect of process parameters on the surface morphology of AISI 304 stainless steel during plasma beam polishing. The main arc current, nozzle height, scanning speed and plasma gas flux that have the greatest influence on the arc plasma characteristics and energy input values are considered to be the key experimental parameters. While the measured roughness values and their intensity autocorrelation functions are applied to evaluate the polished surfaces. The capacity of plasma beam polishing to produce glossy and defect-free surfaces on AISI 304 has been successfully verified. Under the specific combination of main arc current, nozzle height and scanning speed, the energy input could reach approx. 90 J/mm ∼ 110 J/mm, corresponding to the more than 85.9% fall in areal roughness S a. Although the plasma gas flux had no discernible impact on the energy input, it was observed that the surface uniformity varied from 0.81 ± 0.01 μm S a to 1.04 ± 0.04 μm S a with the increase of plasma gas flux. Possible changes of chemical composition and nano-hardness during plasma beam polishing of AISI 304 stainless steel are analyzed. [ABSTRACT FROM AUTHOR]

Energy beam polishing technologies have revolutionised the metal surface precision finishing procedure of various materials owing to their specific capability to provide non-contact, selective, and automated polishing processes; further, these technologies help enhance the mechanical properties of the materials. However, the inadequate understanding of the fundamental mechanism of the energy beam finishing process may limit the practical application of such technologies. Furthermore, to cope with the rapid development in various technologies associated with energy beam surface finishing, there is a strong requirement for researchers to highlight the current obstacles and predict promising trends. In this review paper, the main benefits and characteristics of energy beam surface finishing are first summarised. The various parameters that influence the energy beam finishing results are then discussed. For different metallic materials, the experiment parameters are not very identical; therefore, the discussions will be conducted in a sequence. The surface roughness level that can be obtained for the assigned material is considered a key aspect for evaluating the finishing effect, and the variation in mechanical property is considered as a supplement. We also review the fundamental evolution of the energy beam finishing models, existing applications in various fields, and limitations that stand in the way of satisfying industry requirements. The paper will be concluded by introducing the future development trends in energy beam surface finishing technologies, for instance, the large-area and short-time finishing, which play a decisive role on whether the current research can be applied for factory production and provide economic benefits. Image 10 • The fundamental aspects of the energy beam polishing mechanisms are introduced. • The key developments of the energy beam polishing technology are enumerated. • The innovation and optimisation of energy beam finishing model are discussed. • The existing applications of energy beam polishing in various fields are reviewed. • The future trends of energy beam metal surface treatment are prospected. [ABSTRACT FROM AUTHOR]

• Plasma beam multi-track polishing can reduce the surface roughness of AISI 304. • S a decreased from 8.67 ± 0.12 μm to 0.79 ± 0.06 μm representing a 90.9% reduction. • A two-dimension transient model was developed to simulate the polishing process. • Modelling results verify that the offset distance affects the molten pool flow. The adoption of high energy beam in the area of metallic surface polishing is a novel technique to produce glossy surfaces. In particularly, plasma beam finishing is emerging as a workable and economical option to realize high quality irradiated surfaces. In this paper, effect of overlap between the adjacent tracks on surface morphology during plasma beam finishing was researched through using different offset distances, and AISI 304 austenitic stainless steel was selected to be the test material. Based on considering surface roughness, certain statistic features (e.g. material ratio function and power spectral density function) were analyzed to comprehensively reflect the influence of offset distance. To aid a deeper understanding of the molten pool flow and surface evolution during polishing process, a two-dimension transient model, which consists of magnetic field, electric field, laminar flow and heat transfer, was developed by means of the finite element approach. The multi-track polishing tests showed that although the irradiated surface became smoother, significant bulges were found in the overlapped region between adjacent tracks. The simulation model then verified that these bulges varied with different offset distances, reflecting different dynamic processes. Therefore, the selection of offset distance is important for the optimization of plasma beam polishing. [ABSTRACT FROM AUTHOR]

The effects of linear heat input (LHI) on the microstructure and corrosion behavior of an austenitic stainless steel processed by wire arc additive manufacturing (WAAM) are studied. It is found that the primary dendrites spacing of the studied steel increases from 18 μm to 23 μm, when the LHI increases from 219 J ⋅ mm - 1 to 590 J ⋅ mm - 1 . But the δ-ferrite contents under high and low LHIs are similar. Moreover, the passivation voltage range decreases and the corrosion current density increases with the increase of LHI. The poor corrosion resistance for the studied steel under high LHI is highly related with the segregation of Cr and Mo atoms in the Ni-poor δ-ferrite. • The effect of linear heat input on a stainless steel processed by WAAM is studied. • The primary dendrites grow quickly during the WAAM under high linear heat input. • The poor corrosion property is related with the segregation of Cr/Mo in δ-ferrite. [ABSTRACT FROM AUTHOR]

The hot tensile deformation and fracture behaviors of a typical ultrahigh strength steel are investigated by isothermal uniaxial tensile experiments over the tensile temperatures of 900–1150 °C and strain rates of 0.01–10 s - 1 . By combining the results of tensile tests and microstructural observation, the effects of deformation processing parameter (strain rate and tensile temperature) on the macroscopic plastic deformation behaviors, microscopic fracture morphologies and microstructural evolution are clarified. The studies indicate that the hot tensile behavior of the studied steel is greatly influenced by the competition of damage evolution, work hardening, and dynamic softening. With the increased strain rate, the strain rate sensitivity coefficients decrease, but the value of elongation to fracture increase from 0.38 to 0.50. The variations of microvoids or cracks in amounts and sizes show an increasing trend with the increased strain rate or the decreased tensile temperature. At the low tensile temperature (900 °C), the poor capability of necking diffusion hardly prevents the strain localization, and the mesoscopic damages easily take place. As the strain rate increases from 0.01 to 10 s - 1 , the neat fracture morphologies gradually change to the serrated. The main ductile fracture mechanisms transform from the intergranular fracture at the high tensile temperature and low strain rate to the transgranular fracture at the low tensile temperature and high strain rate. • The hot tensile deformation and fracture behaviors of a typical ultrahigh strength steel are studied. • With the increased strain rate, the strain rate sensitivity coefficient decreases, but the elongation to fracture increases. • The main fracture mechanisms transform from the intergranular to transgranular fracture with the decreased tensile temperature. [ABSTRACT FROM AUTHOR]