Key Engineering Materials Vol. 815

Abstract: This study combines spectroscope diagnostics of the plasma jet under APS and the characterization of sprayed coatings by using nanoparticles yttria partially stabilized zirconia (YSZ) powder. The absolute intensities of ArⅠneutral species spectrum lines were used to estimate the electron excited temperature of the thermal plasma jets by the Boltzmann method. The effects of the detection distances, current intensities and H₂ flow rates on electron temperature were analyzed. The plasma sprayed YSZ coatings were heated to 1200°C for 6 min under the atmosphere, and then the micro-structure of the nanostructured powder and coatings were analyzed by using Field Emission Scanning Electron Microscope. The results showed that the increase of input power and H₂ content considerably increased the electron temperature, while the electron temperature decreased a lot with the increase of the detection distance. The coatings were consisted of the equiaxed grains and a small-size grain shape pores.

Abstract: Chemical vapor transport deposition (CVTD) is an effective method for preparing large tungsten coatings for space thermionic reactors. In this study, a high-density, high-work-function polycrystalline tungsten coating was prepared using a WCl₆ transport agent in a concentric tube-type closed transport system. The relationship between the kinetics and the microstructures of the CVTD polycrystalline tungsten coating at the substrate temperature of 1593 K-1793 K and system pressure of 15.93 Pa-106.8 Pa was studied, which provided a basis for the preparation of high-quality tungsten coatings. At a low temperature or a low pressure, the activation energy was approximately 2 kJ/mol, the deposition rate was almost independent of the temperature changes, and the control mechanism was mass transport limited. The tungsten coating had nodules on the surface with pores in the grain boundaries and grew preferentially along <111>. At a high temperature and a high pressure, the apparent activation energy was approximately 90 kJ/mol, the value of order was approximately 1, and the control mechanism in this process range was surface limited. The tungsten coating exhibited a hexagonal pyramidal structure, and the growth direction was preferred along <110>. The average work function of the tungsten coating prepared at a temperature of 1673 K and a system pressure of 106.80 Pa was as high as 5.20 eV.

Abstract: In order to improve the wear resistance of H13 steel, a layer of cobalt-based cladding layer was deposited on the surface of H13 steel by plasma transfer arc welding technology. High-temperature wear test was carried out on H13 steel and cladding layer under 300N loading force, and the two materials were ground at different temperatures with 300M steel. The experimental results show that under 300N loading force, the wear resistance of the cladding layer and H13 steel decreases first and then increases with the increase of temperature, which is related to the softening and oxidation of the material. At 350°C,the material softens and the oxide layer fails to entirely cover the wear surface, so the wear resistance of the cladding layer and H13 steel is lowered. At 500°C and 650°C, the wear surface is covered by a dense oxide layer, which protects the surface of the material from direct wear. The higher the temperature is, the thicker the oxide layer is and the better the protection effect is. At various experimental temperature, the wear resistance of the cladding layer is better than that of H13 steel. The surfacing of a cobalt-based cladding layer on the surface of H13 can improve the wear resistance of H13 steel.

Abstract: This paper studies the atmospheric corrosion characteristics of grid metal frame equipment in Chongqing. Through the standard field test method of atmospheric corrosion -"exposure" method, this study carried out the substation site hanging test of the Q₂₃₅ steel of the power transmission and transformation engineering structural material in the atmospheric environment, and mastered the corrosion data of Q₂₃₅ steel in different corrosion stages. It was found that the morphology, quantity and characteristics of corrosion products on the metal surface varied greatly with the progress of corrosion. According to the metal corrosion morphology of different corrosion time, combined with image processing technology and wavelet transform algorithm, the parameters such as gray mean M, corrosion standard deviation σ_m, corrosion energy E, and energy percentage of wavelet image coefficient were selected as corrosion characteristic variable. At the same time, the BP neural network algorithm was used to qualitatively evaluate the corrosion state of the electrical equipment metal. By testing the on-site samples of the two substations, the corrosion state values of the samples were 0.946 and 0.8071, respectively, which is consistent with the actual corrosion degree, and the system had a good evaluation result.

Abstract: Hot deformation behavior of as-cast 300M steel was investigated in the temperature range of 850-1200°C and strain rate range of 0.01-10 s^-1 using Gleeble-3800 thermo-mechanical simulator. Based on the true stress-strain curves corrected for friction, flow stress behavior and deformation mechanism were analyzed, and the constitutive model of as-cast 300M steel was established based on the Arrhenius model and Zener-Hollomon parameter (Z). The microstructure after deformation was observed by Olympus GX51 microscope. The experimental results show that the flow stress of as-cast 300M steel decrease with the increase of deformation temperature and the decline of strain rate. The dynamic recrystallization is more likely to occur at higher temperatures and lower strain rates. By regression analysis, the hot deformation activation energy (Q) of the as-cast 300M steel was calculated to be 360.332 kJ/mol. Microstructure evolution is greatly affected by deformation temperature and strain rate. The dynamic recrystallized grain size increases with the enhancement of deformation temperature and the decrease of strain rate.

Abstract: The spheroidization mechanism from different initial microstructures during spheroidizing heat treatment was studied in Fe-0.68C-2.33Mn alloy. Two types of initial microstructures, i.e. pearlite and martensite, were obtained by varying the cooling rate. The microstructure and property evolution during spheroidizing annealing was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The DICTRA software, assuming local equilibrium conditions, was used to simulate the carburizing process of different initial microstructures through different cooling rate. The results indicate that the spheroidization mechanism of cementite was related to the initial microstructures and the smaller lamellar spacing of pearlite inhibited the coarsening of cementite, resulting in the size of cementite smaller than that of martensite as the initial structure.

Abstract: In this paper, the properties of the base metal of the low-alloy high-strength steel 20MnTiB, the welding process and the microstructure and properties of the welded joints were studied. The results are as follows: post-heat treatment below 400°C, the strength change of the steel decreases slowly, the elongation does not change significantly, and the metallographic structure is not obvious. When the temperature is above 400, the strength is greatly reduced. And its plasticity increases remarkably, and precipitates on the grain boundary are precipitated and grown on the metallographic structure. When the line energy is in the range of 9.6~12.0kJ/cm, the mechanical properties and microstructure of the welded joints meet the requirements, and the welding process that meets the requirements is studied. Finally, the mechanical properties and microstructure of the welded joint are studied. Provide a reference for the research and application of steel.

Abstract: A new heat source model consisted of inverted conical heat source and rotary Gauss body heat source is established using the CAE software for the keyhole effect of laser-TIG hybrid welding. The inverted conical heat source is used for analyzing the wide upper part of weld pool due to the rapid heat up by the laser and arc. The rotary Gauss body heat source model is used for analyzing the long and narrow lower part of weld pool formed by the laser. The result showed that, compared with other single source mode, this new heat source model may get a better simulation of the weld pool morphology, especially the inflection point near the keyhole. It provides a new method to predict the morphology and size of the weld pool of magnesium alloy laser-TIG welding.

Abstract: The casting process design of large cast steel supports is carried out, and the special integrated sand core and forged steel cast lugs are used to simplify the cavity manufacturing process. The ProCAST software was used to simulate the casting process of the stent, simulating the filling and solidification of the casting, and predicting the occurrence of defects such as shrinkage and shrinkage of the casting. According to the simulation results, the cause of the defects is analyzed, and the casting process of the stent is optimized. The simulation results show that the optimization scheme effectively reduces the casting defects and the surface of the stent is free from defects.

Abstract: In order to study the effect of UHMWPE fiber laminate on effective protection area of ceramic composite target, a dynamic analysis software ANSYS/LS-DYNA was used to calculate the effective protection area of ceramic/aluminum/aluminum composite target and ceramic/UHMWPE/aluminum/ aluminum composite target, both of whose areal density were 139.5kg/m², against 14.5mm armor piercing projectile at the speed of 1000m/s. The result showed that UHMWPE fiber laminates will increase the effective protection area by 64.4% for ceramic composite targets under the same areal density.