The Precipitation of Niobium Carbide and Its Influence on the Structure of HT250 for Automobile Wheel Hubs

Improving the strength of grey cast iron wheel hubs will improve the safety of automobiles and have a great significance for energy saving and environmental protection. This paper systematically compares the calculation results of Python-based precipitation calculation and JmatPro software simulation with experiments. The results show that with a low mass fraction of niobium (0.098%) cuboid Niobium Carbide (NbC) precipitates do not form in the liquid phase; however, an elongated NbC niobium-rich phase may form during the solidification process and in the solid phase. However, cuboid NbC precipitates can be precipitated from the liquid phase when the niobium mass fraction is higher (0.27%, 0.46%). These results indicate that with the increasing niobium content the amount, particle size, and initial precipitation temperature of niobium carbide precipitated in the matrix structure will increase. According to the observation and statistical analysis of the microstructure, it is found that tensile strength will be improved with an increase in niobium content due to the refinement of the graphite and pearlite interlamellar spacing. In this paper, adding less than 0.32% of Nb to grey cast iron is recommended, considering the comprehensive cost and the effect of niobium in the material structure.

Highly Enhancing Electrical, Thermal, and Mechanical Properties of Polypropylene/Graphite Intercalation Compound Composites by In Situ Expansion during Melt Mixing

Polypropylene/graphite intercalation compound (PP/GIC) composites are prepared via melt mixing at three different temperatures (180, 200, and 220 °C). The dispersion of GICs in the composites is clearly improved due to the increased interlamellar spacing caused by in situ expansion of GICs at higher temperatures, which facilitates the intercalation of PP molecular chains into the GIC galleries. As a result, the PP/GIC composite with 10 wt% GICs prepared at 220 °C (PG220) presents a dielectric constant of about 1.3 × 108 at 103 Hz, which is about six orders higher than that of the composite prepared at 180 °C (PG180). Moreover, the thermal conductivity of the PG220 sample (0.63 Wm−1K−1) is 61.5% higher than that of the PG180 sample. The well-dispersed GICs accelerates the crystallization of PP by increasing the nucleation point and enhances the thermal stability of the composites. The PG220 sample shows a Young’s modulus that is about 21.2% higher than that of the PG180 samples. The results provide an efficient approach for fabricating polymer/GIC composites without complex exfoliation and dispersion processes.

Numerical Simulation of Wire Rod Cooling in Eutectoid Steel under Forced-Convection

A coupled thermal-microstructural simulation model was developed to estimate the thermal history in a eutectoid steel wire rod under continuous cooling and forced-convection. The model coupled the phenomena of heat transfer, phase transformation and estimation of the cooling boundary condition. The thermal histories were analyzed at different cooling rates to emulate the forced-convection conditions by air-jet as in the controlled cooling conveyor. The thermal histories were acquired and used to calculate the forced-convection heat transfer coefficients through the solution of the Inverse Heat Conduction Problem, while the phase transformation was approximated with the Johnson–Mehl–Avrami–Kolmogorov (JMAK) kinetic model. From the heat transfer coefficients and the kinetic parameters, a user-defined function (UDF) was coded and employed in the ANSYS Fluent® software. The model results were compared and validated with the experimental histories, obtaining a good agreement between both responses, while the microstructural evolution of the pearlite was validated using Scanning Electron Microscopy (SEM) and Vickers microhardness. It was found that specimen diameter and air velocity are the main variables to modify the undercooling and therefore the pearlite interlamellar spacing.

Microstructure Factor of Creep Behavior in Near-α Ti Alloy

Heat-resistant Ti-Al-Nb-Zr alloys, which don’t contain Sn, have been designed to obtain good oxidation resistance above 600 °C. In addition, to design Ti alloys with best balance of creep and fatigue properties, prior β grain size which affects fatigue properties and lamellar microstructure which affects creep properties were controlled by heat treatment. In the present study, the effect of microstructure on creep properties of one of the alloys, i.e., Ti-7.5Al-4Nb-4Zr alloy, with the bimodal (B), the lamellar structures in small prior β grains (LS), and the lamellar in large prior β grains (LL) were investigated at 600 °C. The creep deformation mechanism for each microstructure was a power-law creep. However, the creep life varied depending on the microstructures. The longest creep life was obtained in LS with prior β grain size of 90 μm and interlamellar spacing of approximately 10 μm, while the shortest creep life was obtained in LL with prior β grain size of 550 μm and fine interlamellar spacing of less than 2~3 μm. This suggests that creep life is more affected by interlamellar spacing than by prior β grain size.

Effect of the initial austenitization temperature on the structural microheterogeneity during drawing of carbon steel with a pearlite structure

During wire production, strain fields can be distributed inhomogeneously over the section during drawing and cause structural micro-inhomogeneity, which significantly affects the stability of the process. However, during plastic deformation of carbon steel with a pearlite structure, the interlamellar spacing in the ferrite-carbide mixture and the size of pearlite colonies, which determine the deformation behavior of steel, are of great importance. In addition, in the wire manufacturing technology, heat treatment operations are used with heating the steel to the austenitic state, the temperature of which significantly affects the formation of the structure and properties of the steel. The paper investigates the effect of the austenitization temperature on the structural microheterogeneity of a wire made of carbon steel with a pearlite structure after drawing. The results of studying the microstructure, determining the interlamellar spacing, the anisotropy coefficient of pearlite colonies, as well as the distribution of microhardness over the cross section of the sample during drawing after different temperatures of preliminary austenitization are presented. It is shown that after preliminary austenitization at temperatures of 900, 950 and 1000 °C in a wire made of carbon steel with a pearlite structure, microstructural inhomogeneity in the dispersion of the ferrite-pearlite mixture is observed. It manifests itself as a difference of the interlamellar spacing in pearlite at the surface and in the center of the sample cross section and is retained during subsequent drawing with a total reduction of 8 to 15%. It has been established that the temperature of preliminary austenitization has practically no effect on the anisotropy coeffi cient of pearlite colonies in the initial state after austenitization, and it does not change over the cross section of the sample. However, with subsequent drawing with an increase in the total reduction, the anisotropy coefficient increases, while it increases from the surface to the center of the sample. It is revealed that with an increase in the preliminary austenitization temperature from 900 to 1000 °C, the microstructural inhomogeneity in the drawn wire is manifested to a greater extent, which can be associated with an increase in the grain size of the initial austenite, the size of pearlite colonies, and the interlamellar spacing in pearlite. Microstructural inhomogeneity is confirmed by the nature of the distribution of microhardness over the cross section of the sample. The research was carried out with the financial support of the Russian Foundation for Basic Research and DNT within the framework of the scientific project No. 18-58-45008 IND_a.

Effects of Molybdenum Addition on Rolling Contact Fatigue of Locomotive Wheels under Rolling-Sliding Condition

Rolling contact fatigue (RCF) damages often occur, sometimes even leading to shelling on locomotive wheel treads. In this work, the RCF damage behaviors of two locomotive wheel materials with different molybdenum (Mo) contents were studied, and the influence of depth of wheel material was explored as well. The result indicates that with the increase in the Mo content from 0.01 wt.% (wheel 1, i.e., a standard wheel) to 0.04 wt.% (wheel 2, i.e., an improved wheel), the proeutectoid ferrite content and the interlamellar spacing of pearlite decreased, the depth and length of the RCF cracks increased and the average RCF live of locomotive wheel steel improved by 34.06%. With the increase in the depth of material, the proeutectoid ferrite content and the interlamellar spacing of pearlite increased, the depth of RCF cracks increased, the length of RCF cracks of wheel 1 increased and then decreased whereas that of wheel 2 decreased, the RCF live showed a decrease trend for wheel 1, while the RCF life increased and then decreased for wheel 2. The processes of shelling can be divided into three patterns: cracks propagating back to the surface, crack connection and fragments of surface materials.

Numerical simulation of manufacturing process chain for pearlitic and bainitic steel rails

Computer system for the design of technology of the manufacturing of pearlitic and bainitic rails was presented in this paper. The system consists of the FEM simulation module of thermal–mechanical phenomena and microstructure evolution during hot rolling integrated with the module of phase transformation occurring during cooling. Model parameters were identified based on dilatometric tests. Physical simulations, including Gleeble tests, were used for validation and verification of the models. In the case of pearlitic steels, the process of subsequent immersions of the rail head in the polymer solution was numerically simulated. The objective function in the optimization procedure was composed of minimum interlamellar spacing and maximum hardness. Cooling in the air at a cooling bed was simulated for the bainitic steel rails and mechanical properties were predicted. The obtained results allowed us to formulate technological guidelines for the process of accelerated cooling of rails.