Quantitative Analysis of Work Hardening and Dynamic Softening Behavior of low carbon alloy Steel Based on the Flow Stress

In this study, the constitutive equation and DRX(Dynamic recrystallization) model of Nuclear Pressure Vessel Material 20MnNiMo steel were established to study the work hardening and dynamic softening behavior based on the flow behavior, which was investigated by hot compression experiment at temperature of 950 °C, 1050 °C, 1150 °C and 1250 °C with strain rate of 0.01 s⁻¹, 0.1 s⁻¹ and 10 s⁻¹ on a thermo-mechanical simulator THE RMECMASTOR-Z. The critical conditions for the occurence of dynamic recrystallization were determined based on the strain hardening rate curves of 20MnNiMo steel. Then the model of volume fraction of DRX was established to analyze the DRX behavior based on flow curves. At last, the strain rate sensitivity and activation volume V^* of 20MnNiMo steel were calculated to discuss the mechanisms of work hardening and dynamic softening during the hot forming process. The results show that the volume fraction of DRX is lower with the higher value of Z (Zener–Hollomon parameter), which indicated that the DRX fraction curves can accurately predicte the DRX behavior of 20MnNiMo steel. The storage and annihilation of dislocation at off-equilibrium saturation situation is the main reason that the strain has significant effects on SRS(Strain rate sensitivity) at the low strain rate of 0.01 s⁻¹ and 0.1 s⁻¹. While, the effects of temperature on the SRS are caused by the uniformity of microstructure distribution. And the cross-slip caused by dislocation piled up which beyond the grain boundaries or obstacles is related to the low activation volume under the high Z deformation conditions. Otherwise, the coarsening of DRX grains is the main reason for the high activation volume at low Z under the same strain conditions.     

Waste recycling of coal fly ash: A novel approach to prepare hierarchically porous coal fly ash/Al2O3 ceramic composite with high porosity and high strength templated by emulsion-assisted self-assembly

The coal fly ash (CFA) produced from coal-fired power generation is classified as a common solid waste; thus, improving the recovery and utilization rate of CFA is highly desirable. In this study, a novel strategy using CFA and Al₂O₃ as raw materials, to prepare hierarchically porous ceramic composites that serve as potential candidates for future building materials is developed. In this process, the well-developed self-assembly method in which an anionic modifier is used to prepare hydrophobic powders that form an attractive oil/water network via electrostatic interactions, thereby yielding honeycomb-like structures. In order to explore the mechanism of preparation, five samples with different mixture ratios of alumina and CFA were prepared according to 1: 0, 2: 1, 1: 1, 1: 2, and 0: 1 (Alumina: CFA). Compared with the sample prepared with pure CFA, the as-prepared CFA/Al₂O₃ composite exhibited both superior porosity and high mechanical property. When the porosity is as high as 73 ± 0.17%, the compressive strength is as high as 80.9 ± 3.4mpa (alumina: CFA = 1:1). As the porosity decreases to 49.3 ± 0.7%, the compressive strength reaches 159.33 ± 36.89mpa (alumina: CFA = 1:2). Moreover, this work obtains the highest compressive strength-porosity related B-value in comparison to previously reported CFA-based composites and provides a new insight into the effective recycling of CFA and offers a novel approach to prepare CFA/Al₂O₃ composite with excellent overall mechanical properties.     

Effects of temperature and strain rate on microstructure and mechanical properties of high chromium cast iron/low carbon steel bimetal prepared by hot diffusion-compression bonding

The objective of this study is to develop a hot diffusion-compression bonding process for cladding low carbon steel (LCS) to high chromium cast iron (HCCI) in solid-state. The influence of temperature (950–1150 °C) and strain rate (0.001–1 s⁻¹) on microstructure, hardness and bond strength of the HCCI/LCS bimetal were investigated. The interface microstructure reveals that the unbonded region can only be found for 950 °C due to lack of diffusion, while the intergrowth between the constituent metals occurred at and above 1100 °C. When bonding temperature increases to 1150 °C, a carbide-free zone was observed near the interface on the HCCI layer, and the thickness of the zone decreases with an increase of bonding strain rate. These evolutions indicate that the bond quality was improved by raising temperature and reducing strain rate due to the increase of element diffusion. The hot compression process of the bonding treatment not only changes the carbide orientation of the HCCI, but also increases the volume fraction of Cr–carbide. Based on the microstructural examinations and mechanical tests, the optimum bonding temperature and bonding strain rate are determined to be 1150 °C and 0.001 s⁻¹, respectively.     

Effect of denucleating glass composition on undercooling of Fe83Ga17 alloy melts

Adopting glass fluxing combined with superheating cycling method, the undercooling and its stability of Fe₈₃Ga₁₇ alloy melts were investigated using different kinds of denucleating glass: B₂O₃, 90% NaSiCa + 10% B₂O₃ (simplified as Na–Si–Ca–Al–B) and 70% Na–Si–Ca–Al–B + 30% Na₂B₇O₄. The results showed that different glass has different denucleating mechanism. The purification of B₂O₃ glass is only a physical process, by which the stable bulk undercooling cannot be obtained during superheating–cooling cycles. While taking Na–Si–Ca–Al–B glass as purifying agent, its denucleating mechanism is a comprehensively physicochemical process. But the stability of undercooling is still undesirable because of the separation between melt and glass during cooling process in superheating cycling. A stable bulk undercooling can be obtained by physicochemical denucleating process in the case of 70% Na–Si–Ca–Al–B + 30% Na₂B₇O₄ molten glass owing to its suitable viscosity.     

Slag resistance mechanism of MgO–Mg2SiO4–SiC–C refractories containing porous multiphase aggregates

The slag resistance of MgO–SiC–C (MSC) refractories should be improved because of the mismatch in the thermal expansion coefficient between the aggregates and matrix, as well as the defects caused by the affinity between periclase and slag. In this study, MgO–Mg₂SiO₄–SiC–C (MMSC) refractories were prepared using porous multiphase MgO–Mg₂SiO₄ (M-M₂S) aggregates to replace dense fused magnesia aggregates. Compared to MSC, the slag penetration index of MMSC decreased by 43.5%. The structure of the porous aggregates increased the surface roughness, and the multiphase composition of the aggregates decreased the mismatch of the thermal expansion coefficient with the matrix, thus reducing debonding between the aggregates and matrix. The aggregates and matrix in the MMSC formed an interlocking structure, which bound them more tightly to improve the slag resistance. The slag viscosity at different depths from the initial slag/refractory interface was calculated using the Ribond model. The M-M₂S aggregates increased Si_xO_y^z− in the slag, which increased the slag polymerization and slag viscosity. The aggregates and matrix in the MMSC reacted with the slag to form high melting point phases, which reduced the channel of the slag. In addition, the penetration depth and velocity derived from the Washburn Equation were modified for the CaO–SiO₂–Al₂O₃–MgO–FeO slag and magnesia based refractory to accurately evaluate slag penetration.     

稀薄氨气的燃烧特性及动力学研究

文章通过稀薄氨气在固定床反应器中的燃烧,研究了反应温度、停留时间、氨气浓度和氧气浓度对低浓度氨气燃烧特性的影响,并描述了氨气在氧气过量条件下在陶瓷蜂窝蓄热体中燃烧的动力学过程。研究结果表明:提高反应温度、延长停留时间以及增大氧气浓度和氨气浓度均可以提高NH3转化率,氧气浓度过高会促进NO生成;当反应温度为740~770℃、氨气浓度为1%、氧气浓度为15%时,氨气在陶瓷蓄热体中燃烧的活化能为253.56 kJ/mol;与氨气在自由空间内的燃烧相比,氨气在陶瓷蜂窝蓄热体中主要发生表面燃烧反应。     

时效处理对铸轧7050铝合金组织和性能的影响

采用透射电镜、X射线衍射、硬度测试等实验方法,研究了不同时效处理工艺对铸轧7050铝合金组织和性能的影响。结果表明：随着单级时效温度的升高,合金达到峰值硬度所需的时间缩短,合金的峰值硬度降低,晶内和晶界析出相也随温度的提高发生不同程度的长大;双级时效时,晶内有较多粗大的析出相,晶界析出相已不再连续且已经发生长大,晶界无沉淀带更加明显;回归再时效处理时,晶内析出相与单级时效时相比发生粗化,晶界上为断续的粗大长条状析出物,无偏析带加宽。     

基于电脉冲技术抑制连铸水口结瘤堵塞的研究

 浇注过程中连铸水口表面存在自生电场,由此所引起的电润湿、电化学反应等界面作用严重影响水口结瘤堵塞。在连铸平台进行了工业试验,研究结果表明无电场处理的水口内腔结瘤物厚度可达10 mm,吐钢孔堵塞严重;电脉冲处理后的水口内腔平滑,吐钢孔无明显结瘤物。电脉冲处理可减弱或消除水口表面自生电场引起的界面作用,从而有效抑制水口结瘤,延长水口使用寿命。     

Syntheses and characterizations of two novel Ln(III)–Cu(II) coordination polymers constructed by Pyridine-2,4-dicarboxylate ligand

Two novel three dimension Ln(III)–Cu(II) coordination polymers [Gd₂Cu(pydc)₄(H₂O)₆]_n and [Sm₂Cu₃(pydc)₆(H₂O)₆]_n were prepared by the hydrothermal reactions of CuO, Ln₂O₃ (Ln = Gd, Sm), H₂pydc (H₂pydc = 2,4-pyridinedicarboxylic acid) and characterized by single-crystal X-ray diffraction analysis.