Influence of the Preheat Treatment on the Microstructure and Properties of X37CrMoV5-1 Steel

The technological properties of the hot-work tool steels depend on their microstructural quality induced by the production process and the heat treatment. One of the frequently applied methods to improve microstructure is preheat treatment. The treatment of carbonitriding X37CrMoV5-1 steel has been investigated in fluidized bed furnaces. Thickness and phase composition have been conducted using optical microscopy Neophot 32 and scanning microscopy, JEOL 5400, after etching in nital.     

热轧卷温度场模拟在CSP生产HP295钢相变分析中的应用

采用有限元模拟软件Ansys，模拟了3mmHP295气瓶钢热轧卷温度场的变化，作出不同节点的温降曲线，并与该钢连续冷却转变(CCT)曲线结合，分析了HP295钢卷材不同部分冷却过程的组织转变。模拟结果表明，卷取温度为660-680℃时卷材心部平均冷却速度仅为0．04℃/s，远远低于出现碳化物离异共析的临界冷却速度0.1℃/s，使卷材深冲性能急剧降低。避免碳化物离异共析谚碳体沿铁索体晶界分布的有效措施是将卷取温度降至600～620℃。     

Niobium-containing catalysts—the state of the art

This review article is devoted to the materials containing niobium, which have been discovered or developed in the past few years and exhibit the potential application in heterogeneous catalysis. Niobium oxides and mixed oxides as well as sulfides, nitrides (oxynitrides), carbides (oxycarbides), and phosphates are considered. Among the catalytic processes in which Nb-containing materials were tested, liquid and gas phase oxidation is described in details, and the role of niobium in the prevention of the catalyst from SO₂ poisoning is mentioned.     

Oxidation of ultrahigh temperature ceramics: kinetics,mechanisms, and applications

Materials capable of oxidizing in a protective manner at ultrahigh (>1700 °C) temperatures are needed to push beyond this barrier defined by SiC. Although possessing attractive mechanical properties and oxidation resistance, SiC-based materials are ultimately temperature limited by the melting point of SiO₂. The vast array of ultra-high and high temperature ceramic literature indicates the majority of these materials, like borides, carbides, MAX-phases, and high-entropy ceramics, fall woefully short regarding oxidation resistance. However, for specific applications, like low-orbit aeropropulsion, high ballistics coefficient atmospheric re-entry, and hypersonic cruise, there are a few promising materials. In the present review, oxidation criteria are gathered to build application specific heuristics and are then applied to a multitude of ultra-high temperature ceramics to gauge material efficacy. Discussion of oxidation kinetics, mechanisms and reaction products is offered for each material, identifying strengths, weaknesses, and the remaining gaps in our knowledge.     

Effect of the starting materials on the reaction synthesis of Ti3SiC2

Ternary carbide of titanium and silicon was produced via mechanical milling and following heat treatment. Effects of the starting materials, milling time and heat treatment temperature were studied. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were utilized to evaluate the structural and morphological evolutions of the ball-milled and annealed powders. Results showed that the ball milling of TiO–Si–C as the starting materials failed to synthesize Ti₃SiC₂. Additionally, ball milling the elemental powders for shorter milling times resulted in the activation of the powders. However, after longer milling times, Ti–TiC nanocomposite was obtained. Furthermore, during annealing the milled powders, Ti₃SiC₂–TiC nanocomposite with the mean grain size of 16 nm was synthesized. After 20 h of milling, a very fine microstructure with narrow size of distribution and spheroid particles was achieved.     

Boron-modified phenol formaldehyde resin-based self-healing matrix for Cf/SiBOC composites

C_f/SiBOC was fabricated from 2D carbon fabric as reinforcement and slurry-containing boron-modified phenol formaldehyde (BPF) resin with silicon as matrix resin using reaction-bonded silicon carbide method. The processing involves synthesis of (BPF) resin by reacting various amount of boric acid with phenol formaldehyde resin, polymer to ceramic transformation at 1450°C under argon atmosphere, with and without silicon, thermal transformation of the polymer matrix composite into a ceramic matrix composite and evaluation of isothermal oxidation for ceramics and its composites at 1000, 1250 and 1500°C. The ceramic studies, confirmed the formation of B₄C, SiC and SiB₄ (SiBOC) mixed phase and the role of boron as a catalyst for graphitisation of free carbon present in the ceramic. Oxidation of C_f/SiBOC composite at various temperatures leads to the formation of borosilicate glass which heals the cracks, hindering the inwards diffusion of oxygen.     

Fusion welding of refractory metals and ZrB2-SiC-ZrC ceramics

Molybdenum and a molybdenum alloy were fusion welded to ZrB₂-based ceramics to determine if the electrical and thermal properties of the metals and ceramics affected their weldability. Commercial ceramic powders were hot pressed, machined into coupons, and preheated to 1600 °C before joining the ceramics to commercial metals using plasma arc welding. Weldability varied as indicated by the range of porosity observed within the fusion zones. Measured thermal and electrical properties appeared to have little to no effect on the weldability of metal-ceramic welds despite the large range of values measured across each property. Differences in melting temperatures between metal and ceramic coupons did affect weldability by changing the weld penetration depth into ceramic coupons. Future studies on metal-ceramic welds are suggested to investigate the effect that work function, melt viscosity, wetting, or other properties have on weldability.     

Fabrication of magnesium based composites reinforced with carbon nanotubes having superior mechanical properties

Magnesium (Mg) composite reinforced with carbon nanotubes (CNTs) having superior mechanical properties was fabricated using both pure Mg and AZ61 Mg alloy matrix in this study. The composites were produced via powder metallurgy route containing wet process using isopropyl alcohol (IPA) based zwitterionic surfactant solution with unbundled CNTs. The produced composites were evaluated with tensile test and Vickers hardness test and analyzed by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM) equipped with energy dispersive spectroscopy (EDS) and electron back scattered diffraction (EBSD). As a result, only with AZ61 Mg alloy matrix, tensile strength of the composite was improved. In situ formed Al₂MgC₂ compounds at the interface between Mg matrix and CNTs effectively reinforced the interfacial bonding and enabled tensile loading transfer from the Mg matrix to nanotubes. Furthermore, it was clarified that the microstructures and grain orientations of the composite matrix were not significantly influenced by CNT addition.     

Microstructural analysis of fusion and heat affected zones in electron beam welded ALLVAC 718PLUS™ superalloy

The fusion zone and heat affected zone (HAZ) microstructures of electron beam welded superalloy 718PLUS™ (718 Plus) that has been newly developed by ATI ALLVAC were examined. The microsegregation pattern during solidification of the fusion zone indicated that while Fe, Co, W, and Cr segregated to the core of the gamma dendrites, Nb, Ti, and Al were extensively rejected into the interdendritic liquid. Electron diffraction and X-ray microanalysis using transmission electron microscopy (TEM) of the fusion zone showed that the major secondary phases that formed from the interdendritic liquid were gamma/MC type carbide eutectic and gamma/Laves eutectic constituents. HAZ microstructure showed partially melted zone immediately adjacent to the fusion zone and intergranular microfissuring associated with resolidified products which suggested that HAZ cracking in this alloy occurred by liquation cracking. Microstructural examination of the HAZ using analytical scanning electron microscope showed resolidified gamma/Laves eutectic on the cracked and backfilled grain boundaries. Fine resolidified MC type carbide particles were also observed in the HAZ. Causes of grain boundary liquation were identified and the solidification of intergranular liquid in the HAZ was discussed.