Metal dusting

This introductory review paper summarizes shortly the research on metal dusting, conducted in the MPI for Iron Research during the last dozen years. Metal dusting is a disintegration of metals and alloys to a dust of graphite and metal particles, occurring in carburizing atmospheres at a_C > 1 and caused by the tendency to graphite formation. The cause of destruction is inward growth of graphite planes into the metal phase, or in the case of iron and low alloy steels into cementite formed as an intermediate. The kinetics of metal dusting on iron and steels was elucidated concerning dependencies on time, temperature and partial pressures. High alloy steels and Ni‐base alloys are attacked through defects in the oxide scale which leads to pitting and outgrowth of coke protrusions, after initial internal formation of stable carbides M₂₃C₆, M₇C₃ and MC. A dense oxide layer prevents metal dusting, but formation of a protective Cr‐rich scale must be favored by a fine‐grain microstructure and/or surface deformation, providing fast diffusion paths for Cr. Additional protection is possible by sulfur from the atmosphere, since sulfur adsorbs on metal surfaces and suppresses carburization. Sulfur also interrupts the metal dusting mechanism on iron and steels, causing slow cementite growth. Under conditions where no sulfur addition is possible, the use of high Cr Nickelbase‐alloys is recommended, they are largely protected by an oxide scale and if metal dusting takes place, its rate is much slower than on steels.     

Improvements in reliability and serviceability of cemented carbides with wear-resistant coatings

Typical defects in microstructure at a surface-coating interface in chemical vapor deposition coated cemented carbides are described. Results on applying new technologies developed to improve reliability and serviceability of coated cemented carbides are presented. Using WC-based cemented carbide substrates with a uniform, fine microstructure etched before coating eliminates various defects at the surface-coating interface. This results in improvement in serviceability and reliability of the coated cemented carbides in metal-cutting.     

Influence of Laser Welding on the Alumina Growth on a Thin FeCrAl-RE Foil at High Temperature

We study isothermal oxidation of laser welded FeCrAl-RE samples containing specific fractions of seams in a bead-on-plate configuration at approximately 900°C using thermogravimetric analysis (TGA), field emission scanning electron microscope (FEG-SEM), transmission electron microscope (TEM), electron probe microanalysis (EPMA) techniques. An important reduction in the alumina-growth rate over the fusion zone compared to the base material occurs at 900°C, thereby, suppressing the discontinuous increase in mass gain commonly observed for alumina-forming alloys when the temperature decreases from 1000°C to 900°C. This phenomenon is mainly related to the concomitant dramatic chromium carbide precipitation at the fusion zone/oxide film interface and possible earlier injection of the rare earth elements into the oxide layer. On one hand, chromium carbide precipitation, which is linked to the laser melting-induced high free carbon, contributes to improve the effectiveness of the diffusion barrier provided by the thermally growing scale. On the other hand, due to their initial high enrichment at the fusion zone surfaces, rare earth elements can penetrate in the oxide layer and promote the elimination of detrimental phase transformation of metastable platelets (γ,θ-Al₂O₃) to α-Al₂O₃ during the initial stages of oxidation.     

4Cr5MoSiV1热作模具钢热疲劳中碳化物粗化动力学分析

用透射电子显微分析了4Cr5MoSiV1钢热疲劳后的显微结构变化，结果表明，在热疲劳过程中M23C6碳化物粒子发生粗化，M23C6的尺寸r和热疲劳循环次数间并不服从简单的指数关系。表明经典理论不完全适合M23C6碳化物在热疲劳过程中的粗化行为。从经典扩散理论出发，考虑多组元扩散问题，得到了球状碳化物在热疲劳过程中的粗化方程。根据Thermo-calc热力学计算结果，进一步证实了富Cr的M23C6碳化物在热疲劳过程中为易粗化碳化物，与实验结果吻合。     

API X80管线钢异常碳化物形成机理

针对API X80管线钢在生产实际中遇到的落锤性能较低的问题进行了分析。结果表明,该管线钢在落锤撕裂试样断口处存在大量的异常球形析出物,这些析出物为Mo、Ti、Ni等合金元素的碳化物,尺寸约为几个微米,不但削弱了这些合金元素在钢中的弥散强化作用,而且降低了管线钢的落锤撕裂性能。结合热处理实验,探讨了这些异常长大碳化物的生成机理,即钢板的冷却速度影响碳的扩散,并最终对碳化物的形成产生决定性的作用。减少析出物较好的办法是终轧后空冷至碳化物形成温区的上限附近,然后再强制水冷,保证钢板快速通过碳化物形成区间,从而抑制碳化物的异常长大行为。     

粗晶钨粉碳化过程中粒子间的烧结现象

采用Fsss粒度分别为9μm和21μm的两种钨粉在相同的工艺条件下进行高温碳化,利用扫描电子显微镜、金相显微镜等对样品进行了分析。结果表明,高温碳化过程中会发生粒子间的烧结,碳化温度越高,粒子间的烧结越厉害;钨粉粒度越细越容易烧结;粒度较小的钨粉高温碳化烧结后所得的WC晶粒尺寸虽然会明显增大,但是晶粒度的均匀性下降。     

Characterizations of tungsten carbide as a non-Pt counter electrode in dye-sensitized solar cells

Tungsten carbide particles as a counter electrode for dye-sensitized solar cells are prepared by heating layered tungsten oxides at 700, 800, and 900 °C. In particular, the sample heated at 800 °C reveals dominant WC structure and relatively high specific surface area. The improved photoelectrochemical properties of the sample prepared at 800 °C may be due to both catalytic activity of WC phase and high active sites for iodide reduction in comparison with other electrodes.     

WC—Co硬质合金热处理强化机理作用的研究

本文探讨了WC-15wt%Co硬质合金的力学性能与热处理工艺的关系。并以X射线衍射和透射电子显微镜为主要手段,研究了合金在烧结后的冷却或淬火中粘结相由面心立方β—Co→密排六方α-Co转变的马氏体相变过程。分析了热处理前后WC硬质相的表层宏观应力及其对合金性能的影响。比较了合金在热处理态和常规烧结态下显微结构的差异。结果表明,粘结相面心立方β-Co含量的升高;WC相表层宏观压应力的增加以及WC晶粒间邻接度的减少是该合金热处理的主要强化机理。     

Effect of different reinforcements on composite-strengthening in aluminium

In the development of metal-matrix composites, reinforcements of aluminium and its alloys with ceramic materials has been pursued with keen interest for quite sometime now. However, a systematic comparison of the effect of different reinforcements in powder-processed aluminium and its alloys is not freely available in the published literature. This study examines the influence of SiC, TiC, TiB₂ and B₄C on the modulus and strength of pure aluminium. B₄C appears slightly superior as a reinforcement when comparing the effect of SiC, TiC, B₄C and TiB₂ on specific modulus and specific strength values of composites. However, TiC appears to be a more effective reinforcement, yielding the best modulus and strength values among those considered in this study. The differences in thermal expansion characteristics between aluminium and the reinforcements do not seem to explain this observation. The other advantage of TiC is that it is economically a more viable candidate as compared to B₄C and TiB₂ for reinforcing aluminium alloys. It is suggested that the superior effect of TiC as a reinforcement is probably related to the high integrity of the bond at the Al-TiC interface.     

Modelling of metal-binder migration during liquid-phase sintering of graded cemented carbides

Sintering of cemented carbides with graded metal binder content is modelled in this article. Production of graded cemented carbides by traditional liquid-phase sintering is hampered by high rate of liquid-binder migration with inevitable subsequent homogenisation of binder content in specimens. The driving forces of the migration are analysed. It is shown that the migration is governed not only by capillary forces but also by additional stresses that arise due to surface tension at solid–liquid interfaces. As a result, the migration takes place even in pore-free graded specimens. A numerical approach for the prediction of binder flow with coupled deformation of the refractory skeleton during liquid-phase sintering is put forward. Calculations predict that complete densification of specimens with retention of graded-binder distribution can be achieved by sintering with long solid-state sintering stage ended by comparatively short high temperature liquid-phase stage.