Simultaneous Aluminizing and Chromizing of Steels to Form (Fe,Cr)3Al Coatings

A cementation pack involving halide activatorsand elemental Al and Cr powders has been used to achievethe codeposition and diffusion of aluminum and chromiuminto low-alloy steels. A two-step treatment at 925°C and 1150°C yields dense anduniform ferrite coatings of about 400-m thickness,with surface compositions of approximatelyFe₃Al plus several percent Cr. The two stepheating schedule prevents the formation of a blocking chromium carbide atthe substrate surface. An attempt was made to add atrace of Ce to the Al + Cr content of the coating byintroducing Ce oxide into the pack, but there is no evidence that this doping was achieved. Uponcyclic oxidation in air at 700°C, the coated steelexhibits a negligible 0.085 mg/cm² weightgain for 1900 one-hour cycles. Virtually no attack wasobserved on coated steels tested in a simulated boileratmosphere at 500°C for 500 hr. But coatings with asurface composition of only 8 wt.% Al and 6 wt.% Crsuffered limited sulfidation attack in the simulated boiler atmosphere at temperatures higher than500°C for 1000 hr.     

Pack formation and long term oxidation kinetics of TiAl3 coating on γ-TiAl

Abstract

The aim of this study is to produce microcracking free TiAl₃ coatings on γ-TiAl alloy by the pack cementation process and to determine the long-term oxidation kinetics and thermal stability of the coating at high temperatures. It was shown that microcracking free coatings could be prepared in the AlCl₃ activated packs containing 4 wt-%Al as the depositing source. The conditions required for the formation of a microcracking free coating are discussed in relation to the pack chemistry at high temperatures. The TiAl₃ coatings formed were oxidised in air for more than 6200 h, during which weight gains were measured at regular intervals. The major oxide in the scale was Al₂O₃. It was observed that a TiAl₂ phase zone formed in the subsurface of the scale as a result of preferential oxidation of Al in the TiAl₃ coating. It was found that a linear relationship existed between the weight gain and logarithm of time of oxidation at 800°C: Δm _t = k _lln(αt+ 1). The thermal stability of the coating was assessed by measuring the growth kinetics of the TiAl₂ interlayer at the boundary between the TiAl₃ coating and substrate, which was determined to be d^1·4 = 0·1t (d and t in μm and h respectively) at 800°C using the experimental data measured over a diffusion annealing period of more than 6200 h.     

高清时代后期制作的新选择——大洋POST PACK后期制作软件套装

随着高清时代的到来,电视节目无论从视觉上还是听觉上都有了比以往更大的表现空间,这就对电视节目制作的工艺水平提出了更高的要求.本文详细介绍了大洋新一代节目制作软件套装POST PACK的诸多特色,其在统一的平台架构体系中,将非线性剪辑的概念扩展到前所未有的宽广领域,使得剪辑师、调色师、录音师和动画设计师更加亲密无间地合作...     

Formation of nickel aluminide/nickel hybrid coatings on alloy steels by two step process of nickel plating and low temperature pack aluminisation

Abstract

The present study investigates the conditions required for forming a hybrid coating consisting of an outer nickel aluminide layer and an inner nickel layer on alloy steels. A commercial alloy steel of 9Cr–1Mo was used as a substrate. Electroless and electronickel plating processes were used to form an initial nickel layer on the steel. The AlCl₃ activated packs containing pure Al as a depositing source were then used to aluminise the nickel deposit at temperatures ≤650°C. The effect of phosphorus or boron content in the initial nickel layer deposited with the electroless nickel plating solutions using hypophosphite or boron–hydrogen compound as reducing agent was investigated in relation to the spallation tendency of the coating either immediately after the aluminising process or during the thermal annealing post-aluminising process. Under the aluminising conditions used, the outer nickel aluminide layer formed was Ni₂Al₃. For the electroplated nickel deposit, the growth kinetics of the outer Ni₂Al₃ layer during the pack aluminising process was found to obey the parabolic rate law with a parabolic rate constant being 12·67 μm at 650°C for 2 wt-%AlCl₃ activated pack containing 4 wt-% pure Al as a deposition source.     

Ultrasensitive technique for detection of hydrogen emanating from steel and other solid surfaces

Abstract

Measurement of diffusible hydrogen in steel by destructive testing is, in many cases, impractical. A new non-invasive surface measurement technique is presented, whereby hydrogen emanating from a well defined steel surface is captured in a stream of air and analysed in situ. The apparatus embodying the technique is evaluated with respect to critical measurement parameters, indicating, for a high sensitivity version, flux resolvable to <2 pL cm^-2 s^-1 hydrogen at 298 K and 1013 mbar, and linear measurement of flux to 1000 pL cm^-2 s^-1. Flux measurements were obtained under ambient conditions by magnetically attaching the apparatus to one face of coupons expected to contain a low and uniform concentration of hydrogen, and 2 mm steel plate cathodically charged at the opposing face. The results suggested that hydrogen emanated into air at a rate of 50–30 000 pL cm^-2 s^-1, and was controlled by hydrogen diffusion through the steel bulk. The reproducibility and responsivity of the technique is also illustrated.     

泰科电子最新90°安装IGBT模块flow90在变频器中的应用

在变频器的应用中,有些场合对变频器的体积要求很严格,例如纺织机械,要求变频器做成宽度很窄的"书本"式结构。这样就迫使变频器中散热器必须做特殊设计,增加了散热器设计和安装的难度,同时也增加了成本,影响了散热器的散热效果。泰科电子最新推出的flow90功率模块通过模块内部管脚和散热底板90o设计,很好地解决了这个问题。     

基于网络编码的802.11 DCF信道接入机制

提出一种基于网络编码的802.11 DCF信道接入机制(NCA-DCF)。采用冲突避免的主动确认策略,使相应节点形成编码意识,通过编码包的机会转发,提高数据包传输效率,结合无线网络编码的特性,运用马尔科夫链模型对NCA-DCF进行数学建模。分析结果表明,与传统的DCF信道接入机制相比,NCA-DCF不仅可以避免节点发送分组的碰撞,有效提高信道利用率,而且能够明显提高全网吞吐量。     

Low temperature aluminisation of alloy steels by pack cementation process

Abstract

The pack aluminisation process is normally applied at temperatures >973 K at which the mechanical properties of alloy steels would degrade. Thus, the present study was undertaken to apply this process to aluminising the alloy steels at temperatures <973 K in order to increase their high temperature oxidation resistance while maintaining their microstructure and hence mechanical strength and creep resistance. A type of commercial alloy steel P92 (9Cr–1Mo) was used for the present study. Pack powder mixtures consisting of Al, AlCl₃ (anhydrous) or NH₄Cl and Al₂O₃ were used to carry out the process. The aluminising temperature was varied from 773 to 973 K, pack Al content from 1 to 30 wt-% and aluminising time from 1 to 16 h to investigate their effects on the coating growth kinetics in the AlCl₃ activated packs. It was observed that all the coatings formed in the AlCl₃ activated packs were of a single layer structure with Fe₂Al₅ as the main coating phase. It was established that the interrelationship between the thickness h (in μm) of this coating layer and aluminising temperature T (in K), time t (in h) and pack Al content W (in wt-%) can be described by h=83005·9W^1/2t^1/2e^?73330/(RT). In the NH₄Cl activated packs, it was found that coating formation and dissolution took place simultaneously at 923 K and stable growth of a coating layer was only possible when the pack Al content was sufficiently high. However, the coatings formed in these packs had highly uneven regions.     

泰科电子最新90^0安装IGBT模块f1ow90在变频器中的应用

在变频器的应用中，有些场合对变频器的体积安求很严格，例如纺织机械，要求变频器做成宽度很窄的“书本”式结构。这样就迫使变频器中散热器必须做特殊设计，增加了散热器设计和安装的难度，同时也增加了成本，影响了散热器的散热效果。泰科电子最新推出的flow90功率模块通过模块内部管脚和散热底板900设计，很好地解决了这个问题。     

Formation of aluminide coatings on low alloy steels at 650°C by pack cementation process

Abstract

The pack aluminising process is normally conducted on alloy steels at temperatures higher than 900°C at which mechanical properties of steels would degrade. This study aims to investigate the feasibility of pack aluminising a commercial 9Cr – 1Mo alloy steel at 650°C in an attempt to increase its high temperature oxidation and corrosion resistance without adversely affecting its mechanical properties and consequently to increase its long-term structural operating temperatures to up to 700°C. It was demonstrated that this could be achieved using packs containing AlCl₃ as an activator and elemental Al as a depositing source. The coatings formed under these conditions consisted of an outer Fe₁₄ Al₈₆ layer and an inner FeAl₃ layer with an abrupt interface between the coating and substrate, suggesting that the coating is formed via a mechanism of the inward Al reaction – diffusion. The pack Al content was varied from 1 to 6 wt-% to investigate its effects on the coating formation process. It was found that the pack Al content in this range affected only the coating thickness and therefore the growth rate of the coating, but not the surface Al concentration. A post-aluminising heat treatment study was also undertaken for an aluminised specimen at 650°C under an argon atmosphere to investigate the kinetics of converting the brittle Fe₁₄ Al₈₆ and FeAl₃ phase layers to a more ductile FeAl phase layer. It was observed that this was a slow process requiring 1132 h for an initial coating layer thickness of 33μm. The coating after the conversion consisted of a uniform top FeAl layer with all other alloying elements in the solid solution and a diffusion zone underneath.