Z.D. Xiang

Halide Activators Suitable for Low Temperature Pack Aluminisation of Electroplated Nickel on Creep Resistant Ferritic Steels

This study aims to identify suitable activators for pack aluminising electroplated Ni in order to provide processing flexibility for forming Ni-aluminide/Ni hybrid coatings on creep resistant ferritic steels at temperatures below 700 °C. Experiments are made using NH4Cl, AlCl3 (anhydrous), NaCl and NH4F activated packs prepared from Al and Al2O3 powders. Both NH4Cl and AlCl3 (anhydrous) are found to be suitable activators. NaCl may be used as an activator if the amount of Al added is sufficiently high. But, the fluoride salts cannot be used as activators in the low temperature range concerned here due to their strong tendency to deposit AlF3.

Air and Wet Air Oxidation of 9Cr-3Co-3W Creep Resistant Ferritic Steel at 650 °C

The new experimental creep resistant ferritic steel of the 9Cr-3Co-3W type was oxidised at 650 °C in air and wet air. The oxidation kinetics was measured by intermittent weight measurement. The scales formed were analysed using techniques of XRD, SEM and EDS. The results showed that the oxidation rate was more than a magnitude faster in wet air than in air. The oxidation kinetics in air obeyed the parabolic rate law of oxidation only in a limited oxidation period of up to 1726 h whereas it did not follow any power rate law of oxidation in wet air. The steel cannot form a protective Cr2O3 scale either in air or in wet air at 650 °C. Instead, the scale formed in air consisted of an outer (Fe0.6Cr0.4)2O3 layer and an inner Cr-rich (Fe,Cr)2O3 layer containing Cr2O3 particles, but in wet air it consisted of an outer Fe3O4 layer and an inner (Fe,Cr)3O4 layer.

Alloy Elements Added in Fe-6.5wt% Si Silicon Steel and their Effect on Ductility and Magnetic Properties

Fe-6.5wt%Si silicon steel has excellent soft magnetic properties, but its ductility in room temperature is near zero. A lot of researchers tried to improve the ductility by adding alloy elements. In this paper, we summarized the action mechanism and the content of these alloy elements according to the overseas and domestic research status. Al, B, Cr, RE, et al. improved the ductility of Fe-6.5wt%Si silicon steel in low temperature, but reduced its magnetic properties simultaneously. Ni, Ti, Mn, et al. improved the ductility of Fe-6.5wt%Si silicon steel inconspicuously.

Minimum Al Content for Forming Alumina Scale on Creep Resistant Ferritic Steels at 650 °C

This study was carried out to determine the minimum Al content needed to form an Al2O3 scale on creep resistant ferritic steels at 650 °C. Two steels differing mainly in Al content were oxidized in air at 650 °C for 3000 h. One of the steels contained 2.3 wt% Al and the other 1.9 wt% Al. Oxidation resistance of the two steels was also compared with that of the commercial P92 steel at the same temperature. The oxidation was monitored by weight gain measurement. XRD, SEM and EDS techniques were used to analyze the scale formed on the surface of the steels. For the steel containing 2.3 wt% Al, a continuous Al2O3 scale was observed after 3000 h of oxidation and growth of the scale was parabolic with an extremely low rate constant of 0.00058 mg cm-2 h-1/2. For the steel containing 1.9 wt% Al, however, only a non-protective scale was formed, which exhibited a layer structure that consisted of an outermost porous Fe2O3 layer, followed by a relatively dense intermixed Fe2O3 and FeCr2O4 inner layer and then by an internal oxidation layer containing voids, Al2O3 and un-reacted metal particles in addition to Fe and Cr oxides; growth of this type of non-protective scale followed the logarithmic kinetics Δmt = klln(αt + 1) for oxidation times up to 3000 h.

A Feasibility Study on Increasing Surface Hardness of Austenitic Stainless Steels by Pack Co-Deposition of N and Cr

This study is an attempt to codeposit N and Cr into the surface of austenitic stainless steels by pack cementation process to simultaneously increase their surface hardness and corrosion resistance. The pack powders were prepared using Cr2N powder as a source of both N and Cr, NH4Cl as activator and Al2O3 as inert filler. Specimens of the AISI204 austenitic stainless steel were treated in the 2 wt% NH4Cl activated 15Cr2N-85Al2O3 (wt%) pack at 1100 °C for different times. It was demonstrated that a top Cr2N layer with a Cr enriched zone underneath can be formed on the steel surface via the vapour phase generated in the activated powder pack. The effect of adding Cr powder into the pack powders on the surface layer formation and on the hardness profile at the cross-section of the specimen surface was also investigated. Hardness values of more than 1800 HV were obtained at the outermost surface of the treated specimen.

Effect of Pack Al Content on Growth Kinetics at 650 °C of Ni2Al3 Coating Layer on Nickel-Electroplated Creep Resistant Ferritic Steels

The hybrid Ni2Al3/Ni coating was formed on creep resistant ferritic steels by firstly nickel electroplating and then partially aluminising the Ni layer at 650 °C by pack cementation process using powder mixtures of Al, AlCl3 and Al2O3. The effect of pack Al content (W) on growth kinetics of the outer Ni2Al3 layer of the coating was investigated by varying it from 2 to 10 wt% whilst keeping the pack AlCl3 content constant at 2 wt% and aluminising conditions at 650°C/4h. It was revealed that, once W was above a minimum level, the growth of the outer Ni2Al3 layer thickness depended linearly on W1/2. The possible reasons for such growth kinetics were discussed.

Enhancing Steam Oxidation Resistance of Ferritic Steels through Formation of Nickel Aluminide Diffusion Coatings at Low Temperatures

A nickel aluminide coating was formed on P92 steel substrate using a two step process of electro-Ni plating followed by pack aluminising at 650 °C. The coating was tested in 100% steam to assess its resistance against steam oxidation at 650 °C using a purpose-built rig for steam oxidation tests. The data obtained were compared with those measured from air oxidation test at the same temperature. It was revealed that steam is a more severe oxidising environment than air for the coating. Oxidation kinetics and degradation mechanisms affecting the lifetime of the coating were discussed.