Papers by Keyword: Pack Cementation

Abstract: A simple metallurgical process for fabricating oxide nano-rod array structures via internal oxidation is described. Some dilute alloys such as Ni(Al) and Fe(Al) solid solutions develop rod-like oxide precipitates after their internal oxidation at high-temperatures and under low oxygen partial pressures. The oxide nano-rod array structure can be developed on the metal substrate by removing the metallic matrix of the internal oxidation zone. Al₂O₃ or MAl₂O₄ (M=Ni or Fe) spinel nano-rod array structures were prepared by using M(Al) solid solutions. Pack cementation process to develop M(Al) solid solution surface layers was used for the fabrication of nano-rod array structures on substrates with desired shape. Near-net shape Ni substrates with oxide nano-rod array structures on their surfaces can be prepared by using pack cementation and internal oxidation.

Abstract: The oxidation resistance of ferritic-martensitic 9% chromium steels in water vapour containing atmospheres is not yet satisfactory. The chromia layer provides little protection because water vapour in the atmosphere is known to promote the formation of the volatile chromium species CrO₂(OH)₂. If a chromium manganese spinel is formed instead, the vapour pressure of the oxy-hydroxide is greatly reduced and evaporation can largely be avoided. Enrichment of the substrate with manganese was achieved using three different processes: using (i) a sputtering technique, (ii) electrochemical deposition both followed by a diffusion heat treatment, (iii) the pack cementation method. Uniform diffusion of manganese was obtained with all of the investigated processes. The improved oxidation behaviour of the coated samples in synthetic air with 10% water vapour at 650°C was demonstrated.

Abstract: 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.

Abstract: In order to improve the oxidation resistance of silicide coatings on an Nb silicide based alloy, Si-B co-deposition coatings were prepared at 1150°C for 8 h by using pack mixtures containing different amounts of B powders. The microstructural characteristics of the coatings were studied. The results showed that all coatings had a triple-layer structure. The middle layers of all coatings were composed of (Nb,X)Si₂ (X represents Ti, Cr and Hf elements) and the inner layers of all coatings were composed of (Nb,X)₅Si₃, but the outer layers of the coatings had different phase constituents and morphologies when the amount of B powders in the pack mixture changed. When the amount of B powders in the pack mixture ranged from 0.5 wt% to 12 wt%, the outer layers were composed of (Nb,X)B_2­, with a few CrB₂ particles. When the amount of B powders in the pack mixture was in the range of 0.05~0.1 wt%, the outer layers were composed of (Nb,X)B₂. However, when the amount of B powders in the pack mixture was in the range of 0.02~0.03 wt%, the outer layers were composed of (Nb,X)Si₂, with needle-like (Nb,X)B₂ particles dispersed in it.

Abstract: An energy-saving and high-efficient powder Al and Si codeposition process was employed on 1045 steel, the new technology was mainly based on applying direct current field (DCF) with proper parameters between the treated samples and the powder. The microstructure, thickness and the phase constituents of the coating were analyzed by OM and XRD. The high temperature oxidation resistance for a total duration of 120h and the corrosion resistance in 10% H₂SO₄ for 8h were investigated. The results showed that DCF could lower heating temperature and accelerate the diffusion rate. The phase of the coatings with DCF was composed of Al_0.7Fe₃Si_0.3 and AlFe_3. The treated samples had good high temperature oxidation resistance and corrosion resistance in 10% H₂SO₄ solution.

Abstract: A low-temperature silicon pack cementation has been developed by applying direct current field (DCF) between the treated sample and the powder agents. The cross-sectional microstructures, coating thickness and high temperature oxidation resistance after siliconizing under different processes were investigated. The results show that DCF can significantly reduce the treating temperature and heating duration comparing with the conventional silicon pack cementation, the heating temperature could be lowered to 750°С from higher than 1050°С normally used. When the treating temperature was 800°С with appropriate intensity of DCF, the siliconizing coatings with thickness around 40μm was produced, while at this temperature, almost no coating could be formed by conventional silicon pack cementation，and the coating rapidly formed at lower temperature had good oxidation resistance below 700°С .

Abstract: In the present work the phases of the zinc coatings deposited with hot-dip galvanizing, pack cementation and wire flame spraying are examined with Scanning Electron Microscopy and Transmission Electron Microscopy. The different phases which are observed are identified with the combined results of electron and X-Ray diffraction. From the results it is concluded that pack cementation coatings are consisted by two different layers while hot dip galvanized coatings are composed by the same phases and additionally two extra phases of the Fe-Zn phase diagram. Flame sprayed coatings are composed by pure zinc, in the form of thin lamellae, together with nanocrystaline zinc oxide which is formed from the oxidation of liquid metallic droplets during the spray procedure.

Abstract: Samples of 316L austenitic steel were submitted to a thermochemical treatment which implies surface diffusion of Al and Ti. The technique of pack cementation with NH4Cl as activator was employed. The powder mixture was made of aluminium, titanium, aluminium oxide and ammonium chloride. The same ratio of Al : Ti = 1 : 5 was used in all experiments. The variables were temperature and time. As a function of these parameters, diffusion layers of different thicknesses were obtained. The samples were analyzed by optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDX), X-ray diffraction and Vickers microhardness trials. All layers were formed by diffusion with reaction and present two zones with different structures and compositions and therefore different properties. The Ti3NiAl2N compound was identified by X-ray diffraction. The presence of this compound in the diffusion coatings increases the superficial hardness of the samples.

Abstract: Magnesium alloys have been received an attention for structural applications due to their low density compared to other alloys, and intensive studies have been focused for enhancing mechanical strength and surface protection as well. Especially, for environmental reasonings, coating processes in a dry condition have been recently received a great attention. In this study, diffusion coatings via Al powders with an aluminum chloride activator have been investigated in order to examine surface protection effect on magnesium alloys. The commercial AZ31 magnesium alloy has been subjected to diffusion coatings in an Al alloy powder for various time frames. An intermediate layer of Mg17Ag12 was successfully synthesized via diffusion annealing. The underlying mechanisms for surface layer formation are discussed together with growth kinetics and microstructural observations