Preparation of aluminide coatings is used to increase the heat resistance. The coating must be frequently applied to details that were previously welded. The description of the structure of the coating in the area of the welded joint can be important when choosing the proper welding techniques. Butt joints were made using tungsten inert gas, arc welding — the method 141. The main differences between the joints are a kind of parent material and production technology (welding with or without a filler material). Parent materials for making the joins are 1.4749 (X18CrN28) and 1.4404 (X2CrNiMo17–12–2) steels. On prepared samples with joints the silicon-aluminide coating by the slurry method were produced. Samples covered by the slurry were annealed in a furnace with a protective atmosphere of argon at two temperatures 800 and 1000°C for 2 hours. To characterize the structure of the coatings electron microscopy, SEM and EDS X-ray microanalysis were used. It was found that the coatings were formed on the whole test surface. There are strong similarities between the structure of coatings produced in a given temperature despite the use of different substrates and various welded joints. Generally, the coatings produced at a temperature of 800°C are characterized by a three-layer structure, while those at 1000°C have two layers. The thickness of the coating produced at 800°C is from a range of 40 to 65 μm depending on the substrate. The thickness of coatings annealed at 1000°C is in the range of 100 to 200 μm. It is noted that the thickness of the coatings on the parent material is in any case higher than on the weld.
Słowa kluczowe: aluminide coating, slurry method, alloy steel, welded joint
Keywords: warstwy aluminidkowe, metoda zawiesinowa, stale stopowe, połączenia spawane.
1. INTRODUCTION Preparation of aluminide coatings is used to increase the heat resistance of Fe and Ni based alloys. The iron and nickel aluminides improve the resistance to many aggressive environments [1÷4] including oxidising [5÷10], sulphidising  and carburising [12, 13] agents. The method of the coating formation should be easy to apply on complex shaped parts and the cost should be as low as possible. One of these methods is the slurry cementation. Commercially available slurries manufactured on the base of aluminium powder (as an active component) and of the solution of an organic binder are widely used . The slurry method used by the authors is a modification of the classical one. The slurry used in this modified method consists of a metal powder (aluminium and silicon), a binding material (the aqueous solution of sodium silicate) and halides as a flux. The aqueous solution of sodium silicate (inorganic binder) in the slurry allowed for annealing in one single step without additional heating when the organic binder is used. The main parameters determining the aluminide coating properties and usefulness are its chemical and phase composition and the thickness. The chemical composition of the aluminide coatings is characterized mainly by the aluminum content of these coatings. The thickness assessment and the analysis of the aluminum content of the coating are of primary importance for testing coating quality and protective properties . The majority of final products working under a high temperature regime like equipment of furnaces for thermochemical treatment is made of high-alloyed steels and welding is the basic technology of joining of these materials. From a technological point of view, the first operation should be welding followed by manufacturing of a protective coating. Therefore, the characterization of the coating in the area of the welded joint can be important when choosing the proper welding [...]
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