Abstract
Effects of various factors on the formation, structure and state of the diffusion layer obtained by heating Aluminium-sprayed iron and steel, were studied by microscopic observations. There are a number of factors responsible for the formation of the diffusion layer, among which were discussed the temperature and time of annealing, thickness, porosity and adhesiveness of the sprayed layer, and the carbon content and pre-treatment of the base metal.
Some preliminary experiments were made on the usefulness of sealers, together with alternative availability of differently processed sprayed layers. From these results, some fundamental surveys on high temperature behaviors of the diffusion layer are given; hence, conditions for high temperature services of the diffusion-annealed coatings were discussed.
Results are summarized as follows:
1) Structure and state of the diffusion layer are essentially dependent on the temperature and time of annealing and thickness of the sprayed layer, although other factors have to do with the diffusion-anneal to some extent.
2) Porosity and poor adhesion of the sprayed layer are the fatal defects for the production of highly resistant coatings, since they affect the uniformity of the diffusion layer, resulting in partial exposure of the base metal.
3) Optimum conditions for the diffusion-anneal were determined from the practical point of view. The most satisfactory diffusion layer is obtained when 0.3mm thick Aluminium-sprayed layer is heated at 900°C for 30min.
4) Use of sealar is advantageous when the thickness of the Aluminium-sprayed layer is less than 0.2mm.
