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Title: Development of materials with improved resistance to molten metal attack in hot-dip zinc coating lines for strip steel products
Author: Brunnock, M. S.
Awarding Body: University of Wales Swansea
Current Institution: Swansea University
Date of Award: 1998
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A study has been carried out of the resistance to molten zinc of a series of candidate materials. The object was to identify those that might be capable of increasing the service life of hot-dip galvanising bath hardware rolls. The testing involved immersing samples in a molten zinc alloy, representative of that used in commercial operations, at 455° and 480°C for periods up to 360 hours. For some programmes of tests the samples were rotated during immersion. The candidate materials ranged from uncoated low carbon steel, commercial and experimental stainless steels and cobalt-base superalloys to some of the same products covered with a variety of non-metallic and ceramic coatings. Although an Fe-18%Cr experimental alloy had poorer resistance to liquid zinc than low carbon steel, the further addition of 9%Ni to produce an experimental Type 304 stainless steel generated better liquid zinc resistance than low carbon steel. It is suggested that structure rather than compositional effects play an important part in this. Microstructures containing significant amounts of martensite appear to be particularly susceptible to zinc attack. There is, however, a compositional effect from silicon which, when varied in experimental cases of a Type 316 stainless steel, showed the same ability to influence coating microstructure as in carbon steels. The effectiveness of coatings was related to two factors: their inertness and their integrity. Some coatings, for example nitrided and titanium nitrided steel, appeared to be attacked by liquid zinc. Tungsten carbide/cobalt were also attacked unless an inert sealant had been applied. Ceramic oxide coatings consisting of a mixture of fine particles of alumina, chromia and silica proved highly effective in countering zinc attack. However discontinuities in inert coatings arising from the manufacturing process or from microsfissuring during periods of sharp temperature change promoted opportunities for zinc attack and undercutting of the coating. An unexpected phenomenon was observed on some semi-immersed samples. Liquid zinc attack occurred progressively above the liquid zinc meniscus to the formation of thick intermetallic layers. In some cases these layers engulfed the sample and sample holder. The phenomenon has been termed 'supermeniscus intermetallic climb' (SMIC).
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (D.Eng.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available