Çelik bantların sıcak daldırma yöntemi ile alüminyum kaplanması

Abstract

Furnace construction industry; Furnace linings, heat reflectors, heat protection grids, baffle plates, combustion chambers, heat exchangers, air preheaters, flue gas pipes. - Building industry; Hot-air shafts, fire walls, pipe insulation for internal use, suction plants. - Apparatus construction; Parts for heating, drying, and roasting installations; parts for refuse incineration facilities. - Tank and container construction; Tanks, containers, cans. - Heaters and cookers/household appliances; Stove linings parts for night current storage heaters, heat radiators, housings and inner elements for baking ovens, grill units, camping grills, toasters, hot-water apparatus. In this study, the effects of the variables of hot- dip aluminizing process on the thickness and microstructure of coating layers were investigated. The variables of the present investigation were bath temperature, holding time, chemical composition of the coating bath and steel substrate. After removal of oil and oxide films from the surfaces by using alkaline and HC1 solution respectively, steel strip specimens (St 12, SAE 1020, AISI 304 and AISI 409) were dipped into three different baths (pure aluminum, 2.70% Si-Al alloy and 6.70% Si-Al alloy) at different temperatures (from 660°C to 820°C) and for different periods of time (from 30 sec to 25 min). The coating layers then were examined by using metal lographic techniques. The results obtained from the experiments were as follows: - When the bath temperature increased from 660 to 740°C, both interfacial alloy layer and aluminum layer thickness increased a little. However, further increase in temperature had no significant effect on the coating layer thickness. - With an increase in holding time from 30 sec to about 15 min, thickness of the overall layer including that of the interfacial alloy layer increased. With further increase in holding time, thickness of overall layer remained almost the same.

Aluminum-coated steel is a special composite material in which the strength and economy of steel and the durability of aluminum are combined. Properties of aluminum coated steel are beneficial för products which require the advantages of good corrosion resistance, bright metal l iç appearance, receptiveness to finishes, high reflectivity and good electrical conductivity. Difficulties were encountered in the last century due to the high chemical reactivity of aluminum. in the twentieth century these problems vere largely overcome and a number of process have been suggested which enable such a coating operation to be carried out. These include batch ör continuous höt dipping, pack diffusion, slurry process, thermal spraying, vacuum ör chemical vapor deposition, ion vapor deposition, electroplating and electrophoresis. Choice of method is determined by coating performance required, intended use of coated product, size and shape of the product, production volume and cost. The most economical for the production of coated sheet and strip, on a continuous basis, is that of hot- dipping. Furthermore, hot-dip process is fast and simple compared to other coating technigûes. So, aluminum coated steel is produced by the hot-dip process on modern high speed continuous coatings lines. Two important factors in successful coating are, proper preparation of steel surface and control of the formation and growth of the intermetallic compound of aluminum and iron that forms at the interface of the aluminum coating and the steel substrate. vi The typical coating l ine utilizes an oxidizing furnace to carbonize any oils on the strip before subsequent reduction of the surface in a hydrogen atmosphere. Oxidizing is carried out in a gas fired furnace at 450-500°C. At this temperature, any oils ör grease on the strip are completely removed. The strip is then exits an annealing furnace operating at 800- 850*C with a reducing atmosphere of cracked ammonia. This unit reduces any oxides present on the strip surface and features an enclosed downtake dipping beneath the level of aluminum in the bath. This downtake ensures that no re-oxidation occurs. The bath itsel f is usually constructed of a steel crucible lined with refractory brickwork. it may contain either aluminum of commercial purity ör aluminum alloyed with silicon which reduces the thickness of the layer of Fe-Al intermetall iç phase. The aluminizing bath is mounted on hydraulic jacks which prevent iron dissolution during plant shut-dovm by lowering the bath so that the strip immersion rol l and its associated hardware are clear of molten aluminum. The overall thickness of the coating may be controlled by the use of wiper rol Is ör air knives. These remove excess aluminum before the strip is cooled and passed the section. There are two distinct grades of hot-dip aluminized steel. The first, Type l, is produced from an aluminum bath alloyed with up to 9% silicon. The silicon reduces the thickness of the iron-aluminum intermetall iç layer thus improving formability. The coating weight of this material is approximately 80-150 g/m2 including both sides (14-25 Mm). Type 2 material is produced from a bath of commercial purity aluminum. The coating thickness on the Type 2 grade is normally in excess of 40 ym (230 g/m2 including both sides). in both cases, about 2-4% of iron is present in the coating alloy. This is iron dissolved from the strip and the coating bath hardvare. The single most important concept in ali metal coating reactions is that of diffusion. This phenomenon can be defined as the thermally activated transport of matter through matter. Aluminum coating of steel is also diffusional controlled reaction. Therefore the rate of thickening of the coating follows a parabol iç law being inversely proportional to its thickness. Thus the empirical equation: x=ktn vii Then truly diffusional controlled reaction kinetics, n should be 0.5. When sol id iron comes in contact with molten aluminum interdiffusion takes place at the contact surface and a diffusion layer is formed in each metal. When iron concentration in aluminum is increased intermetallic compound FeAl3 is formed. The layer reaches a given thickness as the interdiffusion of metals progresses and Fe2Als type compounds appear. Growth of columnar crystals (Fe2Als) take place towards the iron base and iron diffusing through the adjacent layer of FeAla penetrates the aluminum. With further diffusion of iron, Fe2Als changes into FeAl3. As a result of the growth of phase Fe2Als and the increase in diffusion rate of iron in aluminum, the zone of solid solution of aluminum in iron disappears. Thus the diffusion of aluminum in iron is almost exclusively determined by the behavior of phase Fe2Als which appears practically alone during the interaction of these two metals. According to the data, the Fe2Al0 phase possesses an orthorhombic type unit celi. it has also c-axes oriented vertically and situated öne över the another. Lattice positions along the c-axis are occupied exclusively by aluminum atoms. The population density of atoms in these chain structural elements of the lattice may reach 70%. Such a high concentration of voids in mutually parallel detached chain elements of the structure accounts for the increased, selective crystallographic planes. Thus both crystallographic anisotropy of the diffusion rate and the relative high speed interaction between the two metals with the formation of Fe2Al«3 phase can be explained simply. The coating produced by höt dip aluminizing has two layers. A layer of iron-aluminum alloy (intermetallic phase) forms on the steel surface with a layer of aluminum över it. When a deformed area is produced on the aluminized steel the coating along the brittle layer of the intermetallic phase can be easily peeled away, therefore its thickness should be at a minimum. Long term weathering tests have shown that höt- dip aluminized coatings have better corrosion resistance than hot-dip galvanized ör Zn-Al-alloyed coatings. Unalloyed steels can be used at temperatures up to about 550°C but, above this temperature, formation and growth of wustite phase in combınation with scaling occur which can led to complete destruction of steel. viii in the case of hot-dip aluminized sheet, the structure of the coating remains the unchanged and intact after long term temperature exposure up to about 450"C. Only with service temperatures of 450 to 550 °C is growth of an FeAlSi-alloy layer initiated leading ultimately to transformation of the whole coating. As a result of the FeAl-coating formed, the material can be used at temperatures up to 700"C. Hot-dip aluminized steel sheet has a thermal reflection factor of up to 80%. Hot-dip aluminized steel sheet can be processed using conventional sheet matalworking methods. Aluminized steel can be welded using conventional methods. Several fusion welding methods are suitable för use with hot-dip aluminized steel sheet, in particular the TI? (tungsten inert gas) and the plasma welding processes. Spot, roller seam and projection welding are especially suited for joining hot-dip aluminized steel sheet. The high frequency welding process has found wide acceptance for manufacturıng longitudinally welded pipes from hot-dip aluminized steel sheet. The strength of the steel core of hot-dip aluminized steel sheet permits effective fastening with bolts, screws and rivets. Presupposing proper pretreatment and processing conditions, hot-dip aluminized steel sheet has an excellent surface for painting. Hot-dip aluminized steel sheet is suitable for single-layer enamelling. it thus additionally benefits from the advantages offered by enamel, such as mechanical and chemical resistance, decorative appearance, variety of colors, and easy cleaning. Hot-dip aluminized steel sheet is used predominantly for applications where elevated temperatures and corrosion occur simultaneously. Therefore aluminum-coated steel has been a standard material for automotive exhaust systems for about 25 years. Throughout this period, the product has outstanding performance record. Its fields of applications are as follows: - Automotive industry; Silencers, exhaust pipes, fuel tanks, petrol cans, thermal shields. ix - Furnace construction industry; Furnace linings, heat reflectors, heat protection grids, baffle plates, combustion chambers, heat exchangers, air preheaters, flue gas pipes. - Building industry; Hot-air shafts, fire walls, pipe insulation for internal use, suction plants. - Apparatus construction; Parts for heating, drying, and roasting installations; parts for refuse incineration facilities. - Tank and container construction; Tanks, containers, cans. - Heaters and cookers/household appliances; Stove linings parts for night current storage heaters, heat radiators, housings and inner elements for baking ovens, grill units, camping grills, toasters, hot-water apparatus. In this study, the effects of the variables of hot- dip aluminizing process on the thickness and microstructure of coating layers were investigated. The variables of the present investigation were bath temperature, holding time, chemical composition of the coating bath and steel substrate. After removal of oil and oxide films from the surfaces by using alkaline and HC1 solution respectively, steel strip specimens (St 12, SAE 1020, AISI 304 and AISI 409) were dipped into three different baths (pure aluminum, 2.70% Si-Al alloy and 6.70% Si-Al alloy) at different temperatures (from 660°C to 820°C) and for different periods of time (from 30 sec to 25 min). The coating layers then were examined by using metal lographic techniques. The results obtained from the experiments were as follows: - When the bath temperature increased from 660 to 740°C, both interfacial alloy layer and aluminum layer thickness increased a little. However, further increase in temperature had no significant effect on the coating layer thickness. - With an increase in holding time from 30 sec to about 15 min, thickness of the overall layer including that of the interfacial alloy layer increased. With further increase in holding time, thickness of overall layer remained almost the same. in fact it decreased a little due to dissolution of aluminides. Furthermore, a significant amount of aluminides were torn away from the interfacial layer when the layer thickness increased with holding time. - Among the steel specimens used in experiments, AISI 304 had the thinnest interfacial alloy layer. Steel alloys with decreasing alloy layer thickness were as follovs; St 14, SAE 1020, AISI 409. in general, it was found that highly alloyed steel had thinner alloy layer than that of plain carbon steel. - The thickness of the intermediate layer of the intermetallic phase sharply decreased when up to 2.70% Si added in the bath. From the experiments, it was found that further increase in silicon content up to 6.70% had a little effect on the thickness. The appearance of the interface between the substrate and the intermetallic phase showed some changes as well. Aluminizing in commercial purity aluminum bath resulted in a serrated interface while silicon addition gave rise to a planar öne.