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Effect of silicon concentration in bath on the structure and thickness of grey cast iron coating after alphinising

Pietrowski S., Szymczak T.

Archives of Materials Science and Engineering

2007

Vol. 28, nr 7

437-440

Purpose: This paper presents the results of studies on the effect of silicon content in alphinising bath on the thickness of diffusion layer "g1" and of coating "g" produced by immersion on the grey cast iron, grade EN-GJL-250. The temperature of alphinising bath was 750 ± 5°C, and the time of holding the cast iron insert was ζ=180s. Design/methodology/approach: It has been proved that in the bath of commercially pure aluminium and of AlSi1,5 alloy, the diffusion layer and coating produced on grey cast iron are of the highest thickness, comprised in a range of g1=40 ÷ 50µm and g = 220 ÷ 230µm., respectively. The diffusion layer produced in these baths is characterised by a compact, columnar structure of the Al3Fe phase. Findings: An increase in silicon concentration in the alphinising bath to about 11% changes the structure of diffusion layer into the faceted one and makes other phases appear as well. Right on the surface of the cast iron insert, the Fe4CSi carbide is present, followed by the Al12Fe3Si2 phase, on which the Al9Fe3Si2 phase is superimposed. Raising further silicon concentration in bath to a level well above 11% Si, i.e. to about 17% Si, does not change the phase constitution of the diffusion layer. What it causes is only a fragmentation (breaking of continuity) of the layer, thus making it useless in the process of producing a layered casting. An increase in silicon concentration in the alphinising bath in a range of 7 ÷ 17% Si reduces the thickness of diffusion layer in a range of g1=38 ÷ 22µm and that of coating in a range of g=155 ÷ 92 µm. It is supposed that the decrease in thickness of the diffusion layer and of coating with the increasing concentration of silicon in alphinising bath is due to a reduced rate of the reactive diffusion. Originality/value: This is an immediate effect of the enlarged number of silicon atoms and of the presence of the precipitates of primary silicon crystals adhering to the surface of cast iron insert. The reason of occurrence of a monophase structure in the diffusion layer produced in aluminium bath or in bath of AlSi1,5 is the lack of silicon atoms or low concentration of this element in the region of the surface layer dissolved on cast iron insert.

Model of the alphinising coating crystallisation on iron alloys

Pietrowski S., Szymczak T.

Archives of Foundry Engineering

2007

Vol. 7, iss. 3

123-128

The study presents a hypothetical model of crystallisation of the alphinising coating produced on iron alloys by immersion in the bath of silumin. Basing on a wide-range of experiments and investigations, the effect of the type of inserted material ("armco" iron, C45 steel, grey cast iron and nodular graphite cast iron) and of the technological regime of the alphinising process (chemical composition of silumin bath, its temperature, the time of holding an insert in the bath, and the insert surface roughness height "Rz") on the coating structure was determined. The type of the coating structure was established by metallographic examinations carried out by optical microscopy, electron transmission microsopy and scanning electron microscopy, using additionally an X-ray microanalyser and X-ray diffraction patterns. The results of these investigations were described in [1-7]. Basing on the obtained results, a probable model of the crystallisation of an alphinising coating on iron alloys, produced by immersion in the alphinising bath, was developed. It has been stated that, most probably, the alphinising process begins when the insert reaches its contact temperature "ts".. Since that moment, due to the wetting process and convection movement of bath around the insert surface, an intense process of the dissolution starts. A reactive diffusion of the atoms of Fe and Si from the insert to the bath and of the atoms of Al and Si from the bath to the insert takes place. An intermetallic Al3Fe phase is crystallising on the steel, while on the cast iron, a silicon carbide Fe4CSi is growing, probably due to carbon diffusion from graphite. Then, on the steel, as an effect of the peritectic reaction, are successively crystallising the phases of Al12Fe3Si2 and Al9Fe3Si2. The Al3Fe phase probably crystallises on the cast iron to be transformed later, due to peritectic reaction, into an Al12Fe3Si2 phase on which the Al9Fe3Si2 phase will be growing. When the insert is taken out from the bath, on the Al9Fe3Si2 phase are crystallising the phase constituents present in the silu-min bath. It has been proved that the process of crystallisation of the alphinising coating and its thickness are also affected by the roughness height "Rz". There is a critical value of "Rz", different for the steel and cast iron, up to which the thickness of the coating is growing, and which - when exceeded - makes the coating thickness decrease.