Constrained/unconstrained solidification within the massive cast steel/iron ingots
Journal Title: Archiwum Odlewnictwa - Year 2010, Vol 10, Issue 2
Abstract
Some properties of the ingot and especially of the steel forging ingots depend on the ratio of a columnar structure area to an equiaxed structure area created during solidification. The CE transition is fundamental phenomenon that can be applied to characterize massive cast steel ingots produced by the casting house. The mentioned ratio is created spontaneously due to the rate of heat transfer towards the ceramic mould and then to the environment. The ceramic mould operates as an isolator. So that the thickness of the mould together with a growing solid fraction control the heat transfer and finally the ratio of the columnar structure area to the equiaxed structure area. At first the increase of heat accumulation within the ceramic mould is observed. Next the stationary state for heat transfer is created and finally a gentle abatement of the mould temperature associated with the heat output to the environment is expected. The steep thermal gradients correspond to the increase of heat accumulation in the ceramic mould. The steep thermal gradients are required to promote the columnar structure formation. The full heat accumulation in the mould corresponds well with the CE transformation while the appearance of the moderate thermal gradients is referred to the gentle temperature abatement within the ceramic mould. The equiaxed structure is expected within this period of heat transfer behavior. The steep thermal gradients involve the activity of viscosity gradient in the liquid. As the result a sedimentary cones are formed at the bottom of the ingot. The CE transformation is associated with competition between columnar and equaixed structure formation. At the end of competition a fully equiaxed structure is formed. The viscosity gradient is replaced by the thermophoresis which is the driving force for the deposition of some equiaxed grain layers onto the surface of C+E zone. The convection together with the gravity allow the layers to be uniform along the whole height of the ingot. Some equiaxed grain layers are also deposited at the bottom of ingot onto the surface of sedimentary cones due to activity of the gravity. Additionally, some macro-segregation effects are observed in the ingot. There are “A” – type macro-segregation phenomenon and „V” – type macro-segregation phenomenon. Both mentioned phenomena result from the so-called canal micro-segregation which appears due to the deposition of the equiaxed grains onto the bottom of ingot. Micro-fissures, porosity and micro-shrinkage are the result of the segregation phenomena occurring during the ingot solidification. Three ranges within the temperature filed created in the ceramic mould are to be distinguished: a/ for the formation of columnar structure (the C – zone): ( T 0 and steep thermal gradients in the liquid), b/ for the C E transition (columnar to fully equiaxed structure): ( T 0 and the so-called critical thermal gradient in the liquid), c/ for the formation of fully equiaxed structure (the E – zone): ( T 0 and moderate thermal gradients in the liquid).
Authors and Affiliations
W. S. Wołczyński
Keywords
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