Density and thermal expansion coeffcients of liquid and austenite phase in lamellar cast iron


Volume change related defects formation mechanisms are an important detracting phenomenon in production of complex shaped cast components. Among different technical alloys, cast iron behaves in a complex manner due to the combined volume change of the formed phases. The liquid and the austenitic phase are contracting while the graphite phase is expanding during the solidifcation. The complex volume change in combination with complex casting shapes causes a considerable deviation from isotropy in the solidification domain. The mentioned difficulties are considered the main reason why an extensive research work is condensed in the literature within this topic. The multitude of reported experimental set up and the various efforts to interpret the volume change phenomena in terms of density and thermal expansion coeffcients makes the results diffcult to compare from different sources. With these diffculties in mind, the present paper presents a broad experimental series and measures unidirectional linear deformation of an industrially spread lamellar cast iron alloy system (Fe-C-2Si) using the push-rod based dilatometer technique. The measurements are divided into two major groups with respect to the liquid iron deformation over the liquidus temperature line, and the austenite deformation below the solidus temperature line. The obtained results are interpreted as thermal expansion coefficients, density variation slopes, and density data at the liquids and solidus temperature.

 

The following conclusions were drawn through complex experiments

 

(1)A complex experimental and interpretation procedure was applied to calculate the density variation of the liquid and austenite phase in lamellar cast iron. The complexity of the interpretation procedure is caused by the incorporation of gaseous porosities in the liquid and the presence of shrinkage porosity in the austenitic matrix when the volume change was measured. Use of elements such as aluminum contributes to deoxidate the liquid and reduces the tendency for gas porosity formation in the solidifying sample. The shrinkage porosity forms generally in the low carbon-containing samples and is reduced by producing the samples under rapid solidifying conditions.

 

(2)The thermal expansion coefficient, which is the primary result of the applied experimental measurement, was compared with the calculated thermal expansion coeffcient by the Thermo-Calc software. The measured and calculated data for the liquid phase expansion were in the same range, while a slight difference was observed between the measured and calculated thermal expansion coefficient of the austenite phase. The observed differences are interpreted as being dependent on the general assumption of material isotropy in both the liquid and austenite phase. While the observed unidirectional length deformation in the dilatometer is nearly isotropic in the case of the liquid phase measurement, the length deformation registered only in axial direction for the austenite sample is anisotropic. In the case of the calculated data in Thermo-Calc a perfect isotropy is considered.

 

(3)The slope of the density variation as a function of temperature for the various alloys and the density data in the close vicinity of the liquidus and solidus line are calculated data derived from the primary measurement of the unidirectional length deformation. Literature data are available only for the density slope over the solidus temperature and the density data at the solidus temperature for comparison. The literature and the interpreted measurement data are found to be in the same range. The calculated data based on the experimental measurement in the austenite phase and in the vicinity of the solidus line can be considered as novel data since no literature data are found.

 

(4)he major beneft of the obtained data is considered to be the wide range of used carbon content for unextensively used industrial cast iron alloy investigated and interpreted under similar experimental conditions.

 

The present work was realized partly within the SPOFIC I & II project founded from the Swedish VINNOVAagency and partly within the project Lean Cast fnanced by the Swedish Knowledge Foundation. All the members of these projects are greatly acknowledged. The authors also wish to thank Peter Gunnarsson, Jönköping University, for the machining of the dilatometer samples.

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