Lennart Elmquist

Shrinkage Porosity and its Relation to Solidification Structure of Grey Cast Iron Parts

Abstract The purpose of this work was to investigate the relation between shrinkage porosity and the macrostructure. Based on results from an earlier investigation regarding shrinkage porosity and its characteristic features, a test casting was developed. The macrostructure was preserved using direct austempering after solidification heat treatment. Shrinkage porosity was found in regions where it was designed to occur. It was also found that this type of porosity is connected to the atmosphere via defects located on the surface, confirming earlier findings. Beneath the surface, it extends as a three-dimensional network through the casting. The shrinkage porosity was, in some cases, found along the boundaries separating primary crystals, and in some cases, it encircled separate primary crystals. A shrinkage porosity formation mechanism is proposed based on these observations, discussing the importance of a rigid columnar zone, movement of a local thermal centre and how pressure differences developing during solidification influence the formation of shrinkage porosity.

Inoculation and its effect on primary solidification structure of hypoeutectic grey cast iron

Abstract The solidification of grey cast iron is controlled by the addition of inoculants. This is done in order to provide nucleation sites and hence facilitate the formation of eutectic cells and decrease the degree of undercooling. The number of eutectic cells and the graphite morphology affect the final properties of the casting. Preceding the nucleation of graphite and the eutectic cells is the nucleation of the primary austenite. It was found that the addition of inoculants also influences the primary solidification. The largest effect on the primary dendrites is obtained by inoculation using pure iron powder. It was also shown how the columnar to equiaxed transition (CET) depends on the number of equiaxed dendrites per unit volume. In addition, the primary structure was found to influence the eutectic solidification. The relationship between the secondary dendrite arm spacing and the eutectic cell size was found to correlate well with the work of others.

Mechanical properties of Solid Solution-Strengthened CGI

Despite the increased usage of pearlitic compacted graphite iron (CGI) in heavy vehicle engines, poor machinability of this material remains as one of the main technical challenges as compared to conventional lamellar iron. To minimise the machining cost, it is believed that solution-strengthened CGI material with a ferritic matrix could bring an advantage. The present study focuses on the effect of solution strengthening of silicon and section thickness on tensile, microstructure and hardness properties of high-Si CGI materials. To do so, plates with thicknesses from 7 to 75 mm were cast with three different target silicon levels 3.7, 4.0 and 4.5 wt%. For all Si levels, the microstructure was ferritic with a very limited pearlite content. The highest nodularity was observed in 7 and 15 mm plate sections, respectively, however, it decreased as the plate thickness increased. Moreover, increasing Si content to 4.5 wt% resulted in substantial improvement up to 65 and 50% in proof stress and tensile strength, respectively, as compared to pearlitic CGI. However, adding up Si content to such a high level remarkably deteriorated elongation to failure. For each Si level, results showed that the Young’s modulus and tensile strength are fairly independent of the plate thickness (30–75 mm), however, a significant increase was observed for thin section plates, particularly 7 mm plate due to the higher nodularity in these sections.

Solidification Study of Gray Cast Iron in a Resistance Furnace

Solidification of hypoeutectic gray cast iron has been investigated in a resistance furnace. A cylindrical shaped sample with three thermocouples, two inside and one in the surrounding, was solidified and compared to samples quenched at different stages during the solidification interval. An additional sample in the series has been produced to reveal the primary grain structure using Direct Austempering After Solidification (DAAS) treatment. The present paper collect for the first time under the same experimental procedure the solidification of a hypoeutectic gray cast iron where both the growth of the primary and eutectic phase are observed. An increase of the secondary dendrite arm spacing (SDAS) was observed until the end of the solidification interval. A comparison of the measured solidified fractions of the phases with those predicted using Fourier Thermal Analyses evidenced the quenching effectiveness. The eutectic cell size is in agreement with the literature. The method presents good consistency.

The relation between SDAS and eutectic cell size in grey iron

Abstract The solidification of cast components is a complex and important process as this is the moment when the final properties are established. For hypoeutectic grey iron, solidification starts with nucleation and growth of the primary austenite followed by the eutectic reaction forming eutectic cells. In this work, the microstructure and significance of the different constituents formed during solidification has been examined. It was found that the size of the eutectic cells is a function of secondary dendrite arm spacing (SDAS). The SDAS, on the other hand, was found to depend on the solidification time and hence the growth rate of the dendrites. The effect of chemical composition on SDAS and eutectic cell size was found to depend on cooling rate. It is suggested that the relationship between the eutectic cells and dendrite arm spacing is based on segregation effects and the nucleating capacity of the melt.

Cast Iron Solidification Structure and how it is Related to Defect Formation

In this work, the meaning of the solidification structure and how it is related to defect formation in grey cast iron will be discussed. The work also confirms observations made earlier. In previous work the formation of shrinkage porosity in grey cast iron cylinder heads was investigated. It was found that the defect is located around solidification units resembling primary austenite grains. The solidification of grey cast iron starts with the formation of primary austenite grains, followed by the eutectic solidification. The primary grains nucleate and grow either as columnar or equiaxed grains, creating a columnar to equiaxed transition between the two zones. Based on the presence of a migrating hot spot, and other characteristics found on the cylinder heads, a geometry was developed that promote the formation of shrinkage porosity. The primary solidification structure, normally transformed during the solid state transformation, was preserved using a technique called Direct Austempering After Solidification (DAAS). After solidification, the samples were cut and prepared for investigation using a Scanning Electron Microscope (SEM) equipped with a detector for Electron Back Scattered Diffraction (EBSD). Individual grains were identified and the primary solidification structure around the defects was revealed. The investigation shows how shrinkage porosity is formed and located between primary austenite grains. This confirms that the primary solidification structure has a large influence on the formation of defects in grey cast iron. The investigation also confirms the correctness of earlier results as well as the validity of the DAAS technique.

Defect Formation of Gray Iron Casting

Cast iron is one of the oldest technical alloys (engineered cast materials) used for creating objects. From the very beginning of casting, foundrymen were fighting to avoid casting defects. In the beginning a successfully produced casting was associated with witchcraft. Cast component producers suffer substantial yearly expenses due to rejecting or repairing castings. This present work will summarize research efforts to understand the formation mechanisms of defects, performed in collaboration with Swedish foundries during the last few years. It will focus on defects specific to the casting of gray iron components and the studied defects: gas porosity, shrinkage porosity and metal expansion penetration. Novell experimental set up has been developed or existing methods have been improved to study defect formation mechanisms. Today we can realize that casting without defects is possible only by approaching the defect formation mechanism with multidisciplinary science.

An approach to foster integrative skills during the engineering studies

This paper presents an approach to overcome the drawbacks associated with education programs developed on the basis of domain-specific knowledge only. The approach is based on establishment of mean ...

On the Problems of a Migrating Hot Spot

In this work the meaning of a migrating hot spot during solidification will be discussed. Initially, just after mould filling, global as well as local thermal centres, or hot spots, are established. This occurs at mass concentrations and heavy sections as the mould wall initially is at ambient temperature. However, these hot spots are found to be migrating during solidification. This migration influences any temperature measurement and subsequent thermal analysis, introducing uncertainties. It has also been found that common casting defects such as shrinkage porosity and metal expansion penetration can be associated with a migrating hot spot. In these cases the final position of the hot spot is located across the interface between the casting and the mould. The solidification starts at the mould walls and it was found that the columnar zone is affected by the migrating hot spot. The macrostructure was preserved using DAAS technique and it was revealed that an otherwise normal columnar zone because of the migrating hot spot had been affected. Based on these findings it is suggested that the migrating hot spot is making the columnar zone weaker and more susceptible for pressure differences arising during solidification.

On the thermal conductivity of CGI and SGI cast irons

Abstract The thermal conductivity of Compacted Graphite Iron (CGI) and spheroidal graphite iron (SGI) was established in the temperature range from room temperature up to 500 °C using the experimental thermal diffusivity, density and specific heat values. The influence of nodularity, graphite amount, silicon content and temperature on the thermal conductivity of fully ferritic high-silicon cast irons was investigated. It was found that the CGI materials showed higher thermal conductivity than the SGI materials. The thermal conductivity tended to increase with increasing temperature until it reached a maximum followed by a subsequent decrease as temperature was increased up to 500 °C. Conventional models were applied to estimate thermal conductivity and the predictive accuracy of each model was evaluated. The thermal conductivity could be estimated by the Helsing model. The Maxwell model, Bruggeman model and Hashin–Shtrikman model were also in fair agreement using the thermal conductivity value of graphite parallel to the basal planes in graphite.