M. Kawalec

Role of Sulfur on the Transition from Graphite to Cementite Eutectic in Cast Iron

In the current study, an analytic solution is considered to explain the influence of sulfur on the transition from graphite to cementite eutectic in cast iron. The outcome from the current study indicates that this transition can be related to (a) the graphite nucleation potential (directly characterized by the cell count and indirectly by the nucleation coefficients; (b) the eutectic graphite growth rate coefficient; (c) the temperature range between the equilibrium temperature for graphite eutectic and the formation temperature for cementite eutectic; and (d) the liquid volume fraction, after pre-eutectic austenite solidification. In addition, the absolute and the relative chilling tendencies, as well as critical cooling rates including the chill width of the cast iron can be predicted from the current study. The analytic model was experimentally verified for castings with various sulfur contents. It is found that the main role of sulfur on the transition from graphite to cementite eutectic is through its effect on lowering the growth coefficient, and hence, the graphite eutectic growth rate. In addition, it is found that with the increasing sulfur content, the critical cooling rate is significantly reduced, thus increasing the absolute and the relative chilling tendency values, including the chill width.

Influence of carbides morphology on the mechanical properties for Fe-C-V alloys

The paper presents the results of tests on the spheroidising treatment of vanadium carbides VC done with magnesium master alloy and RE. The conducted metallographic studies have shown that introducing the magnesium master alloy to an Fe–C–V system of eutectic composition causes the crystallisation of spheroidal carbides. The content of these carbides is about 5.6%, representing 33% of all the crystallised vanadium carbides. Adding RE to the base alloy melt caused 31% of the vanadium carbides crystallise as dendrites. Testing of mechanical properties has proved that the spheroidising treatment of VC carbides in high-vanadium cast iron increases the tensile strength by about 60% and elongation 14–21 times, depending on the type of the spheroidising agent used. Tribological studies have shown that high-vanadium cast iron with eutectic, dendritic and spheroidal carbides has the abrasive wear resistance more than twice as high as the abrasion-resistant cast steel.

Effect of Different Molding Materials on the Thin-Walled Compacted Graphite Iron Castings

This article addresses the effects of six mold materials used for obtaining thin-walled compacted graphite iron castings with a wall thickness of 3 mm. During this research, the following materials were analyzed: fine silica sand, coarse silica sand, cerabeads, molohite and also insulated materials in the shape of microspheres, including low-density alumina/silica ceramic sand. Granulometric and SEM observations indicate that the sand matrix used in these studies differs in terms of size, homogeneity and shape. This study shows that molds made with insulating sands (microspheres) possess both: thermal conductivity and material mold ability to absorb heat, on average to be more than five times lower compared to those of silica sand. In addition to that, the resultant peak of heat transfer coefficient at the mold/metal interface for microspheres is more than four times lower in comparison with fine silica sand. This is accompanied by a significant decrease in the cooling rate of metal in the mold cavity which promotes the development of compacted graphite in thin-walled castings as well as ferrite fractions in their microstructure.

Effect of cooling rate and of titanium additions on the microstructure of thin-walled compacted iron castings

This article addresses the effect of cooling rate and of titanium additions on the exhibited microstructure of thin-walled compacted graphite iron (TWCI) castings as determined by changing moulding media, section size and ferrotitanium. Various moulding materials were employed (silica sand and insulating sand ‘‘LDASC’’) to achieve different cooling rates. This study shows that the cooling rates exhibited in the TWCI castings varies widely (70–14 °C/s) when the wall thickness is changed from 2 to 5 mm. In turn, this is accompanied by a significant variation in the compacted graphite fraction. The resultant cooling rates were effectively reduced by applying an insulating sand in order to obtain the desired graphite compactness. Ti additions in combination with LDASC sand moulds were highly effective in promoting the development of over 80% compacted graphite in castings with wall thicknesses of 2–3 mm as evidenced by quantitative metallographic analyses.

Thermophysical Properties of Thin Walled Compacted Graphite Iron Castings

The thermal conductivity, diffusivity and specific heat were investigated in thin walled compacted graphite iron castings. The research was conducted for thin-walled iron castings with a 3-mm wall thickness. This study addresses the effect of cooling rate and of titanium additions on the exhibited microstructure and thermophysical parameters of thin-walled compacted graphite iron (TWCI) castings as determined by changing the molding media (silica sand and insulating sand LDASC), and Ferro Titanium. The tested material represents the occurrence of graphite in the shape of nodules, flakes (C and D types) and compacted graphite with a different shape factor and percent nodularity. Thermophysical parameters have been evaluated by the laser flash technique in a temperature range of 22–600°C. The results show that the cooling rates together with the titanium content largely influence the microstructure, graphite morphology and finally thermophysical properties of thin walled castings.