Effects of nodularity on thermal conductivity of cast iron

Both spheroidal graphite iron and compacted graphite iron are used in the automotive industry. A recently proposed mixed graphite iron exhibits a microstructure between the conventional spheroidal graphite iron and compacted graphite iron. Evaluation results clearly indicate the suitability and benefits of mixed graphite iron for exhaust component applications with respect to casting, machining, mechanical, thermophysical, oxidation, and thermal fatigue properties. A new ASTM standard specification (A1095) has been created for compacted, mixed, and spheroidal graphite silicon-molybdenum iron castings. This paper attempts to outline the latest progress in mixed graphite iron published.

This manuscript summarizes recent investigation of the authors into the solidification and shrinkage defect formation of ductile and compacted graphite cast irons. The study makes use of the DAAS (direct austempering after solidification) technique, developed earlier by some of the authors, to reveal the solidification grains. In addition, color metallography is used to reveal the microsegregation pattern. The formation of shrinkage defects is related to both the solidification macrostructure and the microsegregation pattern. The examination of the macrostructure shows that large solidification grains, composed by dendritic austenite and graphite, characterize the solidification of compacted graphite irons of carbon equivalent values ranging from hypoeutectic to hypereutectic. Similar results are observed for ductile iron, but in this case, the hypereutectic irons display a finer macrostructure. The characteristics of the primary austenite dendrites are revealed by the microsegregation pattern, which is exposed by the color etching reagent. This pattern also clarifies the nature of the interaction between the growing austenite and the compact and spheroidal graphite particles. The microshrinkage cavities in both ductile and compacted graphite irons form at the interior of the austenite grains, where fluid feeding cannot compensate the contraction of the last-to-freeze portions of the melt. Schematics of the solidification of ductile and compacted irons are proposed, accounting for the experimental observations.

The performance of coatings on furan resin sand moulds [P-Toluol Sulphonic Acid (PTSA) as hardener] [FRS-PTSA], was compared to 'no sulphur' Novolak resin coated sand [NRS] moulds by analysing the graphite characteristics in the surface layer of Mg-treated irons (0.020―0.054%Mg) and over the entire section. The extent of an abnormal surface layer is influenced by the different Mg contents for compacted or nodular graphite irons, the sulphur content of the moulding system and the type of mould coating (containing sulphur or desulphurization materials). With lower Mg content, more graphite degeneration was apparent in the cast surface layer, especially at less than 0.03%Mg res [typical Mg content for vermicular/ compacted graphite cast irons] with increasing differences between FRS-PTSA and NRS moulds, and uncoated and coated moulds results. Un-coated FRS-PTSA moulds prompted five times more degenerate graphite in the surface layer than NRS moulds. Sulphur bearing coatings increased the layer up to five times the thickness in NRS moulds and up to 50% in FRS-PTSA moulds, especially for less than 0.030%Mg residual. Desulphurization type coatings based on MgO appear to be efficient for high Mg-content iron (0.05%Mg, ductile iron) but not active enough for less than 0.03%Mg (compacted graphite iron type). The change in graphite characteristics in the centre of samples evolved in a clear relationship with degenerate graphite in the surface layer, for the experimental solidification conditions.

Compared with 2D microstructure characterization, quantitative characterization of graphite morphology from 3D perspective can make it more possible to predict and improve properties of cast iron with relatively high accuracy. In the present study, based on 2D metallographic photos, X-ray tomography was utilized to investigate 3D morphology of graphite clusters in different cast iron and 3D quantitative characterization of graphite morphology was carried out by Avizo software. The results show that basically all nodular graphite in ductile iron are isolated and almost all graphite phase is interconnected in gray iron. The interconnectivity of compacted graphite iron is closely related with vermicularity. The flake-shaped graphite in gray iron is different with coral-shaped graphite in compacted graphite iron from 3D perspective. The quantity proportion of graphite clusters smaller than 10000μm3 is as high as 99.7% and 97.1% respectively in gray iron and compacted graphite iron with 91.3% vermicularity. The volume of graphite clusters larger than 100000μm3 basically accounts for most of graphite volume and the proportion is over 95% in gray iron and compacted graphite iron with high vermicularity. The difference of graphite volume distribution among different cast iron is mainly concentrated in the volume range from 10000 to 100000μm3.

Investigation on the graphite nucleation and growth mechanism of the compacted graphite iron

Influence of alloying elements and its effect on austempering of compacted graphite iron – A review

Tribological Characterization of Centrifugally Cast Graphite Cast Iron under Dry and Wet conditions

Foundrymen have long recognized the value of adding special additives to gray and ductile cast irons to modify structure and control properties. While graphite growth mechanisms are better understood today than just a few years ago, comprehensive theories have yet to be established and accepted. Nevertheless, efforts to comprehend and control the inoculation and nodularizing processes have resulted in a variety of products and treatment techniques. This paper endeavors to summarize the present status of the alloys used to modify cast iron. A very diverse product line of inoculants exists in today's market. While most inoculants are based on ferrosilicon and contain some aluminum and calcium, they may also contain barium, titanium, strontium, rare earths, or other elements. The method of addition, as well as the location and time of addition of special alloys to cast iron, plays a prominent role in determining the addition levels and the effectiveness of a particular treatment. For example, inoculants may be added to the ladle, or later in the process as the metal stream enters the mold, or in the mold itself. Inoculants used in the ladle are normally added in the stream entering the ladle and are usually granular in nature. In late inoculation, on the other hand, the inoculant may be added as a powder, an encapsulated powder, or a bonded or cast insert. Today, the bulk of inoculants are added in the ladle. However, due to metallurgical and economical considerations plus production restrictions, the practice of late inoculation has grown in recent years. The nodularizing treatment for ductile iron is commonly accomplished by ladle treatment with magnesium-ferrosilicon alloys. A variety of other additives, including pure magnesium, is finding use. Recently, particularly in the automotive industry, many foundries have begun to convert to the Inmold Process. Efforts are currently being made within the foundry industry to produce an intermediate iron to gray and ductile iron, known as compacted graphite iron. At least one additive is available commercially for this purpose.

The influence of nitriding on the fatigue resistance of grey and nodular (ductile) cast iron piston rings was studied for six chemical compositions. The selected materials include three grey cast irons: one with a mixed initial microstructure (martensite plus bainite), the second martensitic and another martensitic alloyed with niobium; and three nodular cast irons: one common nodular cast iron, the second alloyed with Nb and another alloyed with Al and V. These materials were gas nitrided for 3 and 6 h. The relationship between the mechanical behaviour and the treatment time (depth of nitrided layer) was established. Samples were tested under a cyclical bending load. The stresses were determined in the cross-section, where the biggest moment was experienced. Results were statistically treated by means of Weibull analysis. Nitriding showed that fatigue resistance of grey cast iron with a martensitic structure is higher than that of a mixed structure. Fatigue resistance of nodular cast irons is red...

The influence of alloying elements on the chill formation in Compacted Graphite Iron (CGI) is investigated. Chill wedges cast in an industrial foundry were used to investigate the chill formation. A total number of 19 chemical compositions were studied, including three trials of varying nodularity treatment level; four trials of varying copper content; four trials of varying silicon content; four trials of varying tin content and four trials of varying carbide promoter content. Three wedges were cast for each alloy composition, of which one was used for measuring the temperature at three different heights in the wedge. Contrary to some previous reports, the results indicate that low-nodularity CGI is not more prone to chill formation (columnar white) than high-nodularity CGI. Trends regarding the effect of alloying elements on chill formation are shown to generally be in agreement with previous work on spheroidal graphite iron and lamellar graphite iron. Most of the samples also show carbide formation in centre line areas of the wedge (inverse chill), this occurrence is also discussed in the paper.

Wear resistance of cast irons used in brake disc rotors

Two types of compacted vermicular graphite (CVG) cast iron having ferritic and pearlitic structures have been studied to determine the effects of CVG on fatigue strength, fatigue crack growth rate and fracture appearance of cast iron. The results obtained were compared with those of two types of spheroidal graphite (SG) cast iron (corresponding to JIS FCD) having ferritic and pearlitic structures. The significant conclusions obtained are as follows: The fatigue limit of the CVG cast iron was lower than that of the SG cast iron independent of matrix structure. This tendency was prominent for ferrite matrix. However, the fatigue limit ratio of the CVG cast iron was not less than that of the SG cast iron regardless the shape of graphite. The fatigue crack growth rate of the CVG cast iron was higher than that of the SG cast iron. This trend was significant for the CVG cast iron having ferritic matrix. This could be due to the fact that the fatigue crack propagation in the CVG cast iron occurs through a zig-zag path between graphites more than that in the SG cast iron when compared at the same level of Kmax.

The mechanical properties of ductile iron (DI) and compacted graphite iron (CGI) are measured and reported on standard machined specimens (as per ASTM). However, most castings retain most of the as-cast surface. This surface layer (the casting skin) includes both surface and subsurface features. Because of the casting skin, the mechanical properties of the part are typically significantly lower than those found on standard ASTM machined specimens.

In sand casting, the surface microstructure often differs from the bulk microstructure due to mold–metal interactions. In nodular and compacted graphite iron, these interactions manifest mostly by a variation in graphite morphology, often attributed to magnesium depletion due to the release of sulfur and oxygen from the molding materials. Additionally, previous research suggests that solidification time, surface condition, and metallostatic height also affect the degree of surface microstructure deviations, though the mechanisms remain poorly understood. Therefore, the present case study targeted an in-depth analysis of the casting skin phenomena in two cylinder blocks sourced from two different foundries to ascertain its occurrence, severity, and main characteristics. Interfaces with molding materials were evaluated under varying solidification times and metallostatic heights, and measurements of the degenerated graphite layer (DGL) thickness and nodularity were performed; average surface roughness was estimated through image analysis. Correlations between these variables and the DGL thickness were assessed. Nital and Motz’s etchants were also employed to examine the DGL characteristics in greater detail, while scanning electron microscopy and energy-dispersive x-ray spectroscopy further explored the presence of particles at the mold–metal interfaces. Results revealed a wide range of DGL structures without a clear correlation to the studied process variables. In fact, some trends observed were opposite to those reported in the literature. A deeper understanding of the key mechanisms is still needed as the casting skin continues to challenge foundries in daily production.

Effects of titanium addition and section size on microstructure and mechanical properties of compacted graphite cast iron