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Ductile nodular cast iron with compacted (center and lower left) and flake graphite (right), binary image processed based on Reference 84.
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Ferritic spheroidal graphite cast iron (SGI) materials have a remarkable technical potential and economic impact in modern industry. These features are closely related to the question of how the cast materials can be produced without structural defects and graphite degenerations such as, for example, chunky graphite. Although the chunky graphite de...
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The classification of grades inside a material family should be based on the properties required by design procedures. This paper proposes a reclassification of spheroidal graphite ferritic pearlitic and ausferritic (ADI) ductile cast irons grades based on yield strength (YS), strength ratio (SR) UTS/YS and elongation at fracture (EF). In fact, the...
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... In the case of the faster retracted sample 1.2, the switch from controlled cooling conditions into quenching was evidenced by a rapid increase in the amount of pearlite up to the point at which the ferrite remained only in the form of thin envelops surrounding fine graphite flakes ( Figure 5e). The VC areas of both ingots presented very similar microstructures characterized by the presence of elliptical islands of ferrite phase (slashed dendrite arms) surrounded by channels filled with eutectic formed by fine twisted graphite flakes in a ferritic matrix, i.e., chunky graphite [27] (Figure 5a,d). Aside of that, the occasional presence of coarse compacted graphite particles was noted. ...
... In the case of the faster retracted sample 1.2, the switch from controlled cooling conditions into quenching was evidenced by a rapid increase in the amount of pearlite up to the point at which the ferrite remained only in the form of thin envelops surrounding fine graphite flakes (Figure 5e). The VC areas of both ingots presented very similar microstructures characterized by the presence of elliptical islands of ferrite phase (slashed dendrite arms) surrounded by channels filled with eutectic formed by fine twisted graphite flakes in a ferritic matrix, i.e., chunky graphite [27] (Figure 5a,d). Aside of that, the occasional presence of coarse compacted graphite particles was noted. ...
... This agrees with the literature data documenting that Al not only has pro-graphitizing properties but also acts as a strong anti-spheroidizer [24,25,32]. According to ASTM A247, their shape was the closest to the VII-type in all samples [27,33,34]. Regarding the spatial distribution, graphite type C showed the best match with that represented by samples 1.1 and 1.2, while graphite type A was closest for others. ...
SiMo ductile cast iron combines ease of part fabrication with good mechanical properties, including a usable plasticity range. Its poor corrosion resistance inherited from grey cast iron could be alleviated through alloying with Al or Cr additions capable of forming a dense oxide scale protecting the substrate. However, the presence of Al and Cr in cast iron tends to make the material brittle, and their optimum alloying additions need to be studied further. The present work was aimed at investigating the effect of crystallization rates on microstructure changes during directional crystallization of SiMo-type alloys with up to 3.5% Al and 2.4% Cr. The experiment was performed using the Bridgman–Stockbarger method. The tubular crucible was transferred from the hot section to cold section at rates ranging from 5 mm/h to 30 mm/h with a 4/5 crucible length and then quenched. The introduced Al promoted graphitization up to a point, wherein, at the highest applied addition, the graphite precipitation preceded crystallization of the rest of the melt. A rising level of Cr in these alloys from 1% to 2.4% resulted in the formation of low and high contents of pearlite, respectively. The higher crystallization rates proved effective in increasing the ferrite content at the expense of pearlite. In the investigated cast iron samples with smaller applied alloying additions, Widmanstätten ferrite or ausferrite, i.e., fine acircular phase, were often found. The switch from directional crystallization to quenching caused a transition from a liquid to solid state, which started with nucleation of islands of fine austenite dendrites with chunky graphite eutectic separating them. As these islands expanded, they pushed alloying additions to their sides, promoting carbide or pearlite formation in these places and forming a super-cell-like structure. The performed experiments helped gather information concerning the sensitivity of the microstructure of SiMo cast iron modified with Al and Cr to crystallization rates prevailing in heavy cast structures.
... The variation of graphite and MC carbide mole fractions as a function of silicon content for the studied compositions are calculated and given in Figure 4. It is a fact that silicon promotes nucleation of graphite, increasing the number of graphite nodules, [30,31] however, as the silicon content increases graphite mole fraction decreases due to the decreasing carbon content in the studied compositions (Figure 4(a)). On the other hand, increasing silicon content causes a decrease in cooling rate during solidification which slows down the carbon diffusion to the graphite nodules that are surrounded by austenite. ...
... The graphite content decreases as silicon content increases in the cast irons as calculated by Thermo-Calc. The studies have shown that silicon takes place in graphite crystal and promotes its nucleation causing an increase in the number of graphite, [30,31] besides graphite morphology depends on the solidification rate. [32] The decrease in cooling rate causes slower encapsulation of graphite by austenite, thus graphite content in the matrix decreases due to the inhibited growth of nucleated graphite, besides nodular morphology of graphite is deteriorated. ...
... [32,43] As silicon content in the liquid composition increases the supercooling at the austenite-liquid interface is affected and increased silicon content causes lower cooling rate. [26,[30][31][32]44] Chen et al. have reported that besides silicon effect, graphite nodularity is also affected by niobium which decreases carbon diffusion. [34] Besides the chemical composition of the cast iron, [14] quality and efficiency of inoculation and spheroidization treatments during casting also affect the graphite nodularity as well as the composition of the agents used. ...
The low oxidation resistance of SiMo ductile cast irons used as exhaust manifold material at high temperatures necessitates the development of new generation ductile cast iron compositions. New alloy designs can be made using CALPHAD methodology, and solidification sequence, segregation and critical phase transformation temperatures can be determined, especially for the solidified bulk materials. Thus, in commercial practicality, castable compositions with a raised A 1 temperature can be obtained. In this study, novel SiNb cast irons with varying silicon contents were developed as candidate materials for exhaust manifolds. Solidification sequence, microsegregation, phase transformations, equilibrium phases of hypereutectic compositions containing 4 to 7 wt pct Si were calculated by CALPHAD-based modeling. The bulk materials of the studied compositions were cast as Y blocks and metallurgical analyzes were carried out. Studies revealed that; (i) in the ferritic matrix of the cast irons, graphite, Nb-rich carbides and some pearlite existed, (ii) pearlite formation was due to the negative segregation of silicon and positive segregation of manganese during solidification, (iii) as silicon content increased the amount of silicon dissolved in ferrite phase increased in the solidified structure and as a result pearlite formation decreased at the cell boundaries, and amount of vermicular graphite increased, (iv) depending on the silicon content the critical A 1 temperature varied between 860 °C to 1013 °C and these values were higher than that of SiMo cast iron. All these findings revealed that SiNb cast irons had phase stability at higher temperatures compared to SiMo cast iron.
... When the wall thickness increased, regardless of the Ti content, there was a tendency to decrease nodularity because lower cooling rates caused lower nucleation rates of graphite during solidification [23]. Since vermicular graphite occurs through the radial growth of pre-nucleated graphite nodules, preventing C diffusion during solidification by increasing the cooling rate could increase the nodularity [24]. Accordingly, considering the 0.5 wt% Ti-alloyed specimens, the graphite nodularity at the thickest section was 70% less than that of the thinnest wall. ...
... Significant progress has been made in the understanding of cast irons in the last two decades, with numerous investigations focused on developing new cast irons with improved properties such as strength, ductility, toughness, hardness, and wear resistance [9][10][11][12][13][14][15][16]. Other areas of research include testing mechanical properties [17][18][19][20][21][22][23][24][25][26][27], thermal-mechanical fatigue resistance [28][29][30][31][32][33][34][35][36][37][38][39], thermal fatigue resistance [39,[42][43][44][45][46][47][48][49][50][51], oxidation resistance [8,, and determining the conditions for achieving desirable graphite and matrix microstructures [74][75][76][77][78][79][80][81][82][83]. ...
... Defects and metallurgical discontinuities form during metallic alloys solidification and can affect significantly the magnitude and the variability of mechanical properties. In castings of Ductile Irons (DIs), for instance, the main defects and discontinuities can be degenerated graphite agglomerates, dross, inclusions, gas and solidification shrinkage porosities, etc., which can have a detrimental impact on several mechanical properties, such as the room-temperature tensile strength, ductility, fatigue resistance, and fracture toughness [1][2][3][4][5][6][7]. In the continuous casting production of steels, for instance, the metallurgical discontinuities, such as cracks forming during solidification at the slab surface, influence appreciably the material quality resulting from the subsequent slab straightening [8,9]. ...
The microstructure and tensile behavior of two heavy section castings that had chemical compositions typical of GJS400 were investigated. Conventional metallography, fractography, and micro-Computer Tomography (μ-CT) were employed, enabling the quantification of the volume fractions of eutectic cells with degenerated Chunky Graphite (CHG), which was identified as the major defect in the castings. The Voce equation approach was exploited to evaluate the tensile behaviors of the defective castings for integrity assessment. The results demonstrated that the Defects-Driven Plasticity (DDP) phenomenon, which refers to an unexpected regular plastic behavior related to defects and metallurgical discontinuities, was consistent with the observed tensile behavior. This resulted in a linearity of Voce parameters in the Matrix Assessment Diagram (MAD), which contradicts the physical meaning of the Voce equation. The findings suggest that the defects, such as CHG, contribute to the linear distribution of Voce parameters in the MAD. Furthermore, it is reported that the linearity in the MAD of Voce parameters for a defective casting is equivalent to the existence of a pivotal point in the differential data of the tensile strain hardening data. This pivotal point was exploited to propose a new material quality index assessing the integrity of castings.
... The higher strength, associated with better machinability, could also decrease production costs. 1,2,[4][5][6][7][8][9][10][11][12][13][14] Despite the elongation at rupture being progressively reduced with Si content, its impact is less significant than that promoted by increased pearlite fraction in standard SGI. For instance, EN-GJS-600-3 grade is a ferritic-pearlitic grade with similar tensile strength as SSF DI 600-10 grade (min. ...
... In the studies of Méndez et al., 38 it was also verified that cast irons with 5.21 wt% Si presented degenerated graphite, identified as chunky graphite by the authors. 7,15,[39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57] Several investigators concluded that various ways of preventing chunky graphite formation include increasing the solidification rate, using Ni-Mg or pure Mg as a nodularizer, or adding Sb (0.02 wt%). Chunky graphite precipitation can also be prevented by adjusting some foundry process parameters, such as the melt's overheating. ...
This work focused on producing and characterizing high silicon cast irons (microstructural analysis, tensile properties, damping capacity, elastic properties). Moreover, it also evaluated the influence of RE alloying on damping capacity. The samples present a step-casting geometry to verify the cooling rate’s impact on their microstructures and, consequently, on the mechanical properties. The high Si SGI presented a fully ferritic matrix ( i.e. pearlite < 5%) with uniform dispersed graphite nodules; however, those also RE alloyed showed a lower nodularity. The results show an average increase, relative to the base SGI, of 33% for tensile strength and 55% for yield strength (section thicknesses between 45 and 25 mm). A damping capacity increase was observed, reaching a maximum increase of 32% with the addition of RE. The beneficial impact lies in the possibility of increasing the damping capacity of SGI, without compromising its tensile strength, by balancing the graphite degeneration (induced by RE alloying) and strengthening the ferritic matrix (promoted by the high Si content).
... The carbide content of the white cast iron acts as an impurity which allows crack propagation, while the flakes of graphite initiate several new cracks which eventually lead to the fracture of the material. However, enhancement in ductility can be achieved by the production of ductile cast iron with spherical graphite, where the crack propagation resistance is accomplished [16]. ...
This paper aims to examine the microstructure, corrosion resistance and hardness of bone particles reinforced cast iron for advanced applications in marine and automotive industries, especially for the fabrication of engine blocks. In most marine and automobile industries, cast irons remain the major material used for the manufacture of engine components such as flywheels, piston rings, cylinder heads and engine blocks. Nevertheless, several categories of cast iron possess poor impact strength, low corrosion resistance, easy crack propagation and brittle failure over time. As a result of these challenges, cast iron scraps melted and reinforced with particles from goat bone. Samples of reinforced and unreinforced cast iron were produced. The microstructure of the samples was studied using SEM/EDS (scanning electron microscope equipped with energy dispersive spectroscope) and XRD (X-ray diffractometer), while the corrosion rate of the samples was studied using potentiodynamic polarization technique following ASTM G102 standard and using 3.65% NaCl solution as the corrosive medium. The SEM micrographs indicated that the goat bone particle (GBP) reinforced cast iron possessed more refined microstructure, smoother morphology and microstructural homogeneity. For the unreinforced cast iron sample (control), the corrosion rate (Cr) and Brinell hardness of 1.427 mm year⁻¹ and 275.3 HBN, respectively. On the other hand, the goat bone particle (GBP) reinforced cast iron samples possessed a Cr and hardness ranging from 0.510-1.006 mm year⁻¹ and 288.9–297.4 HBN, respectively. These results indicated that the goat particle reinforced cast iron sample (control) possessed higher passivation and superior indentation resistance relative to the unreinforced cast iron sample.
... As mentioned before, the more carbide formation enhanced the nodularity since the growth of graphite particles under slower cooling rates was the main reason for its decrease. It was possible to reduce the amounts of degenerated morphologies by impeding C diffusion because they occurred due to the radial growth of graphite nodules [24]. ...
Spheroidal graphite cast irons (SGCIs) have been widely used in automotive and energy industries requiring excellent mechanical strength and wear resistance. This study aims to investigate the effects of vanadium content and cooling rate on the performance of these alloys. The SGCIs alloyed with different amounts of V (0, 0.25, 0.5, and 1 wt.%) were fabricated at different section sizes (10, 20, 30, 40, and 50 mm) by the sand casting process for this purpose. The microstructural evaluation showed that V8C7 carbide density and pearlite ratio increased while ferrite volume decreased with increasing V content and decreasing section size. The number of graphite particles seemed to be inversely proportional to carbide precipitation. The more carbide caused the coarser graphite nodules with decreased nodularity. Thinner-wall castings provided a higher cooling rate resulting in better mechanical strength. Yield strength (YS), ultimate tensile strength (UTS), and hardness of SGCIs increased with increasing V and cooling rate. The minimum wear losses were achieved in 0.5 wt.% V-alloyed SGCIs for all section sizes. Increased V8C7 carbide density in 1 wt.% V-alloyed SGCIs significantly reduced impact toughness, causing decreased wear resistance despite their higher mechanical strength.
... [15][16][17]23,24 Bi and Ce already exert a major influence on the morphology of graphite at low concentrations, even compared with other interfering elements. 21,[25][26][27] Hence, their admissible concentrations are in a low ppm range. 27 Cerium is commonly used as an alternative nodularising agent to magnesium in concentrations up to 500 ppm by weight in thin-walled castings. ...
... Ce does, however, inevitably increase the risk of formation of chunky graphite. 16,17,20,24,26 Bismuth is used in inoculating agents, as its intermetallic compounds are effective nucleation sites for graphite and the nucleating effect fades very slowly. 11,30 However, the formation of graphite degeneration has been observed already at contents of 30 wt ppm Bi. 31 Like Ce, it is used to prevent graphite degenerations by elements from the opposing group, hence preventing CHG formation. ...
... Each sample was ground and polished, and a graphite analysis according to ISO 945 1, 2 and 4 4,5,35 was performed with 10 micrographs at 1009 magnification each. Areas with degenerated graphite were manually marked as described in, 26 and that area was counted as degenerated. These marked areas were excluded from the pictures for further automated analysis of graphite particles. ...
The impact of combined addition of high levels of bismuth up to 120 wt ppm and cerium up to 2000 wt ppm on the graphite morphology in GJS 450-18 with 3.2 wt% carbon and 3.2 wt% silicon was studied. Experiments were conducted with insulated keel blocks with a solidification time of 40 min. Samples from the thermal centre of the castings were analysed by optical microscopy, and the forms and sizes of graphite particles were characterised. Bismuth addition, even at 25 wt ppm, resulted in an altered graphite form in the last-to-freeze regions resembling intercellular lamellar graphite (ILG). Additions of 45 wt ppm or more Ce to these Bi-containing melts prevented the formation of ILG and produced chunky graphite (CHG) instead. ILG did not appear for ratios Bi/Ce > 1.5, while CHG could not be found for ratios Bi/Ce < 0.7. Only one type of graphite degeneration (either ILG or CHG) was present in each sample, thus rendering their formation mutually exclusive. Larger amounts of Ce between 300 and 500 wt ppm resulted in the formation of predominantly graphite form V, while no degenerations could be observed. The addition of Ce to a Bi-contaminated cast iron melt to avoid ILG formation and achieve a regular graphite structure with mainly form VI graphite is not possible. Instead, form V replaces form VI as the predominantly formed morphology. Ce levels higher than 1000 wt ppm resulted in the formation of large areas of undercooled graphite in the last-to-freeze regions regardless of the Bi content in the cast iron melt.
... Prenucleated nodules can become chunky graphite with a high level of branching due to radial growth along their caxis (22). Alloying with Al decreased the C diffusion rate, causing deteriorating graphite nodularity (23). ...
... Due to the barrier effect of this oxide layer on the C diffusion, the ferrite formation was interrupted, and the pearlite ratio dramatically increased as the Al content reached 4 wt%. Because of the same reason, some irregular graphite shapes also developed in the cast iron structure, causing decreased nodularity (22). ...
Spheroidal graphite cast irons with 0-4 wt.% aluminum addition fabricated by green sand casting were austenitized at 900 °C for 90 min and subsequently austempered at 300 °C for 60 min. The samples were subjected to dry sliding wear tests against Al2O3 balls under various conditions. Microstructural analysis showed that increasing Al decreased the acicular ferrite ratio and disrupted graphite nodularity. Accordingly, UTS and hardness decreased while impact strength increased except for the 4 wt.% Al-alloyed cast iron. Al2O3 layer formed around graphite nodules could not be removed during the austempering and caused an increase in the mechanical strength of this sample. Sliding tests revealed that the wear resistance of the samples increased with sliding speed and decreased with applied load. Worn surfaces predominantly suffered from abrasion, adhesion, and delamination. Although lower Al addition increased wear losses, 4 wt.% Al-alloyed cast iron showed the best wear resistance.
