High Silicon Ductile Iron Research Articles

Impact of Quenching and Aluminium on Si-Segregation and B2 Superstructure Formation in Solid Solution Strengthened Ferritic Ductile Cast Iron

Solid solution strengthened ferritic ductile iron proves to have a better ratio of tensile strength to elongation than conventional ductile iron grades. This applies up to a maximum silicon content of 4.3 wt%, beyond which it leads to an abrupt decrease in ultimate tensile strength and elongation at fracture. During solidification of high silicon ductile iron, negative segregation of silicon occurs, and the highest silicon concentration is observed near the graphite nodules. This high silicon concentration leads to long-range ordering of iron and silicon, and this ordering results in formation of superstructures like BCC_B2 and D03. The presence of super structures restricts the mobility of dislocations and leads to abrupt fracture of the material. This research focuses on the investigation of silicon homogenization by the addition of aluminium and heat treatments to avoid the formation of brittle iron–silicon superstructure. Thermodynamic–kinetic simulations as well as experimental investigations, including variation in alloy composition and quenching, are performed. The results provide a promising understanding to control the micro-segregation of silicon in ductile cast iron based on heat treatments and alloy composition.

Determination of Corrosion Resistance of High-Silicon Ductile Iron Alloyed with Nb

In this study, the effects of Nb on the microstructural characteristics, hardness, and corrosion resistance of high-silicon ductile cast iron (HSDI)-3.6 wt.% Si were investigated. Samples from different castings with 0–0.9 wt.% Nb were obtained and compared to a commercial ductile iron. Microstructures showed that the amount of ferrite in the matrix increased with increasing Nb content, from 34% for unalloyed HSDI to 88% for HSDI-0.9 wt.% Nb. The presence of randomly distributed NbC carbides was identified by EDX for all the samples alloyed with Nb, and the hardness of the HSDI increased with the Nb content. To evaluate the influence of the Nb content on the corrosion resistance of HSDI, potentiodynamic tests were carried out in a solution of H2SO4. The highest corrosion rate on HSDI was obtained for the HSDI-0.3 wt.% Nb sample, with 2802 mills per year, due to the amount of pearlite present and the lowest presence of NbC carbides, compared to the HSDI-0.9 wt.% Nb, with 986 mills per year. This behavior was attributed to the ferrite matrix obtained because of a high Si content in the DI, which delayed the anodic dissolution of the alloy and suppressed the pearlitizing effect of Nb for contents greater than 0.3 wt.%, as well as to the effect of NbC carbides, which acted as inhibitors.

Pseudo-passive films on cast irons: A strategy to mitigate corrosion by acting directly on microstructure

The effect of the microstructure on the corrosion resistance of four grades of cast irons was studied in an alkaline 0.03 M NaCl solution. The research was performed by combining microstructural analysis, electrochemical measurements and the characterization of the corroded surfaces. The structure underneath the corrosion layer determines the corrosion resistance of cast irons. It increases according to the following order: grey < chunky < spheroidal < high silicon ductile cast iron. The graphite morphology governs the rate of galvanic corrosion whereas the ferrous matrix controls the formation of non-disrupted oxide layers. A pseudo-passive layer explained the lowest corrosion rate of the high silicon ductile cast iron.

Influence of Graphite-Phase Parameters on the Mechanical Properties of High-Silicon Ductile Iron

High silicon solid solution-strengthened ductile iron exhibits advantageous combinations of static strength and ductility due to elevated silicon contents. However, the Charpy impact toughness decreases rapidly with increasing silicon content. In particular, the transition temperature between ductile and brittle fracture behaviour is shifted significantly to higher temperatures. Therefore, the optimum adjustment of the graphite phase is of major importance in order to achieve sufficient toughness properties. So far, little is known about the influence of the graphite phase on the toughness properties of high-silicon ductile iron. In the present study, the graphite particle distribution was therefore systematically varied for ductile iron grade EN-GJS-500-14. The effect of the graphite nodule count in the range from 77 to 273 1/mm2, and the influence of the average particle size and particle distance on the Charpy impact energy and the characteristics of the resulting transition curves were investigated by means of statistical analyses. The results that were obtained indicate that the ductile-to-brittle transition temperature in Charpy impact tests can be reduced by about 22 °C by increasing the average nodule count by 100 1/mm2. Additionally, both the lower and upper shelf energies are raised by increasing the nodule count by 100 1/mm2. Based on these results, it becomes apparent to producers that the inoculation technology for toughness-stressed components needs to be specifically considered and adapted.

Microstructural Characterization and Evaluation of Corrosion Resistance in a Sulfuric Acid Solution of High-Silicon Ductile Iron Alloyed with Niobium

Microstructural Characterization and Evaluation of Corrosion Resistance in a Sulfuric Acid Solution of High-Silicon Ductile Iron Alloyed with Niobium

Influence of Strain Rate, Temperature and Chemical Composition on High Silicon Ductile Iron

Today, the use of solution hardened ductile iron is limited by brittleness under certain conditions. If chassis components are subjected to loads having high strain rates exceeding those imposed during tensile testing at sub-zero temperatures, unexpected failure can occur. Therefore, it is the purpose of this review to discuss three main mechanisms, which have been related to brittle failure in high silicon irons: intercritical embrittlement, the integrity of the ferritic matrix and deformation mechanisms in the graphite. Intercritical embrittlement is mainly attributed to the formation of Mg- and S-rich grain boundary films. The formation of these films is suppressed if the amount of free Mg- and MgS-rich inclusions is limited by avoiding excess Mg and/or by the passivation of free Mg with P. If the grain boundary film is not suppressed, the high silicon iron has very low elongations in the shakeout temperature regime: 300 to 500 °C. The integrity and strength of the ferrite are limited by the reduced ordering of the silicumferrite with increasing silicon content, once the “ordinary” ferrite is saturated at 3% silicon, depending on the cooling conditions. Finally, the graphite damaging mechanisms are what dictate the properties most at low temperatures (sub −20 °C).

Mechanical Properties and Impact Toughness of Molybdenum Alloyed Ductile Iron

Grades of high silicon ductile iron offer excellent combinations of static strength and ductility as well as good machinability due to their fully ferritic, solution strengthened matrix. As a result of elevated silicon contents, however, the ductile-to-brittle transition temperature in the Charpy impact test is significantly increased. Thus, minimum required Charpy impact energies cannot be met for many applications by using high silicon ductile iron. Therefore, alloys with lower strength and higher toughness properties are commonly used for many technical applications. The enormous lightweight construction potential of high silicon ductile iron can therefore not be fully exploited. The present investigation pursues the metallurgical approach of partially substituting silicon with molybdenum as an alternative strengthening element in order to improve the toughness properties while maintaining similar static mechanical properties. Molybdenum serves as a carbide-stabilising element in ductile iron, while simultaneously promoting ferrite formation and is therefore regarded to be suitable alloying element. In Charpy impact tests, the ductile-to-brittle transition temperature could be reduced by about 55 °C by reducing the silicon content to 2.95 wt% and adding 0.21 wt% molybdenum compared to a high silicon alloy. Additionally, it was possible to mathematically describe the transition behaviour of the studied alloys using nonlinear regression functions and to achieve a sufficient correlation of empirically determined and calculated data. This present metallurgical concept offers a promising metallurgical tool for further improving the toughness properties of alloyed ductile iron.

Effects of cobalt on mechanical properties of high silicon ductile irons

High silicon ductile irons are being developed due to their advantages relating to pearlitic-ferritic grades (high ductility, fully ferritic structures, good machinability, etc.). Recent studies reported that silicon contents higher than 5.2 wt-% originates drastic embrittlement due to chemical ordering. For improving the mechanical properties, the addition of other alloying elements becomes an interesting way of work. This study focuses on the cobalt effect on as-cast microstructures and mechanical properties of ductile irons with silicon contents that maximise ultimate tensile strength. The results obtained show that addition of 4 wt-% cobalt increases the ultimate tensile strength by about 10% and decreases the silicon content at the maximum in this property respecting the unalloyed alloys because cobalt enhances ordering as does silicon.

Influence of Carbide-Promoting Elements on the Microstructure of High-Silicon Ductile Iron

Because of its low cost, steel scrap is one of the most important raw materials for the production of ductile iron (DI). The amount of carbide-promoting elements in steel scrap, such as chromium, manganese, molybdenum, niobium and vanadium, is expected to increase in the future. Most of these elements have a negative impact on the microstructure and mechanical properties of DI. The solubility of carbide-promoting elements in solid solution-strengthened DI materials, standardized in DIN EN 1563:2011, is modified by the high silicon content. For these new materials, the tolerance limits for carbide-promoting elements and their mutual influence must be known to ensure a sustainable production process. To investigate the individual and combined impact of carbide-promoting elements on the carbide content in high-silicon ductile iron EN-GJS-500-14, experimental investigations and thermodynamic–kinetic microstructure simulations were carried out. Microstructure was characterized using metallographic analysis, and quantitative relations between chemical composition and microstructure were developed by means of regression analysis. Besides this quantitative analysis, it was found that the formation of grain boundary carbides can be detected via thermal analysis. Furthermore, experiments and simulations showed that vanadium promotes the formation of chunky graphite in high-silicon DI castings.

Effect of Cooling Rate on Graphite Morphology and Mechanical Properties in High-Silicon Ductile Iron Castings

This paper investigates the effects of silicon content above 3 wt% and different cooling rates on the graphite morphology and the mechanical properties of ductile cast iron. To research and determine these effects, three grades of high-silicon ductile iron were cast with silicon content of 3 wt%, 3.5 wt% and 4.5 wt%. In order to investigate the effect of different cooling rates during solidification, four sections with different thicknesses were cast in the same mold, i.e., cube with wall thickness 180 mm and standard Y2, Y3 and Y4 keel blocks. Microstructure analysis and tensile test were performed on the as-cast samples. In the second step of analysis, detailed metallographic observations on specially prepared samples were performed in order to investigate the risk of graphite degeneracy appearance, especially chunky graphite formation due to high silicon content and its effect on mechanical properties. The results showed that graphite content, ferrite fraction, yield strength and tensile strength were increased by increasing the silicon content. On the other hand, elongation decreases with increasing silicon content. Also, based on the metallographic observations of the present work, the risk of chunky graphite formation increases with increase in silicon content and with decrease in the cooling rate. Chunky graphite formation affects negatively mechanical properties, especially elongation in ductile cast irons with silicon content higher than 3.5 wt%.

Effect of Bismuth on Preventing Chunky Graphite in High-Silicon Ductile Iron Castings

The effect of Bi level on the macro- and microstructure of thick-walled high-silicon ductile iron castings has been investigated, as well as the influence of chunky graphite on the mechanical properties. Influence of cooling rate on graphite morphology was also investigated. High silicon level has caused the formation of chunky graphite, in very high amounts, along the entire cross section of the casting. Microstructure reveals that the addition of Bi affects not only the reduction of chunky graphite amount, but also the distribution along the casting cross section which is not uniform any more as in the casting without Bi. When Bi is added, the zone with chunky graphite appears independently of the cooling rate. Positive effect of Bi on the reduction of chunky graphite has been shifted towards higher levels, from 0.006 to 0.01 wt% Bi. Higher levels of Bi are more efficient also for the improvement in spheroidal morphology of graphite. The addition of 0.01 wt% Bi gave the best results, while the Ce/Bi ratio amounted to 0.6. This paper has proven that Bi acts on the increase in the strength values also in the high-silicon ductile iron casting. According to microstructure observations, it can be concluded that eutectic cells of chunky graphite are formed during bulk eutectic reaction. First, the primary graphite nodules are precipitated, followed by the initial phase of eutectic reaction during which the austenite dendrites precipitate and grow encapsulating at the same time primary graphite nodules.

Effect of Silicon and Graphite Degeneration on High-Temperature Oxidation of Ductile Cast Irons in Open Air

The use of high silicon ductile irons is increasing as they offer some advantages with respect to conventional pearlitic–ferritic grades such as high elongation at rupture for a given tensile strength value and a fully ferritic matrix. Besides addressing mechanical requirements, some castings must fulfil corrosion and high-temperature oxidation requisites. Two different ductile cast irons with silicon contents of 2.04 and 5.21 wt% were used so as to comparatively study their mechanical properties and high-temperature oxidation responses. The structure of both alloys contains nodular graphite and a fully ferritic matrix. Some samples of the alloy with 5.21 wt% silicon also contain abundant degenerated graphite which was identified as Chunky graphite. Oxidation resistance of both materials was evaluated by exposures to air at 650 °C for 720 h using a tubular furnace. The alloy with 5.21 wt% silicon showed an oxidation resistance about three times higher than the low silicon alloy. Although both alloys showed similar oxidation mechanisms, the oxidation scale formed on the high-silicon alloy stands out for having lower thickness, higher silicon content in the metal/oxide interface and more compact and adherent layers. Samples with Chunky graphite showed a similar evolution to those with graphite nodules, so no negative effect from this graphite degeneration on oxidation process was observed. The analyses performed by XRD revealed the presence of fayalite in the 5.21 wt% Si alloy, which is responsible for the better oxidation resistance.

Mechanical Properties and Impact Toughness of Nickel and Aluminum Alloyed High Silicon Ductile Iron

High silicon grades of ductile cast iron are known to be highly advantageous in regard to technically relevant properties and economic efficiency. In particular, the outstanding mechanical properties lead to an increasing demand since 2011, the year of incorporation to the EN 1563 standard. However, low impact resistance and spontaneous failure are concerns that limit the application, especially at lower temperatures. Silicon serves as a solid solution strengthener. By the addition of cobalt, aluminum and nickel as additional solid solution strengthener, an improvement in mechanical properties compared to only silicon could be obtained. Previous studies showed that the addition of 1.5 wt.% Ni to an EN-GJS-500-14 grade with 3.8 wt.% Si resulted in a tensile strength of 650 MPa at 15 % elongation. In the present study, silicon was substituted stepwise by nickel and aluminum, simultaneously aiming at the retention of the mechanical properties of the EN-GJS-500-14 grade. By decreasing the silicon content to 3.3 wt.% Si at 1.1 wt.% Ni and 0.2 wt.% Al, EN-500-14 was obtained. Even though, the presence of pearlite in the matrix was observed, this substitution of silicon led to an increase in Charpy-V-notch toughness by 4 Joule at room temperature. For further alloy design of high silicon ductile cast iron for simultaneously substituting silicon and improving the mechanical properties and notch toughness, the restrictions for pearlite formation must be complied.

Variation of Tensile Properties of High Silicon Ductile Iron

The casting processes are characterized by complex relationships between predictors and responses. It is the fundamental understanding of these complex relationships that often involves hundreds of factors, which improves quality without losing productivity and raising cost. In this work, cast solid solution strengthened ferritic spheroidal graphite irons GJS-500-14 and GJS-600-10 (EN 1563:2012) have been evaluated. These materials offer stronger components with good machinability owing to their even hardness properties. In this case the predictors are chemical composition, gating layout, foundry set-up, testing procedure and equipment etc. and the responses are the tensile properties (Rp0.2, Rm, A5). Here 200 tensile specimens compiled from industrial foundry melts from over 30 years of research have created a state-of-the-art platform for statistical engineering in order to perform Exploratory Data Analysis (EDA) and data visualization. This statistical platform has provided new insight on how foundries should treat complex relationships between predictors and responses in order to identify sources of variation and interaction effects.

Microstructure of As-Cast High-Silicon Ductile Iron Produced via Permanent Mold Casting

Permanent mold casting of ductile iron is an environmentally friendly alternative to sand mold casting. The high cooling rate leads to a homogeneous microstructure with low grain size, but can also lead to cementite formation. In the work presented, the as-cast microstructure of solid solution-strengthened ductile iron (SSDI) cast in permanent mold made of gray iron is analyzed. The high silicon content in SSDI decreases chilling tendency and promotes the formation of ferrite. Silicon contents in the range 3.2–4.3 wt% and mold preheating temperatures in the range 300–450 °C were analyzed for section thicknesses of 10, 20 and 30 mm. Solidification time and cooling rate at the casting sections analyzed were measured. The results show that an as-cast microstructure free from carbides is achieved in sections of 20 and 30 mm for all castings analyzed, whereas for 10 mm a partial chilled microstructure is produced. As expected, the pearlite content decreases with increasing silicon content and increasing preheating temperature. A higher cooling rate increases eutectoid undercooling and thus promotes pearlite formation. From the experiments carried out, a quantitative correlation is deduced for the pearlite content as a function of eutectoid undercooling and Si content. The nodule count is approximated as a linear dependency on solidification time.

Microstructural strain mapping during in-situ cyclic testing of ductile iron

This paper focuses on local strain distribution in the microstructure of high silicon ductile iron during cyclic loading. In-situ cyclic test was performed on compact-tension (CT) samples inside the scanning electron microscope (SEM) to record the whole deformation and obtain micrographs for microstructural strain measurement by means of digital image correlation (DIC) technique. Focused ion beam (FIB) milling was used to generate speckle patterns necessary for DIC measurement. The equivalent Von Mises strain distribution was measured in the microstructure at the maximum applied load. The results revealed a heterogeneous strain distribution at the microstructural level with higher strain gradients close to the notch of the CT sample and accumulated strain bands between graphite particles. Local strain ahead of the early initiated micro-cracks was quantitatively measured, showing high strain localization, which decreased by moving away from the micro-crack tip. It could be observed that the peak of strain in the field of view was not necessarily located ahead of the micro-cracks tip which could be because of the (i) strain relaxation due to the presence of other micro-cracks and/or (ii) presence of subsurface microstructural features such as graphite particles that influenced the strain concentration on the surface.

Mechanistic approach to new design concepts for high silicon ductile iron

Due to its superior mechanical properties, castability, machinability and price, the number of applications for high silicon solid solution strenghtened ferritic ductile cast iron is strongly increasing. However, the solid solution strengthening is limited to a maximum of 4.3wt% silicon. A higher silicon content leads to a decline of UTS, YTS and to a strong decline in elongation at fracture. In order to analyze the fundamental mechanism, Y2 and Y4 standard DI specimens were casted with silicon contents varying between 3.95 and 5.63wt%. For a comprehensive analysis, all investigations were conducted on specimens manufactured from the same castings. With increasing silicon, the fracture type changes from ductile to brittle, whereas the graphite nodularity, nodule size and nodule count as well as the amount of harmful phases cannot be connected to the observed embrittlement phenomena. Therefore a ductile (3.95wt% Si) and a brittle specimen (5.36wt% Si) where investigated via TEM diffraction. It was shown that the amount of B2-ordered phases in the matrix strongly increases with increasing Si-Content. Moreover, reflexes of DO3-ordered phases could only be observed in the brittle specimen. Silicon long-range-ordering can be concluded as the main reason for the upper limit of silicon solid solution strengthening in ductile iron grades in accordance to previous findings for silicon steels. The presented study paves the way for mechanistically based new alloy design concepts for high silicon ductile iron.

Study of dimensional change of high-silicon ductile iron with ADI and Dual-Phase-ADI microstructures starting from different as-cast structures

In this work the percent dimensional change (DC%) and its dispersion in high silicon Ductile Iron with ADI and Dual-Phase-ADI structures obtained through heat treatment starting from different as-cast microstructures, was studied. Specimens were obtained from three different melts with Si content ranging from 3.1 to 4.2%. Dual-Phase-ADI structures were obtained using special heat treatment, starting from as-cast structures. Regarding dimensional change, DC% in ADI and Dual-Phase-ADI decrease as Si% and the proportion of ferrite phase in as-cast specimens increased. On the other hand, DC% increased with austenitizing temperature and ausferrite content in the final microstructure. A fuzzy model to predict DC% in ADI parts was adapted to include DC% prediction in Dual-Phase-ADI. The fuzzy model allowed to predict the DC% for ADI and Dual-Phase-ADI parts with an root mean square <0.05%.

Characterization and modeling of the mechanical behavior of high silicon ductile iron

This paper investigates the effect of the solidification conditions and silicon content on the mechanical properties of ductile iron and presents empirical models for predicting the tensile behavior based on the microstructural characterizations. Two ductile iron grades of GJS-500-7 and GJS-500-14 were cast with silicon content of 2.36% and 3.71%, respectively. The cast geometry consisted of six plates with different thicknesses that provided different cooling rates during the solidification. Microstructure analysis, tensile and hardness tests were performed on the as-cast material. Tensile behavior was characterized by the Ludwigson equation. The tensile fracture surfaces were analyzed to quantify the fraction of porosity. The results showed that graphite content, graphite nodule count, ferrite fraction and yield strength were increased by increasing the silicon content. A higher silicon content resulted in lower work hardening exponent and strength coefficient on the Ludwigson equation. The results for 0.2% offset yield and the Ludwigson equation parameters were modeled based on microstructural characteristics, with influence of silicon content as the main contributing factor. The models were implemented into a casting process simulation to enable prediction of microstructure-based tensile behavior. A good agreement was obtained between measured and simulated tensile behavior, validating the predictions of simulation in cast components with similar microstructural characteristics.

高Si球状黒鉛鋳鉄の切欠き強度とその構造部材としての考察

高Si球状黒鉛鋳鉄の切欠き強度とその構造部材としての考察