Ductile Iron Grade Research Articles

The effect of silicon on the critical resolved shear stress of solid solution strengthened ferritic ductile iron

Solid solution strengthened ferritic ductile iron (SSFDI) exhibits improved mechanical properties compared to conventional ductile cast iron (DCI) grades, however, its potential widespread application is hindered by unpredictable brittle fracture which might be attributed to microstructural silicon segregation and associated superstructure formation. The aim of the present study is therefore to deepen the understanding on the effect of local silicon segregation on the mechanical properties of SSFDI, which is crucial especially for the common applications of DCI in cyclically loaded structures. Micropillar compression tests were carried out on three different casts to investigate the solution strengthening effect of silicon in the ferritic matrix. An almost perfect linear relationship between critical resolved shear stress (CRSS) and global silicon content was found. It was also found that the variation of CRSS with silicon content corresponds well to the variation of the macroscopic yield limits (under tension and compression) with global silicon content of different SSFDI alloys. This indicates that the ferritic matrix dominates the yield limit of the DCI alloys investigated in this study, while the morphology of the graphite nodules plays a minor role under monotonic loading conditions.

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.

Influence of Cooling Rate and Alloying by “Cr, V, and Ni” on Microstructure and High-Temperature Wear Behavior of SiMo Ductile Iron

AbstractSiMo ductile irons, typical heat-resistant materials, are subjected to varied wear environments during operation in high-temperature applications. SiMo ductile iron castings of different thicknesses were cast in investment and greensand molds, achieving a wide range of cooling rates. The present work aims to investigate the effect of the cooling rate and alloying elements (Cr, V, and Ni) on the microstructure and the abrasive wear behavior of these grades of SiMo ductile iron at high-temperature 700 °C under different loads. Thermodynamic calculations were used to propose the phase diagrams, critical transformation temperatures, and phase volume fractions in all SiMo alloys by using the Thermo-Calc software then verified by and Differential Scanning Calorimetry (DSC). The microstructure of unalloyed SiMo ductile cast iron consists of graphite nodules and carbides embedded in the precipitates at the grain boundary regions in a ferrite matrix. The alloyed SiMo microstructure contains nodular graphite and the carbides promoted by the alloying elements (Cr and V). The alloyed SiMo alloys exhibit higher wear resistance than unalloyed ones. These wear results support that the microstructure plays a chief role in wear loss. The combination of M6C, VC, and M7C3 carbides embedded in a ferrite-pearlite matrix (alloyed SiMo) seems to be more resistant to wear than the ferritic matrix with lamellar pearlite and eutectic M6C carbides (unalloyed SiMo).

A strain energy density design approach for large cast iron components: From microstructural analysis to multiaxial fatigue response

A new criterion for predicting multiaxial fatigue in plain and V-notched specimens made of ductile cast iron is proposed. The criterion is based on volumetric strain energy density (SED) and separately accounts for the effect of normal and shear mean stresses with two material-dependent coefficients. The criterion also considers the impact of out-of-phase loading through a strengthening factor, assuming that fatigue damage is initiated by the total SED peak in the cycle. The criterion is validated using a large amount of independent multiaxial fatigue data, obtained from testing three different ductile cast iron grades.

Influence of Microstructure on Static and Cyclic Fracture Toughness of EN-GJS-600-3 Nodular Cast Iron

In this paper, the effect of microstructure of a thick-walled rotor shaft for wind turbines on fracture toughness properties has been investigated. The relevant nodular cast iron grade EN-GJS-600-3 was processed using chill casting technology. Due to different solidification conditions over the wall thickness, heterogeneous microstructures were formed. To illustrate the influence of the microstructure gradient caused by chill casting technology, specimens were taken from different sample positions in the cross section of the casting component. A detailed metallographic analysis revealed essential differences in microstructure. The crack growth resistance under quasi-static loading conditions and the fatigue crack propagation under cyclic loading conditions were measured. The results of the static fracture mechanics investigations revealed that fracture toughness is strongly influenced by the microstructure of this pearlitic ductile iron grade. On the other hand, cyclic fracture mechanics analysis showed that the complex formation of the microstructure has only a minor effect on the fatigue threshold value, but microstructure has a significant effect on the stable crack growth. For the assumed load cases, it was shown that microstructure can be a dominant factor on the mechanical and fracture toughness properties.

Influence of Free Ferrite on the Mechanical Properties of High Strength Intercritical Austempered Ductile Iron

The first stage of this study investigates the precipitation of free ferrite from the austenite on fully austenitized ductile iron. Several sets of samples of low alloy ductile iron are fully austenitized and then cooled down to different temperatures and different times within the intercritical austenite-ferrite-graphite phase field. Based on these results, heat treatment cycles aimed at obtaining microstructures composed of free ferrite and ausferrite are carried out. Tensile, impact and toughness tests are performed to characterize the mechanical properties. The results show that, related to the high strength austempered ductile iron grades, the best combinations of properties were obtained from the mixed structures composed of 5% free ferrite and 95% ausferrite, resulting from the austempering at 280°C. These amounts of free ferrite allow obtaining an increase of the elongation (about 50%) and impact toughness (about 10%) while the tensile strength and fracture toughness decrease by about 1.5 and 15% respectively.

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.

Tensile properties and fracture toughness at service temperatures of an optimized pearlitic ductile iron alloy for automotive crankshafts

Cast iron and forged steel have competed against each other for many years for different industrial applications. Crankshafts are one of the products where this competition is still active, both for heavy duty and light machinery applications. For the specific case of automotive crankshafts, high strength pearlitic cast iron grades have found a strong position in passenger car market niches. The mechanical performance data available for these specific ductile cast iron grades mostly come from room temperature tensile and impact testing. Nevertheless, the failure mode of cast iron crankshafts involves fracture mechanics, and their usual range of working temperatures goes from −20°C for cold starts to 120°C for top maximum lubricating oil temperature. This work covers this range by studying a high strength pearlitic cast iron employed to manufacture crankshafts both in terms of tensile, impact and fracture mechanics testing at −20°C, 22°C and 120°C. A specific molding pattern was designed to produce the desired materials, namely crankshafts and Y2 and Y3 wedges, which were cast in an industrial crankshaft manufacturing line. The results indicate that there is a relevant difference between the low temperature and the high temperature mechanical response of the studied cast iron. This finding is interpreted in terms of safety, as the crankshafts should not fail in service after the engine has started running and the lubrication has reached its steady state working temperature.

Ultrafine Ductile and Austempered Ductile Irons by Solidification in Ultrasonic Field

In this research, ultrasonic melt treatment (UST) was used to produce a new ultrafine grade of spheroidal graphite cast iron (SG iron) and austempered ductile iron (ADI) alloys. Ultrasonic treatment was numerically simulated and evaluated based on acoustic wave streaming. The simulation results revealed that the streaming of the acoustic waves propagated as a stream jet in the molten SG iron along the centerline of the ultrasonic source (sonotrode) with a maximum speed of 0.7 m/s and gradually decreased to zero at the bottom of the mold. The metallographic analysis of the newly developed SG iron alloy showed an extremely ultrafine graphite structure. The graphite nodules’ diameter ranging between 6 and 9 µm with total nodule count ranging between 900 to more than 2000 nodules per mm2, this nodule count has never been mentioned in the literature for castings of the same diameter, i.e., 40 mm. In addition, fully ferritic matrix was observed in all UST SG irons. Further austempering heat treatments were performed to produce different austempered ductile iron (ADI) grades with different ausferrite morphologies. The dilatometry studies for the developed ADI alloys showed that the time required for the completion of the ausferrite formation in UST alloys was four times shorter than that required for statically solidified SG irons. SEM micrographs for the ADI alloys showed an extremely fine and short ausferrite structure together with small austenite blocks in the matrix. A dual-phase intercritically austempered ductile iron (IADI) alloy was also produced by applying partial austenitization heat treatment in the intercritical temperature range, where austenite + ferrite + graphite phases coexist. In dual-phase IADI alloy, it was established that introducing free ferrite in the matrix would provide additional refinement for the ausferrite.

Microcracks Reduction in Laser Hardened Layers of Ductile Iron

A study of surface hardening of Ductile Iron (DI) with and without austempering heat treatment was developed. The chemical composition of the material contains alloying elements such as Cu and Ni, that allow to obtain a Ductile Iron Grade 120-90-02, based on ASTM A536, which was heat treated to be transformed to Austempered Ductile Iron (ADI). Specimens of 10 × 10 × 5 mm3 were obtained for application of surface hardening by Nd:YAG UR laser of 150 W maximum power. The parameters used were advance speed of 0.2 and 0.3 mm/s and power at 105, 120, 135 and 144 W; the departure microstructures were fully pearlitic in the samples without heat treatment, and ausferrite for austempered samples. Microstructural characterization of hardened samples was performed were analyzed and martensite and undissolved carbides were identified in the pearlitic samples, while in ausferrite samples it was found finer martensite without carbides. The depth of hardened surface to different conditions and their respective microhardness were measured. The results indicate that the surface hardening via laser is a suitable method for improving wear resistance by means of hardness increment in critical areas without compromising the core ductility of DI components, but the surface ductility is enhanced when the DI is austempered before the laser hardening, by the reduction of surface microcracks.

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.

The Effects of Austempering Temperature and Time on Mechanical Properties of Ductile Cast Iron Grade FCD450

The purpose of the study was to inspect microstructure, mechanical properties and impact toughness of ductile cast iron grade FCD450 produced by austempering process. The study focused on austempering parameter, which effected impact toughness of material at low temperature. The FCD450 was initially temperature austenized at 885°C (1625˚F) for 2 hours. Austempering was carried out at three different temperatures of 271°C (520˚F), 313°C (560˚F) and 357°C (675˚F). The austempering temperature were varied at 1.5, 2.5 and 3.5 hours. X-ray diffraction was showed that the austempered ductile cast iron (ADI) microstructure consists of austenite and ferrite. The results showed that when austempered at 357°C (675˚F) for 2.5 hours has highest hardness and impact energy at low temperature. The dimple ductile fracture of ADI fracture surfaces was revealed by scanning electron microscope (SEM).

Optimization Study on Sliding Wear Characteristics and Heat-Treatment Conditions of Different Grades of Ferritic Ductile Cast Iron

The present work reports an experimental investigation on influence of microstructures on dry sliding wear performance of different grades of ferritic ductile iron. Ductile cast iron samples of ferritic grade have been subjected to different heat-treatment processes at different temperatures and times. Taguchi optimization technique (L16) has been applied to evaluate the influence of different process variables (load, time, heat treatment, and grade) during ball-on-plate wear test. Meanwhile, analysis of variance (ANOVA) method was adopted to know the significance of aforesaid process variables. ANOVA results confirmed that the heat-treatment process has highest significance (54.76%) within all process variables. Among heat-treated specimens, austempered samples have outstanding wear resistance while the DMS samples have lower wear resistance. In addition, overall utility values have been evaluated by using individual utility values of weight loss and hardness. An obtained overall utility value gives the optimum combination for achieving higher wear resistance and hardness. Additionally, morphology of wear surfaces was examined in scanning electron microscope and the micrographs confirm the existence of inferior surface in terms of abrasive wear, adhesive wear, particle pullout and delaminated sheets on wear track. Enrichment of oxygen element has been observed on the worn path through energy-dispersive X-ray spectroscopy. X-ray diffraction analysis confirms the existence of different compounds like iron and silicon oxides on the wear track surface which may improve its hardness.

The Importance of SiC in the Process of Melting Ductile Iron with a Variable Content of Charge Materials.

The article presents issues related to melting ductile iron grade EN-GJS-400-15, with different proportions of feedstock (steel scrap and pig iron). The main attention was paid to determining the impact of silicon carbide on the structure and properties of melted cast iron. In the conducted melts, carbon and silicon deficiencies were supplemented with a suitably chosen carburizer, ferrosilicon, and SiC metallurgical silicon carbide. The percentage of silicon carbide in the charge ranged from 0 to 0.91%. The basic condition for the planning of melts was to maintain the repeatability of the chemical composition of the output cast iron and cast iron after the secondary treatment of liquid metal with various charge compositions. Based on the tests, calculations, and analyses of the results obtained, it was concluded that the addition of SiC may increase the number and size of graphite precipitates. Increasing the SiC content in the charge also caused a change in the solidification nature of the alloy and the mechanism of growth of spheroidal graphite precipitates, causing their surface to form a scaly shell. The influence of the addition of silicon carbide on the reduction of the temperature of liquidus in the alloys was also observed. Silicon carbide had a positive effect on the structure and properties of melted alloys. The introduction of SiC into the melting in the studied range caused an increase in the content of carbon and silicon without causing an increase in the amount of impurities in the alloy.

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%.

ASSESSING THE VOLUME FRACTIONS OF THE PHASES, NODULARITY AND NODULE COUNT OF SPHEROIDAL GRAPHITE CAST IRON USING IMAGEJ SOFTWARE

In this study, a simple method for assessing the microstructure of ductile cast iron by using a freeware digital image processing software is described. The method is applied to three different ductile iron grades to assess their ferrite, pearlite and graphite volume fractions as well as graphite nodularity, nodule count, nodule size. All specimens were found to show good nodularity (~ 88 % by area) with different graphite average size and perimeter morphology. The amount of graphite was also found to be similar (11±2 % by area) in all specimens. On the other hand, ferrite percentages were found as 85.8, 57.1, and 52.5 % respectively for ASTM A536 grades 60-40-18, 65-45-12, and 80-55-06. It was also found that among these three grades, the higher the pearlite content, the higher the hardness. The agreement with the standards confirms that quantitative metallography through image processing is a powerful tool in order to estimate the mechanical properties of cast irons.

Influence of aluminium on silicon microsegregation in solution strengthened ductile iron

ABSTRACTThe excellent static mechanical properties of solid solution strengthened grades of ferritic ductile iron (SSFDI) are adjusted by elevated silicon contents and undergo a rapid loss above a critical content of about 4.3 wt-% silicon. This phenomenon is attributed to the formation of iron-silicon superstructures that is intensified in particular by the formation of silicon microsegregation. In order to affect silicon microsegregation, the alloying concept of SSFDI has been modified. In the present investigations, an inversion of the silicon microsegregation profile could be achieved by alloying a nodular cast iron melt with 1.2 wt-% aluminium. The results provide a metallurgical tool to shift the silicon embrittlement to higher silicon contents and thus to further enhance the mechanical properties of SSFDI.

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.

Grey relational analysis coupled with principal component analysis to optimize the machining process of ductile iron.

The strict specifications for specific applications of the ductile iron casting must qualify the defined standards with limiting tensile strength and the correct material composition limits. The undesired combinations of properties are reported during casting of ductile iron components because of the graphite presence as spheroids rather than as flakes. The design specific grades of ductile Iron comprising desired matrix microstructure can be produced by controlling the matrix microstructure with alloy additions around the graphite either by heat treatment or as-cast. The present research investigates the machinability of ductile iron grade EN-GJS-500-7and the development of model and optimized design for the major machining characteristic indexes viz. metal removal rate, cutting forces and surface roughness in turning process. The cutting parameters selected are cutting speed, feed rate and depth of cut. The grey relational analysis adopting grey relational grades as performance index is used to determine the optimal combination of cutting parameters. The set of experiments on the basis of response surface methodology (RSM) are employed in preparing the objective model to study the effect of the main turning parameters. Further principal component analysis is applied to calculate the weighting values corresponding to the selected machining characteristic indexes in order to determine their relative importance during the process. The results obtained from the confirmation experiments revealed that grey relational analysis coupled with principal component analysis is an appropriate approach and thus proposed to be a useful tool to improve the cutting performance in turning process.

Diagnosis of Missed Ductile Iron Melts with Process Modelling

AbstractThe paper presents an application of advanced data-driven (soft) models in finding the most probable particular causes of missed ductile iron melts. The proposed methodology was tested using real foundry data set containing 1020 records with contents of 9 chemical elements in the iron as the process input variables and the ductile iron grade as the output. This dependent variable was of discrete (nominal) type with four possible values: ‘400/18’, ‘500/07’, ‘500/07 special’ and ‘non-classified’, i.e. the missed melt. Several types of classification models were built and tested: MLP-type Artificial Neural Network, Support Vector Machine and two versions of Classification Trees. The best accuracy of predictions was achieved by one of the Classification Tree model, which was then used in the simulations leading to conversion of the missed melts to the expected grades. Two strategies of changing the input values (chemical composition) were tried: content of a single element at a time and simultaneous changes of a selected pair of elements. It was found that in the vast majority of the missed melts the changes of single elements concentrations have led to the change from the non-classified iron to its expected grade. In the case of the three remaining melts the simultaneous changes of pairs of the elements’ concentrations appeared to be successful and that those cases were in agreement with foundry staff expertise. It is concluded that utilizing an advanced data-driven process model can significantly facilitate diagnosis of defective products and out-of-control foundry processes.