Graphite Nodules Research Articles

Pearlitic ductile cast iron: fatigue crack paths and damaging micromechanisms

The influence of the graphite nodules morphology (shape, dimension and distribution) on ductile cast irons (DCIs) mechanical properties is experimentally confirmed both in static, quasi static and cyclic loading conditions. According to the most recent results, these graphite elements cannot be merely considered as “microvoids embedded in a metal matrix”, but their presence implies a modification of the damaging micromechanisms and this modification I s influenced by the metal matrix microstructure.In this works, the different damaging mechanisms that are active in the graphite nodules in a pearlitic DCI are semi-quantitatively analyzed using light optical microscope observations of the fracture surface profiles.

Fatigue and fracture resistance of ferritic ductile cast iron: the effect of Sb and solidification time

In this paper, we explore the effect of inoculants and solidification time on the mechanical properties of an EN-GJS-400-type ferritic ductile cast iron (DCI). For this purpose, static tensile, rotating bending fatigue, fatigue crack growth and fracture toughness tests are carried out on three different material conditions. They are produced under fast cooling (solidification time 2h45min), representative of thin walled castings, and very slow cooling (solidification time 10—13h), representative of thick walled castings, this latter with and without the addition of Sb. It has been found that the long solidification time leads to an overgrowth and degeneration of the spheroidal graphite nodules. The addition of Sb avoids the formation of chunky graphite observed in the slowly cooled condition but results in large exploded graphite nodules. These effects impact negatively on tensile strength, total elongation and fatigue strength. Conversely, the resistance to fatigue crack growth is even superior and the fracture toughness comparable to that of the fast cooled condition. Metallurgical and fractographic analyses are carried out to explain this behaviour.

Influence of graphite morphology on static and cyclic strength of ferritic nodular cast iron

High silicon alloyed nodular cast iron consists of a purely ferritic matrix and graphite nodules, mainly. Varying wall thicknesses and manufacturing conditions result in different graphite morphologies throughout a structural component. From an experimental point of view, axial fatigue and tensile tests were carried out on specimens with differently degraded graphite. From a numerical point of view, the microstructure has been modelled using a finite element (FE) approach with representative volume elements (RVE). The RVE models were built according to micrographs of fatigue specimens. The generated RVEs determine effective material properties through elasto-plastic homogenization and were subsequently analysed using a shakedown approach. In shakedown theory, the material re-enters the elastic regime after a few cycles of initial plastic deformation. This work uses the shakedown theorem to derive a lower bound estimation of the endurance limit from a non-incremental simulation. Here, the material has to be modelled elastic-perfectly plastic. The major challenge in modelling nodular cast iron is to determine suitable material parameters for the graphite and ferrite phase, revealed by parameter studies on the static and cyclic model. By using reasonable material parameters, fundamental effects, observed in the fatigue tests, were reproduced on the model level.

Effect of some inoculants on the structure and properties of thin wall ductile iron

The production of ductile iron parts as thin as (2-3 mm), known generally as Thin Wall Ductile Iron (TWDI) casting. Inoculants play a great role in changing the structure–properties of ductile iron. In the present work, the effect of inoculation on thin wall ductile iron castings is analyzed. Three types of potent inoculants were used namely Ce-Ca-Al-S-O-FeSi, Zr-Mn-Ca-Al-Ba-FeSi and Sr-Al-Ca-FeSi. The in-mould inoculation was done. It is observed that Ce-Ca-Al-S-O-FeSi inoculant provides highest nodule count and a unique wide distribution of nodule size in comparison to thin wall ductile iron (TWDI) inoculated with Zr-Mn-Ca-Al-Ba-FeSi and Sr-Al-Ca-FeSi. By increasing the weight of Ce-Ca-Al-S-O-FeSi from 0.2% to 0.4% for in-mould inoculation leads to increase in the nodule count. It was also observed that thin wall ductile iron (TWDI) samples inoculated with Ce containing 0.2% to 0.4% inoculant contain pearlitic matrix, whereas the samples inoculated with 0.6% inoculant have ferritic matrix. The as-cast microstructure of thin wall ductile iron (TWDI) containing 0.6% inoculants contain islands of pearlite in ferritic matrix whereas the samples inoculated by 0.2% to 0.4% inoculants had ferrite in the form of concentric shells around graphite nodules.

Bending damages in galvanized ductile cast irons

Ductile cast irons (DCIs) are characterized by mechanical properties close to low and medium carbon steels. Carbon atoms are mainly localized in graphite nodules, which are dispersed in a metallic matrix. The microstructure of metallic matrix can be ferritic, austenitic, pearlitic, martensitic or their mix, depending on chemical composition and heat treatment. Thanks to the high castability and low production costs, DCIs are used in many fields (e.g., automotive and pipes). The wide utilization of DCIs in many fields and critical application leads to particular attention to the corrosion phenomenon. Hot dip galvanizing is one of most important protection process, used to protect metallic materials (mainly steels) against corrosion in many aggressive environments.In this work, a ferritic-pearlitic DCI (GS500) was galvanized by using a pure Zn bath at 440°C to generate a zinc coating. Bending tests on galvanized specimen were performed to generate crack damage in the coating phases. The bending cracks path propagation in zinc coatings were observed using both a light optical microscope and scanning electron microscope. A damage parameter, defined as a number of radial crack for a millimeter of the deformed arc, was evaluated for each zinc coating intermetallic phases.

Pearlitic Ductile Cast Iron: mechanical properties gradient analysis in graphite elements

Ductile Cast Irons (DCIs) are able to combine a good versatility and high performances with a low cost, especially if compared to steels with analogous performances. For these reasons, although these grades have been relatively recently developed, DCIs applications are more frequent. Analyzing the damaging micromechanisms in static, quasi-static or cyclic conditions, the analysis of the role played by the graphite elements is not univocal. Sometimes, they are considered as voids embedded in a more or less ductile matrix, sometimes they are considered as a soft but homogeneous material. In this work, the role played by the graphite nodules in pearlitic grains is reviewed and their mechanical properties are investigated by means of nanohardness tests.

Identification of Mg2Cu particles in Cu-alloyed austempered ductile iron

In the present work, the Mg2Cu precipitates in copper-alloyed austempered ductile iron (ADI) were identified by analyzing techniques such as TEM and SEM with EDS. It was revealed that, in castings made of ADI-containing copper, highly dispersed particles of Mg2Cu are formed, whose size does not exceed <1 μm. The research work was carried out on ductile iron that was austenitized at 900 °C, followed by austempering at 380 °C. The microstructure was investigated using various techniques, including optical microscopy, XRD, SEM, and TEM. In addition to this, the exhibited impact properties of castings with Cu, Ni, and Cu+Ni were also determined. This study casts a new light on the formation of the structure of Cu-alloyed ADI. The highly-dispersive and brittle Mg2Cu particles that are located in the vicinity of the graphite nodules have a negative effect on the impact properties of ADI. It has also been shown that impact strength decreases from levels of 160-180 J (for copper-free ADI) to 90-120 J (for copper-and copper-nickel-alloyed ADI).

Prediction of Graphite Nodule Count and Shrinkage Tendency of Spheroidal Graphite Cast Iron by One Cup Thermal Analysis

Three cup thermal analysis is a good method for predicting chill depth, tensile strength and graphite type in gray cast iron. However, it cannot be used in ductile cast iron due to several reasons. For this reason, we have developed a method for investigating the graphite eutectic temperature (hereafter TEG) and cementite eutectic temperature (hereafter TEG) in ductile cast iron, from the chemical composition of the molten metal, as follows

Theory of the Stereological Analysis of Spheroid Size Distribution – Validation of the Equations

AbstractThe paper presents validation tests for method which is used for the evaluation of the statistical distribution parameters for 3D particles’ diameters. The tested method, as source data, uses chord sets which are registered from a random cutting plane placed inside a sample space. In the sample space, there were individually generated three sets containing 3D virtual spheres. Each set had different Cumulative Distribution Function (CDF3) of the sphere diameters, namely: constant radius, normal distribution and bimodal distribution as a superposition of two normal distributions. It has been shown that having only a chord set it is possible, by using the tested method, to calculate the mean value of the outer sphere areas. For the sets of data, a chord method generates quite large errors for around 10% of the smallest nodules in the analysed population. With the increase of the nodule radii, the estimation errors decrease. The tested method may be applied to foundry issues e.g. for the estimation of gas pore sizes in castings or for the estimation of nodule graphite sizes in ductile cast iron.

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.

Rolling contact fatigue resistance of austempered ductile iron processed at various austempering holding times

The rolling contact fatigue resistance of austempered ductile iron (ADI) produced at various holding times was evaluated by using a twin-disk rolling configuration rig. The resulting fatigue life of ADI samples was compared with that of conventional quenched and tempered (Q&T) ductile iron samples with the same chemical composition. It was found that the macro hardness of ADI samples decreased and the fatigue life became shorter for longer holding time. For similar macro hardness, ADI samples had higher fatigue resistance as compared with Q&T ductile iron samples. Optical microscopy and scanning electron microscopy indicated that the graphite nodules acted as crack nucleation sites for the ductile iron. The cracks propagated towards either the surface or adjacent graphite nodules until the formation of pits or spalls occurred. The excellent fatigue resistance of ADI samples could be attributed to the increase of surface hardness which was caused by the strain induced transformation of retained austenite into martensite. By using X-ray diffraction, the difference of retained austenite present in the wear track and off the wear track ranged from 4.42% to 26.15%.

Genetic Algorithm-Based Optimization Methodology of Bézier Curves to Generate a DCI Microscale-Model

The aim of this article is to develop a methodology that is capable of generating micro-scale models of Ductile Cast Irons, which have the particular characteristic to preserve the smoothness of the graphite nodules contours that are lost by discretization errors when the contours are extracted using image processing. The proposed methodology uses image processing to extract the graphite nodule contours and a genetic algorithm-based optimization strategy to select the optimal degree of the Bézier curve that best approximate each graphite nodule contour. To validate the proposed methodology, a Finite Element Analysis (FEA) was carried out using models that were obtained through three methods: (a) using a fixed Bézier degree for all of the graphite nodule contours, (b) the present methodology, and (c) using a commercial software. The results were compared using the relative error of the equivalent stresses computed by the FEA, where the proposed methodology results were used as a reference. The present paper does not have the aim to define which models are the correct and which are not. However, in this paper, it has been shown that the errors generated in the discretization process should not be ignored when developing geometric models since they can produce relative errors of up to 35.9% when an estimation of the mechanical behavior is carried out.

Effect of Heat Treatments and SiC Content in the Mechanical Properties and Microstructure of Self-Lubricating Steels

The objective of the present work was to study the effect of heat treatments in the microstructure, graphite nodules and mechanical properties of self-lubricating steels, to achieve this, self-lubricating steels (Fe + 0.45C + 4Ni + 1Mo %wt) and additions of 2 and 3 %wt SiC were fabricated. They were consequently heat treated in 3 different conditions: martempering at 180 °C and tempering at 530 °C and 300 °C respectively and austempering at 300 °C. Hardness, yield strength, tensile strength and work hardening behavior were studied in as-sintered and heat-treated samples. The microstructure was analyzed by optical microscopy, scanning electron microscopy, Raman spectroscopy and micro-hardness. The transformation temperatures were determined using dilatometric tests. Results show that the presence of dissolved Si in the matrix due to SiC dissociation notably affects the morphology of the microstructure and transformation temperatures also affecting post heat treatment mechanical properties. The structure of graphite nodules produced by SiC dissociation is nott significantly affected by the heat treatments.

Discrete laser spot hardening of austempered ductile iron

This study investigates the effect of discrete spot laser surface hardening (LSH) on austempered ductile iron (ADI) specimens. ADI surfaces were irradiated with single laser pulses, having different beam powers and diameters with the intent of establishing the parameters that yield a surface with the best combination of maximum hardness and hardened depth, whilst exhibiting minimal distortion and no cracking. A process parameter map was plotted and showed that to produce a hardened microstructure without melting and cracking, one must operate within a very small range of parameters achieving surface hardness values between 700 and 800HV and depths of approximately 150μm. The resulting microstructure consisted of martensite with unaltered graphite nodules. After laser hardening, the mean surface roughness increased from 0.15 to 0.43μm. When applying laser powers higher than 780W, melting was observed within the laser spot. Laser surface melting (LSM) resulted in a severely distorted surface, with an increase in the mean surface roughness from 0.15 to 1.34μm. Within the melted zone, transverse cracks and the dissolution of the graphite nodules could be observed. The rapid self-quenching associated with laser melting resulted in an austenitic microstructure having a surface hardness of approximately 400HV. A martensitic structure with hardness values peaking around 700–800HV, was observed at a depth 250–300μm below the molten structure.

On the fatigue damage micromechanisms in Si‐solution–strengthened spheroidal graphite cast iron

AbstractGraphite nodules in spheroidal graphite cast iron (SGI) play a vital role in fatigue crack initiation and propagation. Graphite nodules growth morphology can go through transitions to form degenerated graphite elements other than spheroidal graphite nodules in SGI microstructure. These graphite particles significantly influence damage micromechanisms in SGI and could act differently than spheroidal graphite nodules. Most of the damage mechanism studies on SGI focused on the role of spheroidal graphite nodules on the stable crack propagation region. The role of degenerated graphite elements on SGI damage mechanisms has not been frequently studied. In this work, fatigue crack initiation and propagation tests were conducted on EN‐GJS‐500‐14 and observed under scanning electron microscope to understand the damage mechanisms for different graphite shapes. Crack initiation tests showed a dominant influence of degenerated graphite elements where early cracks initiated in the microstructure. Most of the spheroidal graphite nodules were unaffected at the early crack initiation stage, but few of them showed decohesion from the ferrite matrix and internal cracking. In the crack propagation region, graphite/ferrite matrix decohesion was the frequent damage mechanism observed with noticeable crack branching around graphite nodules and the crack passing through degenerated graphite elements. Finally, graphite nodules after decohesion acted like voids which grew and coalesced to form microcracks eventually causing rapid fracture of the remaining section.

Effects of Magnesium Variation and Heat Treatment on Mechanical and Micro-Structural Properties of Ductile Cast Iron

The effects of magnesium variation and austempering heat treatment on mechanical and microstructural properties of ductile iron produced using the rotary furnace were investigated. Varied quantity of magnesium-ferrosilicon in the range of 0.03 % to 0.06 % were used as nodulirizer to treat 4 kg mass of molten metal per ladle by sandwich process and poured into sand mould. Mechanical test (tensile, hardness, fatigue, impact and wear) and micro-structural examinations were carried out on the four samples produced. Samples C and D of 0.056% and 0.061% magnesium showed an improvement in their micro-structural properties due to the presence of more graphite nodules. Hence, they were observed to have exhibited better tensile strength of 598.07MPa and 609.03MPa. The fatigue strength also increased to 501.91MPa and 509.27MPa respectively. These two samples were further subjected to austempering heat treatment by heating to 850ºC for austenitization and soaked for homogenization for one hour at the temperature before quickly transferred into a salt bath of 50 % NaNO3 : 50 % NaCl maintained at 3600C and quenched for transformation for 1 hour before finally air cooled. Mechanical tests and micro-structural examinations were thereafter carried out. Sample C had an outstanding increase in tensile strength, from 598.07MPa to 891.22MPa, while specimen D increased from 609.03MPa to 898.76MPa. The results of abrasion test indicated that samples C and D had abrasion resistance increase from 2.20×10¹¹m² and 2.39×10¹¹m² to 2.35×10¹¹m² and 2.68×10¹¹ m² respectively after austempering. There were also relative increase in fatigue resistance and impact toughness for the two samples but with relative reduction in hardness from 47.7 to 44.2 and 50.3 to 47.4.

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.

Structure and properties of in situ-generated two-dimensional turbostratic graphite nodules

Powder injection moulding and sintering were used to obtain iron-based self-lubricating composites in which SiC particles were used as precursors to generate graphite nodules dispersed in the composite matrix. Using thermodynamic simulations with DICTRA®, it was possible to predict the SiC dissociation in the ferrous matrix studied (Fe+0.6C). The evolution of the dissociation of SiC particles during sintering was investigated by X-ray diffraction (XRD) and dilatometric analysis, whereas the structure and morphology of the graphite nodules were studied by field emission gun-scanning electron microscopy (FEG-SEM), micro-Raman spectroscopy, and transmission electron microscopy (TEM). To compare the characteristics of the graphite nodules, micro-Raman and tribological analyses (friction coefficient and wear rate) were carried out for nodular cast iron and graphite powder. Reductions in the friction coefficient and wear rate of the tribological pair can be associated with the turbostratic characteristics of the graphite nodules.

Numerical simulation of microstructure evolution on near eutectic spheroidal graphite cast iron

A multiphase cellular automaton model was developed to simulate microstructure evolution of near eutectic spheroidal graphite cast iron (SGI) during its solidification process, and both dendritic austenite and spheroidal graphite growth models were adopted. To deduce the mesh anisotropy of cellular automaton method, the composition averaging and geometrical parameter were introduced to simulate the spheroidal graphite growth. Solute balance method and decentered square algorithms were employed to simulate austenite dendrites growth with different crystallographic orientations. The simulated results indicate that the graphite nodule grows in a spherical morphology when the surrounding environment of a single graphite nodule is same. However, for two adjacent graphite nodules, the environment is different. The higher the carbon concentration, the faster the growth of graphite. By comparison with experimental results, it is found that the microstructure evolution of near eutectic spheroidal graphite cast iron during solidification process can be reproduced quantitatively by numerical simulation with this model.

Multi-Scaling Homogenization Process for Nodular Cast Iron Using BEM

In this work, a multi-scaling homogenization process using boundary element formulation (BEM) for modeling a two-dimensional multi-phase microstructure containing irregular’s inclusions is presented. The BEM is very attractive for multiscale modeling tools for heterogeneous materials. In this approach, the iterative inhomogeneity discretization of the external boundary is disregarded, leading to a computational low cost. This approach was used for solving the elastic problem of a representative volume element (RVE) and the field theory medium. The main goal relies on finding the effective properties of micro-heterogeneous materials within a homogeneous and orthotropic matrix. Expressions for evaluating the effective properties under Plane Stress (PT) for orthotropic materials were also presented. Generally, the numerical models consider the graphite nodules as voids for GGG-40 and the roundness is close circular geometry. In this sense, a nodular cast iron GGG-40 microgram was obtained by X-ray computed tomography and Laser Confocal Microscope System, allowing the modeling of the true nodule shape. The numerical results showed good agreement with the experimental tests. The inclusions of graphite were considered as voids in the material matrix. Experimental stress–strain tests and micrographic analysis were used to determine the Young’s modulus, spatial distributions, as well as, nodule shape. The numerical in this work was compared with the obtained experimental results in this work. The comparison between the obtained experimental data with those available in the literature also showed good agreement.