Ferritic-pearlitic Ductile Cast Irons Research Articles

Investigation on dynamic strain aging behaviour of ferritic-pearlitic ductile cast irons

The influence of ferritic and pearlitic microstructures to dynamic strain aging (DSA) of ductile iron was investigated by means of strain-controlled cyclic tests performed at temperatures ranging from room temperature to 450°C, strain rates of 10−5 to 10−2 s−1 and at three different strain amplitudes along with tensile tests. Results indicated that all tested microstructures exhibit strain aging effects at elevated temperatures in some form, similar to ferritic-pearlitic steels. Additionally, static tensile tests show hardening at intermediate temperatures, minima in elongation and serrations in stress–strain curves. Cyclic data indicates DSA related changes in stresses at 250°C and 350°C at smaller strain rates, while the effects become more suppressed at higher strain rates.

Intermetallic phase kinetic formation and thermal crack development in galvanized DCI

A new class of material characterized by mechanical properties close to the traditional low and medium carbon steel is the Ductile Cast Irons (DCIs). High castability and low production costs allow them to be used in many fields, ranging from automotive to piping. In this class of iron-based alloy the content of carbon is mainly localized in the graphite nodules, which are dispersed in a metallic matrix characterized by different phases: ferrite, austenite, pearlite, martensite or their mix, depending on the chemical composition and the heat treatment. Metallic matrix is subjected to the corrosion phenomenon, and, for this reason, the study and the development of traditional and innovative protection techniques are important, mainly in critical applications. Hot dip galvanizing is one of the investigated protections because the zinc is able to protect the iron-based alloys thanks to the reduction potential that is lower than the iron one. In this work, a ferritic-pearlitic DCI (GS500) is galvanized by using a pure Zn bath at 440°C in order to generate a zinc coating at different dipping time. The presence of thermal cracks is observed by means of a Scanning Electron Microscope (SEM).

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.

Fatigue Strength Evaluation of Ferritic-Pearlitic Ductile Cast Iron with Notches and Holes of Various Sizes

Fatigue Strength Evaluation of Ferritic-Pearlitic Ductile Cast Iron with Notches and Holes of Various Sizes

Fatigue limit prediction of ferritic-pearlitic ductile cast iron considering stress ratio and notch size

The mechanical behavior of ductile cast iron is governed by graphite particles and casting defects in the microstructures, which can significantly decrease the fatigue strength. In our previous study, the fatigue limit of ferritic-pearlitic ductile cast iron specimens with small defects () could successfully be predicted based on the parameter model by using as a geometrical parameter of defect as well as the tensile strength as a material parameter. In addition, the fatigue limit for larger defects could be predicted based on the conventional fracture mechanics approach. In this study, rotating bending and tension-compression fatigue tests with ferritic-pearlitic ductile cast iron containing circumferential sharp notches as well as smooth specimens were performed to investigate quantitatively the effects of defect. The notch depths ranged 10 ∼ 2500 μm and the notch root radii were 5 and 50 μm. The stress ratios were R = −1 and 0.1. The microscopic observation of crack propagation near fatigue limit revealed that the fatigue limit was determined by the threshold condition for propagation of a small crack emanating from graphite particles. The fatigue limit could be successfully predicted as a function of R using a method proposed in this study.

Influence of mean stress on fatigue strength of ferritic-pearlite ductile cast iron with small defects

Because of their excellent mechanical properties, low cost and good workability, the application of ductile cast iron has been increased in various industries such as the automotive, construction and rail industries. For safety designing of the ductile cast iron component, it is necessary to understand the effect of stress ratio, R, on fatigue limit of ductile cast iron in the presence of small defects.Correspondingly in this study, rotating bending fatigue tests at R = −1 and tension-compression fatigue tests at R = −1 and 0.1 were performed by using a ferritic-pearlitic ductile cast iron. To study the effects of small defects, we introduced a small drilled hole at surface of a specimen. The diameter and depth of a drilled hole were 50, 200 and 500 μm, respectively.The non-propagating cracks emanating from graphite particles and holes edge were observed at fatigue limit, irrespective of the value of stress ratio. From the microscopic observation of crack propagation behavior, it can be concluded that the fatigue limit is determined by the threshold condition for propagation of a small crack. It was found that the effect of stress ratio on the fatigue limit of ductile cast iron with small defects can be successfully predicted based on parameter model. Furthermore, a use of the tensile strength, σB, instead of the Vickers hardness, HV, is effective for fatigue limit prediction.

The effect of pearlite on the hydrogen-induced ductility loss in ductile cast irons

Hydrogen energy systems, such as a hydrogen fuel cell vehicle and a hydrogen station, are rapidly developing to solve global environmental problems and resource problems. The available structural materials used for hydrogen equipments have been limited to only a few relatively expensive metallic materials that are tolerant for hydrogen embrittlement. Therefore, for the realization of a hydrogen society, it is important to expand the range of materials available for hydrogen equipment and thereby to enable the use of inexpensive common materials. Therefore, ductile cast iron was, in this study, focused as a structural material that could contribute to cost reduction of hydrogen equipment, because it is a low-cost material having good mechanical property comparable to carbon steels in addition to good castability and machinability. The strength and ductility of common ductile cast irons with a ferritic-pearlitic matrix can be controlled by the volume fraction of pearlitic phase. In the case of carbon steels, the susceptibility to hydrogen embrittlement increases with increase in the pearlite fraction. Toward the development of ferritic-pearlitic ductile cast iron with reasonable strength for hydrogen equipment, it is necessary to figure out the effect of pearlite on the hydrogen embrittlement of this cast iron. In this study, the tensile tests were conducted using hydrogen-precharged specimens of three kinds of ferritic-pearlitic ductile cast irons, JIS-FCD400, JIS-FCD450 and JIS-FCD700. Based on the results, the role of pearlite in characterizing the hydrogen embrittlement of ductile cast iron was discussed.

Fatigue crack propagation in Ductile Cast Irons: an Artificial Neural Networks based model

Abstract: All the available “Paris-like” models (analytical relationships between da/dN, crack growth rates, and ΔK, stress intensity factor amplitude) are not able to take into account the possible influence of all the parameters that influence the fatigue crack propagation process. Among them, the stress ratio R (e.g., K min /K max ) is one of the most investigated and, although in the last decades the influence of R on the different propagation mechanisms has been widely investigated (e.g., crack closure effect), this parameter is often considered as an independent variable in the “Paris-like” models. A different approach can be followed using the Artificial Neural Networks that are able to consider all the possible parameters, with the condition of a satisfactory training stage. In this work, an artificial Neural Networks based model is optimized considering the influence of the stress ratio on the fatigue crack propagation in a ferritic-pearlitic Ductile Cast Iron.

Fatigue crack propagation and overload damaging micromechanisms in a ferritic–pearlitic ductile cast iron

AbstractRecently discovered (1948), ductile cast irons (DCIs) are able to combine the good castability of grey irons with the interesting mechanical properties, both static and cyclic, of carbon steels, allowing to directly produce components with complex shape that are requested to be characterized by high mechanical performances, especially considering toughness and fatigue resistance. DCIs mechanical properties and damaging micromechanisms depend on the matrix (ferritic, pearlitic, ferrite pearlitic, ausferritic, etc.) and on the graphite nodules morphological peculiarities (shape, dimension, distribution, etc.).In this work, the fatigue‐crack propagation micromechanisms in a ferritic–pearlitic DCI were investigated, focusing both the role played by the microstructure and the contribution of the graphite nodules. In addition, the overload effects on the composite microstructure of the investigated ferritic–pearlitic DCI were analysed.

Overload effects on fatigue cracks in ferritic-pearlitic ductile cast irons

Matrix microstructure (e.g., phases volume fraction, grains size and grain distribution) and graphite nodules morphology peculiarities (e.g., nodularity level, dimension, distribution etc.) strongly affect the mechanical behavior and damaging micromechanisms in Ductile Cast Irons (DCIs). Concerning the influence of the graphite nodules, it depends both on the matrix microstructure and the loading conditions (e.g., static, quasi-static or cyclic loadings). The influence of graphite nodules on the damaging micromechanisms is not univocally identified. Some authors proposed to consider the graphite nodules as voids embedded in a more or less ductile matrix; other authors recently proposed a more complex contribution of the graphite nodules, suggesting a mechanical properties gradient inside the graphite nodules, with the graphite elements – matrix debonding as only one of the possible damaging micromechanisms.In this work, three different ferritic-pearlitic DCIs were investigated, focusing the damaging micromechanisms due to overloads applied on fatigue cracked Compact Type specimens. Scanning Electron Microscope (SEM) and Digital Microscope (DM) observations were performed on the lateral surfaces of the overloaded specimens following a step by step procedure: SEM observations were mainly focused on the damaging mechanisms in graphite nodules; DM observations were mainly focused on the damaging mechanisms in the ferritic-pearlitic matrix.

Fatigue crack tip damaging micromechanisms in a ferritic-pearlitic ductile cast iron

Due to the peculiar graphite elements shape, obtained by means of a chemical composition control (mainly small addition of elements like Mg, Ca or Ce), Ductile Cast Irons (DCIs) are able to offer the good castability of gray irons with the high mechanical properties of irons (first of all, toughness). This interesting properties combination can be improved both by means of the chemical composition control and by means of different heat treatments(e.g. annealing, normalizing, quenching, austempering etc). In this work, fatigue crack tip damaging micromechanisms in a ferritic-pearlitic DCI were investigated by means of scanning electron microscope observations performed on a lateral surface of Compact Type (CT) specimens during the fatigue crack propagation test (step by step procedure), performed according to the “load shedding procedure”. On the basis of the experimental results, different fatigue damaging micromechanisms were identified, both in the graphite nodules and in the ferritic – pearlitic matrix.

Damaging micromechanisms characterization in a ferritic-pearlitic ductile cast iron

The analysis of the damaging micromechanisms in Ductile Cast Irons is often focused on ferritic matrix. Up to ten years ago, for this grades of DCIs, the main damaging micromechanism was identified with the graphite elements – ferritic matrix debonding. More recent experimental results showed the presence of an internal gradient of mechanical properties in the graphite elements and the importance of other damaging micromechanisms, with a negligible importance of the graphite elements – ferritic matrix debonding mechanism. In this work, damaging micromechanisms development in a ferritic – pearlitic DCI have been investigated by means of tensile tests performed on mini-tensile specimens and observing the specimens lateral surfaces by means of a scanning electro microscope (SEM) during the tests (“in-situ” tests). Experimental results have been compared with the damaging micromechanisms observed in fully ferritic and fully pearlitic DCIs

Sliding wear behavior of a ferritic–pearlitic ductile cast iron with different nodule count

The effect of graphite nodule characteristics (nodule count, nodule size and nodularity) on the wear behavior of ductile iron has been the subject of many researches to date to optimize its wear performance. The present work aims to illustrate the effect of nodule count on the sliding wear behavior of a ferritic–pearlitic ductile iron. Different ductile cast iron test samples, from 10 to 50mm in thickness, with the nodule counts varying between 150 and 450nod/mm2, were used. The samples for wear tests were heat treated in order to adjust the matrices to a ferritic–pearlitic microstructure. Wear tests were carried out using a pin-on-disk type apparatus based on ASTM-G99 standard under dry sliding conditions. It was found that the oxidative wear is operative at the lower load while adhesive associated with delamination wear is predominant mechanism at the higher load. Furthermore, at the lower applied loads, the specimens with the high nodule count exhibit lower wear rate than those having the low nodule count, while at the higher loads wear resistance deteriorates with increasing nodule count. The obtained results were discussed based on the effect of nodule count on oxidative, adhesive and delamination wear.

Damaging micromechanisms in ferritic–pearlitic ductile cast irons

Microstructure influences on damaging mechanisms in ferritic–pearlitic ductile cast irons (DCIs) were investigated. Four different ferrite/pearlite volume fractions were considered, and in situ scanning electron microscope observations were performed during step by step tensile tests in a microtensile holder. SEM observations allowed to identify an evident microstructure influence on ferritic–pearlitic DCIs damaging mechanisms and the key role played by graphite constituents. Phase volume fractions and distribution affect both crack initiation and crack propagation. The importance of the damage mechanism based on graphite nodules–matrix interface cracking, usually indicated as the most relevant, is only partially confirmed, depending on the DCI microstructure.

Fatigue crack propagation damaging micromechanisms in ductile cast irons

Ductile iron discovery in 1948 gave a new lease on life to the cast iron family. In fact, these cast irons are characterized both by a high castability and by high toughness values, combining cast irons and steel good properties. Ductile cast irons are also characterized by high fatigue crack propagation resistance, although this property is still not widely investigated. In the present work, three different ferritic–pearlitic ductile cast irons, characterized by different ferrite/pearlite volume fractions, and an austempered ductile cast iron were considered. Their fatigue crack propagation resistance was investigated in air by means of fatigue crack propagation tests according to ASTM E647 standard, considering three different stress ratios ( R = K min/ K max = 0.1; 0.5; 0.75). Crack paths were investigated by means of a crack path profile analysis performed with an optical microscope. Crack surfaces were extensively analysed by means of a scanning electron microscope both considering a traditional procedure and performing a quantitative analysis of 3D reconstructed surfaces, mainly focusing graphite nodules debonding.

BAM Production Control Programme for Containers for Transport and Storage of Nuclear Materials

In the process of testing spent fuel casks, BAM is gaining a lot of relevant data regarding the quality level of ductile cast iron (DCI). The basic parameters governing the material behaviour of ferritic and ferritic pearlitic DCI are dicussed and the development of container quality over recent years is summarised. The high quality level of German DCI containers is outlined. The effect of microstructure, sample size and loading rate on the fracture toughness of DCI is discussed in the second part of the paper.