Graphite Nodules Research Articles

A Microscale Model for Ausferritic Transformation of Austempered Ductile Irons

This paper presents a new metallurgical model for the ausferritic transformation of ductile cast iron. The model allows predicting the evolution of phases in terms of the chemical composition, austenitization and austempering temperatures, graphite nodule count, and distribution of graphite nodule size. The ferrite evolution is predicted according to the displacive growth mechanism. A representative volume element is employed at the microscale to consider the phase distributions, the inhomogeneous austenite carbon content, and the nucleation of ferrite subunits at the graphite nodule surface and at the tips of existing ferrite subunits. The performance of the model is evaluated by comparison with experimental results. The results indicate that the increment of the ausferritic transformation rate, which is caused by increments of austempering temperature and graphite nodule count, is adequately represented by this model.

Comparison of pyrolytic graphite spheres from propylene with spheroidal graphite nodules in steel

Two types of the spherical growth form of graphite from different precursors, pyrolytic graphite spheres (PGSs) formed by pyrolysis of propylene and spheroidal graphite nodules (SGNs) formed by heat treatment of the medium carbon steel, are investigated by scanning electron microscopy and transmission electron microscopy. Microstructural similarities and differences of these graphite spheres are drawn. The results show that PGSs and SGNs have a similar core-shell microstructure, which comprises a low graphitic core in the center and conical grains growing outward radially from the core with the cone axis along the c-axis direction of graphite. The role of hydrogen on pyrolytic graphite sphere formation is discussed, and we suggest layer-by-layer growth of the spheroidal graphite nodules by two-dimensional nucleation on the basal plane based on the parallel arrangement of graphene layers in the sectors. It is shown that the state of the precursor, gas, liquid, or solid, is not a determining factor on governing the resultant microstructure of graphite spheres.

Modeling the elastic behavior of ductile cast iron including anisotropy in the graphite nodules

This paper presents a micro-mechanical approach to model the intrinsic elastic anisotropy of the graphite particles in ductile iron. Contrary to most of the published works in the field, the constitutive behavior is directly derived on the basis of the nodule characteristic internal structure, composed of graphite platelets arranged into conical sectors. In this way, the large uncertainty traditionally associated with local mechanical measurements of micro-hardness is eliminated. The proposed anisotropic description is validated by simulating the macroscopic ductile iron elastic response by means of a 3D periodic unit cell model. In this respect, an explicit procedure to enforce both periodic displacement and periodic traction boundary conditions in ABAQUS is presented, and the importance of fulfilling the traction continuity conditions at the unit cell boundaries is discussed. It is shown that localized inelastic deformation is likely to develop for loading conditions which can still be considered as elastic at the macroscopic scale. The presence of a weak interface between the graphite and the matrix is also investigated, and it is found to affect the results to a limited extent only.

Three-dimensional local residual stress and orientation gradients near graphite nodules in ductile cast iron

A synchrotron technique, differential aperture X-ray microscopy (DAXM), has been applied to characterize the microstructure and analyze the local mesoscale residual elastic strain fields around graphite nodules embedded in ferrite matrix grains in ductile cast iron. Compressive residual elastic strains are measured with a maximum strain of ∼6.5–8 × 10−4 near the graphite nodules extending into the matrix about 20 μm, where the elastic strain is near zero. The experimental data are compared with a strain gradient calculated by a finite element model, and good accord has been found but with a significant overprediction of the maximum strain. This is discussed in terms of stress relaxation during cooling or during storage by plastic deformation of the nodule, the matrix or both. Relaxation by plastic deformation of the ferrite is demonstrated by the formation of low energy dislocation cell structure also quantified by the DAXM technique.

Effect of laser surface treatment on the dry sliding behaviour of the EN-GJS400-12 ductile cast iron

Laser surface treatment (LST), using two levels of laser energy intensities, was carried out on ductile cast iron GJS400; the corresponding effect on microstructure and dry sliding wear behaviour was studied. The tests were carried out against a carburized steel also on the untreated GJS400 and lamellar GJL300 cast irons, for comparison. LST enhanced the wear resistance of GJS400, while leading to an increase of its coefficient of friction (COF). The best performance was observed in the cast iron treated at the lower energy intensity, leading to a lower surface hardness and related superior toughness. Among the untreated cast irons, the GJS400 presented the best tribological behaviour, due to the positive effect of graphite nodules in respect to flake graphite.

Dry sliding wear of Ni alloyed austempered ductile iron

Measurements of dry sliding wear are presented for ductile irons with composition Fe-3.56C-2.67Si-0.25Mo-0.5Cu and Ni contents of 0.8 and 1.5 in wt.% with applied loads of 50, 100 and 150 N for austempering temperatures of 270, 320, and 370 °C after austenitizing at 870 °C for 120 min. The mechanical property measurements show that the grades of the ASTM 897M: 1990 Standard can be satisfied for the selected austempering conditions. The results show that wear resistance is independent of austempering temperature with an applied load of 50 N, but there is a strong dependence at higher austempering temperatures with applied loads of 100 and 150 N. Observations indicate that wear is due to subsurface fatigue with cracks nucleated at deformed graphite nodules.

DEGRADATION ASSESSMENT OF NUCLEAR POWER PLANT EXTRACTION STEAM PIPING AFTER LONG TERM SERVICE

Degradation due to ageing in a Nuclear Power Plant’s Extraction Steam Piping has been assessed. Samples of twelve years old seamless carbon steel SA 106B pipe have been taken and subjected to metallographic examination, hardness testing, radiographic examination and ultrasonic thickness measurement to investigate and analyze ageing in piping material. Metallographic examination of aged sample revealed irregularly distributed graphite nodules in ferrite grains and on grain boundaries as well, the reduction in pearlite phase has also been observed from 40% to 25%. This Transformation in microstructure has reduced hardness of steel. Reduction in hardness has found 20.4% and 0.7% on inner and outer layer of the pipe material respectively. Corrosion pits having average depth of 0.07 mm have been found on external surface. Radiograph of aged pipe revealed material removal and wall thinning due to erosion on inner surface of pipe. The extent of erosion had been checked, which has found 8.7%.

Observation of Nanometric Silicon Oxide Bifilms in a Water-Atomized Hypereutectic Cast Iron Powder

This study investigated the reasons for the irregular structure of primary graphite nodules that were formed in a hypereutectic cast iron powder during water atomization. The graphite nodules contain a significant amount of micron-sized pores and multiple nanometric voids that formed from silicon oxide bifilms. The bifilms theory is often used to explain the mechanisms responsible for the presence of pores in castings. However, even if many results presented in the literature tend to corroborate the existence of bifilms, to this date, only indirect evidences of their existence were presented. The observations presented in this paper are the first to show the double-sided nature of these defects. These observations support the bifilms theory and give an explanation for the presence of porosities in castings. The bifilms were used as substrate for graphite growth during solidification. The irregular structure of the graphite nodules is a consequence of the rather random structure of the bifilms that were introduced in the melt as a result of turbulences on the surface of the melt during pouring. The confirmation of the existence of bifilms can contribute to the understanding of the mechanical properties of various metallic parts.

A micro-mechanical analysis of thermo-elastic properties and local residual stresses in ductile iron based on a new anisotropic model for the graphite nodules

In this paper, the thermo-elastic behavior of the graphite nodules contained in ductile iron is derived on the basis of recent transmission electron microscopy investigations of their real internal structure. The proposed model is initially validated by performing a finite element homogenization analysis to verify its consistency with the room-temperature elastic properties of ductile iron measured at the macro scale. Subsequently, it is used to investigate the formation of local residual stresses around the graphite particles by simulating the manufacturing process of a typical ferritic ductile iron grade, and the results are compared with preliminary measurements using synchrotron x-rays. Finally, the obtained accurate description of the stress & strain field at the micro scale is used to shed light on common failure modes reported for the nodules and on some peculiar properties observed in ductile iron at both micro and macro scale.

Effect of nodule count and austempering heat treatment on segregation behavior of alloying elements in ductile cast iron

The equilibrium partition ratio, k, has been measured for Mn, Mo, Si, Ni and Cu in a ductile iron with composition (wt.%): 3.45C, 0.25Mn, 0.25Mo, 2.45Si, 0.5Ni and 0.5Cu with different nodule counts obtained from different section sizes of 13, 25, 75 mm in the as cast, austenitized (at 870 °C for times 1, 4 and 6 hours) and austempered (at 375 °C for times 1 to 1,440 min) samples. Results show that Mn and Mo segregate positively at cell boundaries, but Si, Ni and Cu concentrate in an inverse manner in the vicinity of graphite nodules and there is a depletion of these elements at cell boundaries. Segregation curves for Ni and Cu are more smooth than for Si. Carbide formation has been observed at cell boundaries. Based on the results, the partition ratios for all elements decrease with increasing the nodule count. More carbide with coarser morphology has been observed in the microstructure with a lower nodule count. Austenitization for a longer time can decrease partition ratio, but cannot eliminate it entirely. Increasing the austenitization temperature has the same efect. Austenitizing parameters have no significant effect on carbides volume fraction. The kinetics of austempering is faster in higher nodule counts and subsequently better mechanical properties including higher ductility, strength and toughness have been observed for all austempering conditions studied.

Solidification Kinetics of Graphite Nodules in Cast Iron and Shrinkage Porosity

The shrinkage porosity of castings made from cast iron with spherical graphite (SGI) depends on a combination of intrinsic (density and volume of phases, solidification kinetics) and extrinsic conditions related to casting-mold thermo-mechanical interactions. Precipitation of graphite nodules increases the specific SGI volume, and control of the nucleation rate in solidified castings can be used for improving casting soundness. In this article, the method of structural reconstruction of solidification kinetics was used to link the nucleation rate of graphite nodules to experimentally observed shrinkage porosity in a specially designed test casting. An automated SEM/EDX system was used to determine the “true” two-dimensional graphite nodule distributions in the casting sections. These two-dimensional distributions were converted into the volume particle distribution functions (PDF), and the solidification kinetics were reconstructed by applying inverse simulations. The experiments were performed with variations in inoculation and pouring temperature. The shrinkage porosity was compared to the restored sequence of graphite nodule nucleation in the specific casting volumes. It is shown that the second nucleation wave in low-temperature poured and inoculated SGI eliminated interdendritic microporosity. The suggested method could be used in industry to improve the soundness of SGI castings.

Austempering Parameters and Machinabilityof Austempered Ductile Iron: AComprehensive Review on EffectiveParameters

Austempered ductile iron (ADI) formed by austempering of nodular cast iron (NCI). ADI has good mechanical properties due to the matrix of ausferrite (austenite, ferrite and graphite nodules). It gives good combination of high strength to weight ratio, fatigue resistance, ductility and toughness. Austempering process offer ability to access to a wide range of characteristics with appropriate selection of heat treatment parameters. Graphite nodules give ductility to material and also act as crack arrester and give good fatigue strength. ADI is strain hardening or work hardening material, it has property to go for strain induced transformation (SIT) while straining. When high normal force is applied to ADI while machining, austenite on surface undergoes to SIT and form martensite. This makes further machining even more difficult and reduces performance of cutting tool. This paper deals with detail study of ADI, effective austempering parameters and performance of different cutting tools at various input machining parameter while machining ADI.

A new approach to calculate the nodule density of ductile cast iron graphite using a Level Set

This paper proposes a new approach with digital image processing techniques to determine the number of graphite nodules on samples of nodular cast iron by following the NBR 6913 standard. Counting errors and excessive inspection time are common problems if the procedure is performed without the assistance of computational systems. To overcome these drawbacks, we propose here an algorithm for graphite nodule segmentation based on the Level Set technique. The proposed approach and two other computational methods, Watershed and Region Growing, are compared with the results given by experts using optical microscopy (OM). The results of the proposed method were closer to those of the experts using OM than the other two computational methods. The proposed method presented greater accuracy and faster execution time than the traditional method by visual inspection.

Influence of the graphite elements morphology on the fatigue crack propagation mechanisms in a ferritic ductile cast iron

Ductile cast irons (DCIs) are commonly considered as really interesting materials, due to their interesting combination of mechanical properties and technological peculiarities. Characterized by the high castability that is a cast irons technological peculiarity, DCIs are characterized by a very interesting combination of good mechanical properties (e.g., tensile strength and fatigue resistance). These properties are strongly influenced by the DCIs microstructure, that is defined both by the matrix (considering the morphological peculiarities like phases distribution, grain dimension, etc.) and by the graphite nodules elements, that are characterized by shape, dimension and distribution. In addition, the presence of defects (like pores, both micro and macro) can strongly affect the DCIs mechanical behaviour (e.g., considering large castings).In this work, a ferritic DCI with degenerated nodules was obtained and the fatigue crack propagation resistance was investigated by means of fatigue crack propagation tests and compared with the behaviour of a commercial ferritic DCIs.

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.

On the isotropic elastic constants of graphite nodules in ductile cast iron: Analytical and numerical micromechanical investigations

A comprehensive description of the mechanical behavior of nodules in ductile iron is still missing in the published literature. Nevertheless, experimental evidence exists for the importance of such graphite particles during macroscopic material deformation, especially under compressive loading. In the present paper, the nodules’ elastic properties are thoroughly investigated by means of both analytical and numerical techniques. The analysis takes into account the influence of several non-linear phenomena, as local residual stresses arising during solid-state cooling, interface debonding and limited particle strength. It is shown that if the nodule internal structure is considered, the traditional isotropy assumption leads to the definition of a domain of admissible values for the effective elastic constants. However, micromechanical calculations indicate that values within the domain do not provide mesoscopic moduli in agreement with Young's modulus and Poisson's ratio recorded for common ferritic ductile iron grades. This suggests that graphite nodules may not be considered isotropic at the microscopic scale, at least from a mechanical viewpoint.

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.

An Approach Based on the Exploratory Data Analysis to Relate the Wear Behavior with the Microstructure of Ductile Cast Irons

The aim of this work is to propose a new methodology to relate Ductile Cast Irons (DCIs) wear behavior with the separation distances and sizes of the graphite nodules through an Exploratory Data Analysis (EDA). This methodology consists of morphological image processing tools (compacity and size distribution curves), an EDA performed by the use of box plots and an EDA-based section classifying algorithm. This algorithm classifies the microstructure of DCIs into classes and levels grouping different behaviors of the separation distances and sizes of graphite nodules. Finally, it was found, through a number of tribological tests, that the obtained classes and levels have a different wear behavior. The results achieved by this methodology were compared with those of traditional techniques used to characterize the microstructure of the material.

Effect of Quenching and Partitioning Heat Treatment on Ductile Cast Irons

In this study, the ductile cast irons were prepared by the melting process and then carried out the quenching and partitioning heat treatment. The microstructures were observed by both an optical microscopy and a field emission scanning electron microscope and the tensile strengths were measured by a uniaxial tensile tester. The effect of partitioning time on the microstructures and the tensile strength were investigated. The results showed that the matrix of as-cast samples contain about 26% ferrite and 74% pearlite and the microstructure quenched without partitioning consisted of a mixture of graphite nodules plus lath martensite plus retained austenite. With the increase of the partitioning time, the retained austenite became stable and could transform into lower bainite or carbide gradually, which gave rise to the fact that the tensile strength changed nonlinearly and reached a maximum at the partitioning time about 120 minutes.

Effects of Number of Graphite Nodules on Fatigue Limit and Fracture Origins in Heavy Section Spheroidal Graphite Cast Iron

Effects of Number of Graphite Nodules on Fatigue Limit and Fracture Origins in Heavy Section Spheroidal Graphite Cast Iron