Graphite Nodules Research Articles

Development and investigation of austempered ductile iron (ADI) for thick-walled gear components

Austempered ductile irons (ADI) are attractive for thick-walled and highly stressed gear components due to the possibility to design complex structures requiring a combination of high strength and moderate ductility. In this study, ADI was developed and optimized for industrial planetary carriers with shaft diameters ranging from 120 to 460 mm. The material was characterized by correlating fatigue properties to microstructural parameters using samples prepared from cast and heat treated carriers. The optimized castings exhibit a tension-compression fatigue strength enhanced by 25% compared with castings of the same size of nominally the same material grade. A scaling behavior was developed in order to estimate fatigue strength on the basis of the maximum defect size in the tested volume. The maximum defect can be either a graphite nodule or a micropore. By optimizing the microstructure of a thick-walled casting according to the typical loading conditions, a structurally graded component with a uniform degree of utilization can be obtained. This allows efficient castings and heat treatment of thick-walled ADI parts.

Influence of casting heterogeneities on microstructure and mechanical properties of austempered ductile iron (ADI)

An investigation was carried out to examine the influence of as-cast heterogeneities on the resulting microstructure and mechanical properties of an austempered ductile cast iron (ADI). A commercial hypereutectic ductile iron alloy with 3.59% C, 2.68% Si, 0.24% Mn and 0.46% Cu (in wt. %) was produced in a coreless induction furnace. The specimens were obtained by casting the alloy into Y-block molds. The austempering heat treatments consisted of pre-heating at 500°C, followed by austenitizing step at 900°C and 930°C during 60 minutes. Austempering was carried out in molten metal baths at temperatures of 300°C and 370°C for 30 minutes. Microstructural characterization was carried out by light optical microscopy (LOM) with image analysis, scanning electron microscopy (SEM-FEG) and X-ray diffraction with Rietveld refinement. The mechanical properties were evaluated by tensile and Vickers hardness tests. The results of this investigation indicate that as-cast heterogeneities have a detrimental effect on the transformed microstructure and mechanical properties of ADI. During austempering, non uniform distribution of graphite nodules and segregation of alloying elements at cell boundaries resulted in transformation gradients throughout the metallic matrix. Intercellular regions displayed inhomogeneous microstructures containing eutectic carbides and martensite. Yield strength, tensile strength and hardness decrease for increasing austempering temperature. Fracture surfaces showed a rapid transition from ductile to cleavage mode at intercellular regions, containing solidification carbides.

Numerical study of microstructural effects on chip formation in high speed cutting of ductile iron with discrete element method

Microstructural deformation and fracture process is important for chip formation and finished surface quality during metal cutting. In this paper, discrete element method (DEM) is introduced to establish a heterogeneous material model for cutting simulation to understand the microstructural deformation and fracture behaviors. A typical heterogeneous engineering material, ISO 450-10 ductile iron, was selected for modeling and experiments. Graphite nodules and ferrite grains were modeled respectively for studying their deformation behaviors. Cutting force and chip morphology obtained by simulation were compared with the experimental results. It shows that lamellar structure and unequal segments form at the chip free surface, which was also observed by optical microscope (OM) and scanning electron microscopy (SEM). The deformed degree of graphite nodules is much higher than that of ferrite grains. In addition, cracks are prone to produce and the chip size becomes smaller as the cutting speed increases. The velocity field and stress distribution of material near the rake face were investigated and the relationship between stress and cutting speed was further discussed. The velocity fluctuation of discrete particles in heterogeneous model is more obvious due to the microstructures compared with that in homogeneous model. Furthermore, the stress of material changes significantly with the increase of cutting speed since velocity vortexes may occur, resulting in the occurrence of the fracture. The results demonstrate that the influences of microstructure on crack initiation and chip formation are more significant at high cutting speeds.

A high-resolution study of graphite nodule formation in experimental medium-carbon machining steel

Alloying a medium-carbon steel has accelerated graphite formation during annealing, for example, during high-temperature tempering. Electron microscopy and related techniques have been used to characterise the graphite particle structure and dispersion from pre-anneal starting microstructures of ferrite-pearlite, bainite and martensite. Graphitisation in the solid state in steels (and also, for useful comparison, in cast irons) has previously received little attention. However, FIB/FEGSEM has permitted specimen preparation of the relatively coarse microstructures, thus enabling high-resolution observation previously limited for graphite formation in cast irons and steels. The study has importance to the potential development of more economic and user-friendly machining steels, which would rely upon internal lubrication by graphite nodules within the microstructure. Many recently developed advanced steels are reduced in carbon but expensively alloyed to produce the desired properties, whereas an aim in this project is simply to use carbon, which is a very cheap and abundant alloying element.

Effect of microstructure on fatigue behaviour of advanced high strength ductile cast iron produced by quenching and partitioning process

The fatigue behaviour of high strength ductile cast iron produced by a quenching and partitioning process (Q&P) is evaluated. The Q&P process is receiving increased attention as a new way to produce ultra-high strength steels with multiphase microstructures composed of a martensite matrix and substantial amounts of carbon-stabilized retained austenite. Currently, the mechanical properties arising from applying the Q&P process in ductile cast irons have not been investigated. Thermal treatments consisted of heating the material to 880°C for a 2h soaking time followed by quenching in oil at 140°C and 170°C, intermediate temperatures between Ms and Mf allowing the formation of a controlled amount of athermal martensite. The material was reheated to 300°C and 375°C (the partition treatment) for different times between 15 and 120min and subsequently air-cooled to room temperature. Microstructural evolution and carbon partitioning was investigated by in situ synchrotron X-ray diffraction. With a microstructure composed of tempered martensite, bainitic ferrite and carbon-enriched stabilized austenite, a new class of properties was obtained, with an enhanced strength when compared to ADI (austempered ductile iron) while still maintaining reasonable elongation. Fatigue testing was undertaken using polished plain bend bars (no stress concentration features) assessed under four-point bending. Uninterrupted tests at varying loads show that the higher partitioning temperature is beneficial for fatigue life. Fracture initiation sites are primarily from pores and a number of decohered graphite nodules. A strong influence of the microstructure on subsequent fatigue crack growth is observed from interrupted testing with replica records and SEM examination of tested samples, with cracks exhibiting significant tortuosity, at times even appearing to grow approximately parallel to the tensile axis. Using these results, the effects of the Q&P treatment, particularly the role of the partitioning time and temperature, on the fatigue properties of ductile cast iron are assessed.

Synchrotron measurements of local microstructure and residual strains in ductile cast iron

The local microstructure and distribution of thermally induced residual strains in ferrite matrix grains around an individual spherical graphite nodule in ductile cast iron (DCI) were measured using a synchrotron X-ray micro-diffraction technique. It is found that the matrix grains are deformed, containing dislocations and dislocation boundaries. Each of the residual strain components in the matrix grains exhibits a complex pattern along the circumferential direction of the nodule. Along the radial direction of the nodule, strain gradients from the interface to the grain interior are seen for some strain components, but only in some matrix grains. The observed residual strain patterns have been analysed by finite element modelling, and a comparison between the simulation and experiments is given. The present study of local residual stress by both experimental characterization and simulation provide much needed information for understanding the mechanical properties of DCI, and represent an important contribution for the microstructural design of new DCI materials.

3D Quantitative Analysis of Graphite Morphology in Ductile Cast Iron by X-ray Microtomography

In this article, X-ray microtomography and color metallographic techniques have been used to perform three-dimensional quantitative characterization of graphite nodule morphology in a step-shaped ductile cast iron casting. Statistical analyses of the graphite nodule count, diameter, sphericity, and spatial distribution have been processed for three samples in detail. The results reveal that graphite nodules in ductile cast iron can be categorized into two categories. The first types are nodules located in eutectic cells (NIECs), and the other one refers to nodules located between the eutectic cells (NBECs). The NIECs possess a larger average diameter but smaller sphericity compared with the NBECs, and the sphericity decreases along with the increasing of diameter. The increasing casting thickness results in an increasing count and percentage of NBECs. In addition, most nodules are NIECs in thin walls instead of NBECs in thick walls. Nonuniform spatial distributions of graphite nodules caused by the existence of NBECs have been found to become more obvious along with the increase of cast thickness.

Graphite nodules in fatigue-tested cast iron characterized in 2D and 3D

Thick-walled ductile iron casts have been studied by applying (i) cooling rate calculations by FVM, (ii) microstructural characterization by 2D SEM and 3D X-ray tomography techniques and (iii) fatigue testing of samples drawn from components cast in sand molds and metal molds. An analysis has shown correlations between cooling rate, structure and fatigue strengths demonstrating the benefit of 3D structural characterization to identify possible causes of premature fatigue failure of ductile cast iron.

Friction and wear performance of functionally graded ductile iron for brake pads

Due to its wear resistance, functionally graded ductile iron (FGDI) is a potential material for automotive brake pads. In this investigation it was studied (i) under well controlled sliding conditions in a typical tribological model-test to show in detail the mechanism behind improved wear resistance, namely the “property on-demand” functionality in the sub-surface zone and (ii) in a validation experiment on a disc-brake machine that mimic realistic brake-contact conditions, at various elevated temperatures against a carbon-reinforced ceramic disc in order to understand its tribological behaviour in a real automotive brake application. The results revealed a positive effect of the functional gradient zone (FGZ) on the wear performance of the functionally graded ductile-iron pads. Most notably, improved wear resistance was observed under sliding model-test conditions as well as a more-stable wear and wear that is less sensitive to the contact temperature during a disc-brake test. In addition, the sliding model-tests revealed that FGZ leads to faster running in and a more stable coefficient of friction. The disc-brake tests also show that FGZ ensures a relatively constant coefficient of friction for all contact temperatures. Surface analysis of the pads further revealed that oxidized, smoothened plateaus containing graphite nodules were formed on the pads. On the other hand, a patchy and layered transfer film resulting from the pads was formed on the disc's surface.

Fatigue and Fracture Resistance of Heavy-Section Ferritic Ductile Cast Iron

In this paper, we explore the effect of a long solidification time (12 h) 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 specimens extracted from the same casting. The obtained results are compared with those of similar materials published in the technical literature. Moreover, the discussion is complemented with metallurgical and fractographic analyses. It has been found that the long solidification time, representative of conditions arising in heavy-section castings, leads to an overgrowth of the graphite nodules and a partial degeneration into chunky graphite. With respect to minimum values prescribed for thick-walled (t > 60 mm) EN-GJS-400-15, the reduction in tensile strength and total elongation is equal to 20% and 75%, respectively. The rotating bending fatigue limit is reduced by 30% with respect to the standard EN-1563, reporting the results of fatigue tests employing laboratory samples extracted from thin-walled castings. Conversely, the resistance to fatigue crack growth is even superior and the fracture toughness comparable to that of conventional DCI.

Shaping optimal zinc coating on the surface of high-quality ductile iron casting. Part I – Moulding technologies vs. zinc coating

Abstract Studies have demonstrated that in the process of hot dip galvanizing the decisive influence on the mechanism of zinc coating formation and properties has the quality of the mechanically untreated (raw) surface layer of the galvanized product. The terms “casting surface layer” denote various parameters of the microstructure, including the type of metal matrix, the number of grains and the size of graphite nodules, possible presence of hard spots (the precipitates of eutectic cementite) and parameters of the surface condition. The completed research has allowed linking the manufacturing technology of ductile iron castings with the process of hot dip galvanizing.

Growth stages and hexagonal-rhombohedral structural arrangements in spheroidal graphite observed in ductile iron

Crystallographic structures of spheroidal graphite particles (graphite nodules) were examined using transmission electron microscopy (TEM). Structures of graphite nodules were investigated relative to different stages of nodule growth in ductile iron. Curved graphene layers were observed during the early growth of the graphite nodules. Thin layered stacking faults give rise to streaking in the basal reflections, which give rise to curvature of the nodule and growth steps on the surface. Columnar substructures consisting of parallel peripheral subgrains were found in the outside region of graphite nodules that were formed during the intermediate growth stage. Straight planar graphene layers were observed in the columnar substructures. Crystallographic orientation of graphite showed little change through the subgrain and the c-axes of multiple subgrains in a single columnar substructure were parallel. A method for characterizing the crystal structures of graphite based on the selected area diffraction pattern was introduced. Both hexagonal structure and rhombohedral structure were found in the spheroidal graphite particles. Possible crystallographic defects associated with hexagonal-rhombohedral structure transition were discussed.

Effects of niobium addition on microstructure and tensile behavior of as-cast ductile iron

The effects of niobium addition up to 0.11wt% on the microstructure and tensile properties of as-cast ductile iron (ACDI) were investigated. Metallographic analyses by both optical microscopy (OM) and scanning electron microscopy (SEM) indicated that niobium (Nb) promoted the formation of pearlite, reduced pearlite lamellar spacing and decreased the extent of graphitization taking place in the Nb-alloyed ACDI. The nodularity and nodule counts of graphite changed insignificantly when the Nb content was less than 0.08wt% in the ACDI. The analysis of precipitates by transmission electron microscopy (TEM) revealed that nano and micro sized (Nb, Ti)C carbides acted as nucleation site for graphites, and promoted the formation of large graphite nodules with low roundnesses as Nb content rose above 0.08wt%. The results of tensile testing showed that the yield strength, ultimate tensile strength and elongation of the ACDI with 0.08wt% Nb increased by 12.1%, 11.2% and 14.3% over those of the Nb-free ACDI, respectively. The optimum values of the yield strength, tensile strength and elongation of the Nb-alloyed ACDI were found to be 418MPa, 746.0MPa and 8.0%, respectively, at the Nb content of 0.08wt%. The high strain hardening rates of the Nb-containing ACDIs implied that they were capable of spontaneously strengthening itself increasingly to a large extent, in response to a slight plastic deformation after yielding.

Damage mechanisms in silicon-molybdenum cast irons subjected to thermo-mechanical fatigue

The damage mechanisms active in silicon-molybdenum cast irons, namely EN-GJS-SiMo5-1 and SiMo1000, under thermo-mechanical fatigue and combined thermo-mechanical and high-cycle fatigue conditions have been investigated. The studied load conditions are those experienced at critical locations in exhaust manifolds of heavy-vehicle diesel engines, namely a temperature cycle of 300–750°C with varied total mechanical and high-cycle fatigue strain ranges. It is established that oxide intrusions are formed in the early life from which macroscopic fatigue cracks are initiated close to the end-of-life. However, when high-cycle fatigue loading is superimposed, small cracks are preferentially initiated at graphite nodules within the bulk. In addition, it is found that both the oxidation growth rate and casting defects located near the surface affect the intrusion growth.

Communication: Characterization of Spatial Distribution of Graphite Nodules in Cast Iron

Important properties of cast iron, such as fatigue strength, wear resistance, and low-temperature toughness, relate to spatial distribution of graphite nodules. Characterization of spatial distribution can also provide insight into the solidification sequence in casting. An automated SEM/EDX analysis was utilized to distinguish graphite nodules from other structural features (pores and inclusions). The two-dimensional near-neighbor distance (NND) between nodule centers was calculated for three equal sets of nodule diameters (small, medium, and large) in each cast iron. Comparison of measured spatial distributions and ideal random distribution was executed by plotting the mean and variance ratios of NND on a spatial distribution quadrant. This method was used to clarify clustering or ordering tendencies of graphite nodules in studied cast irons. The suggested procedure was used to verify the effects of inoculation and the cooling rate on spatial distribution of graphite nodules. Inoculation of sand casting increased nodule counts, decreased mean NND, and eliminated clustering of small graphite nodules precipitated at the solidification end. Intensive surface cooling of a continuously cast bar significantly increased nodule count near the external surface and decreased NND without changing spatial distribution. The suggested analysis can be used as a tool for cast iron quality control and process development.

Damaging micromechanisms in an as cast ferritic and a ferritized ductile cast iron

Abstract: Mechanical behavior and damaging micromechanisms in Ductile Cast Irons (DCIs) are strongly effected by matrix microstructure (e.g., phases volume fraction, grains size and grain distribution) and graphite nodules morphology peculiarities (e.g., nodularity level, nodule size, nodule count, etc.). The influence of the graphite nodules depends on both the matrix microstructure and the loading conditions (e.g., quasi-static, dynamic or cyclic loadings). According to the most recent results, these graphite nodules show 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, two different ferritic DCIs were investigated (a ferritic matrix obtained from as-cast condition and a ferritized matrix) focusing on the damaging micromechanisms in graphite nodules due to tensile stress. Specimens lateral surfaces were observed using a Scanning Electron Microscope (SEM) during the tests following a step by step procedure.

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

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

Experimental and analytical investigation of the TMF-HCF lifetime behavior of two cast iron alloys

The combined loading Thermomechanical Fatigue (TMF) with High Cycle Fatigue (HCF) has been experimentally investigated for two cast iron alloys. Both alloys contain globular graphite nodules but the first one has a ferritic structure while the second one has an austenitic crystal structure. In particular, the influences of the HCF frequency, of the HCF loading amplitude and of the location of the superposed HCF cycles have been investigated. It was observed that the HCF frequency has a limited impact on the fatigue life. On the other side, the HCF-strain amplitude has a highly non-linear influence on the fatigue life. The experimental results can be understood in terms of a fracture mechanics based damage mechanism: Cracks quickly initiate due to the TMF loading and the growth of the cracks up to a few mm controls the fatigue life. If HCF-loading cycles are superposed, cyclic crack propagation dramatically accelerates after a threshold has been reached. This threshold is regarded as controlling the fatigue life reduction. The previous ideas have been expressed in a model that can be very simply applied to estimate the fatigue life reduction ratio due to the superposed HCF cycles. It only contains two adjustable parameters and can be combined with any TMF life assessment model.

Prediction of Graphite Nodule Count and Shrinkage Tendency in Ductile Cast Iron, with 1 Cup Thermal Analysis

Although 3 cup thermal analysis is a good method to predict the molten metal’s quality in gray cast iron, it could not be used in ductile cast iron due to several reasons. We have succeeded in investigating the graphite eutectic temperature (hereafter simply shown as TEG) and cementite eutectic temperature (hereafter simply shown as TEC) in ductile cast iron, from the chemical composition of the molten metal, as follows: $$ {\text{TEG}} = 1149.1\left( {^\circ {\text{C}}} \right) + 4.7{\text{Si mass\% }} - 4.0\left[ {{\text{Sol}}.{\text{Mn mass\% }}} \right] - 44{\text{P mass\% etc}}. $$ $$ {\text{TEC}} = 1142.6\left( {^\circ {\text{C}}} \right) - 11.6{\text{Si mass\% }} - 0.75\left[ {{\text{Sol}}.{\text{Mn mass\% }}} \right] - 46.2{\text{P mass\% etc}}. $$ (hereafter, mass% is simply shown as %). We added various kinds of inoculants into the molten metals and then cast them at various fading time into 1 cup. Using the cooling curve of 1 cup and above equations, we calculated eutectic graphitization ability (hereafter simply shown as EGA). The graphite nodule count (N) of CE cup T.P. is determined by $$ {\text{EGA}}\left( { = 100 \times {\text{DT}}1/{\text{DTE}}} \right) $$ as follows: $$ N = 6.13 \times \left( {100 \times {\text{DT}}1/{\text{DTE}}} \right) - 126 $$ . In addition, when EGA becomes over 70 %, shrinkage tendency (θ) becomes low near 60°. Consequently, 1 cup thermal analysis is a good system to predict graphite nodule count and shrinkage tendency (θ) correctly in ductile cast iron, before pouring it.

Fatigue Characterization and Optimization of the Production Process of Heavy Section Ductile Iron Castings

In heavy section cast iron components, anomalous microstructures can be formed especially at the core, because of low cooling rate, with a consequent decrease in mechanical properties. The matrix microstructure, the presence of surface defects, such as inclusions and porosities, and the size and morphology of graphite nodules are critical issues to ensure the adequate performance of the component in fatigue applications. Proper design of heavy section ductile iron castings requires a database of information about microstructures and mechanical properties that nowadays is missing or is very poor. In this paper, we presented the mechanical characterization of ductile irons with solidification times up to 20 h, carried out to identify the metallurgical and the process parameters that most affect the improvement in fatigue strength. An innovative production line was designed, aimed at reproducing on a small-scale sample the characteristics of heavy walled components. A standard ferritic ductile cast iron compared to a ferritic cast iron with high silicon content and a fully pearlitic ductile cast iron were studied by tensile, Brinell hardness and rotating bending fatigue tests. Optical microscope analysis and scanning electron microscope observations of the most significant fracture surfaces were also performed. It was concluded that the ferritic cast iron is more suitable for the production of large thickness castings undergoing fatigue loads, because the decrease in ductility and its effect on other properties is less pronounced.