Graphite Nodules Research Articles

Austempered Ductile Iron with Dual Microstructures: Effect of Initial Microstructure on the Austenitizing Process

This work compares alternatives for the production of high ductility ADI, austempered from an intercritical austenitizing temperature range, with a microstructure of ferrite + ausferrite. Different initial microstructures were selected, including ferrite, pearlite, pearlite + ferrite, martensite and ausferrite. The samples were austenitized within the intercritical zone (795 °C) for different times (up to 12 h) and then austempered at 350 °C. The evolution of the formation and distribution of ferrite and austenite during the austenitizing process for the different initial microstructures was studied. For the selected austenitizing temperature, all of the initial microstructures produced 30% ferrite (70% ausferrite) in the final microstructure (after 12 h). The mechanical properties depend upon the distribution and refinement of the ferrite and ausferrite areas. Spheroidized carbides from pearlite are not completely dissolved during intercritical austenitizing for 2 h. Graphite nodules are an important source of carbon for the austenite formed from starting microstructures of ferrite, pearlite + ferrite and pearlite with grain boundaries being the main pathway for carbon diffusion. Austenitizing started away from the graphite nodules at eutectic cell boundaries due to Si segregation. For starting microstructures of ferrite, pearlite and pearlite + ferrite, a homogeneous distribution of austenite does not occur. Rather, it is concentrated on eutectic cell boundaries and ferrite grain boundaries with large areas of ferrite around the graphite nodules. For starting microstructures of martensite and ausferrite, carbon is evenly distributed and quickly dissolved, resulting in a homogeneous distribution of austenite and ferrite with the best combination of strength and ductility.

Strain Energy Decomposition Influence in the Phase Field Crack Modelling at the Microstructural Level of Heterogeneous Materials

The prediction of a crack initiation and propagation occurring on the microstructural level of heterogeneous materials can be a very demanding problem. According to the results of recent investigations, the emerging phase field approach to fracture has a strong potential in modelling the complex crack behaviour in a simple manner. In this study, recently developed phase field staggered solution scheme with the residual norm stopping criterion has been employed for the fracture analysis of heterogeneous microstructure exhibiting complex crack phenomena. The microstructural geometries based on the metallographic images of the nodular cast iron and the material properties of an academic brittle material have been used in numerical simulations where the graphite nodules have been considered as porosities. Two commonly used energy decomposition models, the spectral decomposition and the spherical-deviatoric split, and their effects on the results of the phase field modelling are investigated. Numerical results show that the proposed algorithm recovers the complicated crack path driven by the complex microstructural topology.

Experimental Analysis of Mechanical Properties, Wear and Corrosion Characteristics of IS500/7 Grade Ductile Iron Casting

In the present research investigation, ductile iron castings conforming to IS500/7 grade casting were produced using green sand moulds. The castings were produced in a regular production foundry using 1.25T Induction Furnace. The molten metal was subjected to standard desulphurisation and modularisation to get graphite in the nodular form. Based on the earlier investigation by the above group the castings were austenitised at 900oC for 120minutes and quenched in a salt bath consisting of potassium nitrate and sodium nitratein the ratio of 55:45 and maintained at 310oC for 120 minutes to bring about austempering treatment. The result in austempered ductile iron will consist of graphite nodules in a matrix of bainite[1,2]. The effect of austempering heat treatment on microstructure, mechanical properties[13,14] andwear characteristics were studied and compared with the as-cast condition. Mechanical properties such as ultimate tensile strength, % elongation, hardness and impact strength were determined. Wear tests were carried out to determine the weight loss of specimens; using pin on disc type machine. Wear test was held for different loads viz. 300 gm, 400 gm and 500 gm and different speeds conditions viz. 950 r.p.m, 1430 r.p.m and 2130 r.p.m. Corrosion tests were carried out to determine corrosion rate at different viz. 35°C & 45°C temperature for increasing duration viz. 2 hours to 24 hours. The results of the research investigation indicate that the heat treated castings shows higher Ultimate Tensile Setength values (54.86% increase), elongation values (64.32% increase) and Brinell hardness values (19.03% increases) as compared to the ascast condition of casting. From the dry sliding wear studies, it is seen that heat treated specimen’s shows higher resistance to wear compared to the ascast specimens and corrosion rate & weight loss due to corrosion in heat treated condition is less compared to ascast ones.

The Role of Microstructure on Tensile Plastic Behavior of Ductile Iron GJS 400 Produced through Different Cooling Rates, Part I: Microstructure

A series of samples made of ductile iron GJS 400 was cast with different cooling rates, and their microstructural features were investigated. Quantitative metallography analyses compliant with ASTM E2567-16a and ASTM E112-13 standards were performed in order to describe graphite nodules and ferritic grains. The occurrence of pearlite was associated to segregations described through Energy Dispersive X-ray Spectroscopy (EDS) analyses. Results were related to cooling rates, which were simulated through MAGMASOFT software. This microstructural characterization, which provides the basis for the description and modeling of the tensile properties of GJS 400 alloy, subject of a second part of this investigation, highlights that higher cooling rates refines microstructural features, such as graphite nodule count and average ferritic grain size.

Microstructure Changes During Solidification of Cast Irons: Effect of Chemical Composition and Inoculation on Competitive Spheroidal and Compacted Graphite Growth

Amongst the most important graphite shapes, nodules and compacted particles are of particular interest as they can coexist in castings with relevant changes in properties. In this context, it has been often reported that the compacted shape is an intermediate form between nodules and graphite lamellas that may be seen as degeneracy from the nodular one. The present work shows the microstructure evolution of an initially ductile iron alloy with a silicon content of about 2.4 wt% when reducing progressively the magnesium content by holding a large melt batch in a nitrogen-pressurized pouring unit for 8 h. Thermal cups with and without inoculant were cast at a regular time interval together with a sample for chemical analysis. Interestingly, the thermal records of the inoculated samples show no significant changes with time while the structure evolved from fully spheroidal to half-spheroidal half-compacted graphite. Conversely, the thermal curves of the non-inoculated samples showed two arrests, one at nearly the same temperature as for inoculated alloys and a second one at a temperature decreasing with holding time until being below the metastable eutectic temperature. Microstructure observations showed the presence of a limited number of compacted cells which decreases as well with holding time. These observations suggest that these cells start developing during the temperature interval between the first and second arrests, leading to a bulk eutectic transformation either above or below the metastable eutectic temperature. These results support the view that a fully compacted structure can be obtained only with a controlled inoculation which should not be too high to avoid too high nodularity.

Surface transformation hardening of ductile cast iron by a 600w fiber laser

Laser transformation surface hardening of the forming edges of large mold made of nodular cast iron has gained momentum in recent years. In this investigation a 600 w fiber laser was employed to harden GGG-60 cast iron. It is found that to achieve the optimum surface properties, the combination of laser power density and travel speed has to be set such that to dissolve the carbides but not that high to dissolve the graphite nodules. The response of a hardened track to a second thermal cycle by a neighboring laser track in terms of both tempering and re-hardening showed that by increasing the overlapping, a more uniform surface microstructure and hardness is obtained. In addition by employing differential scanning calorimetry (DSC) analysis the maximum “Hardening Ratio” by the fiber laser as the power source for GGG-60 cast iron was found to be 15.3 %.

3D characterization of structure and micro-porosity in two cast irons with spheroidal graphite

Formation of low density graphite phase during solidification increases the volume of semi-solid cast iron with spheroidal graphite (SGI) and resulted pressure could help eliminate shrinkage micro-porosity in casting. Therefore, micro-porosity in casting can be linked to 3D graphite nodule structure. Two experimental SGI's with different levels of shrinkage micro-porosity in castings were intentionally produced by variation in inoculation parameters which influenced graphite nodule structure. 3D-morphological characteristics of graphite nodules and micro-porosity level were determined by μCT scanning. The obtained statistical data was discussed with respect to graphite nodule solidification mode and casting soundness. The capabilities of μCT scanning in predicting micro-shrinkage in SGI casting were compared to the other traditionally used techniques.

Experimental Analysis of Mechanical Properties, Wear and Corrosion Characteristics of IS400/12 Grade Ductile Iron Casting

In the present research investigation, ductile iron castings conforming to IS400/12 grade casting were produced using green sand moulds. The castings were produced in a regular production foundry using 1.25T Induction Furnace. The molten metal was subjected to standard desulphurisation and modularisation to get graphite in the nodular form. The molten metal was poured into the sand mould and allowed to solidify. Based on the earlier investigation by the above group the castings were austenitised at 900oC for 120minutes and quenched in a salt bath consisting of potassium nitrate and sodium nitrate in the ratio of 55:45 and maintained at 310oC for 120 minutes to bring about austempering treatment. The result in austempered ductile iron will consist of graphite nodules in a matrix of bainite[1,2]. The effect of austempering heat treatment on microstructure, mechanical properties and wear characteristics were studied and compared with the ascast condition. Mechanical properties such as ultimate tensile strength, percentage elongation, hardness and impact strength were determined. Wear tests were carried out to determine the weight loss of specimens; using pin on disc type machine. Wear test was held for different loads viz. 300 gm, 400 gm and 500 gm and different speeds conditions viz. 950 r.p.m, 1430 r.p.m and 2130 r.p.m. The results of the research investigation indicate that the heat treated castings shows higher Ultimate Tensile Setength values (35.93% increase), elongation values (24.2% increase) and Brinell hardness values (12.05% increases) as compared to the as-cast condition of casting. From the dry sliding wear studies, it is seen that heat treated specimens shows higher resistance to wear compared to the ascast specimens.

About Equilibrium Mode Ruling Ferritic Transformation in Low-Alloy SGI

Ferrite precipitating around the graphite nodules shaping the typical bull’s-eye microstructure could grow under negligible partitioning local equilibrium or under paraequilibrium conditions, as both imply that ferrite inherits the composition of the parent austenite. The first mechanism has been rejected by other researchers by means of simple calculations of the silicon spike width necessary for local equilibrium conditions to take place. Nevertheless, experimental analyses are necessary to verify this conclusion. In this study, transmission electron microscopy has been used to assess the presence of a silicon spike in front of the growing ferrite interface. The outcome allowed the authors to confirm that a paraequilibrium mode governs the transformation, supporting the conclusions of previous calculations. In addition, some issues about ferrite growth modeling are discussed.

Influence of Graphite Nodules on the Microstructure and Mechanical Properties of Hollow Ductile Iron Pipe after Austempering Treatment

Herein, the microstructure and mechanical properties of ductile iron with different numbers of graphite nodules before and after austempering treatment are systematically investigated. The results show that the amounts of graphite nodules affect the microstructure and resulting mechanical properties of ductile iron. The ferrite fraction in an as‐cast matrix decreases with the increase in graphite nodules; this outcome is mainly attributed to the cooling rate. The matrix of ductile iron after austempering treatment is composed of acicular ferrite and retained austenite. The acicular ferrite is refined with the increase in graphite nodules, because the increase in interface between graphite/austenite provides more nucleation sites during austempering treatment. Meanwhile, the formation of ferrite causes carbon atoms to diffuse into the surrounding matrix, giving rise to the increase in carbon concentration of untransformed austenite and further stabilizing the retained austenite. Hence, the volume fraction of retained austenite and the carbon content in it both increase with the increase in graphite nodules. The amounts in graphite nodules affect the mechanical properties significantly after austempering treatment; this outcome is related to the phase constituents of ductile iron after austempering treatment. Herein, the detailed mechanisms are discussed.

The Role of Microstructure on the Tensile Plastic Behaviour of Ductile Iron GJS 400 Produced through Different Cooling Rates—Part II: Tensile Modelling

Tensile testing on ductile iron GJS 400 with different microstructures produced through four different cooling rates was performed in order to investigate the relevance of the microstructure’s parameters on its plastic behaviour. Tensile flow curve modelling was carried out with the Follansbee and Estrin-Kocks-Mecking approach that allowed for an explicit correlation between plastic behaviour and some microstructure parameters. In the model, the ferritic grain size and volume fraction of pearlite and ferrite gathered in the first part of this investigation were used as inputs, while other parameters, like nodule count and interlamellar spacing in pearlite, were neglected. The model matched very well with the experimental flow curves at high strains, while some mismatch was found only at small strains, which was ascribed to the decohesion between the graphite nodules and the ferritic matrix that occurred just after yielding. It can be concluded that the plastic behaviour of GJS 400 depends mainly on the ferritic grain size and pearlitic volume fraction, and other microstructure parameters can be neglected, primarily because of their high nodularity and few defects.

Development of high strength austempered ductile iron (ADI) from conventional pig iron

An alloy ductile iron can be transformed into high strength austempered ductile iron (ADI) by transforming ferritic-pearlitic matrix into ausferrite after subjecting to austempering heat-treatment process. ADI can be used for making complex shapes components for high strength applications owing to its castability. The present work aims to develop ductile iron containing Ni and Mo using low cost conventional pig iron rather than expensive Sorel pig iron by ladle sandwich method in a commercial high-frequency induction furnace. Effect of austempering time ranging from 30 to 120 min at austempering temperatures of 265 °C and 330 °C on microstructure and mechanical properties of resulting ADI was evaluated. Results showed that ADI developed from the cost-effective ductile iron produced by pig iron have mechanical properties comparable to ADI developed using Sorel pig. Optical and scanning electron microscopy revealed that large size graphite nodules were formed in the matrix of ausferrite and retained austenite at 265 °C austempering for 30 min. Increasing austempering time resulted in an increase in ausferrite fraction with the decrease in retained austenite fraction. ADI developed by austempering at 330 °C resulted in a small fraction of ausferrite and a large fraction of retained austenite compared to that developed at 265 °C. Austempering heat treatment process considerably improved mechanical properties of ductile iron at both tempering temperatures (265 and 330 °C). The tensile strength, elongation and hardness of ADI produced by austempering at 265 °C for 90 min soaking time increased by 42, 83 and 26% compared to that produced at 330 °C for 90 min.

Slip-Rolling Behavior of Ductile and Austempered Ductile Iron Containing Niobium or Chromium

Abstract The use of high niobium alloyed cast iron alloys is a relatively new approach in which the niobium addition intends to improve the properties of the material by the precipitation of hard niobium carbides during solidification. Steels can be replaced by ductile cast iron in some rolling applications, such as gears and cams, in order to reduce material costs. The aim of this work is to evaluate ductile iron alloyed with 1 weight percent (wt.%) niobium for the as cast specimens and with 1.8 wt.% and 2.4 wt.% niobium for the austempered specimens under lubricated slip-rolling tests using mixed/boundary conditions in an Amsler-type machine. Austempered ductile iron (ADI) alloyed with 1 wt.% chromium, or Carbidic ADI, was tested for comparison. For the as cast conditions, the niobium addition resulted in an increase of wear resistance owing to the low contact pressure of these tests. However, for the austempered specimens, the best performance was found for unalloyed ADI. The main factor acting in the initiation and propagation of cracks in ductile iron is the presence of the graphite nodules. The coarse carbides also contributed to the initiation of cracks and spalling of the material.

Wear behavior of CADI obtained at different austenitizing temperatures

Abstract The microstructure, mechanical properties and wear resistance of carbidic austempered ductile iron (CADI) obtained at different austenitizing temperatures were investigated. The dry sliding wear failure mechanism in block-on-ring tester and erosive wear failure mechanism of CADI in the slurry of pH = 4, pH = 10 were analyzed. The results show that with the increase of austenitizing temperature, the hardness and tensile strength of CADI first increase and then decrease, and the wear loss decreases first and then increases. The shedding of carbides in corrosive media and the deformation of the matrix around the graphite nodules are the main failure mechanisms in corrosive wear. In addition, abrasive wear and adhesive wear are the main failure mechanism under dry sliding wear condition.

Ductile cast irons: Microstructure influence on the fatigue initiation mechanisms

AbstractIn the last decades, the combination of high mechanical performances and low production costs increased the industrial interest on ductile cast irons. These grades are often used for applications where the fatigue resistance can be a critical issue (eg, machine frames for the wind‐power industry or crankshaft used in trucks) and the investigation of the main damaging mechanisms during both the crack initiation and the crack propagation stage could offer new perspectives about these alloys.Ductile cast irons can be considered as a natural composite, being characterized by graphite elements (nodules) embedded in a more or less ductile matrix (ranging from fully ferritic to pearlitic, from martensitic to austempered). In this work, the fatigue crack initiation mechanisms were investigated considering different matrix microstructure and the presence of a mechanical properties gradient in the graphite nodules.

Microstructure and properties of Cu-bearing carbidic austempered ductile iron

AbstractIn this paper, the effects of copper addition (0–2.0 wt.%) on the microstructure and properties of carbidic austempered ductile iron were studied using an optical microscope, a scanning electron microscope, a Rockwell hardness tester, a pendulum impact tester, an X-ray diffractometer, a block-on-ring wear testing machine and an electrochemical tester. The amount of carbides and graphite nodules simultaneously decreased with the increase in Cu content. Cu also improved the nodularity of graphite, made the graphite nodules grow, increased the pearlite content and refined the pearlite. After austenitizing (900°C / 100 min) and austempering (300°C / 100 min), the amount and size of the acicular ferrite significantly reduced with the addition of Cu. In contrast, the amount of retained austenite dramatically increased, and the carbon content in retained austenite slightly increased. The hardness of carbidic austempered ductile iron had no obvious change with the increase in Cu content. The wear resistance of carbidic austempered ductile iron first increased and subsequently decreased, and the impact toughness and corrosion resistance were improved with the increase in Cu content. The comprehensive properties of carbidic austempered ductile iron were optimal, with the Cu content of 1.0 wt.%.

Micro-CT Quantitative Evaluation of Graphite Nodules in SGI

The quality of spheroidal graphite cast iron (SGI) castings is directly related to the morphology of graphite nodules. Therefore, accurate and comprehensive quantitative evaluation of microstructure is critical for industrial practices. The most influential morphological characteristics of the graphite phase in SGI can be divided into three categories: (1) shape and size of individual graphite nodules, (2) total volume of graphite phase and nodule number, and (3) spatial distribution. During the last decades, SGI structure analysis has progressed from visual observation of polished cross section and quantitative 2D techniques (metallography and automated SEM/EDX) to observation of the real 3D structure using micro-computed tomography (µCT) scanning. In this article, 2D and 3D morphological characteristics of graphite nodules were determined by µCT scanning and compared. 3D µCT provides accurate characterization of the real shape factor and diameter of graphite nodules. It was shown that large and small graphite nodules in inoculated SGI have the different modes of space distribution which could be linked to casting solidification.

A practical method for fatigue limit prediction in ductile cast irons

AbstractA simple, reasonably accurate method was proposed for fatigue limit prediction in ductile cast irons (DCIs) containing small defects, employing an easy‐to‐use prediction equation. A technique was also presented for the estimation of the statistical effects of complex structural discontinuities, as characterized by graphite and casting defects. The validity of these approaches was confirmed by the experimental results obtained via rotating‐bending fatigue tests on six DCIs, with varying distributions of graphite nodule size and different matrix structures.

Overload effects on fatigue cracks in a ferritized ductile cast iron

Ductile cast irons offer an interesting combination of good castability and high mechanical properties, with acceptable costs. This is due to the peculiar graphite nodules shapes obtained not by means of long and expensive heat treatments but by means of the chemical composition control. The relevant mechanical properties of these grades in the as cast conditions can be further modified and improved controlling the matrix microstructure, both modifying the alloys chemical compositions and/or optimizing different heat treatments (e.g., austempering). High temperature heat treatments can imply a modification of the graphite nodules shape due to the carbon atoms solid diffusion process. In this work, the influence of the modification of graphite nodules shape, induced by heat treatment for DCI ferritization, on the damaging mechanisms is investigated, focusing their role on the fatigue crack tips damaging mechanisms after overloadings.

Material defects as cause for the fatigue failure of metallic components

The paper provides an overview on material defects which may serve as fatigue crack initiation sites and can cause final fatigue failure of a component. These comprise non-metallic inclusions in steel and aluminum alloys, cavities such as pores, micro-shrinkages and un-welded regions in cast, sinter and additively manufactured alloys, graphite nodules, shrinkages and other items in nodular cast iron, regions of defective microstructure, microcracks and secondary notches such as undercuts and surface roughness. Besides their origin, the effect and mechanisms on fatigue crack initiation and propagation are discussed. The considerations are proceeded and accompanied by a brief discussion of some basic aspects such as the stages of crack propagation along their length scale, the overcoming of crack arrest and the question when a secondary notch can be treated as a crack.