This work focuses on the effect of cooling rates controlled by the different crystallizer inner diameter on microstructure and properties of the of cast iron and the ADI. After the same austempering process, the effect of graphite quantity and size on the properties of cast iron and ADI and the effect of the original microstructure are investigated. The number of graphite nodules, the nodular graphite size, the number and morphology of the ferrite plates, and the high carbon austenite determine the final properties of the ADI castings. For this purpose, five kinds of ductile cast irons are fabricated by melting and casting with different crystallizer inner diameters. A microstructural investigation and the mechanical testing of the ductile iron and ADI are performed.
The exact morphology of the ferrite phase and the relative amounts of ferrite and austenite can be controlled by adjusting the austempering temperature and time, which will further determine the mechanical properties of the ADI [ 7 15 ]. When ADI is austempered at lower temperatures, different bainite ferrite morphologies, volume fractions of high-carbon austenite and carbon contents in austenite can be obtained [ 16 ]. When the ADI is austempered at higher temperatures, an ausferritic microstructure is produced, which is a combination of carbide-free ferrite and austenite. The ferrite becomes feathery (coarse) and it has a lower yield strength, which decreases the yield strength of the ADI [ 17 18 ]. In addition, different morphologies of graphite also have a great influence on the mechanical properties of the material. T. Borsato [ 19 ] studied that degenerated graphite particles (spiky graphite) increase the notch effect of crack nucleation, thereby reducing the fatigue strength of the material. Y. Liu [ 8 ] has researched that the tensile strength of compacted graphite iron with low nodularity rate is lower than that of ductile cast iron with nodularity rate greater than 95%. Because the stress concentration is easily formed at the tip of the vermicular graphite, cracks are easily formed here. The ADI has good mechanical properties, and its strength is one to two times higher than the ductile iron. However, isothermal quenching requires a salt bath treatment and a large initial investment in the production equipment, yields serious nitrite pollution, and has a low productivity, high energy consumption, and a high cost.
The excellent properties of ADI are related to its unique microstructure. During austempering, ductile iron undergoes a two-stage phase transformation. In the first stage, austenite () decomposes to ferrite (α) and high carbon austenite () because of the high silicon content of ductile iron, which suppresses carbide formation. As the carbon-rich austenite is very stable, the combination of ferrite and residual austenite is the most desirable microstructure and gives ADI excellent properties. If ductile cast iron is held at this temperature for a prolonged period, a second reaction occurs. During the second stage,can decompose to ferrite (α) and carbide (ε) [ 5 ]. Because this structure contains ε carbide, which makes the material brittle, this reaction must be avoided. The optimum austempering period is the duration between the end of the first reaction and the beginning of the second reaction, which is sometimes referred to as the “processing window”. The Si content in ductile cast iron is high, which can promote the separate nucleation of ferrite in austenite, inhibit the formation of cementite, and accelerate the diffusion of carbon atoms in austenite. The toughness and stability of austenite increase significantly with an increase of carbon content in austenite. The formation of acicular ferrite without carbide precipitation and a significant increase of carbon content of austenite are important features of this austempered transition. High-carbon austenite and acicular ferrite have an excellent toughness, and the matrix does not contain brittle phase cementite.
The cast iron industry has developed rapidly and has played an important role in iron and steel research [ 1 ]. Microstructural characteristics of cast iron, such as the graphite shape and distribution, and the metal matrix microstructure, depend on the chemical composition, the casting process, and the cooling condition [ 2 ]. The nodularity rate varies in a cast component because of variations in the crystallizer inner diameter and heat dissipation in different regions of the crystallizer [ 3 ]. For ductile iron with nodular graphite particles, the overall graphite volume, area fraction of the nodular graphite (nodularity), number of nodules per unit area (nodule number), nodular graphite diameter and matrix microstructure are influenced by the solidification condition and addition of alloying elements [ 4 ]. The microstructural characteristics, such as the amount of ferrite/pearlite, the nodularity rate, the nodular size and morphology, affect the mechanical properties of ductile iron significantly [ 5 8 ].
To quantify the volume fraction of residual austenite () and the carbon content of austenite () in the alloys, the samples were analyzed by the X-ray diffractometry (XRD). A D8 ADVANCE X-ray diffractometer with a Cu target, a tube current of 40 mA, a tube voltage of 35 kV, and a Lynxeye XE detector were used. Samples were scanned over a 2θ range from 40° to 100° with a step size of 0.02°. The profiles were analyzed by using Jade 6 software (MDI, USA) to obtain the peak positions and integrated intensities of (200), (220), (311), and planes of FCC austenite and (110) and (211) planes of BCC ferrite. Thewas calculated quantitatively by using Jade 6.0. The volume fractionwas determined by the direct comparison method by using the integrated intensities of the above planes. Three samples were examined from each heat-treated condition and the data reported are the averages from these samples. The carbon content of the austenite was determined by the following formula:whereis the lattice parameter of austenite in nanometers andis the carbon content of austenite [ 12 ].
The microstructure was revealed by etching in 4% Nital for approximately 15 s. A Zeiss 40MAT microscope (Zeiss, Oberkochen, Germany)was used for low magnification observation of the microstructures in different states. A scanning electron microscope (SEM, FEI Quanta 650FEG, FEI, Hillsboro, OR, USA) was used to analyze the microstructures. Microstructural parameters, such as the nodular graphite number, the nodular graphite diameter, and the nodularity rate were evaluated according to the standard ASTM-A247-16a and measured by using Photoshop and Image-Pro Plus (IPP, Media Cybernetics, Rockville, MD, USA).
Tensile tests were carried out on a servo-hydraulic MTS test machine according to the standard ASTM E8M [ 21 ], with a gauge length of 30 mm. Three samples were tested under each heat-treated condition and the average values were reported. The tensile properties, including the tensile strength σ(MPa), yield strength σ(MPa), reduction of area φ (%) and elongation δ (%), were calculated as the averages of three tests. Room temperature impact tests were carried out in a domestic JBN-300B pendulum chain-impact testing machine. The impact toughness was measured based on the average of five U-shaped Charpy impact tests according to the standard ASTM E23-2016b. Each was considered a valid Kvalue. Samples were cut into 55 mm × 10 mm × 10 mm. The hardness testing was carried out by using a Brinell Hardness tester at a 750-kg applied load and using a 5-mm diameter steel ball. The reported microhardness values are the average from five measurements.
Castings with different crystallizer inner diameters in the as-cast condition were marked as AC 60, AC 75, AC 95, AC 120, and AC 150, respectively. After fabrication, castings from the core of the original rods were cut into 60-mm-thick cylinders, which were austenitized at 900 °C for 2 h, then quenched into a salt bath (that contained 50% KNO 3 + 50% NaNO 3 ) at 250 °C and kept for 2 h. ADI samples were marked with different crystallizer inner diameters as ADI 60, ADI 75, ADI 95, ADI 120, and ADI 150, respectively.
The hot metal components were configured according to mass percentage: C: 3.4–3.7%, Si: 1.5–1.7%, Mn: 0.3–0.5%, S ≤ 0.05%, P ≤ 0.05%, Fe and other impurities formed the balance. After the molten iron was melted in an intermediate frequency induction furnace nodulizer, the 75 FeSi inoculant was added into the melt. The temperature of the molten iron that emerged from the furnace during casting was 1480 °C, and the starting solidification temperature was ~1250–1300 °C. Because ductile iron is a multi-element Fe-C-Si alloy, the cooling rate of continuous casting is very fast, so its crystallization process occurs within a certain temperature range, rather than as a certain critical point temperature, as indicated on the two-dimensional equilibrium phase diagram. Crystallizers with diameters of 60, 75, 95, 120, and 150 mm were used. After incubation and nodularization, the final material composition was determined by infrared absorption method. The chemical compositions of samples were C 3.51, Si 2.8, Mn 0.32, S 0.0085, P 0.023, and Fe balance (wt.%).
The as-cast structure of the vertical continuous casting profile is dendritic on the outside, and equiaxed crystals begin to appear at about 15 mm from the outside wall. However, in the subsequent annealing and quenching process, multiple phase transformations and recrystallization occur, making the dendrites become equiaxed crystals that have been greatly refined, and the internal and external microstructures tend to be similar and consistent.
The liquid–solid interface is a three-dimensional tapered tube shape. Under the action of the free energy of phase change provided by cooling, crystal nuclei are continuously generated on the interface, which then grow up, thickening the tube wall. However, under the effect of the convective erosion of molten iron, a part of the initial crystal nuclei will float up and adhere to the upper tube wall. The higher the tube wall, the more floating crystal nuclei can be obtained, which is equivalent to increasing the nucleation rate, making the deep part of the tube wall or the core of the bar finer. Offsetting the slower cooling rate of the core of the profile causes coarse microstructures.
The material was cast via the vertical continuous casting of ductile iron profiles by using the bread pig iron, the scrap steel, and the cast-iron alloy. Compared with other casting methods, the microstructure obtained by vertical continuous casting is more uniform. During vertical continuous casting of hollow profiles, the distribution of liquid and solid phases in the longitudinal and transverse sections of the crystallizer occurs as shown in Figure 1 . The figure on the left in Figure 1 is a schematic diagram of the convection of molten iron in the crystallizer, and the figure on the right is a schematic diagram of the centripetal solidification of the crystallizer. The phase transformation and solidification in the crystallizer is similar when the solid bar is continuously cast vertically.
As shown in Figure 6 a, the ultimate tensile strength (σ) and yield strength (σ) of the ADI samples decrease initially, increase, and fluctuate with an increase in the crystallizer diameter, but little difference has been observed for AC 75–AC 150, which shows the same trend of hardness in Figure 6 a. After the isothermal quenching, the average ultimate strength improved from 500 to 1100 MPa, and the average hardness increased by 140%. Figure 6 c,d shows the elongation, reduction of the area, and the impact energy of the crystallizer inner diameters 60–150 mm in the as-cast and ADI states. The trend of ADI 120 was different, which may be because of the dispersion nature of the graphite; big dispersion is expected. The final trend was that as the crystallizer inner diameter increased, the elongation and impact energy decreased in both states.
The thinner crystallizer showed that a higher number of graphite particles resulted in a shorter distance between the particles and thus, a greater overlap between the ferrite regions that were formed around the graphite particles. With an increase in distance between the graphite particles, the ferrite no longer overlapped, and the ferrite area fraction increased. When the inner diameter of the crystallizer exceeded 95 mm, more time was provided for the carbon atoms to diffuse from the melt to graphite, so that larger graphite particles, and consequently, larger ferrite regions could form with a lower ferrite fraction. This trend was associated with a significant increase in distance between the graphite nodules (an increase in austenite diffusion paths for carbon), which resulted in a greater pearlite fraction in the areas between the graphite nodules (see Table 2 ).
Five pictures were selected, the area fraction of the different phases was determined, and the average was taken to yield the data in Table 2 for each as-cast specimen at the same magnification. The pearlite content first decreases and then increases with an increase of crystallizer inner diameter, and reaches a minimum at a diameter of 95 mm, and the ferrite content first increases and then decreases. Approximately the similar amount of nodular graphite exists in the first four specimens, while that in the AC 150 is the lowest.
Javaid et al. [ 24 ] reported that samples with lower-diameter crystallizers had a significantly stronger tensile and yield strength because of a more homogeneous microstructure, a higher nodule number, and a better graphite nodularity rate. A smaller crystallizer inner diameter yields a larger supercooling, a finer pearlite spacing, and smaller ferrite particles [ 25 26 ]. A pearlite content for the AC 60 specimen shows a smaller difference from that for the AC 150 sample, but σand σare higher than that of the AC 150 samples. In the AC 95, the pearlite volume content is 17.6%, which is almost half of that for the AC 150 (33.4%). Pearlite is strong, but σand σof AC 95 and AC 150 show little difference. The AC 60 specimen has a nodular graphite diameter class of 8, with a nodularity rate of 100% which is the highest. The nodular graphite is distributed uniformly in the matrix. The number of nodular graphite decreases for the AC 150 specimen. When the nodular graphite diameter class is 6, the nodularity rate decreases to 70%, and chunks of graphite are produced, which appear to be small graphite scraps that are dispersed in the matrix. The emergence of chunky graphite decreases the tensile strength, the elongation, and the area reduction [ 27 28 ]. The higher nodularity rate of graphite in ductile iron does not cause an obvious stress concentration, which therefore results in a significant improvement of the strength, plasticity, and toughness [ 29 ]. The improvement in graphite nodularity rate is an important way to improve the ductile iron properties.
Vγ
and theCγ
. A more irregular bainitic ferrite distribution yields a more disordered orientation, a larger interface energy and higher tensile strength, yield strength and hardness. TheCγ
and theVγ
should be as high as possible to improve the impact toughness [The mechanical properties of ADI are affected by several factors. The tensile and yield strength of materials are related to the content and morphology of bainitic ferrite, whereas the toughness is related to theand the. A more irregular bainitic ferrite distribution yields a more disordered orientation, a larger interface energy and higher tensile strength, yield strength and hardness. Theand theshould be as high as possible to improve the impact toughness [ 17 ].
Vγ
increases, the bainitic ferrite content decreases initially and then increases, with an overall decrease. For the ADI 60, the bainitic ferrite particles are small with a maximum interfacial area, and the largest transition kinetics. After austempering, the highest content of acicular bainite ferrite and the lowestγ
hc content is obtained. For the ADI 75, 95, 120, and 150 samples, theCγ
is higher than that in the ADI 60 specimen, the bainitic ferrite is lower, and the strength, hardness, and impact toughness of the ADI 60 specimen are higher than that of the ADI 75-, 95-, 120-, and 150 samples. Because the toughness and strength of fine acicular bainite ferrite are better than that of coarse bainite ferrite, and the nodular graphite is distributed uniformly in the crystallizer with an inner diameter of 60 mm, the nodular graphite and the stress concentration are small, the cutting and reducing effect of the matrix is reduced, therefore, the mechanical property of the ADI 60 sample is the best. A comparison of the effect of nodular graphite on the properties of the as-cast specimens shows that the influence of nodular graphite remains the same. In the ADI 75 and ADI 95 samples with different graphite nodule numbers, the diameter class and nodularity rate of the nodular graphite are the same. With an increase in crystallizer inner diameter, theV
γ
increases and the acicular bainite orientation tended to be straight and parallel. Compared with the ADI 120 sample, the ADI 150 sample shows an increase in the residual austenite content, a decrease in the nodularity rate and the amount of nodular graphite, and the presence of chunky graphite brings poor mechanical properties.The as-cast specimens after austempering have higher tensile strength and yield strength but show a decrease in elongation. Figure 5 shows the ADI microstructures and the XRD analysis diagram. The results show that with an increase in crystallizer inner diameter, the overallincreases, the bainitic ferrite content decreases initially and then increases, with an overall decrease. For the ADI 60, the bainitic ferrite particles are small with a maximum interfacial area, and the largest transition kinetics. After austempering, the highest content of acicular bainite ferrite and the lowestcontent is obtained. For the ADI 75, 95, 120, and 150 samples, theis higher than that in the ADI 60 specimen, the bainitic ferrite is lower, and the strength, hardness, and impact toughness of the ADI 60 specimen are higher than that of the ADI 75-, 95-, 120-, and 150 samples. Because the toughness and strength of fine acicular bainite ferrite are better than that of coarse bainite ferrite, and the nodular graphite is distributed uniformly in the crystallizer with an inner diameter of 60 mm, the nodular graphite and the stress concentration are small, the cutting and reducing effect of the matrix is reduced, therefore, the mechanical property of the ADI 60 sample is the best. A comparison of the effect of nodular graphite on the properties of the as-cast specimens shows that the influence of nodular graphite remains the same. In the ADI 75 and ADI 95 samples with different graphite nodule numbers, the diameter class and nodularity rate of the nodular graphite are the same. With an increase in crystallizer inner diameter, theincreases and the acicular bainite orientation tended to be straight and parallel. Compared with the ADI 120 sample, the ADI 150 sample shows an increase in the residual austenite content, a decrease in the nodularity rate and the amount of nodular graphite, and the presence of chunky graphite brings poor mechanical properties.
In this study, because of the crystallizer inner diameter difference, different original as-cast microstructures and ADI microstructures are obtained, under the same isothermal quenching condition. The initial as-cast microstructure affects the microstructures and properties of the austempered ductile iron obtained after isothermal quenching. Ferrite has a low carbon content, pearlite has a high carbon content, and the graphite has the highest carbon content. Because of the different carbon contents of the initial structure, when the temperature increases, the material transforms into austenite with different carbon contents, which results in carbon-rich and carbon-poor areas in austenite and promotes carbon diffusion [ 30 ]. An increased number of phase interfaces yields a better austenite nucleation. In the as-cast initial structure with a crystallizer inner diameter of 60 mm, the ferrite particles are small, the pearlite layer spacing is small, the number of nodular graphite per unit area is the largest, and the diffusion distances for carbon are shorter in the grain-refined microstructure, which facilitates carbon partitioning. When the temperature increased to the austenitization temperature, more phase interfaces are favorable for austenite nucleation. Hillert et al. [ 31 ] has reported that the process of forming austenite from pearlite is controlled by carbon diffusion and that the effective distance of diffusion is approximately equal to the interlamellar spacing of pearlite. The finer pearlite will provide more nucleation sites for austenite formation. The refined austenite microstructure provides more nucleation sites for bainite and the process of ferrite formation is rapid, which refines the bainite packet structure [ 32 ]. For the larger crystallizers, the nodular graphite micro structure gets coarser, as well as the bainite microstructure after austempering heat-treatment. After austempering, bainite ferrites with different morphologies and different contents form with a high austenite carbon content, and determine the mechanical properties of the ADI. This study gives a qualitative investigation of the relationship between heterogeneous microstructure [ 33 ] and mechanical properties, and the detailed and systematic investigations are for the future investigation, which may include advanced characterization methods such as in situ mechanical testing in microscopes [ 34 35 ].
Different crystallizer inner diameters correspond to different as-cast microstructures and yield different ADI microstructures under the same austempering conditions, as well as the morphology of bainite ferrite, the volume fraction of high-carbon austenite, and the carbon content in austenite. With the increasing crystallizer inner diameter, the cooling speed of casting decreases, the number of nodular graphite decreases, the spacing between nodular graphite increases, and sometimes chunky graphite appears, which reduces the toughness. In this work, the similar microstructure as the traditional isothermal quenching is obtained after the isothermal quenching of specimens from different-inner-diameter casting crystallizers, but with different mechanical properties. Different crystallizer inner diameters can be selected according to different mechanical property requirements.
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