Since its inception in 1946, ductile iron casting has experienced rapid development due to its excellent castability and relatively low production cost, finding widespread application in sectors such as automotive, mining machinery, and wind power equipment. However, when compared to carbon steels, conventional ductile iron casting often exhibits limitations including higher brittleness, lower strength, inferior corrosion resistance, and reduced wear performance, which constrains its potential applications. To broaden the scope of its use, significant efforts are directed towards enhancing the properties of ductile iron. Generally, two primary pathways are employed to improve the performance of cast iron: alloying, which involves the addition of specific elements to enhance characteristics, and heat treatment, which modifies the matrix microstructure to achieve a better combination of properties. Common heat treatment processes for ductile iron casting include quenching and tempering, annealing, thermomechanical processing, and austempering to produce austempered ductile iron (ADI).
Beyond these established methods, the Quenching and Partitioning (Q&P) process, a novel heat treatment concept introduced for advanced high-strength steels in 2003, has shown remarkable potential for improving the strength-ductility balance. The fundamental principle involves quenching an austenitized steel to a temperature between the martensite-start (Ms) and martensite-finish (Mf) points to form a controlled fraction of initial martensite. This is followed by a partitioning step, typically at a higher temperature, where carbon diffuses from the carbon-supersaturated martensite into the surrounding untransformed austenite. This carbon enrichment stabilizes the austenite against transformation to martensite upon final cooling to room temperature, resulting in a final microstructure comprising martensite and retained austenite. The introduction of this Q&P process to ductile iron casting is considered highly beneficial for performance enhancement. While preliminary studies have explored Q&P in ductile iron, a comprehensive understanding of the phase evolution and carbon diffusion mechanisms during the process, particularly the influence of key parameters like quenching temperature, requires further elucidation.
The quenching temperature (QT) is a critical parameter in the Q&P process for ductile iron casting. It directly dictates the initial volume fraction of martensite (fM, initial) formed upon the first quench. If QT is too low, a high fraction of martensite is formed. During the subsequent partitioning stage, carbon from this abundant martensite diffuses into a relatively small volume of untransformed austenite, leading to a high degree of carbon enrichment in the austenite. Although this austenite becomes highly stable, its initial volume is small, resulting in a potentially low final volume of retained austenite (Vγ). Conversely, if QT is too high, the initial martensite fraction is low. During partitioning, the limited carbon available from this small amount of martensite is insufficient to adequately enrich the large volume of untransformed austenite. Consequently, the austenite remains unstable and transforms into fresh martensite (secondary martensite) during the final cooling, again yielding a low Vγ. Therefore, selecting an optimal quenching temperature is paramount to maximizing the retained austenite content and achieving an optimal combination of strength and ductility in ductile iron casting. This study aims to systematically investigate the effects of quenching temperature on the microstructure evolution, phase constituents, and resulting mechanical properties, specifically hardness, of a ductile iron casting subjected to the Q&P process, thereby contributing to a deeper mechanistic understanding of this promising treatment.
1. Experimental Procedures: Material and Methods
1.1 Material Preparation and Casting
The ductile iron casting used in this investigation was prepared using Q10 pig iron as the base material. Alloying elements were added using high-purity copper (99.99%), ferromanganese, and 75% ferrosilicon. Melting was conducted in air using a medium-frequency induction melting furnace with a power of 30 kW and a capacity of 30 kg. The charge sequence followed the order of element absorption rates. The molten metal was superheated to approximately 1450°C and held for 3-5 minutes after complete melting. Following this, a rare-earth magnesium alloy was added for nodularization treatment (spheroidization) over approximately 2 minutes. After slag removal, a ferrosilicon-based inoculant was added for eutectic cell refinement. The treated molten ductile iron casting was then poured at 1380°C into a sand mold to produce standard Y-block castings. The chemical composition of the final ductile iron casting, measured via optical emission spectrometry, is presented in Table 1.
| C | Si | Mn | P | S | Cu | Mg | RE | Fe |
|---|---|---|---|---|---|---|---|---|
| 3.6-3.8 | 2.40 | 0.259 | 0.015 | 0.015 | 0.762 | 0.044 | 0.027 | Bal. |
1.2 Heat Treatment: Quenching and Partitioning (Q&P) Process
Samples were sectioned from the Y-block castings using wire electrical discharge machining (EDM). The Q&P heat treatment cycle was applied as follows:
- Austenitization: Samples were austenitized at 900°C for 120 minutes in a muffle furnace to ensure a homogeneous austenitic matrix.
- Quenching (Q): Immediately after austenitization, samples were rapidly transferred and quenched into a bath of agitated oil maintained at various quenching temperatures (QT). The quenching temperatures investigated ranged from 100°C to 260°C (373 K to 533 K) at intervals of 20°C. The samples were held at the quenching temperature for 2 minutes to ensure temperature uniformity and to allow for the isothermal formation of the initial martensite fraction.
- Partitioning (P): Following the quench hold, samples were transferred to a salt bath furnace held at a constant partitioning temperature of 300°C. They were held at this temperature for two different times: 0 minutes (i.e., directly cooled after quenching) and 30 minutes. This allows for the study of the effect of the carbon partitioning step itself.
- Final Cooling: After the partitioning step, all samples were air-cooled to room temperature.
The complete matrix of heat treatment parameters is summarized in Table 2.
| Sample Group | Austenitization | Quenching Temp., QT (°C) | Quenching Hold (min) | Partitioning Temp. (°C) | Partitioning Time (min) |
|---|---|---|---|---|---|
| A (No Partitioning) | 900°C / 120 min | 100, 120, 140, 160, 180, 200, 220, 240, 260 | 2 | – | 0 |
| B (With Partitioning) | 100, 120, 140, 160, 180, 200, 220, 240, 260 | 300 | 30 |
1.3 Microstructural and Mechanical Characterization
The as-cast microstructure was examined using optical microscopy (OM) on both unetched and etched (with 4% nital) samples to assess graphite morphology and matrix structure, respectively. For heat-treated samples, microstructural analysis was performed using field emission scanning electron microscopy (FESEM). Phase identification and quantification were carried out using X-ray diffraction (XRD) with Cu-Kα radiation. The volume fraction of retained austenite (Vγ) was calculated from the integrated intensities of the martensite/ferrite (α) and austenite (γ) diffraction peaks using the direct comparison method. The lattice parameter of the retained austenite (aγ) was determined from the diffraction peak positions, and the average carbon content in the retained austenite (Cγ, in wt.%) was estimated using the following relationship, which accounts for the influence of substitutional alloying elements present in the ductile iron casting matrix:
$$ C_{\gamma} = \frac{a_{\gamma} – 3.578 – 0.00095\text{Mn}_{\gamma} – 0.002\text{Ni}_{\gamma} – 0.0006\text{Cr}_{\gamma} – 0.0031\text{Mo}_{\gamma} – 0.0018\text{V}_{\gamma}}{0.033} $$
For simplification in this study, given the low alloying content beyond Si, Mn, and Cu, the primary influence of silicon is considered indirectly, and the formula provides a reliable estimate. The macro-hardness of the samples was measured using a Rockwell hardness tester (HRC scale). A minimum of five indentations were taken per condition to ensure statistical relevance.
2. Results and Discussion
2.1 As-Cast Microstructure of the Ductile Iron Casting
The unetched microstructure revealed a well-dispersed distribution of graphite nodules with a nodule count exceeding 250 nodules/mm² and a nodularity level above 95%, classifying the graphite shape predominantly as Type I (spheroidal). The etched microstructure of the as-cast ductile iron casting showed a typical ferritic-pearlitic matrix with “bull’s eye” ferrite surrounding the graphite nodules. Image analysis indicated the matrix consisted of approximately 26% ferrite and 74% pearlite. This confirms the successful production of a high-quality ferritic-pearlitic ductile iron casting suitable for subsequent heat treatment studies.
2.2 Influence of Quenching Temperature on Phase Constituents
XRD patterns for all samples subjected to the full Q&P process (Group B: 30 min partitioning) at different quenching temperatures are presented. All patterns consistently show diffraction peaks corresponding to the body-centered cubic (bcc) structure of martensite/ferrite (α) and the face-centered cubic (fcc) structure of retained austenite (γ). The presence of these peaks confirms that the Q&P treatment successfully produced a dual-phase microstructure of martensite and retained austenite in the ductile iron casting, regardless of the specific QT.
The volume fraction of retained austenite (Vγ), calculated from the XRD data, is plotted as a function of quenching temperature in Figure 1. The data reveals a non-monotonic relationship. As QT increases from 100°C, Vγ initially increases, reaching a maximum value of approximately 27.1% at QT = 200°C, before decreasing with further increases in QT. This trend can be explained by the competing mechanisms governing austenite stabilization:
- Low QT (e.g., 100-160°C): The driving force for martensite transformation is high. Therefore, a large initial fraction of martensite (fM, initial) forms upon quenching. During partitioning at 300°C, carbon from this abundant martensite source diffuses into a relatively small volume of untransformed austenite. This leads to a high carbon enrichment in the austenite (Cγ), making it very stable. However, because the initial volume of austenite available for stabilization was small, the final Vγ, though highly enriched, is limited.
- Optimal QT (e.g., 180-220°C): At these intermediate temperatures, a more balanced ratio between fM, initial and the untransformed austenite volume is achieved. There is sufficient martensite to supply carbon, and a sufficient volume of austenite to receive it. The carbon enrichment level is adequate to stabilize a significant portion of this austenite against transformation during final cooling, resulting in the peak Vγ observed at 200°C.
- High QT (e.g., 240-260°C): The undercooling below Ms is reduced, leading to a low fM, initial. During partitioning, the limited carbon source from this small martensite fraction is insufficient to enrich the large volume of untransformed austenite to a level high enough for stabilization. Consequently, during final cooling, this under-enriched austenite transforms to secondary martensite, leading to a decrease in the final Vγ.
This behavior underscores the critical role of QT in balancing carbon supply and austenite volume for optimal stabilization in ductile iron casting.
Concurrently, the average carbon content in the retained austenite (Cγ), calculated from the lattice parameter, shows an inverse trend with Vγ, as summarized in Table 3. The highest Cγ values are observed at the lowest and highest QT, while the minimum Cγ coincides with the temperature range (180-220°C) where Vγ is maximized. This inverse correlation is logical: when the carbon donor (martensite) is abundant relative to the austenite acceptor (low QT), the enrichment per unit volume of austenite is high. When the austenite volume is large relative to the carbon donor (high QT), the enrichment is low. At the optimal QT, a compromise is reached where a moderate level of enrichment stabilizes a large volume of austenite.
| Quenching Temperature, QT (°C) | Volume Fraction of Retained Austenite, Vγ (%) | Carbon Content in Retained Austenite, Cγ (wt.%) |
|---|---|---|
| 100 | 18.5 | ~2.05 |
| 140 | 22.1 | ~1.75 |
| 180 | 26.3 | ~1.35 |
| 200 | 27.1 | ~1.30 |
| 220 | 25.8 | ~1.40 |
| 260 | 19.8 | ~1.90 |
Furthermore, at the partitioning temperature of 300°C and a hold time of 30 minutes, a competing phase transformation must be considered: the isothermal decomposition of austenite to bainite. The partitioning stage can, therefore, also involve a bainitic transformation, especially from the carbon-enriched austenite. Carbon rejection from the forming bainitic ferrite into the remaining austenite provides an additional carbon enrichment mechanism. At lower QT, the high-carbon martensite is the primary carbon source. At higher QT, the lower-carbon bainite may become a more significant contributor to carbon partitioning. This complex interplay between martensite tempering, carbon partitioning, and bainite formation contributes to the observed trends in Cγ. The relationship can be conceptually described by considering the carbon mass balance during partitioning, where the total carbon partitioned (Cpart) enriches the untransformed austenite:
$$ C_{\text{part}} \approx f_{M,initial} \cdot \Delta C_{M \rightarrow \gamma} + f_{B,iso} \cdot \Delta C_{B \rightarrow \gamma} $$
where \( f_{B,iso} \) is the fraction of bainite formed isothermally, and \( \Delta C \) terms represent the average carbon transferred from each phase to austenite.
2.3 Microstructural Evolution with Quenching Temperature
FESEM analysis of the Q&P treated ductile iron casting samples (Group B) confirms the XRD findings. The matrix microstructure consists of a mixture of lath-like martensite (both initial and potentially secondary) and interlath/blocky regions of retained austenite. The graphite nodules remain intact and unchanged. With increasing QT, several microstructural trends are observed:
- The prior austenite grain boundaries become more distinctly outlined.
- The apparent size of the martensite laths or packets tends to increase. This is attributed to the reduced nucleation density of martensite at higher quenching temperatures (closer to Ms), allowing individual martensite units to grow larger before impingement.
- The contrast from the retained austenite regions varies, correlating with the calculated Vγ and Cγ. At the optimal QT of 200°C, a significant amount of interlath austenite is observed.
It is challenging to unequivocally distinguish between tempered initial martensite, secondary martensite, and possible lower bainite in the final microstructure using conventional microscopy, as all can exhibit acicular morphologies. This highlights the value of combined XRD and detailed electron microscopy for complete phase analysis in Q&P processed ductile iron casting.
2.4 Mechanical Property Response: Hardness
The Rockwell hardness (HRC) of the ductile iron casting samples is significantly influenced by both the quenching temperature and the application of the partitioning step, as graphically summarized in Figure 2 and detailed in Table 4.
Effect of Partitioning: For any given QT, samples that underwent only quenching (Group A, 0 min partitioning) exhibit substantially higher hardness than their counterparts subjected to the full Q&P process with 30 minutes of partitioning (Group B). This is a direct consequence of the partitioning step. In Group A, the microstructure consists of high-carbon, untempered primary martensite and potentially some unstable austenite transformed to secondary martensite. This leads to high hardness. In Group B, the partitioning step at 300°C causes:
- Tempering of Initial Martensite: Carbon diffuses out of the initial martensite, reducing its lattice strain and carbon supersaturation, thereby lowering its hardness.
- Stabilization of Austenite: The carbon-enriched austenite is retained as a soft, ductile phase at room temperature, further reducing the overall hardness of the ductile iron casting.
Thus, the Q&P process deliberately sacrifices some hardness to introduce a stable ductile phase.
Effect of Quenching Temperature within Each Group:
- Group A (No Partitioning): Hardness shows a general, though modest, decreasing trend with increasing QT. At very low QT, the microstructure is dominated by very fine, high-carbon primary martensite, resulting in peak hardness. As QT increases, the fraction of this hard martensite decreases, while the fraction of (initially) softer untransformed austenite increases. Upon final cooling, this austenite transforms to secondary martensite, but its carbon content is lower than that of the primary martensite formed at lower temperature, contributing to a slight overall hardness decrease.
- Group B (With 30 min Partitioning): Hardness decreases more noticeably as QT increases from 100°C to around 180°C. Beyond this point, the hardness stabilizes or shows only minor fluctuations. This trend is directly linked to the microstructural evolution described earlier. At low QT (e.g., 100°C), the final structure contains a significant amount of tempered martensite (from the high initial fraction) and a small amount of very high-carbon retained austenite. The high martensite fraction maintains relatively high hardness. As QT approaches the optimal range (180-220°C), the fraction of tempered martensite decreases, and the volume of softer retained austenite increases significantly (reaching ~27%), leading to a pronounced drop in hardness. At even higher QT, although Vγ decreases slightly, the martensite present is likely tempered and potentially mixed with other transformation products, keeping the hardness in a similar range. This demonstrates the ability of the Q&P process to tailor the hardness of ductile iron casting over a considerable range by controlling QT.
The hardness (H) can be approximated by a rule of mixtures considering the constituent phases:
$$ H \approx V_{\alpha’} \cdot H_{\alpha’} + V_{\gamma} \cdot H_{\gamma} + V_{B} \cdot H_{B} $$
where \( V_{\alpha’} \), \( V_{\gamma} \), and \( V_{B} \) are the volume fractions of tempered martensite, retained austenite, and bainite, respectively, and \( H_{\alpha’} \), \( H_{\gamma} \), and \( H_{B} \) are their corresponding hardness values. The trends in Figure 2 reflect the changing values of these volume fractions with QT.
| Quenching Temperature, QT (°C) | Hardness, HRC (0 min Partitioning) | Hardness, HRC (30 min Partitioning) |
|---|---|---|
| 100 | 60.5 ± 0.5 | 56.0 ± 0.7 |
| 140 | 59.8 ± 0.6 | 52.5 ± 0.8 |
| 180 | 59.2 ± 0.4 | 49.0 ± 0.5 |
| 200 | 58.9 ± 0.7 | 48.5 ± 0.6 |
| 220 | 58.5 ± 0.5 | 49.2 ± 0.7 |
| 260 | 58.0 ± 0.8 | 50.1 ± 0.9 |
3. Conclusions
This systematic investigation into the application of the Quenching and Partitioning (Q&P) process to ductile iron casting, with a focus on the effect of quenching temperature (QT), leads to the following key conclusions:
- The Q&P process successfully produces a multiphase microstructure in ductile iron casting consisting of martensite (tempered) and retained austenite. The volume fraction of retained austenite (Vγ) exhibits a strong non-monotonic dependence on QT, reaching a maximum of approximately 27.1% at an optimal QT of 200°C for the studied alloy and partitioning conditions (300°C/30min). This maximum corresponds to the most favorable balance between the initial martensite fraction (carbon source) and the untransformed austenite volume (carbon sink) for effective carbon partitioning and austenite stabilization.
- The average carbon content in the retained austenite (Cγ) shows an inverse relationship with Vγ. The highest carbon enrichment (>2.0 wt.%) occurs at low QT where the austenite volume is small, while the minimum carbon content (~1.3 wt.%) is found near the optimal QT where the austenite volume is largest. This trade-off is fundamental to the Q&P mechanism in ductile iron casting.
- Microstructural analysis confirms the matrix evolution from predominantly martensitic at low QT to a mixture with significant interlath retained austenite at intermediate QT. Prior austenite grain boundaries become more distinct, and martensite lath size appears to increase with increasing QT due to reduced nucleation density.
- The mechanical response, as measured by macro-hardness, is effectively tailored by both the partitioning step and the QT. The partitioning step consistently reduces hardness compared to quench-only conditions due to martensite tempering and austenite retention. For Q&P processed samples, hardness decreases with increasing QT up to the optimal range, primarily due to the increasing volume of the soft retained austenite phase, and then stabilizes at higher temperatures. This demonstrates the potential of the Q&P process to engineer a range of hardness levels in ductile iron casting.
In summary, the quenching temperature is a pivotal parameter in the Q&P heat treatment of ductile iron casting. It governs the initial phase distribution, the kinetics and extent of carbon partitioning, and the final balance between hard and soft phases. Mastering this parameter is essential for designing Q&P cycles aimed at achieving specific combinations of strength, ductility, and toughness in ductile iron casting components, potentially opening new avenues for its application in demanding engineering contexts. Future work should correlate these microstructural findings with tensile and impact properties to fully establish the property profiles achievable through Q&P processing of ductile iron casting.