Superior Resistance and Ductility Through Novel Quench- and Partitioning-Path in Complex Refined Microstructure

Abstract

A well-designed complex microstructure containing both soft and hard micro-constituents can enhance the mechanical properties of steel. In this study, commercial AISI 9254 steel was annealed at 900 °C, rapidly cooled to 550 °C for 500 s to promote approximately 50% fine pearlitic transformation, quenched to 125 °C for partial martensitic transformation, and finally heated to 375 °C for 1800 s to complete the partitioning stage in a novel quench and partitioning (Q&P) process. Tensile testing revealed a yield strength (YS) of ≈1500 MPa, an ultimate tensile strength (UTS) of ≈1570 MPa, and a total elongation of ≈13.85%. This high yield strength indicates the ability of the material to support the development of lightweight, yet high-strength components for demanding applications. Additionally, the balanced total elongation helps mitigate the risk of brittle failure, enhancing fracture toughness and reducing the likelihood of premature failures in critical structural applications. These results indicate an increase of approximately 8.3% in strength and 34.5% in ductility compared to the as-received 9254 steel. X-ray analysis revealed that the complex microstructure had fewer crystallographic defect densities than the as-received sample. Secondary electron images showed ultrafine martensite laths and cementite lamellae within the body-centered cubic (BCC) matrix, with some proeutectoid ferrite found at prior austenite grains. Electron backscattered diffraction (EBSD) analysis estimated low internal distortion in martensite laths, with average crystal defect densities around 2.25 × 10¹⁴ m⁻². The BCC matrix contained ferrite and martensite, with carbide particles and a small amount of retained austenite detected by transmission electron microscopy (TEM). These findings confirm the enhanced mechanical properties of commercial 9254 steel through the novel Q&P processing.

1. Introduction

The development of advanced materials for demanding engineering applications has driven extensive research into optimizing the mechanical properties of spring steels. These materials are integral to industries such as automotive and aerospace, where a balance of high strength, ductility, and durability is paramount for safety-critical components. Traditional heat treatment processes, such as quenching and tempering, have long been used to improve the strength of spring steels [1,2]. However, these methods often face limitations, including susceptibility to hydrogen embrittlement and reduced fatigue resistance at ultrahigh-strength levels [3,4,5]. Consequently, innovative approaches, such as quenching and partitioning (Q&P) heat treatment, have garnered significant attention for their potential to overcome these challenges and unlock new performance benchmarks [6,7,8,9].

The Q&P process is designed to produce a unique multiphase microstructure composed of tempered martensite and stabilized retained austenite [10]. This is achieved by precise control of thermal cycles, where steel is first partially or fully austenitized, then quenched to a temperature between the martensite starting and finishing temperatures, and subsequently reheated to facilitate carbon partitioning. Retained austenite, stabilized through carbon enrichment, contributes to enhanced ductility by transforming into martensite under mechanical stress, a phenomenon known as the transformation-induced plasticity (TRIP) effect [11]. While early applications of Q&P focused on high-purity prototype alloys, recent research has extended its scope to commercial grades, such as AISI 9254 spring steel, revealing complex relationships between retained austenite fraction, its distribution, and overall mechanical performance [12,13,14,15,16]. Despite these advancements, controversies persist regarding the optimal conditions for stabilizing retained austenite and its impact on property trade-offs, such as strength versus ductility [17,18,19]. Furthermore, integrating cost-effective alloying strategies with scalable industrial practices remains an ongoing challenge.

The role of alloying elements such as silicon and manganese in the development of high-performance spring steels is important, particularly in the context of multiphase microstructures. Silicon, often present in concentrations exceeding 1 wt.%, suppresses the formation of carbides during heat treatment, enhancing the stability of retained austenite and promoting carbon partitioning [11]. This effect not only prevents cementite precipitation but also facilitates the TRIP effect, contributing to improved ductility and toughness [20]. Manganese, on the other hand, acts as a potent austenite stabilizer by lowering the martensite start temperature (Ms), thereby increasing the retained austenite fraction at room temperature [12,21]. The synergy between silicon and manganese further refines the microstructure, enabling a fine dispersion of martensite, bainite, and retained austenite, which collectively improve the strength and toughness of spring steels under dynamic loading conditions [22]. Despite these advantages, the combined solute drag effects of these elements on phase transformation kinetics require careful optimization to avoid adverse effects on mechanical properties, such as delayed recrystallization and uneven phase distribution [23,24].

In metal processing, the microstructure of steels is a critical determinant of their mechanical properties, with retained austenite playing a prominent role. In Q&P steels, the stability of retained austenite is largely influenced by carbon partitioning during the partitioning stage, which enriches the austenite with carbon, enhancing its resistance to transformation under stress [25,26,27]. This process is integral to the TRIP effect, as mentioned, and improves ductility and toughness. Morphological characteristics of retained austenite, such as grain size and aspect ratio, significantly impact its transformation behavior, with lenticular grains transforming more rapidly compared to globular [22,28]. Additionally, the presence of martensite and other phases surrounding retained austenite contributes to stress shielding, which affects the stability of retained austenite [23,29]. Nanoscale carbide precipitation is another key factor in the microstructure of Q&P steels. While silicon is traditionally added to suppress carbide formation and promote carbon partitioning, recent studies have observed precipitation occurring even in silicon-enriched steels [23,30]. These carbides reduce the carbon available for austenite stabilization, potentially compromising the desired microstructural balance. Advanced characterization techniques, such as atom probe tomography (APT) and electron backscatter diffraction (EBSD), have revealed the complex interactions between phases and the localized effects of carbon and solute partitioning, which are crucial for understanding the mechanical performance of Q&P-treated steels [31,32,33]. Nanoprecipitates and solute clusters within martensite contribute to strength, but require precise control to avoid undermining ductility. This complex interaction between microstructural features, including retained austenite, martensite, and nanoprecipitates and their characterization highlights the importance of optimized heat treatment processes in tailoring the mechanical properties of metallic materials for demanding industrial applications.

This study aimed to address critical gaps in the application of Q&P treatments by exploring a novel pathway tailored for commercial AISI 9254 steel. The objective was to achieve superior mechanical performance, balancing enhanced ductility and wear resistance through a refined microstructure. The main hypothesis of this research is that the formation of a multicomplex microstructure—consisting of hardened and refined fine pearlite (resulting from pure diffusional transformation), pure displacive martensite for exceptional strength and wear resistance, displacive–diffusional bainite for enhanced toughness and stress redistribution, and high-stability retained austenite to leverage the TRIP effect for improved ductility—can synergistically optimize the mechanical performance of commercial AISI 9254 steel. Leveraging advanced heat treatment protocols, the study incorporated microstructural characterization techniques, including EBSD and transmission electron microscopy (TEM), to investigate phase interactions, carbon partitioning, and retained austenite stability. These insights can be critical for developing spring steels capable of meeting the demanding requirements of the automotive [34,35] and aerospace industries [14,36], where components must endure high cyclic stresses and abrasive conditions without compromising performance. The findings could contribute to the broader field of material science by demonstrating the feasibility of integrating Q&P processing into scalable manufacturing, offering a cost-effective approach to improve the strength, toughness, and durability of industrial spring steels. This work underscores the transformative potential of Q&P treatments, advancing the design of high-performance materials for safety-critical and load-bearing applications.

2. Materials and Methods

2.1. Material Selection and Preparation

Commercial AISI 9254 silicon–manganese spring steel with a chemical composition of approximately 0.54 wt.% carbon, 1.58 wt.% manganese, 1.05 wt.% silicon, and 0.41 wt.% chromium was selected as the material for this study due to its favorable mechanical properties and potential for enhancement through advanced heat treatment. The material was obtained as hot-rolled plates with a thickness of approximately 8 mm, exhibiting superficial oxidation caused by high-temperature fabrication processes. Hot rolling was conducted at around 800 °C, followed by air-cooling. To ensure oxide-free surfaces and avoid potential decarburization, approximately 2 mm from each surface of the hot-rolled plates was ground using #80 silicon carbide (SiC) abrasive paper. This step ensured the removal of surface oxides and provided a clean metallic surface for subsequent processing. Specimens were then machined into cylindrical shapes with a diameter of 10 mm and a length of 100 mm to ensure uniformity in heat treatment and mechanical testing. All specimen dimensions were measured with a precision of ±0.01 mm to minimize variability in the experimental results.

2.2. Heat Treatment

The heat treatment process followed a novel Q&P protocol designed to refine the microstructure and optimize mechanical properties. First, specimens were austenitized at 900 °C for 3 min (Figure 1) in a controlled atmosphere furnace to achieve a homogeneous austenitic phase. According to prior dilatometry studies [37], the austenite starting (A₁) and finishing (A₃) temperatures were determined to be 780 ± 2 °C and 810 ± 2 °C, respectively, under a heating rate of 5 °C/s. This ensured complete austenitization before quenching and thermal control during the austenitization and partitioning stages. Following austenitization, the samples were rapidly cooled to 550 °C and held isothermally for 500 s to induce partial transformation into fine pearlite. This step was critical to achieving a balance of hardness and ductility by promoting diffusion-based phase transformation. Subsequently, the samples were quenched to 125 °C for 20 s to initiate the displacive transformation of approximately 50% of the remaining austenite into martensite. Considering the martensite starting temperature (Ms) of approximately 260 °C [37], this cooling step was carefully designed to retain sufficient austenite for subsequent stabilization. Finally, the specimens were reheated to 375 °C for 1800 s to enable carbon partitioning from martensite into the retained austenite, stabilizing the latter phase. This step enhanced the microstructural complexity and mechanical performance of the treated steel. All thermal treatments were precisely monitored and controlled using a high-precision dilatometer (Bähr DIL-805A model), which recorded dimensional changes throughout the phase transformation processes.

The kinetics of phase transformations were evaluated indirectly using dilatometry, which recorded real-time dimensional changes throughout the thermal cycles. The start and end points of key transformations, such as martensite formation during quenching and carbon partitioning during the partitioning stage, were determined from the dilatometry data. Heating and cooling rates, maintained at 5 °C/s and 30 °C/s, respectively, as shown in Figure 1, ensured precise control of thermal conditions, enabling consistent phase transformation kinetics. The correlation between the rate of phase transformations and the resulting microstructure was further analyzed through advanced characterization techniques, such as XRD and EBSD, to provide insights into the microstructural evolution and its influence on mechanical properties.

2.3. Microstructural Characterization

Microstructural analysis of the as-received and Q&P-treated samples was conducted using a combination of scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Each technique provided complementary insights into the microstructural evolution induced by the Q&P process, enabling a comprehensive understanding of phase transformations, crystallographic features, and mechanical property relationships.

Field-emission scanning electron microscopy (FE-SEM): SEM analysis was conducted using a field-emission scanning electron microscope (FE-SEM, JEOL JSM-7001F) at an accelerating voltage of 20 kV and a working distance of 8 mm. Specimens were mechanically polished using progressively finer abrasives, including 0.5 µm and 0.3 µm alumina suspensions, followed by a final polishing step with diamond paste (0.3 µm and 0.1 µm granulometry). The polishing process was performed manually to achieve a mirror-like surface. Chemical etching was carried out using a 3% nital solution (97% ethyl alcohol and 3% nitric acid) for 10–20 s, ensuring uniform coverage without sample–container contact. SEM imaging allows clear differentiation of microstructural features such as martensite laths, bainitic regions, and retained austenite, providing detailed insights into the material’s phase composition and morphology.

Electron backscatter diffraction (EBSD): To enhance the quality of EBSD scans, samples were carefully polished using a 50 nm colloidal silica slurry for three hours to reduce surface deformation and improve indexing accuracy. EBSD analysis was performed using an accelerating voltage of 20 kV, a spot size of 5, a working distance of approximately 12 mm, and a step size of 250 nm in hexagonal scan grid mode. This technique provided detailed analyses of grain boundaries, crystallographic texture, and kernel average misorientation (KAM) to quantify residual strain and dislocation densities. The EBSD results contribute to understanding the grain-level microstructural refinement achieved through the Q&P process.

X-ray diffraction (XRD): XRD analysis was employed to determine phase fractions, with particular attention to retained austenite content. Data were collected using Cu-Kα radiation at a scan rate of 2° per minute. Crystallographic defects were also assessed using diffraction peak broadening. Williamson–Hall (W-H) analysis [38,39] was applied to separate the contributions of crystallite size and microstrain to peak broadening, with microstrain serving as an indicator of lattice distortions and dislocation densities. These results provide critical insights into the microstructural integrity and phase stability of the material following the Q&P process.

Transmission electron microscopy (TEM): Thin foils for TEM analysis were prepared using focused ion beam (FIB) milling to ensure electron transparency. TEM analysis was performed using a JEOL JEM-2100 microscope operating at 200 kV. This technique enabled nanometer-scale characterization of carbide morphology, interphase boundaries, dislocation density, and the distribution of retained austenite. TEM provided detailed insights into microstructural refinement and the nanoscale mechanisms contributing to the material’s enhanced mechanical properties.

2.4. Mechanical Testing

Tensile tests were performed using a universal testing machine at a constant strain rate of 10⁻³ s⁻¹ and at room temperature. The specimens were prepared with a gauge length of 25 mm and a diameter of 5 mm, following ASTM E8/E8M standards [40]. For statistical reliability, a minimum of three specimens were tested for each condition, and the average results are reported. These tests provided essential insights into the mechanical performance, including yield strength, ultimate tensile strength, and elongation. Vickers microhardness measurements were carried out using a load of 500 g and a dwell time of 10 s to assess hardness gradients across different microstructural regions.

Nanoindentation tests were performed using a Triboindenter TI 950 (Hysitron Inc., Eden Prairie, MN, USA) in load-controlled mode to evaluate the phase-specific mechanical properties of the complex microstructure. A Berkovich indenter, known for its triangular pyramid geometry, was employed for these measurements. The maximum load applied during the tests was 1.5 mN, ensuring precise evaluation of individual phases within the multiphase microstructure, including martensite laths, bainite sheaves, and retained austenite. The tests were carried out at room temperature, and the load–displacement curves were recorded to extract mechanical properties such as hardness (H) and reduced modulus (Er). Data were analyzed to correlate the nanoindentation behavior with the microstructural features.

2.5. Carbon Content Analysis

The carbon content in retained austenite was determined using lattice parameters derived from XRD data. The lattice parameter of the face-centered cubic (FCC) phase was calculated from the (111) diffraction peak using Cu-Kα radiation, and the carbon content was estimated using the established relationship between the lattice parameter and carbon concentration. To validate these results, thermodynamic simulations of carbon partitioning were performed using Thermo-Calc software with the TCFE13 database. The simulations modeled the equilibrium concentrations of carbon in martensite and austenite during the partitioning stage. Discrepancies between experimental and calculated values were attributed to factors such as carbon trapping at defects and non-uniform partitioning during rapid quenching.

2.6. Statistical and Data Analysis

All experimental data were analyzed statistically to ensure reproducibility and reliability. Microstructural phase fractions were determined from at least five randomly selected fields for each sample, and mechanical property tests were conducted on three replicate specimens. The results are reported as mean values with their corresponding standard deviations. Statistical significance was evaluated using analysis of variance where applicable.

3. Results and Analysis

The heat treatment protocol utilized a novel quenching and partitioning (Q&P) process designed to achieve superior resistance and ductility through a complex refined microstructure. The process involved austenitization at 900 °C for 3 min, followed by isothermal holding at 550 °C for 500 s to refine the pearlitic structure. Subsequent quenching to 125 °C facilitated partial martensitic transformation, while partitioning at 375 °C for 1800 s enabled carbon diffusion and stabilization of retained austenite. This controlled thermal cycle promoted a balance of diffusional and displacive transformations, resulting in a multiphase microstructure tailored for enhanced mechanical performance.

3.1. Determining the Isothermal Time for Development of Partially Fine Pearlite Structure

To optimize eutectoid transformation kinetics, the sample was heated to 900 °C and then rapidly cooled to 550 °C to promote a fine pearlite structure. This ensures mechanical resistance and ductility by transforming partially fine alternating ferrite–cementite lamellae through diffusion transformation. Rapid cooling from 900 °C to 550 °C refines the nuclei for eutectoid transformation, allowing growth by a diffusional mechanism with controlled timing to avoid excessive growth. A study by Ghosh et al. [41] supports the significance of controlled cooling rates and precise temperature management in optimizing microstructural features, which directly influence the mechanical properties of steels. Figure 2 shows the dilatation curve and the first derivative of length changes, particularly during the isothermal step at 550 °C. It indicates that within the initial 500 s, the pearlitic transformation occurs in approximately 50% of the volumetric fraction, leaving the remaining austenite untransformed for further phase transformations, which is utilized in the subsequent complex Q&P processing. Toji et al. [42] emphasize the role of isothermal treatments in fine-tuning the pearlite morphology to achieve desired mechanical outcomes.

3.2. Change in Length and First Derivative of Length Change to Monitor Phase Transformations During the Novel Q&P Process

Figure 3 presents a detailed dilatation curve of the heating and quenching stages of the novel Q&P process. Initially, a fully austenitic structure was achieved by annealing the sample at 900 °C and maintaining this temperature for 30 min to ensure homogenized austenite grains. The sample was then rapidly cooled to 550 °C to form fine eutectoid ferrite nuclei, holding at this temperature for 500 s to promote the formation of a fine ferrite–cementite pearlite structure. This fine pearlite structure was expected to offer superior work hardening capabilities and enhance plastic deformation [43]. Additionally, the formation of proeutectoid ferrite at the austenite grain boundaries could also occur during this stage. Once approximately 50% of the parent austenite had transformed into pearlite, the sample was rapidly quenched to 125 °C for 20 s to induce displacive martensitic transformation. Previous studies on this sample demonstrated that martensite starts forming at around 260 °C. By applying the leverage rule, quenching to 125 °C was shown to induce approximately 50% of displacive martensitic transformation, resulting in the formation of ultrafine martensite laths [8,14,15]. Subsequent gradual heating to 375 °C facilitated martensite lattice relaxation by expelling excess carbon into the neighboring untransformed austenite, thereby stabilizing the austenite. Maintaining the sample at this temperature allowed the untransformed austenite to decompose into ferritic-bainite and carbide particles. This process involves a combination of displacive bainite nucleation and diffusional growth during the elevated temperature, driven by effective carbon diffusion [44,45]. The final microstructure was expected to consist of well-dispersed ultrafine carbide particles and retained austenite within a ferritic–martensitic matrix.

The dilatation curves presented in Figure 2 and Figure 3 provide valuable insights into the phase transformation kinetics during the Q&P process. Figure 2 illustrates the initial pearlitic transformation at 550 °C, where a pronounced change in length is observed during the first 500 s. This is attributed to the nucleation and growth of pearlite via a diffusional mechanism, with approximately 50% of the eutectoid transformation occurring during this stage. The dimensional stability observed after 500 s confirms the completion of the pearlitic transformation. Figure 3 demonstrates the complex interplay of phase transformations during the Q&P process, with distinct events captured through both the dilatation curve and the first derivative of the change in length. The sharp contraction observed during the initial quenching stage reflects the displacive martensitic transformation. As the temperature is elevated during the partitioning stage, evidence of bainitic transformation and martensite lattice relaxation through carbon partitioning is observed, indicated by gradual expansion. These transformations are essential for stabilizing retained austenite and refining the microstructure, ultimately enhancing the material’s mechanical properties.

Figure 4 shows uniaxial tensile tests conducted on both as-received and complex designed microstructure samples. The term “complex microstructure” in this study refers to the coexistence of multiple phases within the treated AISI 9254 steel, including fine pearlite, martensite, bainite, and retained austenite. Each phase plays a specific role in optimizing mechanical performance: fine pearlite contributes to wear resistance, martensite provides exceptional strength, bainite enhances toughness and stress redistribution, and retained austenite leverages the TRIP effect to improve ductility. This multicomponent microstructure was validated through advanced characterization techniques, such as EBSD, XRD, and TEM, which confirmed the presence and morphology of these phases and their synergistic contribution to the material’s superior mechanical properties.

The yield strength (YS) of the as-received sample increased from approximately 1355 MPa to 1500 MPa, while the ultimate tensile strength (UTS) increased from approximately 1450 MPa to 1565 MPa, showing improvements of 10.7% and 9.7%, respectively. Interestingly, the total elongation also increased from approximately 10.3% to 13.85%, an improvement of about 34%. This significant enhancement in both mechanical resistance and ductility can be attributed to the presence of soft retained austenite dispersed among hard microconstituents such as martensite and bainite, along with the dispersion of hard carbide particles in the novel Q&P sample. The increase in n-values and the energy absorption before rupture (area under the stress–strain curves) further demonstrates the effectiveness of the novel Q&P processing in optimizing the mechanical properties.

X-ray diffraction (XRD) patterns for both samples are presented in Figure 5a. The XRD patterns show that the as-received sample exhibited a slight shift to the right, indicating higher internal energy due to increased crystallographic defects. However, the application of the novel Q&P process resulted in noticeable lattice relaxation accompanied by some phase transformations [46,47]. Additionally, approximately 8.7 ± 1.1 vol% of retained austenite was estimated by analyzing the relative peak intensities.

Table 1. Lattice parameters, microdeformation, crystalline size, and dislocation density in BCC matrix for both samples.

	Lattice Parameter (a, Å)		Microdeformation (ε × 10−3)		Crystalline Size (D, nm)		Dislocation Density (ρ, 1014)
	Value	Error	Value	Error	Value	Error	Value	Error
As-received sample	2.8814	0.0048	0.445	0.094	1.631	0.356	4.463	0.863
Complex microstructure sample	2.8323	0.0076	0.362	0.075	1.966	0.417	2.752	0.678

summarizes the key microstructural parameters obtained from XRD analysis of the resulting BCC ferrite–martensite structure (Figure 5b). These parameters include the lattice parameter (a), microdeformation (ε), crystalline size (D), and dislocation density (ρ). The values were derived using the Williamson–Hall method, which separates the contributions of microstrain and crystallite size to the broadening of diffraction peaks.

The Q&P-treated sample exhibited a reduced lattice parameter (2.8323 Å compared to 2.8814 Å for the as-received sample), which reflects the redistribution of carbon and the stabilization of retained austenite during the partitioning process. The decrease in microdeformation (0.362 × 10⁻³ vs. 0.445 × 10⁻³) suggested improved lattice integrity and stress relaxation. Notably, the crystalline size increased from 1.631 nm in the as-received sample to 1.966 nm in the Q&P-treated sample, indicating grain refinement due to the thermal cycles. Dislocation density also decreased significantly (from 4.463 × 10¹⁴ to 2.752 × 10¹⁴), pointing to a reduction in lattice distortions and defects, which contributes to improved toughness and ductility. These observations correlate with the enhanced mechanical performance of the Q&P-treated steel, demonstrating the effectiveness of the heat treatment in refining the microstructure.

The lattice parameter was slightly reduced by applying the novel Q&P process, which facilitated the partitioning of excess carbon out of the tetragonal matrix, thereby reducing both the lattice parameter and dislocation densities [47]. The as-received sample, subjected to a quenched and tempered condition, exhibited a wide range of carbide particles due to the local variations in cooling and heating rates. These variations significantly affected the crystalline size and microdeformation, which can influence the critical nucleation radius during the phase transformation under the nucleation mechanism [48].

As previously mentioned, approximately 8.7 ± 1.1 vol% of retained austenite was characterized in the complex microstructure, which effectively contributed to enhanced ductility. The carbon content in the retained austenite plays an important role in determining its stability and mechanical performance. During the Q&P process, carbon partitioning from martensite to austenite enhances the carbon concentration in the austenite, which increases its stability at room temperature. This stabilization promotes the TRIP effect under mechanical stress, contributing to improved ductility and toughness. Furthermore, understanding the carbon content allows for the optimization of the partitioning stage, preventing excessive carbide formation, which can reduce the retained austenite fraction and adversely affect the desired balance of mechanical properties. In this way, calculating the carbon content in the retained austenite is essential to justify its stability within the complex microstructure. The carbon content of retained austenite (C in wt%) is determined using the austenite lattice parameter ($a_{\gamma }$), in angstroms, derived from FCC {111} peaks, according to Equation (1) by Chhajed et al. [49]:

$$C_{\left(wt\%\right)}={\displaystyle \frac{a_{\gamma }-3.357}{0.0467}}$$

(1)

The 2θ peak position of the (111) austenite was 43.1008 using Cu (K_α = 1.5406 Å) radiation. Thus, aγ is calculated by Equation (2):

$$a_{\gamma }={\displaystyle \frac{\lambda \sqrt{3}}{2.sin{\theta }_{111}}}$$

(2)

Therefore, the carbon content of the retained austenite is estimated to be approximately 1.82 wt% (0.40 at%). The model simulates the microstructural evolution and carbon partitioning in a two-dimensional grid composed of elementary volumes, each defined by a position vector, phase parameter (indicating austenite or martensite), carbon concentration, and defect concentration [50]:

$$X_{\left(C,i\right)}^{total}=X_{\left(C,i\right)}^{free}+X_{\left(C,i\right)}^{trapped}$$

(3)

The carbon concentration in the martensite microstructure is partially trapped at defects and partially interstitial in the BCC iron lattice. Initially, the material is fully austenitic, and during quenching, a fraction transforms into martensite. The initial microstructure has homogeneous overall carbon concentration. In martensite, carbon is either trapped at defects or free as an interstitial solid solution. The total carbon concentration in each element is the sum of trapped and free carbon. Trapped carbon does not diffuse but remains in equilibrium with the free carbon within the same volume, allowing the model to simulate carbon redistribution and phase transformations during heat treatment.

The equilibrium condition requires that the chemical potentials of carbon in the adjacent phases are equal, ensuring a balanced distribution of carbon. This model is particularly useful in understanding carbon partitioning during the first quenching stage, where inducing 50% prior austenite in martensite results in a rich pre-martensitic lath structure. Upon reheating to 375 °C for the partitioning stage, excess carbon migrates out of martensite, partitioning into the nearest untransformed martensite. During the partitioning time, the enriched austenite partially transforms into ferritic-bainite and carbide particles. The model helps explain carbon redistribution and the resulting phase transformations, providing insights into optimizing heat treatment processes to achieve the desired microstructural characteristics and properties (Figure 6). By simulating these processes, researchers can predict the behavior of carbon-rich pre-martensitic structures and their evolution during heat treatments, leading to improved alloy design and performance. In the present study, we assumed an Fe-C material comprising either only martensite or martensite and austenite. The thermodynamic input for interphase partitioning at 400 °C was calculated for all possible local compositions by setting equal the chemical potential of carbon in solid solution in austenite and martensite (i.e., ${\mu }_C^{\gamma }={\mu }_C^{\left({\alpha }^{\prime },free\right)}$), calculated from Thermocalc using the TCFE13 database. The resultant relationship is shown in Figure 6 and is applied locally in the numerical description in adjacent elementary units and per boundary. The relationship plotted in Figure 6 expresses the equilibrium concentrations $X_C^{\left({\alpha }^{\prime },free\right)}$ and $X_C^{\gamma }$, which are read as carbon concentration in martensite solid solution in equilibrium with austenite and carbon concentration in austenite in equilibrium with martensite solid solution, respectively. The difference between the experimental and calculated values for the carbon content in retained austenite can be attributed to crystal defects, such as carbon trapped at sites during rapid quenching.

Figure 7a illustrates the pearlitic microstructure of as-received AISI 9254 steel formed during air-cooling following hot rolling. This process induced a diffusional eutectoid transformation, resulting in alternating layers of cementite and ferrite. Notably, the presence of alloying elements such as Cr, Mn, and Si contributed to an increased volumetric fraction of cementite compared to the predictions of the Fe-C phase diagram. Then, a successful pearlite–martensite matrix was achieved, as shown on scanning electron microscopy (Figure 7b,c). The dark regions in the images result from differences in contrast between the highest and lowest altitudes in the microstructure, which correspond to variations in corrosion resistance during etching. The ferritic matrix and proeutectoid ferrite, which nucleated at prior austenite grain boundaries, are visible in Figure 7b. The ultrafine pearlite, characterized by an alternating ferrite–cementite eutectoid diffusion phase transformation, offers excellent hardenability through parallel elongated cementite (achieved during the 550 °C isothermal stage, suggesting quenching and partitioning, or Q&P). It is worth noting that harder microconstituents, such as carbides and retained austenite, exhibit greater resistance to nital etching solutions. As a result, they appear at higher altitudes with brighter coloration, allowing them to be distinguished more easily.

Additionally, tempered martensite is observed following the quenching stage, resulting in approximately 50% volume of nanostructured martensite laths (formed during the 125 °C quenching stage, suggesting Q&P). Given the considerable carbon content in the steel (≈0.54 wt.%), the presence of tetragonal martensite can adversely affect mechanical properties. Therefore, aging as part of the partitioning process (conducted at the 375 °C partitioning stage, suggesting Q&P) facilitates the partitioning of carbon atoms out of the martensite, allowing them to combine with the nearest untransformed austenite. The size, FCC-stabilizing elements, and location of these phases determine their stability and phase transformation behavior during the partitioning stage. Alloying elements such as Cr may form precipitates, and Fe-based carbides can also form. In Figure 7c, acicular ferrite, upper bainite, and lower bainite (featuring nanoscale carbide particles within the ferritic sheaf) are shown, confirming a mixed transformation mechanism involving both diffusional and displacive nucleation and growth processes.

Figure 8 provides transmission electron microscopy (TEM) images that offer nanometric characterization of the complex microstructure following Q&P, complementing the findings shown in Figure 7. These images reveal the diffusional growth of cementite and the presence of fine carbide particles within the ferritic matrix. The nanometric scale of these observations highlights the intricate distribution and morphology of carbides, which play a crucial role in enhancing the mechanical properties of the steel. The ability of cementite to grow through diffusional processes is evident, providing insight into the complex interactions between phases during the Q&P process. This detailed view of the microstructure further supports the presence of a mixed transformation mechanism involving both diffusional and displacive processes, as seen in the formation of lower bainite and other phases in the microstructure.

The martensitic transformation in the studied AISI 9254 steel was achieved by rapid cooling from the austenitization temperature, ensuring a diffusionless displacive transformation. This transformation is governed by the martensite starting and finishing temperatures, reflecting the thermal range within which the phase transformation occurs. The resulting martensite exhibited a lath morphology, confirmed through TEM, with lattice distortions indicative of high tetragonality due to trapped carbon atoms. These distortions contribute to the superior resistance of the steel. In addition, the crystallographic shear mechanism inherent to martensitic transformations minimizes energy barriers, favoring the rapid transformation kinetics observed in this work. Additionally, the interplay between the retained austenite and martensite phases, coupled with nanoscale precipitation, underscores the complex microstructure that contributes to the enhanced mechanical properties of the steel.

Figure 9 presents bright- and dark-field TEM images alongside selected area electron diffraction (SAED) analyses conducted to characterize the microconstituents and explore their interrelationships. The comparison of bright- and dark-field images confirms the presence of blocky retained austenite. The green and blue circles indicate the SAED analysis of two neighboring BCC laths, the first with an orientation of (011)[0 −1 1] and the second with (−1 −1 −1)[0 1 1]. This confirms the formation of high-angle boundaries in the martensite/bainite laths, which hinder atomic plane slippage, thereby enhancing mechanical resistance. Furthermore, the presence of retained austenite with an orientation of (2 −1 10)[0 1 1] in this region demonstrates that the martensite laths surrounding the retained austenite can support and stabilize it under external loading conditions. This stabilization of retained austenite contributes to the simultaneous improvement in mechanical strength and ductility observed in the complex microstructure achieved through the novel Q&P process. The balance between retained austenite and martensite/bainite phases is critical for optimizing these properties, aligning with findings from similar studies [14,51,52,53].

Figure 10 presents the EBSD measurements conducted to obtain the crystallographic data of the analyzed material at approximately 4140× magnification with a step size of 0.28 µm. The initial Kikuchi pattern analysis revealed the presence of 2.09% retained austenite (RA) in the FCC pattern. It is important to note that the mechanical preparation of samples for EBSD measurements may reduce the fraction of RA due to martensite-induced transformation, known as the TRIP effect. The synergistic interaction between retained austenite and the martensitic matrix contributes significantly to the mechanical performance of the material. The retained austenite at a volume fraction of 2.09% embedded within martensitic laths undergoes the TRIP effect under external loading. As demonstrated in our previous work [14,16,54], nanohardness testing revealed the “pop-in” behavior under the TRIP effect, providing direct evidence of this mechanism. This transformation alleviates localized stress concentrations, delaying crack nucleation and propagation, which is particularly important in enhancing both strength and ductility. As a result, the material achieves a UTS of approximately 1570 ± 10 MPa with an elongation of ≈13.85%, demonstrating a superior balance of mechanical properties. This enhancement underscores the importance of optimizing the retained austenite fraction and distribution for applications requiring high-performance materials with excellent toughness and durability.

A Z-direction inverse pole figure (Z-IPF) map is also presented to illustrate the variation in crystallographic orientation across the analyzed region. Statistical data for both the BCC matrix and RA (FCC) are shown separately. Analysis of the BCC phase, which includes the ferritic matrix, tempered martensite, and bainite, revealed a predominant (112) crystal orientation parallel to the rolling plane. Furthermore, detailed analysis using the ODF (orientation distribution function) at constant φ₂ = 45° shows that the (112) [110] texture component is dominant. In contrast, the FCC structure exhibits a distribution of (112)–(102) crystal planes in the retained austenite. Therefore, pole figures for the (111) and (110) planes of each phase were calculated and presented to determine the possible orientation relationship between the parent austenite (FCC) and the daughter products (BCC). Analysis of the pole figures demonstrates a clear Kurdjumov–Sachs (K-S) orientation relationship [55,56,57,58], where (110)_BCC is parallel to (111)_FCC.

Figure 11 presents the analysis of the BCC (body-centered cubic) structure grain size, focusing on regions with point-to-point misorientation angles greater than 15°, allowing a tolerance of 5° on martensitic laths. As shown in Figure 11 (top left), the average size of martensite laths or bainitic leaves is approximately 2.11 ± 1.6 µm. In contrast, some larger ferritic grains, averaging about 7.7 µm, were observed and are likely associated with the ferritic matrix within pearlite colonies formed during isothermal holding at 550 °C. The kernel average misorientation (KAM) with a threshold of 3° is presented in Figure 11 (top half). This analysis indicates that the average distortion within the laths is about 2.64 ± 1.1°, with a maximum distortion reaching 3.87° primarily at the lath interfaces. These highly distorted sites are well dispersed and contribute to superior mechanical strength. Dislocation densities were calculated using the entry-wise norm of the Nye tensor, as shown in Figure 11. The average dislocation density in the BCC matrix is approximately 3.7 × 10¹⁴ m⁻², reaching a maximum of about 2 × 10¹⁵ m⁻². This detailed analysis of both BCC and FCC structures via EBSD effectively explains the material’s superior mechanical resistance, attributed to the fine martensite lath-bainite leaf structure, enhanced ductility from larger ferritic matrices within pearlite colonies, and the well-distributed soft retained austenite.

Figure 12 illustrates the nanoindentation load–displacement curves obtained from the investigated AISI 9254 steel sample, showcasing distinct mechanical responses corresponding to different phases in the microstructure. The curves highlight three distinct behaviors related to martensite, bainite, and retained austenite. The detailed mechanical properties derived from these measurements are summarized in

Table 2. Nanoindentation results.

Nanoindentation	m	H (GPa)	Er (GPa)	heff (nm)	hmax (nm)	A (nm2)	S (µN/nm)	Pmax (µN)
1	1.28	3.87	182.14	132.85	133.16	387,927.21	128.04	1499.73
2	1.58	3.88	189.59	132.27	132.68	386,685.12	133.07	1499.75
3	1.32	4.25	186.64	126.01	126.34	352,766.97	125.12	1499.77
4	1.52	4.26	199.33	125.32	125.94	352,158.61	133.51	1499.74
5	1.45	6.44	195.59	100.88	101.39	232,773.31	106.51	1499.77
6	1.4	6.52	210.44	99.55	99.95	230,136.41	113.95	1499.8

, and include parameters such as maximum load (Pmax), stiffness (S), projected contact area (A), maximum indentation depth (hmax), effective depth (heff), reduced modulus (Er), hardness (H), and work hardening coefficient (m).

The nanoindentation data revealed a complex interaction between phases within the microstructure, corroborating the findings from other characterization techniques. The load–displacement curves in Figure 12 and the mechanical properties in Table 2 illustrate distinct phase-specific behaviors, as follows.

• Retained Austenite and TRIP Effect: Nanoindentations 1 and 2 exhibited the lowest hardness values (3.87–3.88 GPa) and showed clear pop-in behavior during indentation, indicative of the TRIP effect. The retained austenite, identified by its lower hardness and higher ductility, undergoes stress-assisted transformation into martensite during deformation. This transformation alleviates localized stress concentrations, enhancing ductility and delaying crack propagation.
• Medium Hardness Bainite: Nanoindentations 3 and 4 demonstrated medium hardness values (4.25–4.26 GPa), characteristic of bainite sheaves formed during the partitioning stage. Bainite represents a combination of diffusional and displacive phase transformation mechanisms, where carbide precipitation and ferritic sheaf formation contribute to moderate hardness while retaining some plasticity.
• Martensite with Highest Hardness: Nanoindentations 5 and 6 recorded the highest hardness values (6.44–6.52 GPa), corresponding to pure displacive martensitic laths formed during the quenching stage. Martensite, with its high carbon content and tetragonal distortion, provides exceptional strength and hardness, albeit with limited ductility.

The findings of this study demonstrate the significant enhancement of mechanical properties in commercial AISI 9254 steel through a novel quenching and partitioning (Q&P) pathway. The observed increase in yield strength (YS) to 1500 MPa and ultimate tensile strength (UTS) to 1570 MPa, coupled with a 34% improvement in total elongation, underscores the effectiveness of this tailored heat treatment. These mechanical improvements align with previous studies that emphasize the role of a multiphase microstructure, including martensite, retained austenite, and nanoscale carbides, in achieving a balance of strength and ductility [1,11,24,59]. The TRIP effect of stabilized retained austenite, facilitated by carbon partitioning during the partitioning stage, plays an important role in enhancing ductility without compromising strength [13,60].

The stability and morphology of retained austenite are pivotal in optimizing the mechanical performance of Q&P-treated steels. This study observed approximately 8.7 ± 1.1 vol% of retained austenite, which contributed to improved ductility through the TRIP effect. This volume fraction is consistent with previous work, which report that retained austenite fractions between 8% and 12% provide an optimal balance of mechanical properties [7,13,53]. The lenticular morphology of retained austenite, along with its dispersion within a martensitic matrix, has been shown to enhance its transformation kinetics under mechanical stress, further contributing to the steel’s work-hardening capacity [49,54].

The partitioning stage facilitated carbon migration from martensite into retained austenite, stabilizing it while minimizing carbide precipitation. Although silicon is incorporated to suppress carbide formation, this study identified nanoscale carbide precipitates that enhance secondary hardening. These findings agree with prior studies that highlight the dual role of nanoscale carbides in contributing to strength while reducing carbon availability for retained austenite stabilization [24,46,48]. Advanced characterization using TEM and EBSD revealed fine martensite laths and low dislocation densities, suggesting that carbon redistribution also mitigated residual stresses, further improving ductility [47,58].

The results of this study hold significant implications for the application of Q&P-treated steels in safety-critical components. The enhanced ductility and wear resistance demonstrated by the novel Q&P pathway make it a promising candidate for automotive suspension systems and aerospace structural elements, which demand high cyclic stress tolerance and durability. Furthermore, the scalability of this heat treatment protocol presents an opportunity for cost-effective industrial adoption, aligning with the goals of lightness and sustainability in modern manufacturing.

Future research could focus on fine-tuning the partitioning parameters to maximize retained austenite stability while minimizing carbide formation. Investigating the interplay of additional alloying elements, such as chromium and molybdenum, could further enhance the wear resistance and high-temperature stability of these steels. Additionally, modeling the carbon partitioning process using advanced computational tools, such as phase field or molecular dynamic simulations, may provide deeper insights into optimizing microstructural evolution during Q&P treatment.

This study demonstrates the critical role of fine pearlite and carbide precipitates in enhancing the mechanical performance of commercial AISI 9254 steel. The incorporation of fine cementite–ferrite lamellae during the 550 °C isothermal stage significantly contributed to the observed improvements in mechanical properties. The ultrafine lamellae enhanced work hardening behavior, enabling an increase in total elongation from approximately 10.3% in the as-received material to 13.85% in the Q&P-processed sample, an improvement of 34%. Furthermore, the interplay between fine pearlite and the carbon-enriched untransformed austenite facilitated carbide precipitation within martensite and bainite, leading to a yield strength of approximately 1500 MPa and an ultimate tensile strength of 1565 MPa. This simultaneous enhancement of mechanical resistance and ductility underscores the technological significance of incorporating fine pearlite into the Q&P processing pathway. These findings open new avenues for designing advanced steel microstructures tailored for demanding engineering applications.

Systematic quantification and analysis of void content can significantly enhance the understanding of microstructural evolution and its impact on mechanical performance, particularly in complex processes like quenching and partitioning (Q&P). Studies have demonstrated that void size, distribution, and volume fraction play a critical role in influencing material properties and failure mechanisms. For instance, Sherzer et al. [61] highlighted how void spaces in concrete mesostructures lead to stress concentration and fracture localization, affecting macroscopic behavior. Similarly, Little et al. [62] emphasized the accuracy of micro-CT in characterizing voids in fiber-reinforced composites, showing its superiority over conventional techniques for assessing the three-dimensional distribution and shape of voids. Tosco et al. [63] utilized micro-CT for internal void analysis in bulk-fill composites, providing detailed 3D reconstructions and quantitative measurements of voids that revealed the morphology and extent of defects. Moreover, Ge et al. [64] demonstrated how micro-CT-based analysis captures pore defects and their impact on the tensile behavior of 3D braided composites using reconstructed void data to establish trans-scale models for damage prediction. These studies collectively underscore the potential of micro-CT as a powerful tool for quantifying voids, offering insights into their role in microstructural evolution and mechanical behavior, which is particularly relevant for understanding void formation and its effects during Q&P processing.

Finally, thermal gradients are inevitable during the Q&P heat treatment process due to varying heat transfer rates between the surface and the core of steel specimens. Differences in cooling rates can cause microstructural heterogeneities, with faster cooling near the surface promoting martensitic formation, while slower cooling in the interior may lead to the retention of pearlitic or bainitic structures. The competition between diffusional and displacive phase transformation mechanisms during Q&P processing further amplifies these effects, as the extent of carbon partitioning and retained austenite stabilization depends on precise thermal management. The formation of a complex multiphase microstructure inherently requires careful control of heat treatment parameters to minimize these gradients. However, such heterogeneities can also be advantageous in specific engineering applications. For example, surface-hardened components subjected to constant contact stresses may benefit from a harder martensitic layer for wear resistance while maintaining a tougher, more ductile core for improved mechanical performance. Future work could explore advanced thermal modeling and controlled heat treatment designs to mitigate thermal gradients and further enhance microstructural uniformity in commercial-scale production.

4. Conclusions

This study demonstrated the effectiveness of a novel quenching and partitioning (Q&P) pathway tailored for commercial AISI 9254 steel in achieving superior mechanical properties. The proposed heat treatment process resulted in a multiphase microstructure comprising ultrafine martensite laths, retained austenite, and fine carbide particles. These features contributed to a yield strength of approximately 1500 MPa, ultimate tensile strength of 1570 MPa, and total elongation of 13.85%, reflecting significant improvements in both strength and ductility compared to the as-received material. The stabilization of retained austenite through carbon partitioning was found to enhance ductility via the TRIP effect, while the suppression of excessive carbide formation maintained a balance between hardness and toughness.

The dilatation curves provided key evidence of the phase transformation mechanisms driving the enhanced properties of Q&P-treated AISI 9254 steel. The kinetics of the pearlitic transformation were clearly identified at 550 °C, while the interplay of martensitic and bainitic transformations during the Q&P process was observed through dimensional changes and their derivatives. These findings emphasize the importance of precise thermal management in controlling phase transformations and optimizing the balance between strength, ductility, and toughness in the treated steel.

Microstructural characterization using EBSD and TEM provided critical insights into the phase distribution, crystallographic orientations, and defect densities within the processed steel. The findings highlighted the importance of controlled thermal cycles in optimizing carbon partitioning and refining the microstructure, particularly the role of silicon and manganese in stabilizing retained austenite and minimizing phase transformation inconsistencies. These results underscore the transformative potential of the Q&P process in advancing the performance of commercial spring steels, particularly for applications in automotive and aerospace industries where enhanced ductility, wear resistance, and high cyclic stress tolerance are essential.

Future work could explore the integration of additional alloying elements, such as chromium and molybdenum, to further enhance high-temperature performance and wear resistance. Computational modeling of carbon partitioning dynamics and phase transformations could also provide deeper insights into optimizing the Q&P process for scalable industrial applications. This study sets the stage for the broader adoption of Q&P treatments in the design of high-performance materials, emphasizing the importance of advanced heat treatment processes in modern materials engineering.