The pursuit of high-performance materials for critical marine applications, such as ship hulls and structural components, consistently drives research in metallurgy and materials engineering. Among these materials, high-strength low-alloy (HSLA) cast steels are indispensable due to their design flexibility, ability to form complex geometries, and excellent weldability. A primary challenge lies in achieving an optimal balance between strength and toughness, particularly low-temperature toughness, which is paramount for ensuring structural integrity in the harsh, cold environments encountered at sea. Yield strengths exceeding 510 MPa are often mandated for such applications, but this must not come at the expense of fracture resistance.
This investigation focuses on a specific class of marine-grade cast steel and explores advanced heat treatment strategies to tailor its microstructure and, consequently, its mechanical properties. Traditional quench and temper (Q&T) processes can achieve high strength but may compromise toughness. Intercritical, or two-phase region, heat treatment presents a sophisticated alternative. This technique involves heating the steel into the temperature range where both ferrite (α) and austenite (γ) coexist, followed by quenching and tempering. The resulting microstructure is a controlled mixture of soft ferrite and hard tempered martensite (or its transformation products like tempered sorbite), offering a pathway to superior strength-toughness combinations.
The fundamental principle hinges on the phase transformation behavior defined by the Fe-C phase diagram and the steel’s continuous cooling transformation (CCT) diagram. The critical temperatures, Ac1 (austenite formation start) and Ac3 (austenite formation finish), define the intercritical window. The volume fractions of ferrite and austenite at the quenching temperature are governed by the lever rule, approximately expressed for a given composition:
$$V_{\gamma} \approx \frac{C_0 – C_{\alpha}}{C_{\gamma} – C_{\alpha}}$$
where $V_{\gamma}$ is the volume fraction of austenite, $C_0$ is the overall carbon content of the steel casting, $C_{\alpha}$ is the carbon content in ferrite (very low, ~0.022 wt% at 727°C), and $C_{\gamma}$ is the carbon content in austenite at the intercritical temperature, read from the phase boundary. Upon quenching from this two-phase field, the austenite transforms to martensite, while the ferrite remains essentially unchanged. Subsequent high-temperature tempering decomposes the martensite into a fine, tough microstructure of tempered sorbite (ferrite with finely dispersed carbides).
The precise thermal path to reach and process within the intercritical region significantly influences the final outcome. This study systematically compares three distinct two-phase heat treatment routes for a marine cast steel, analyzing their impact on microstructure evolution, strength, and low-temperature impact toughness.
I. Experimental Methodology: Material and Processing Routes
The subject of this study is a low-carbon, nickel-chromium-molybdenum-vanadium alloyed cast steel, representative of grades used for demanding marine components. Prior to the intercritical heat treatments, the steel casting underwent a standard homogenization and normalizing cycle to eliminate casting segregation and establish a uniform starting microstructure primarily consisting of bainite.
1.1 Material Composition
The chemical composition of the investigated steel casting is provided in Table 1. The balanced alloying system is designed to provide hardenability (via Cr, Ni, Mo), grain refinement (via V), and solid solution strengthening while maintaining good weldability and toughness.
| C | Mn | Si | Ni | Cr | Mo | V | P | S |
|---|---|---|---|---|---|---|---|---|
| 0.14 | 0.55 | 0.30 | 2.90 | 1.10 | 0.23 | 0.05 | 0.010 | 0.006 |
1.2 Determination of Phase Transformation Temperatures
The critical temperatures Ac1 and Ac3 were experimentally determined using dilatometric analysis according to standard methods. The values were found to be:
$$Ac_1 = 722^\circ C$$
$$Ac_3 = 820^\circ C$$
These temperatures define the boundaries for designing the intercritical heat treatment schedules.
1.3 Designed Two-Phase Heat Treatment Schedules
Three different thermal processing routes were conceived to understand the effect of the path to and through the intercritical region. All schedules conclude with a high-temperature tempering step at 620°C for 100 minutes, followed by water cooling. The parameters are summarized in Table 2.
| Process Designation | Thermal Schedule | Key Concept |
|---|---|---|
| Process 1 | 890°C x 60 min (WQ) + 750°C x 60 min (WQ) + 620°C x 100 min (WQ) | Full Austenitization + Intercritical Quenching (Conventional Two-Step Quench) |
| Process 2 | 890°C x 60 min (FC to 750°C) + 750°C x 60 min (WQ) + 620°C x 100 min (WQ) | Full Austenitization + Slow Cooling into Intercritical Region |
| Process 3 | 750°C x 60 min (WQ) + 620°C x 100 min (WQ) | Direct Intercritical Quenching from Sub-Critical State |
Process 1 (Conventional Two-Step Quench): This is the classic two-phase heat treatment. The steel casting is first fully austenitized at 890°C (well above Ac3) and water-quenched (WQ) to produce a fully martensitic structure. It is then reheated into the intercritical region at 750°C (between Ac1 and Ac3), held to establish an equilibrium mixture of ferrite and austenite, and water-quenched again. Finally, it is tempered.
Process 2 (Slow Cool into Intercritical): This process modifies Process 1 by eliminating the first quench. After austenitization at 890°C, the steel casting is furnace-cooled (FC) slowly (approx. 60 minutes) directly to the intercritical holding temperature of 750°C, held, then quenched and tempered. This simulates a potential industrial simplification.
Process 3 (Direct Intercritical Quench): This is the most simplified route. The steel casting, starting from its initial normalized (bainitic) condition, is directly heated into the intercritical region at 750°C, held, then quenched and tempered, bypassing any full austenitization step.
1.4 Mechanical Testing and Microstructural Analysis
Samples for testing were extracted from the 1/4-thickness location of heat-treated blocks. Room-temperature tensile tests were performed to determine yield strength ($R_{p0.2}$), tensile strength ($R_m$), elongation (A), and reduction of area (Z). Charpy V-notch impact tests were conducted at both -20°C and -80°C to assess low-temperature toughness. Microstructural characterization involved optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) to analyze phase morphology, distribution, and substructure. Grain size measurements were performed using the linear intercept method.
II. Results: Microstructure and Mechanical Properties
The three heat treatment schedules produced markedly different microstructural states and mechanical property profiles in the steel casting.
2.1 Microstructural Evolution
Process 1 (Two-Step Quench): The final microstructure consisted of a finely dispersed, dual-phase mixture of approximately 40% polygonal ferrite and 60% tempered sorbite (Figure 1a, schematic). The ferrite grains appeared equiaxed and were interspersed with islands of the tempered sorbite, which under TEM revealed a fine dispersion of carbides within a ferritic matrix (Figure 1b). The prior austenite grain structure exhibited a “mixed grain” characteristic, with some larger grains surrounded by finer ones, averaging about 4.5 µm.
Process 2 (Slow Cool): The microstructure was dominated by tempered sorbite (~95%), with only about 5% ferrite present (Figure 2a). The sorbite retained a discernible lath morphology from the prior martensite, with carbide precipitation along lath boundaries (Figure 2b). The grain structure was more uniform and equiaxed compared to Process 1, with an average grain size of approximately 8.6 µm.
Process 3 (Direct Quench): This process yielded a microstructure with a high volume fraction (~55%) of blocky, proeutectoid ferrite, with the remaining 45% being tempered sorbite (Figure 3a). The ferrite grains were significantly larger and more contiguous. Carbides were observed decorating the ferrite grain boundaries (Figure 3b). The overall grain size was the largest among the three, averaging about 11.2 µm.
2.2 Mechanical Property Evaluation
The complete set of mechanical properties is tabulated in Table 3. The data reveals clear correlations between the processing route, the resulting microstructure of the steel casting, and its performance.
| Process | $R_{p0.2}$ (MPa) | $R_m$ (MPa) | A (%) | Z (%) | $KV_2$ (-20°C) (J) | $KV_2$ (-80°C) (J) |
|---|---|---|---|---|---|---|
| Process 1 | 634 | 757 | 24.0 | 75 | 188 (182, 175, 207) | 181 (173, 182, 188) |
| Process 2 | 912 | 965 | 18.0 | 68 | 137 (127, 142, 143) | 67 (79, 69, 52) |
| Process 3 | 544 | 683 | 26.0 | 72 | 191 (187, 194, 192) | 131 (66, 165, 162) |
Strength: Process 2 produced the highest strength, with a yield strength of 912 MPa, far exceeding the 510 MPa requirement. Process 1 offered a robust yield strength of 634 MPa with good margin. Process 3 resulted in the lowest yield strength of 544 MPa, barely meeting the threshold with minimal safety margin.
Toughness: Process 1 demonstrated exceptional and consistent low-temperature toughness, with impact energies at -80°C (181 J average) nearly matching those at -20°C. Process 3 showed good average toughness at -20°C but exhibited severe inconsistency at -80°C, with one value as low as 66 J, indicating potential embrittlement risks. Process 2 displayed significantly lower impact energies at both temperatures, with a pronounced drop at -80°C (67 J average).
Strength-Toughness Balance: The superiority of Process 1 is evident when plotting yield strength versus -80°C impact energy (Figure 4). It occupies the optimal region of high toughness combined with adequate strength. Process 2 is in the high-strength, low-toughness quadrant, while Process 3 is in the lower-strength region with unreliable toughness.
III. Analysis and Discussion: Mechanisms Governing Property Outcomes
The divergent properties stem directly from the microstructural states engineered by each thermal path. The underlying mechanisms can be analyzed through the lenses of phase fraction control, carbon redistribution, grain growth, and final precipitate distribution.
3.1 Process 1: The Mechanism of Optimal Balance
This process provides the greatest level of microstructural control for the steel casting.
1. First Quench (890°C WQ): Creates a homogeneous, carbon-supersaturated martensitic matrix. This resets the entire microstructure, erasing any chemical or structural memory from the prior normalized state.
2. Intercritical Reheat (750°C): Upon heating to 750°C, the martensite decomposes. Carbon-rich regions transform to austenite ($\gamma$), while carbon-poor regions become ferrite ($\alpha$). The system moves towards the equilibrium phase fractions predicted by the lever rule. The fine prior martensitic structure provides a high density of nucleation sites, leading to a fine dispersion of both phases.
3. Second Quench (750°C WQ): The austenite (now enriched in carbon relative to the bulk composition) transforms to fresh, high-carbon martensite. The ferrite remains unchanged.
4. Tempering (620°C): The second-generation martensite tempers into tough, fine-carbide-containing sorbite. The pre-existing ferrite is thermally stable and soft.
The final duplex structure leverages composite strengthening. The strength can be approximated by a rule of mixtures:
$$\sigma_y \approx V_{\alpha}\sigma_{\alpha} + V_{sorbite}\sigma_{sorbite}$$
where $V$ and $\sigma$ represent volume fraction and yield strength of each phase. More importantly, the finely interspersed soft ferrite acts as a potent crack arrester. During impact loading, propagating cracks are repeatedly forced to change direction at ferrite/sorbite interfaces, consuming more energy through crack tip blunting and bifurcation. This significantly enhances low-temperature toughness, explaining the excellent and consistent impact values. The “mixed” prior austenite grain size, averaging a fine 4.5 µm, further contributes to toughness.
3.2 Process 2: The Consequence of Simplified Thermal Path
Eliminating the first quench and slowly cooling from the austenitizing temperature fundamentally alters the process for the steel casting.
1. Slow Cooling (890°C → 750°C): As the temperature drops through the intercritical range, proeutectoid ferrite begins to form at prior austenite grain boundaries. The slow cooling rate allows for substantial ferrite growth, depleting the adjacent austenite of carbon. However, because the cooling is not isothermal, the system is not at equilibrium. The final phase fractions at the 750°C hold are not the same as in Process 1.
2. Intercritical Hold & Quench: The hold at 750°C may not fully re-austenitize the carbon-depleted regions. The resulting microstructure before the final quench likely contains less austenite (and it is less uniformly distributed) than in Process 1. Upon quenching, this leads to a final structure dominated by tempered martensite/sorbite with very little free ferrite.
3. Result: The high volume fraction of the hard sorbite phase ($V_{sorbite} \approx 0.95$) drives the yield strength to a very high 912 MPa according to the rule of mixtures. However, the lack of the soft, crack-arresting ferrite phase drastically reduces the energy absorption capacity during fracture. The impact toughness, especially at -80°C, plummets. The more uniform, larger grain size (8.6 µm) also offers less resistance to crack propagation compared to the finer, mixed structure of Process 1.
3.3 Process 3: Challenges of Direct Intercritical Processing
Starting from a normalized bainitic microstructure for the steel casting introduces complications.
1. Initial State: The bainite contains a non-uniform distribution of carbides and carbon in solid solution.
2. Heating to Intercritical (750°C): During heating, austenite nucleates preferentially in high-carbon regions, such as at carbide interfaces. Concurrently, the existing ferrite in the bainite can grow. The carbon gradient accelerates ferrite formation, leading to excessive growth of blocky ferrite grains ($V_{\alpha} \approx 0.55$).
3. Quench & Temper: The resulting austenite, now confined to localized areas, transforms to martensite upon quenching. The large, contiguous ferrite grains remain. Tempering can lead to carbide coarsening, particularly at ferrite grain boundaries.
4. Result: The high ferrite content lowers the overall strength (544 MPa). The large grain size (11.2 µm) is detrimental to toughness. Most critically, the carbide decoration on ferrite boundaries and potential chemical segregation create weak paths for crack propagation. This leads to unstable fracture behavior, manifested as the scattered, occasionally very low, impact values at -80°C. The low single value of 66 J is a clear warning sign of potential brittle fracture initiation under severe conditions.
The toughness behavior can be modeled considering the effect of microstructural barriers. The ductile-to-brittle transition temperature ($T_{db}$) is influenced by grain size ($d$) and stress concentrators:
$$T_{db} \propto \ln \left( \frac{1}{d^{1/2}} + k \cdot \sigma_{conc} \right)$$
where $k$ is a constant and $\sigma_{conc}$ represents stress concentration from features like grain boundary carbides. Process 1 has a fine effective $d$ and low $\sigma_{conc}$. Process 3 has a large $d$ and higher $\sigma_{conc}$ from boundary carbides, raising $T_{db}$ and causing unstable fracture near -80°C.
IV. Discussion: Implications for Marine Steel Casting Production
This systematic comparison underscores that not all “two-phase” or “intercritical” heat treatments are equivalent for this class of steel casting. The thermal history—the precise path taken to create the ferrite-austenite mixture—is as critical as the final temperature itself.
Process Optimization vs. Operational Simplification: While Processes 2 and 3 offer apparent simplifications by reducing the number of heating or quenching steps, they do so at a severe cost to the final property profile of the steel casting. Process 2 leads to potential over-strengthening and embrittlement, which could compromise damage tolerance in a marine structure. Process 3 leads to under-strengthening and introduces unacceptable scatter and risk of low-temperature brittle fracture.
The Non-Equilibrium Nature of Industrial Processing: The study highlights the deviation from ideal equilibrium assumptions. The starting microstructure (martensite vs. bainite) and the heating/cooling rate dramatically affect phase transformation kinetics, carbon diffusion, and final phase distribution. The assumption that holding at a single intercritical temperature (like 750°C) will produce the same microstructure regardless of prior state is incorrect for this steel casting. The first full austenitization and quench in Process 1 is essential to create a homogeneous, reproducible starting point for the critical intercritical treatment.
Role of Alloying Elements: Elements like Ni and Mo in this steel casting not only enhance hardenability but also influence the stability of the phases during tempering and retard the recovery processes, contributing to the stability of the fine sorbite structure achieved in Process 1. Vanadium contributes to secondary hardening during tempering through fine V(C,N) precipitation, which can be optimized in the 620°C tempering step to further strengthen the sorbite phase without significantly harming toughness.
Generalizability: The principles elucidated here—the necessity of microstructural homogenization prior to intercritical treatment for consistent duplex structure formation—are likely applicable to a broad range of HSLA steel castings where an optimal strength-toughness synergy is required. The specific temperatures and phase fractions will vary with composition, but the fundamental importance of the processing path remains.
V. Conclusion
Through a detailed investigation of three distinct two-phase region heat treatment schedules, this work establishes the critical link between thermal processing path, microstructure, and final mechanical properties in a high-strength marine steel casting.
- The conventional two-step quenching process (Process 1: Full Austenitization + Intercritical Quench + Temper) produces an ideal microstructure comprising a fine, interspersed mixture of approximately 40% ferrite and 60% tempered sorbite. This structure delivers an outstanding balance of properties: a yield strength (~635 MPa) that comfortably exceeds the 510 MPa requirement for marine applications, coupled with exceptionally high and consistent low-temperature impact toughness (exceeding 180 J even at -80°C).
- Simplified routes compromise this balance. Process 2 (Slow Cool into Intercritical), by avoiding the initial quench, results in a sorbite-dominated microstructure with minimal ferrite. This leads to excessive strength (~910 MPa) and poor, low-temperature toughness (~67 J at -80°C). Process 3 (Direct Intercritical Quench), starting from a non-homogeneous state, produces a coarse, ferrite-rich structure with boundary carbides, resulting in marginal strength (~545 MPa) and unstable, potentially poor low-temperature toughness.
- The key mechanistic insight is that the initial full austenitization and quench in Process 1 are indispensable for resetting the microstructure and chemical homogeneity of the steel casting. This creates a reproducible and uniform starting condition for the subsequent intercritical anneal, enabling precise control over the final dual-phase structure that is responsible for the superior strength-toughness synergy.
Therefore, for the industrial production of critical marine components from this grade of steel casting, where reliability and performance in extreme environments are non-negotiable, the adoption of the complete “single-phase zone quenching + two-phase zone quenching + high-temperature tempering” process (Process 1) is strongly recommended. This ensures the consistent achievement of the robust mechanical property profile necessary for the safety and longevity of marine structures.