The continuous cooling transformation (CCT) diagram of a steel is a fundamental tool in metallurgy, providing critical insight into the phase transformations that occur during cooling from the austenitic state. It maps the onset and finish temperatures of various transformation products—such as ferrite, pearlite, bainite, and martensite—against cooling rate and time. This diagram is indispensable for predicting the final microstructure and mechanical properties of a component after a given thermal cycle, making it a cornerstone for designing heat treatment processes. For steel castings intended for critical applications, especially those operating in cryogenic environments, a precise understanding of the CCT behavior is paramount. It allows foundries and engineers to tailor cooling rates, whether in the mold or during subsequent heat treatment, to achieve the desired balance of strength, toughness, and dimensional stability.
The production of high-integrity steel castings for low-temperature service presents unique challenges. Materials like the ASTM A352 LC3 grade, a 3.5% Ni steel, are commonly specified for components such as valves, pumps, and piping systems that must withstand temperatures down to -101°C. The excellent low-temperature toughness of this grade stems from its nickel content, which stabilizes the austenite phase and refines the transformation microstructure. However, the achievable microstructure and properties are not solely a function of chemistry; they are profoundly influenced by the cooling history. In a steel casting, cooling rates can vary significantly from the surface to the core of a thick section. Without a detailed CCT diagram, it is difficult to predict whether these varying cooling rates will lead to undesirable microstructural gradients, such as soft ferrite-pearlite formations in slow-cooling regions or untempered, brittle martensite in fast-cooling regions. Despite the widespread use of LC3-grade steel castings, publicly available, experimentally determined CCT data for this specific material is scarce in the literature. This lack of data forces engineers to rely on generic guidelines or estimations, potentially compromising the optimization of heat treatment cycles and the consistency of mechanical properties.
This study aims to fill this knowledge gap by experimentally determining the complete CCT diagram for an LC3-type low-temperature steel casting. The approach combines the dilatometric analysis, which tracks dimensional changes during phase transformations, with comprehensive microstructural characterization and hardness testing. This multi-faceted methodology ensures accurate identification of transformation start and finish points and correlates them with the actual resulting phases. The generated CCT curve will provide a reliable scientific basis for defining appropriate normalizing, quenching, and tempering parameters. Furthermore, identifying the critical cooling rates for the suppression of soft phases and the onset of martensitic transformation is crucial for designing the casting process itself, especially for heavy-section steel castings where thermal management is key. The findings will contribute to a more robust and predictable manufacturing framework for high-performance cryogenic components, enhancing their reliability in demanding service conditions.
1. Materials and Experimental Methodology
The material for this investigation was sourced from the bulk section of a commercially produced steel casting. The casting was manufactured using an alkaline phenolic no-bake mold process with zircon sand to achieve good surface finish and dimensional accuracy. The melt was prepared in a medium-frequency induction furnace using selected scrap steel and nickel plate additions. Special attention was paid to minimizing trace impurities like Pb, Sn, As, Bi, and Sb, which can segregate to grain boundaries and impair low-temperature toughness. A combination of strong deoxidation and rare earth treatment was employed to enhance the cleanliness of the steel casting. The final chemical composition of the cast material, verified by optical emission spectroscopy, is presented in Table 1 alongside the specification limits of ASTM A352 LC3.
| Element | C | Mn | Si | P | S | Ni | Ce | Al |
|---|---|---|---|---|---|---|---|---|
| Steel Casting (Actual) | 0.14 | 0.74 | 0.38 | 0.010 | 0.014 | 3.12 | 0.016 | 0.063 |
| ASTM A352 LC3 (Max.) | 0.15 | 0.50-0.80 | 0.60 | 0.04 | 0.045 | 3.00-4.00 | – | – |
From the cast block, a larger test piece was extracted and subjected to a normalizing pre-treatment at 920°C for 2 hours to homogenize the microstructure and eliminate any residual casting segregation. Cylindrical specimens with dimensions of Ø4 mm × 10 mm were then machined from this normalized stock for dilatometric analysis.
The core of the experimental work was performed using a DIL 805A fully automatic quench dilatometer. Each specimen was subjected to the following thermal cycle: heating to 950°C at a constant rate of 1°C/s, holding at this austenitizing temperature for 15 minutes to ensure complete homogenization and grain growth, followed by continuous cooling to room temperature at a predetermined constant rate. A wide range of fifteen cooling rates was investigated: 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 30, 50, 80, 100, 120, and 150°C/s. This range spans from very slow, furnace-like cooling to very rapid, water-quench-like cooling, covering all practically relevant scenarios for steel casting heat treatment.
The dilatometer records the relative change in length (ΔL/L0) of the specimen as a function of temperature and time. During cooling, phase transformations cause deviations from the normal thermal contraction line due to the different specific volumes of the transformation products. These deviations were analyzed using the tangent method according to standard YB/T 5128-2018 to determine the critical transformation temperatures: the start and finish of ferrite/pearlite formation ((F+P)s, (F+P)f), bainite start and finish (Bs, Bf), and martensite start and finish (Ms, Mf).
After the dilatometric tests, all specimens were mounted, ground, polished, and etched with a 4% nital solution for metallographic examination. Their microstructures were observed and documented using optical microscopy. To quantitatively link microstructure to properties, Vickers microhardness (HV10, load 10 kgf, dwell time 5 s) was measured on each specimen, with the reported value being the average of three indents.
The final CCT diagram was constructed by plotting the measured transformation start and finish temperatures against the logarithm of cooling time (derived from cooling rate) using data analysis software. The microstructural observations and hardness values were overlaid on the diagram to define the fields of different transformation products.
2. Experimental Results
2.1 Critical Temperatures and Dilatometric Data
The heating portion of the dilatometric cycle was used to determine the equilibrium transformation temperatures Ac1 and Ac3. The average values calculated from multiple measurements using the tangent method on the heating curve at 1°C/s were found to be:
$$ A_{c1} = 700^\circ C $$
$$ A_{c3} = 849^\circ C $$
These values represent the temperatures at which austenite begins to form during heating and is completely formed, respectively, for this specific steel casting composition.
The detailed transformation data extracted from the cooling dilatometry curves are summarized in Tables 2 and 3. Table 2 lists the start and finish times and temperatures for the ferrite and pearlite transformations at lower cooling rates. Table 3 provides the data for bainite and martensite transformations at higher cooling rates. Notably, the martensite start temperature Ms showed a dependence on cooling rate, increasing from approximately 384°C at 20°C/s to around 427°C and stabilizing at cooling rates of 100°C/s and above.
| Cooling Rate (°C/s) | (F+P)s | (F+P)f | ||
|---|---|---|---|---|
| Time (s) | Temp. (°C) | Time (s) | Temp. (°C) | |
| 0.05 | 4721 | 714 | 7881 | 556 |
| 0.1 | 2411 | 709 | 3961 | 554 |
| 0.2 | 1216 | 707 | 2036 | 543 |
| 0.5 | 495 | 703 | 823 | 539 |
| 1 | 246 | 705 | 418 | 533 |
| 2 | 133 | 686 | 235 | 482 |
| 5 | 56 | 675 | – | – |
| 10 | 29 | 672 | – | – |
| Cooling Rate (°C/s) | Bs | Bf | Ms | Mf | ||||
|---|---|---|---|---|---|---|---|---|
| Time (s) | Temp. (°C) | Time (s) | Temp. (°C) | Time (s) | Temp. (°C) | Time (s) | Temp. (°C) | |
| 5 | 91.16 | 499 | 97.00 | 471 | – | – | – | – |
| 10 | 43.71 | 523 | 49.00 | 470 | – | – | – | – |
| 20 | 16.95 | 634 | 24.50 | 480 | 29.30 | 384 | 35.00 | 270 |
| 30 | 12.60 | 602 | 16.87 | 474 | 19.73 | 388 | 24.20 | 254 |
| 50 | 8.60 | 570 | 12.14 | 443 | 12.00 | 400 | 16.06 | 247 |
| 80 | – | – | – | – | 7.84 | 403 | 10.20 | 214 |
| 100 | – | – | – | – | 6.20 | 430 | 8.38 | 212 |
| 120 | – | – | – | – | 5.36 | 427 | 7.18 | 208 |
| 150 | – | – | – | – | 4.49 | 427 | 5.95 | 207 |
2.2 Microstructural Evolution
The microstructures observed after cooling at different rates revealed a clear progression of transformation mechanisms, characteristic of a medium-carbon, low-alloy steel casting.
- 0.05 to 2°C/s: At these very slow to moderate cooling rates, the microstructure consisted entirely of proeutectoid blocky ferrite and pearlite colonies. The fraction of pearlite increased with cooling rate as the transformation was shifted to lower temperatures.
- 5°C/s: A transitional microstructure was observed, comprising blocky ferrite, pearlite, and the first appearance of granular bainite.
- 10°C/s: The pearlite formation was fully suppressed. The microstructure was primarily composed of blocky ferrite and lath-like upper bainite.
- 20°C/s: Ferrite formation was restricted primarily to grain boundaries. The dominant microstructure was a mixture of lath bainite and lath martensite.
- 30 to 50°C/s: The microstructure was a complex mixture of bainite and low-carbon martensite. The proportion of martensite increased steadily with increasing cooling rate.
- 80°C/s: The microstructure was predominantly martensitic with a small amount of retained austenite. This cooling rate is near the critical value for suppressing all high-temperature transformations.
- 100 to 150°C/s: At these very high cooling rates, the microstructure was fully martensitic, indicating that the cooling rate exceeded the critical value needed to bypass both diffusion-controlled (ferrite, pearlite, bainite) transformations.
2.3 Hardness Trends
The microhardness of the steel casting specimens showed a strong and systematic dependence on the cooling rate, directly reflecting the increasing hardness of the constituent phases. The results are summarized in Table 4 and graphically represented below.
The hardness increased monotonically from 168 HV10 at the slowest cooling rate (0.05°C/s, ferrite-pearlite structure) to 453 HV10 at the fastest rate (150°C/s, fully martensitic structure). A significant jump in hardness occurred between 50°C/s (mixed bainite-martensite) and 80°C/s (near-fully martensite), after which the hardness plateaued, indicating that a fully martensitic structure had been achieved.
| Cooling Rate (°C/s) | Microhardness (HV10) | Predominant Microstructure |
|---|---|---|
| 0.05 | 168 | Ferrite + Pearlite |
| 0.5 | 183 | Ferrite + Pearlite |
| 2 | 201 | Ferrite + Pearlite |
| 5 | 228 | Ferrite + Pearlite + Granular Bainite |
| 10 | 262 | Ferrite + Lath Bainite |
| 20 | 302 | Lath Bainite + Martensite |
| 30 | 345 | Bainite + Martensite |
| 50 | 389 | Bainite + Martensite |
| 80 | 431 | Martensite + Retained Austenite |
| 100 | 450 | Fully Martensitic |
| 150 | 453 | Fully Martensitic |
2.4 Constructed CCT Diagram
Integrating all the experimental data—dilatometric transformation points, microstructural analysis, and hardness values—the continuous cooling transformation (CCT) diagram for this LC3-type low-temperature steel casting was constructed. The diagram clearly maps the regions of ferrite-pearlite transformation, the bainite bay, and the martensite start and finish lines.
A key parameter derived from the diagram is the critical cooling rate for martensite formation (Vc). This is defined as the minimum cooling rate required to avoid the nose of the ferrite-pearlite transformation curve and, in this case, also the bainite transformation, resulting in a fully martensitic structure. By analyzing the dilatometric curve at 80°C/s and fitting the transformation kinetics, it was determined that the bainite start (Bs) temperature of approximately 565°C is nearly tangent to the 80°C/s cooling curve. Therefore, the critical cooling rate for this steel casting is determined to be approximately:
$$ V_c \approx 80^\circ C/s $$
3. Discussion
3.1 Validation of Critical Temperatures
The experimentally determined Ac1 and Ac3 temperatures can be compared against values predicted by empirical formulas that account for chemical composition. Using the regression equations proposed by Trzaska et al., which are of the form:
$$ Ac_1 = 739.3 – 22.8C – 6.8Mn + 18.2Si + 11.7Cr – 15Ni – 6.4Mo – 5V – 28Cu $$
$$ Ac_3 = 937.3 – 224.5C – 17Mn + 34Si – 14Ni + 21.6Mo + 41.8V – 20Cu $$
and inserting the composition from Table 1 (with Cr, Mo, V, Cu assumed negligible or zero), the calculated values are:
$$ Ac_{1(calc)} = 691^\circ C \quad (\text{vs. experimental } 700^\circ C, \Delta = 9^\circ C) $$
$$ Ac_{3(calc)} = 861^\circ C \quad (\text{vs. experimental } 849^\circ C, \Delta = 12^\circ C) $$
The close agreement (within ~10-12°C) validates the accuracy of the dilatometric measurements for this specific steel casting composition and provides confidence in the subsequent cooling transformation data.
3.2 Analysis of the Critical Cooling Rate
The experimentally determined critical cooling rate of ~80°C/s is a vital parameter for heat treatment design. It can be compared to predictions from theoretical models. The Kasai model provides a semi-empirical formula relating composition to the critical cooling rate (V1):
$$ \log V_1 = 9.81 – (4.62C + 0.78Mn + 0.41Ni + 0.80Cr + 0.66Mo + 0.0018P_A) $$
where P_A is a parameter related to austenitizing conditions. Using the nominal composition, the calculated critical cooling rate is:
$$ V_{1(calc)} \approx 77.5^\circ C/s $$
The deviation from the measured value of 80°C/s is only 2.5°C/s, which is remarkably close. This consistency indicates that the hardenability of this steel casting is well-predicted by its chemical composition and aligns with expectations for a 3.5% Ni steel. For practical purposes in a foundry, this means that to achieve a fully martensitic structure throughout a section during quenching, the cooling rate at the core of the steel casting must exceed 80°C/s. This has direct implications for selecting quenchants (e.g., water, polymer, or oil) and for designing the geometry of the casting to avoid excessively thick sections that cool too slowly.
3.3 Microstructure-Property Relationships and Practical Implications
The progression of microstructures and the corresponding hardness profile offer a classic demonstration of how cooling rate controls the properties of a steel casting. The gradual increase in hardness from 168 to 453 HV10 is a direct consequence of the transition from the soft, ductile ferrite-pearlite aggregate to the strong, hard lath martensite. The presence of nickel in this steel casting plays a crucial dual role: it lowers the transformation temperatures (shifting the CCT curves to the right), thereby increasing hardenability, and it also imparts solid solution strengthening and enhances the toughness of the resulting martensite and bainite, which is essential for cryogenic applications.
The CCT diagram reveals several important zones for heat treatment of this steel casting:
- Normalizing (Air Cooling): Cooling rates between 0.2 to 5°C/s, typical for air cooling of moderate sections, will produce a ferrite-pearlite or ferrite-bainite mixture with hardness in the 200-230 HV10 range. This is suitable for obtaining a uniform, refined grain structure prior to a final quench and temper.
- Quenching for High Strength: To achieve the high strength associated with a tempered martensite structure, the cooling rate during the quench must exceed the critical 80°C/s. The diagram shows that at 100-150°C/s, a uniform martensite with hardness >450 HV10 is guaranteed. Subsequent tempering will adjust this hardness to the desired level while improving toughness.
- Avoiding Mixed Microstructures: The diagram clearly shows a range of intermediate cooling rates (e.g., 20-50°C/s) that produce mixed bainite-martensite microstructures. While these may offer a good combination of properties, they can be inconsistent if cooling is not uniform. For reliable and predictable properties in a steel casting, it is often preferable to aim for a cooling path that leads to a single, dominant transformation product—either fully bainitic (for high toughness) or fully martensitic (for high strength), followed by appropriate tempering.
The shift in Ms temperature with cooling rate is also noteworthy. The increase in Ms from ~384°C to ~427°C as the cooling rate increases from 20 to 100°C/s is likely due to the suppression of carbon diffusion and the formation of small amounts of very early bainite or other transition phases at the slower rates, which slightly depletes the carbon content of the remaining austenite, thereby raising its Ms point. This phenomenon underscores the complex interplay between different transformation mechanisms during continuous cooling of a steel casting.
4. Conclusions
This comprehensive investigation successfully determined the continuous cooling transformation (CCT) behavior of an LC3-type low-temperature steel casting through a combined methodology of dilatometry, metallography, and hardness testing. The following key conclusions are drawn:
- The critical austenite transformation temperatures for this specific steel casting composition are Ac1 = 700°C and Ac3 = 849°C, which align well with values predicted from empirical compositional formulas.
- The microstructure evolves systematically with cooling rate: from ferrite-pearlite (≤2°C/s), to mixed structures with bainite (5-20°C/s), to bainite-martensite mixtures (30-50°C/s), and finally to a fully martensitic structure (≥100°C/s). The transition near 80°C/s yields a martensitic structure with some retained austenite.
- The microhardness of the steel casting increases continuously from 168 HV10 to 453 HV10 as the cooling rate increases and the microstructure transitions from soft ferrite-pearlite to hard martensite.
- The constructed CCT diagram provides a definitive map of these transformations. The critical cooling rate required to obtain a fully martensitic microstructure in this steel casting is approximately 80°C/s. This value is in excellent agreement with predictions from the Kasai hardenability model.
The experimentally derived CCT diagram fills a significant gap in the available data for this important class of cryogenic materials. It serves as an essential scientific and engineering reference for optimizing the heat treatment of LC3-grade steel castings. By using this diagram, manufacturers can precisely define normalizing, quenching, and tempering parameters to achieve the target microstructure and mechanical properties consistently, ensuring the reliability and performance of critical components in low-temperature service environments. The methodology and findings also provide a template for characterizing the CCT behavior of other grades of specialized steel castings.