Abstract
This paper presents an experimental study on the continuous cooling transformation curve (CCT curve) of A487-4B low-alloy cast steel using the Gleeble 1500D thermal simulation testing machine. The effect of various cooling rates on the microstructure and hardness of the coarse-grained heat-affected zone (HAZ) was investigated. The results revealed that the Ac1, Ac3, and Ms temperatures of A487-4B steel are 747°C, 875°C, and 422°C, respectively. At cooling rates of 0.2, 0.5, 1, and 5°C/s, the primary transformation products consist of ferrite, pearlite, and lath bainite. At a cooling rate of 10°C/s, the microstructure transforms into a mixture of lath bainite and lath martensite. Further increasing the cooling rate to 20–50°C/s results in a microstructure dominated by lath martensite and acicular martensite. As the cooling rate increases, the martensite content within the microstructure rises, leading to a corresponding increase in microhardness. The obtained CCT curve provides a fundamental basis for optimizing the mechanical properties and refining the welding and heat treatment parameters of A487-4B cast steel.
1. Introduction
With advancements in technology, the performance requirements of cast steels have significantly increased, particularly in low-temperature, heavy-duty, and impact-resistant applications. These steels must exhibit not only high strength but also excellent low-temperature toughness and weldability. Low-alloy cast steels are achieved by incorporating alloying elements into traditional cast carbon steels, thereby enhancing their mechanical properties and service life . Welding is a crucial application for low-alloy cast steels, yet the heat-affected zone (HAZ), particularly the coarse-grained HAZ, often represents a weakness in the joint, leading to reduced overall mechanical properties. Studies have shown that the high residual heat and extended duration of welding can significantly enlarge the austenite grain size in the HAZ, thereby impairing toughness and promoting brittleness and cracking.
Given the intricate microstructure and properties of the HAZ, accurately assessing its mechanical behavior is challenging. To precisely evaluate the coarse-grained HAZ, welding thermal simulation techniques are employed. These techniques facilitate the investigation of welding-induced phenomena such as cold cracking, hot cracking, reheat cracking, and stress corrosion cracking, alongside the formation of microstructures and assessment of fracture toughness and continuous cooling curves in the HAZ .
The continuous cooling transformation curve (CCT curve) serves as a pivotal theoretical foundation for process planning and parameter selection in steel processing. Numerous scholars have utilized thermal simulation to determine and analyze CCT curves for various steel grades, providing valuable insights for practical applications. However, research on the CCT curves of low-alloy pressure-bearing cast steels remains scarce.
This study aims to determine the CCT curve of A487-4B low-alloy cast steel using the Gleeble 1500D thermal simulation testing machine. The investigation examines the transformation characteristics and resultant microstructures and hardness at various cooling rates in the coarse-grained HAZ. The obtained CCT curve offers a reliable reference for optimizing the mechanical properties and refining welding and heat treatment parameters of A487-4B cast steel.
2. Materials and Methods
2.1 Experimental Material
The experimental material was A487-4B low-alloy cast steel with a plate thickness of 20 mm. The chemical composition of the base metal is detailed in Table 1. Cylindrical specimens with dimensions of φ6 mm × 75 mm were machined from the steel block using wire electrical discharge machining (EDM).
Table 1: Chemical Composition of the Experimental A487-4B Cast Steel (wt.%)
| Element | C | Mn | Si | P | S | Cr | Ni | Mo | Cu | V | W |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Content | 0.23 | 0.63 | 0.51 | 0.01 | 0.01 | 1.49 | 0.54 | 0.24 | 0.08 | 0.01 | 0.01 |
2.2 Thermal Simulation Procedure
The Gleeble 1500D thermal simulation testing machine was employed for the welding thermal simulation. The CCT curve was determined using the thermal dilation method. A thermocouple was welded at the center of each specimen to monitor the temperature. To prevent oxidation during heating, the specimen ends were coated with graphite powder, and the tests were conducted under vacuum conditions.
The simulation protocol comprised the following steps:
- The specimen was heated from room temperature to 500°C at a rate of 2°C/s.
- Further heating to 1000°C was achieved at 0.05°C/s, followed by a 3-minute soak at this temperature.
- Rapid cooling to room temperature was then performed at 10°C/s.
- The Ac1 and Ac3 temperatures were determined using the tangent method on the resultant dilation-temperature curve.
For the CCT curve determination, the specimen was heated to a peak temperature of 1300°C at 100°C/s and held for 2 seconds to ensure complete austenitization. The specimen was then cooled to Ac3 at 40°C/s, followed by cooling to 200°C at various rates (0.2, 0.5, 1, 5, 10, 20, 25, 30, 40, and 50°C/s). Uncontrolled cooling continued below 200°C. Temperature and dilation data were continuously recorded during the tests.
2.3 Microstructure and Hardness Analysis
After thermal simulation, specimens were ground, polished, and etched with a 4% HNO3 in alcohol solution for 4-5 seconds. Microstructural observations were conducted using an OLYMPUS-BX51M optical microscope. The Vickers hardness of the specimens was measured using an HV-50A Vickers hardness tester with a load of 100 N and a dwell time of 15 seconds.
3. Results and Discussion
3.1 Effect of Cooling Rate on Microstructure
The microstructures observed at various cooling rates are depicted in Figure 1. At low cooling rates (0.2, 0.5, 1, and 5°C/s), the transformation products primarily consisted of ferrite, pearlite, and lath bainite. As the cooling rate increased, the proportion of lath bainite grew, with a corresponding decrease in ferrite and pearlite. The lath bainite became denser and more uniformly distributed within the grains.
At a cooling rate of 10°C/s, the microstructure transformed into a mixture of lath bainite and lath martensite. Further increases in the cooling rate to 20–50°C/s resulted in a microstructure dominated by lath martensite and acicular martensite. As the cooling rate rose, the lath martensite became finer, with a decrease in lath bundle diameter and sub-bundle width. This refinement was attributed to the increased undercooling and subsequent restriction of atomic diffusion at higher cooling rates.
3.2 Effect of Cooling Rate on Hardness
The Vickers hardness values measured at various cooling rates are summarized in Table 2 and depicted in Figure 2. At a cooling rate of 0.2°C/s, the microstructure comprised primarily of ferrite and pearlite, resulting in a low average hardness of HV200. As the cooling rate increased to 5°C/s, the enhanced lath bainite content led to a significant hardness increase, reaching HV273. Above 10°C/s, the dominant martensitic microstructures imparted substantial hardness, with the highest average hardness of HV473 recorded at a cooling rate of 50°C/s.
Table 2: Vickers Hardness Values at Different Cooling Rates
| Cooling Rate (°C/s) | Vickers Hardness (HV) | Average Vickers Hardness (HV) |
|---|---|---|
| 0.2 | 203, 197, 201 | 201 |
| 0.5 | 220, 223, 219 | 221 |
| 1.0 | 238, 241, 236 | 238 |
| 5.0 | 266, 281, 273 | 273 |
| 10 | 313, 310, 308 | 308 |
| 20 | 417, 406, 420 | 414 |
| 25 | 416, 414, 413 | 414 |
| 30 | 450, 487, 484 | 473 |
| 40 | 474, 450, 479 | 468 |
| 50 | 484, 470, 455 | 470 |
The marked hardness increase at cooling rates exceeding 10°C/s highlights the severe quench hardenability of A487-4B cast steel. However, this heightened hardness may lead to embrittlement and cold cracking within the HAZ during welding applications. Consequently, low heat input welding practices accompanied by preheating and post-weld cooling are recommended to mitigate these issues.
3.3 CCT Curve of A487-4B Cast Steel
Based on the thermal dilation data, the phase transformation temperatures at various cooling rates were determined and summarized in Table 3. The CCT curve constructed from these data points is presented in Figure 3. The critical transformation points were identified as Ac1 = 747°C, Ac3 = 875°C, and Ms = 422°C.
Table 3: Phase Transformation Temperatures at Different Cooling Rates
| Cooling Rate (°C/s) | Fs (°C) | Ps (°C) | Bs (°C) | Bf (°C) | Ms (°C) |
|---|---|---|---|---|---|
| 0.2 | 747 | 684 | 649 | 600 | – |
| 0.5 | 732 | 676 | 595 | 496 | – |
| 1.0 | 719 | 673 | 600 | 497 | – |
| 5.0 | 710 | 668 | 526 | – | – |
| 10 | – | – | 555 | 465 | 380 |
| 20-50 | – | – | – | – | 406-422 |
At cooling rates below 5°C/s, the primary transformation products were ferrite, pearlite, and lath bainite, with the transformation temperatures shifting to lower values as the cooling rate increased. This shift was attributed to the diffusion-controlled nature of ferrite and pearlite transformations, which became increasingly hindered at higher cooling rates. Conversely, the martensitic transformations observed at cooling rates exceeding 10°C/s were not significantly affected by the cooling rate, likely due to their non-diffusional nature.
4. Conclusion
This study investigated the CCT curve and microstructural-hardness relationships of A487-4B low-alloy cast steel under various cooling rates using the Gleeble 1500D thermal simulation testing machine. The key findings are summarized below:
- At cooling rates of 0.2–5°C/s, the microstructures primarily comprised ferrite, pearlite, and lath bainite. With increasing cooling rates, the proportion of lath bainite rose, accompanied by a corresponding decrease in ferrite and pearlite.
- At a cooling rate of 10°C/s, the microstructure transformed into a mixture of lath bainite and lath martensite. Further increases in the cooling rate to 20–50°C/s resulted in a microstructure dominated by lath martensite and acicular martensite.
- The Vickers hardness increased with the cooling rate, reaching a maximum of HV473 at 50°C/s due to the higher martensite content. However, the severe quench hardenability necessitates the adoption of low heat input welding practices, preheating, and post-weld cooling to prevent embrittlement and cold cracking.
- The CCT curve of A487-4B cast steel was successfully determined, revealing critical transformation points of Ac1 = 747°C, Ac3 = 875°C, and Ms = 422°C. The phase transformation behaviors and resulting microstructures were thoroughly analyzed and discussed.
These findings provide valuable insights into the welding and heat treatment of A487-4B cast steel, facilitating the optimization of process parameters and the enhancement of material properties.