In my work as a materials engineer, I have long been fascinated by the challenge of developing steel castings that can endure harsh environmental conditions. Atmospheric corrosion, driven by factors such as temperature fluctuations, humidity, and chemical pollutants like oxygen, carbon dioxide, and sulfur dioxide, is a primary cause of degradation in metal structures exposed to the elements. While weathering steels produced via continuous casting and rolling are widely used, there is a significant gap for steel castings with complex geometries or poor weldability. These steel castings require tailored compositions and heat treatments to achieve both excellent corrosion resistance and robust mechanical properties, particularly at low temperatures. This article details my research and production experiences in creating three novel low-temperature atmospheric corrosion-resistant steel castings, leveraging computational simulation to optimize their performance.
The development of these steel castings began with a thorough analysis of different atmospheric environments: rural, urban, industrial, and marine. Each environment presents unique corrosion mechanisms. For instance, industrial atmospheres contain sulfur dioxide that forms acidic electrolytes on metal surfaces, accelerating corrosion, while marine atmospheres feature chloride ions that promote pitting. Traditional weathering steels often rely on phosphorus for corrosion resistance, but high phosphorus content can induce low-temperature brittleness and welding issues in steel castings. To overcome this, my team designed three new steel casting alloys—designated as Alloy A, Alloy B, and Alloy C—with balanced additions of chromium, nickel, copper, and controlled low levels of carbon, phosphorus, and sulfur. The chemical compositions are summarized in Table 1.
| Steel Casting Designation | C | Si | Mn | P | S | Cr | Ni | Cu |
|---|---|---|---|---|---|---|---|---|
| Alloy A (Similar to ZGS10CrNiCu) | 0.07–0.12 | ≤0.60 | 0.60–1.40 | ≤0.015 | ≤0.015 | 0.80–1.20 | 0.50–1.00 | 0.30–0.50 |
| Alloy B (Similar to ZGS10Cr3NiCu) | 0.07–0.12 | ≤0.60 | 0.60–1.40 | ≤0.015 | ≤0.015 | 2.50–3.50 | 0.40–0.60 | 0.30–0.50 |
| Alloy C (Similar to ZGS10Cr5NiCu) | 0.07–0.12 | ≤0.60 | 0.60–1.40 | ≤0.015 | ≤0.015 | 4.50–5.50 | 0.40–0.60 | 0.30–0.50 |
These compositions aim to achieve a dense, protective rust layer through alloying element synergies. Alloy C, with the highest chromium content, is designed for bare use in all atmospheric environments, including marine. Alloy B is suitable for bare use in rural, urban, and industrial atmospheres but requires coating in marine settings. Alloy A, with lower alloy content, is intended for coated use in rural, urban, and industrial atmospheres. The low carbon and impurity levels are critical for enhancing toughness and weldability in these steel castings.
The production of these steel castings involved meticulous foundry practices. We employed a 5-ton electric arc furnace for melting to achieve precise control over composition, especially for low carbon, phosphorus, and sulfur contents. Given the high chromium content in Alloys B and C, which increases gas absorption tendency, a combination of diffusion and precipitation deoxidation was used. Aluminum was added for final deoxidation to promote fine grain structure. After pouring, the steel castings were allowed to cool slowly in molds to prevent cracking, and risers were removed using thermal cutting to avoid stress concentrations. The complex shapes of these steel castings necessitated such careful handling to ensure integrity.
Heat treatment is paramount for optimizing the microstructure and properties of steel castings, especially with high alloy content that can lead to segregation. We utilized JMatPro software to simulate phase transformations and determine optimal heat treatment parameters. For Alloy A, with moderate alloy content (around 4%), normalizing was selected to achieve a fine ferrite-pearlite structure. The simulation provided the Ac3 temperature, which is the point where austenite transformation completes during heating. Using JMatPro, the Ac3 for Alloy A was predicted to be 801°C. The normalizing temperature was set using the formula: $$T_{\text{normalizing}} = Ac_3 + \Delta T$$ where $\Delta T$ is 90°C for fine-grain steel, resulting in $T_{\text{normalizing}} = 801^\circ\text{C} + 90^\circ\text{C} = 891^\circ\text{C}$. Cooling rate simulations from CCT curves indicated that a cooling speed ≤0.1°C/s (360°C/h) was needed to complete pearlitic transformation. In production, we controlled cooling by furnace cooling to 500°C at 100°C/h, followed by air cooling to prevent cracking in thick-section steel castings.
For Alloys B and C, with higher alloy content (around 5.3% and 7.2%, respectively), quenching and tempering were necessary to eliminate segregation and achieve a tempered sorbite structure for balanced strength and toughness. JMatPro simulations gave Ac3 temperatures of 791°C for Alloy B and 781°C for Alloy C. To ensure homogenization, the quenching temperature was set to 900°C for both, with heating rates of 180°C/h to avoid distortion. The保温 time was calculated based on section thickness: 1 hour per 25 mm. Cooling media selection was critical; simulations from CCT curves showed that to avoid bainite formation, cooling rates needed to exceed 10°C/s. Given the thickness of our steel castings (up to 80 mm), we chose NaCl aqueous solution as the quenching medium to achieve sufficient hardenability. The tempering temperature was set to 680°C based on JMatPro predictions to obtain optimal mechanical properties without approaching the pearlite formation range.
The mechanical performance of these steel castings was evaluated against the standard for ZG270-500, a common cast steel for structural applications like bridge bearings. Our target was to exceed its properties, especially in low-temperature impact toughness. The simulated and actual mechanical properties are summarized in Table 2 and Table 3. JMatPro predicted yield and tensile strengths for Alloy A as 314 MPa and 488 MPa, respectively, which aligned closely with experimental results. For Alloys B and C, simulations indicated strengths above 500 MPa after tempering, but actual tests showed a good balance with toughness.
| Property | Yield Strength (MPa) | Tensile Strength (MPa) | Elongation (%) | Reduction of Area (%) | Impact Energy at -40°C (J) |
|---|---|---|---|---|---|
| Average Value | 332 | 538 | 28.5 | 66.5 | 36.5 |
| Standard Deviation | ±20 | ±30 | ±2.5 | ±10 | ±10 |
| Steel Casting | Yield Strength (MPa) | Tensile Strength (MPa) | Elongation (%) | Reduction of Area (%) | Impact Energy at -40°C (J) |
|---|---|---|---|---|---|
| Alloy B | 452 | 583 | 25.5 | 71.0 | 38.0 |
| Alloy C | 477 | 589 | 23.5 | 64.5 | 54.5 |
All three steel castings met or surpassed the ZG270-500 requirements (yield strength ≥270 MPa, tensile strength ≥500 MPa, elongation ≥18%, impact energy ≥22 J at room temperature). Notably, the low-temperature impact energies at -40°C were exceptional, demonstrating the efficacy of our design for cryogenic applications. The properties can be related to composition via empirical formulas, such as the yield strength approximation for low-alloy steels: $$\sigma_y = \sigma_0 + \sum k_i \cdot C_i$$ where $\sigma_0$ is the base strength and $k_i$ are coefficients for elements like Cr, Ni, and Cu. Our alloys optimize these contributions to enhance both strength and toughness in steel castings.
Microstructural analysis revealed the success of our heat treatments. For Alloy A steel castings, normalizing produced a uniform mixture of fine ferrite and pearlite, as intended. For Alloy B and Alloy C steel castings, quenching and tempering resulted in a tempered sorbite structure with dispersed carbides, which imparts high strength and good ductility. The microstructure evolution during heat treatment can be described using phase transformation kinetics. For instance, the austenite decomposition during cooling follows the Johnson-Mehl-Avrami-Kolmogorov equation: $$X = 1 – \exp(-k t^n)$$ where $X$ is the transformed fraction, $k$ is a rate constant dependent on temperature, $t$ is time, and $n$ is an exponent. Our simulations helped tailor cooling rates to achieve desired microstructures in these steel castings.
Corrosion performance was assessed through atmospheric exposure tests, though detailed data is beyond this article’s scope. The high chromium content in Alloys B and C promotes the formation of a dense, adherent rust layer rich in Cr-oxides, which acts as a barrier against further corrosion. The corrosion rate $R$ in atmospheric environments can be modeled as: $$R = a \cdot \exp(b \cdot [\text{SO}_2]) + c \cdot [\text{Cl}^-]$$ where $a$, $b$, and $c$ are constants, and $[\text{SO}_2]$ and $[\text{Cl}^-]$ are pollutant concentrations. Our steel castings show reduced $R$ values due to alloying effects.
In conclusion, this research demonstrates a comprehensive approach to developing advanced steel castings for low-temperature and atmospheric corrosion resistance. By integrating computational tools like JMatPro with traditional foundry and heat treatment practices, we successfully produced three steel casting alloys with tailored compositions and microstructures. These steel castings exhibit superior mechanical properties, including excellent low-temperature toughness, and are suitable for various corrosive environments. The work highlights the potential of simulation-driven design in overcoming challenges in steel casting production, paving the way for more durable and reliable components in infrastructure and industrial applications. Future work will focus on long-term corrosion monitoring and further optimization of welding procedures for these steel castings.