In the field of power generation and industrial machinery, steam turbines play a critical role, and their components must withstand extreme operational conditions. Among these components, the cylinder is paramount, as it houses the rotor and contains high-pressure steam. The performance and safety of the turbine heavily depend on the material properties of the cylinder, particularly under elevated temperatures. This study focuses on ZG22Mo, a low-alloy steel casting commonly used for steam turbine cylinders in industrial applications. Steel castings like ZG22Mo are favored for their ability to be formed into complex shapes, offering design flexibility and cost-effectiveness. However, despite its widespread use, comprehensive data on the high-temperature tensile strength of ZG22Mo steel casting has been lacking in domestic technical manuals and standards. This gap poses challenges for designers aiming to comply with international standards such as API 612:2014, which specifies that the design stress for pressure-containing parts should not exceed 0.25 times the minimum tensile strength at the maximum operating temperature. Therefore, this research aims to systematically investigate the high-temperature mechanical properties of ZG22Mo steel casting through experimental testing and statistical analysis, providing reliable guaranteed values for engineering design.
The importance of steel castings in high-temperature applications cannot be overstated. Steel castings are integral to many industrial sectors due to their durability and adaptability. For steam turbine cylinders, the material must exhibit excellent creep resistance, thermal stability, and mechanical strength at temperatures ranging from ambient to over 500°C. ZG22Mo, as a cast steel, is designed to meet these demands with its composition of carbon, silicon, manganese, chromium, and molybdenum. Molybdenum, in particular, enhances high-temperature strength by promoting solid solution strengthening and carbide formation. This study delves into the behavior of ZG22Mo steel casting under thermal stress, contributing to the broader understanding of material performance in critical environments. By addressing the data deficiency, we support the advancement of steam turbine technology, ensuring that designs are both safe and efficient. The following sections detail the materials, methods, results, and analyses conducted in this investigation, with an emphasis on statistical rigor and practical applicability.
The manufacturing process of steel castings, including ZG22Mo, involves melting, pouring into molds, and heat treatment to achieve desired microstructures and properties. For this research, the test material was ZG22Mo steel casting, with its chemical composition specified in Table 1. The composition is crucial as it directly influences the mechanical behavior, especially at high temperatures. The casting process ensures homogeneity and integrity, but variations can occur, necessitating rigorous testing. The specimens used in this study were single-cast blocks with dimensions of 35 mm × 100 mm × 250 mm, fabricated alongside actual cylinder castings to simulate real production conditions. These specimens underwent heat treatment consisting of normalizing at temperatures not exceeding 960°C and tempering at a minimum of 680°C, which is standard for optimizing the toughness and strength of steel castings. Prior to testing, ultrasonic inspection was performed to detect any cracks or defects, ensuring the quality of the specimens. This step is vital for steel castings, as internal flaws can compromise test results and material reliability.
| Element | C | Si | Mn | Cr | Mo | P | S |
|---|---|---|---|---|---|---|---|
| Range | 0.18–0.23 | 0.30–0.60 | 0.50–0.80 | ≤0.30 | 0.35–0.45 | ≤0.030 | ≤0.030 |
The room-temperature mechanical properties of ZG22Mo steel casting are well-documented and serve as a baseline for comparison. As per standard requirements, the material should exhibit a tensile strength (Rm) between 440 and 590 MPa, a yield strength (Rp0.2) of at least 245 MPa, an elongation (A) of 22% or more, a reduction of area (Z) of 40% or more, and impact energy (KV2) of 27 J or higher. Table 2 summarizes these requirements, which were verified through initial tests on specimens from ten different heats. The consistency of room-temperature properties is essential for ensuring that the steel casting meets quality benchmarks before subjecting it to high-temperature evaluations. In this study, one specimen from each heat was tested at room temperature according to GB/T 228.1-2010, and the results confirmed compliance, as shown in Table 3. The fracture surfaces of these specimens displayed typical ductile characteristics, indicative of good material integrity. This preliminary validation is critical for any steel casting intended for high-temperature service, as it establishes a foundation for extrapolating behavior under thermal stress.
| Property | Tensile Strength (Rm), MPa | Yield Strength (Rp0.2), MPa | Elongation (A), % | Reduction of Area (Z), % | Impact Energy (KV2), J |
|---|---|---|---|---|---|
| Requirement | 440–590 | ≥245 | ≥22 | ≥40 | ≥27 |
| Specimen | Tensile Strength (Rm), MPa | Yield Strength (Rp0.2), MPa | Elongation (A), % | Reduction of Area (Z), % |
|---|---|---|---|---|
| 1 | 493 | 327 | 33 | 63 |
| 2 | 545 | 365 | 27 | 55 |
| 3 | 478 | 307 | 32.0 | 65 |
| 4 | 496 | 319 | 27.0 | 55 |
| 5 | 513 | 330 | 29.5 | 60 |
| 6 | 502 | 319 | 28.0 | 60 |
| 7 | 495 | 311 | 29.0 | 62 |
| 8 | 495 | 308 | 27.0 | 59 |
| 9 | 490 | 311 | 27.0 | 63 |
| 10 | 514 | 324 | 29.0 | 63 |
High-temperature tensile testing was conducted to evaluate the performance of ZG22Mo steel casting under operational conditions. The tests were performed at four temperature points: 300°C, 400°C, 450°C, and 500°C, with ten specimens tested at each temperature, corresponding to ten different heats to account for material variability. The testing procedure followed GB/T 228.2-2015, with specific rates: pre-load at 10 MPa, yield range at 0.3 mm/min, elastic modulus at 0.3 mm/min, test speed at 1.2 mm/min, and yield point at 0.3 mm/min. This standardized approach ensures reproducibility and accuracy, which is crucial for characterizing steel castings. The results, presented in Table 4, include tensile strength (Rm), yield strength (Rp0.2), elongation (A), and reduction of area (Z) at each temperature. The fracture surfaces from high-temperature tests showed variations compared to room-temperature specimens, often exhibiting more pronounced necking and dimple patterns, reflecting the material’s response to thermal softening. These data form the basis for statistical analysis to derive guaranteed values, as raw test results naturally scatter due to inherent uncertainties in steel casting processes.
| Temperature | Specimen | Tensile Strength (Rm), MPa | Yield Strength (Rp0.2), MPa | Elongation (A), % | Reduction of Area (Z), % |
|---|---|---|---|---|---|
| 300°C | 1 | 470 | 345 | 22.0 | 52.0 |
| 2 | 515 | 370 | 22.0 | 47.5 | |
| 3 | 455 | 330 | 25.5 | 60.5 | |
| 4 | 490 | 340 | 24.0 | 48.5 | |
| 5 | 495 | 360 | 22.5 | 53.0 | |
| 6 | 470 | 350 | 23.0 | 55.0 | |
| 7 | 460 | 345 | 23.5 | 54.0 | |
| 8 | 465 | 330 | 19.5 | 40.5 | |
| 9 | 465 | 335 | 24.0 | 54.0 | |
| 10 | 490 | 355 | 22.0 | 41.5 | |
| 400°C | 1 | 440 | 230 | 32.0 | 69.0 |
| 2 | 470 | 275 | 28.0 | 63.5 | |
| 3 | 415 | 225 | 30.5 | 68.0 | |
| 4 | 435 | 245 | 28.5 | 68.5 | |
| 5 | 455 | 245 | 28.5 | 66.5 | |
| 6 | 435 | 250 | 36.0 | 67.5 | |
| 7 | 425 | 240 | 29.5 | 55.0 | |
| 8 | 430 | 240 | 31.5 | 59.5 | |
| 9 | 425 | 240 | 33.5 | 68.0 | |
| 10 | 445 | 260 | 36.0 | 65.5 | |
| 450°C | 1 | 395 | 220 | 25.0 | 73.5 |
| 2 | 435 | 270 | 31.0 | 73.5 | |
| 3 | 385 | 215 | 31.5 | 74.5 | |
| 4 | 390 | 235 | 35.5 | 72.5 | |
| 5 | 415 | 235 | 34.0 | 76.0 | |
| 6 | 390 | 240 | 29.5 | 68.0 | |
| 7 | 385 | 230 | 30.0 | 71.5 | |
| 8 | 385 | 230 | 31.0 | 70.0 | |
| 9 | 385 | 225 | 32.0 | 73.5 | |
| 10 | 410 | 230 | 32.5 | 69.0 | |
| 500°C | 1 | 345 | 200 | 34.0 | 78.0 |
| 2 | 370 | 280 | 31.0 | 74.0 | |
| 3 | 330 | 225 | 37.0 | 79.0 | |
| 4 | 340 | 220 | 32.0 | 72.0 | |
| 5 | 360 | 225 | 36.0 | 78.0 | |
| 6 | 350 | 230 | 33.5 | 73.5 | |
| 7 | 345 | 220 | 35.0 | 77.0 | |
| 8 | 330 | 225 | 30.5 | 75.5 | |
| 9 | 335 | 215 | 37.0 | 76.0 | |
| 10 | 355 | 235 | 36.0 | 75.5 |
To derive reliable guaranteed values from the test data, statistical analysis based on the normal distribution principle was employed. This approach is essential for steel castings, where material properties can vary due to casting defects, heat treatment inconsistencies, or compositional fluctuations. The process involves calculating the arithmetic mean and standard deviation of the test results, then applying the 3σ criterion to identify and remove outliers. For instance, consider the tensile strength data at 300°C for ZG22Mo steel casting. Let the ten measured values be denoted as \(X_1, X_2, \ldots, X_{10}\). The arithmetic mean \(\delta\) and standard deviation \(\sigma\) are computed as follows:
$$\delta = \frac{1}{n} \sum_{i=1}^{n} X_i$$
$$\sigma = \sqrt{\frac{1}{n} \sum_{i=1}^{n} (X_i – \delta)^2}$$
For the 300°C data: \(X_1 = 470\), \(X_2 = 515\), \(X_3 = 455\), \(X_4 = 490\), \(X_5 = 495\), \(X_6 = 470\), \(X_7 = 460\), \(X_8 = 465\), \(X_9 = 465\), \(X_{10} = 490\) MPa. The initial calculations yield:
$$\delta_1 = \frac{470 + 515 + 455 + 490 + 495 + 470 + 460 + 465 + 465 + 490}{10} = 477.5 \text{ MPa}$$
$$\sigma_1 = \sqrt{\frac{(470-477.5)^2 + (515-477.5)^2 + \ldots + (490-477.5)^2}{10}} = 18.1 \text{ MPa}$$
According to the 3σ criterion, values beyond \(\delta \pm 4\sigma\) are considered outliers and removed. Here, \(\delta_1 + 4\sigma_1 = 477.5 + 4 \times 18.1 = 549.9\) MPa and \(\delta_1 – 4\sigma_1 = 477.5 – 4 \times 18.1 = 405.1\) MPa. All values fall within this range, so none are removed. However, if outliers existed, they would be excluded, and the mean and standard deviation recalculated. The guaranteed value \(U\) is then determined using the formula:
$$U = \delta – \frac{U_p \cdot \sigma}{\sqrt{n}}$$
where \(U_p\) is the standard normal deviate corresponding to a desired confidence level. For a confidence level of 0.999, \(U_p = 3.09\). Substituting the values for 300°C:
$$U = 477.5 – \frac{3.09 \times 18.1}{\sqrt{10}} = 477.5 – 17.7 = 459.8 \approx 460 \text{ MPa}$$
This process was repeated for all mechanical properties at each temperature, resulting in the guaranteed values summarized in Table 5. The use of normal distribution ensures that the derived values account for statistical variability, providing a conservative estimate suitable for engineering design. This methodology is particularly relevant for steel castings, where safety factors must accommodate potential material inconsistencies. By applying these statistical techniques, we enhance the reliability of data for critical applications like steam turbine cylinders.
| Temperature | Tensile Strength (Rm), MPa | Yield Strength (Rp0.2), MPa | Elongation (A), % | Reduction of Area (Z), % |
|---|---|---|---|---|
| Room Temperature | 486 | 309 | 26.9 | 57.3 |
| 300°C | 460 | 232 | 22.0 | 44.9 |
| 400°C | 423 | 221 | 28.6 | 60.9 |
| 450°C | 382 | 218 | 30.0 | 69.84 |
| 500°C | 334 | 206 | 32.0 | 73.8 |
The results indicate a clear trend of decreasing tensile and yield strengths with increasing temperature, which is typical for steel castings due to thermal activation of dislocation motion and microstructural changes. For ZG22Mo steel casting, the tensile strength drops from 486 MPa at room temperature to 334 MPa at 500°C, while yield strength declines from 309 MPa to 206 MPa over the same range. In contrast, ductility metrics like elongation and reduction of area generally improve at higher temperatures, reflecting enhanced plasticity. This behavior is crucial for designers, as it highlights the need to balance strength and ductility in high-temperature environments. The guaranteed values provide a safety margin, ensuring that steam turbine cylinders operate within allowable stresses as per API 612. For example, at 500°C, the minimum tensile strength is 334 MPa, so the design stress should not exceed \(0.25 \times 334 = 83.5\) MPa. This conservative approach mitigates risks associated with material degradation over time.
Steel castings offer unique advantages in high-temperature applications, but their performance must be thoroughly characterized. The study of ZG22Mo steel casting reveals that its high-temperature properties are satisfactory for steam turbine cylinders up to 500°C. The molybdenum content plays a key role in maintaining strength at elevated temperatures by forming stable carbides and reducing temper embrittlement. Compared to other cast steels, ZG22Mo provides a good combination of cost, manufacturability, and performance. However, designers should consider factors like creep and fatigue, which were not covered in this tensile study. Future research could explore long-term exposure tests to assess creep resistance, as well as impact toughness at high temperatures. Additionally, advancements in steel casting technology, such as improved melting practices and heat treatment controls, could further enhance the consistency of ZG22Mo properties. By integrating statistical analysis with experimental data, this work contributes to the database for steel castings, supporting innovation in turbine design and maintenance.
In conclusion, this research systematically investigated the high-temperature tensile properties of ZG22Mo steel casting for steam turbine cylinders. Through extensive testing at 300°C, 400°C, 450°C, and 500°C, and application of normal distribution principles, we derived guaranteed values for tensile strength, yield strength, elongation, and reduction of area. These values fill a critical gap in material data, enabling designers to comply with API 612 standards and ensure safe operation. The methodology underscores the importance of statistical rigor in evaluating steel castings, where variability is inherent. The findings demonstrate that ZG22Mo steel casting retains adequate strength at high temperatures, making it a viable choice for industrial steam turbines. As demand for efficient and reliable power generation grows, continued research into steel castings will be essential for pushing the boundaries of performance and sustainability. This study serves as a foundation for further exploration into the behavior of cast materials under thermal and mechanical loads, ultimately contributing to safer and more advanced engineering solutions.