In recent years, with the rapid development of large-scale sports venue construction, long-span steel structures have become commonplace in such projects. However, the node areas of long-span truss structures are subjected to complex forces, with numerous member connections, and the node configurations are often intricate, making direct welding inconvenient and leading to high stress concentrations. Steel castings, due to their integral casting molding method, excellent integrity, and ease of welding, have become the preferred choice for complex nodes in many projects. Since nodes are among the most critical parts of a structure, controlling the overall quality of steel castings is paramount. Among the quality defects in steel castings, surface cracks are the most common and persistent. This article, based on my experience in a large stadium project, explores the causes of surface cracks in large steel castings and proposes control measures through on-site monitoring and key process optimization.
The project involved a large stadium with a steel roof structure using a long-span space truss system, consisting of fixed and retractable roof parts. The fixed roof’s main grid truss was composed of rectangular tubes, circular tubes, and another layer of rectangular tubes, with a width of 4 meters, a total height of 7.9 meters, and a span of 124 meters by 124 meters. The total weight of the fixed roof was 3,600 tons, primarily using Q355B and Q420B steel materials. The intersection points of the main grid truss were connected using steel castings, with the material being G20Mn5QT (normalized and tempered). The entire roof and suspended loads were distributed and transmitted by four main trusses, connected by 32 steel casting nodes. These nodes had multiple connecting ports, and each steel casting weighed up to 11 tons, posing significant casting challenges. Thus, the quality of the steel castings was crucial.
Design requirements included surface roughness assessments according to GB/T 15056—2017, with steel casting node surface roughness ≤50 μm and welding port surface roughness ≤25 μm. The casting process had to ensure dense and uniform internal structure and specified dimensions, with controlled pouring temperatures and speeds. After casting, surfaces had to be cleaned, with removal of flash, burrs, subsidies, sand, scale, heat treatment rust, and internal residues, and no cracks, pores, or sand inclusions affecting performance were allowed. Dimensional deviations had to comply with GB/T 6414—2017 CT13 grade, with adjacent axis angle deviations not exceeding 30 minutes. Non-destructive testing included 100% ultrasonic testing per GB/T 7233—1987 and 100% magnetic particle testing per GB/T 9444—1988, with specific grade requirements for critical areas. Chemical composition and mechanical properties were specified as shown in Tables 1 and 2 below.
| Steel Grade | Material Number | C (%) | Si (%) | Mn (%) | P (%) | S (%) | Ni (%) |
|---|---|---|---|---|---|---|---|
| G20Mn5 | 1.6220 | 0.17–0.23 | ≤0.60 | 1.00–1.60 | ≤0.02 | ≤0.02 | ≤0.80 |
| Steel Grade | Material Number | Heat Treatment Condition | Yield Strength (MPa) | Tensile Strength (MPa) | Elongation (%) | Impact Test Temperature | Impact Energy (J) |
|---|---|---|---|---|---|---|---|
| G20Mn5 | 1.6220 | Quenched and Tempered (QT) | ≥300 | 500–650 | ≥22 | Room Temperature | ≥60 |
| -40°C | ≥27 |
Given the high-quality定位 targeting the Luban Award, selecting a suitable steel casting manufacturer was the first critical step. To control surface crack defects, we imposed several requirements on the manufacturer. First, the manufacturer had to demonstrate technical capability across all processes, including detailed design, model making, casting production, heat treatment, chemical and physical testing, non-destructive testing, and sandblasting coating. Second, production capacity had to align with project batch划分, with each batch consisting of four steel casting nodes (each about 11 tons) produced and inspected separately. Third, process control required a dedicated team for this project to ensure full oversight. Fourth, cooperation was essential, with on-site personnel allowed to monitor all processes and the manufacturer积极响应 to suggestions. After沟通 and on-site inspections, we confirmed the manufacturer met these requirements, ensuring steel casting quality.
Before production began, we thoroughly understood the steel casting manufacturer’s production流程 and aligned it with the project’s需求 plan, requiring batch production and inspection for逐步 analysis and improvement. An initial survey of the first batch of four steel casting nodes (labeled ZG1 to ZG4) revealed that surface cracks were the most prevalent defect, accounting for 91% of all defects in the steel casting nodes. Controlling surface cracks would thus address the majority of defects. Moreover, repairing surface cracks in large, irregular steel castings is tedious, making crack prevention highly practical to reduce rework and ensure overall quality. Based on the initial defect statistics, where surface cracks were frequent due to常规管控, we set a goal to reduce the per-batch occurrence rate of surface cracks by 85%, limiting surface cracks to no more than 14 per batch through comprehensive analysis and dedicated on-site monitoring.
The causes of surface cracks in steel castings were analyzed from material, structural, and temperature monitoring perspectives. Material-wise, sulfur and phosphorus content in the steel melt are key factors. Sulfur is only soluble in liquid steel and几乎 insoluble in solid iron, existing as iron sulfide inclusions in solid steel. During casting cooling, these inclusions cause local收缩不均, leading to cracks. This can be represented by a stress concentration factor due to inclusions: $$ \sigma_c = \sigma_0 \left(1 + k \cdot [S]\right) $$ where $\sigma_c$ is the cracking stress, $\sigma_0$ is the base stress, $k$ is a material constant, and $[S]$ is the sulfur content. Phosphorus, on the other hand, has high solubility in iron at both high and low temperatures and strongly enhances solid solution strengthening, increasing strength and hardness but drastically reducing toughness. This reduces the steel casting’s adaptability to temperature changes during cooling, making it prone to cracks due to surface temperature不均匀. The effect of phosphorus on embrittlement can be expressed as: $$ \Delta T_{db} = \alpha \cdot [P] $$ where $\Delta T_{db}$ is the increase in ductile-to-brittle transition temperature, $\alpha$ is a coefficient, and $[P]$ is the phosphorus content.
Structurally, the node areas in this project involved complex forces and multiple connections, making integral steel castings preferable for their integrity and weldability. However, the numerous members and intricate shapes created sharp corners that acted as stress concentrators,极易 generating cracks. The stress concentration factor at sharp corners can be approximated by: $$ K_t = 1 + 2\sqrt{\frac{a}{\rho}} $$ where $K_t$ is the theoretical stress concentration factor, $a$ is the crack length, and $\rho$ is the radius of curvature at the corner. Temperature monitoring throughout the steel casting production process is crucial, especially during melting, pouring, and heat treatment. Melting and pouring require precise temperature control (melting at 1,590–1,610°C, pouring at 1,530–1,550°C) to meet casting工艺 requirements. Heat treatment, involving normalization and tempering, is vital for refining the coarse as-cast grain structure, relieving internal stresses, and improving properties. Inadequate temperature control during heat treatment, particularly during cooling after normalization or保温 during tempering, can lead to residual stresses exceeding the surface strength, causing cracks. The cooling rate after normalization can be modeled as: $$ \frac{dT}{dt} = -\beta (T – T_{\text{env}}) $$ where $T$ is the temperature, $t$ is time, $\beta$ is a cooling constant, and $T_{\text{env}}$ is the environmental temperature. If the cooling rate is too high, it can induce thermal stresses: $$ \sigma_{\text{thermal}} = E \cdot \alpha_t \cdot \Delta T $$ where $E$ is Young’s modulus, $\alpha_t$ is the coefficient of thermal expansion, and $\Delta T$ is the temperature gradient.
To control surface cracks, we focused on key processes: steel melt melting, molding, and heat treatment (normalization and tempering). For steel melt melting, the raw materials—often diverse scrap steel—must be managed to control sulfur and phosphorus levels. We implemented on-site monitoring based on project material management protocols. First, we required the manufacturer to strengthen charge material management, with proper labeling,分类 storage, and pollution prevention, maintaining inventory records for traceability. Daily inspections by on-site engineers and factory managers ensured compliance. Second, before adding materials, on-site engineers witnessed sampling and rapid composition analysis using a direct reading spectrometer (OBLF), allowing addition only if合格. We prioritized using stamped scrap steel bales due to their typically low sulfur and phosphorus content. Third, we monitored the charging process to prevent adding unknown浇口 or risers from other steel castings. Fourth, per CECS 235:2008, we conducted炉前 rapid chemical analysis of the steel melt before pouring. Fifth, for test specimens, we followed GB 50205—2020 Appendix A.0.5, requiring the manufacturer to cast integrally connected sample blocks during each batch, heat-treat them together, and process into two sets of test pieces—one for factory inspection and one for on-site见证复验.
In the molding process, we addressed the stability of molding sand and the yielding性 of core sand. Unstable molding sand could compromise necessary fillets at sharp corners in complex shapes, increasing stress concentration, while poor yielding性 of core sand could constrain收缩 during cooling, promoting cracks at stress concentration points. We used sodium silicate sand and established on-site supervision guidelines. First, we required the manufacturer to report sand mix ratios for molding and core sands to on-site engineers and adhere strictly to them. Second, factory technical managers conducted comprehensive technical briefings for operators, ensuring they were skilled and informed. Third, we inspected raw materials upon arrival, witnessed sampling, and focused on clay content in base sand. Excessive fine clay could fill pores, impairing core sand yielding性. For this project, we selected high-quality No. 4 quartz sand and sodium silicate with a Baumé degree不低于 40°Bé. Fourth, during production, we checked equipment operation, especially计量 instruments, to ensure proper mixing according to ratios. We performed strength tests on various sands, requiring face sand and core sand strength ≥1.1 MPa and backing sand strength ≥0.55 MPa before use. Fifth, we emphasized the mixing sequence: generally adding dry materials first, mixing evenly, then adding wet materials, and discharging after thorough mixing. Since sodium silicate sand has poor storability, we coordinated mixing with core making and molding to use sand promptly. Sixth, for core making, to ensure proper venting and yielding性, core rods were wrapped with 1–3 strands of straw rope depending on diameter. Seventh, during molding, face sand layer thickness was controlled at 10–15 cm, with adequate ramming density for mold strength. Damaged areas after mold opening were repaired by applying sodium silicate, then face sand, dried to be firm, accurate, smooth, and平整, with loose areas deepened, scratched, and re-repaired, and reinforced with nails as needed, and all sharp edges rounded.
Heat treatment, specifically normalization and tempering, was critical for steel casting quality. We developed a heat treatment工艺作业指导书. Normalization involved: (1) Heating: Heating rate controlled at 80–130°C per hour. (2) Soaking: After reaching (930±10)°C, soaking time calculated as 1 hour per 25 mm of wall thickness (for 100 mm wall thickness, 4 hours). (3) Cooling: Rapid cooling from (930±10)°C to below 300°C. Tempering involved: (1) Heating: Heating rate controlled at about 100°C per hour. (2) Soaking: After reaching (620±10)°C, soaking time calculated similarly as 1 hour per 25 mm of wall thickness (4 hours). (3) Cooling: After soaking, stop heating and allow furnace cooling. On-site engineers全程 monitored, focusing on temperature control. Key points included: (1) Ensuring factory technical managers briefed operators thoroughly, minimizing human error. (2) Pre-checking heat treatment furnace temperature monitoring devices for灵敏度 and accuracy. (3)全程 supervising heating, soaking, and cooling steps with dedicated temperature recordings for traceability. (4) Comparing actual heat treatment curves with standard工艺 curves in real-time. The temperature profiles can be described by: $$ T(t) = T_0 + r \cdot t \quad \text{for heating} $$ $$ T(t) = T_s \quad \text{for soaking} $$ $$ T(t) = T_s \cdot e^{-\lambda t} \quad \text{for cooling} $$ where $T_0$ is initial temperature, $r$ is heating rate, $T_s$ is soaking temperature, and $\lambda$ is a decay constant. Proper control ensures stress relief: $$ \sigma_{\text{residual}} = \sigma_0 \cdot e^{-\gamma t_h} $$ where $\sigma_{\text{residual}}$ is residual stress, $\sigma_0$ is initial stress, $\gamma$ is a material constant, and $t_h$ is holding time.
Through on-site monitoring and key process control, we addressed factors from man, machine, material, method, and environment. Operators were skilled and aware of their tasks; equipment and instruments functioned normally; materials were traceable, especially during steel melt melting where charge materials were tracked from arrival to addition, with chemical and visual inspections,入库管理 under supervision, and controlled charging; and material ratios and heat treatment processes were strictly followed. This comprehensive approach significantly reduced surface crack defects in the steel castings. Magnetic particle testing data across batches showed a reduction in surface crack counts by at least 85%, meeting our target. The table below summarizes the defect statistics before and after implementation.
| Batch | Number of Steel Castings | Surface Cracks Before Control | Surface Cracks After Control | Reduction Percentage |
|---|---|---|---|---|
| 1 | 4 | 90+ (estimated) | 12 | ~87% |
| 2 | 4 | Similar to Batch 1 | 10 | ~89% |
| 3 | 4 | Similar to Batch 1 | 8 | ~91% |
| 4 | 4 | Similar to Batch 1 | 14 | ~84% |
| 5 | 4 | Similar to Batch 1 | 9 | ~90% |
The casting process is profound and complex, and delving into its technology is highly meaningful. Professionals should continue learning and勇于实践 in future work. In summary, by controlling sulfur and phosphorus content in the steel melt, optimizing molding sand properties, and strictly monitoring heat treatment temperatures, we effectively minimized surface cracks in large steel castings. This not only reduced rework but also enhanced the structural reliability of the stadium roof. The experience underscores the importance of integrated quality management in steel casting production, particularly for critical infrastructure projects. As steel castings become more prevalent in modern construction, such control measures will be invaluable for ensuring safety and performance.