In the production of large steel castings, achieving high-quality surface finish and minimizing defects such as surface flow marks and shrinkage porosity in the riser are critical challenges. Our experience in manufacturing complex steel castings has shown that these issues significantly impact product quality, repair costs, and overall production efficiency. This article explores the influence of pouring temperature and pouring methods on the formation of defects in large steel castings, based on extensive historical data analysis and experimental investigations. The primary focus is on optimizing these parameters to enhance the fluidity of molten steel during mold filling and improve feeding efficiency in the riser, thereby reducing defects like water marks and shrinkage cavities. Through systematic experiments, we have identified optimal ranges for pouring temperature and refined pouring techniques, leading to substantial improvements in product quality and cost reduction in steel casting operations.
The manufacturing of large steel castings involves intricate processes where even minor deviations can lead to significant defects. In our facility, one particular large steel casting component, characterized by its complex geometry and multiple machined surfaces, frequently exhibited surface flow marks and shrinkage porosity in the top riser after pouring. These defects not only necessitated extensive rework, increasing labor intensity and production costs, but also resulted in scrap rates that adversely affected operational efficiency. Statistical analysis of production data from recent years revealed an average shrinkage porosity rate of approximately 66.48%, highlighting the severity of the issue. This high defect rate underscores the need for a detailed examination of the pouring process parameters in steel casting.
To understand the root causes, we analyzed the existing pouring process, which specified a starting pouring temperature range of 1555°C to 1565°C and a bottom-pouring method with a mold filling time of 65-85 seconds per casting. However, this approach often led to poor fluidity of the molten steel as it reached the upper surfaces, causing surface flow marks. Additionally, the original pouring method, which involved fully opening the nozzle at the start and then performing intermittent point pouring after the mold was filled to the riser top, failed to provide adequate feeding for solidification shrinkage, resulting in shrinkage porosity. The relationship between temperature and fluidity can be described using a simplified model where the fluidity index $F$ is proportional to the temperature difference above the liquidus temperature $T_l$: $$F = k \cdot (T – T_l)^n$$ where $k$ is a constant dependent on composition, $T$ is the pouring temperature, and $n$ is an exponent typically between 1 and 2 for steel castings. At lower temperatures, $F$ decreases, leading to incomplete filling and surface defects.
| Production Period | Number of Castings Produced | Number with Shrinkage Porosity | Shrinkage Porosity Rate (%) |
|---|---|---|---|
| 2022 | 550 | 367 | 66.73 |
| 2023 | 475 | 312 | 65.68 |
| 2024 (Jan-Jun) | 237 | 160 | 67.51 |
| Total | 1262 | 839 | 66.48 |
The defects in steel casting are primarily attributed to inadequate fluidity and improper solidification feeding. Surface flow marks occur when the molten steel loses momentum and forms ripples due to reduced viscosity and surface tension effects at lower temperatures. The critical temperature for avoiding these marks can be derived from empirical observations, where a minimum pouring temperature $T_{min}$ is required to maintain sufficient superheat. For instance, if $T_l$ is the liquidus temperature of the steel alloy, then $T_{min} = T_l + \Delta T_{superheat}$, with $\Delta T_{superheat}$ typically ranging from 20°C to 50°C for large castings. In our case, the original temperature range was often below this threshold, leading to frequent defects. Shrinkage porosity, on the other hand, arises from volumetric contraction during solidification, which is exacerbated by insufficient feeding from the riser. The feeding efficiency $\eta_f$ can be modeled as $\eta_f = \frac{V_f}{V_s}$, where $V_f$ is the volume of feed metal supplied and $V_s$ is the shrinkage volume. If $\eta_f < 1$, porosity forms, and optimizing the pouring method is essential to maximize $V_f$.
To address these issues, we designed a series of experiments to evaluate the effects of pouring temperature and pouring method on defect formation in steel castings. The experiments involved varying the starting pouring temperature and modifying the pouring technique, with each condition tested on multiple castings to ensure statistical reliability. The pouring method was adjusted from the original intermittent point pouring to a continuous细流补浇 (fine-stream feeding) approach when the molten steel reached one-quarter of the riser height. This change aimed to enhance feeding by maintaining a liquid channel for longer durations, reducing the risk of premature solidification.
| Experiment Scheme | Starting Pouring Temperature (°C) | Pouring Method | Number of Castings | Observations |
|---|---|---|---|---|
| Scheme A | 1555 | Original method: full nozzle opening, then point pouring after filling to riser top | 2 | Severe surface flow marks and shrinkage porosity in both castings |
| Scheme A | 1555 | Modified method: full opening initially, then fine-stream feeding at 1/4 riser height to top | 2 | Surface flow marks in first casting, minor shrinkage; severe defects in second casting |
| Scheme B | 1565 | Original method | 2 | No surface marks in first casting, minor shrinkage; slight marks and shrinkage in second |
| Scheme B | 1565 | Modified method | 2 | No surface marks or shrinkage in first casting; slight marks but no shrinkage in second |
| Scheme C | 1570 | Modified method | 2 | No surface flow marks or shrinkage porosity in either casting |
| Scheme C | 1575 | Modified method | 2 | No defects observed in both castings |
| Scheme C | 1580 | Modified method | 2 | No surface marks or shrinkage, but minor cracks in first casting; no issues in second |
| Scheme C | 1585 | Modified method | 2 | No surface or shrinkage defects, but significant increase in localized cracks |
The results from these experiments demonstrated a clear correlation between pouring temperature and defect occurrence in steel casting. At starting pouring temperatures below 1565°C, surface flow marks and shrinkage porosity were prevalent, consistent with the fluidity model where lower temperatures reduce $F$. As the temperature increased to 1570°C and above, surface flow marks were eliminated, indicating that the superheat was sufficient to maintain fluidity throughout the mold filling process. However, when temperatures exceeded 1580°C, we observed an increased tendency for cracking, which can be attributed to higher thermal stresses and reduced ductility at elevated temperatures. The risk of cracking $R_c$ can be approximated by $R_c = \alpha \cdot (T – T_{crit})^m$, where $\alpha$ is a material constant, $T_{crit}$ is a critical temperature threshold, and $m$ is an exponent. For our steel casting, $T_{crit}$ appears to be around 1580°C, beyond which $R_c$ rises sharply.
In terms of pouring method, the modified approach involving fine-stream feeding at the riser significantly improved feeding efficiency. By switching to a continuous细流补浇 when the molten steel reached one-quarter of the riser height, we ensured a steady supply of liquid metal to compensate for solidification shrinkage. This method enhances $\eta_f$ by preventing the formation of a solidified skin that blocks feeding paths. The optimal pouring sequence can be summarized as: initially open the nozzle fully to achieve rapid mold filling, then transition to a fine, continuous stream when the steel level reaches 25% of the riser height, and continue until the mold is filled to the top. This technique aligns with principles of directional solidification, where the riser remains liquid longest to feed the casting adequately.
Further supporting measures were implemented to stabilize the steel casting process. For instance, we ensured that the ladle was thoroughly preheated to a bright red color before tapping, minimizing heat loss and maintaining temperature consistency. Additionally, the tap weight was controlled to exceed the planned amount by at least 100 kg, guaranteeing sufficient molten steel for complete filling and feeding. These steps are crucial in large steel casting operations, where small variations in temperature or volume can lead to defects. The overall improvement in process parameters is summarized in the table below, comparing the original and optimized conditions for steel casting.
| Parameter | Original Process | Optimized Process |
|---|---|---|
| Starting Pouring Temperature (°C) | 1555 – 1565 | 1565 – 1580 |
| Pouring Method | Full nozzle opening initially, then point pouring after filling to riser top | Full opening initially, then fine-stream feeding at 1/4 riser height to top |
| Typical Mold Filling Time (s) | 65 – 85 | 56 – 75 |
| Defect Incidence | High rates of surface flow marks and shrinkage porosity | Minimal defects, with occasional issues only under insufficient steel conditions |
Since implementing the optimized process, we have produced 268 units of this large steel casting, with only three instances of shrinkage porosity due to inadequate steel volume, resulting in a defect rate of 1.12%. This represents a dramatic improvement over the previous average of 66.48%, demonstrating the effectiveness of the adjustments. The reduction in rework and scrap has not only lowered production costs but also enhanced operational efficiency in our steel casting facility. The relationship between pouring temperature and defect formation can be further analyzed using statistical models. For example, a regression analysis of our data suggests that the probability of surface flow marks $P_{sf}$ decreases exponentially with temperature: $$P_{sf} = e^{-\beta (T – T_0)}$$ where $\beta$ is a constant and $T_0$ is a reference temperature, approximately 1560°C in our case. Similarly, the probability of shrinkage porosity $P_{sp}$ is influenced by both temperature and pouring method, with the modified method reducing $P_{sp}$ significantly.
In conclusion, the optimization of pouring parameters is essential for high-quality steel casting production. Our findings indicate that a starting pouring temperature above 1565°C markedly improves surface quality, with temperatures of 1570°C and above eliminating surface flow marks entirely. However, temperatures exceeding 1580°C increase the risk of cracking, necessitating careful control. The refined pouring method, involving fine-stream feeding at the riser, effectively prevents shrinkage porosity by ensuring continuous feeding during solidification. These insights, combined with auxiliary measures like ladle preheating and adequate steel volume, have transformed our steel casting process, leading to superior product integrity and cost savings. Future work could involve computational modeling to predict fluidity and solidification patterns, further refining the process for various steel casting applications.
The principles discussed here are broadly applicable to other large steel castings, where similar defects may arise. By prioritizing temperature management and pouring techniques, manufacturers can achieve consistent quality in steel casting operations. The continuous pursuit of process optimization, supported by data-driven analysis, is key to advancing the field of steel casting and meeting the demands of complex industrial components.