Reduce Material Thickness and Improve Reliability of Necked Steel Drum Packaging Ma Yuming [Abstract] The author discusses the main causes of steel drum failures from the perspectives of metal materials science, drum structure, technology, and process, supported by extensive production experience and detailed data. The primary reason for these issues is the use of necking technology to reduce material thickness while improving the reliability of steel drums. [Keywords] steel barrel necking technology There are multiple approaches to enhance reliability, such as increasing material thickness, replacing low-grade materials with high-grade ones, or simply upgrading old products. However, this article focuses on a technique that reduces material thickness while ensuring or even improving reliability. This method has been successfully applied in production practices. For example, in the aerospace industry, titanium alloys are often used to reduce weight by creating a "honeycomb" structure inside parts. This approach decreases material thickness, adds reinforcing ribs, maintains or enhances strength, and improves overall reliability. Similar techniques like cast ribs and cold stamping embossing are widely used in mechanical manufacturing, showcasing the effectiveness of reducing material thickness while increasing strength and reliability. In the steel drum industry, could this method be adopted if the material is thin and the shape is simple? Steel drums are commonly used to transport and sell dangerous goods, and their reliability directly determines the safety of the contents. Under similar conditions, closed or medium-open steel drums generally have higher reliability than straight-open ones, resulting in better safety. Reliability is a key quality indicator for steel drums, and drop tests are typically used for evaluation. In closed or medium-open drums, the most vulnerable area is not immediately visible. The lid and bottom are joined to form an integrated structure, whereas in straight-open drums, the base and body are connected, and the lid is secured with a hoop. When filled, the lid must remain tightly sealed, making the joint between the lid and the top of the drum the most critical point. Reducing the material thickness can exacerbate this issue. When a steel drum containing dangerous goods falls or is impacted, the lid and body are prone to collapse, even with a hoop in place. This can lead to serious safety incidents. Specifically, regardless of whether the material is thinned, the most dangerous part of a straight-open drum is located 180° opposite the impact point and about 150° on either side. According to national and international standards, this area experiences the most deformation and is therefore the most hazardous. The root cause lies in the strength and reliability of the three key components: the drum body, the lid, and the hoop. While the drum body is the strongest, the hoop is less so, and the lid is the weakest. When impacted, the drum deforms slightly first, followed by the hoop, which may become looser at the 180° opposite point. The lid, being more flexible, may then separate from the body, leading to collapse. On the sides of the impact point, the deformation varies, but the lid tends to deform the most, causing it to separate from the hoop. Therefore, ensuring that the deformation of the drum mouth remains consistent across all parts is crucial to prevent separation and accidents. Figure 1 illustrates this concept. Figure 1 So how can we develop a technology or process that ensures reduced material thickness while maintaining or enhancing the reliability of steel drums when they fall or are subjected to impact? The answer lies in the "necking" technique. By controlling the deformation in the circumferential direction and minimizing it in the axial direction, the reliability and safety of the drum can be significantly improved. Thus, the success of the "necking" technology is essential to achieving thinner materials without compromising reliability. "Necking," also known as "shrinking," refers to the process of reducing the diameter of the drum's mouth through rolling or other methods. This technique creates a difference between the original diameter (D) and the necked diameter (d), represented by C = D - d, as shown in Figure 2. Figure II C = D - d Through extensive testing, it was found that this difference causes greater deformation in the transition area, with the drum mouth and lid experiencing more circumferential deformation and less axial deformation, ensuring that both parts deform similarly and preventing collapse. Figure III Figure 3 shows the results of a destructive drop test according to relevant national standards. A 50-liter necked steel drum made with 0.6 mm material was compared to a straight-open drum made with 0.8 mm material. The results showed that the necked drum exhibited significantly more circumferential deformation and a larger load-bearing area, while the axial deformation remained similar. This proves the effectiveness of the C value. The axial deformation being nearly identical indicates that the non-deformable part is minimally affected, ensuring the same axial strength for both types of drums. This confirms that a 25% reduction in material thickness does not lower the drum's reliability. In the circumferential direction, the necked drum shows greater deformation, ensuring uniform deformation across the mouth and lid, thereby improving overall reliability. As a result, a 0.6 mm necked drum can replace a 0.8 mm straight-open drum. Figure 4 Figure 4 displays the deformation of a 0.6 mm necked drum versus a 0.8 mm straight-open drum during a 180° straight weld drop test. These two comparison tests were conducted on various types and sizes of straight-open drums. The deformation and material thinning patterns largely followed the same rules. Deformation in closed and medium-open drums also followed similar trends. Through practical application, a C value range of 10–25 mm is most suitable. When the material is thin, a smaller C value is preferred. If the C value is too large, the "necks" may stretch and thin further, lowering the overall strength. The material thickness can be adjusted based on the C value, allowing for flexibility in choosing the appropriate size. The material thickness can be reduced by up to 25%. However, in practice, the material is usually reduced by 15–20%, with some margin left for safety. The necking technology is compatible with a wide range of steel plates specified by national standards, with thicknesses ranging from 0.5 to 1.4 mm. The appearance before and after the "necking" process is shown in Figure 5: Figure 5 After the steel drum is "necked," why does its overall reliability increase? This must be explained by examining the structure of the steel drum, particularly the material and design. First, let's look at the materials used in steel drum manufacturing. In metallurgy, carbon steel with a carbon content below 0.20% is classified as low-carbon steel. Thin sheets made from this type of steel are called low-carbon steel sheets. Low-carbon steel sheets are divided into ordinary carbon structural steel and high-quality carbon structural steel based on the amount of harmful impurities (P, S) and non-metallic inclusions. The former contains more impurities and is typically used for less demanding applications, such as A3 sheet. The latter has fewer impurities and superior mechanical properties, such as ST12 and 08F, which are widely used in high-performance steel drums due to their good strength, rigidity, cost-effectiveness, and workability. In addition to strength, high-quality carbon structural steel for steel drums requires good plasticity, weldability, and formability to allow for the creation of various shapes. The main factor affecting the mechanical properties of steel drum materials is the carbon content. Changes in carbon content alter the metallographic structure of the steel, which in turn affects its performance. Other chemical elements also influence the mechanical properties of the material, but carbon content remains the most significant factor. The carbon content in low-carbon steel sheets used for steel drums typically ranges from 0.05% to 0.20%. Steel within this range is hypoeutectic, consisting of a mixture of ferrite and pearlite. As carbon content increases, the proportion of pearlite rises and ferrite decreases. Conversely, lower carbon content leads to more ferrite and less pearlite. Increased pearlite improves strength but reduces plasticity, while increased ferrite enhances plasticity but lowers strength. Therefore, higher carbon content increases strength but decreases plasticity. During the necking process, ferrite becomes elongated and flattened, accompanied by minor carbide precipitation, while pearlite grains are refined slightly. This strengthens the pearlite structure, increasing the strength of the "necked" drum through a process known as "work hardening." This phenomenon occurs to some extent in all types of steel drums, but it is most pronounced and effective in necked drums. Next, consider the structure of the steel drum. 1. When subjected to impact, the stress is distributed among the hoop, lid, and body. The body bears most of the impact force. As long as the thinned material can withstand the same impact force in the axial direction, the problem is solved, and the lid and hoop do not affect this. This is determined by the drum's structural design. 2. The lid and hoop do not undergo "work hardening" during the manufacturing process, except for the body. From a metallurgical perspective, this only applies to the body. 3. After processing, the diameter of the top or bottom of the necked drum is 10–25 mm smaller than the original. The material is thicker and performs better in the "necked" area. Additionally, the "transition part" experiences significant deformation, ensuring consistent deformation at the top or bottom of the drum, thus enhancing overall reliability and ensuring the safety of the contents. Fourth, the "necking" technology differs from general cold-die shrinking: 1. The former uses thinner material, while the latter uses thicker or more ductile material. 2. The former uses roll forming, while the latter uses molding. The former employs rotating equipment, and the latter uses linear motion equipment, limiting the former to one molding cycle and the latter to multiple cycles. 3. The former has more difficulty controlling dimensions and larger tolerances, while the latter offers easier control and smaller tolerances. Fifth, the advantages of necked steel drums include: 1. Significant economic benefits Reducing material thickness saves raw materials and brings substantial economic benefits to companies. For example, a small company producing 200,000 50-liter steel drums annually would need 1,256 tons of steel per year. A 25% reduction in material usage saves 314 tons, valued at approximately 1.727 million yuan at 5,500 yuan per ton. Imagine the scale for the entire country. 2. Convenient stacking and transportation Necks make the bottom of the drum more stable when stacked, improving safety. Conversely, un-necked tops are easier to stack, but the strength of the un-necked top isn't enhanced, so the advantage lies in convenience. Tapered drums can be transported more efficiently, increasing capacity and reducing costs. However, the drum mouth cannot be necked, meaning its strength isn't improved, and only the bottom can be shrunk. Otherwise, the benefit of the tapered drum is lost. 3. Preparation for disposable steel drums Disposable steel drums are increasingly popular globally. As long as the goods reach their destination safely, the packaging mission is complete. A one-time-use steel drum is produced using thinner steel plates while still meeting safety requirements. This reduces the weight of the drum, saving materials and costs, and minimizing environmental pollution from secondary use. Compared to foreign countries, foreign steel drums use 10–25% less steel than domestic ones, which is quite impressive. This shows that the material thinning achieved through the "necking" technique aligns with global market trends. It is feasible and reliable, supporting the development of disposable steel drums. The "necking" technology is traditional but highly effective. When properly implemented, it can solve real-world production problems, drive technological progress, improve productivity, and contribute to enterprise development.
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