Durable lightweight container
Winfa, s. r. o. of Trnava have designed a roller container produced using Hardox 450.
The abrasion-resistant and at the same time high-strength steel makes it possible to manufacture lighter and more resistant containers.
Containers with a volume of 30 to 40 m³ and a weight of about 2700 to 3100 kg are normally considered to be heavy; they are made of S235 or S355 grade structural steel plates with a 5-mm thick bottom and 3 mm thick sidewalls and front. The lifespan of a container is termed its durability.
There are, in fact, 3 basic container failure mechanisms. Plastic deformation (a dent) possibly resulting in a tear in the sidewalls or bottom; fatigue cracks in welded joints and abrasion. Of these three damage mechanisms, deformation resulting from bending of the sides or the bottom of a container is the most critical. Significant damage to a container caused by abrasion is an issue only in very specific cases, and fatigue cracks in welded joints are mainly influenced by the welding quality, rather than the material used. There is a simple way to increase the resistance of a steel structure to plastic deformation – use stronger steel. The standard material that is currently for producing containers is S235 steel (with a yield strength of Re = 235 MPa), and S235 steel is considered to be high-strength. Nowadays, however, steel comes in the form of plates or strips with a yield strength of up to Re = 1 500 MPa.
Of these types of steel, Hardox 400 or Hardox 450 steel with a yield strength of Re 1000 and 1 200 MPa respectively has normally been used for truck beds, as well as some containers. Other commonly used materials include Strenx 700 steel with a yield strength of Re = 700 MPa. Based on good experience with the application of high-strength steel on truck beds, Winfra wanted to validate the production of a container that would primarily be used for transporting scrap steel, and would be more durable and lighter than the existing containers of this type. The result was a roller container with a volume of 30 m³ and a weight of 2 388 kg. The resulting weight is 300-400 kg lower that a standard container. The bottom, sidewalls, front and door of the container are made from 3-mm Hardox 450 steel. The resistance of this steel to plastic deformation due to impact is approximately the same as the resistance of 7.5-mm S355 steel. A similar difference was found in their resistance to abrasion. Hardox 450 steel is about 2.5 to 3 times more resistant to abrasion than S355 steel. When constructing the container, the reinforcement in the container walls and bottom were removed (except for one transverse reinforcement across the bottom of the container). This solution both decreases the weight of the container and increases the resistance of the walls and the bottom to deformation due to impact. Increasing resistance by removing reinforcement might sound paradoxical, but a free area without a rib framework eliminates more deformation in an elastic rather than plastic way, thus decreasing any permanent damage to the walls or the bottom of the container. Another construction improvement is the integration of the top edge in the container wall. It means, the edge is made and bent using the metal sheet that forms the container sidewall. This solution increases the rigidity of the container while decreasing the number of welded joints. The primary motivation in the construction of thiscontainer was to increase its resistance to deformation and lifespan, by decreasing the weight by around 400 kg results in potential fuel savings and the possibility to transport more cargo. There are various calculations that demonstrate the savings when a lower weight is transported, or that demonstrate how much more it is possible to produce, if a greater payload is used. In general, two extreme scenarios might be considered: First, it is not possible to fill the container to such an extent that its maximum payload is used to its full potential (common situations often include when scrap steel or an empty container is transported). Therefore, the only saving here is a decrease in diesel consumption due to the fact that the “load” is 400 kg lighter. The second scenario is to use 100% of the payload, thus transporting “400 kg extra”. The “saving” in this case consists in the amount of extra production for a greater amount of cargo.
The first option – saving fuel. Distance travelled annually: 70,000 km. The amount of diesel saved by adecrease in the weight of the assembly by 1000 kg: 0.5 l/100 km, i.e. 350 l of diesel per year. This means, if diesel is about EUR 1/l (without VAT), the savings will amount to EUR 350 a year.
The second option – using a greater payload. Distance travelled annually: 70,000 km. The transport cost for 400 kg cargo/1 km = 1.5 eurocents. This means, EUR 1 050 more per year.
In the case of containers that may be fully loaded to their maximum load capacity and travel more kilometres, it is possible to use high-strength steel in another way. That is, to decrease the thickness of the high-strength steel sheets to the lowest possible thickness from a technical point of view. In these cases, wall thickness is reduced to 2 mm and the bottom of the container to 2-3 mm. Although this solution does not introduce a significant increase in resistance, it produces a remarkable decrease in weight. In this way it is possible to decrease the weight and increase the payload by 800–1 200 kg for 30 m³ to 40 m³ roller containers.