Interview with Dr. Michael Hellmann
SORBITEX®, a special material developed by Waelzholz, offers an enormous directional spring tension of up to 3,000 MPa. Combined with its excellent durability, this textured steel strip has proven itself in many applications that require the use of heavy-duty springs. In this interview, Dr. Michael Hellmann, head of materials technology at Waelzholz, explains the potential offered by SORBITEX® textured steel strip.
Dr. Hellmann, up until now, SORBITEX® has been known as spring steel strip for heavy-duty springs. Why is the textured steel strip so well suited for this?
This material is produced in a multi-stage manufacturing process. First, the special fine pearlitic microstructure forms in continuous furnaces specially designed for these products. The strip is then rolled once again under significant forming force on our Sendzimir mills. This can change the shape of the material by over 90%. The result is a strip with exceptionally high tensile strength and an extremely high yield/tensile ratio, which results in outstanding durability, even under significant stress. And these are precisely the properties that are required for such applications, like the retractor springs used in car seat belts, for example. SORBITEX® offers the necessary degree of reliability during processing and in the final product.
Now one could think that SORBITEX® is completely unsuitable for complex forming processes due to its high tensile strength. Is this the case?
This is exactly one of the surprising characteristics of SORBITEX®: The material is also suitable for deep-drawing processes, indeed. This is, of course, an advantage for the customers of ours who need to use these kinds of forming processes for their products and who are also dependent on high-strength, yet very lightweight materials.
Representation of drawability of SORBITEX®
What are the capabilities of SORBITEX® when it comes to the forming process?
We demonstrated this textured steel strip’s extremely wide range of forming capabilities in a comprehensive test using what is known as the cross tensile testing method. This was conducted at the Automotive Center Südwestfalen GmbH (acs) testing institute. At the same time as the experiment, we also performed calculations using numerical simulations. A 1,000-ton press was used to reproduce over four different radii in one stroke during this cross tensile test. The result was that we achieved a deep drawability of 18 mm at maximum radii of 20/25 mm. This is an enormous figure for a material with such high tensile strength.
Cross tensile test with variable radii; Photo credit: acs
What does this result mean for real-world production processes?
The test proved that SORBITEX® is indeed suitable for deep drawing. This is a strong argument for us to increasingly offer the textured rolled material for applications in which deep drawing needs to be carried out during the production process, but at the same time, maximum tensile strength is required in the final product. This means our customers receive a sophisticated material that combines seemingly contradictory properties and therefore opens up completely new possibilities for them.