For many applications, the traditional method of selecting materials based on their main properties does not go far enough. This is because despite having similar properties, such as the same tensile strength, steel materials can differ considerably when it comes to the finer details – particularly with respect to processing characteristics. This is especially true for high-strength materials, where material grades of the same tensile strength can exhibit widely differing levels of weldability, bendability, or isotropy. The resulting potential can be fully harnessed through the application-oriented implementation of cutting-edge steel materials. This is made possible by involving Waelzholz as a material supplier in the product development process at an early stage. As a result, the materials developed in this manner exhibit properties specifically tailored to both the end application and the customer’s manufacturing processes.
Time and flexibility are key factors in product development today. “Short product life cycles increase the pressure on all stages of the value chain, from steel production to steel strip production to manufacturing the final product,” explains Dr. Michael Hellmann from Waelzholz’s Materials Technology department. In light of this fact, there is an increasing demand for materials that offer customized performance, both in terms of the final application and the manufacturing processes within the individual stages of the value chain. This requires close collaboration between the engineering team and the material supplier at a very early stage of product development.
More than just the typical material properties
When selecting materials, the focus is often on the typical properties like spring force, resistance to wear, corrosion resistance, magnetizability, or a favorable strength-to-weight ratio. Dr. Hellmann explains why this approach often falls short in many projects: “The conventional properties are really just the tip of the iceberg. These are the factors that are immediately considered relevant. These core features often mask other requirements, however, some of which directly become more important during the development stage, but sometimes also indirectly after the start of mass production.” In order to precisely identify the aforementioned factors and customize the material accordingly, Waelzholz has developed a three-step procedure:
1. Development project
2. Processing behavior
3. Series testing
1. Development project: early involvement for higher degrees of flexibility
A development project often begins with the conventional properties that are presumed to be necessary for an application. The fact that these only cover a fraction of the materials that may be suitable is illustrated by the chart in Figure 1 showing elongation as a function of tensile strength. According to Dr. Hellmann: “The chart shows our range of materials for the properties of yield strength and tensile strength in the range 150 – 1,700 MPa (22 – 247 ksi) at an elongation of 10 – 60%. This results in a total of 280 different material variations available from Waelzholz.” The overlaps in the material properties are of particular importance to us in this context, however. They show that several material variations fall into identical yield strength and tensile strength ranges. This means that we need to look at further properties to differentiate between them. “We have tremendous potential here to perfectly cover the entire range of requirements by taking other factors into account when selecting materials,” says Dr. Hellmann. The earlier the material supplier is involved in the development process, the more flexibility and freedom the manufacturer will gain through this holistic approach.
Range of materials
Figure 1: Representation of the elongation as a function of the strength
2. Planned processing behavior and reproducibility
Example spring steel: When manufacturing springs, the spring steel strip is often formed using complex, multi-stage punching/bending processes. According to Dr. Hellmann: “Due to the complicated tooling arrangements, manufacturers can’t use process technology that allows them to react to changes in material properties. This makes predictable processing behavior and guaranteed reproducibility that much more important.” In order to achieve this, Waelzholz tests different material and implementation concepts in a process that closely replicates the final production process. “For spring steel strip, a bending test that involves analyzing spring-back behavior has proven to be a good indicator (Figure 2). We conduct this material test on different production batches and examine random samples over the entire length of the core of a coil,” explains the materials engineer. In Figure 3, we see the spring-back behavior of two materials – both with very narrow scattering across the samples. This ensures predictable processing behavior with a high degree of reproducibility.
Determination of spring-back behavior
Figure 2: Schematic diagram of the experimental setup of the bending test
Figure 3: Example of two materials with very narrow scattering of spring-back behavior
3. Optimization of property combinations in serial production
The third step focuses on assessing uniformity in serial production. According to Dr. Hellmann: “When integrated into mass production, we inspect the tolerance uniformity over the length and width of each coil. In this process, our highly sensitive measuring devices can even detect when a parameter tends to move in the direction of a tolerance limit, but is still well away from it. This gives us the ability to fine-tune the process at a very early stage.” In connection with numerous analyses of the mechanical properties as well as statistical tests over the entire coil length, this guarantees serial production quality and identifies potential areas of improvement early on. “Our comprehensive testing technology offers a considerable advantage, both during the development stage as well as in serial production. Our laboratories are capable of conducting all of the essential testing methods, from macroscopic examinations to microscopic testing using a scanning electron microscope.”
Analysis integrated into serial production
Figure 4: Example from an analysis of continuous strip thickness from one of the cold rolling lines conducted during the production process
Left: The results of an analysis of the mechanical properties conducted over the length of the core of a produced coil of micro-alloyed high-strength steel
Right: A statistical CAQ long-term analysis of the mechanical properties of this material
Successful development partnerships
“In the past ten years, Waelzholz has almost tripled the percentage of custom-developed materials. And more than 85% of them were created working directly with our customers,” says Dr. Hellmann. Highly collaborative development partnerships lead to material solutions that perfectly meet all of the respective customer’s requirements, from the process to the application – creating real value in the process. According to Dr. Hellmann: “Our customers develop groundbreaking products and inspire us with their ideas. We use our materials expertise to help them bring these ideas to life. And thanks to having locations on four continents, our materials expertise is available worldwide exactly where our customers need it – locally.”