Casting Breakthrough In Steel Casting Technology

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Fraunhofer-Institut für Werkzeugmaschinen und Umformtechnik IWU

FAULHABER Drive Systems

Wuxi InfiMotion Technology Co., Ltd.

Researchers at Fraunhofer IWU and TU Bergakademie Freiberg (Germany) have developed a cold-formable, copper-alloyed austenitic cast steel with TRIP/TWIP properties. The new alloy combines strength and ductility: it is highly resilient and can still undergo plastic deformation.

The core of this innovation lies in the so-called TRIP/TWIP effect, which gives the new cast steel its extraordinary properties. TRIP stands for "transformation-induced plasticity" and TWIP for "twin-induced plasticity". These mechanisms cause the microstructure of the material to change under load, which leads to a significant increase in strength and ductility.

• TRIP effect: Under mechanical stress, part of the austenite, a soft and tough structural phase, transforms into martensite, i.e. a hard and solid phase. This transformation leads to local hardening of the material and increases its resistance to cracking.

• TWIP effect: So-called deformation twins are formed in the austenite, which also contribute to strengthening and increasing the toughness of the material.

The combination of these two effects significantly increases the strength of the material and delays component failure under dynamic load.

Nadine Lehnert

Both effects increase the tensile strength of the material and its ability to absorb mechanical energy: "The combination of these two effects significantly increases the strength of the material and delays component failure under dynamic load. In addition, the forming capacity and energy absorption capacity improve considerably in the event of an impact," explains Nadine Lehnert, who has taken over the project management of the DFG-funded research project "Cold forming of cast steel" at Fraunhofer IWU.

And this is how it works: The initial shape made from the cast steel alloy under consideration is formed by cold forging into a product with a fine-grained, back-transformed austenitic microstructure. The production route begins with a coarse-grained, austenitic structure. The workpiece is first reduced in diameter in an extrusion die. This mechanical stress leads to a partially martensitic structure due to the TRIP/TWIP effect. The subsequent heat treatment in the furnace reduces the grain size (fine grain size) in the component, thanks to the transformation of martensite back into austenite. Under high loads, cracking can occur in the component, specifically in the austenite structure, but this does not lead to failure, but is stopped by a martensitic transformation of the structure. The renewed hardening (martensite) even increases the load-bearing capacity of the material.

Areas of application with high safety potential

The unique properties of the new cast steel make it ideal for use in safety-critical applications where the highest demands are placed on strength, toughness and reliability.

• Automotive engineering: screws, chassis components, crash absorbers and body structures benefit from the material's high energy absorption and crash safety.

• Aerospace: Structural components and fastening elements can be made lighter and more resistant with the new cast steel.

• Medical technology: Implants and surgical instruments can be optimized thanks to the high biocompatibility and strength of the material.

• Construction and infrastructure: Rock bolts and fastening elements for bridges and tunnels can be made safer thanks to the material's high crack resistance. This is because the alloy shows its advantages where the durability of the material is important, even under extreme loads.

Energy-efficient cold forming as a key technology

The new cast steel is also suitable for cold forging. This process enables components to be manufactured at room temperature, making energy-intensive processes such as hot rolling superfluous. "The cold forming process chain is significantly shorter and more efficient. We start with a pre-cast workpiece, which is then formed directly. This eliminates numerous energy-intensive steps such as heating, rolling and descaling, which are required for hot forming," explains Lehnert.

Focus on sustainability and cost-effectiveness

In addition to the technical advantages, the development of the new cast steel also contributes to sustainability and cost-effectiveness.

• Conservation of resources, health aspects: The partial replacement of nickel with copper reduces the use of expensive and scarce resources as well as the health risks during processing.

• Energy saving: Cold forming consumes significantly less energy than hot forming, which leads to a reduction in CO2 emissions.

• Cost efficiency: The simplified process chain, lower material usage and lower gas consumption (cold forging) reduce production costs.

Our aim is to fully exploit the potential of the TRIP/TWIP effect and enable the cost-effective production of high-performance components for a wide range of applications.

Nadine Lehnert

The team's research results form the basis for the targeted use of the TRIP/TWIP effect for safety-critical applications. Future research work at Fraunhofer IWU will focus on optimizing the forming process and the targeted adjustment of material properties. "Our aim is to fully exploit the potential of the TRIP/TWIP effect and enable the cost-effective production of high-performance components for a wide range of applications," says Lehnert.