Researchers at the Fraunhofer Institute for Machine Tools and Forming Technology (IWU) and TU Bergakademie Freiberg have made a significant breakthrough in steel casting technology. They have developed a cold-formable, copper-alloyed austenitic cast steel with TRIP/TWIP properties, opening new possibilities for safety-critical applications. This alloy combines high strength with exceptional ductility, making it both highly resistant and formable.
The newly developed cast steel is particularly well-suited for applications such as rock bolts used to stabilize rock walls along traffic routes, tunnel walls, or underground mining chambers. When rock material impacts the protective netting, it can damage the anchors. The cold-formed alloy responds to such stress with renewed hardening of the material, which also benefits connecting elements.
TRIP and TWIP Effects as Key Mechanisms
The outstanding properties of the new steel casting are based on the TRIP/TWIP effect:
- TRIP Effect (Transformation-Induced Plasticity):
Under mechanical stress, part of the austenite—a soft and tough microstructure phase—transforms into martensite, a harder phase. This transformation leads to local hardening of the material and increases its resistance to cracking.
- TWIP Effect (Twinning-Induced Plasticity):
In this case, deformation twins form within the austenite, also leading to hardening and increased toughness.
Both effects boost the material’s tensile strength and its ability to absorb mechanical energy.
“By combining these two effects, the material’s strength is significantly increased and component failure under dynamic load is delayed. In addition, the formability and energy absorption capacity in the event of impact are considerably improved,” explains Nadine Lehnert, who leads the project “Cold Forming of Cast Steel” at Fraunhofer IWU, funded by the German Research Foundation (DFG).
Manufacturing Process Involving Cold Forging
The production process begins with a coarse-grained austenitic structure. The workpiece is initially reduced in diameter using an extrusion die. This mechanical stress triggers the TRIP/TWIP effect, leading to a partially martensitic structure. A subsequent heat treatment in a furnace reduces the grain size within the component by reversing martensite back into austenite. Under high stress, cracks may begin to form in the austenitic structure, but these do not result in component failure.
