Meaning
Iron based alloys characterized by a body centered tetragonal crystal structure provide the extreme hardness and tensile strength required for intrusion protection in battery packs. Martensitic steel is produced by heating the metal to an austenitic phase and then quenching it rapidly in water or oil to trap carbon atoms within the lattice. This process prevents the formation of softer phases and results in a material with very high resistance to deformation and abrasion.
In the context of vehicle safety, these steels are used for the side impact beams and cross members that prevent external objects from penetrating the battery compartment. Its application is limited by its low ductility, which makes it prone to cracking if it is not tempered correctly.
Microstructural Transformation
Diffusionless transformation of the austenite into martensite creates high internal stresses that contribute to the strength of the alloy. Martensitic steel requires a specific carbon content to achieve the desired hardness, often supplemented by manganese or chromium to improve the depth of hardening. If the quenching is not uniform, the part may warp or develop surface cracks.
This phase is extremely brittle in its as quenched state and must be tempered to restore a small amount of toughness.
Mechanical Property
Ultimate tensile strength of these alloys can exceed two thousand megapascals, making them some of the strongest materials used in automotive manufacturing. Martensitic steel allows for the design of very thin structural components that can still support massive loads during a rollover event. This high strength to weight ratio is essential for offsetting the mass of the battery cells in electric vehicles.
The hardness of the material also makes it resistant to the wear and tear of long term structural loading.
Processing Requirement
Fabrication of parts usually involves hot stamping because the material is too hard to be formed at room temperature. Martensitic steel must be joined using specialized welding techniques that manage the heat input to avoid softening the metal or creating brittle zones. Laser welding is often used because it provides a precise and fast join with minimal thermal impact on the surrounding area.
If the cooling rate after welding is too slow, the strength of the joint will be significantly lower than the base metal. These components are often coated with an aluminum silicon layer to prevent oxidation during the high temperature forming process.
