Coextrusion of steel–steel materials for manufacturing stress-optimised massive components

Global challenges like depleting natural resources, rising prices of fossil fuels, and climate change are increasing the significance of lightweight construction in vehicle development and thereby reducing CO₂ emissions. Thus, cost-effective weight reduction leads to the development of new high-strength steel materials and processes that enable the achievement of component properties through adapted thermal and mechanical process control. This eliminates time- and cost-intensive post-heat treatments to achieve component properties, reducing process and energy costs. Among these steels are high-strength ductile bainitic (HDB) steels and precipitation-hardening ferritic-pearlitic (AFP) steels, distinguished by their good strength-toughness ratio. Using these steels, existing geometries can be dimensioned with considerable weight reduction.

However, drive and suspension parts must have profiles with properties tailored to their use. Many components, require high hardness values alongside high strength and toughness. In order to achieve geometries and structures of such components in the context of lightweight construction, the usual case-hardening steels lack the necessary strength and toughness. This yields the scientific-technical problem that, due to the low carbon content of HDB and AFP steels, the hardness of gears, running surfaces, or cams isn’t high enough. Although alloying variants with higher carbon content exist that can achieve the required hardness values, these exhibits reduced notch impact toughness or ductility, which leads to shorter lifetimes. Consequently, the developed high-strength steels are limited in monolithic construction by component restrictions.

Promising in terms of the outlined conflict of objectives is the development of a process chain for the production of hybrid components with locally tailored properties. High-strength materials (38MnVS6 / 18CrNiMo7-6) can thus be used in areas with high strength requirements, while in component areas with high hardness requirements, a steel material with a high carbon content (100Cr6) can be used. A composite forming with compound extrusion process is used for the simultaneous deformation and joining of the two materials, in which materials are heated to a predefined forming temperature and then joined together within the forming die. The simultaneous presence of high contact pressures and temperatures promotes diffusion processes and the formation of cohesive bonds.