ColdSpring

Industrial context of the study

At SOGEFI Suspension, whether for stabilizer bars, coil springs or other suspension components, the key idea behind any product development process is continuous weight reduction, combined with improved performance. SOGEFI's perfect mastery of materials, whether in design, manufacture or development, enables us to manufacture products that combine performance with optimum lightness. This, in turn, helps to reduce vehicle weight and, consequently, fuel consumption, without compromising the vehicle's road-holding qualities.

During the production of coil springs, the winding stage of the heat-treated SAE 9254 (54SiCr6) steel wire, with its martensitic structure,  sometimes causes the appearance of a surface defect generally referred to as a "white layer" due to its appearance under the optical microscope. This phase on the surface of the wire has a hardness of 750 Hv, whereas the core of the wire has a hardness of 600 Hv. This "white layer" hardened phase is the result of a surface tribological transformation (STT), i.e. a change in the surface properties of a material due to friction.
At present, the presence of a white layer in the surface of a wire is not known. At present, the presence of these defects mainly represents a brake on product productivity and an overall threat to quality. Even if, from experience, they have no influence on service life, depending on the location of this phenomenon, breakage can occur during spring manufacture. This has a significant economic impact, since it requires a temporary halt to production. In addition, as part of its product lightening program, SOGEFI is using steels with higher mechanical properties, particularly in terms of hardness. This will increase the risk of this phenomenon occurring, and thus limit the development of lighter solutions.

Thesis objectives and scientific approach

From an industrial standpoint, the aim of this project is to seek optimal production conditions for reducing or even eliminating surface tribological transformation, particularly with steels currently in use (SAE 9254), but also for future grades. This will involve developing a methodology for analyzing, understanding and experimentally reproducing the conditions under which surface tribological transformation occurs. This will involve analyzing the contact conditions encountered during the shaping process. These will then have to be reproduced experimentally to characterize the conditions under which the white layer appears, and to understand its mechanical behavior and metallurgical structure. Finally, the aim of this approach will be to optimize the choice of surface treatments and the associated process parameters to obtain an optimum final product.