Abstract
Heat-treated and shot-peened lightweight steels with demanding requirements for durability are applied in high-performance automotive leaf springs. Due to their heat-treatment they exhibit degraded properties in the surface-near area compared to the core. This area, which may extend until 300 μm from the surface to the core, experiences the highest bending stresses at operation. The microstructure in the surface and sub-surface layers determines the mechanical performance as well as the wear resistance. The present study refers to the material properties of a stress shot-peened 51CrV4 steel at various depths from the surface. The effect of the manufacturing process has been captured both by Vickers micro-hardness measurements and nanoindentation. The latter combined with a Fine Element Method (FEM)-based algorithm enables the determination of variations in the material’s stress–strain curves over the affected layers, which translate to internal stress changes. The nanoindentation technique has been applied here successfully for the first time ever on leaf springs. The combination of microstructural analysis, microhardness and nanoindentation captures the changes of the treated material, offering insights on the material characteristics, and yielding accurate elastoplastic material properties for local, layered-based analysis of the components’ mechanical performance at operational loading scenarios, i.e., in the framework of stress shot-peening simulation models.
Highlights
High strength steels with ultimate tensile strength >1600 MPa after quenching and tempering are used in technological areas, where the superior strength and enhanced fatigue life are essential for the overall safety of the structure
The results presented here have been extracted from investigations performed within the framework of the LIGHTTECH project (LIGHTTECH—Innovative Approaches of Stress Shot Peening and Fatigue Assessment for the Development of Lightweight, Durability-Enhanced Automotive Steel Leaf Springs) [6]
The decarburization depth and qualitative intensity can be estimated according to the standards [17,18], while the reasons for its occurrence should be sought among the surface treatments applied during the leaf production, i.e., the heat treatment and the hot tapering, as both are high-temperature processes that may promote the reaction of carbon with oxygen towards the formation of CO2
Summary
High strength steels with ultimate tensile strength >1600 MPa after quenching and tempering are used in technological areas, where the superior strength and enhanced fatigue life are essential for the overall safety of the structure. The SSP process is considered the most promising and effective way to enhance the fatigue properties of suspension components on an industrial scale. It has been adopted by all major manufacturers as a time- and cost-effective treatment for at least doubling the fatigue life of their products, especially leaf springs [1]. The SSP process control and optimization are very challenging tasks of high complexity: shot material, geometry and hardness, shot velocity and incidence angle, degree of coverage and tensile prestress interact with the already uncertain state of the area of application (decarburized surface with reduced strength, inevitably induced by the preceding heat treatment). Over the past two decades research activities were focused on the SSP simulations
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