Integrated Design of a Steel Wheel Hub

This study deals with the optimization of a 42CrMo4 steel wheel hub used by FIAT and was conducted by EnginSoft on behalf of the Fiat Group Automobiles (FGA). The aim of the study was to optimize the design and manufacturing process to improve the strength of the hub, reduce defects and save on material and material waste. The first part of the study rotated around an improved geometry for the part and an optimized forging process. The second part identified how to obtain the best possible mechanical properties for the metal part following heat treatment and machining.

The Analysis of the Forging Process

The object of this study was a wheel hub in 42CrMo4 steel from FIAT. FGA provided the geometric CAD models and the rheological characteristics of the 42CrMo4 steel used in production. The study included:

  • the computation of extra-metal required, taking into consideration thermal expansion, machining for chip removal and the draft angles, and an estimate of the total metal needed
  • the calculation of fillets and undercut
  • the flash gap design
  • an analysis and selection of the most suitable kinematics, aimed at streamlining the process and minimizing the force and energy used
  • the selection of the best lubricant that would extend the life of the molds, resulting in both financial and ecological gains
  • an evaluation of the total forging load
  • an estimate of the size of the mold needed
  • an analysis of friction phenomena
  • an analysis of heat transfer during manufacturing
  • prediction of the mold temperature

An Integrated Approach to Engineering Design

42CrMo4 Rheological characteristics

Rheological characteristics of the 42CrMo4 steel used in production. The description used came from the Forge database.

The second part of the study analyzed the influence of heat treatment and specifically the quenching process on the finished product. The output parameters of the first study were used as input parameters for a structural study of the part providing the distribution of local mechanical properties and residual stresses.

The forged wheel hub was given an austenitizing treatment at 900°C followed by oil quenching at 50°C. The rate of cooling associated with different quenching methods and the different obtainable phase transformations were analyzed.  We continued to optimize the quenching process for the part until we obtained the desired mechanical characteristics and specifically the local HB (hardness) to evaluate actual behavior in case of fatigue.

The Heat Treatment study showed that it is possible to estimate the best microstructure (Martensite, Bainite, Ferrite and Perlite) and the desired mechanical properties (HB, E, Ys, UTS, A%) through the interpolation of the  Chemical Composition data, the CCT Transformation Diagram and the Temperature-Time history.

Residual stresses often lower the yield stress and are also the cause of dimensional instability and the ultimate shape of the part after machining. Tensile residual stresses were evaluated after heat treatment as well as after machining. The study highlighted the well-know effects of residual stresses:

  • tensile residual stresses on the surface reduce the mechanical properties of the metal and their resistance to stress corrosion, render the part more susceptible to fatigue, and cause minor cracks and other usage phenomena
  • compressive residual stress, resulting from heat treatment, has a beneficial effect in that it delays the start and propagation of cracking and it reduces the level of tension in the layers where the load is greater

By finally integrating the structural verification with the manufacturing design chain, the forged component and the design we proposed overall achieved:

  • a reduction in forging defects, in compliance with the DMADV Six Sigma methodology used at FGA
  • reduced material waste by 16%
  • improved mechanical static characteristics (E, Ys, UTS, A%), homogeneous radial distribution of mechanical properties (E, Ys, UTS, A%),  and resistance to wear and fatigue
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Wheel hub