February 2024 Volume 6

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MATERIALS

Seeing Is Believing: Visualizing How Metal Flows in Hot Working By Patrik Holm

Hot working processes such as forging not only modify the shape of a steel workpiece, but they also improve its mechanical properties such as strength, fatigue life and impact resistance. This is because it breaks down the as-cast structure, closes up porosity and redistributes segregated elements. The actual effect of hot working is determined largely by the process used to deform the steel workpiece. This is illustrated by Figures 1 and 2 that show the significant differences between forging and rolling.

Visualizing Ring Rolling Ovako’s R&D team focused its initial efforts on developing a practical method for the visualization of material flow on the ring rolling process. This is used to manufacture rolled and forged rings used in demanding, heavy-duty applications such as bearings. The process starts with upsetting and piercing of the steel billet. This results in a donut-shaped pre-form. Next, this is passed to the ring rolling mill, where the donut becomes a ring. In order to optimize the process to ensure the final product has the high level of quality required, it is vital to be able to understand what is happening to the metal at each stage. A useful tool for building this understanding is finite element method (FEM) simulations. For this study, an FEM simulation of the upsetting, piercing and ring rolling steps was performed. This provided a theoretical approach for comparison with the new industrial visualization method. Table 1: Input parameters used for the FORGE simulation. Input parameters Values Steel grade 100Cr6 Flow stress model Hansel – Spittel with parameters from Forge Temperature billet 1200°C Friction during upsetting Coulomb limited Tresca model 0.4 and 0.8 HTC air 10 W/m2K HTC tool-work piece 2000 W/m2K Tool speed 30 mm/s Tool temperature 250°C Creating the Simulation Commercial CAD software was used to create a 3D model of the process. FORGE software was used, with the input parameters shown in Table 1. As part of this simulation, a pattern of rods was introduced within the 3D modeled billet to show the material flow, see Figure 3. Figure 4 shows the tool and billet setup in FORGE.

Figure 1: Simulation of strain imposed on bar by forging (left) and rolling (right).

Figure 2: Simulation of strain across the cross-section shows the more intense effect of forging (left) on the internal parts of the bar. The key aim of the process is to achieve the correct flow of the steel grain. That is why it is very important to be able to visualize exactly how the material flows.

FIA MAGAZINE | FEBRUARY 2024 39