Machine hammer peening and deep rolling of austenitic ferritic cast iron

Machine hammer peening and deep rolling of austenitic ferritic cast iron

 

Motivation

The use of (advanced) high strength steels in the automotive industry leads to an increased wear of the drawing tools. For this reason, highly loaded areas of the tools made from cast iron are hardened by laser treatment or substituted by steel inlays. Since cast iron is a cheap material with valuable properties for the use in deep drawing, it would be advantageous to qualify the grey cast iron EN-JS2070 (former GGG70L) for the forming of high strength steels.

 

Objective

The goal of this research project is to improve the wear behavior of drawing tools made of cast iron and to reduce the time need in die and mould production at the same time.

In this respect, the cast iron is heat treated in order to create an austenitic ferritic microstructure (austempered ductile iron – ADI). By inducing a (mechanical) stress the austenite is transformed into a martensite, which hardens the surface layer significantly. The required stress is produced by the mechanical surface treatments “mechanical hammer peening” and “deep rolling”. Both processes allow the smoothing of surface asperities (from milling) and therefore a reduction of manual polishing operations. According to the demands of the die and mould making industry the resulting surface needs to be both hard and smooth.

By changing of the process chain it is possible to substitute a) the manual polishing operation and b) the laser or induction hardening of the cast iron.

 
Figure 1: Initial state of EN-JS2070
Figure 1: Initial state of EN-JS2070
Figure 2: As ADI after heat treatment
Figure 2: As ADI after heat treatment

Approach

In the first step different heat treated ADI samples are hammered and deep rolled with varied parameters. On basis of these results an optimized machining strategies for the hammering / deep rolling is derived, which leads to smooth and hard martensitic surfaces. In a parallel work package the machining strategy are further optimized with the help of finite element simulations.

Ensuing, the most suitable samples are tested in the strip drawing test, which serves to characterize the wear behavior of the ADI material in direct comparison to a reference tool machined according to the state of the art. Additionally, the tribological behavior of a nitrided ADI material is investigated. The experimental results are verified in a real process. Based on the finings, a guideline for industrial use is derived and the project thus completed and by deriving relevant criteria for an industrial use the project is completed.

Acknowledgment

The PtU would like to thank the der European Research Association for Sheet Metal Working (EFB) and the following companies for their support.

  • Daimler
  • Volkswagen
  • FILZEK TRIBOtech
  • ECOROLL
  • F&K Werkstoffprüfung
  • Konstruktion und Werkzeugbau Löcker /Accurapuls
  • Schwartz Wärmebehandlungen
  • Rhenus Lub