Austempered ductile iron heat treatment for machine hammer peening of milled tool surfaces

Austempered ductile iron (ADI) has become a competitive material to conventional steels. In addition to its favorable price the main reasons are its mechanical properties can be adjusted over a wide range via heat treatment. Austempered ductile iron consists of ferrite, graphite and metastable austenite. Tailoring its microstructure (phase fractions, stability) with regard to the application is an important challenge. A cast iron used for forming dies is EN‐JS2070. In earlier studies it could be shown that EN‐JS2070 can be transformed into austempered ductile iron 1]. Machine hammer peening, causes martensitic transformation of the metastable austenite and leads to hard and smooth surfaces. Focus of this study is to optimize the microstructure with regard to machine hammer peening process. Before and after machine hammer peening the sample surfaces were characterized using optical and laser microscopy, X‐ray diffraction and hardness measurement. It could be shown that a combination of high amount of metastable austenite with a high carbon content leads to the best results in surface roughness and hardness.