Machine hammer peening of austempered ductile iron: Microstructural investigation, surface roughness and mechanical properties

Author:  J. Scheil, C. Müller, L. Ahmels
Source:  Conf Proc 2014: ICSP-12 Goslar, Germany (pgs.523-529)
Doc ID:  2014131
Year of Publication:  2014
Abstract:  
Austempered ductile iron (ADI) has become a serious competitive material to conventional steels. In addition to its favorable price the main reason is that its mechanical properties can be adjusted over a wide range by different heat treatments. The unique microstructure of ADI consists of ferrite, graphite and metastable austenite. Adjusting the microstructure (phase fractions, stability) with regard to the application is one important challenge. The key phase hereby is the metastable austenite because it can be transformed into hard martensite under external forces. EN-JS2070 is a cast iron widely used for forming dies. Part of this study is to investigate whether a suitable heat treatment leading to ADI microstructures can be found for this material. Therefore the cast iron was austempered choosing austenitizing temperature of 950 °C and different austempering conditions. Additionally the effect of a mechanical surface treatment, machine hammer peening (MHP), on the microstructural changes in this ADI was examined. Before and after MHP the sample surfaces were characterized using optical microscopy, XRD, hardness measurement and laser microscopy. It could be shown that by a suitable heat treatment an ADI microstructure in ENJS2070 can be produced. MHP of the heat treated samples results in hard martensitic surfaces. This is accompanied by a significant smoothing of the surfaces. By combining MHP with the ADI heat treatment of EN-JS2070 a further qualification of this tool material becomes feasible.