The benefit of the presence of Cr containing inserts inside ductile iron on the tribological properties of these materials has been recently demonstrated. In order to understand these improved properties, a secondary ion mass spectrometry (SIMS) and transmission electron microscopy investigation of these wear-resistant ductile cast irons has been performed. The results indicate that the principal change in the microstructure induced by inserts occurs in the inserts and in their surroundings where the formation of carbides with different sizes and compositions is observed: M3C, M7C3 and M23C6 (M stands for Fe and/or Cr). These are not pure binary carbides since among the different elements studied by SIMS, Cr, Fe, Mn and, to a lesser extent, Ni are soluble in these carbides. Copyright © 2012 John Wiley & Sons, Ltd.