Influence of steel type on electrical discharge machined surface integrity

This paper studies the influence of type of steel on electrical discharge machined (EDM) surface integrity. Tests were performed with two hardenable steels (tool steel type X155CrMoV12 and high carbon content steel type C90) and two non-hardenable steels (austenitic stainless steel type X2CrNiMo17-12-02 and ferritic stainless steel type X6Cr17). Surface integrity was characterized by roughness, micro-hardness, residual stress distribution and near surface damage. In the case of hardenable steels, EDM leads to a structure of three layers, white, martensite quenched and transition layers. High carbon diffusion induces important near surface hardening and a high tensile residual stress level that leading to crack generation. In the case of non-hardenable steels, metallurgical transformations result only on the formation of the recast layer with a dendritic structure and a slight increase in grain size. Near surface hardening due to carbon enrichment depends strongly on the initial structure (BCC or FCC). Finite element method has been used to assess temperature and residual stress fields in the case of X2CrNiMo17-12-02 steel. Experimental and calculated residual stress profiles were compared. It was found that the shapes of the profiles are different in the surface layers and quite similar in the deeper layers, and that the calculated residual stress values are greater than those obtained experimentally.