The technology of E.D.M. wire cutting has been well received by tool & die makers and rightly so. However, certain points are worthy of discussion.

As the temperature at the point of wire erosion (wire cutting) can reach some 50000°C / 90000°F, or approximately twenty times hotter then the welting point of the steel, a resolidifacation layer accurs in witch micro cracks develop, (they develop also in the re-hardened zone but to a lesser extent) it follows that these act as crack initiation sites from which larger cracks can progress, a condition not helped by the fact that hardenesses in the re-hardened zone have been found to be as high as 1200 vickers  / HRC 74 in steels such as D2

I deally, these layers should be removed by polishing but this is not always possible. This is when a post EDM stress relief (usually 25°F /15° to 25°C) lower than the tempering temperature of the steel can be very useful in restoring some ductility / toughness to the tools.