EDM (Electrical Discharge Machining) is an accepted and economical method of machining many tool steels. This method of machining can produce very intricate shapes to very accurate sizes. However, there can be problems associated with the EDM process in relation to the final heat treatment of the die or tool. It is most important to understand the resulting metallurgy and surface-layer construction in relation to the final heat treatment. The final EDM layer is generally seen (microscopically) as follows:
- The surface layer, which is also known as the recast layer, is at the immediate surface and is generally very hard and brittle. (Remember it has seen a very high temperature, up to the point of melting).
- The objective of EDM is to vaporize the surface layer and remove it. But, in reality, it is a melted layer that cools rapidly at the surface of the tool.
- The surface is now in a highly residual-stress-full condition, which can and very often does lead to cracking, heat checking and (ultimately) premature surface failure.
- More often than not, it is the heat treater who is blamed, no matter how careful the heat treater has treated the tool.
- Below the recast layer is a layer of untempered martensite. The untempered martensite transformation has occurred due to the heat sink that takes place because of the generated heat from the EDM procedure, which will diffuse into the body of the tool and create the appropriate transformation from austenite into martensite because of the rate of cooling.
- The untempered martensite gives rise to a stressful potential crack-occurrence condition. Hardness values of the untempered martensite layer can be as high as 62 HRC (depending on the chemistry of the steel).
- Below the untempered martensite layer will be another layer of what can be a tempered martensite layer with hardness values in the region of 52-54 HRC (once again depending on the steel chemistry).
- Below that layer of tempered martensite will be the original hardness and microstructure of the base material.
The recast layer is the dangerous layer because not all of the melted layer has been flushed away. When a fail sample is etched, the recast layer will be seen as a white layer (do not confuse with the white layer of a nitrided surface).