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Electrical discharge machining (EDM) has been utilized and accepted as a standard method of machining practice today. It is performed using the arc discharge or the wire EDM method. Engineers and machinists alike are not always aware of the metallurgy of the process and the problems that it can cause for the heat treater.
EDM has been the root cause of many die failures. Most of the failures are attributed to the heat treatment procedure. Failures include:
- Surface cracking of the machined part
- Catastrophic cracking in the cavity area of a machined sunken cavity
- Rounding over of punch forms that have been subjected to wire EDM (in other words, the punch form is mushrooming over)
- Surface machining difficulties after the EDM procedure
The same thing occurs with the EDM procedure, only this time the cathode is immersed in an oil-based cooling medium while the cavity is eroded electrically by melting the steel surface. The steel is heated to a temperature where its surface melts, and the molten residue is flushed away by the oil coolant.
As a result of the melting, the steel surface is raised to a minimum temperature of at least 3300°F. There is heat migration from the molten steel surface into the body and toward the core of the material. The surface of the steel will melt and leave what is known as a recast layer. The immediate layer is very soft but can still lead to crack initiation and crack growth.
Below the recast layer, there can be any metallurgy that you wish to see, with varying degrees of hardness values at different depth levels. The metallurgical phase can be pearlite, upper bainite, lower bainite, untempered martensite, retained austenite in untempered martensite or other mixed metallurgical phase layers.
The layer of most concern will be the area of untempered martensite. The fresh martensite will form due to the steel temperature, and the appropriate rate of cooling that will occur as the heat from the recast layer migrates into the body of the steel.
The fresh, untempered martensite is both an unstable and brittle phase of the steel metallurgy and is at risk of cracking.