There are many possible causes of tool-steel failures in their operating environment. As has been stated in previous blogs, the causes of catastrophic failures can be numerous. The failure of the die or mold can be attributed to machining methods of the mold or die form.

One aspect of mold failure during manufacture is the sinking of the mold or form into the die block. The form in the die is sunk typically by EDM or wire EDM.

The method of creating the die form by the above methods involves the melting of the surface of the form. The electrode, be it copper or graphite, is immersed in an oil-based cooling liquid.

The principle of metal removal is by localized melting of the surface. The electrode generates heat to melt the immediate surface to remove metal to follow the die form. The energy of heat that is generated by the electrode needs to dissipate itself.

The theory is that the liquid in which the electrode is immersed will be the source that will absorb the generated heat from the surface of the steel. Please remember, that the die itself acts as a heat sink. The heat generated at the surface of the steel will transform the steel phase from ferrite into austenite at the surface and below the surface.

The cooling medium and the body of the die will act as a quench medium and transform the austenite surface into fresh untempered martensite. Fresh untempered martensite is the most unstable phase.

The result of this phase is that the surface of the eroded die form is totally unstable, and there is a very strong likelihood that the surface would generate what could be interpreted as quench cracks. The first layer is generally known as the:
  • Melted layer – This is the immediate surface that has been primarily melted to arrive at the necessary die form. This layer will generally etch out using a Nital etchant as an immediate white layer. Obviously during the melting of the surface, significant overheating has taken place.
  • Rehard layer – This is the layer immediately below the melted layer. It is the layer where martensite has formed. This will etch out using a Nital etchant as a martensitic layer. The amount of formed martensite will be dependent upon the steel chemistry, particularly carbon content.
  • Tempered layer – This is a layer immediately below the martensite layer. It will appear as tempered martensite due to the heat dissipation from the EDM surface of the die.
  • Core – This is the area of the original substrate of the dye material. The core hardness will not change from the starting hardness.
If one were to conduct a micro-hardness survey of a cross traverse through the EDM, one would see a low immediate surface hardness followed by the high hardness of the untempered martensite followed by the tempered layer and, of course, down to the core hardness.

Great care is necessary by the EDM operator in power selection. Equally so, it will be necessary to temper the immediate surface of the die by tempering at an appropriate tempering temperature. This is to reduce the risk of cracking and to temper the untempered martensite. Any crack will initiate from the immediate surface and down, (most likely) into the core.

The depth of these layers will be dependent on: erosion power setting, dwell time of the surface immediately beneath the electrode and achieved surface temperature.

The act of tempering immediately after the EDM erosion is no guarantee that cracking will not occur. It will reduce the risk of cracking, however, if the die is placed into temper immediately after the EDM process.