 Eliminates EDM Stresses
Electrical Discharge Machining while providing a relatively inexpensive means for producing die casting die inserts, unfortunately, at the same time, sets up some very high and detrimental EDM surfaces stresses that if not properly tended to will actually accelerate thermal stress cracking. Since metal is removed by a series of electrical spark discharges, the steel in the
contact area melts or vaporizes then solidifies on the surface of the cavity. Each spark erodes a tiny bit of metal, leaving a small crater in the surface of the tool. This leaves the surface in a high residual stress condition which can lead to early heat checking and possible gross cracking. The
top most or recast layer is a brittle, non-etch white layer
containing cracks. This is the material that has melted and rapidly solidified and is not flushed away by the die electric fluid. This layer is densely infiltrated with carbon and has a distinct separate structure to that of the parent metal.
Below this layer is the heat affected zone that has been structurally altered by the heat produced during EDM which reaches the austenizing temperature of the steel. This zone may contain re-hardened or hard, brittle untempered martensite which is formed during the rapid cooling from this temperature. This can be expected to increase crack susceptibility since such a microstructure stores
considerable strain energy that decomposes with heat.
After mandatory removal by polishing of the top "white cast" layer" it is important to protect the next exposed layer. MetaLL ifeÒ compressive stress
topography closes the cracks that have propagated below the recast layer into the heat affected untempered martensite zone and restores the desired residual compressive stress benefits to the tool.
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