An interesting experiment to demonstrate the various hardness values would be to take a piece of tool steel (such as D2) after it has been subjected to EDM, stop-grind the surface in three or more different locations, conduct very light-load surface-hardness checks and note the hardness variations.
Another experimental method would be to cut an EDM sample (using an abrasive
cut-off machine with a strong coolant flow) and then pre-grind and polish the
cut sample. This would be followed by a cross-sectional hardness
traverse from the surface through to the core. Then note the hardness values.
You will find that the hardness value will be in the
range of 63-66 HRC (and possibly higher) just below the surface. This would be indicative of a region
of untempered martensite. Follow the hardness test with an etch using 5%
(approximately) solution strength nital, and etch the surface for approximately
4-6 seconds. This should be followed by a wash and rinse (in cold, clean running
water), and then the etched surface should be suitably dried. The sample should then be examined
under a microscope at 400-500X.
You will observe many different microstructures (including untempered martensite).
You will also (probably) see micro-crack propagation in the recast layer
(immediate surface). The question is then asked, “What can be done to prevent
this?” The answer is very short and sweet! Nothing can be done to prevent its occurrence. It is a process
chance that you take.
The only thing that can be done to minimize
the risk of crack propagation, either from the recast layer or the untempered
martensite, is to temper the steel immediately after the EDM process. The
temperature selected should be at least 380°F. The tempering temperature can go
as high as 50°F below whatever is the final tempering
temperature of the steels’ final heat treatment.
If you don’t have a tempering furnace, simply put the piece
(if it is small enough) into an ordinary domestic oven at 380°F and temper it
there. While the EDM process is an accepted method of machining, it is a method
that will (and does) affect the EDM surface metallurgy.
It would behoove some of the metallurgical research establishments to conduct
an investigatory research program into the metallurgical disadvantages and
possible advantages of EDM.
The question often arises as to when the EDM process should be conducted – before final heat treatment or immediately after final heat treatment? There is
no categorically correct answer to that question, other than to do what works best for you.
Heat treaters, please be aware of the potential problems you face if
you heat treat any tool steel that has been machined by the EDM process. A
potential surface crack may well be pre-existing, and you accelerate the
pre-existing crack by the subsequent heat-treatment procedure. Then you are blamed
for cracking the tool! To avoid that surprise, it may behoove the heat treater
to conduct at least a dye-penetrant check before proceeding with the heat-treatment
Electrical Discharge Machining (part 2)
By David Pye
David Pye is the owner and operator of Pye Metallurgical International Consulting, Saint Anne's on Sea, Lancashire, U.K. He has 25 years of practical experience in captive and commercial heat treatment, metallurgical laboratory operation and industrial furnace sales. He also has teaching experience on a very wide range of heat-treatment and metallurgical subjects.
He can be reached at email@example.com.
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