Cracking problem on 440C durring wire EDM 2

[Mononcue]

I have some parts wire edm cut from a round blank of 440C. The material is hardened to 52-56 HRC per AMS 2759/5 type 3 with sub zero cooling. The certs say it is 54-55 HRC. During the edm process the tips of my part tend to crack. This has only occurred twice out of many parts, but since it happens on the final process it is expensive as it scraps the whole blank. Last time this happened I had the remaining blanks drawn back a little and had no further problem. But now I have had the problem reoccur and I’m looking for ideas. The cracking only happens on a sharp edge near the center of the blank. Suggestions as to what may be the cause are welcomed as well as cures.

 

[israelkk]

Probably hydrogen embrittlement. The Wire EDM process produces hydrogen. We had similar problem when we machined a spring from heat treated bar, the spring shuttered to pieces first time it was loaded. The hydrogen penetrates the base metal and makes it more brittle than it already is. The internal stresses in the part cause the cracks. Maybe you can do stress relieve before the EDM and bake the parts immediately following the EDM to remove the hydrogen. However, all military and aerospace specifications forbid using hydrogen emitting processes such as coatings the produce hydrogen for metals in hardness over 40RC or ultimate strength higher then 180,000psi. It is better to first cut and them heat treat and final machine.

 

[Robertmet]

He said during EDM the part cracks. That would eliminate hydrogen embrittlement. I would try a stress relief before EDM to remove possible problems in the material. You should of course be tempering after EDM.

 

[Mononcue]

Let me clear this up a bit. I am assuming the cracks are happening during EDM. I have a 4.5" dia X 2.5 " blank that 9 curved pieces are being cut from. The shape is critical thus they are cut after heat treat. I have an outside shop doing the EDM. They rough cut all the pieces then do all of the finish passes,holding the parts by a small strip opposite end of the tip. As they snip off the parts one by one, and they remove them from the blank, they say some parts are already cracked . The only place they tend to crack is right on the tip which is in the center of the 4.5" dia. I just sent 2 more blanks back to heat treat to have them drawn back to around 52 RC to see if this helps.

 

[Robertmet]

What does the microstructure look like in the center ? Different from the OD ,coarse carbides ??

 

[Mononcue]

We have not done a micrograph exam of the structures as yet. I am considering trying a different material that may have a more uniform microstructure than 440C. This part is used in manufacturing medical devices (corosion resistant) and needs to be very stable to hold shape during the EDM process. Any sugestions?

 

[tripleZ]

Have you tried rough cutting a part prior to heat treat to hog out the majority of the stock, then running the finish passes post heat treat? Die inserts that I made from tool steel would sometimes crack on me due to the internal geometry and the stress induced during heat treat. But dropping a slug out in the soft state and leaving around 0.050" per side, we could set up and run finish passes post heat treat without experiencing cracking. Definitely more expensive with regards to setup time and processing time if you needed to increase the size of internal corners to avoid stress risers.

 

[EdStainless]

What temper temperature are you using? How may temper cycles?

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[Robertmet]

http://www.crucibleservice.com

has their CPM process used on a good assortment of steels. They have smaller , more evenly distributed carbides. Stability will also depend on the HT process.

 

[Mononcue]

These are the specs right off the HT certs.
Degrease, 1925 deg. 1.5 - 3 hrs, Argon, Oil 60 - 160 deg., -90 deg. or lower, 3 hrs. 675-775 deg.,3 hrs. Air 675-775 deg.3 hrs., air rem oxide, HRC 52-56
Actual HRC 54-55
I think next time I will try Crucibles material and perhaps another heat treat company.

 

[EdStainless]

good thought, I really suspect flaws in the original material

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