A while back I bought a used, like-new ZT 0550 and I've carried it regularly since.  One thing I noticed is that this knife doesn't retain an edge worth a damn.  I've got a couple S30V Spyderco knives that get hair-popping sharp and then stay that way for a while.  The ZT 0550 can get just as sharp, but it seems like after a day or two of minimal use (cutting a steak, maybe an apple and some tape on a box) it's already too dull to cut the hair on the back of my hand.  Considering that the S35V blade should be at least as hard as the S30V on my spyderco knives, I can't help but wonder if this one wasn't heat treated properly.  Anybody have any issues like this with their ZT?

If you bought it used you have no idea what the previous owner might have done to it.  Are you sure is not a counterfeit?

I'm not aware of any counterfeit 0550s, but I suppose you can't be 100% sure if you're not the original buyer. That said, I see absolutely no signs that this knife isn't 100% authentic. Likewise, I can't be sure that the previous owner didn't do something to it, but when I got the knife it appeared to be in like-new, never carried condition, and never even appeared to have been sharpened.  I would be very surprised if the previous owner did some sort of back-yard hack that involved heating the knife to the point where it could effect the original blade tempering.

I've seen some factory knives with a soft edge before usually from burning the edge on final sharpening. Most of the time it only takes a few sharpenings to get back into hard steel though.

Sharpening a knife with a belt sander can overheat it and ruin the heat treat.    Either that or the blade or knife is a fake.

My money is on a knock off, however if you feel strongly enough about it contact ZT/Kai USA and see what your options are.

Always a possibility that a blade got shipped without heat treatment or poor heat treatment.  Know anyone with a Rockwell hardness tester?  That would verify the blade hardness.

Actually...

Has a matter of fact...

I happen to have a ZT 562 with Elmax steel might suffer from the same issue.

I purchased it new and it had a decent edge at first.  But after an extremely short period of time, it lost much of its edge.  I suspected a soft blade or improper heat treating and sent it back to ZT for sharpening and to ensure the blade treatment was good.  So far, its been okay since getting it back.  But it still seems to lose its sharpness faster then other Elmax ZT blades I have.