Pitting in ZDP-189 Core Steel

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aboynamedsuita

aboynamedsuita

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I was surprised (still am actually) to see that this happened given the high chromium content of ZDP-189. I had washed and rinsed the knife and probably had it resting on a towel for a few minuets while I washed a few other things prior to drying (I wash several things at a time for economy of motion, rather than wash/dry repeat).
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I've done this with R2 and even misono Carbon for way longer without issues, and with stuff other than tap water for rinsing. Ive left my cheap J.A. Henckels International knives in the sink overnight when I'm too lazy to wash them without problems (what are beater knives for?), and after this ZDP incident I even left my itinomonn wa butcher sitting for about 4 hours with chicken juice as a test and not a spot of rust to be seen – pics or it didn't happen:
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Anyone care to hazard a guess as to whether there may be metallurgical issues with this particular knife or if I should just not be so lazy and dry immediately?
 
Pitting is scary stuff. Seems like a problem with that steel, not you.
 
Wonder if you may have put it away last time and missed a small trace of water? Seems like it would take at least an overnight before anything so drastic...your other experience with the carbon is more of what I would expect...bet you got some nice color on that butcher now?
 
rick_english said:
Pitting is scary stuff. Seems like a problem with that steel, not you.
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Not what I was hoping to hear, but that said, I've never encountered anything like this before so it's all new to me.


chinacats said:
Wonder if you may have put it away last time and missed a small trace of water? Seems like it would take at least an overnight before anything so drastic...your other experience with the carbon is more of what I would expect...bet you got some nice color on that butcher now?
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I'd be really surprised if that was the case, I keep this one out on my mag rack so it's not in a confined space or anything like that. Surprisingly this happens with ZDP-189 being a stainless steel but the itinomonn and other carbons don't have this develop.

I remember when I picked it up to dry there were a few beads of water that were on the surface and I suppose this is where they were in contact. So shocked that this would happen from tap water and in a matter of minutes, not like cutting something acidic like lemons (but ZDP-189 is stainless anyways).

The butcher is actually kinda meh for color, there's some typical grey/beige patina forming on the cladding but this exercise didn't phase it much (planning to sharpen soon so no big deal if something happened). Kinda surprised by that too lol
 
Yes ZDP189 can be pretty reactive stuff. Do not regard %Cr as an absolute measure of stain resistance; chemistry is a bit more involved than that.
 
XooMG said:
Yes ZDP189 can be pretty reactive stuff. Do not regard %Cr as an absolute measure of stain resistance; chemistry is a bit more involved than that.
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What are your experiences with it being reactive?

My ZDP knife has been coated in water, citrus juice, and general grime for more than a few minutes and I haven't found it to be reactive in the slightest. I am surprised to see the OP's experience.
 
JDA_NC said:
What are your experiences with it being reactive?

My ZDP knife has been coated in water, citrus juice, and general grime for more than a few minutes and I haven't found it to be reactive in the slightest. I am surprised to see the OP's experience.
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As I said, the chemistry is a bit more involved. I have had three ZDP knives and two were moderately reactive, and it is not simply down to chemical composition as %weight.
 
And to backpedal a bit, I don't mean reactive as in discolored onions, but local corrosion like pitting.
 
ZDP-189 has 20% chromium, but it also has 3% carbon, which ties up most of the chromium in carbide formation. The corrosion resistance is pretty low; technically, it would be more accurately classified as a semi-stainless steel... Spyderco uses it in some of its knives, and users often comment on seeing rust spots after carrying them in their pockets whilst sweating, or even patina from fingerprints. Think of ZDP-189 as powdered metal D2 on very serious steroids. Other PM stainless's like R2 or SRS-15 have much more corrosion resistance by comparison.

The spotting will polish out, and the micro-chip on the edge (Which shouldn't be from the spotting) should sharpen out normally. True 'pitting' is actually a divot eaten into the steel, which I can't see in the photos (It just looks like 'patina' spotting, but could just be the photographs.), so it should polish out with a bit of metal polish. I wouldn't worry about it, if it was me, but the steel is teaching you about its character. I don't see what happened as a metallurgical defect. As said above, corrosion is a chemical reaction, and there are quite a few variables that influence the process. Different heat treatment practices can noticeably influence how reactive a steel is, which is an explanation for why one knife maker's steel reacts differently than the exact same steel from another smith.

Hopefully this helps...

- Steampunk
 
I agree with steampunk, 3% carbon makes ZDP 189 almost semi-stainless.
 
A high temperature tempering can have been used to get rid of retained austenite instead of cryo, this makes for lower corrosion resistance and can maybe explain the differences from different makers.
 
RDalman said:
A high temperature tempering can have been used to get rid of retained austenite instead of cryo, this makes for lower corrosion resistance and can maybe explain the differences from different makers.
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I have Miyabi 7000 MCD out of ZDP-189. This knife is cryo tempered and I do not have any issues with any kind of reactivity. This knife behaves just like standard Wusthof stainless.
 
Cool :)

Just to clarify. A high temp temper is usually around 500 c, with cryo you'd use the recommended lower tempering temperature to reach desired hardness after cryo, usually around 200 c.
 
krx927 said:
I have Miyabi 7000 MCD out of ZDP-189. This knife is cryo tempered and I do not have any issues with any kind of reactivity. This knife behaves just like standard Wusthof stainless.
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I had the same knife and mine was more sensitive to pitting and rusting than my yoshi skd ( semi stainless)
I am not sure if they have done a cryo on that blade , cryo on soft steel / hard core steel San mai may result in delamination
 
In the Zwiling catalogue the MC66 (=ZDP 189) is described as one * (out of 4****) SS. I have no piting pb with Miyabi 7000 MC but have some with the Ceramax with same alloy comming from the same factory.
 
chefcomesback said:
I had the same knife and mine was more sensitive to pitting and rusting than my yoshi skd ( semi stainless)
I am not sure if they have done a cryo on that blade , cryo on soft steel / hard core steel San mai may result in delamination
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That is interesting. My yoshi skd petty is much more sensitive.
About cryo, at the time when I was buying this knife it was said on their site that they are cryo tempered.
 
I think this might have been a droplet that persisted through drying... I have seen it before on my stainless knives sometimes I don't do a good wipe down at the end of the day... As long as you aren't constantly subjecting your knife to harsh conditions it will eventually become less noticeable.
 
RDalman said:
A high temperature tempering can have been used to get rid of retained austenite instead of cryo, this makes for lower corrosion resistance and can maybe explain the differences from different makers.
Click to expand...

Where can I find some info about this to read up on? I haven't heard of anything like it and it sounds interesting. The closest thing I know of that fits your description is martempering.
A quick google search didn't give me anything.
 
Thanks for all the responses. It sounds like it isn’t a problem afterall; I was mostly concerned if there was perhaps a metallurgical defect. The knife is a Miyabi 600MCD (same blade as the 7000MCD but with the full tang western handle) https://www.zwilling.ca/miyabi/knives-and-accessories/knives/600mcd

MAS4T0 said:
Is your water hard?
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Our water isn’t noted as hard, no issues requiring softeners or with soaps (although sometimes you can get water spots if you let dishes air dry).

XooMG said:
Yes ZDP189 can be pretty reactive stuff. Do not regard %Cr as an absolute measure of stain resistance; chemistry is a bit more involved than that.
Click to expand...
Steampunk said:
ZDP-189 has 20% chromium, but it also has 3% carbon, which ties up most of the chromium in carbide formation. The corrosion resistance is pretty low; technically, it would be more accurately classified as a semi-stainless steel... Spyderco uses it in some of its knives, and users often comment on seeing rust spots after carrying them in their pockets whilst sweating, or even patina from fingerprints. Think of ZDP-189 as powdered metal D2 on very serious steroids. Other PM stainless's like R2 or SRS-15 have much more corrosion resistance by comparison.

The spotting will polish out, and the micro-chip on the edge (Which shouldn't be from the spotting) should sharpen out normally. True 'pitting' is actually a divot eaten into the steel, which I can't see in the photos (It just looks like 'patina' spotting, but could just be the photographs.), so it should polish out with a bit of metal polish. I wouldn't worry about it, if it was me, but the steel is teaching you about its character. I don't see what happened as a metallurgical defect. As said above, corrosion is a chemical reaction, and there are quite a few variables that influence the process. Different heat treatment practices can noticeably influence how reactive a steel is, which is an explanation for why one knife maker's steel reacts differently than the exact same steel from another smith.

Hopefully this helps...

- Steampunk
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Appreciate the detailed information. The pictures aren’t very good as I took them with my iPhone with the max zoom (1st and 4th), max zoom through my loupe at 30X (2nd, 3rd and 6th), max zoom and my loupe at 60X (5th). The microchip came about as a result of the pitting (you can see best it in the 4th picture about 2/5ths up the edge from the left), and although it looks more severe in the pictures, it is actually eaten thru right at the edge as it now feels to have the chip in the edge rather than just the tarnish. Likewise the other spot in the 1st picture does in fact have a small divot. At first they were a bit darker but I suppose some of that tarnish/rust got removed with subsequent cleaning.

All this said it is good to know it isn’t a defect, I’ll just be cognizant of this moving forward.
 
Huh, interesting. I'm guessing that the retained austenite transforms into bainite with this kind of temper (at about 500°C), as opposed to turning into tempered martensite? I'm surprised at how little hardness gets lost in a temper that high of a temperature, compared to the drop in hardness in a plain carbon steel at even 300°C.

For whatever reason, both Uddeholm and Hitatchi suggest cold/cryo treatment over high temperature tempering to deal with retained austenite. I wonder if tempering embrittlement has anything to do with it?
I shouldn't derail the thread... :spankarse:


Edit:
TB_London said:
Google 4th stage of tempering for lots of detail

Verhoeven's seminal text discusses it briefly in chapter 9 http://www.hybridburners.com/documents/verhoeven.pdf
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Hah! I was right about the bainite!

Thanks a bunch for the resources, now I've got something new to read up on.
 
tjangula said:
Our water isn’t noted as hard, no issues requiring softeners or with soaps (although sometimes you can get water spots if you let dishes air dry).
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I asked because I've only ever experienced what you described in my hometown, where the water is super hard and has significant iron content.
 
I have a few Spyderco's A Super Blue, M390, & a CPM S30V. I bought a little ladybug about a year ago think it was on sale. I never even checked the steel on this miniature blade was sharpening it saw it is ZDP-189. A solid hunk of steel 1.75" long:biggrin:

In order of easy to sharpen to slightly harder. Easy Super Blue--CPMS30V--ZDP-189---M390.
 
DamageInc said:
I have a little pitting on my 7000MCD as well.

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How do you like the 7000MCD? I see it is discontinued and you cannot get it any more.

There were some quite bad reviews specially about the handle.

I quite like mine. Now after a thinning session it cuts great. But I never got any pitting nor even a slightest spots on this knife. I many times I am lazy and I do not wipe it and just leave it on the counter...
 
It's a nice knife, but the handle is atrocious. It's ungoldy handle-heavy. Terrible balance. But the blade is a nice middleweight.
 
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