S90V vs K390. Makes no sense

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LostCause

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Apr 1, 2022
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I had 2 K390 knives so far. Delica and Endura. Sold them both (each to different friend, one of which has several K390 Spyderco models now).

I just got Manly City in S90V.
I cut same carpet I cut with K390 Delica and it also lost the shaving edge after 12 cuts, just like Delica.

But unlike K390 Delica which lost it's shaving edge in the meaning it got "smoothed out" or "rounded" and later struggled with fibrous material until sharpening, S90V lost it's shaving edge but still remained rough and had bite to it.

Then I stapled some cardboard and cut the cardboard and into the staple with S90V the same way I did with K390 that I had - and neither of steels suffered any damage to the edge.

Manly S90V is between 59-61HRC.
Spyderco K390 is at 64-65HRC.

I also remember K390 being easier to sharpen.


I contacted my friend and we'll meet to compare K390 and S90V toe to toe like real nerds. This hopefully gives us more insight in how these steels behave.

What I did was not scientific by any means, but it is rather confusing.

Here is what I'm trying to find out:
1. How does S90V have comparable edge retention to K390 while being at lower hardness?
2. Why is K390 easier to sharpen despite higher rockwell hardness and having the same edge retention as S90V?
3. Why is S90V not brittle despite being stainless steel with high edge retention and hardness?
4. Why does S90V keep it's "bite" even when seemingly dull?
5. Why does cutting with K390 feels smoother while cutting with S90V feels more agressive?
 
Not a metallurgist, but there are a lot of different factors in play in addition to hardness. For example:

1. Edge angle;
2. Final grit used in sharpening;
3. Carbide content;
4. Grain size.

Sharpening with a coarser grit gives a toothier edge that may be less acute (shaving sharpness), but has more bite and may retain working sharpness longer.

Sharpening at a lower angle will give you a sharper, longer lasting edge, although it will be more subject to chipping and rolling. Chipping would be more of an issue with K390 and s90v, although the former is tougher, even at higher hrc.

Carbide content is distinct from percentage of a particular metal in an alloy. Both K390 and s90v are composed of 9% Vanadium, but I don't how much of that Vanadium forms Vanadium carbide in the two steels. Since s90v is expected to have slightly better edge retention, despite being at a lower hrc, I would guess that it has a higher carbide volume.

In short, I would say that to do a more realistic comparison, you would have to sharpen both blades the same way (angle and grit progression).
 
I recall reading somewhere that when S90V first became available in knives, manufacturers didn't have the equipment to heat treat the steel to the hardness that's recommended today. So there's a pretty broad spread in Rockwell measurements in all the knives out there in S90V. Apparently the lower hardnesses still perform pretty well; it's possible yours might be lower than the stated Rockwell and thus less brittle.

I'm not familiar with the manly city knife, but I can say that a flat ground Delica is pretty thin behind the edge, which makes them quicker to sharpen than most other knives with the same steel (and even the same bladestock thicnkess).
 
As luck would have it I own both the knives being discussed.

1000017609.jpg

My calipers are at my shop but the max spine thickness is pretty similar.
1000017608.jpg


The grinds and bevel angles, however, are quite different:
1000017611.jpg
And the Spyderco bevel is a little "toothier."

I'll pull out my WorkSharp later and figure out the angles. I'd guess the Manly is 20° and the Spyderco is.... less.
 
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Before making determinations on either steel, did you make sure that both knives are at the same edge angle and edge finish, are properly deburred, and have been sharpened a handful of times to get through the burnt steel from the factory edge (possibly goes without saying that the edge you put on shouldn't be put on by a powered grinder, either).

I say all of that because your observations on edge wear don't track with the expected behavior of the steels, assuming both knives are heat treated correctly.
 
marrenmiller said:
Before making determinations on either steel, did you make sure that both knives are at the same edge angle and edge finish, are properly deburred, and have been sharpened a handful of times to get through the burnt steel from the factory edge (possibly goes without saying that the edge you put on shouldn't be put on by a powered grinder, either).

I say all of that because your observations on edge wear don't track with the expected behavior of the steels, assuming both knives are heat treated correctly.
Click to expand...
When I did that with Delica it was sharpened several times. Manly knife was sharpened once out of the box, right before this was done.

I sharpened both at their factory edge angles.

Both were sharpened on same stone and stropped with same strop and same compound.
 
Spillproof said:
As luck would have it I own both the knives being discussed.

View attachment 2359592

My calipers are at my shop but the max spine thickness is pretty similar.
View attachment 2359593


The grinds and bevel angles, however, are quite different:
View attachment 2359596
And the Spyderco bevel is a little "toothier."

I'll pull out my WorkSharp later and figure out the angles. I'd guess the Manly is 20° and the Spyderco is.... less.
Click to expand...
I only have this one at the moment. But should meet my friend this weekend to test it directly compared to Delica that he has.
20231017-102333.jpg


This is first S90V steel knife for me and so far the steel behaves unexpectedly.

I expected it to be very hard and very brittle. I also expected edge retention to be significantly lower than K390. And I thought it'd be easier to sharpen because of lower HRC.

My expectations did not come true.
 
Relative to other hard stainless steels, S90V is not brittle. Lots of carbides, and keeps biting for a long time. One of my favorites ...

It's on Larrin's "blue line", identifying ideal compromises between edge retention and toughness.

stainless-toughness-edge-retention2.jpg
 
I lust after knives in S90V. They stay sharper longer, aren't brittle, and are stainless enough for almost everything I need it for.
I also have knives in K390 (Delica, Stretch 2), and while they are very good cutters, they develop a patina quite readily whenever I'm cutting almost any type of moist material.
My knives in S90V (quite a few Sypdercos and several customs) cut for longer before needing a touchup, and don't require much maintenance at all except for the occasional strop on leather with some diamond spray. They are all also patina-free.
 
LostCause said:
When I did that with Delica it was sharpened several times. Manly knife was sharpened once out of the box, right before this was done.

I sharpened both at their factory edge angles.

Both were sharpened on same stone and stropped with same strop and same compound.
Click to expand...
A difference of a few degrees at the edge might be accounting for much of what's going on here.
 
Delgadonine said:
Nothing to say really, other than this is first time I've heard of K390. Thank you for this information.
Click to expand...
It's awesome in edge retention, but stains easily; akin to 1095 carbon steel. It makes D2 seem stainless, by comparison.

Spillproof said:
As luck would have it I own both the knives being discussed.

View attachment 2359592

My calipers are at my shop but the max spine thickness is pretty similar.
View attachment 2359593


The grinds and bevel angles, however, are quite different:
View attachment 2359596
And the Spyderco bevel is a little "toothier."

I'll pull out my WorkSharp later and figure out the angles. I'd guess the Manly is 20° and the Spyderco is.... less.
Click to expand...
Great post, and I think the shallower edge grind on the K390 Delica explains everything.


gooeytek said:
I lust after knives in S90V. They stay sharper longer, aren't brittle, and are stainless enough for almost everything I need it for.
I also have knives in K390 (Delica, Stretch 2), and while they are very good cutters, they develop a patina quite readily whenever I'm cutting almost any type of moist material.
My knives in S90V (quite a few Sypdercos and several customs) cut for longer before needing a touchup, and don't require much maintenance at all except for the occasional strop on leather with some diamond spray. They are all also patina-free.
Click to expand...
For my uses, it is The Perfect Blade Steel. I know sharpening will take a few more strokes, but with a good edge grind to start with, it will still be no sweat.

I have a Spyderco Leaf Jumper in K390, and it is awesome, but as you say, it stained immediately. I expected it to stain, but was surprised how quickly it happened, compared to D2. I don't consider it a downside, just something to be aware of and something that makes it less than perfect.
 
I'm going to have to re read this post again, and do more research....
I was thinking of making a few fixies this winter out of k390, or 10V, or something....?

But maybe I should also include S90V in my consideration?
 
First, Manly’s website specs the S90V City at 60-62 HRC, so that’s the range I would consider accurate. Per Sal, “K390 runs about 64.” So, the specs are closer than you thought.

Second, it’s generally accepted that you don’t get a true representation of a steel’s edge retention until you’ve sharpened the knife a few times to get past the factory edge. Pretty much every factory edge is machine ground and “burnt” to some degree.

Third, Larrin’s testing had S90V at 61.5ish HRC out-cut K390 at 63ish HRC by about 7%. So I think it’s safe to say that your experience is not necessarily unexpected or unprecedented. Carbide type/hardness and carbide volume are very influential for edge retention, and I believe S90V has higher carbide volume than K390. This would also track with K390 having significantly higher toughness.
 
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