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Why is carbon steel easier to sharpen?

Joined
Dec 12, 2013
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191
This is a question that's been bugging me for a while, and I haven't been able to find a concrete answer yet. It's generally acknowledged that Carbon steel is easier to sharpen than stainless, but I've never really understood why. My understanding is that if you have a carbon steel and stainless steel with equivalent edge-retention, the carbon steel will be easier to sharpen. I presume it has to do with the carbides formed by the Chromium added to stainless, but wouldn't the wear resistance from that translate to better edge retention as well? How exactly does this work?
 
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In order to make steel stainless, chromium must be added to steel in excess of around ~13%. Some of this chromium becomes "tied up" and forms chromium carbide while the free chromium provides added corrosion resistance. Carbides tend to be very hard, much harder than the average 58-62 Rockwell range of the steel. This is what provides added edge retention and wear resistance. Another point of note is the hardness of the steel itself, carbon steels such as 1095 typically clock in around 57-59 HRC, while high alloy stainless steels can get into the mid 60's. The harder the steel, the longer edge retention. This comes at the cost of reduced ease of sharpening.
 
Joined
Jan 9, 2007
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Simple answer: Carbon steel isn't easier to sharpen than stainless steel.

Some carbon steels, depending on their heat treat are easier to sharpen than some stainless steels depending on their heat treat.
 

EChoil

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I know what you mean. To answer your question, it is because carbon steel is softer than other steels. Hardness is of prime consideration in sharpening.

Edge retention involves other factors in addition to hardness.

You'll get plenty of technical from the experts here.... :)
 
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Certainly hardness is a factor, but if I understand correctly, high alloy steels with a large volume of carbides at the same hardness of a carbon steel, are more difficult to sharpen because of the higher hardness of these carbide clusters. Big Chris, a knifemaker here on the forum, states that he gets a 5 second life span on his belts whens he's grinding on the high carbide steels he prefers to make knives out of.
 
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Joined
Jan 7, 2007
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I know what you mean. To answer your question, it is because carbon steel is softer than other steels. Hardness is of prime consideration in sharpening.

Edge retention involves other factors in addition to hardness.

You'll get plenty of technical from the experts here.... :)

This isn't inherently true. 1095 can be hardened to 66 HRC. Hardness is dependent on heat treatment.

the short answer is in general most ”carbon steel” blades aren't super hard or full of vanadium or.chromium carbides so they require less abrasive materials to cut away the steel.
 

sodak

Gold Member
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Mar 26, 2004
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... It's generally acknowledged that Carbon steel is easier to sharpen than stainless, ...
20 or 30 years ago, this would have been a true statement, but steels have come a very long way since then. It's really no longer possible to partition steels into these 2 groups and make general statements about them.

IMO, the big game changer has been the new technology (PM) for adding and distributing different carbides, in both stainless and non-stainless steels. There are also steels (ZDP 189) with around 3% carbon, which would have been impossible in the past. They would have become cast iron at that percentage.
 

daizee

Knifemaker / Craftsman / Service Provider
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Dec 30, 2009
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Stainless is also harder to sharpen because it loses grains and carbides in large chunks rather than small ones, so if you have bad edge geometry or the wrong abrasive, you will keep tearing your edge apart rather than sharpening it.

The PM steels reduce this problem to some degree. I've found S30V and S35VN easy to sharpen at production-knife hardnesses.
 

EChoil

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This isn't inherently true. 1095 can be hardened to 66 HRC. Hardness is dependent on heat treatment.

Which of what I wrote isn't inherently true...which of my five sentences? You lost me here.

the short answer is in general most ”carbon steel” blades aren't super hard or full of vanadium or.chromium carbides so they require less abrasive materials to cut away the steel.

Yeah, or in layman's terms: softer.

I know what he meant. Just making it simple for the man, man. If you want to expound it, be my guest.... :)
 
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There was a thread discussing carbon steel and other simpler steels vs. high carbide steel that went supernova some time ago. The discourse was mostly civil but interminable. So, here's how I see that. This is my opinion and I will deny I ever thought, much less said it if challenged at all.

Generally carbon steel is softer and tougher. It can be sharpened more easily and the edge can be recovered quickly partially because you are just re-aligning it to an extent. It responds to steeling (like, with a steel) like a kitchen knife if it isn't too hard. It will take a fine edge.

Your higher carbide stainless steels take longer to sharpen but hold that edge better due to the carbides. On simpler high carbide steels they may not hold the fine edge super well because the carbides are large. They will hold a toothy edge very well.

The PM (powdered metallurgy) steels have very fine carbides and are hard to sharpen as well and will hold a fine, or toothy edge well. I suspect they glow in the dark.;)

With the proper equipment the high carbide steels are not hard to sharpen. Carbon steel, with no carbides responds much better to simpler methods.

Carbon steel is also forgiving for so-so sharpeners (like myself) since the burr is easier to remove and probably other reasons I hope somebody will chime in with.

So, softer, does better with simple equipment, does better with lower sharpening skills.
 
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Dec 12, 2013
Messages
191
Thanks for all the replies. I was originally wondering this because I had picked up a Dragonfly2 in Super Blue a while back after being impressed by the ZPD-189 version, and when I compared the two, it seemed like the Super Blue held an edge about as well as the ZDP-189, but was significantly easier to sharpen. I'd heard it mentioned several times that carbon steel was easier to sharpen, so I had assumed that was the difference.

So, from what I'm hearing hear, it seems that:
1. Carbon steels are easier to sharpen because they lack carbides (makes sense)
2. Those same carbides contribute to edge holding by resisting wear on the edge, including sharpening (makes sense)

So, if I took a simple carbon steel and whatever it's stainless equivalent would be, assuming we don't get any additional elements outside of chromium mixed in, and they're equally adept at holding an edge, then they would be roughly the same difficulty to sharpen. Correct?

So, how is it that the Super Blue can hold an edge similarly to ZDP-189, considering that it (I assume) doesn't have as many carbides, while sharpening better? Is it simply a superior mixture of alloys, or better grain structure? Because right now, it seems like magic.
 
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Hypereutectoid steels have carbides. It isn't just having carbide, it is the amount of carbide, the type of carbide, and the average size. Super blue has carbide, it's what some of the alloying elements are there for. Similar hardness and matrix will give similar performance with similar geometry.
 
Joined
Jan 7, 2007
Messages
3,067
I know what you mean. To answer your question, it is because carbon steel is softer than other steels. Hardness is of prime consideration in sharpening.

Edge retention involves other factors in addition to hardness.

You'll get plenty of technical from the experts here.... :)

Which of what I wrote isn't inherently true...which of my five sentences? You lost me here.

Yeah, or in layman's terms: softer.

I know what he meant. Just making it simple for the man, man. If you want to expound it, be my guest.... :)

I went ahead and put what I was responding to from you in bold so you could see it. You said that carbon steels are softer than other steels. That is not universally true. 1095 is a carbon steel that can be hardened to over 65 HRC. In general the simple carbon steels are not run very hard, but that is not an inherent material property. Many non-stainless steels can be run very hard, like CPM-M4, REX-121, 10V, etc. Saying "carbon steel is softer than other steels" simply isn't true. 440A, 3cr and other stainless steels are often run in the low 50s HRC.

It all depends on what the manufacturer does.
 

chiral.grolim

Universal Kydex Sheath Extension
Joined
Dec 2, 2008
Messages
6,422
Thanks for all the replies. I was originally wondering this because I had picked up a Dragonfly2 in Super Blue a while back after being impressed by the ZPD-189 version, and when I compared the two, it seemed like the Super Blue held an edge about as well as the ZDP-189, but was significantly easier to sharpen. I'd heard it mentioned several times that carbon steel was easier to sharpen, so I had assumed that was the difference.

So, from what I'm hearing hear, it seems that:
1. Carbon steels are easier to sharpen because they lack carbides (makes sense)
2. Those same carbides contribute to edge holding by resisting wear on the edge, including sharpening (makes sense)

So, if I took a simple carbon steel and whatever it's stainless equivalent would be, assuming we don't get any additional elements outside of chromium mixed in, and they're equally adept at holding an edge, then they would be roughly the same difficulty to sharpen. Correct?

So, how is it that the Super Blue can hold an edge similarly to ZDP-189, considering that it (I assume) doesn't have as many carbides, while sharpening better? Is it simply a superior mixture of alloys, or better grain structure? Because right now, it seems like magic.

A couple possibilities (btw, the Dragonfly2 SuperBlue was HT'd to ~62 and the ZDP-189 to ~65 iirc):

ZDP relies on softer, larger chromium carbides while SuperBlue relies on finer, harder Tungsten carbides. While the ZDP total carbide volume may be higher, I'd guess ~25% to Blue's ~15% +/-3, that advantage in abrasive cutting is lessened by the nature of those carbides. For example, compare the wear-resistance of D2 to D7 below.

More important however, if the edges of each are finely polished for push cutting (e.g. 6000 grit), the rate of abrasive wear allows for a quickly blunting apex and makes it harder to discern any difference between the two (e.g. http://www.bladeforums.com/forums/s...based-on-Edge-Retention-cutting-5-8-quot-rope).

Furthermore, if the edges were not finished identically (same angle, same grit) and used in a relevant test (i.e. abrasive cutting vs wood-carving or meat-cutting), then a direct comparison of the steels cannot be made as those other factors have a much greater impact than steel-type in most circumstances.

Most 'carbon steel' blades are simple spring-steels like 1084 or 1095 with minimal carbide volume and usually low hardness. But even those with higher carbide-content and hardness tend to feature softer carbides easily cut/shaped by traditional hones (e.g. Arkansas). High-vanadium steels present harder carbides that require harder abrasives to sharpen. If the abrasive is sufficiently hard and sharp (e.g. diamonds, SiC), there is no discernible difference between the steel types. If it is not, sharpening difficulty may be proportional to total carbide volume even though wear-resistance cutting abrasive materials doesn't show it.

CPM3Vtotalcarbidevol100kb.jpg
 

sodak

Gold Member
Joined
Mar 26, 2004
Messages
5,997
1. Carbon steels are easier to sharpen because they lack carbides (makes sense)
2. Those same carbides contribute to edge holding by resisting wear on the edge, including sharpening (makes sense)
Go look at Ankerson's thread on edge testing, and see the latest knife that he tested. It's my Phil Wilson Coyote Meadow in CPM 10V, a non-stainless steel. See how it did for edge holding and how easy it is to sharpen.

You're trying to generalize in areas that you cannot.

Hypereutectoid steels have carbides. It isn't just having carbide, it is the amount of carbide, the type of carbide, and the average size. Super blue has carbide, it's what some of the alloying elements are there for. Similar hardness and matrix will give similar performance with similar geometry.
Nice to see you back and posting!!! :D
 
Joined
Dec 6, 2011
Messages
4,156
I think some of this idea is historical.
People used to sharpening a soft 1095 on Arkansas, & often steeling sometimes, buy a new knife in 440c, & find it hard to sharpen using the same technique. Hence, the legend starts.
 

David Martin

Moderator
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Apr 7, 2008
Messages
19,520
Ok, looking for the grain from the chaff on this and needing some straight forward information. I can buy the less carbides in the matrix thinking. I can also swallow the simple steel make up, thinking. Nothing real complex. Hence, easier to cut with a decent stone. (not a diamond or Arkansas) Just a mid-range. Heat treating the carbon to a higher hardness than a stainless and this gives equal edge retention but the non-stainless is still easier to sharpen. Causes me to cast a look of askance. DM
 
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EChoil

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May 22, 2014
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I went ahead and put what I was responding to from you in bold so you could see it. You said that carbon steels are softer than other steels. That is not universally true. 1095 is a carbon steel that can be hardened to over 65 HRC. In general the simple carbon steels are not run very hard, but that is not an inherent material property. Many non-stainless steels can be run very hard, like CPM-M4, REX-121, 10V, etc. Saying "carbon steel is softer than other steels" simply isn't true. 440A, 3cr and other stainless steels are often run in the low 50s HRC.

It all depends on what the manufacturer does.

"Can be," "can be...." coulda, woulda, shoulda, dude. I was keeping it simple. I stand by what I posted.

*LOL*

Go nuts---tech the guy to death with jargon....
 
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