What makes one "hard" edge better than another?

[Stacy E. Apelt - Bladesmith]

In recent talks off-forum as well as ones here I have been asked why some things, like lawn mower blades, are bad for knives if they can be hardened enough to get sharp.

First, hard is not exclusively a part of sharp. Everything from paper, to tin foil, to wood, to steel, to glass can be sharp enough to cut you. How many times in a row it can do that is another thing.

"Hard" is relative. It is also only one of many things that makes a knife blade good or bad. The things that need balance are hardness, toughness, brittleness, and wear resistance. I'm not going to go into the details about these, but they are why one metal is better than another for specific tasks.

When a steel with .84% carbon is hardened, all the carbon gets locked in the iron matrix, hopefully making martensite. There is no excess carbon left. Think of this as mortar cement in a perfect blend ( Portland/sand/water). Strong and reasonably hard, it makes excellent knives. Steel in this range is called eutectoid, and takes a very fine edge. it has reasonable wear resistance. Toughness is balanced by the tempering point.

When steel with more than .84% carbon hardens, the excess carbon combines with other things available, and forms other carbides...chromium, manganese, vanadium, tungsten, etc. these carbides are hard balls. Think of this as concrete, strong mortar with stones added to make it even more wear resistant. The size and type of the stones determines the roughness and wear resistance. Steel in this range is called hyper-eutectoid, and make very hard and tough knives. The edge can be fine or more coarse, depending on the size of the carbides. the harder the carbide type, the more wear resistant. Larger carbides may be dislodged from the matrix easier than smaller ones, so even though they may be harder, large carbides make for a shorter edge life.

Now, when the steel has less than .84% carbon, the iron eats up the carbon to make as much martensite as it can, and the rest of the iron remains as ferrite and similar iron structures, and unbounded alloy ingredients. This means that even though the steel got harder, it has little wear resistance due to all the soft stuff the harder martensite is in. This is like cement made with too much sand. The Portland cement combines with as much sand as it can, but there is still lots of unbounded sand in the mix. It will seem hard, but have very low strength. These steels are called hypo-eutectoid, and are tough, but have low edge life and wear resistance. Chromium is often added to toughen the steel up a bit.

When the carbon gets below .45% there just isn't enough martensite to overcome the weaker gaps in the matrix, and the steel will not become hard enough to be usable at all. This makes for a very tough steel, because it doesn't get hard enough to become brittle. These are called low-carbon steel or structural steel. It is excellent for construction of things that need strength and will not receive wear....like bridges and building frames. It is lousy for knives........ regardless of how much alloying is added.
This is where lawn mower blade steel generally falls. With all things done to the max, it still will not harden near what a eutectoid steel will...and won't work well as a knife.




I hope this simplified explanation helps newer smiths ( and a few older ones who swear by their low carbon blades) understand the reasoning behind the recommendation against using mystery and low carbon steel like mower blades.

Lets keep discussion pleasant and educational, as good discourse is how we learn best.

 

[LAHST Handmade knives]

This should be in the stickies IMO

 

[tryppyr]

Stacy,

Alloying is one thing... layering is another. If we're setting up a discussion of various ways steels can be combined to make the best knives, we probably ought to mention the two most obvious candidates in the mixed steel category... san mai and damascus. I won't pretend to be the expert... just thought I should introduce the discussion. That way someone might understand why mixing a tough steel with a hard steel could result in good (or bad) knives.

 

[Triought]

In recent talks off-forum as well as ones here I have been asked why some things, like lawn mower blades, are bad for knives if they can be hardened enough to get sharp.

First, hard is not exclusively a part of sharp. Everything from paper, to tin foil, to wood, to steel, to glass can be sharp enough to cut you. How many times in a row it can do that is another thing.

"Hard" is relative. It is also only one of many things that makes a knife blade good or bad. The things that need balance are hardness, toughness, brittleness, and wear resistance. I'm not going to go into the details about these, but they are why one metal is better than another for specific tasks.

When a steel with .84% carbon is hardened, all the carbon combines with iron and makes iron carbides ( cementite). Iron carbide is much harder than iron. Think of this as mortar cement in a perfect blend ( Portland/sand/water). Strong and reasonably hard, it makes excellent knives. Steel in this range is called eutectoid, and takes a very fine edge. it has reasonable wear resistance. Toughness is balanced by the tempering point.

When steel with more than .84% carbon hardens, the excess carbon combines with other thongs available, and forms other carbides...chromium, manganese, vanadium, tungsten, etc. these carbides are much harder than iron carbide. Think of this as concrete, strong mortar with stones added to make it even more wear resistant. The size and type of the stones determines the roughness and wear resistance. Steel in this range is called hyper-eutectoid, and make very hard and tough knives. The edge can be fine or more coarse, depending on the size of the carbides. the harder the carbide type, the more wear resistant. Larger carbides may be dislodged from the matrix easier than smaller ones, so even though they may be harder, large carbides make for a shorter edge life.

Now, when the steel has less than .84% carbon, the iron eats up the carbon to make as much iron carbide as it can, and the rest remains as ferrite and similar iron structures, and unbounded alloy ingredients. This means that even though the steel got harder, it has little wear resistance due to all the soft stuff the harder carbides are in. This is like cement made with too much sand. The Portland cement combines with as much sand as it can, but there is still lots of unbounded sand in the mix. It will seem hard, but have very low strength. These steels are called hypo-eutectoid, and are tough, but have low edge life and wear resistance. Chromium is often added to toughen the steel up a bit.

When the carbon gets below .45% there just isn't enough carbides to overcome the weaker gaps in the matrix, and the steel will not become hard enough to be usable at all. This makes for a very tough steel, because it doesn't get hard enough to become brittle. These are called low-carbon steel or structural steel. It is excellent for construction of things that need strength and will not receive wear....like bridges and building frames. It is lousy for knives........ regardless of how much alloying is added.
This is where lawn mower blade steel generally falls. With all things done to the max, it still will not harden near what a eutectoid steel will...and won't work well as a knife.




I hope this simplified explanation helps newer smiths ( and a few older ones who swear by their low carbon blades) understand the reasoning behind the recommendation against using mystery and low carbon steel like mower blades.

Lets keep discussion pleasant and educational, as good discourse is how we learn best.

This is great info. Thank you so much!

 

[Stacy E. Apelt - Bladesmith]

Stacy,

Alloying is one thing... layering is another. If we're setting up a discussion of various ways steels can be combined to make the best knives, we probably ought to mention the two most obvious candidates in the mixed steel category... san mai and damascus. I won't pretend to be the expert... just thought I should introduce the discussion. That way someone might understand why mixing a tough steel with a hard steel could result in good (or bad) knives.

Yes, selection of steel types for mixing is important, but the primary attribute need to be matching HT. This is probably best left for a different discussion, as this one has enough to grasp in itself.

 

[Triought]

Just trying to make sure I understand.

So I'm using 5160 High Carbon Steel. It says it has a typical chemistry of C .63, Mn .86, Si .23,CR .83

That would mean it has Carbon of .63, which means it's a hypo-eutectoid and will have low edge life and wear resistance? Is that correct? I actually bought it because on the website it said It has good wear resistance.

 

[Stacy E. Apelt - Bladesmith]

5160 is not high carbon. It is medium carbon steel with 1% chromium added for toughness. It has enough carbon to make good tough knives. The edges will wear faster than higher carbon steel, but they are good workers and very tough. Sharpening is simple and fast. Nothing wrong with 5160 for camp knives, choppers, and hard use knives. It would not be a good choice for kitchen knives, slicers, or fine edge cutting tools.

One thing that makes 5160 popular is the ease of HT and how forgiving it is if the HT is done with minimal equipment. This is due to the alloy content, particularly the 1% Mn.

I'll toss out another fact that is affected by the carbon content - the higher the carbon content the lower the tempering temperature. 1095 tempers ate 350-400F, 5160 tempers at 450-500F, and 1045 tempers at 800-1000F.

 

[samuraistuart]

"the excess carbon combines with thongs available" LOL. I'd like to see that centerfold! In all seriousness tho....great post Stacy. I always love the portland cement / sand analogy. It is what helped me understand this stuff!!!!

 

[me2]

Your 5160 should make a fine blade, provided you shape it properly and harden it to suit the purpose.

I would like to both agree and disagree with Stacy. In general, no it is not every steel that hardens that is sufficient for knives. I don't have a problem with anyone making blades from lawnmower blades and such. They will cut, and they will need sharpening on a regular basis. I have issue with people who dig around in their local junk yard and then claim to make a superior knife out of something they found there. They might, but it's highly unlikely.

My disagreement will be in the details.

"When a steel with .84% carbon is hardened, all the carbon combines with iron and makes iron carbides ( cementite). Iron carbide is much harder than iron. Think of this as mortar cement in a perfect blend ( Portland/sand/water). Strong and reasonably hard, it makes excellent knives. Steel in this range is called eutectoid, and takes a very fine edge. it has reasonable wear resistance. Toughness is balanced by the tempering point."

This is kinda the opposite of what happens. When the steel is annealed, the vast majority of the carbon is combined with iron in the form of iron carbide. Heating it for quenching breaks the bonds and in a 0.84% C steel and removes all or virtually all of the carbides. When quenched, the carbon which was separated from the iron gets trapped and can't recombine. This trapped carbon is what give steel the hardness. Hypereutectoid steel has too much carbide to break all the bonds at normal temperatures, so some carbide is left to give extra wear resistance. In hypoeutectoid steel, all the carbides will be dissolved, as in the eutectoid 0.84% steel. However, there isn't as much carbon to start with, so there is less trapped, and the maximum attainable hardness is decreased related to the carbon content.

It's a little known piece of information, but annealed steel has more carbides, iron or otherwise, than hardened steel. 1095 has roughly 15% carbide by volume when annealed, but less than 5% when hardened. Exactly the opposite of what you'd think.

 

[Triought]

5160 is not high carbon. It is medium carbon steel with 1% chromium added for toughness. It has enough carbon to make good tough knives. The edges will wear faster than higher carbon steel, but they are good workers and very tough. Sharpening is simple and fast. Nothing wrong with 5160 for camp knives, choppers, and hard use knives. It would not be a good choice for kitchen knives, slicers, or fine edge cutting tools.

One thing that makes 5160 popular is the ease of HT and how forgiving it is if the HT is done with minimal equipment. This is due to the alloy content, particularly the 1% Mn.

I'll toss out another fact that is affected by the carbon content - the higher the carbon content the lower the tempering temperature. 1095 tempers ate 350-400F, 5160 tempers at 450-500F, and 1045 tempers at 800-1000F.

Perfect, Thanks for the clarity. I called it High Carbon Steel as that is what Jantz called it. (http://www.knifemaking.com/product-p/xj418.htm)

 

[samuraistuart]

I'd like to hear more about carbides in the annealed state vs tempered martensite carbides if that's OK. Always wondered about carbide formation in annealed steel. Is the carbon separate from the iron annealed, is the carbon separate from the vanadium in the annealed state, etc. My guess was that no, the carbon is separate for all the elements until it is brought to critical were it combines with what it is going to combine with, and then trapped by quench. But I have no idea. I get mad at myself for not being able to visualize this stuff and figure it out.

 

[samuraistuart]

There is a cut off point that industry uses when talking about High carbon steel. Is it .5 points and above is considered high carbon? for us knife makers, it's .8 or so. Below that is relatively low carbon, right at it is uh....carbon...uh....and above it high carbon.

 

[Triought]

There is a cut off point that industry uses when talking about High carbon steel. Is it .5 points and above is considered high carbon? for us knife makers, it's .8 or so. Below that is relatively low carbon, right at it is uh....carbon...uh....and above it high carbon.

Interesting, this is good to know. It will make me pay more attention to the chemistry labels on the sites. I just got 5160 as it said it was easy to work and ht. I thought for first knife from a steel billet that it'd be the way to go. Just wish I wouldn't of gotten 1/4" thick.

 

[Stacy E. Apelt - Bladesmith]

me2 Thanks for the catch. I worded it wrong completely....don't know why? I changed it.

 

[Triought]

So how does the Chromium interact to add to the hardness along with the Carbon? If you don't mind me asking.

 

[Willie71]

Chromium increases hardeneability. It makes the steel deeper hardening, and slows the required rate of quenching to beat the nose (to the point of air hardening in high chromium steels.) Increasing hardness, and hardenability are different things and easy to confuse. Chromium also increases the austentizing temp.

 

[Willie71]

There is a cut off point that industry uses when talking about High carbon steel. Is it .5 points and above is considered high carbon? for us knife makers, it's .8 or so. Below that is relatively low carbon, right at it is uh....carbon...uh....and above it high carbon.

I always understood up to 0.4%C as low carbon, 0.4-0.7(.5)%as medium carbon, and 0.75% and up as high carbon. 5160 is a medium carbon steel as is 1066, but they get lumped in with other higher carbon knife making steels.

 

[Triought]

Chromium increases hardeneability. It makes the steel deeper hardening, and slows the required rate of quenching to beat the nose (to the point of air hardening in high chromium steels.) Increasing hardness, and hardenability are different things and easy to confuse. Chromium also increases the austentizing temp.

Perfect, thanks for the info!

 

[John Katt]

Thanks stacy

My steel IQ just went up 5 points ;0)

 

[me2]

The references I've seen give the cutoff point at 0.6% C. I believe it is done this way because there are some changes in the way things look under magnification that dont start until 0.6% C or above.