Using the chemistry, RE: sword steel

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Kevin R. Cashen

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Sep 9, 2003
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I thought I would add my $ .02 to the question posed in the thread “Kevin, or Mete', sword of 01”, however I have always been uncomfortable with thread titles addressed to specific individuals due to the exclusionary overtones, and I really don’t wish to be another little tin god on the knifemaking scene, so I have reposted here where I invite all with some input to also give answers. Posts within threads addressed specifically to me are different and work fine but whole threads with my name in the title make me out to be much more important than I am. But don’t sweat it LRB, I am not upset and am more than happy to share what I can, I just want anybody else to feel welcome to do so as well.

But so as not to make this some odd thread that is really just an out of place post from another, I thought I would share some of my notes on the “Steel Selection” part of one of my classes.

I divide knives into three using categories, slicers, choppers and piercers. The piercers are a touchy category that I chose to name piercers instead of stabbers or thrusters due to the touchy and politically charged overtones, and are really not at versitile in legitimate uses outside of combat so I will focus on the other two.

Within slicers and coppers I recognize two types of cutting mechanisms on the microscopic level. There are saws and wedges. Wedges excel in the push cut by separating materials and smoothly displacing them. Saws use rakers to tear materials fibers out the way in a draw cutting action. In both of these edge geometry will play a very significant role but for the other major factor, heat treating, we will discuss the chemistry of the steel.

True choppers work within a realm dominated by different factors than many knives and are primarily used as wedges more than saws. While some swords can be used well for draw cuts almost all see their most cutting potential in a cleaving wedge action. In such use impact toughness is more critical than abrasion resistance and your steel choice may be determined with this in mind.

The simplest and oldest way to get better toughness is to simply lower the hardness, this is the way it was done for centuries and how the majority of sword makers still approach things today. The problem with sticking with this ancient method is that it doesn’t seem to take into account the development alloying. With a straight carbon steel going above HRC 55 can mean brittleness and catastrophic failure in a sword. However with the inclusion of other elements to help reinforce the iron matrix most properties change dramatically and one can get much higher toughness at a significantly higher hardness.

The next simplest way to obtain toughness is through carbon content, but this method is very similar to the hardness method in that it will always be a compromise between strength and ductility. Steels with carbon content less than .45% will suffer enough from insufficient hardness that I will not cover them here, but a blade made from steel that has less than .6% Carbon will still noticeably harden with a slightly different makeup. Those who have looked into it will have encountered the two morphologies of martensite- lathe and plate (did you really think it was as simple as one martensite and that is it?). Plate martensite forms in steels that are .6% carbon or less due to the temperatures at which this ratio makes the stuff. Lathe martensite consists of nicely symmetrical lathe packets that look almost like fern fronds in the steel, but more importantly it forms along more order rational habit planes so the arrangement is much less chaotic and stressed. In steel with more than 1% carbon you will form plate martensite that looks more like a shattered window piled in a bucket than pretty fern fronds, and works off from irrational habit planes that impinge upon each other at high angles creating a much more stressful situation, so this stuff is very brittle. In the range from .6 to 1.0% carbon you will get varying mixes of lathe and plate. So staying at .6% or lower in carbon content will automatically give you a tougher steel.

Now for the elements, if toughness is what you are looking for you are looking for elements that favor iron more than carbon. Those that like carbon will form carbides and carbide is always more brittle than iron based solutions. Two elements that come to mind immediately for this are nickel and silicon. Neither care much for carbon and will want to nestle in between iron atoms instead. This will enhance the softer ferrites ability to resist deformation and coming apart. However some elements, such as chromium, can also lend a hand in propping up the ferrite, I know what you are saying – hey Kevin isn’t chrome a carbide former? Yes it is, but chromium is also capable of dissolving somewhat in ferrite even in high carbon levels (see “Alloying Elements in Steel” by E. C. Bain).

So with these thoughts in mind we can help narrow our sword steel choices by looking at the chemistry first. Steel with nickel or silicon will allow us to go higher in hardness while maintaining good toughness. Steel with carbon levels around .6% will lend even more toughness, and as long as we avoid carbide formers there will be enough to reach a formidable martensite hardness. If we see things like vanadium, tungsten, columbium etc… then the carbon must go up accordingly to feed these greedy elements and still have enough to reach the hardness levels we desire. And when this happens many more carbides are formed and this will affect toughness if not carefully dealt with. And then you will end up with all kinds of whacked out things like intentional alloy banding and segregation that would not have been necessary if the maker just would have chosen a steel suited for the task.

It is not that O1 would make a bad sword, and many of the arguments I have heard against O1 over the years could be best handled by the grumblers learning how to heat treat this steel properly. But O1 has some added features that are overkill in areas not critical for sword use. O1 is a slicer steel more than a chopper steel due to it focus on abrasion resistance. It can make a nice sword, but L6 would beat it in that capacity. 5160, with its lower carbon and chromium boost would probably also be more suitable, not because it is a better steel but just simple efficiency, it is not loaded with a bunch of unnecessary features.

As far as the tempering range of O1 being a problem with a drop in impact strength, a phenomenon known as TME (tempered martensite embrittlement) I doubt I have pointed much to O1 for this issue. In the range of 450F to 550F L6 shows a significant dip in the toughness curve while O1 shows a plateau at the most. So in this case O1 does actually have the upper hand
 
In choosing a steel for slicers, you will want strength. It will need to resist deformation as much as possible, and then stand up to wear even more so now we look at other elements.

Higher carbon levels will result in a little more martensite hardness over and above the .6% previously discussed but only to a point. And that point is the eutectoid, at .8% carbon little more is gained by piling on more carbon except higher levels of plate martensite and increased possibilities of retained austenite. Above the eutectoid you will have extra carbon to deal with so that it does not become more of liability than an asset. One way to do this is to remove it from solution and tie it up in compounds. Something like 1095 will do this in the simplest form of cementite (iron carbide), if you keep it fine and evenly dispersed this will result in a steel that is like a fine aggregate similar to concrete, a rather hard material filled with very hard particles, making the entire object quite strong and very hard to abrade away.

However if you loose control of that carbide spacing and size instead of a fine hard concrete, you can end up with something that more resembles a crumbling old barn foundation, with huge chunks of rock separated by softer mortar. To keep things fine and very strong we go with grain refining, carbide forming elements. One of the best is vanadium (there are two stronger carbide formers but they are less common in the steels we are most familiar). Vanadium in quantities up to .2 to .3% will from very nice fine carbides that keep grain boundaries from moving resulting in very strong steel that wears pretty good, but in larger quantities will drastically increase the wear resistance of steel to levels that will challenge the most patient stock remover (it will also stiffen things significantly under the hammer).

With strong carbide formers you will need to bump up the carbon levels as they will lock onto free carbon and not let go, and can significantly reduce hardenability because of it. Adding .5% vanadium to a .8% carbon steel could reduce the available carbon level enough to make it behave like a .45% carbon steel in heat treating unless one heats it above 1950F and deals with the hazards that would bring.

So with simple carbon steel there will be excess carbon to deal after maximum martensite hardness is achieved, but with the addition of a carbide former that excess can be put into a more stable form that can be quite beneficial in both grain refinement and abrasion resistance. If one has a good grasp of the concepts and equipment that will allow the necessary control, one can see how it could be possible to evenly distribute very fine carbides throughout a matrix that has just enough trapped carbon for maximum hardness. Such an edge will not deform or roll on the micro level and will stand up to anything softer than those carbides, and there is a whole world of things softer than those carbides.
Will this edge top out in impact strength? No, but that is not very important in a slicer. Here things like high nickle and silicon are the uneccesary overkill.

But then we get into the slippery slope of specialization. The concept of moderation in all things is lost on some folks and they see a slicer as such a saw that everything is seen as a ripping saw instead of crosscut and they go for the most aggressive rakers they can. This will indeed result in very impressive cutting ability but only with rather specific materials due to the inherent weaknesses of this extreme.

For such steel higher quench hardness levels will be aimed for in order to maximize strength over impact toughness, and the guy who decides to chop things with a fine edged skinner gets what he deserves.
 
Thank you for sharing your knowledge and an interesting read. I have learned so much more about the effects of chemistry and what actually happens when hardening and tempering from you and some others. I appreciate your efforts in educating myself and others and doing it in a way that is easily understandable. I can not express enough how I believe this will effect my knives and their performance.
 
i just puilled some thing from all this

cpm3v is .8 carbon and has 2.75 V in it so is that why the temps are over 2000 for most heat treating

it was my thought that having a "cpm2v" beign something on the order of 1.0 carbon and 2.0 V with the same other as 3v would make for a great steel (fine grain higher wear and higher hardness) as i could then harden it up to 64-65 and temper down a hair like i can with cpm10v. as fine as 1v sounds it to me jsut seems like its too soft being .55 carbon and 1.0 V
mind you im looking for a good high carbon kitchen knife steel (air quench)and love 3v but think i should be able to get it harder
 
The elements V, W, and Mo have a very strong bond with carbon thus require higher temperatures to dissolve those carbides.High V content makes for harder to grind blades. Have you tried CPM154 ? Makes for a very fine blade for both the maker and user.
 
yep mete i love the cpm154i run it at 62 in the kitchen but im looking for a great high carbon steel to go with as an option
 
Kevin, I did not mean to exclude anyone, but you and Mete' are the two I trust the most for questions involving metallurgy. Thankyou both, and all others, for your input. I will use 5160, or L6. Does anyone know where I can buy it precision ground?
 
It's nice to be discussing at senior college level again. may i add another factor to mr cashen's list: mechanics. take a box cutter. it's a classic slicer made of fairly high carbon steel. the blade is mounted so as to as to drag the point at least two inches above the hand when exucuting a draw cut. the 2-inch extension magnifies the hand's leverage, thereby adding pressure to the work being cut. also, it is the point that bears down on the work so it is not strictly a perpendicular slicer. given the above situation, the basic design of both the blade handle is to minimize resistance by the work from both piercing and sawing action. quite simply one has to offer a fresh tip against the work as often as possible. so brittleness is not avoided. it is necessary.
 
Thanks for the wonderful information, guys. This confirms what I thought I already knew from reading bits & pieces over the years.

I have another question to bring up since we're on this topic though. I recently participated in a discussion with a gentleman who had differing thoughts on some things... He seemed to agree that toughness and impact resistance could be increased by going with a lower carbon steel, but he maintained that you needed lots of carbides to provide strength. He said the lower carbon steels or alloys made for shock resistance would suffer from plastic deformation vs. high alloy (such as high carbide stainlesses) steel, seemingly regardless of hardness. Is there such a tradeoff involved (a difference that could be realistically noticed in real world blade use, that is)? Do lots of carbides increase strength at the cost of impact resistance? Or is this more a function of the hardness the matrix is heat treated/tempered to?
 
Very nice thread, more information than one could find in a whole day of seaching. Thanks for sharing the knowledge.

Rob
 
LRB, why do you want precision ground ? You're going to grind it anyway so you lose the benefit [but not the price !!!] of precision ground.....Possum, carbides do not add strength , they only add wear resistance. Strength of a blade is dependant on strength of the matrix. To increase strength of the matrix you would saturate with carbon [~.80 %] and add other alloying elements.Nickel BTW has the ability to increase impact strength and we see this in L6. I've never been able to find exactly what the mechanism was. As far as carbides and impact strength - fine carbides as in the CPM steels offer higher impact strength than conventionally made steels with larger carbides.
 
Thank you, Mete. Further details-

To increase strength of the matrix you would saturate with carbon [~.80 %] and add other alloying elements.
Click to expand...

Does this assume the higher carbon steel/matrix would be stronger because it's capable of reaching higher hardness? I realize there are some differences in things like tensile strength between alloys, but realistically speaking... Would 1080 be noticeably more resistant to rolling and plastic deformation than 1055 if both were tempered to the same hardness?

Regarding other alloying elements in the mix, I understand this can be used to increase impact resistance, but do they also have a noticeable effect on static measures of strength, such as tensile, compressional, etc.?
 
Actually, while we're at it, I had a couple of points of curiosity, for anyone who could answer them.

First, Does the presence of carbide forming elements impact the morphology of the martensite as based on the carbon content? Basically, is the tyoe of martensite dependant on the free carbon taken up hardening the martensite, or is it more simply dependant on the presence of carbon atoms, even if they are being tied up in, say, vanadium carbide?

Second, where elements like Nickel and Silicon are concerned, I have read that they have a similar ferrite strengthening effect, thus both increasing toughness post heat treat. What I have not been able to find out (admittedly with a minimum of detached effort thus far) is weather they are equally as effective, and weather there would be a point to having both. Do Nickel and Silicon have any sort of synergistic or exclusionary effect? While I know the action, effect, and etc are very different, Chromium and Molybdenum (for example) are added together to a good many steels, apparantly both to aid in hardenability. Since this is done so often, I must assume that in that case, there is some inherent advantage in using two such elements, or, at least avoiding using too much of one. Is this the case for Nickel and Silicon? I noticed that Crucible adds both to their L6... Why not just more of one or the other?
 
Dan Zawacki said:
Actually, while we're at it, I had a couple of points of curiosity, for anyone who could answer them.

First, Does the presence of carbide forming elements impact the morphology of the martensite as based on the carbon content? Basically, is the tyoe of martensite dependant on the free carbon taken up hardening the martensite, or is it more simply dependant on the presence of carbon atoms, even if they are being tied up in, say, vanadium carbide?
Click to expand...

This is a matter of easily confused cause and effect. More recent research into martensite morphologies have revealed that they are probably the result of the temperature at which they are formed, however this is inextricably tied to the alloy content. Most alloying will lower Ms and carbon is one of those, so the more carbon you have, the lower will be Ms and this will make the martensite more plate like in nature. So this gives the possibility of raising Ms by not putting as much carbon in solution, but allowances would also have to be made for other alloying elements, things like moly have a very profound effect on Ms. But much of this is a mute point to most bladesmiths since the controls necessarry to put precise predetermined ammounts into solution, and no more, is not exactly realistic with a forge:( All the same if you add this to the hazard of retained austenite, it you get all the more reason not to overheat your steel before the quench.

Second, where elements like Nickel and Silicon are concerned, I have read that they have a similar ferrite strengthening effect, thus both increasing toughness post heat treat. What I have not been able to find out (admittedly with a minimum of detached effort thus far) is weather they are equally as effective, and weather there would be a point to having both. Do Nickel and Silicon have any sort of synergistic or exclusionary effect? While I know the action, effect, and etc are very different, Chromium and Molybdenum (for example) are added together to a good many steels, apparantly both to aid in hardenability. Since this is done so often, I must assume that in that case, there is some inherent advantage in using two such elements, or, at least avoiding using too much of one. Is this the case for Nickel and Silicon? I noticed that Crucible adds both to their L6... Why not just more of one or the other?
Click to expand...

CHAMPALOY
(AISI L6)
Typical Chemistry
Carbon 0.75%
Manganese 0.70%
Silicon 0.25%
Chromium 0.80%
Nickel 1.50%
Molybdenum 0.30%

The chemistry that Crucible gives for the steel they buy and sell shows nickel as the primary element of the two, with silicon almost falling in the trace element range, but not quite. In this case I would say that nickel is there for the toughening effect and silicon is there for deoxidizing effects and perhaps some toughening without stabilizing austenite as much as nickel could. This steel can be prone to oxidizing issues in uncontrolled furnace atmospheres with a fine balance between oxidizing and decarburizing.
 
I love this discussion, it is like being back in school. I really have to concentrate as it has been 25 years since my last Met Lab. Steven
 
That's a wonderful find hardheart made. I have Bain's book in print but the wealth of geek info on the site is incredible.

Thanks, hardheart!
 
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