Perhaps a basic curve discussion?

[Kevin R. Cashen]

On the chart above you will see the curve for soft steel yields at a very low level of force, this is because it is “ductile” (easily deformed) that is after all the meaning of soft. While the fully hardened steel requires the most force to deform because it is “strong”, able to resist deformation. Fully hardened steel resists deformation to the point that the only way for it to give when its limits are reached is to come apart. This is the ultimate strength, in fact that is what it is referred to on the chart when a steel has yielded and just can’t take any more. Many graphs will show the ultimate strength as a point beyond the peak in ductile strength but will recognize that in reality ultimate strength is the point of total failure where the steel will just come apart.

Measuring the force required to flex a piece of steel while in the proportional range is a waste of time! Any sharp engineer could crunch some numbers and give you the actual force without ever holding the blade as long as he had the dimensional cross section measurements. Which brings up the point that if one wants to change the amount of force to flex , we have established that they can forget about the heat treatment, but they can simply change the thickness versus length and get instant results. It takes little force to take a fillet knife to 90 degrees, and it may even avoid taking a permanent bend. On the other hand a blade that is over ¼ “ thick will require a cheater bar and will be more prone a permanent set because of it. If you want a decided advantage in the bend test without even knowing how to heat treat, make a blade that is wide and thin.

This is why when person tells me they made a blade that “flexed” to a certain degree and returned to true, and then insist on feeding me a line about their heat treating- I don’t care! It isn’t relevant, and the only thing they have given me information on is how they grind their blades.

Under the accurate definition of “strength”, soft spines do not make stronger knives, they make them weaker (a better term would be ductile or tough). And this takes us to the next issue- the huge confusion over tensile ductility and impact toughness…

 

[Kevin R. Cashen]

What falls in line with the function of a knife more, slow prying with the flat of the blade, or sudden loading into the edge (such as chopping)? Long ago I shifted my emphasis in testing away from bending and onto impacting. I am making knives, not crowbars, and if I were making a crowbar, I wouldn’t want the dang thing to bend! How many crowbars have you seen that easily bend? They wouldn’t be able to perform prying tasks if they just yielded under the force. A bent blade is neither a good knife nor a pry bar, it is just bent! Using what we have discussed here, if you have a customer that can’t tell the difference between a $500 knife and a $10 crowbar, don’t change your heat treatment, make him a knife that is thick enough to do the job. The edge may no be as keen but if you are going to use a fishing pole to drive nails one has to expect to give a little in its trout catching abilities.

What we really want is a piece of steel that is a proper compromise between the extremes of brittleness and ductility, if we find that sweet spot where the edge does not deform in use and the blade does not fail under sudden load, we have success! What would be worse, some bozo doing something ridiculously abusive that has nothing to do with knife use and breaking or bending your blade or having the edge fold over or the blade break in a simple chopping task?

All this being said one cannot ignore the marketing value of a differentially hardened blade. This canard has so thoroughly saturated the knife market that suggesting that it is anything but the gospel may not be wise marketing. Temper lines do indeed have aesthetic value and I will still occasionally incorporate them purely for that reason. So let me say that I have never been a shrewd businessman or a slick salesman, and would willingly lose sales by proudly proclaiming my blades through hardened. The fact that so many knifemakers believe that “soft” is “strong” means that even more knife buyers don’t get it, and so many people have pushed the idea that it is the sign of a great blade that for sales it probably is a better way to go. Heck most knife uses will not show a difference in performance anyhow, so this gives us the latitude of having so many different ways of making knife that still satisfies the public.

So I would say make the knives you wish to make, but I think that honesty, facts and proper terms that don’t mislead, is not too much to ask.

 

[Jose Reyes]

Hi Kevin,

Not sure I understand your graph but is a. total resistance, b. where the steel would "flex" and c. when the steel will bend? Is it possible that a properly fully harneded blade will exhibit some degree of flex or bend before it fails? Is the force required to edge chip or crack such a blade proportional to the increased force required to bend it? Will this have any effect on the tendency for a crack to propagate across the width of the blade?

I also wonder whether 52100 and 5160 are considered simple steels and it's just a coincidence that using a magnet during HT will work or if they are not and that's what you find so perplexing about Ed's claims. Does your ability to determine the precise structure of the steel throughout the blade play a significant role in your preference to go the fully hardened route? Do you consider Ed's methods more fraught with peril, more chance to have something go wrong or to make a miscalculation, or do you believe there are structures forming that should not be present? In your opinion would etching and testing the blade reveal any mistakes that were made?

Have you given any consideration to the cross section of the hardened steel? As I understand it with edge quenches or multiple edge quenches the hardened area rises towards the spine in a diamond cross section. I know there are other methods like drawing back the spine after a full HT but I imagine there would be internal differences. I read on one of these threads that Ed reffered you to some published work mentioning multiple quenches used in industry and wonder if you found anything enlightening.

I probably have a few more questions but it's past my bedtime so I'll leave those for another day. If some of these questions are off topic, please forgive me, but they stem from your writings on this and other threads that I've read over the last few days but didn't respond to. My avatar and sig line might be enough for you to consider me coming at you with both barrels blazing but that is not the case, and I guess would not be much different than me assuming you have some ingrained dislike for Ed personally or his work.

What a beautiful sword you have available on your site, congrats, all the hard work paid off. Very nice work on the acorns, they are fit for the most discriminating cheek pouch... Something that previous commitments keep me from buying but that I would love to have in my collection. :thumbup:

Take care,
Jose

 

[Jose Reyes]

Hi Kevin,

Okay, I see you have made additional posts while I hunt and pecked my response but I'll have to read them tomorrow!

Jose

 

[Kevin R. Cashen]

Jose, to conclude that I have any personal axe to grind with anybody would be a false assumption. I have encountered very few people in this business that inspired a dislike on a personal level and none of them have been mentioned here, nor would I deal with them in a public manner. You see I am a fact and logic junky, I love these things because there can be no personal feelings wrapped up in them, they just are.

You will notice that I have pointed out no particular maker’s method or school of thought in my posts regarding the stress/strain, for indeed no one person can be focused on in a concept that has been tried by virtually every smith in the business today. I myself got a fancy little stamp given to me for making edge quenched blades and bending them.

If I have been too aggressive on this topic an apology may be in order to Alan Molstad, for it was the confusion on some of the concepts in his thread http://www.bladeforums.com/forums/showthread.php?t=447945 That made me think that a stress-strain curve should be included in this thread.

I will be happy to discuss any of the questions raised in your post after I have actually put some time in the shop this day. After all, discussion is what this is all about.

But there will be no personal attacks by me, and I would urge that folks not try to turn this into an "us vs. them" sort of thing as that is pointless and nonproductive. Personal methods or schools of thought are not on trial here, and focusing any one maker’s ways would have little to do with the principles involved. Since I am a fact junky the best way to defeat me is with facts.

As for your avatar, the maker presented has made such an impact on our business that one needs only to see them from afar to instantly know his work, we all should aspire to such achievements. This would not be the first time I have said that I would be very happy myself to own any one of those blades, and any shrewd collector would recognize your good fortune.

 

[thombrogan]

Since I am a fact junky the best way to defeat me is with facts.

Hiding my vial of holy water with bainitic stopper (that is, the stopper is made of bainite, not that the vial contains something which will stop me or others from discussing bainite - only Jerry can prevent bainite).

 

[Kevin R. Cashen]

What a beautiful sword you have available on your site, congrats, all the hard work paid off. Very nice work on the acorns, they are fit for the most discriminating cheek pouch... Something that previous commitments keep me from buying but that I would love to have in my collection.

Jose, let’s start by allowing me to make up for my bad manners and thank you for your very kind words about my hunting sword. That sword has actually just been sold, but I get too side tracked on the internet to visit my own site when I should. I am conscious that your mere mentioning of it is an unexpected favor in bringing attention to my available work, as I mentioned previously, I am a terrible businessman and despite the encouragement of others cannot bring myself to much self promotion on these forums, so the mention was a nice perk to your joining the discussion.

Next I would also like to apologize if I left the impression that anybody would intentionally mislead others in the description of their work or testing. Alan Molstad has impressed me with his friendly, straight forward posts and highly constructive questions, my impression is that his knives will quickly be as fine as the gentleman he appears to be.

One troubling aspect of this thread is how, while it is based upon very fundamental metallurgical basics to working steel, it is difficult to discuss these foundational premises without inspiring assumptions that you are at odds with specific individuals. I think this in itself says a lot about our business.

But enough of the formalities and pleasantries, lets get to the rest of your post.

 

[Kevin R. Cashen]

Not sure I understand your graph but is a. total resistance, b. where the steel would "flex" and c. when the steel will bend? Is it possible that a properly fully hardened blade will exhibit some degree of flex or bend before it fails? Is the force required to edge chip or crack such a blade proportional to the increased force required to bend it? Will this have any effect on the tendency for a crack to propagate across the width of the blade?

"A" corresponds to the point where deformation begins to become plastic (permanent) instead of elastic and the area from “A” to “B” is about continued deformation even if no further force is applied; when pushing steel until it bends we have all noticed that we can feel the moment flexing stops and bending begins, because we can feel it “give” slightly. That small area is only included because I made this curve originally for another discussion and gets very specific in the mode of yielding in tensile testing specimens, it would be inaccurate to apply these very specific mechanisms directly to something as unspecific as bending a knife blade, that is why the differences between the proportional range and yielding is all that is really relevant to our discussion. “C” is the continuation of plastic deformation with its distinctive arch describing other things specific to tensile specimens, like necking, but here the high point is described as the ultimate strength since it is basically all down hill from there to total failure. Then end of the line is exactly that… “snap”!

Yes, a properly through hardened blade will flex like crazy and exhibit some plastic deformation before failure. Tempering is the process that allows the blade to have some give so that it’s only option is not to just snap, as it would if it was brittle. Hardening takes the extreme of the bottom curve and changes it to the top curve, tempering then takes it to a point between. But in none of the cases is the slope of the proportional limit changed, heat treating only moves the yield point up or down and nearer or farther from the failure point. I realize that this stuff is kind of convoluted at first glance, and critics could point out all kinds of fine point inaccuracies in my attempts to simplify it as much as possible. A whole thread could be devoted to all the things happening in a stress-strain curve and all the technical fine points and descriptions, hopefully mete will pop in and give one of his concise and to the point descriptions to give an alternative to my endless blathering.

Edge chips and cracks are not easily plugged into the same numbers as the spine since they are of a much thinner cross section, this is why one can bend an edge quenched blade and not crack the edge even though it is significantly harder. Fractures move very differently in ductile materials than in strong ones, so indeed cracking can be stopped at the interface of the two, something to consider if you expect an edge to crack. But here we also get into impact toughness, ductility and how our alloying can help us out with these issues.

I also wonder whether 52100 and 5160 are considered simple steels and it's just a coincidence that using a magnet during HT will …

That is quite relative to your perspective, to a fellow who is used to stainless alloys they are indeed quite simple, but in comparison to simple iron carbon alloys, as applicable to the second diagram here, they are not simple at all. 52100 has rather elevated levels of carbon for simpler steel, creating a few issues all on its own, but adding the chromium really changes the whole game. The magnet reads over 200F. lower than the suggested temperature to get the carbon into solution. A true convenient coincidence would be the magnet for something like 1095 or W2, where the temps fall close enough to each other to be quite useful.

Does your ability to determine the precise structure of the steel throughout the blade play a significant role in your preference to go the fully hardened route?

Actually from what we have discussed, one need not bother looking inside the steel for products of heat treatment as the principles determining these considerations are independent of it. If you know what cross section you want in your blade for its intended use and how it will effect the stiffness, then you move on to the heat treatment to work on other properties that will be affected.
For considerations of propriety I would prefer to not address any specific makers methods. After all, we all have to do our own thing, and the market will bear endless variations on how to make a knife. As personal rule I never criticize anybody else’s knives, that is just beyond acceptable to me, and a makers methods are like opinions, everybody is entitled to have their own, but people are not entitled to have their own facts, so that is where I prefer to focus.

In your opinion would etching and testing the blade reveal any mistakes that were made?

etching for macroscopic viewing of the outer blade surface will only reveal overall phases, if I wanted to see if the temper line ran off the edge at some point perhaps it would be useful, beyond that I have no reason to do it myself.

Testing is the only way to determine whether gains or mistakes have been made. Each maker has to determine what it is he wishes to test for, how he wishes to test for it, and how he will interpret those results. Reading a test wrong can be worst than not testing so one needs to consider all of these things and proceed carefully.

Have you given any consideration to the cross section of the hardened steel? As I understand it with edge quenches or multiple edge quenches the hardened area rises towards the spine in a diamond cross section. I know there are other methods like drawing back the spine after a full HT but I imagine there would be internal differences. I read on one of these threads that Ed reffered you to some published work mentioning multiple quenches used in industry and wonder if you found anything enlightening.

The differences would be a pearlitic/martensitic interface, versus varying degrees of tempered martensite, with the overall result being the differentially tempered blade would have more play in the proportional range thus being more “springy”. Ed was unable to give me a specific reference to a particular paper so I dug through some of my files and found information on the carbide refining techniques proposed by Grange in the mid 60’s and expanded upon since by others such as Stickles. These papers roughly fit the description that he gave. Their work was geared towards developing carbide conditions conducive to wear life in bearings while avoiding some of the quirks offered by bearing steels in traditional heat treatments, such as retained austenite. It was not so much multiple quenching as finding another condition than tradition anneals to austenitized from.

 

[hardheart]

Is there some value in flexing just for a maker to show that he was able to quench and temper without creating cracks? It would have to be repeated to show consistent ability, but I'm guessing that the 'test' has to be done with multiple blades no matter what it is supposed to show.

 

[Jose Reyes]

Hi Kevin,

Your graph would have made much more sense to me if I had waited a few more minutes before I started my reply and read your following posts. I appreciate your scientific approach and the steadfastness of your convictions and willingness to share them in a manner that makes it easier for the layman to understand, so thanks for the thorough reply.

Don't get me wrong, I can see the benefits of hardening the blade all the way through, but in my mind it does still come with the trade off that the blade will ultimately break. I know that once a blade takes a bend it affects the structure of the steel, and especially if you're not able to bend it relatively straight again it might be much less efficient, but you still have something to work with. Unless the edge on a diff HT'd blade inherently has a greater propensity to crack that is another instance where it could prevent a broken blade. It just seems unreasonable to me to say that there is no circumstance under which a diff HT would be a more suitable choice.

I'd like to make it clear that my intent is not to turn this into an "us vs. them" debate, but I have interpreted your comments to mean that a diff HT has no value other than for aesthetics. If I read you right, do you mean to say that it's the grind and the type of steel that Howard Clark uses in his bainite blades that give them their performance and a full HT on the same blade would be just as good and have no significant effect on having it fail during hard use? Please excuse my using names but these are the people I am familiar with. They may very well come on here, call me a dope, and totally agree with you. I am not trying to ruffle any feathers or start any fueds. I never discussed HT with Howard when I ordered my blade but he assured me that any of his steels/methods would work just as well. I'm sure he has to change the recipe or dimensions a bit but I assume he has a set goal and knows what changes to make depending on the materials he's using.

One of my favorite knives is a fully HT'd rope cutting blade made by Ed Schempp. I have no bias against the concept and let the maker decide how to realize his desires for the perfomance characteristics of his creation, taking the intended usage into consideration. I have confidence in his ability or would not be interested in owning his knives. I'll find out what type of HT he decided to use when the next blade I have on order with him arrives. There are a couple of other comments I want to make that are more off topic so I'll send you an e-mail. Thanks again for your time.

Take care,
Jose

PS Edited to include my question on whether "springy" in your quote refers to flex or not. If the other question I asked in e-mail is pertinent to the performance of your blades please feel free to add those comments as well.

 

[Kevin R. Cashen]

Is there some value in flexing just for a maker to show that he was able to quench and temper without creating cracks? It would have to be repeated to show consistent ability, but I'm guessing that the 'test' has to be done with multiple blades no matter what it is supposed to show.

Due to the popular concepts, flexing any blade is great for selling the blade, I just have no use for it because it doesn’t tell me anything about my heat treatment until after the yield point is exceeded, and then the blade is either bent or broken. This goes back to my comments about the importance of properly choosing and interpreting your tests. Many of the misconceptions common in knifemaking today are a result of honest misinterpretation of observations (e.g. heat treatment effects blade stiffness).

Flexing may reveal a crack, or it may not, depending upon the conditions. One trick for this I once read that Bill Moran did, that I thought was really clever, was to give a sharp rap with the spine of the blade on the horn of the anvil to check for cracking. Sudden loading is very revealing of things like cracks.

 

[Kevin R. Cashen]

Unless the edge on a diff HT'd blade inherently has a greater propensity to crack that is another instance where it could prevent a broken blade. It just seems unreasonable to me to say that there is no circumstance under which a diff HT would be a more suitable choice.

It may have a greater propensity to crack if the soft blade body doesn’t supply the same support that a stronger structure would. Many blades that I have seen edge quenched (not all, but many) have a lower chance of edge cracking because of fine pearlite in the martensite as a result of insufficient cooling in the quenching method used. What keeps these edges from showing plastic deformation is often the geometry. A maker can tell you a lot about his heat treatment by how thin his edges can be and still take a good knife use workout.

If I read you right, do you mean to say that it's the grind and the type of steel that Howard Clark uses that give his blades their performance and a full HT on the same blade would be just as good and have no significant effect on having it fail during hard use?...

Howard has an excellent grasp of blade geometry, and I am sure he would agree with me that his choice of steels plays a large role in his results, we have both had a long term love affair with L6 for its ability to handle impact. Howard also has well understood the concepts we are discussing here for some time, and may even be quicker than I to tell you the difference between plastic and elastic deformation. But let’s not get confused between a soft, pearlitic spine resulting from an edge quench and something like a bainitic spine, which is the result of an entirely different process altogether. Both still have to obey the physics in the right hand slope of the proportional range, but lower bainite will give more resistance to plastic deformation while still offering toughness. A bainite/martensite blade would have much more in common with a differentially tempered blade than a differentially hardened blade.

…I'm sure he has to change the recipe or dimensions a bit but I assume he has a set goal and knows what changes to make depending on the materials he's using…

I am positive Howard does, he is a good maker, but neither I, Ed, or Howard can change the rules governing that curve, only one maker can do that but I don’t think he has taken any orders in a LONGtime, and I don’t believe he has ever* taken custom orders.

It just seems unreasonable to me to say that there is no circumstance under which a diff HT would be a more suitable choice….

… but I have interpreted your comments to mean that a diff HT has no value other than for aesthetics…

If I left that impression, my actual intentions were different. Absolutes, when dealing with human preferences and tastes, can be problematic. For me differential heat treating is purely a matter of aesthetics, however if I thought it would produce an unacceptable blade I wouldn’t use it at all. Although I repeat myself, one cannot ignore the market value of it today, and if one wants a blade that bends it is the only way to go. It think it is possible to point out the bad information and false concepts that has arisen around it without being opposed to it, and such a popular construction should be able to work without myths and falsehoods.

A full hardened and properly tempered blade will withstand much more force before yielding and probably breaking, an edge quenched blade will yield much quicker and bend, these are just the facts that none of us can change, but we do have the choice and it is up to us which one best suits our tastes.

Misinformation can be bad, facts are neither good nor bad, they just are.

*if ever there were an “ever” this guy knows about it, heck he invented the concept

 

[Jose Reyes]

I am positive Howard does, he is a good maker, but neither I, Ed, or Howard can change the rules governing that curve, only one maker can do that but I don’t think he has taken any orders in a LONGtime, and I don’t believe he has ever* taken custom orders.

Ahahaha, I'm not so sure. If Adam was anything like the men of today I'm sure he at least whispered something about big **** when one of his ribs was being harvested...

Thanks for clarifying your position, I can see that I mistook some of what you were saying. Personally I don't find Ed's knife to be so easily bendable that it'll make a difference in use but as you explained it that might have more to do with the geometry than the HT. I appreciate your attention and will now try to extricate myself from your "sticky" without leaving a boot behind. I'm going to keep my eye on this thread but if you have anything further you wish to share feel free to e-mail me.

Take care,
Jose

 

[Kevin R. Cashen]

Ahahaha, I'm not so sure. If Adam was anything like the men of today I'm sure he at least whispered something about big **** when one of his ribs was being harvested... ...

As a long time married man, I would say that if that was a custom order, there are days I think it would be a prime example as to why the customer should trust that the maker knows what he is doing, and leave the fine details to an expert .

 

[Kevin R. Cashen]

There is always all this chatter about whether a knife blade will fail. All kinds of aggrandizing of this heat treatment or that combination of structures over a blade simply through hardened and tempered properly. Let me tell you one of the reasons I can’t bring myself to get caught up in any of it. As rule I will never again test un-notched L6 specimens lower than 60HRC in my impact tester; I put a bit of time and finances into having that machine in my shop and I don’t care to damage it. I watched a through hardened piece of L6 tempered in the mid 50’s HRC bounce 240ft lbs. I have four heads for it and that was the heaviest at the highest height, it almost gives me a hernia to lift the arm into position. For those not familiar with the test, or still are not getting why I have full confidence in the proper alloy through hardened, let me put it like this- how much do you weigh? Imagine putting on ice skates and jumping onto a 1cm x 1cm rod of steel with one foot. Not slowly pulling on it until it gives, but throwing everything at it in a split second only to have it laugh at you. This is so far beyond what any knife use should subject steel to that I would focus more on avoiding any defects or problems responsible for a failure than overcompensating for a problem that just isn’t there.

 

[jdm61]

Oy!!!! My head hurts now Kevin....I did the full quench with clay coating on a 6 inch W2 blade and tempered at around 450. WOW!!!! Even with my amazingly bad grinding skills, that thing is pretty darn sharp and will take some abuse. i have been whacking at every piece of old scrap wood i can find to see if the edge will deform and not a single ding. By comparison, the semi-successful blades that I have made from 5160 and 1080 don't seem to get quite as sharp. The W2 blade alswo seems to "bite" better even though it has a thicker profile than some of the other blades. The next one going to heat treat shouldbe even more of an adventure. It is a 12 inch x 1/4 thick "fighting" bowie that feels very light for it's size and should be very well balanced......assuming that it doesn't warp like a pretzel...lol. You and some other folks ahve said that you should learn how a particular steel works and concentrate on getting the most out of it. I think I have found mine:thumbup: Thanks for the assistance.

 

[Kevin R. Cashen]

W2 was always one of my favorites, I just wished it wasn't one of the first ones on that ever growing lists of great simple steels that are going the way of the dinosaur. I just got off the phone with one of my suppliers who said that their company is phasing out O1. O1, what the heck is this world coming to? Soon the only steels left out there will require more techy equipment than I have gathered in my shop. The sad world I envision on the horizon is one where America is at the mercy of the other countries that still have a funtioning steel mill in them, and almost every bladesmith has a clay stack next to a pile of charcoal behind his shop and his own secret vein of iron ore. For now Michigan is too cold in the winter to pick for ore, and my clay stack is covered with a tarp.

 

[Deweyknives]

01? O1?!?!? That really will suck!! I started on 01 and with your guidence here,Kevin,have become more and more impressed with it!! Is that just that particular supplier or is that going industry wide??

I guess after I stock up on W2 from Don,I better get a supply of 01 also!

 

[Phillip Patton]

Soon the only steels left out there will require more techy equipment than I have gathered in my shop.

I think your not giving yourself enough credit. I just got finished forging a blade from A2. If I can forge and heat treat air hardening steels, you can, trust me. You've got much more complex equipment than I do.

That's a bummer about the O1. I would stockpile steel like that, but in Indiana we have to pay inventory tax, and it would be sky high if I had tons of steel laying around.

 

[lcf]

Kevin, thanks for all the great info. we met this past summer in troy oh. you use l-6 and 0-1 in your pattern, is there other combinations,and would you share them with us. i tried 0-1 and 5160, ended up with little to no distinction in the color. it was almost chrome on chrome.