1095 HT troubles

[Kevin R. Cashen]

Originally Posted by Andrew Garrett

Ron,

I guess I missed where someone wrote that they skated a file at 47RC. I simply do not believe that...

See paragraph 3 in my last post, for an explanation. We really need to get away from unwavering belief in traditional bladesmith testing, and embrace more thorough analysis of all aspects of steel if we are going to make the same progress that all other steel and tool making industries have enjoyed. Don?t take my word for it, the steel was tested with a file by a room full of the biggest names in bladesmithing, it saw no tempering, it was properly prepared for metallographic mounting and examination and the file was ran on a fractured end! My hardness tester is regularly calibrated off from very good test blocks, and further rechecked with good test blocks before any important reading, and no less than 5 HRC readings were taken from the sample.

Of course this is all a moot point if one can read the micrographs provided, the whole story and explanation for the results is quite obvious in the images, mete probably spotted the situation right off. Heck if one took some time at the microscope to measure pearlite percentage they could predict a range of Rockwell hardness before using the tester.

I have a cake of wootz that sets on my bench in the shop, it will tear a file up if you try to cut it, but I have pushed impressions of letter stamps into it clearly with just hand pressure? penetrative vs. scratch hardness.

[Kevin R. Cashen]

Originally Posted by son_of_bluegras

...
And for those of us stuck using primitive methods and equipment, what sort of test can be done to ensure we are getting what we want? I do destructive testing on my knives occacionally and use the others so if someone wants on I can pass it on with confidence that it will serve the purpose. But I would like to do the best I can with what I have available and I know about the file test and can break the occasional blade blank after heat treat, is there something else I should be doing without investing a lot in equipment?

I am still mulling over some of the other things you have brought up (many from past threads so I've been at it a while), so one day other questions may follow.

ron

I am sorry, I got side tracked on other aspects and did not address the question I really wanted to in my previous reply. Fancy testing tools really are not at all necessary in quality control of knives as tools. The best test for any tool is to use it hard in the exact way that it was intended to be used. Cut lots of different things in many different ways, that is simple and something anybody could do.

My fancy testing gadgets actually do very little in finished knife testing; they are designed to test very specific properties in steel very accurately. The main reason I got those gadgets was to understand the limitations of so many of the improvised tests us bladesmiths see as definitive. The one thing we all have to remember is to avoid at all costs believing that one test is definitive, and to know exactly what that test measures and what it cannot measure.

The file test is good but one dimensional, if we combine it with a Rockwell test we then have both bases covered and an excellent overall picture of hardness. Any form of testing that involves ?flexing? (deformation that returns to true afterwards) is actually almost worthless in determining heat treat or hardness. Breaking a blade is useful if we can measure the force and nature of the load required, otherwise we need to fall back on examining the fracture surface for clues to the whole story. Bending can show us that the blade is soft, but how soft is it? I like hitting things with my edge, because I am a big fan of impact toughness, brass or softer steel can work, some really get worried about various woods but wood really shouldn?t threaten steel if it is heat treated properly. One needs to be careful not to get stupid with hacking things though, ask yourself ?is this a legitimate thing to cut with a knife or I am just being silly with unrealistic sensationalism? one can lie to themselves and begin to believe it if they get caught in that trap. You can chop in some pretty outrageous things by playing with edge geometry to do it, but will that same edge shape be any use to the guy who just wants to skin some game?

[Gary Mulkey]

Kevin,

Please explain something for me. As I understand it, a typical Rockwell tester applies pressure to a piece of hardened & sharpened steel to see how much pressure is required to leave the typical "dimple" in the piece to be tested. I have always thought of toughness as impact resistance. If so then this sounds like a test for toughness rather than hardness. Can you explain to a guy who learned steel in a blacksmith shop rather than a classroom the difference between testing hardness & toughness.

Gary

[Kevin R. Cashen]

This is actually a really great question Gary, since there is a lot of confusion about toughness vs. hardness. Force applied at a constant rate or gradually will measure ductility vs. hardness, the Rockwell penetrator will take many seconds to apply the major load, so it is unable to measure impact toughness. Ductility in a bend or under penetration can give clues to tensile strength however. Most bladesmiths would call a blade that can bend ?tough? but this is not entirely accurate as it would be more appropriate to call it ductile. Strength is kind of the opposite of ductility, as strength is defined as the resistance to deformation. A very strong material will resist deformation to the point that it only method of yielding is fracture, so brittleness can often be the undesired partner of strength. So a long time ago, before alloying considerations, the idea arose that you could either have brittleness or you could have ductility, and people began to apply toughness to ductility in a rather absolute way that is not always the case.

The darned thing is that you can have a material that is very ductile and tough under constant load, but very weak and brittle under sudden loading such as impact. A great example of this would be my vinyl siding that I cracked the other day . You can bend the stuff, stretch the stuff and it is quite soft, but if you smack it very fast it breaks in a brittle manner. Steel will also do this. Fully hardened and other brittle steels will break via cleavage with no deformation, while soft steel will break in many small shearing actions and deform, this is why the two will look very different on the broken ends afterwards. But soft steel when loaded very suddenly will not have time for the shearing mechanisms to occur and will break with cleavage very much like hardened steel. Thus it takes much less force to break steel with sudden shock than it does if the load was applied slowly as in a bend. In my opinion if a blade is going to fail it will most likely be from impact than from bend, simply because it is more likely to get used that way and it is an easier to occur.

The kicker is that in the old days the only way to keep a steel tough was to make it more ductile, and often it had to go to overkill in ductility to handle the impact part, so in the old days blades may have had to be softer to survive. But then we developed alloying. Dump some nickel or silicon into the steel chemistry and it will increase toughness in a manner independent of softening in the temper. Thus a piece of 15n20 or L6 will have many times more impact toughness than a piece of 1075 at exactly the same hardness. I no longer test unnotched L6 in my Charpy tester because it will max out at over 240 ft lbs. and possibly damage my equipment!

I many ways bladesmiths are still stuck in the middle ages on their concepts of hardness vs. toughness and thinking the only way to have a tough blade is to make a portion of it, or even all of it much softer than what would be optimal for a cutting tool. Proper selection and use of alloying can allow us to have our cake and eat it too.

[Andrew Garrett]

Kevin,

Thanks for clearing that up for me. I'm no metalurgist, but I see no moss gathering either. I understand now that the scratch resistance could be very different than penetration hardness.

I also understand the ductility factor--that slow bending or reshaping metal is more likely to successfully change shape than the same material being quickly bent. My layman's mind has always atributed such action to heat. If it is allowed to bend slowly, the heat rises and 'protects' from breakage before it happens. This is akin to stretching slowly and 'warming up' before heavy physically activity at the gym. I may be WAY off base.

Back to the file test. If the quenched blade is cleaned of scale, and the cross section of the edge is about 1/16" of an inch, surface or scratch hardness affecting the file test seems unlikely. At that thickness, what would be scratch hardness halfway up the bevel (near where the hamon might be) will likely be through hardness at the thin cross section of the egde.

Now, pat me on the head and tell me where I'm wrong. I'm enjoying this discussion.

[NJStricker]

It sounds to me, if I understand everything, that a lot of this goes back to an idea I've had about knives for a long time. No single knife will do all things well. This concept not only applies to blade shape and edge geometry, but to the temper as well.

The idea of toughness really doesn't mean much unless you understand the trade-offs between hardness and ductility, at least for the simple carbon steels. If a maker or user doesn't understand it, then the concept of toughness is nothing more than a marketing ploy, or an idea to be taken at face value by the person proclaiming a certain degree of toughness.

Interesting that we have developed machines that will enumerate a value along a scale for hardness, ductility, and shear strength. Yet we try to describe all 3 of those (or possibly more) qualities with the single concept of toughness. Does a "toughness scale" even exist?

[Gary Mulkey]

I many ways bladesmiths are still stuck in the middle ages on their concepts of hardness vs. toughness and thinking the only way to have a tough blade is to make a portion of it, or even all of it much softer than what would be optimal for a cutting tool. Proper selection and use of alloying can allow us to have our cake and eat it too.[/QUOTE]

I'm glad that I am not alone in my confusion about strength & toughness. I understand & agree about alloying & strength. In destruction testing of my blades, my best blades have been of alloy steels (I grant that some of this may be my personal abilities in H/T). I still use a fair amount of 1095 as it has been my preference in carbon steel for a long time and still an outstanding steel. I must admit though that I often use it when wanting a hamon for cosmetic reasons.

Thanks for clarifying some things.

Gary

[Andrew Garrett]

Heck Gary, that sounds good to me, because 1095 with a hamon means differential HT, which means... better toughness overall with an awesome edge, right?

[Andrew Garrett]

To be sure though, you cannot edge quench 1095 the way you can 1084 (as seen in Ed Caffrey's video), what with first dunking the tip to a stop plate and rocking back for the rest of the edge. The requirement for the rapid quench kills that idea.

We settle for a straight line hamon or use the 'clay' which is wet Satanite for me.

[Gary Mulkey]

Andrew,

Heck Gary, that sounds good to me, because 1095 with a hamon means differential HT, which means... better toughness overall with an awesome edge, right?

This is what we were discussing. Is it really tough or simply ductile? The clay tempered blade will probably bend more quickly that one of tempered martenzite because of the strength (or lack thereof) of the pearlite formed by the quench. It will probably bend farther without fracture as well. Does this make it tough? You tell me.

I am considering putting a hamon on an Arkansas toothpick that I am in the process of building at this time. If I do, it will mainly be for the cosmetics of the hamon. Will it be a tough blade? Yes (or should I say ductile). Can I build one that is tougher out of an alloy steel without the clay temper? I believe that I can.

Gary

[AcridSaint]

I'm with Gary on this one. I like edge and clay quenches because they can look really nice. I don't think that edge or clay quenching is the way to get a superior knife, however. There's a lot of writing about the bending of Japanese style blades. They will bend and take a set very easily.

The simple steels they were working with and lack of a good, consistent way to temper the whole blade is what I believe led Asian smiths down the road of differential heat treatment. Although there are examples of tempered edges, there are also many examples of full-hard sword edges in Japanese pieces. This is fine when it's only the very edge of the piece and you don't plan on doing much flexing, but how would they have stood up if all of their swords were full hard and straight as an arrow?

For what it's worth, 1084 may have a slower quench requirement, but I don't think it's leaps and bounds behind 1095.

[Kevin R. Cashen]

One of the most crippling things to the progress of modern bladesmithing would almost have to be the idea that bent blade is somehow a good thing. I am not the only ABS certified smith that believes that hamons and differential hardening is mainly for aesthetics with very little functional value. Edge quenching is great to get you through the bend test and win your stamp but on a real using knife it has some serious problems. I have many images from my microscopes of Japanese swords, modern, shinto (kind of old) and koto (real old). The heat treat can be pretty good for what they had to work with but you can immediately see good reason to move that hamon as far away from the edge as you can.

I have resigned myself to accepting that no matter how plain it is laid out; a good percentage of bladesmiths will refuse to accept reality about the rock solid laws of physics that govern strength, ductility, bend and flex. I understand this with many as their whole career and reputation is based upon bending blades, making the facts almost impossible to embrace. However, an edge quenched blade, unless it is very thick (which is the only way to affect this aspect) can be bent with moderate pressure from a mans arm (I know, I have to test people for the ABS often), a fully hardened and tempered blade will take many, many times more force to deform or break. This is just how the universe works, we can ignore it or we can work with it.

I did whole lot of edge quenching into oil on a plate in the years I taught at the ABS school, and after all my experience, testing and studying the results, I have chosen to not use that method at all with my knives, even with a really great quench oil. Especially with really great quench oil, as that method trashes a good oil in short order. If I go with a hamon I clay coat it and go entirely under the surface of the quench, pretty much like the Japanese did. I have a few hamons to do on my order list and I will do them because that is what the customer wanted but I wouldn?t go that route on a knife I to put to heavy use myself.

If a person is realy concerned about breaking a blade (something that none of us actually realize how rare it is), tempering a fully hardened spine back to a lower hardness will easily outperform the dead soft spine in both strength and toughness. In equal hardness, pearlite is not all that tough when compared to well tempered martensite.

Sometimes I get the idea that folks think I am some kind of nerdy know it all (well, I am a nerd), but I don?t see how one has to be well learned or know it all in order to see the common sense in the idea that soft steel is not as strong as hard steel ??? A lot of what I say sounds very radical or hard to believe, but the more incredible thing is that nothing I am presenting is anything more than common knowledge in almost any other metal working industry. Much of what bladesmiths have come to believe is what the rest of the world recognizes as nonsense. I think we have been living on our mystique for too long now.

[Kevin R. Cashen]

Originally Posted by Andrew Garrett

...Back to the file test. If the quenched blade is cleaned of scale, and the cross section of the edge is about 1/16" of an inch, surface or scratch hardness affecting the file test seems unlikely. At that thickness, what would be scratch hardness halfway up the bevel (near where the hamon might be) will likely be through hardness at the thin cross section of the egde.

Now, pat me on the head and tell me where I'm wrong. I'm enjoying this discussion.

I am not sure I understand, surface or scratch hardness should be a matter of affecting filing- it IS filing. The file works by cutting or scraping the surface of the steel, everything it does is based upon the scratch hardness of the steel, regardless of the cross section. What you point out in the thickness would be a matter of thermal conductivity in heating or cooling, i.e. the edge would reach the heating temperature before the spine thus have a bit more time (seconds) to get into solution, or more importantly on cooling be able to lose heat fast enough to avoid making pearlite and thus achieving full hardness.

However, what is shown in the sample in question is insufficient solution to begin with, so one could quench that steel in the fastest quench on earth and still not reach full hardness since there was never enough carbon put into play to begin with. And as for the thinner sections gaining those few seconds to get carbon into play, well soaking is time AT temperature, not coming to temperature. Once you get things to temperature then conditions have been made for the carbon to move, now you will only benefit by giving it the time necessary to do so. This sample was soaked for 5 minutes at non-magnetic, 5 minutes is around five times longer than most smiths working with a forge, or a torch, ever do and the mixed structure was consistent throughout.

I was actually surprised at how much carbon was left out of play in that sample since I did the usual bladesmith thing of making coarse pearlite for the anneal before proceeding with hardening. Pearlite should go into solution faster than spheroidal carbides (the way steel comes from the mill), but going just to nonmagnetic (once again the common practice with many smiths) just was not enough. But we must also remember that we started out talking about 1095 (hardening from 1475F) and this sample is 5160 (supposed to be taken to 1525F), so the nonmagnetic point of 1414F would not show as drastic a difference with such a steel. But then another fatal flaw in bladesmith thinking is the mistake of treating all steels with the same basic recipe.

Andrew, I am glad you are enjoying the discussion, since the back and forth question and answer is the only way to get these concepts out on the table. It sure beats the heck out of everybody turning their backs and doing their own thing without caring either way.

[Gary Mulkey]

Kevin,

I know that this is off from the original topic of the thread but I'm going to ask anyway. I have been doing a fair amount of experimenting with getting the hamon to follow the clay (satinite) more closely. Usually I will quench 1095 in a fast oil which does fair in following the clay but not close enough to get the artistic hamons that I have been after. I am guessing that I need to do water quenches for this purpose but even with lower carbon steels the amount of fracture caused by the water quench makes it undesirable. What steel/quenchant combination do you have the most success with in closely matching the hamon with the clay application?

Gary

[Andrew Garrett]

I can certainly see how fully hardening the entire blade and then drawing back the spine with a torch is the path to a blade which is superior to a simple clay quench. I guess I just never thought about it that much.

The knives I make, usually small and plenty thick--basic pocket knives, are not designed for heavy bending loads. I simply want them to cut very well and maintain a good edge, as that is their function. I also want them to be beautiful.

I've not made many big knives of the type I see ABS guys bending in a vice to prove their forging skill (a practice I have never fully understood). My HT methods would likely not survive such a test.

I have ED Fowlers video which shows him bending a blade back and forth over 180* over a dozen times before it breaks. It's very impresive, but I'm not sure why that makes it superior. Like you I suppose, I see little more benefit in a bent knife than a broken knife--I wouldn't trust either with any important task after that, but at least a broken knife might be re-ground to a new point if enough survives. A bent knife, even if straightened, would leave me thinking that it had a permanent weal spot.

I admit that as primarily a stock removal guy, I am standing on the shoulders of others achievements. I read books and converse here to gather my knowledge. Discussions such as these always remind me to do more testing for myself and not believe everything I hear.

Thanks.