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scherar
12-20-2010, 07:10 AM
I nailed the HT on an O-1 blade yesterday. But this morning I was doing a soft back draw on the spine (I already tempered the whole blade in the oven right after the quench) and got the tip too hot. What is the correct procedure for re-heat treating? Do I need to anneal, etc? What do I need to do to avoid warpage the next go around?

Kevin R. Cashen
12-20-2010, 07:54 AM
A good stress relieving (heat to around 1200F and air cool) may do the trick without decarburizing or scaling things all up. Annealing would be the surest method but overkill, in my opinion.

scherar
12-20-2010, 08:40 AM
Thanks Kevin. Do you think the soft back draw does much for toughening, adding flex or any other added benefit. Currently I am doing this on thinner blades (1/8") that might see rougher use. I read about it in Goddard's book and do it often, but I always think that the heat might run further down towards the edge than one might be able to tell (even though the colors) and soften the original homogenous temper. I have tried the "edge in water" method, but I can't seem to get the spine hot enough when getting closer to the tip of the knife (not really that close). The water is either too much of a heat sink or maybe I need to step up to a hotter flame.

Thanks for your time!

Kevin R. Cashen
12-20-2010, 09:22 AM
Drawing the spine, changing the temper, or any other kind of heat treating will do absolutely nothing for adding flexibility to the steel, flex is exclusively a function of thickness, that is just a fact of physics. These operations will increase toughness via added ductility but it will be at the expense of strength, i.e. the blade will bend much easier than before but will have to bend farther to break. The only reason to be concerned with this kind of toughness, despite what you have read or heard, is in impact loading on larger blades, like when chopping. I personally think it is pointless to differentially temper a blade less than 6" in length or one that will not see heavy loading, and if a person wants a $500 prybar with an edge on it... how many good prybars have you seen that will bend? I would recommend a few other books, as many as you can get your hands on is always good, and if the spine draw is giving you grief save it for the big choppers.

Doug Lester
12-20-2010, 11:33 AM
Let me add to what Kevin said. I think, from my own experience, that a lot of beginning knife makers think that differential hardening or a soft draw on the spine is something mandatory after reading the ABS test requirements, and it is, but only for the ABS test requirements. As Ed Caffrey explained to me, a person only has to make two knives with differential hardening or a soft draw, one for the ABS Journeyman's test and the other for the ABS Master's test and that's only if one ellects to take the tests. Personally, my attitude is that if one needs a knife to get a knife and if one needs a pry bar to get a pry bar. That said, a soft spine may come in handy with a heavy use knife because they may be subjected to a certain amount of twisting and bending doing things like splitting wood or the pelvis of big game. Just another case of form following funtion.

Doug Lester

Cliff Fendley
12-20-2010, 12:45 PM
Very good post Doug. I've never understood the need for a differentially hardened blade that will bend 90 degrees without breaking. The test does show the knowledge of being able to make one do it though.

Has anyone tried to bend a good quality name brand pry bar, strong arm, spud bar, tire tool, etc 90 degrees without it breaking? Maybe some will, I don't know but I've broke a few doing extreme things working on heavy equipment and such. The cheap China ones that just bend don't make a very good pry bar, you just have to know when one starts flexing too much you need to back off and get a bigger bar. Not that you can't spring one but 90 degrees is a little extreme in my opinion. If a pry bar wont then why should a knife blade.

scherar
12-20-2010, 04:37 PM
Thanks for all the input. I will probably lay off the soft back draw unless someone asks specifically for a rugged use knife. I was only doing it since I thought it made a knife that was a touch better. Anytime I sell a knife though, I always think that there is a possibility that someone will do what they aren't supposed to do with it. I, and everyone else who has responded to this knows what a knife is for, but that doesn't mean everyone else does.

With that being said, I can't ever recall telling someone that the pry bar I own pries very well, but it failed me when I tried to cut "whatever". On the other hand, it is well known that someone in the past said that the knife cuts very well, but it failed me when I was quartering my elk. Even though he didn't tell anyone that he was trying to separate a hind leg from the pelvis, it still failed.

This is just what goes through my mind as I am supplying a product that is expected to perform. I try and think all things through when I am charging someone a good price for my product. I want to be confident that it is the best I can make.

Cliff Fendley
12-20-2010, 06:16 PM
Most of the time when someone breaks a knife from abuse I would say it's the tip they break off. At least when I'm destruction testing a knife doing crazy things that's the part I'm likely to break first when digging and prying. I just did it today to a knife blade. To break a blade in the middle you have to very purposely do some serious prying and even without a soft back a properly tempered knife is flexing to the point that it is giving plenty of warning it is being pushed beyond the limit.

When you are prying with a pry bar or screw driver and it's flexing a good amount you back off realizing it's going to take a larger tool, anyone that doesn't do the same with their knife is asking to have it broken.

Doug Lester
12-20-2010, 08:41 PM
It's hard telling what some people will do with their knives. One maker I ran across when I was researching making knives said that he had a man who return a knife for repair on warantee who said the knife didn't hold up to being driven into a tree truck when he used it as a step to get up into his tree stand.

Doug Lester

Kevin R. Cashen
12-21-2010, 09:22 AM
Very good post Doug. I've never understood the need for a differentially hardened blade that will bend 90 degrees without breaking. The test does show the knowledge of being able to make one do it though.

Has anyone tried to bend a good quality name brand pry bar, strong arm, spud bar, tire tool, etc 90 degrees without it breaking? Maybe some will, I don't know but I've broke a few doing extreme things working on heavy equipment and such. The cheap China ones that just bend don't make a very good pry bar, you just have to know when one starts flexing too much you need to back off and get a bigger bar. Not that you can't spring one but 90 degrees is a little extreme in my opinion. If a pry bar wont then why should a knife blade.

Cliff and Doug, as an ABS Master who is called upon often to oversee that test let me just say... You're alright!:punk: You get it and I wished a whole lot more people would. If we approach the bend test as a guage of the smiths abilities with heat, and not the knife, it is a good thing, but the misinterpretation of that test has caused more damage to bladesmithing than almost any other issue. For years I have seen people intentionally doing things that will lessen the cutting ability of a knife just to facilitate a bend and have shaken my head in profound sadness.

As with many things the cure is simple- common sense! What is a knife suppose to do? Cut things! If one does want to abuse a perfectly good knife what does bendability have to do with it? Do good springs easily bend? Do prybars easily bend? The operative word is "strength" which is not the opposite of hard, but the opposite of soft, flimsy and bendy. But then you guys obviously get all that.

Sorry, I get more passionate that I should about this topic, because I saddens me to see so much potential in talented makers squelched by so many false notions and bad information, but then I see people who do get it and my hope is renewed.

Doug Lester
12-21-2010, 10:41 AM
I hear ya, Kevin. Unfortunantly, there are a lot of "experts" out there who push a lot of this stuff. I feel the same way about hamons and stabilized wood or people saying that Diamondwood is only found on crappy knives. Thankfully we have people like you and Ed Caffrey who understand what is going on and can set things straight and I'm happy to pass it on. There is not thing one that I've learned about knife making that I didn't learn from someone else and I'm deeply indebted to the true experts out there.

Doug Lester

scherar
12-21-2010, 12:41 PM
Doug,

Just out of curiosity sake, can you expand a little on the hamon & stabilized wood issue. I have read a lot of opinions about stabilized wood.

Thanks

Doug Lester
12-21-2010, 02:41 PM
Ok, the hamon shows the demarcation between the martinsetic steel on the edge and the pearletic steel of the spine. The martinsite holds the edge but the pearlite gives more strength and toughness to the body of the blade. Now a hamon does give visual evidence that the two areas are there, which can be desibable to some, even useful. However this does not mean that a blade with a hamon is a better knife than one without one. First of all it is the differential hardening, not the hamon, that is important so there's really no difference between a blade with differential hardening, or tempering, that does not desplay a hamon than a differentially hardened blade that does not desplay a hamon. A hamon is only evidence of it. That is to say that a blade with a hamon made from W2 is not by necessity either better or worse than a blade without a hamon that was made of 52100 with a soft draw on the spine and two blades made of W2, both with differential hardening are the same, all other things being equal, even if one is not etched to show it's hamon. Then we get to the fact that differential hardening is not necessary to making a superior blade, especially a short one, as was brought up earlier in this thread.

Stabilized wood is also a place where there is a lot of underserved hype about it. Unstabilized wood handles have been used for millinia on knives and have held up well. Now not all woods are equal. Some woods are too soft by nature to stand up to being used for knife handles. The spalted wood deffinantly fall into this catagory. Some are too prone to cracking and checking, such as most of the beryl woods. Some like hickory, maple, walnut can be used with no more than a good sealing finish but could well benifit from stabilization. Then there are some woods that just simply don't need it like osage orange and some that is so heavy and oily, like African Blackwood, lignum vitae, and some of the rosewoods that are too dense and oily to take the treatment. One top of that, stabilization is not an absolute cure for woods that will not make good handles. Woods that are subject to expansion and contraction will still expand and contract, just not as much. Woods that are prone to cracking will probably still crack, they just might last a little longer before they do and maybe not crack quite as badly. Stabilization is a useful tool for making a superior knife with some woods but it is not the greatest thing since canned beer and sliced rye bread that some want to make it out to be.

Doug Lester

scherar
12-21-2010, 04:10 PM
Thanks for the explanation. I own a custom knife with a hamon and it doesn't cut better than any other knife that I own. I'll probably never try to see if it will bend 90. One thing that I will say is that it looks nice, if one is into that.

Thanks for your opinion on the woods. They say that ironwood is just about the best for stability, not that I disagree, but I also own another custom knife where the tang definitely stands proud. I guess handle materials are going to do what they are going to do, especially when you throw in climate factors.

Thanks again.

Doug Lester
12-21-2010, 06:32 PM
Remember that bending a knife blade to 90 degrees constitutes distructive testing. The blade will not come back to true straighness and you will crack the hardened edge. I would not do that on any blade that I did not intend to break. As you have seen, a hamon does not mean that the blade has superior edge holding ability. What it does do is show how wide the band of hardened steel is at the edge which will give an idea of how long the knife will last before all the hardened steel is ground away by sharpening.

I don't know if desert ironwood is the most stable or not but, from what I've been told, is another wood that will not take stabilization. If you can find a good piece of it it does make a beautiful handle. It's been overharvested and good pieces are starting to become rare. It wouldn't suprise me if it doesn't fall under the CITES treaties soon.

Doug Lester

Doug Lester

Cliff Fendley
12-21-2010, 07:34 PM
I will say those hamons look good whether it serves a purpose or not, I would like to use some regular carbon steel so I could try and do some.

Kevin R. Cashen
12-22-2010, 10:11 AM
Cliff, it is indeed in the beauty of the hamon that its true value rests, and the makers who recognize this seem to do the best with it, but the hype around differential hardening has provided some of the biggest honesty challenges for many a maker in representing their product and methods. Doug, very good explanation and I am glad we are pretty much on the same page on the general conclusions about hamons, there are a few additions I would make since I have spent some time studying these things and have found some tidbits beyond the conventional wisdom of the topic.

No other phases in steel can match martensite for possessing both strength and toughness at the same time, pearlite can lend toughness but only via ductility and at the expense of strength. A soft back blade cannot be stronger than a fully hardened one, but it can be more ductile and thus tougher, but less elastic. In other words a blade with a pronounced hamon will not flex nearly as far without taking a permanent set but will then permanently bend farther before breaking, but the bend will happen with as much as 1/5 the force it takes to bend or break a fully hardened blade. Those fantastic old katanas that everybody defers to when tauting the superiority of the hamon are notoriously easy to bend, they just don't include that tidbit in all the hype.

Also the guys who are emotionally invested in hamons will like to tell you about how "stiff" the blade is and how much force it takes to "flex" it but that is when you need to ask them about how thick the blade is. The amount of force required to elastically flex a blade and still have it return to true is determined exclusively by the thickness and shape of its cross section, heat treatment only comes into play after you have pushed the blade too far in flexing it, and pearlite will be pushed too far long before hardened steel. So a blade that is differentially tempered will actually possess a higher toughness along with a higher strength than one that has a hamon from differential hardening.

Of course the very edge of a differentially hardened blade will not be any better than a blade hardened any other way, and in fact the chances are greater that it could be not as good if the maker gets too caught up in the appearance of the hamon instead of keeping the properties of the edge their #1 priority. Here are just a few examples of how the hamon can be in conflict with a good cutting edge:

If the maker feels only water will give the most striking hamons with modern steels that cannot handle that quench, they will end up under-soaking to accentuate activity, and avoid cracking.

If the maker doesn't use clay but merely edge quenches into oil with simple steels, thus leading to many under-cooling issues.

If the maker applies clay too close to the edge and causes pearlite colonies in the critical edge martensite. This will not necessarily be visible until it reaches much greater volumes ( I have spent many hours studying the inside of the habuchi area under the microscope on both ancient and modern swords)

One that I find most disturbing is when makers are using methods to nail exact soaks at exact temperatures for their given alloy, but abandon them for methods that produce a better hamon regardless of the advantages they are giving up for edge quality.

Along with these, and others, it is a bad omen when makers start pointing to a hamon as proof of a superior heat treat when the only thing it says is that the heat treat was good for hamon. Good hamon and the optimum heat treat for that steel are not necessarily the same, especially when you consider that in order to form a hamon you must do many things that are contrary to a good heat treat, e.g. intentionally making pearlite is not a rational thing to do in any other hardening operation unless you are making a hamon.

Just as a good hamon is no guarantee of a good heat treatment neither is its presence an indication of a poor heat treatment, there are many makers who have their priorities all in line and make stunning hamons with excellent performing edges. Burt Foster, Rick Barrett, and Dan Maragni are just a few of the makers who I would fully trust their edges on blades sporting hamons that are outstanding. These guys know their temperatures, know their quench oils and have worked out exactly how to apply their clay for those mediums. But above all you will never hear one of these guys feeding you a line of bull about how super their blades are just because of the hamon, and since they are able to recognize the limitations they are able to easily work around the pitfalls to produce an excellent edge.

Edited to add: This is one of those topics that is very touchy and can easily brig about hard feelings, the personal investment that so many have in developing their hamon will cause them to take any contrary physical facts rather defensively, and that is too bad, for such a beautiful addition to a blade should have the benefit of rational, objective and factual information to rest on, and be honestly appreciated for what it is.

scherar
12-22-2010, 04:21 PM
I have always thought similarly about hamons, especially if done by edge quenching. If "agitation" in the quench media is so important (I always do it), then you really aren't getting that benefit in an edge quench, are you? I guess if you had some sort of circulation pump working in your tank, then it might happen. Maybe some people have all that figured out.

Just a thought.

Doug Lester
12-22-2010, 09:08 PM
Kevin, thanks for your reply. I was not aware that a soft draw produced a stronger steel than differential hardening, though thinking about it it does make sense. On doing a soft draw one is not going to get the spine of the blade hot enough to austinize the steel there but keep the steel in the edge uneffected. Or at least it would be pretty hard to do. Of course then one has to remember the difference between toughness and strength when talking about steel. My knowledge of steel metallurgy come from "Steel Metallurgy for the Non-Metallurgist so I realize, actually more and more lately, that I am missing some of the finer points on the subject.

Doug Lester

scherar
12-23-2010, 07:10 AM
Doug,

Maybe I am not understanding your reply to Kevin very well, but isn't a soft back draw done often on a blade that is homogenously quenched? The steel in the spine would therefore be austenized before doing the draw with a torch. One different way of doing that is by only bringing the edge of the blade to temperature, as with a torch and then totally quenching. I am sure you and Kevin don't agree with that method since you are not really controlling the temp as with an oven.

I have read your reply a few times will admit that I am probably misinterpreting the soft back draw.

Doug Lester
12-23-2010, 10:55 AM
Thanks for reminding me of another way to harden just the edge of a blade. The thought behind my responce to Kevin was based on it didn't click in my mind that when one did a soft draw on the spine of a blade they were starting out with tempered martinsite and changing it to a softer temper and that the martinsite, even with the soft draw, was still stronger than the pearlite that would result from differential hardening. My comment about heating the spine with a torch until it was austinised to convert the tempered martinsite to pearlite was more to illustrate the impracticallity of doing so.

Doug Lester

Carey Quinn
12-23-2010, 11:50 AM
Man this place is great! Thank you gentlemen for such an exchange of information. Kevin, I’d love to be able to follow you around for a while and just listen. I almost always learn something every time I read one of your posts.

I have always used carbon steels, 5160 and 1084, and done a partial quench because I believed it produced a superior knife. Now, after reading your posts, I am forced to re- evaluate my thinking. This is especially true since I generally make hunter and smaller ‘user’ types of knives that are not generally subjected to the rigors of larger heavy duty blades.

Would I be correct in thinking that, besides the strength vs toughness issue, a full quench would offer somewhat less margin for error than an edge quench?

Thanks,
Carey

Doug Lester
12-23-2010, 10:22 PM
When you ask what makes a superior knife blade you have to ask superior for what. Just about everything in knife making is a trade off between opposing traits. Increasing the edge holding ability of a blade decreases the sharpenability. Softening the spine of a blade by drawing the temper or differential hardening may make a tougher blade but it will also make it weaker. A weaker blade may stand up to the stresses of chopping or limited prying better but will be more likely to bend. A stronger blade may resist deformation better but it is more likely to break if bent too far. A hollow ground blade may slice better but be more likely to chip out on the edge. A convex grind my stand up to chopping better but not be quite as good slicing things. There are just trade off after trade off in knife making and they all support different funtions.

Doug Lester

Kevin R. Cashen
12-24-2010, 09:10 AM
What Doug said is spot on, everything in making a blade is a series of carefully balanced compromises. I tend to say “optimize specific properties” and avoid using the word “superior” whenever I can in order to leave the hyperbole for the hype-mongers and attention hounds this business is rife with.

Optimum performance comes from designing the blade and heat treatment specifically for the task the knife is meant for. In an attempt to make Ronco supermatic ginsu type products to match the tall tales needed to sell them, we have kind of dumbed down our knives, which only worked because we pulled off the greater feat of also dumbing down our clients with the marketing. Of course when I say “we” I am not talking about those present in this discussion, but I think you know some of they guys I am talking about.:3:

But real performance comes from looking at a fine slicing blade and making the edge thin, strong and very abrasion resistant while ignoring the guys who say it needs to bend 90 degrees the be the “ultimate knife”. This optimum performance is the opposite of what you want in a large chopper where you would beef the edge geometry up a little bit and switch priorities from abrasion resistance to impact toughness and strength. But oddly enough even this type of blade gains nothing from being able to easily bend. Bendy steel is nothing to stake your life on. Polybius and others document how those who faced the Romans with swords that bent were summarily slaughtered. When the chips are down a bent blade is no more useful than a broken one.

Determine the task for the knife, then focus on the properties that will perform that task best. That task could be chopping, slicing or a general use knife. The chopping knife will need to sacrifice some of the properties of the slicer in order to gain in others and vice versa, while the general using knife that is expected to do it all will have to compromise on all the properties in order to do everything just O.K. If you go with this specialization approach the tricky part is educating your customers enough to deprogram them from the marketing, so they aren’t trying to chop through an iron bar with a fine skinner because of something they saw in a magazine.:rolleyes:

Kevin R. Cashen
12-24-2010, 09:19 AM
...Would I be correct in thinking that, besides the strength vs toughness issue, a full quench would offer somewhat less margin for error than an edge quench?

Thanks,
Carey

Carey I would love to tackle this question but I am not certain if I understand it? Are you asking if there could be more to go wrong in a full quench or a differential quench? The best way to hedge your bet for complete transformation, good homogeneous structure and condition is to get the steel cooled as evenly and effectively as possible. Thus a full quench done properly will have less to go wrong than differentially hardening. If I have to do a differential hardening I prefer using clay in order to still be able to get the whole thin under the surface of the quenchant and maximize my transformation.