Hello Jeremy, I am glad to see that Lin was able to help you with your other question. It would be hard to say that differential heat treatment would be the way to go as you are thinking without keeping it in context with what it is that you are planning to do. Of course for an ABS test knife it would be the way to go, but a completely pearlitic spine is mostly an aesthetic choice, particularly in the case of the clay and hamon method, when high strength and pure cutting application are involved. In your case with the idea of battoning the spine in use, the pearlitic spine may even become more of a liability for exactly the reason you mention.

It is very easy to confuse ductility and toughness and it sounds like what you want is a very tough blade but not a ductile one. Ductile steel bends or deforms under loads, while tough steel is more about handling sudden loading and shocks. A Ductile blade will avoid breaking but may easily bend or, as you fear, mushroom under a baton. A tough blade will also avoid breaking but may also maintain the strength to resist bending or deforming.

There are two ways to achieve the tough blade you are looking for, alloying or heat treating. With alloying one can choose a steel with the chemistry capable of allowing comparatively high hardness for strength and still be tough, but since you already have a chosen steel let’s look at the heat treatment.

You are correct that the better way to get a tough, rather than ductile, blade is to fully harden it and then draw the spine back softer than the edge in the temper. Most simply harden and temper as normal and then protect the edge with water, wet sand etc… and draw the spine back to a blue color with a torch.

The spine on the clay quenched blade will be rather soft in steels that will respond quite well to it- 1070, 1075, 1080, 1084, 1095, W1, or W2, since these steels will not require much insulation from the quench to make plenty of pearlite. With these steels that demarcation between hard and soft will be abrupt while with other steels, such as your 5160, there may be more of gradient from hard to soft as the hardening creeps up under the clay.

Kevin,

There's so much discussion about making a knife that will pass the JS or MS test, that it is seldom pointed out that a full soft spine may not be the way to go for the average user knife. I dont want mine to be full soft. In a cutting competition, I saw a knife that was bent just by making a bad lick on a 2 X 4. I have no doubt that the maker softened the spine just like he was going to use if for his test knife. I've tested several that were drawn back and I prefer one with a little stiffness in the spine. I can feel the difference in the way a they cut. It seems to have more life in it. Not soul, Life. <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//smile.gi f' class='bbc_emoticon' alt=':)' />

I sometimes will quench my big blades that will be a chopper like this: horizontal full quench to a count of 4-5 (when I see the color gone for a second or two), then raise the top half of the blade back up out of the quench letting the heat still in the spine draw itself some.

Thanks for the replies. Kevin, I always learn a bunch when reading your stuff. Just so I can try and get this all correct-is it Martensite we want in the blade edge and Pearlite in the spine? Lin, is your method giving some good results as far as the spine not being too soft? Seems like it would be a pretty good method-do you end up drawing back with a torch afterwards as well?

Jeremy

Martensite is the good stuff that we are always wanting to make when quenching a blade, it is the strongest phase of steel. Pearlite is what happens when you let carbon do what it wants with insufficient cooling and is a pretty soft stuff. Lin is absolutely correct about the pearlitic spine and the bending. Often people will let the example of the katana guide them in thinking the pearlitic spine is the ultimate heat treatment. But ironically they are actually relying in the katana mystique or what Hollywood has given us rather than reality since in fact katanas are very prone to bending if one isn’t very skilled in their use. Since I regularly work with modern and ancient type steels I have seen the distinct differences and the reason why dead soft spines were an effective solution to many of the limitations in the ancient world. Lin is also quite correct that we spend so much time helping people make blades that will show the ABS their skills with heat are sufficient to earn a stamp that we forget to consider real world applications and how each heat treatment should best fit them.

Ductility is the best way to avoid a crack when slowly, and intentionally, bending a blade so the pearlitic spine is excellent for a test knife. But the resilience of a less ductile blade will show itself in heavy chopping applications; chopping is very sudden loading not gradual flexing or bending and they work off from entirely different mechanisms. For all of my cutting competition blades I always used damascus, and took the alloying route to toughness, so my blades where always just as hard on the spine as they were on the edge. The combination of strength and impact toughness gave me exactly what I was going for in such a blade. But for a competition blade your controls have to be dead on to go with such a heat treatment since blade failure will endanger the public, so without very precise controls I would recommend a differential heat treatment as the safest route.

Jeremy,

I rarely draw the spine, because I rarely full quench and harden the whole blade. I do want the blade to be differentially hardened, harder on the cutting edge and gradually becomming less hard toward the spine. So, I choose to edge quench or apply some clay to the spine and slow the quench in that area of the blade. 5160 is one steel that will air harden, so I dont clay it. I edge quench it and might even fully dunk it at first as I described above and raise it up to relieve the hardness along the spine. I will test every blade with a file right after the quench, edge, spine, and especially the tang just behind the ricasso. I will draw the tang and ricasso juncture on any knife likely to see chopping. If for some reason I harden the whole blade, I would at some point draw the spine to some degree, but unless it's a test knife, not dead soft.

Kevin, I appreciate your balanced viewpoint of things. Congratulations on becoming a new board member btw.

Thanks for the clarifications and definitions. I'm looking forward to the next time I get to heat treat, now <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//smile.gi f' class='bbc_emoticon' alt=':)' />.

Jeremy

One last thing Jeremy, I should probably cover this in your other thread on using what you have, but I will throw it in here since it is much more specific to differential heat treatment. If people will be using an edge quenching technique I always implore them to use a cheaper substitute rather than a formulated quenchant. The surface heating and inevitable flashing will trash your investment in a good quenchant very quickly and with the faster quench oils the flash point will be lower and thus increase the danger even more. Due to the thermal dynamics involved in edge quenching methods I strongly suggest canola oil instead. Both my testing and research has shown that canola oil is more effective on simple steels than peanut oil, peanut oil gained some popularity because if its significantly higher flash point. For the effects of edge quenching canola oil should work and not break your heart when it starts to get black an sludgy.

Clay however can be used well with formulated quenchants since everything is below the surface, although canola should work as well since the cooling action will also be greater under the surface.

Kevin-

Thank you for the information. What would you think of Lin's method of submerging the whole blade then raising the top after a few seconds using my oil? Would that give me a slightly harder spine that I'm looking for rather than an edge quench where I leave the spine out all together? I was also wondering if it would be worth trying a thin layer of clay to the spine and submerging the whole blade but I don't know how much that would insulate it and how soft the spine would end up... Again, thank you for the input-it's very appreciated.

Jeremy

Jeremy,

I realize the question was addressed to Kevin but I thought I might jump into the discussion. I do not know what steel Lin uses and that would make a big difference in analyzing the process. However, if the steel was 1084 AISI and I quenched the entire blade from austenite for four to five seconds submerged in the quenchant, then the blade would have cooled sufficiently to get past the pearlite nose on a time and temperature curve for 1084. The continued cooling rate with half the blade in the quenchant would be faster than in air alone and here is where I am unable to calculate how much of the blade is above the MS point for how long (this is the temperature where martensite starts to form and it can range from roughly 300 to 500 degrees for the most common steels used in bladesmithing. My guess is that Lin is getting a mix of pearlite and martensite in the upper half of the air exposed portion of the blade, but only testing would show how much. The time elements here are very tricky and they would vary with the steel used. Other steels might have more pearlite and some might even have some bainite forming in the mix.

So the question to me is not does Lin's process work but are you sufficiently skilled to make it work. There still is bigger set of questions. The primary one has to do with your intended outcome? Do you want an all martensite structure? Do you want,as others described, a martensite edge and pearlite back? Why do you want one over the other (esthetics and/or toughness)? Each of the processes described works but each produces a different outcome. You need to know the strengths and weaknesses of the various structures we are talking about (e.g., martensite, pearlite, bainite, mixed, etc.).

Here is where I think a lot of smiths get confused. The assumption is there is one best outcome. That is a falsehood unless we know the other half of the equation - one best outcome to do what? In some cases it may not matter, but in some cases it will matter significantly. For example, I want a knife that I can do one thing very well with. I want to cut cardboard. In that case as long as the blade is heat treated to produce the maximum percentage of martensite at the smallest grain size, then for that steel the knife will do what I want. However, if I want a large knife that will be subjected to severe striking force, then I personally would prefer an all martensite blade with a drawn back. If I want a blade that can bend more than 90 degrees with ease, then a pearlite back. If I want a beautiful and complex hamon or temper line, then I will use a proces that differentiates the martensite edge region from the pearlite/martensite and pearlite back.

Actually, I personally would want a martensite core with a bainite exterior, which is what most of my current larger knives will be showing.

Dan

|quoted:

Kevin-

Thank you for the information. What would you think of Lin's method of submerging the whole blade then raising the top after a few seconds using my oil? Would that give me a slightly harder spine that I'm looking for rather than an edge quench where I leave the spine out all together? I was also wondering if it would be worth trying a thin layer of clay to the spine and submerging the whole blade but I don't know how much that would insulate it and how soft the spine would end up... Again, thank you for the input-it's very appreciated.

Jeremy

Thank you for your input, Dan, I have a lot to learn about heat treating. Please forgive my ignorance, but could you explain bainite for me? I'm familiar with the others, but not with bainite. In reference to the steel, I'll mostly be using 5160, but 1080 will hopefully be something I use once I start trying forge welding-would your example of 1084 work the same for 1080?

As for the intended purpose of the blade, I'm wanting a strong spine that will not deformed if hit or hammered using a piece of wood. I guess maybe I should be asking if what I have in my mind is correct. I want a spine that won't deform or mushroom by "batoning" the spine but not so hard as to chip or crack under the same abuse. I have been under the impression that pearlite would be too soft for the spine in this application-would martensite be too hard?

Thanks for your thoughts.

Jeremy

Jeremy,

Please realize that most of us were searching for information just like you are. Ask away.

Bainite is broken into two somewhat distinct groups of upper bainite and lower bainite. Lower bainite is metallic structure that has some interesting properties. It will not reach the same hardness levels as martensite and usually a 58 on a RC is about the best you could hope for in steels such as 52100. The benefit of bainite is that it is able to achieve a notch toughness level that is almost three times as great as a standard quench from the same heat (Charpy V notch toughness test). So at 57 HC with a bainite structure compared to the same steel with a martensite structure at 57 HC you would expect the bainite blade to be tougher and have better impact resistance. Some have argued the edge holding on bainite at relative hardness is fairly equivalent.

To get lower bainite you have to be able to bring the steel down quickly to a temperature just above the Ms point for that steel. Then you have to keep the steel at that temperature until it transitions from austenite into bainite. Depending on the steel that can be hours to days. Some steels somewhat readily want to transform into bainite while others will not. Because of the lengthy hold time for some steels that bladesmiths use it is just not a feasible process. If you are interested there are some great articles out there on bainite and even a couple written in relation to the low alloy steels that some smiths use (e.g., 52100, L6 or its clones, and others). You really need salt tanks to produce bainite.

Bainite probably makes more sense on big knives than it does on small knives. Bainite probably given that logic is probably even better suited for a sword where impact toughness and ability to withstand force when the nicks in the blade weaken the blades structural integrity.

As to the spine deforming with hits to it, pearlite will deform more than martensite. Part of that is the structure but of that is that it is softer. A fully hardened blade with drawn back would probably better accomplish that outcome.

I cannot compliment Keven enough for all the knowledge and information he imparts to the rest of us. I believe Kevin wants each of us to learn more about metallurgy and the heat treatment of the steels we use. He has a nice way of giving just enough information to move you or me forward. There is an empirical base of knowledge underlying pretty much everything we do. If you can learn that then it opens up a whole range of possibilities and reduces the mistakes of learning by trial and error or by myth.

Dan

Thanks for the explanation. There sure is a lot to learn, but one of my favorite things about making knives is the willingness of such knowledgeable people like you to share with guys like me. Have a great weekend.

Jeremy