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Thread: what makes japanese chisel steel so much harder?

  1. #1
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    what makes japanese chisel steel so much harder?

    This is a simple question, but when I try to find the answer via googling there is a low signal to noise ratio. My question is what makes the steel in japanese chisels have such a significantly higher rockwell hardness? Is it the elemental blend (more chrome, vandium, etc.) or is it the forging process which compacts the carbides or is it tempered less or is it something else entirely?
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  2. #2
    Most Japanese chisels are made from a low carbon body with a rather small bit of high carbon steel at the edge. Usually white paper steel which is a Japanese name for a very simple high carbon steel. It has about 1.2% carbon and the rest is mostly iron.

    Like all high carbon steels it can be hardened to a rockwell C value around 66%. That is, directly after heating to 850 degrees Celsius (or thereabouts) and quenching it has such a high hardness. The disadvantage is that after quenching the steel is very brittle and full of stress. Therefore every tools steel is tempered. When you look up for example the tempering curve for 1095 steel you will see that a low tempering temperature (100 degree celsius) results in still very high hrc values. Higher tempering temperatures, like 200 or even 250 degrees brings the hrc value down to about 60. (All numbers from my head, so to be precise you must look it up).

    Westeern chisels are almost always tempered to those values in the high 50's or low 60's. That gives a solid, relatively hard, but not brittle chisel that is easy to sharpen.

    The Japanes smiths temper to much higher HRC values for their top of the line chisels. 63 or 64. The worst of the stress is now gone from the steel but it is still relatively brittle. It is the construction of the chisel, the low carbon body with the steel only a small part of it. The unhardened low carbon body takes the "beating". So the chisel won't break under hammering. But it still needs to be handled with care. Under prying loads you can easilly break small chips from the edge.

    Oh, and before I forget, the smiths are great craftsman. They manage the heat treating and the forging so that the end result is a steel with very fine grain. The coarser the grain, the more brittle the steel is. So by paying attention to this detail, you can get away with more hardness. It is probably a trade secret how they do it exactly, but it is a matter of much experience anyway, it is not simply following a recipe.
    Last edited by Kees Heiden; 11-24-2015 at 7:11 AM.

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    Quote Originally Posted by jamie shard View Post
    My question is what makes the steel in Japanese chisels have such a significantly higher rockwell hardness? Is it the elemental blend (more chrome, vandium, etc.) or is it the forging process which compacts the carbides or is it tempered less or is it something else entirely?
    Japanese chisels tend to be, but are not always, harder than Western style chisels because they are intentionally heat treated to be harder. This is easy to do, but hard to make practical.

    They can tolerate the extra hardness partially because of the laminated structure of high-carbon steel and low carbon, relatively softer and therefore tougher, steel used to make chisel blades. A western style chisel blade made of uniform high-carbon steel structure would simply break if heat treated to the same hardness.

    Another reason is that the steel is typically (but not always) of better quality, and if forged, will have a relatively more uniform crystalline structure less inclined to chip at higher levels of hardness.

    None of this is rocket science or unique to Japan. Laminated chisels and even plane blades have a long history in the West, as does hand forged, high quality steel. The West just abandoned top-quality blade smithing with the industrial revolution. Some of the best-cutting and most durable laminated plane blades I have ever seen were made in Sheffield England in the 1800's, and branded "Cast Steel." The West simply abandoned the techniques still used in Japan.

    After cutlery became commonly mass-produced in sweat shops, the really good blacksmiths could not compete anymore with factory products.

    The mass industrialization of toolmaking in Western Europe, Northern Europe, and America lead inexorably forward to mediocrity. But in Japan, during this same period, a change in laws forbade the wearing of swords in public (the 'haitorei" 廃刀令 of 1876), thereby ruining the livelihood of tens of thousands of the arguably best blacksmiths in the world. Many of these artisans switched to tool making, and applied their excellent forging, heat treating, and shaping skills to woodworking tools.

    In Japan, prior to this change in laws, the more mechanical gun never really replaced the elegant sword. In fact, the government bureacracy and aristocracy of Japan, unlike America and England, have always feared the common man (who until the 1800's were slaves in all but name) owning weapons, and fiercely opposed private ownership of firearms. Therefore, the sword has always been seen as the ultimate weapon. These extremely sharp, tough, and elegant blades have for many hundreds of years been iconic subjects of reverence and lust. This fanatic desire for sharp, tough cutting tools extended to woodworker's as well.

    Consequently, while the average skill levels of tool blacksmiths dropped in the West, they dramatically improved on the average in Japan. It is no wonder Japan developed not only unique tool designs, but improved the quality and effectiveness of its woodworking tools. The West has never been as fanatic about sharpness and excellent steel as the Japanese.

    I hope this helps.
    Last edited by Stanley Covington; 11-24-2015 at 7:37 AM.

  4. #4
    As mentioned by others, before tempering high carbon steel tools are hard and brittle whether Japanese or English. We deliberately temper the tools at a higher temperature because we prefer them that way and we know how to handle a tool that is heat treated this way. I think the English tools made in the early 19th century are fantastic. I have had only one 18th century tool and it was also very fine. These older tools are also laminated. I have been fanatic about sharpness and steel for over fifty years.

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    At the risk of oversimplifying, it sounds like the critical feature is the lamination which allows for more brittle steel to be used at the cutting edge. The quality of the steel is enhanced by the forging. The steel itself is basic high-carbon steel.
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    The lamination is critical, it allows support of the hard steel by softer low carbon steel or plain iron ( chisel or plane ). Japanese do have alloy steels, in fact some makers are quite famous for their various special blend alloys, such as Tasai or Kengo.

    I prefer plain HC for my purposes, but it's worth a mention.
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  7. #7
    Quote Originally Posted by jamie shard View Post
    At the risk of oversimplifying, it sounds like the critical feature is the lamination which allows for more brittle steel to be used at the cutting edge. The quality of the steel is enhanced by the forging. The steel itself is basic high-carbon steel.
    I would not say that. My chisels and plane irons were all made in England or America. The majority are laminated; I would say the better ones are laminated. We prefer higher tempering for these tools and have for over three hundred years.

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    Quote Originally Posted by jamie shard View Post
    At the risk of oversimplifying, it sounds like the critical feature is the lamination which allows for more brittle steel to be used at the cutting edge. The quality of the steel is enhanced by the forging. The steel itself is basic high-carbon steel.
    That's the nutshell summary,

    The steel is not typically plain high-carbon steel nowadays. More alloys with chrome, moly, and some tungsten, among other additives to improve heat treatment characteristics and QC are used. These additives help significantly to reduce warping during heat treatment and to widen the range of successful temperatures for quenching and tempering. Fewer rejects, better productivity.

    You can imagine what chrome and tungsten do for sharpening, though. Not an improvement, but certainly tolerable.

  9. #9
    There are a few big differences between the old English chisels and the Japanese one. The Japanese like to advertise with HRc of 64 or 65. That is hardly tempered at all, just enough to relieve the stress in the steel from the quench. I don't think English ones ever were beyond 61 or 62 (outliers not counted). The English bench chisels were often a lot thinner. Japanese chisels are relatively thick. The Japanese chisels are also shorter. I think both these factors help the stability of the chisel and thus allow a harder steel bit too.

    Some old English chisels were not laminated. Especially the smaller sizes were often made entirely from cast steel. The Seaton chest has two sets that are almost the same but one set is not laminated and is marked "cast steel". The other set is laminated in the larger sizes and is probably made of wrought iron with a blister steel edge.

  10. #10
    Don't get too excited about a chisel marked "cast steel". In old tools, the words "cast steel" actually means "crucible steel", steel that was melted in a relatively small crucible. Prior to the Bessemer process, the only way to melt steel was in a small crucible and that steel was marked as "cast steel" to distinguish it from blister steel. Cast steel was much more homogeneous than blister steel.

    The crucible process continued after the Bessemer process became popular because it could produce better steel than the Bessemer process (that wasn't saying much, because Bessemer steel was not very consistent batch to batch). Today, the electric arc furnace has replaced the crucible process.

    But the steel that came out of a crucible depended on what went into the crucible and different batches were often different. About the best that can be said of crucible steel (or cast steel) is that it may be about as good as some of the modern carbon steel. Beyond that, a lot depended on the processing of the steel into a tool.

    But the important take-away from my posting is that there's nothing special about "cast steel" - it's just plain carbon steel, often with poorly controlled impurities.

    An excellent book on crucible steel is "Steelmaking Before Bessemer, Volume 2, Crucible Steel" by K.C. Barraclough, published by the Metals Society. Volume 1 on Blister steel is also good.

    Mike
    Last edited by Mike Henderson; 11-24-2015 at 8:16 PM.
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  11. #11
    It is very hard today to find pure carbon steel. Even 1095 has about 0.5% manganese. Now, don't get me wrong, I think manganese in steel is not a bad idea, because it binds suplhur and helps with the hardening of the steel. But it is not quite the same as a pure Japanese white paper steel that only has a tiny bit of silicon added. Some people think any addition makes the steel less perfect, so for those people nothing else is good enough.

    About cast steel, Its production was of course difficult to control. Everything depended on the skill of the foundry men. Chisel makers with an intend to make the best were often intimately connected to their steel supplier or were even making their own steel. Only then they could rely on a steady supply of good stuff. Even then, it is a bit of a miracle that they were able to produce such good tools in an environment like that.

  12. #12
    Kees, you mentioned the cast steel chisels in the Seaton chest. I believe that for some time in the 18th century the craftsmen had trouble forge welding cast steel to the rest of the chisel; it took some years to solve the technical problems, but later cast steel chisels were laminated. I have never seen a Japanese chisel from the 18th century; I don't know what the Japanese were doing at the time.

    I have used cast steel tools for forty years now. I feel like I am lucky to have them. I can thank some 19th century craftsmen who were willing to pay a premium for quality.

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  14. #14
    I cannot prove it, but I think the traditional explanation given--that lamination cushions the steel and keeps it from chipping at high RC--is not true. Chips on a chisel, unless it's seriously defective or seriously mistreated, are usually a few thousandths to maybe a 64th deep. Laminations, especially on a japanese chisel, are at least 3/32-1/8 thick. Looking at the geometry, I think the soft steel (or wrought iron) is just too far away from the edge to have any effect. I think if a japanese chisel were made entirely out of White or Blue paper steel, it would still work as well.
    I had an O1 plane iron made by Steve Knight that was very similar in feel to japanese blades. It was so hard I could barely sharpen it on oil stones, yet it didn't chip. I think that the most important factors are high quality steel, properly hardened and tempered by someone who knows what they're doing. Laminating is important for tradition, and because the white or blue steel is very expensive--I bought one stick this summer when I was in Germany, it was almost $40.

  15. #15
    I think we must now start to look at the stress-strain diagram. Bear with me, this is all new stuff for me too and I haven't quite grabbed it all either.



    This diagram pictures how steel behaves when it is pulled apart in a special machine. The machine tries to make the steel rod longer and meassures the elongation and the force. The most important part of the curve is the first rise until the yield point. This is the springy part of the curve, when the force is released before the yield point the steel springs back to its original shape. Above the yield point the steel deforms permanently until it breaks. hardened steel doesn't reach very far beyond the yield point, it snaps long before the top of this curve.

    The same kind of steel can be hardened to several different hardness levels. A spring steel, hardened to 50 HRc will have a much lower yield point then a steel hardened to 60 HRc, but with much more elongation. It also won't snap as easilly beyond the yield point.

    Here is a diagram describing how I think it works with tool steels. The little stars are the break points. (This is just a quicky diagram, nothing is to any kind of scale, imperial or metric ).


    More hardness makes the curve steeper, raises the yield point (more pressure neccessary to stretch the steel), but also reduces the elongation and it makes the steel more brittle, it snaps quicker when you go beyond the yield point. When you manage to reduce the grain size of the steel during the heat treatment, then you make the steel stronger (higher yield point) and tougher (more elongation until you reach the yield point). A steel with very high hardness can withstand a lot of stress, but it is very brittle so it snaps just a little beyond the yield point and it elongates hardly at all before it breaks.

    How do we translate this to chisels? A chisel with low hardness is perfect for rough work. A bit of prying is no problem, the chisel won't break easilly when mistreated. But the edge isn't very hard. When mistreated the edge will give rather quickly and may even fold over.

    A very hard chisel is always at risc when being bend to break in two parts. It is telling that 5 (!) chisels from the Seaton chest are broken or cracked! That is where the lamination comes in. The hard steel is very well supported by the much tougher iron body of the chisel. But when we look at the edge, then things are a little different. When you hammer a chisel into a piece of hard wood, you want to stay below the yield point. A very hard edge has a much higher yield point, so it won't deform as easilly. In the direction of the length of the chisel, this edge is very well supported and thus very strong. But in a prying direction the support of the edge is much less, and it will be easier to pass beyond the yield point resulting in a chipped edge.

    So the lamination helps to avoid a total failure of the chisel. The hardness plays a role in the stability of the edge in the cutting direction, but high hardness makes it vulnerable for chipping when a force perpendicular acts on the edge.

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