I design Butterfly Swords (“BFS”) for personal use and on a professional basis (in the evening, not my day job) under the Modell Design LLC umbrella, http://www.modelldesignllc.com website. You can also see work product at http://www.everythingwingchun.com/wing-chun-butterfly-swords-s/35.htm web-site (look for the integral knives and flagship line). I recommend that anyone who has a serious interest in BFS read the following two articles:
– Jeffrey Modell, Esq., “History & Design of Butterfly Swords”, Kung Fu Tai Chi Magazine (March/April 2010)
– Jeffrey Modell, Esq., “Modern Butterfly Swords”, Kung Fu Tai Chi Magazine (July/August 2011)
BFS were actually used for the village water wars, by militia, security guards and criminal gangs. You can design them for cheap manufacturing or you can design and spec them as real weapons. I only do the latter but once you have weapons-grade design, balance, construction and materials, the knives are dangerous even if blunt.
I recommend in the strongest terms against training with live blades; if the construction is shoddy the blade may go flying and if it is good one slip can cut through tendon and bone. If you are at the Master level and insist on doing so, do it in private for the safety of everyone around, as shiny knives are an attractive nuisance.
BFS are really just big knives. If you have a Chinese long sword, you are not going to risk the relative brittleness of a stainless steel. But for BFS anything done well that works on a big Western style knife will probably work on a BFS and that means the good stainless steels are an option as well as the medium and high carbon steels.
The “best steel” depends on how you plan on using the knives. All of these answers assume you require a weapons-grade steel. ALL weapons-grade steels, including stainless, rust. Weapons-grade stainless is low maintenance, not no maintenance.
If you use the knives just for single person practice, and you transport them to and fro in your trunk (especially in the Winter), the chief steel choice concern is going to be corrosion resistance which is impacted mostly by the balance between the carbon and chromium in the steel. So, you will want a stainless steel, or a high carbon steel that has been blued or covered in a good coating. Avoid chrome coatings, they flake off with age – especially if you store in your trunk during the Winter (expansion and contraction) – and impacts. Stainless steels have certain classifications, and the precise carbon/chromium mix is likely an allowable range to qualify for that classification. Of course, the steel furnace can be just plain dishonest or shoddy. Anyway, the less carbon (other elements also impact this but carbon is the primary one for the affordable steels) the softer and easier to manufacture so generic 420, a cheap non-weapons-grade stainless referred to as “Butter Steel,” is a likely culprit for crap knives. On the other hand, we can get quality Japanese 420J2 with carbon on the high end of the J2 range that can be professionally heat treated to weapons-grade. That would be about the least expensive weapons-grade stainless. Moving up the expense and quality chain, there is 420HC, 440B, 440C and the modern super knife steels.
If someone says 440 and doesn’t have a letter A through C you should get a very bad feeling. If someone says “surgical grade steel” it could be 420J2 or not, all it needs to be is able to be easily sharpened to a fine edge that it need only hold for a single operation and hopefully not leak out carbon molecules into the wound. Basically it is meaningless sales hype. Many knife snobs think very badly of anything that starts with 420, and they are usually right because whatever knife they experienced had a straight 420, or a low honesty/low carbon 420J2, with improper heat treatment. They would be wrong with respect to a 420J2 or 420HC made by a reputable manufacturer, with a good carbon count, and properly heat treated.
A proper heat treatment is more important than the precise steel grade once you have a certain minimum steel quality. It is not about making the knife edge hard, it is about making the whole blade the optimal balance between hardness and resilience, there being a trade off that varies steel-by-steel.
Anyway, Randall knives smiths their stainless steel blades from 440B or 440C (guy over there said there is a lot of overlap but that probably has more to do with their steel source) so you KNOW that a good quality, professionally heat treated 440B is weapons-grade. 440C used to be the Cadillac stainless steel for U.S. custom knife makers due to its excellent balance of characteristics, and a couple of my favorite knife makers still prefer it as do I. The top U.S. custom knife maker does most of his military work in 440C (bead blasted for a non-reflective finish). Many of the U.S. custom knife makers now use the new super-steels, such as ATS-34 and S90V. These super knife steels are tweaks on 440C. They trade off corrosion resistance in return for different factors such as better edge holding capability or lateral stress resistance. A friend of mine left his ATS-34 knife in his car trunk for a relatively short time period – maybe a week, I don’t remember – and it rusted – so if you are using one of these steels on a combat knife seriously consider a Cerkote coating. One of my associates left an Everything Wing Chun (“EWC”) Flagship line knife soaking in water for weeks (not sure how long on that either but I remember when he told me I thought it was a long time for a knife) (not intentionally) and while the handle rotted, the blade was fine. It was made out of Bohler (German) 440C.
Now a comment was made in a thread on a different forum about there being too much brittleness from certain stainless steels. If it is not heat treated correctly, and if the designer did not specify an intelligent hardness for the edge of the specific knife, I can easily see that happening. Heck, I doubt if most manufacturers making Butterfly Knives even know how to use them. How would they know what to ask for even if they were willing to undergo the expense of sending the blades out to a third-party heat treatment specialist?
Just a comment on grind. Most folks who order custom BFS want them to look great, and they specify a wide hollow grind (i.e., concave like the inner surface of a sphere). That is the grind that will take the sharpest edge, best for flesh on flesh, but it is not a good choice for weapon vs. weapon as it nicks easier since there is less steel at the sharp edge and less steel supporting the edge. The traditional lenticular grind (convex like the outside of a sphere) takes a less fine edge so there is more steel at the sharp the edge and more supporting steel behind it so it is better for resisting nicks from bone and other weapons. For a true beater blade for weapon on weapon, you would want a fat rounded edge, but those are pretty heavy (slower on blocks), ugly and they do impact the balance of the knife (likely slower on blocks). Basically for beating, a flat fat piece of steel with a rounded edge blade and a blunt tip will last the longest. And speaking of weapon vs. weapon, it may be safer for your knives to dent and bend by virtue of being soft, or softer than weapons-grade (heat treatment/steel issue), than to be too hard and brittle and break so consider carefully what you want out of a knife. A knife that soft will probably mark up quicker from blade on blade contacts during individual practice (when your two knives touch).
Anyway, if you are doing single person practice, carbon steel is a maintenance intensive choice. It needs to be kept oiled when not in use and stored in a dry environment, not that I would want to store any knife in a wet or humid environment. Before each use you need to remove the oil. After each use you need to clean and re-oil. NEVER store any steel blade in or near leather, whether stainless or carbon steel, as the chemicals and vapors from the tanning process promote rapid corrosion.
Most martial artists who do not own an expensive Japanese Katana do not take care of their weapons the way they should, the way a hunter would. That’s why I shudder at the thought of martial artists using medium or high carbon steel weapons and leaving them in the car overnight, going to and from practice, sweating on them and leaving oil residues on them.
If you plan on using your BFS for weapon vs. weapon practice, you should be looking at a carbon steel, not a stainless steel. 440C is a tool steel used to make dies that cut other steels. So, it can get really hard, but it trades off resilience and flexibility in return for corrosion resistance and is more likely to nick or break than carbon steel. Now first let me say that I advise you against weapon vs. weapon training, it is dangerous and your weapon will eventually fail if you continue at it (so inspect before each use), I assume no responsibility for any such training by you, your use, misuse or abuse of the info herein, etc., etc., etc., — basically you are on your own, use this info at your own risk and you cannot sue me. And really, no sharps, just think about it.
The quality ladder for common carbon steel ranging from medium to high carbon is pretty much AISI 1055, 5160, 1075, then 1095. 5160 is a good medium grade, fine for “beater” swords. AISI 1075 is good high carbon steel and 1095 the top of the normal high carbon steel food chain. While I have seen someone say oil quenching is the best, that’s not my understanding, rather it is the easiest for the forge. To get the most out of 1075 you need to water quench, and water quenching means some of the blades crack (VERY expensive losing a 300 layer hand forged high quality stainless steel lamellar blade) so it is so it is obviously a more expensive practice. Ditto on 1095. There is no sense upgrading to 1075 or 1095 if you don’t water quench. This is an example of my earlier comment about the relative importance of the heat treatment versus the steel quality.
Hmm, speaking of blades cracking during quenching, an expense of manufacturing, whenever you use weapons grade potential steel and heat treat to a weapons grade hardness, even if you oil quench, a certain percentage of the blades are going to get hairline fractures. Our overseas forge associate inspects and rejects these, but hairline fractures are tough to spot with the naked eye and some get through. I suppose we could do what Busse does, coat them with a flourescent dye and inspect under flourescent light, but Busse gets about $600 for each of their Western combat knives and somehow I don’t think you want to pay $1200 for a pair of martial arts swords you would otherwise pay $300 to $500 for from EWC. Heck, hairline fractures have gotten past me but I blame it on the knives being bagged, sealed, and partially hidden with brown packing tape to hold a tip protector in place by the time I get a chance to eyeball them rather than me getting old. Hairline fractures are structural defects and the product should NOT be used, it needs to be returned. If you use crap steel and a piss poor heat treatment so the finished product is great for patties of butter and toast (and cutting cake), hairline fractures can be avoided so please don’t be too mad if one of these slips through to you and you have to go through the return process. It is a high class problem. Anyway, back to the discussion of steel.
A billet of good 440C costs about 3 or 4 times as much as a billet of AISI 1095, and the 440C knives are going to cost more, so you are less likely to want to beat them up. Another reason not to use stainless steel for weapon vs. weapon.
D2 is a specialty carbon steel used to make dies that cut other steels. It is often called “semi-stainless” steel because it is more rust resistant than your typical carbon steel. It is easily resilient enough for a BFS blade if of good quality and heat treated properly. Don’t do this at home but our crazy forge Master did a video of him bending back one of his D2 flagship design blades held in a vice. It snapped right back into place. He also tapped the vice with the edge and cut nice shiny lines in it. D2 is rough and tough and takes a fine edge and holds it well. You can only sharpen the edge with a sapphire or diamond hone, forget about your normal stone, and it takes four times as long to sharpen. It is more expensive to work and costs more than a lot of other steels. It is in the opinion of many top Western knife folks the best carbon steel for knives. In my opinion, if you take a quality D2 and PROPERLY, professionally heat treat it finishing with the edge at an APPROPRIATE hardness for a BFS, it is an outstanding blade choice for a real weapon.
A friend of mine, Master Jay Penfil, makes tonfa out of a lot of the exotic woods including Lignum Vitae. The Lignum Vitae is so hard his tonfa damage partners’ tonfa in weapon on weapon practice. D2 is like that, only for steel. One of the folks who purchased and EWC flagship D2 pair of BFS complained it was destroying the opposing weapons, which is exactly as I would expect.
There are a number of ways to help protect carbon steels from corrosion, including bluing the steel and coatings. The legitimate coatings vary from a powder coat that is baked on at the low end to a two-step Cerakote at the high end. The Cerakote is extremely durable, but you will probably not cover the edge so that still needs maintenance. Plus, over time, you are going to scrape coating off. That is why I think it is better to have a steel that is naturally corrosion resistant through and through if maintenance is a concern. I did a pair of sharp Hung Gar long-stabbers out of 5160 carbon steel, covered in a baked on black powder coat, and they looked so awesome a Sifu who was stopping in to pick up his pair of 440Cs (looked sharp but actually had a 1mm flat unsharpened edge) decided he also “needed” the black pair for “home defense” — at least that is what he told his wife. I steel miss that pair.
Next a comment on lamellar steel (often mistakenly called “Damascus”). Today it is mostly made from two different hardnesses of high carbon steel layered into decorative patterns. Most of what you will see in the way of lamellar today is not weapons-grade. Originally in China even cheap Butterfly Knives were made of lamellar (folded steel, not decorative intent) at the village smith because they had to due to low steel quality – the blades were more resilient just like modern plywood. Good quality modern mono-steels are just as good, and better than most modern Damascus. That said, there are a few custom smiths who make weapons-grade Damascus BFS. Stainless lamellars are harder to find, and cost more. Stainless is tougher to forge than high carbon steel.
If you are not going to actually use your BFS, if you are a collector, then you may want to get one of the knife super-steels for snob appeal, a lamellar for aesthetic appeal, or a differentially tempered high carbon steel with an artistic true hamon line.
Let me ask you this question. Besides Everything Wing Chun, what other purveyor of Butterfly Swords/Knives/BJD actually cares enough about the martial arts community to even think about the steel and heat treatment issues noted above? I can think of exactly one, they only make knives for Moy Yat and a pair end up costing over $1,000 after shipping and adding in the sheath.