Scott, I would go with mete on this. I would see no advantage to adding L6 to your mix. I have always had better results by matching water hardening with water hardening and oil hardening with oil hardening, and thus prefer to do it that way. I have always considered 15n20 and 1084/1095 the shallow hardening version of L6 and O1, not better or worse just different quench speeds. Adding a deep hardening steel to the mix will only put you in a situation where you will have to choose between the lesser of two evils in the heat treat.
However dont fall into the awful trap that SO many fall into when they work under the misconception that 15n20 is just L6 under a different name- they are two entirely different steels. L6 is very rich in chromium and other alloying while 15n20 is pretty much 1075 with nickel added.
There are actually some guidelines I have developed that can help me determine what goes into Damascus and I can share them but Mr. Purple (Matt) should be aware that is one more item that could be developed into a chapter for my book that I am giving away for free
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First, since aesthetics are subjective and performance should be universal in a tool, my first consideration always goes to how my selection effects the performance. And since performance is predominantly determined by heat treating I focus heavily there.
Drop a material into my hands and ask if I would put it into my mix, the first thing I will ask myself is will this make a good knife all by itself? If the answer is no I will toss it aside for fittings materials, since I dont care to water down my good steel with it. The next question I will ask is what are the heat treating requirements of each of my steels?
The obvious part is the quench speed according to the curves, but there are also other points that can have profound effects. Contrary to popular opinion there is no one universal austenitizing temperature, due to carbide levels each steel has its own preferred range to which it will need to be heated for proper hardening. Lets look at a mix of 1095 and 5160. They sound like a good idea but 5160 needs a range around 1525F while 1095 is best served below 1500F. This leaves no window where both steels can be austenitized optimally. Add to this the fact that one is extremely shallow hardening and the other is deeper hardening and you will then encounter differential rates of hardening that can result in distortion or even delamination. Oh you could eliminate a bit of the problem by just quenching as fast as possible but shocking the 5160 to that level could result in worse issues, such as microfracturing and other nasties. AN example of a window is for my O1/L6 steel, the O1 range tops out at 1500F while the L6 starts at 1500F. so I have my perfect temp for that mix already determined for me, it is a narrow window, but it works
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The next thing I look at is weldability. Some alloys are just ornery to stick and should be left for simple projects or somebody who is a real wizard at forging welding. I use O1 and L6 for almost any simple using blade I make but if I need to do some wild twisting and multi barring with stressful forging I will opt for something like 1075 and the Admiral L6 alternative (8670M) because the stuff sticks together like it is made of glue and I will lessen the chances of wasting a week developing a piece only to throw it away when I find a flaw in polishing (one cant make a living that way). The richer the alloy, the more finicky it becomes in welding, I have come to blame chromium for many of my welding woes, it seems anything with higher chromium levels needs to be watched more closely in the welding. Some blame nickel and its oxides but this only seems to be an issue at very high temperatures.
The next issue in weldability is ductility rates and expansion. It is a smaller concern but can effect the rate at which you draw things out at different temperatures. Mix say 52100 (stiff as a rock under the hammer) with 1084 and the one set of layers could move at such a different rate from the other that there is added stress on the welds in working. This will abate a bit with further working as the carbon diffuses more and evens out in the layers. Which brings me to another point
Carbon migration is our friend! I often see it used like it is a term that describes a disease to be avoided in our Damascus, when it actually allows for strengthening of low carbon layers and making for more even treatments later. If you need to keep carbon in certain layers because your blade will be too soft it moves, keep the stuff that wouldnt have made a good blade out of the mix to begin with. I have found layers of varying abrasion resistance to be much more effective than layers of differing hardness (and thus strength).
Contrast and final appearance, while it is last, is still very important for the marketability of the knife. I personally could just never put pretty over performance. Steels the have nickel contents will display the brightest whites and silvers in the etch, the higher nickel content the brighter the color. Chromium will produce lighter grays but not the same bright silvers and whites. Simpler steels without this alloying will produce darker contrasts. Carbon content can have very subtle effects on the darkness but this is mostly due, in the long run, to its effects on heat treatment. The element you want to look at for darkness is manganese, the more Mn, the closer to black you can get those contrasting layers with etching alone.
One final note on color, heat treating will have profound effects on the etch and colors. Annealed steel is about as interesting as watching paint dry in its contrast from an etch. Fully hardened steel with no temper is not much different. But as you increase amounts of tempering on hardened steel the richer those darker layers will get. After all my experience with metallography I now believe a lot of this is due to tempering carbides. All other form of carbide can be easily recognized under the scope due to its white appearance. But very fine tempering carbides are dark and appear as tiny black dots in the matrix, which leads me to believe that more of them you precipitate, the darker the overall steel will etch macroscopically. Eventually if you heat high enough the carbides will just grow into spheroidal cementite and things will wash out again though.
