1.Difficulty of Pattern welding for one who knows the basics like me?
Pattern welding isn't all that difficult....it's all about the alloys your trying to join, the correct heat range, AND a proper soak time. The alloys you choose will play the biggest part in success or failure. The alloy need to have similar expansion/contraction coefficients....meaning that when heated or cooled, they need to expand or contract at similar rates. This is the reason that 1080 or 1084 and 15N20 are so popular...they are nearly identical with the exception of the 15N20 having a 1.5% nickel content. I generally run my forge at 2350F before I ever put the billet in. Then experience and intuition take over.....you have to remember that when the outside layers of an initial weld look like they are at the proper temp, the interior layers are still "cold"...that is where the soak comes in. Once you make the initial weld, all of the tiny air spaces are eliminated, and the billet begins to exhibit the behavior of a single, solid piece of steel as it applies to heating/cooling.
This is basically adding additonal steps to the process. Again, as long as the alloy are closely compatible, it's not all that difficult. For example, if you want a "core" of random pattern damascus, with exteriors of 1080, you simply build a billet of random, take it down to whatever you wise the core thickness to be, then overlay a piece of 1080 on each side, and forge weld the entire billet. Obviously there are a lot of little nuances to it, but that is the general idea.
If you're thinking of doing San-Mai with 416 stainless, that's a whole different ballgame. Because 416 is so different it requires very specific temps and soak times to make it work. The only way I have been successful with 416 laminated to the exterior of billets is to "dry weld" it. All the mating surface must be clean, flat, and kept oxygen free during the process. It took me 6-8 months of experimenting to figure it out. The way I do it is to forge all the parts to the sizes/thickness I want, then surface grind all the mating faces to 120 grit, clean with acetone, then "sandwich" everything together and MIG weld the entire outside edges....this essentially makes the exterior 416 the "can" portion, as if I were doing a "can" weld with powdered steels. The billet goes into the forge at 2350F, and once it appears to reach that temp, I let it soak for 7-9 mins. Then it goes into the press for a single weld. I then grind all of the MIG off, and continue to forge to shape. This combo has a very narrow forging range....which means you have about a 200-300 degree temp range where you can forge it....otherwise it will often come apart. There are a whole host of other issues that arise from mating these two materials.....annealing is a bear. The only way I can anneal this material is to put it in the heat treat oven at 1350F, for a 1 hour soak, then let it cool down in the oven, overnight. There are also unique things that go on with carbon migration, which will effect how the finished blade is hardened/tempered. Lots of hurdles to overcome, and lots of experimenting required if you're gona go with 416 in a San-Mai.
3. Etchant that is easy to buy without a background check? And soak time?
PCB Archer Etchant from Radio Shack. Otherwise known as Ferric Chloride. Dilute it 3 to 1 with DISTILLED water. Blades MUST be finely finished (I go to 600 by hand) and ABSOLUTELY CLEAN prior to etching. (Never buff a blade that is to be etched....if you do, the pattern comes out looking "smeared") I put on latex gloves, clean the blade with acetone, then clean it again with windex. Soak time in the etch depends on many things....temp of etch, dilution of etch, your desired result, etc. I usually like to have my damascus etched to where I can feel the topography with my finger tip. Most of the time that requires 30 minutes or more.
4. Heat treatmeant of 416ss and 1084 san mai and pattern welded 15n20 and 1084?
I won't even get into heat treating 416ss/1084 San-Mai.....there are just way too many variables throughout the entire process to even give you a "close" answer. Like I said before, it took me 6-8 month of experimenting to figure it out, and it changes from billet to billet, depending on how good or bad my technique is for a given billet.
1084/15N20 is easy.....since the steels are nearly identical. 1550F, then quench, temper 3X for 2 hours each time at 400-425F, allowing the blades to cool to room temp between tempering cycles.
