I'm sure everybody has their own specific way of making San-Mai, but this is what I would offer based on how I do it......
I would encourage you to thin down you're components...... with the type of San-Mai, you won't be able to do much forging on it beyond welding..... it will delaminate if you try to draw it out or do much beyond welding.
Depending on what finished thickness you're wanting, you want to take your core, as well as your laminates down..... I generally take my cores to ..070-.100" and the laminates (outsides) to .100-.120". The only way I've been successful with that combination and a dry weld is to surface grind all the mating faces to at least 120 grit, and just prior to MIG welding, clean the mating faces with acetone (and have latex/Nitrile gloves on...because IF you happen to touch ANY of the mating faces with a bare finger it just won't weld). I put everything together, generally clamping it in a vise, and use vise grips on the outer ends to ensure everything is snugged together as best it can be.
Your MIG welder with work fine. MIG it all the way around the exterior edges, making sure you don't leave any air holes or gaps.
Now, here's the most important part for me..... Have your forge running and ready..... AS SOON AS YOU ARE DONE MIG WELDING, get it into the forge. Something I discovered is that if you have even the tiniest air hole in your welds, and allow the billet to cool, it WILL suck atmosphere in, and if that happens, you'll think its forge welded, but when you grind that last bit of MIG off the outside edges, it will just fall apart, and the mating faces will have a nice shiny black residue on them. I found this out when I was MIG welding a billet one evening, the wife got home from work, and I left the billet till the next morning. Everything seemed like it was going fine. I annealed the billet, and the next day ground off the MIG weld....and if just fell into three pieces. I could see exactly where I had a pin hole in the MIG weld..... a tiny dark line that flared out and covered the entire mating faces of each piece. Lesson learned.
As far as welding it, I dial the forge in at 2370F, and once the billet looks like its at welding heat, I let it soak for 7-10 mins. Then its into the press ,and thats it. No more forging. If you try to draw out or otherwise forge to shape, the odds are great that things will delaminate. The reason being, there is a huge difference in how each of those material moves, expands, and contracts when heated and cooled. When I make this type of San-Mai, once the laminating is done, the forging is done.
Annealing this combination can be a challenge too. It took me a couple of months to figure it out, and after bouncing it off folks like Chad Nichols, Steve Kelly, and several others, and failing a number of times, I finally found what works best for me. If you try to anneal it as you would 1095 (in vermiculite), the 416 will be so hard that a cobalt drill will not touch it. My method of annealing this type of San-Mai is into the heat treat oven, 1350F for a 2 hour soak, and let it cool down with the oven to room temp. Another thing I do is put a second LARGE chunk of steel into the oven with it....takes longer to heat up and soak, but it also forces it to cool down more slowly.
When its done, there is going to be a LOT of exterior scale on the 416, and it is some of the toughest scale to remove that I have every dealt with. Generally I can count on loosing at least .80-.125" off each side, just to get rid of the scale. For me this is where I used an angle grinder, because that 416 scale just laughs at ANY type of belt.
Probably the thing I had to change most was with this material was the actual way I create blade from it. I generally forge most everything, but you have to change your mindset to more of a stock removal mentality, but add in thinking in 3D..... Most San-Mai blades simply look bland if folks are successful at forging them to shape. It's simply the nature of the materials. Probably the most sought after feature of this type material is the decarb line that appears...... this is generally a bright "crystal" looking line of almost total decarb, with a darker line above it (when the bevels are ground/etched). This is where you have to think in 3D to understand it..... again, the "crystal" looking line is almost total decarb, while the darker line is where the 416 has absorbed the carbon. Overall its a very cool effect, that doesn't effect the blades edge, because you've ground through the decarb by the time you have finished bevels. Thats also the reason why its generally a better practice to stock remove these types of billets versus forging them to shape. There are also lots of "variables" the Maker can add in..... cut "ladders" or otherwise deform the billet a bit, and you can change the configuration of the decarb line in the finished blade. The more "hot" work you do on one of these billets, the more chances you have for failure, but you can also create some cool effects to...... as I've always said about knifemaking..... you always have to give up something, to get something.
Don't get discouraged if the first effort doesn't pan out. If you've not dealt with it before, there is a learning curve throughout the process. I worked with it for about 6-8 months before I felt like I could make it work the majority of the time, and it was nearly a year of working with it before I was confident enough about my process to use it on a knife that was for sale.
Hope that helps!
