Thanks for all the comments guys, I'm really learning a lot here.
Clarifications:
-- I have an old Century arc welder that lists itself as good for 100% duty cycle up to 80A and 60% up to 150A, so I expect I'll be fine running some heavy/deep passes. I was planning on using 7018 rod for most of the welding, though I was debating 6010 for the root pass and 7018 for cover passes like the structural welding instructors at the community college taught people to do.
-- I also have a great big oxy-acetylene setup with a rosebud tip, so I was planning on doing quite a bit of preheating to get the whole chunk up to 500 degrees or so before starting with the actual welding. All of the welding books I've seen recommend this for large pieces of metal to prevent cracking further away from the weld where the temperature gradient in the material is high. The confusing part is that all of those books then say something like "if appropriate for your specific alloy/weld design/etc." which I can't say is true or false with any real confidence...
-- I was planning on buying something like a 1" thick plate of tool steel for the striking face. I was leaning towards S7, since it's an impact resistant tool steel, rather than a straight hardness one, but I'd be willing to hunt for something else if it will be easier to weld.
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@Cadillac : I like your point. I could setup the bandsaw to trim the end of the large block to a point and round it from there with a grinder rather than trying to get a horn shape out of a round, since that'll be a *lot* of work. That also means that I can pretty directly replicate the "normal anvil shape" and leave the low carbon steel for the horn and just have the tool steel top on the striking face and edges, as
@RJSakowski pointed out.
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@ericc : That's exactly what I'm concerned about. I was planning on one giant solid block for the meat of the anvil with a weld connecting it to a second fairly large solid block that makes up the entire waist of the anvil. I think you're recommending against that design, but I'm not sure exactly how else to do it. Would you cut the waist block into thirds or quarters vertically and weld them on to the meat separately so as to get maximum weld connection/penetration at that joint? Or do you suggest something else?
Thanks again guys!
Will