Making Anvils

You are thinking of the shelf at he base of the horn. On american and london pattern anvils this shelf is below the level of the main face by an inch of so.
A lot of people thought that itr was left unhardened so it could be used for chiseling. WRONG WRONG WRONG. It was often unhardened because the anvil only had a tool steel plate forge-welded on for the main face, so there was no way to harden the shelf or horn. Most wrought iron, and cast iron anvils were made this way. The only hardened area was the main face.
The reason for the shelf was for hammering on the inside of corner bends.
German pattern anvils don't have this lower shelf, since the horn is the same height as the main face. Instead they have a chunky block of steel against the side of the base of the anvil. Once again so you can hammer the inside of right angle bends.
You NEVER chisel into an anvil face or horn or shelf. Always use a sacrificial steel plate for his. A section of steel channel that just fits over the face of your anvil works very well for this.
Actually it is the duty cycle you want not the raw amperage. A 300 amp machine can handle 200 amps all day long. So you can use 5/32" all day or maybe even 3/16" hard face rods for a shorter period, but a 200 amp machine will limit you down to 1/8" rods for extended use and 5/32" for short runs.
With MIG machines the smallest hardface wire you can get is 0.045" and it reguires 190 - 220 amps, so a 250 amp machine is your smallest option.
The larger the wire or rod you can use the faster the job will go.

Very helpful, as always. A couple of more quick questions: What is the actual amperage that you would use for a 5/32" hardfacing rod? And do hardfacing rods always require DC, or are any available for use with AC?
I don't have any access to 4" plate at the moment ... but just in case I stumble across something ... :)

1/8" rod runs between 105 and 130 amps 5/32" rod runs around 150 and 180 amps 3/16" is up around 220 to 250 amps.
Hardfacing rod will come with a chart on the box stating the proper amperage and how thick a layer can be put down at once.
Often it is limited to 3/16" max in one layer. Then you have to let it cool down a bit. Each successive layer will be hard since there will be less diffusion of the base metal.
Manganese hardface materiels require a lot of hammering to reach full hardness so it is best to grind them roughly then beat the crap out of it with some big hammers you don't care about, and then do the finish grinding.

Something I use for an anvil (and that is once in a blue moon) is I have a chunk of railroad track about 18 or so inches long. I beat the hell out of that once in a while to make a small bracket etc.
Jim

Railroad track is a manganese steel so it gets harder the more it gets used.

Only the modern stuff. I've got newspaper cuttings from the '20s describing the new "manganese steel crossing" being installed at Newcastle Upon Tyne central station. This is a major railway route from London to Scotland. Owing to the station's awkward position relative to the river bridges, the crossing was a massive construction where several tracks crossed across each other. Under the heavy traffic of this route (including the Flying Scotsman) wear on the crossing noses was a major problem and replacement was difficult and disruptive.
The solution was a new steel, developed by the battleship armour makers of the nearby Armstrong works. Two crossings were built together, allowing for future replacement, but it lasted for many years and many times longer than the original steels.
I also have some very old "bridge" rail section that's so old (19th century mine tunnels) it's wrought iron. Some of the earliest "fish belly" rail was even made of cast iron.