Copied this from another forum:
From a post I made a dozen years ago on the "Blacksmith's Virtual Junkyard". As true today as it was then:
You're forcing me to get into an explanation I've been avoiding. No problem, just have trouble explaining things sometimes. Well, here goes anyway. A lot of the following is pretty basic physics, but you probably never took the time to think through what is really going on in hammering.
The first thing we need to look at is why a hammer bounces off the anvil. We have all had the experience of striking the anvil, either intentionally or unintentionally. What happens? The hammer bounces back like a super ball, right? Why? Steel is a very elastic material. It can be deformed within certain limits (known as the materials elastic limit) and it will return to it's original shape. This is easier to imagine if you picture a coil spring (like a valve spring) mounted on a handle so it looks something like a hand hammer. Now, when you hit the anvil you can visualize how the end that contacts the anvil will stop and the other end will continue a little ways thus compressing the spring. Once all the energy has been absorbed by the spring, it quickly returns to it's original length pushing it away from the anvil.
Now, by stretching your mind a little, you will be able to understand that the EXACT same thing is happening to your hand hammer when it strikes the anvil! The face of your hammer stops when it contacts the anvil and the rest of the hammerhead continues downward for a moment, compressing the hammerhead. Of course it returns to it's original shape real fast, pushing it back up. Very little energy is lost in this process. A little heat where the two meet and a little internal heating of the hammerhead is the only loss.
If you can believe all of that we can move on to the anvil. In the above explanation I only considered what went on in the hammer. There's more. When the hammer strikes the anvil, it also compresses. The two compress inversely by their mass. If the place where you hit the anvil compresses, what happens to the rest of the anvil? It resists moving in proportion to its mass. When the top of the anvil springs back to its original shape down equally. If the anvil were made of a material with a low elastic limit it would permanently deform. Lead has a low elastic limit, but is malleable; thus it would deform and return little energy. Concrete has a low elastic limit, but is not malleable and will crumble.
What happens when we put a hot piece of steel in between? All three are compressed according to their mass and malleability and all three expand according to their elasticity. The hot steel has high malleability and low elasticity. As the piece cools down you can feel it losing malleability and gaining elasticity. Every time you hit a piece of hot iron it receives two blows. The first when you hit the piece and the second when the hammer and anvil return to shape. This all happens so fast as to seem like one blow.
What does all this have to do with your question? Be patient! I'm getting to that. Relating all this to a power hammer we can now understand a little more about what's going on. When a power hammer hits the anvil all of the same things happen. The ram of a power hammer must be elastic just like the hand hammer to get the best effect. If you have ever used a lead filled hand hammer you can understand why a lead filled power hammer ram is a bad idea. Concrete filled power hammer anvils are another BAD idea. Somebody ought to make a concrete filled hand hammer anvil to show these people just how bad the idea is.
Is concrete UNDER the anvil worthwhile? A qualified, yes! If the anvil has sufficient mass and a larger enough contact surface to prevent damage to the concrete. The top part of the anvil can compress while the bottom and the concrete resist. As concrete has a low elasticity it is necessary to have a large surface of contact. Putting elastic materials like rubber or wood in the interface help to make a better joint but rebound too slowly to help with the forging. Concrete blocks are most effective in isolating the shock from the surroundings.
Hope I didn't bore you guys too much, I skipped over some, might add to it later.
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