There is a controversy in the world of blacksmithing regarding anvil rebound. The idea is to test an anvil by taking a ball bearing and seeing how high it will return when dropped on the face of the anvil. IIUC, a hardened surface would be more elastic than an unhardened one. That said, since iron heated to forging temps is pretty inelastic compared to the cold hammer and anvil and how hard the anvil surface is is immaterial to how much you can deform the workpiece with a given hammer blow.
It is also noted that orienting an anvil to increase the mass under the hammer blow will improve how the anvil performs. It wouldn’t be correct to say that inertia has anything to do with orientation of the material, would it? Now that I think about it, I would guess that it’s actually stiffness, but I wonder how the material allow and heat treatment affect that. Can someone who went further in physics than I did enlighten me?
You don’t want the surface of the anvil to move. Or rather, some amount of movement is inevitable, but you want to minimize it. If there’s less mass directly under the blow, there’ll be more deformation, and that bit of anvil will move more.
And yes, the heated iron will be softer than any anvil, but smithing is still hard, tiring work, and you don’t want to make it any more tiring than you need to.
Sure, but to what extent does the presence of hot steel between the anvil and the hammer mean that the effect of the hardened surface gets washed out in the noise? You’re right that you don’t want the surface to move, but to what extent does the hardness of the surface matter?
Later today I am going to work on my only anvil, but I often see new smiths thinking that they must have an anvil which scores very well on the rebound test. I counsel them to find a chunk of mild steel and firmly secure it to a heavy base in lieu of making an expensive purchase.
Imagine an anvil so soft that it behaves like sand. The workpiece sinks into the sand when struck and much of the force of the blow’s impact is distributed to other surfaces on the piece to displace sand. As anvil hardness and mounting rigidity increase, they become less sandlike and instead the strikes can be used to work the piece as desired instead of displacing and heating the anvil.
Some anvils are made of sand. A leather bag full of sand for hammering shapes in thin metal. There’s aren’t any varieties between loose filled bags and hard anvils that I know of. There are also ASOs (Anvil Shaped Objects) which are hard anvils often made from a piece of railroad rail or an old engine block, and also the ones shaped like anvils but considered more suitable as a boat anchor than an anvil.
As the material and inertia part of the snip above …
Imagine you’ve got a wooden workbench with some amount of give to it. Now place an anvil only 1" tall on top of the bench and give it a good whack. Consider how little the inertia of the teeny anvil slows the progress of the impact forces from the hammer to the wood.
Now replace the anvil with one that’s 3 feet tall. Same footprint as the 1" anvil, just lots taller. And sitting in the same spot on the same wooden bench. So it weighs 36x as much in total, and also 36x as much per unit of top surface area.
Give it exactly the same whack. How do you think the shape of the force over time curve will differ between the thick and thin anvil? The thick anvil will be much more resistant to being moved. With the effect that the peak force applied to the surface will be much greater.
Now consider a workpiece between the hammer and anvil. If you hit metal less hard than its yield strength it doesn’t bend, it just springs back once the force is removed. To actually reshape the material, you must hit it harder than its yield strength. A high inertia backstop (AKA an anvil) does exactly that. By being highly resistant to being moved it increases the peak force delivered to the workpiece during the impact.
Back in the day when hardening big hunks of metal was difficult, one way to get a better more efficient anvil was simply to make them heavier. Very low tech, but effective. Nowadays hardening an anvil can add a bit more of a spike to the force / time curve. But maybe not enough to matter if you’ve got room to just use plain old inertia.
I don’t doubt that there are specialty tools like sandbags that some might call anvils but OP is referring to steel tools used in forging and ball bearing drop tests.
I saw a random youtube video the other day that should provide some insight; in it, a vintage Swedish anvil is re-surfaced, and a ball bearing bounce test is performed before and after to see what difference is makes. Some salient points:
The anvil is cast, and doesn’t have a hardened surface.
The milling cutter that does the facing is a tungsten carbide type that doesn’t noticeably heat the worked piece (most of the heat is transferred to the swarf chips), so the machining operation won’t harden the surface.
The bounce test before machining showed a lot of variability with respect to location. After machining, the coefficient of restitution was significantly greater, and much more consistent with respect to location.
Machining a lovely flat surface certainly improved bouncability, but I couldn’t say for certain why. Possibly the tiny dimples in the original battered surface reduced the CoR, or it could be the thin coating of iron oxide having an effect (the original surface was nearly black, while the freshly-milled surface was shiny metal-coloured). I reckon it was mostly the tiny dimples reducing the bounce.
Have at it: https://youtu.be/s3O2hwLcVUE … with the caveat that the youtube Algorithm now thinks I’ve got some sort of anvil fetish and wants nothing more than to send me down that particular rabbit hole, so watch out for that.
The OP asks if the ductility of the anvil matters compared to it’s lack in heated metal to be shaped. You brought up sand, it does show the extreme end of the scale in anvils, but I also mentioned that other objects may be used as anvils and they will have different characteristics.