Regarding Work Hardening, with some observations about ductile cast iron anvils

[patrick]

I should probably start a separate thread just on work hardening as that is a topic that does not seem to be well understood. There are at least two kinds of work hardening. The first is based on the idea that a crystal of metal can be deformed resulting in "misalignments" of the atoms  within the crystal. These are called "dislocations" by metallurgists because the structure of the crystal has been dislocated relative to its most stable arrangement. Note that this is not the same as the change in structure we get via the transformation from austenite to martensite. This is simply taking the crystals we have at room temp and deforming them. The dislocations are happening at the atomic scale so it is possible to get a very large number of them into one crystal, but the more we have, the harder it is to get more created. This results in an increase in the actual metal hardness and is what we see as "work hardening". The second method also involves plastic deformation, but some metals will transform to martensite with sufficient plastic deformation. This is what we usually think of when dealing with very high manganese (10% or more) steels. These steels are often used in cast components for railroad and mining applications and especially for hard surfacing welding overlays. It is these alloys that are often recommended for anvil repair. 

The questions of work hardening a ductile iron anvil falls into the first category. for that method of work hardening to be effective you have to be able to plastically deform the anvil face. With out that, there really won't be much in the way of increased hardness. However, ductile iron can be heat treated to fairly high harnesses. if that is done, then not only do you not have to rely on work hardening, but you really can't get much work hardening anyway. for example, if you start with a heat treated hardness of HRc 50, you are unlikely to be able to deform the anvil face sufficiently to get any increase in hardness, especially if you are mostly hitting hot metal.

When we see tools like hammers and anvils fail by spalling or chipping, that is not usually evidence of work hardening but failure by some other mechanism. Personally, I don't like to rely on work hardening as a method of hoping my anvil face will get harder over time. The only way that is really going to be effective is if you take a big sledge and intentionally beat on the anvil face. that will work, but it is very hard to get a smooth uniform finish and that increased hardness region is not very thick so you run the risk of grinding it off. My preference is to heat treat tools to the desired working hardness.

You might be wondering why we don't get work hardening when we are forging. We are in fact getting the same crystal misalignments forming, but the high temperatures used in forging provide enough thermal energy to that those misalignments are "reset" automatically, usually by brand new grains of austenite be nucleated. We don't see that happening during forging because it is very fast. It can happen during forging or during re-heating  so we don't really experience the "work hardening" effect during hot forging.

 

[George N. M.]

Patrick, I am enjoying and learning from your posts.  It's nice to read some real metallurgy.

Do you have an estimate of how far work hardening will penetrate into a piece of steel?  My impression is that is like case hardening, not very deep.

Thanks.

GNM

[Frosty]

Thanks Patrick, that's loud and clear. I think most of our side discussion is a matter of terminology and my lay understanding.

Frosty The Lucky.

[Latticino]

Patrick,

Thanks for the exhaustive response to my initial question.  You have confirmed my suspicions regarding DI anvils.

I think some confusion also comes up when some manufacturers list their anvils as being "cast steel" when they are actually ductile iron.  I'm not saying that ductile iron anvils aren't a valid alternative when properly heat treated, just that they are different from steel.

However, don't you think that the anvil surface deforms a small amount (at the microscopic level) during normal use)?  After a century or so of use with sufficient planishing blows, I suspect there is a bit of work hardening going on.  Certainly there are some dislocations involved in the swayback or mushroomed tops we sometimes see on older anvils.  I was just curious whether the different crystalline matrix microstructure of a DI anvil would behave similarly, but I certainly agree it is a minor factor next to proper heat treatment (and that with proper heat treatment less deformation and consequent work hardening will occur).

[patrick]

George-Depth of work hardening is directly related to how far into the item deformation has penetrated. I did an experiment a year ago where I annealed 3/4 square bars of different steel grades, then forged them at room temp under one of my big power hammers. In the case of a pure iron specimen, I was able to forge it cold into a thin sheet, so in that example the work hardening effect penetrates the full thickness of the final part. But in general, the effect is limited to surfaces unless you have a way to induce deformation through the whole cross section. That usually requires massive rolling mills or other very large metal working equipment. Interestingly, if you are considering something like a knife blade, that could be work hardened through the full cross section along the edge and could be made quite hard without heat treatment. this is true of more than just iron. In fact there is a high likelihood this technique was used to improve the strength and hardness of bronze age weapons and tools.

Latticino-you are absolutely right that DI and steel are two very different classes of material. Both are iron based, but DI and some of the other forms of cast iron have so much carbon that some of it exists as graphite. This is not true of any but a very few steels. Ductile iron, especially when properly heat treated, will have properties comparable to many steels, so in that sense it could be a good substitute, but definitely not the same material and is should not be called steel. Cast Steel is in fact just steel that has been cast to shape instead of being forged. Most of the anvils made today are genuine cast steel. There are some ductile iron anvils on the market. A foundry local to me made one as a trial that I got to use. It was not heat treated and was definitely softer than a properly heat treated steel anvil, but it was a perfectly serviceable tool. Had the alloy been adjusted a bit and it been heat treated I think it would have made a very good anvil.

As for the work hardening in old anvils, yes I'm sure that there is some, especially when you see evidence of mushrooming or sway back, but generally I'd say that unless you use the anvil primarily for cold work, such as in the case of a sawyers anvil, the amount of cold work is really pretty minimal. the hot steel is doing the plastic deformation, not the anvil or hammer. Remember, the anvil face has to be soft enough to yield plastically. Even in the sawyers anvil example, if the face is hard enough, the hammer/work piece will just bounce off without inducing plastic deformation. If that is not happening, you are not really getting the cold work I'm describing. I've looked at lots of metal under the microscope and you can see deformation and its effects on individual grains. dislocations are not seen without the aid of exceptionally high magnification, much higher than you can get with a conventional microscope. Something like a tunneling electron microscope is needed.

An excellent example of work hardening on a common item is rail road rails. Those are soft enough that they do plasticly deform on the surface. Over time, that hardened layer will spall off, leaving a pitted  or flaked surface. Eventually such rails have to be replaced.

[JHCC]

14 hours ago, patrick said:

Most of the anvils made today are genuine cast steel. There are some ductile iron anvils on the market.

Our "List of makers currently producing anvils" thread includes info (as reported by manufacturers and vendors) on what their respective anvils are made from; the only DI anvils on that list (at the time of my writing this, anyway) are JHM, NC Tool, Scott Anvils, and TFS. Interestingly, almost all of those are farrier's anvils, although I don't know if there's any particular reason that would be so.

[Buster Bolster]

this is an interesting conversation - I'm tuned in only due to having an older anvil with a hardened steel top and now a Competitor. There's a lot of things I don't know aside from my daughter taking a whack at the new anvil with a hammer when I wasn't looking and she did put a small crescent in it. I don't get upset about that kind of stuff. 

The soderfors-shaped anvil that I have (unmarked) has not taken mark like that and seems harder. But I have no idea why. 

I do have a hardness tester, and it would be interesting when testing something like this to see if some superficial work on a block of ductile cast could trick the diamond cone. 

No plans to hammer anything that isn't hot, so the Competitor seems hard enough and if it doesn't get harder, that's fine. 

[Frosty]

Your Soderfors IS harder, considerably harder. People who miss blows on mine put marks on their hammer's faces, blunt chisels, etc. Mine will skate a new sharp file so it's running around RC60-62. The downside is anvil faces that hard tend to chip if you miss a blow on an edge and is a good reason to break any sharp edges. My largest radius is about 1/8", the places over the heart that weren't radiused are the edges with chips. 

It still has a few edges that are pretty sharp on the heel but nobody hits the heel of a Soderfors anvil very hard . . . twice. It makes your ears ring through muffs and plugs even on the steel stand.

An old (thankfully) ex-associate was bragging up his 400lb.+ top tier modern anvil at a demo I was working with him when he pointed at my beloved Sorceress as a punky old not real anvil. I couldn't help myself his constant misrepresentation of his work, the quality, etc. etc. tended to get to me so I interrupted.

"Really?" and took a sharp chisel off the table and said show us how hard it is. AND egoed him into hitting his anvil with a hammer and chisel cutting it. Then he all but yelled, "Let's see yours do better!" At which I grinned and flattened the edge of HIS chisel without marking the Sorceress enough to find with a fingernail. It was a Kodak moment, feeding him his own brag publicly. Egoists are easily manipulated but be careful, they never forget the need for revenge. I still have that chisel on my little table, I rounded the edge and use it as my main veiner.

Just remember to be careful of missed blows on the edges, that CRACK you hear as a chip separates isn't the sound of the metal snapping, it is the sound of the chip breaking the sound barrier. Sure that jagged un-aerodynamic bit won't go very far that fast, 2'-3' maybe but look at what's at that level. Even a little hole in a femoral artery is a B A D N E S S  thing. Not to mention the other valuables in the region.

Frosty The Lucky.