I have used what Speedy Metals sells, Schedule 40 iron, and have used it for rings and cylinder liners. When I made the liners for my current project, flathead V-8 engine, I used this material because I had a piece of it left from another job. It machines better than just plain iron, closer to Ductile but not quite the same. When it came time to make rings for my engine I needed to order more material so I did a little investigating and comparing, just for my own knowledge.
First I looked at McMaster's information on both the Grey and Ductile that they sell. I wrote down the specs, carbon, silicon, magnesium etc.
I then did a search for Durabar (trade name). http://www.dura-bar.com/products/index.cfm
I was familiar with the material that they sell listed as 65-45-12 because we had used it where I worked and it was wonderful to work with. Here again I wrote down the specs for comparison sake.
I then went to Speedy Metals and knew that they sold what is listed as Class 40 iron. By the description posted it says that it is a Grey iron but with the inclusion of Silicon it nucleates the the Graphite to change it's properties.
The hard part of actually identifying what Class 40 iron is, at least for me, is because other than the amount of alloying elements the charts get into ASTM numbers and it gets confusing.
Here is what Wikipedia says about Ductile Iron.
"The common defining characteristic of this group of materials is the shape of the graphite
. In ductile irons, graphite is in the form of nodules
rather than flakes as in grey iron
. Whereas sharp graphite flakes create stress concentration points within the metal matrix, rounded nodules inhibit the creation of cracks, thus providing the enhanced ductility that gives the alloy its name.
If you go back to what Speedy describes as nucleating the graphite and what Wikipedia states as graphite in nodular form it seems to have a similarity.
I can tell you this. Machining Ductile iron as supplied by McMaster and Class 40 as sold by Speedy show differences. Not a lot mind you but some. The one thing that is recognizable is when you make piston rings from both materials.
The rings I used in my flathead engine are .030 wide (to fit in a .031 groove) They are .040 thick and have a diameter of .830. Although I have no trouble with breakage while installing them on the pistons the ones made from Ductile iron will start to bend at some point whereas the Grey rings will snap. This falls under the category of Modulas of Elasticity, I believe.
I guess the bottom line is either can be used for liners and rings. Which is better or preferred I can't say. In the model engine world liners and cylinders have been made from 4140 steel to 12L14 steel to Iron but as far as my research and experience rings have always been made from iron.
I hope this more helpful than confusing.