The cast iron you got in the kit is quite likely a piece of continuous-cast normalized and annealed cast iron barstock. That stuff almost guaranteed to have better machining qualities than the c.i. that Westbury had to deal with 75 years ago or whatever it was. It shouldn't have any internal stresses or hard skin to speak of.
You may want to consider making "small pattern" nuts. I don't know if the metric system has an equivalent, but here in the U.S. there are "large pattern" and "small pattern" nuts. You'll need to translate this concept into metric if you decide to pay attention to it, but that shouldn't be difficult.
Large pattern nuts are the type commonly available for machine screws, so if you scale a 1" diameter bolt down to 1/8" (0.125") diameter (which happens to be #5 machine screw), the commonly available nuts for #5 machine screws won't be correctly scaled to correspond.
The nuts you typically buy for #5 machine screws are large pattern and 5/16" (0.3125") across the flats. A standard nut for a 1" diameter bolt, however, is "small pattern" and is 1.5" a/f. Scale that down to 1/8 and the scale nut should be small pattern and only 3/16" (0.1875") a/f. Of course you could be modeling a "heavy pattern" nut for a 1" bolt, which is 1 5/8" (1.625" a/f), in which case a 1/8 scale nut wold be 13/64" (0.203125") a.f....
When I made the bolts and nuts for Kiwi, I guessed that Kiwi was about 1/4" scale of a comparable prototype, so with the aid of Machinery's Handbook and some guesses I figured out the scale bolt and nut head sizes for Kiwi's bolts and nuts and made them based on that.
As a side issue, if the 1" diameter bolt one is scaling down is 8 threads per inch, at 1/8 scale the bolt ought to be 64 tpi, and a #5 machine screw is only 40 tpi. The thread pitch, however, is not anywhere near as noticeable as the nut proportions when making it look "right." Using fine instead of coarse threads helps.
