Cast Iron grades

Would SG (ductile) cast iron be better for steam engine cylinder blocks or just standard grey stuff? It's for a borderer engine build.

Regards

Roger

My limited experience of turning SG iron is that it doesn't readily give quite the finish of the ordinary grey stuff. It's enhanced properties aren't required either, so I' stick with grey.

I'd agree with Clive. You don't need the properties of SG iron and in my experience it has a great propensity to chatter, whereas ordinary continuously cast iron machines very nicely, if rather muckily.

Andrew

The only advantage of using SG iron would be that the column mounting lugs would be a bit more durable if the block was dropped but it would take for than that more me to feel the need to go for it. 250 grade grey cast iron will be nicer to work and easier to get hold of.

A lot of the uS casting kits use what they term "ductile iron" I'm not sure of it's exact spec but it seems to fall between our 250 and SG iron castings that I have machined being less steel like than SG and working like 250 yet it is less prone to breaking on long slender castings, smells a bit more too!

Edited By JasonB on 20/08/2020 17:12:14

Jason makes a good point about long slender castings. It's horses for courses where SG was invented to provide more strength like steel instead of a forging but cheap like a casting. Whereas good 'ol grey iron has the graphite flakes for lubrication in service as a cylinder. Also from the composition I would expect SG to rust more inside the cylinder though I don't have experimental evidence for that.

Thanks guys, thought I'd ask the question, now I know . The plans call up for gunmetal but with 2 engines to build I'd need a mortgage so going with alternative metals and not castings.