I took this off another site. He explains the basics for laymen like me. Ron



Quote from: camman3
Also available from India is a "heavy duty" version which is described as "special grade iron containing higher alloying elements known as SG iron"
Graham

I dunno about the "higher alloying elements", Graham - I rather think that is just flannel written by a salesman trying to understand what was written on a techie's spec sheet. The "SG" is different, and stands for "spheroidal graphite". Another name for this is 'nodular cast iron' - and this is a very different kettle of fish from standard cast iron. Normal cast iron is a mix of iron and graphite, and has a crystalline structure - a bit like a sugar lump. It always contains some free carbon in the form of graphite, which is scattered through the crystal structure in a random fashion, and each particle of graphite is shaped a bit like a cornflake.

(I 'ope you lot at the back are taking notes - there'll be a test on this next week! )

This graphite is what gives your cast iron cylinder barrel resistance to wear and tear, even at the top of the cylinder, where there isn't a lot of oil available - it's the steam age equivalent of PTFE! The downside of those little flakes of graphite is that each one is a perfect starting point for a hairline crack; put the cast iron under tension, bending, shear or torsion stresses, and the cracks will start to form at the edges of the graphite flakes, and spread until the casting fails.

However, some time back (pre WWII?) some clever clogs found that, if you got an iron casting and gave it the right sort of heat treatment (cooking it at a certain temperature for a certain length of time, then cooling it at an appropriate rate), without melting the casting, you could get the graphite to change from little cornflake shapes into little round balls of graphite (hence spheroidal graphite cast iron) - and nice round holes in a component do not lead to stress fractures.

This SG cast iron was (I think) initially used by Fords in the 30s / 40s to make crankshafts for their cars, and, in the 50s by AMC for their big twins crankshafts. These lasted very well indeed, and were far cheaper to make than a forged steel crankshaft - though I still think that a cast steel component would be stronger than SG cast iron of the same size. All the same, if SG cast iron could stand up to the thrashing that the 1950s Fords got (with their 3 speed boxes!), as drivers had to tackle anything fairly steep in second gear , it would probably work as a rear sprocket.

BUT, and it's a big but, that heat treatment process is pretty critical - it has to be done right, and the process matched to the size and shape of the component, or else. You only need a few rogue flakes of graphite left to kick off the sort of damage shown above.

HTH, and best regards,

Jack

email (option): ronpier@talk21.com

I note he says 'it would probably work as a rear sprocket' and refers to damage shown in a previous posting.
Well I would hazard a guess that is broken teeth..
I inadvertantly ran an Indian cast iron rear sprocket on my M20 and it rapidly started to shed teeth..at which point it became obvious it was made of basic cast iron.
Close grained cast iron in the spheroidal form (also known as Ductile cast iron and nodular cast iron) is used to manufacture sprockets for some applications but I wouldn't trust the Indians to make it..and for old motorcycle use I think it is unlikely they would anyway.
Basic Cast iron is used as an engineering material because it is cheap and relatively easy to use but it is unsuitable for applications when high loadings are applied to a component with a narrow cross section...such as a sprocket tooth.
Motorcycle sprockets generally are manufactured from steel in one form or another, either from steel castings or machined from solid stock..(and in the case of early Japanese bikes at least from poor quality stampings)
I only mention this in case anyone interprets the information posted as an 'OK' to buy an Indian manufactured rear sprocket..Personally I wouldn't recommend it.
The more general information about cast iron is interesting though if you are not familiar with the material....Ian

email (option): ian@wright52.plus.com

P.S..It should also be noted that SG/Ductile/Nodular iron is not a specific material type but a classification..The material comes in a range of varying specifications and consequently has a range of potential uses, so what was used for Ford and AMC crankshafts wouldn't necessarily be the same SG iron as that used for other parts. Changes to the heat treatment processes after casting or the addition of alloying elements at the casting stage are both used to alter the materials characteristics..So 'alloying' with other materials is a possibility contary to the comments in the original posting. SG iron was a 'wartime' material, the process being developed at that time. ..Ian

email (option): ian@wright52.plus.com

Very interesting, but just had 5 big lumps cast in cast iron, no idea what sort? luckily for my application it doesnt matter too much, but if not machined yet, would it be possible to heat treat it so it gets stronger??

Parts are over 3 feet long, and 6 x 10" square, to give you an indication, and they are still in the UK.

Thanks for the info,

Lex

email (option): welbike@welbike.net

You are right about the sprocket Ian. It's the rear sprocket (integral drum) on a friends Indian Enfield. It now has about 8 of it's 95 teeth left. It's amazing that the bike kept going. I don't think he has much option that to replace it with an Indian made replacement.

Ron

email (option): ronpier@talk21.com

Lex, nodular cast iron is the result of adding magnesium to the molten iron, and not by heat treatment/cooling down as stated above.
Your castings are probably in normal grey cast iron, increasing the tensile strenght by heat treatment is not possible.

Regards, Peter.

i have found from friend that own modern japanese bikes chain sprocket dont last long either

email (option): roger.beck@node6.com