Weird issue with cast iron

[Cogsy]

I was turning a 1" lump of cast iron bar today, leftover from a bar that I previously turned the liner for my Rupnow engine. It turned fine last time I used it, and the O.D. turned fine but left some strange markings on the surface, kind of a spiral pattern, maybe 1/2" wide for the length of the cut. I did get a photo but it's on my phone and getting it to the computer is difficult. It looks duller than the surrounding metal but the surface finish feels the same.

Anyway, I drilled and bored the piece with no problems and I was thinking how nice cast iron is to machine, then I started on the cooling fins.

The first 3 or 4 were no problem but then I started to get a lot of noise and I had to back out of the cut. The HSS parting blade was worn down on both sides and was smoking hot. Instead of cutting a groove it was cutting a V. I resharpened it and tried again and it finished the cut, but it was noisy. Next groove it did the same thing. Resharpened, tried again and it just kept happening. I changed parting blades and it just kept happening.

I have read about cast iron having a hard skin but I was far deeped into the bar than that. Did I find some sort of hard spot? When I was ready to part the piece off in a different spot, I ran into the same issue yet I manually hacksawed it in the same spot and it was easy.

Any ideas?

 

[purpleknif]

If you heat cast iron it can glaze and then you gots trouble. I've been doing this for 30 years and I find that cast iron does cut nice but you can't whang (that's a highly technical shop rat term) on it.
Hard spots as well as sand pockets are common Problems too.

 

[Cogsy]

So I may have dulled off my parting blade which then heated the cast iron and made my own hard spot. Sounds logical and I do have a history of making things difficult for myself. Thanks.

 

[bazmak]

Low carbon steel is below 0.2% carbon.High carbon steel is around 0.5%
so can be hardened and tempered with heat.Cast iron is about 5.0% carbon
and forms a hard skin/spots where carbon content content is lower(ie nearer
0.5%) and hardens easily.Cast iron can be annealed by heating/soaking and cooling slowly.In some circumstances it can be as hard as carbide,unmachinable
Your not doing anything wrong just unlucky

 

[Cogsy]

I had to part some of my new 1144 stressproof today, and I ran into the exact same problem. Obviously it couldn't be the material at fault.

Anyway, after a bit of investigation it turned out to be a tool problem - the big one operating the lathe that is. I'm not used to this lathe yet and it doesn't have variable speed. Turns out I misread/mucked up the belt and pulley combination and I had it turning almost twice as fast as I thought it was . Changed to the correct speed and now my parting blade works again. Oops

 

[gmac]

.... it's good to know I'm not alone in these "learning experiences" .

Cheers
Garry