Softening cast iron weights

[yukon_rose]

I have a couple of 5 LB cast iron weights that seem ideal to make lathe chuck backing plates from. However when I tried to machine one of them I found that it was so hard that even carbide cutters would only remove a little metal before failing.

How do I anneal the weight so I can machine it?

Gary

 

[rwm]

When cast iron is cast it can form areas called "chill." These areas are super hard to machine. I am not aware that it can be corrected without re-melting. Also the addition of ferrosilicon right before pouring is helpful. Most people just re-cast the part. With weights, the manufacturers do not really care about this. Let's see if someone has an answer on annealing.
Robert

 

[pdentrem]

When I was making the backing plates from semi machined, I had the same problem. Switched to HSS and had better success.

 

[MrWhoopee]

Cast iron weights are of inconsistent and unknown composition. Annealing may improve the machinability, but is probably beyond the capability of most home shops. The weight would need to be heated from 1300 - 1750 ° F. and held for a couple of hours, then cooled slowly. Here's a table of of specifics, but not knowing the exact composition of the weight makes it impossible to know which process is applicable.

 

[Winegrower]

I have had good results by tossing hard to machine lumps in the fireplace overnight. Seems like it’s hot enough and the cooling rate slow enough to anneal pretty well.

Plus, it’s really easy to do.

 

[RJSakowski]

Cast iron used for weights has only one desirable property; weight. Any other metallurgical properties are not important. To that end, the castings are usually made in the cheapest possible way and often contain impurities that would not be tolerated in other applications. Typically, they would be white cast iron; high carbon content from the manufacturing process. White cast can be converted to malleable cast through a drawn out process which consists of heating to around 1,000ºC for a day and slowly cooling for another day. Slag is often a constituent in cheap cast iron and it is nit affected by any annealing process.

 

[benmychree]

As I understand it, cast iron weights were made out of all the left over crap at the end of a pour, and did commonly contain slag inclusions. The iron may have been intended for heavy castings, and when poured for light castings, it would chill and harden. For my chuck backing plates, I have made patterns and had them cast at a area foundry, or I have bought them semi finished, that is just rough machined.

 

[addertooth]

MrWhoopee gave a very good answer. Summary: heat until a magnet no longer sticks to it (the Curie point, the metal will likely be cherry red). Then it needs to go through a VERY slow cooling, I am talking 4 to preferably 8 hours where the temperature is dropping no more than 400 F degrees per hour. It is Especially critical to be slow in the 800 to 300 degree range (transition range for Martensite steel).