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auscab
24th Oct 2019, 11:28 PM
If I get a pattern made and a foundry to cast off that in cast iron . I’m told or have read that the cast iron chills or is a lot harder on the outside skin and this has an effect on milling the machined surfaces . So it’s cast thicker where it’s to be milled so your machining through the hard stuff into softer stuff .

Question . If you get a cast part and heat it right back up red hot then anneal the whole thing by burying it and cooling very slowly . Does that get rid of the hard skin ?
Allowing easier machining ?

Rob

BobL
25th Oct 2019, 10:13 AM
My (limited) understanding of doing this is as follows.

The final result depends on the Type of CI and just how slowly the CI is cooled.
These two factors will determine not just the final hardness of "skin" but are likely to also the structure of the whole piece.

So the first thing you'd need to work out is what type of CI you have.

Whatever it is, annealing of a complex material like CI normally requires not just a simple HEAT-COOL process but HOLDING the CI at a specific Temps for a given period (typically 900ºC for ~1 hour per inch of thickness) and controlled cooling (ie reduce by 40ºC per hour) in the furnace to about 300ºC. The annealing cycle may also be repeated 2-3 times to ensure even annealing

This is a good website about heat treatment of CI - a bit technical but it contains a lot of good info;
https://www.industrialheating.com/articles/94644-heat-treatment-of-cast-irons

This graph from that site shows some typical heat treatment process curves - the "normalise" curve is usually done prior to hardening to improve the hardening process.
383024

All that aside, if you have a spare piece of CI of the same type then you have nothing to lose by trying what you suggest although I would look to holding the CI at the 900ºC mark for some time before cooling and you would need to cool far more slowly in the running furnace.

After machining you will presumably have to re-harden the CI - This requires a slow heating ramp time to remove stresses and similar high temp hold times as annealing but temps about 30º higher and of course a much faster cool or quench in something like oil. .

kwijibo99
25th Oct 2019, 10:30 AM
G'day Rob,
If your casting is done properly you should not have to worry about chill zones. Even thin sections will be fine if the mould is allowed to cool properly. Chill zones only occur if the casting is cooled quickly, usually by dumping it in water to cool it.
The other cause of hard zones in castings is impurities in the metal, again this should not be a problem if you are going to a reputable foundry.

If you've had a casting done and found it to have chill areas that are difficult to machine then I'd go back to the foundry and discuss it with them, again if they're reputable, they will probably re-cast it for you.

When I've had stuff cast, the foundry bloke has had a good look at my patterns and highlighted any issues before using them or even quoting the job. I would highly recommend calling in to the foundry you plan to use and having a chat with them, I've generally found them more than willing to help out.
Cheers,
Greg.

auscab
25th Oct 2019, 11:42 AM
Thanks for the great reply’s Bob and Greg .
Rob

auscab
25th Oct 2019, 11:55 AM
Just thinking more about this . You do see cast iron wood plane bodies from Norris that are stamped Annealed on the front of the sole I think ? I’d never thought about it to much . They were probably stamping it to say “ This is not a brittle casting “ My guess ?
I wonder Could it have been that and possibly the need for them to help machining the sole ?

BobL
25th Oct 2019, 03:58 PM
Just thinking more about this . You do see cast iron wood plane bodies from Norris that are stamped Annealed on the front of the sole I think ? I’d never thought about it to much . They were probably stamping it to say “ This is not a brittle casting “ My guess ?
I wonder Could it have been that and possibly the need for them to help machining the sole ?

More likely to stop them shattering when the blade hits a hard knot or similar.
A hardened sole wouldn't bother a major tool maker much as they would be able to employ all manner of machinery to machine or grind it flat.
It does help a user that they are not rock hard because even hardened soles may need eventually correction.

labr@
26th Oct 2019, 07:41 PM
For the original question I'd go with what Greg said.

With regard to annealing, the casting will still be brittle compared to steel - grey cast iron is an inherently brittle material due to the graphite flakes. A CI plane is unlikely to shatter in use, even with knots in hardwood, but will certainly be in danger if dropped on a concrete floor.

If you want something less brittle ask the foundry if they do ductile iron (spheroidal graphite). This has some of the advantages of grey iron but with ductiliy more like steel so is much less brittle. Veritas use ductile for at least some of their planes.