This hot or a bit more

Small part like that at least 1/2hr but the bigger the part the longer it needs to soak. Then let it cool as slowly as possible. may still not be as soft as a nice bit of CI bar buy you should at least he able to drill & tap with HSS

I think some foundry's soak castings for up to two days when there's a requirement for easy machining.

I've often seen it suggested to put in an open fire in the evening and leave overnight to cool in teh ashes. I tried this on a flywheel and it came out like a pretzel with a broken rim

Neil

… 'Apprentices at a foundry at Stockton on Tees had not heated the moulds for plough shares' …

Wasn't Shaw's down the forty foot road, or Harpers by any chance ?… Usual disclaimer

George.

Well, I use my garden incinerator (galvanised type with a lid & chimney) to burn off dried wood/tree cuttings. I get a good fire going with a fairly deep bed and drop the casting(s) in gently. I then continue top up the fire for a while before letting it die back and putting the lid on. Left overnight, the next day the fire is out but the ashes are normally still hot. I've treated both wheel and larger iron castings in this way (especially if they look a bit blackened or burnt) – as well as small welded fabrications and anything from my scrap bin I think may need stress relieving.

I've never had any damage to parts (apart from heat discolouration) and it does seems to help with chill spots. The only screw-up was when I tried wrapping a part in kitchen foil (I know – a stupid idea in retrospect!) to try to keep it cleaner. The foil melted and was a s** to get off completely.

I don't know what temperature a wood fire can reach (the bottom off the incinerator certainly sometimes glows red hot) but I don't think it will get hot enough to damage cast iron.

So whilst Neil had a bad result (open fire/uneven heating?) – I haven't so far – and will continue to use this method when it seems necessary.

Regards,

IanT

A bit more reading seems to suggest you need a temp of about 900deg C and to let it soak for at least 1hr per inch so bigger bulkier castings need longer. A quick test for when you have reached the right temp is that the iron will no longer be magnetic so a magnet on a long stick should allow you to test it while in the furnace. having your own furnice is certainly preferable to a fire or BBQ.

If the fresh casting is cooled to quickly it will become "chilled", you mostly see it on thin castings or corners of larger ones as that is where they loose heat the quickest. The chilled area is extreamly hard and will take the edge straight off HSS tools. Flat surfaces are not too bad as you can usually use carbide tooling to mill or turn them but if you need to drill or tap a chilled area you are in for trouble.

It is easy to see the hard areas as they come up bright and shiny rather than dull grey. The end of this part is a good example where the thin rim cooled too quickly

And even easier to see the shany hard end while still in the lathe

They also chill (deliberately) cast concrete surface which comes out harder than a witches wotsit.
Resists all attempts with normal masonry drill-bits – even in a rotary/hammer drill. Only way to fit wall fixings was to borrow a cartridge gun.
DaveD

Sometimes you start to wonder whether something you have been doing is a waste of time. This is the case where you know that you have no (or very little) 'expertise' – but where you do have some 'experience' (and there is a difference between the two states).

Since some of this thread seems to contradict my actual experience – I looked further. This paper gives more detail about cast irons and their properties: http://www.heat-treat-doctor.com/documents/CastIrons.pdf

"The most common heat treatments applied to gray cast irons are stress relief because of non-uniform cooling of castings and annealing to improve machinability. Subcritical heating is used for both. Stress relief is done at temperatures between 1020 and 1200˚F (550 and 650˚C) without significantly lowering strength and hardness. Heating at temperatures between 1290 and 1400˚F (700 and 760˚C) lowers the hardness for improved machinability."

"Ferritizing (for the most ductile microstructure) is done by austenitizing at 1650˚F (900˚C), followed by holding at 1290˚F (700˚C) to completely transform austenite to ferrite and graphite."

Elsewhere I discovered that dried oak can burn at up to 1600F.

Well, I'm mostly burning dried apple wood (my trees grow faster than I can cut them back) and the incinerator has vent holes around the base and there is usually a bit of a wind blowing here. So this is not like using a (slow burning) wood stove – I put a lot of wood into my incinerator during an evening and it burns fast and gets really hot – I don't know how hot – but I have no doubt that it gets things easily up to the red heat shown in the photo – and holds it there.

So – I whilst I don't know if I'm getting up to 1650F/900C – I do think I'm getting them hot enough to effect some change – at least that is my experience. I'll leave it to others to worry about whether I'm only just 'annealing' and not actually 'ferritizing' – since it seems to work for me in practice.

Regards,

IanT