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Hard to machine CI

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John Rudd:
I just received a replacement casting for a steam engine I’m building….
Trouble is the casting is so hard to machine it beggars belief even with carbide inserts..

Can I heat treat it to make it softer ( anneal? ) or bin it and get another ( more £££££’s)

John You can try heat treating it. I was successful with a hard cast water pipe cap that I wanted to use as a cylinder base in the pipe and nail engine ModUp several years ago. (,10261.msg117030.html#msg117030 )

Bring it to a dark cherry heat, and hold there for say an hour. Then bury it in wood ashes, which will insulate it and allow it to cool over 24 hours. The slow cooling is the trick that prevents the hardness from reappearing

My casting was small so I accomplished all of the above by putting it in the glowing embers of my wood stove.

John Rudd:
I have a gas torch that I use for silversoldering, also some 12 fire bricks. I can put the casting in the fb and heat, then just let cool?

Or I can fire up the bbq…..🤣 👨‍🚒👨‍🚒

Needs to cool slowly hence Steve's suggestion of leaving to cool in wood ash. Even better - leave it in the log burner overnight and rescue in the morning when you rake out.

Yup, it's the slow cooling that's important. Wood ashes are very insulative (I use a bucket full of them) and can take high temperatures (red hot), unlike most other insulation. In a wood stove you can sometimes find glowing embers buried in wood ash two days after a fire is out. Be sure not to disturb your casting as it cools for a day.

Hardening: heat to cherry red and cool rapidl (plunge)
Softening/anealing: heat to cherry red and cool very slowly


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