Softening hardened cast iron

calderp · Feb 23, 2021

I have a casting off a drill press that had a setscrew which had been overtightened and cracked off a piece of the casting. I built it up with brazing rod (Harris Stay-Silv 15) and was going to drill and tap for the new setscrew but it appears I didn't allow it enough cooling time, it's now so hard I can't drill it with HSS. I'm sure I could get through it with a harder drill but I'm worried I will break a tap off in there and/or run into cracking issues in the future from embrittlement. Can I fix my mistake by heating it up and allowing it a proper long cool? I could either go old-school with a torch and a bucket of sand, or put it in my heat treat oven. Thoughts? If I go the oven route can anyone advise on temperatures and times? If torch what is the target, a dull red? In that area or the entire casting? Casting is not large, maybe 12"x8" with average thickness about 1/2".

atex57 · Feb 23, 2021

If it is the cast that got hard then put in your oven at 1250F and let cool slowly. If it is an inclusion or a slag pocket then you may have to grind it out and start over.

Ed.

Strostkovy · Feb 23, 2021

I have in the past drilled an oversized hole, brazed it closed, then drilled and tapped that instead of the base metal. Works okay if you can't change the hardness of the material.

calderp · Feb 23, 2021

atex57 said:
If it is the cast that got hard then put in your oven at 1250F and let cool slowly. If it is an inclusion or a slag pocket then you may have to grind it out and start over.

Ed.

Thanks. The heat treat stuff I've been reading seems to say cool at 100 degrees per hour but my oven doesn't have an automatic cycle so I was thinking bring it to temp, hold for an hour, then cool about 150/hour to 600 or 700 and then bury it in sand so I don't have to keep tending it.

Illinoyance · Feb 23, 2021

calderp said:
Thanks. The heat treat stuff I've been reading seems to say cool at 100 degrees per hour but my oven doesn't have an automatic cycle so I was thinking bring it to temp, hold for an hour, then cool about 150/hour to 600 or 700 and then bury it in sand so I don't have to keep tending it.

Just let it cool in the oven.

atex57 · Feb 23, 2021

1250F is below the lower critical temperature so it will not harden and does not have to be ramped down. This temp is really more of a high "tempering" temp to soften it.Cool it in the oven as Illinoyance said.

Ed.

dgfoster · Feb 23, 2021

I think you are going to need a higher tmep than 1250. You can try 1250, but do not be surprised or disappoointed if it has little effect. 1650 to 1750 will anneal it and what was white iron will be soft and uniform grey iron. Problem is the silver braze will melt well before that. You could weld grey iron into the defect and then anneal the part. Lots of work. Maybe you'd be better off to "cast" a new part with your welder and some steel stock. Most likely you'll be done sooner and have a much more satisfactory part if you just fab a steel replacement. If your welds are not pretty, a little bondo can make a strong but not stack of dimes weld look very nice. Paint hides a lot. Maybe not the answer you were hoping for, but a practical approach I think.

Denis

pavt · Feb 23, 2021

I would stick it in the oven at 1100F and let it bake for an hour. Then let it cool off naturally in the oven, overnight. Your solder has a solidus of 1190 and a liquidus of 1480.

Shaybuilder · Feb 23, 2021

I used to have to anneal some small cast parts that I used to sell. The thin sections would cool down to fast and be extremely hard. To anneal I would put into a furnace and take the temperature up to 1750 degrees, hold for 1/2 hour and then do a controlled cool down of 50 degrees every 1/2 hour until the temperature reached 900 degrees and then I just let it cool as fast as it could.

thermite · Feb 24, 2021

calderp said:
I have a casting off a drill press that had a setscrew which had been overtightened and cracked off a piece of the casting. I built it up with brazing rod (Harris Stay-Silv 15) and was going to drill and tap for the new setscrew but it appears I didn't allow it enough cooling time, it's now so hard I can't drill it with HSS. I'm sure I could get through it with a harder drill but I'm worried I will break a tap off in there and/or run into cracking issues in the future from embrittlement. Can I fix my mistake by heating it up and allowing it a proper long cool? I could either go old-school with a torch and a bucket of sand, or put it in my heat treat oven. Thoughts? If I go the oven route can anyone advise on temperatures and times? If torch what is the target, a dull red? In that area or the entire casting? Casting is not large, maybe 12"x8" with average thickness about 1/2".

WHERE on what size of DP, and for what function? So we have some idea how hard it has to work at its job.

"On speculation" knowning nuthin' else, I'd be more concerned as to cracking again than over threads for a setscrew.

fciron · Feb 26, 2021

You'll want to make sure your softening temperature is well below the melting point of that stay-silv.

Harris' website shows a solidus point of 1190°f.

jim rozen · Feb 26, 2021

Some of the hardest stuff I've seen was the small cast-iron pulley housings for the double-hung widows in my 1850s vintage house.
At one point I tried to shorten one up a bit to fit in a different window, the stuff is harder than billy goat dicks.

atomarc · Feb 26, 2021

jim rozen said:
the stuff is harder than billy goat dicks.

Would you care to share with us how exactly you know this fact?

Stuart

eKretz · Feb 27, 2021

atomarc said:
Would you care to share with us how exactly you know this fact?

Stuart

Totally opened yourself up to a "Yer momma told me" on that one.

rogertoolmaker · Feb 27, 2021

Slow Cooling

calderp said:
Thanks. The heat treat stuff I've been reading seems to say cool at 100 degrees per hour but my oven doesn't have an automatic cycle so I was thinking bring it to temp, hold for an hour, then cool about 150/hour to 600 or 700 and then bury it in sand so I don't have to keep tending it.

Get a sack of lime and bury your part in the lime to cool. It will be even slower than sand.
Roger

dgfoster · Feb 27, 2021

You could bury it in something even more insulating than sand, if desired. But, it may be useful to remember that the part was originally cast in sand with 4% moisture added. I remove my cast parts from their green sand molds 4 hours or so after casting and measure the sand temp at that time. It is usually down to around 200 degrees. Given that I cast at 2580F it has cooled far faster than 100 per hour! And the grey iron parts are soft and machine (and scrape) beautifully. Heating them to an inadequate temp (1200 is too low) with very slow colling will do nothing to soften them. Heating to 1750 with cooling at 300 per hour will anneal them to buttery softness.

Denis

awander · Feb 27, 2021

thermite said:
WHERE on what size of DP, and for what function? So we have some idea how hard it has to work at its job.

"On speculation" knowning nuthin' else, I'd be more concerned as to cracking again than over threads for a setscrew.

Hey, it looks like someone has hacked Thermite's account.

Obviously, this short post (above) couldn't have been by the real Thermite.....

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