Why do I have to set my furnace to over 100 deg C more than recommended temp to harden it?

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Flynth

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Hi,

I have a weird issue with hardening in my DIY furnace. Temperatures where checked with two different K probes and a handheld pyrometer. Up until now I always used an oxy-acetylene torch to harden small items so all I knew about the temperature was that it was orange. Now that I have an electric furnace/oven I can set exact temperature and I failed many times using the "by the book" recipe for my steel until I bumped the temp up 100-130 deg C.

My steel is Polish NC6 somewhat similar to O kind of steels in US. Its hardening temperature is supposed to be around 830 deg C (1560 F). All my parts have so far been fairly thin at 6 and 10mm (1/4th and a bit under a half inch). I'm quenching in less than a second after opening the oven's door.

Does anyone have any idea why I may need a much higher temperature to harden this steel? (Before I accuse the seller of selling me something else than NC6).
 
Thanks mmcmdl. I've read "Heat treatment, Selection and Application of Tool Steels" by William E. Bryson and "Tool Steels, 5th Edition" by George Roberts et al. I think I've got a pretty good amount of theory learned and I was attempting to apply it in practice.

The problem is as described... I'm seriously starting to think I got a different steel than expected. There is another old Polish steel NC10. It is similar to NC6 and the US O1, O2 etc, but its hardening temperature is 980-1000 deg C. I hope this is not the case and I'm making some error.
 
Hmm, I just tried quenching in water from 840 deg C (1544F). It came out very well hardened. To the point where good quality file doesn't even touch it. The mystery deepens. Surely if this was high temperature hardening steel there would be no austenite so even water quench wouldn't work. Is this assumption correct?

If so, then either this is some water hardening steel, or something is horribly wrong with my oil :-( I did try used motor oil for the very first try, but I switched to rapeseed oil now (the smell is much more pleasant) I tried oil temps between 45 - 100 deg C (113 to 212 F).

I'm going to try hardening a thin piece next and see if it cracks in water.
 
Where did the steel come from? Sounds like a mix-up in labeling.
Flynth said:
Hmm, I just tried quenching in water from 840 deg C (1544F). It came out very well hardened. To the point where good quality file doesn't even touch it. The mystery deepens. Surely if this was high temperature hardening steel there would be no austenite so even water quench wouldn't work. Is this assumption correct?

If so, then either this is some water hardening steel, or something is horribly wrong with my oil :-( I did try used motor oil for the very first try, but I switched to rapeseed oil now (the smell is much more pleasant) I tried oil temps between 45 - 100 deg C (113 to 212 F).

I'm going to try hardening a thin piece next and see if it cracks in water.
Click to expand...
 
If you quench in still water (i.e. a bucket) be sure you have 5% to 10% rock salt dissolved per gallon to prevent both cracking and soft spots caused by dissolved gases clinging to the metal surface. Also agitation to dislodge bubbles is highly recommended.
 
Spark test? O1 is very distinct from simple carbon steels.
 
Diecutter said:
If you quench in still water (i.e. a bucket) be sure you have 5% to 10% rock salt dissolved per gallon to prevent both cracking and soft spots caused by dissolved gases clinging to the metal surface. Also agitation to dislodge bubbles is highly recommended.
Click to expand...
Missed an item. 5% to 10% per gallon equals 3/4# per gallon.
 
I added salt as mentioned and I re-hardened all my parts. Currently I'm suspecting the oil as the test piece of steel (1 inch cube) turned into austenite almost exactly at 830 deg C as nc6 should. If this steel was mislabelled I would expect it to be another tool steel rather than simple carbon steel. This makes identifying it more challenging. However, based on the temperature of austenitisation I strongly suspect it is indeed as sold. I bought it locally from a guy that obtained it from a business being liquidated. Supposedly the business had only tool steel in its stock at the time so if it was going to be different it would be another tool steel.

Also, before I bought it I spent 3h with an angle grinder cutting off two 100kg pieces, so although my ability to identify metals by sparks is not best I'm very familiar with how it sparks :) I would describe it as very large volume of medium length small(cool) orange/yellow sparks.

I spent few days making parts of it so I also know how it cuts. It is pretty hard. I had to use low cutting speed with hss (15-20m, 45-60sfm). The milling chips were usually pretty short. It felt definitely different than plain carbon steel.

So this brings me back to the oil.

Perhaps I'm expecting too much from oil quench? Up until now I only used water quenching and I got used to the hardness achieved. Perhaps oil hardened steel simply is that much less hard? By the book the difference should be 63 to 61 hrc, but my oil hardening resulted in hardness that seemed much lower than that, and started over a mm(40thou) below the surface. Initially I thought this was decarburisation, but same pieces of steel hardened very well in water even on the surface. I'm not sure what to think of it.
 
Could be O-1 then. It has more orange sparks like mild steel, but with a little more fine bursting. Not like A-2, active with stars, S-7 with moly spear points, or W-1, more bushy.
 
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