When I was hardening the blades two days ago, I have tossed in there six cut-offs as well and I have used different methods to quench them. After that I had still one piece left so I have heated that up to the 1050 °C and let it cool in the forge. Then I performed some tests and the results are very interesting.

These are the samples and methods used (marked by the number of notches on the edge):

0 – left to cool in the forge
1 – untreated
2 – left to cool in the air
3 – quenched between Al plates only
4 – oil
5 – water
6 – AL plates + water + Freezer fro 2 hours

None of the samples were subsequently tempered, so they should be at peak hardness.

With the sample size just 1 piece per method I cannot of course perform too many tests. The idea was to polish the samples – which I did. And already during the polishing, I have noticed that all samples seem to be hardened, except #1.

The next step was supposed to be to etch the surface and look at the structure under a microscope. Well, that did not work at all, and the reasons are a mystery to me. Just as it happened with the knives last time, it happened again here – the electrochemical method worked on some samples perfectly, but completely failed on others. I was unable to solve the problem. Another thing was that my microscope apparently does not have big enough magnification to see a difference between the original steel and the quenched one. I could re-build it and improve it about ten times, but I am still not sure if that would be enough and I do not want to get sidetracked to that now, it would be probably more than one day of work and I have already spent two days having fun instead of working.

So I did what I could with samples of this geometry. First trying to scratch them with my hardness measuring gages .

The sample # 1 could be scratched by the lowest 38 HRC gage, which was to be expected.

All samples except # 1 could not be scratched by the 62 HRC gage, so they are at hardness 62 HRC or more. Which is something I did not expect, especially not of # 0, which was left to cool in the forge – and that took definitively several hours. I was expecting this sample to be harder than the new steel, but not hard enough for a knife – but it evidently is more than enough hard for a knife, hardness 62 is in fact quite excessive.

Secondly, I have tried scratching the samples against each other, and the results became even more interesting. All samples could scratch # 1, as expected. But none could scratch #4 and #4 did scratch all, whilst the remaining five could not scratch each other, so they are all of the same hardness.

Thirdly I have put the sharpest angle of the triangle approx 10 mm into a vise and break it off. #1 has bent easily, as expected, all others have snapped off.

What can I deduce from this? Several things.

1. Sample #4 was hardened with the method recommended by the manufacturer and did come out as the hardest of them all, possibly somewhere around 63-65 HRC, which is as hard as steel can get. It could be a fluke (remember – sample size 1), but it could be the reason why this method of hardening is recommended. It is not surprising.
2. From a practical standpoint, the method of quenching seems to be quite inconsequential nevertheless. The oil quenched sample would be brought down a few points in heat treatment anyway and for practical purposes, anything above 51 HRC will be usable with just a bit more edge maintenance, anything above 55 HRC will have reasonable edge retention and above 57 HRC we are in the realm of no reason to complain whatsoever. In this light, the difference between the recommended oil quench and all the other methods seems to be so small as to be trivial and only interesting from a nerd standpoint.
3. The freezer step does not seem to have done anything for this one piece, but this does not rule it out from use on larger pieces that could not be so thoroughly and consistently heated in my setup. Did not do any harm either.
4. Although tempering was not tested, this experiment does indicate that it is just not possible to really destroy the edge on this steel by overheating it during grinding/polishing since even cooling it from the 1050°C to room temperature over the course of several hours hardened it very nearly as well as the recommended oil quenching. I will not test tempering temperatures with regard to this specifically since there are graphs to be found on the internet that show already that the hardness of N690 does not get below 56 HRC up to 900°C.
5. If I want to peen the end of the tang, or do any other work with it, I must be careful to not heat it above the critical temperature at any point in the process. Because once heated above certain threshold, this steel hardens, I cannot prevent it and I probably cannot anneal it again.

It would be interesting to see what is the exact influence on toughness/strength once tempered. I could not find it, so I will have to test it myself. But for that, I will need another sample geometry. So maybe next time.

All in all, the N690 seems to be pretty remarkable steel. It does not have the label (and price tag, otherwise I could not use it) of “super steel”, but it is no wimp either and apparently is not very fussy about the heat treatment, apart from the requirement to heat it above 1050°C.