I lied to myself when I said that I will continue even though the blade might not be hardened properly. I just could not do it. Even though the knife will in the end be just ornamental thing, a wall hanger that will never be used, I could not bring myself to making a knife with improperly hardened blade. So I had another go at it. Fail again, the steel was still soft and it was not only surface – I tried to break of the tip and it bent and straightened as if it were copper. So I had another go at it, using water as quenchant – a big risk because in high carbon steel this can lead to the blade cracking or even exploding into bits. Fail again, it remained soft.
At this point I had to reconsider. I was convinced that I did everything correctly, testing with magnet for the austenite transformation etc. and I am already relatively good at assessing the temperature by the glow color, so I did not think the failed quench was due to wrong temperature. The quenchant also could not be the problem, since I hardened three blades in it without problems and one at the same time as this one failed.
So I surmised the problem lies in the steel. Perhaps it was surface hardened file and when I did the test with ferric chloride I had all the hardened steel ground off already, so it could not show in color. Or perhaps I burned off the carbon due to the not-so-well functioning protective coating. I consider the first option to be more likely. Anyway, I could throw this blade away and start anew, or I could do what I mentioned before – surface harden it.
Theoretically this makes for a very good dagger, because under the hard and brittle surface remains soft and tough steel, which means the dagger would not break easily when hitting something hard – like an armor. But it is a long process that burns through a lot of charcoal with results unsure. And the layer might be too thin and get ground off during polishing.
I succumbed, knowingly, to the sunk cost fallacy and decided to go for it in an attempt to save the blade. This is what I have done:
First I have cleaned the whole blade thoroughly with angle grinder and a twisted knot wire brush wheel. After that I also scrubbed the whole blade with abrasive pad (similar to Scotch-Brite, only different manufacturer). Whilst doing this I noticed that the blade got slightly blotchy and pitted, so there was definitively more material burned off than I am happy with. Inevitable after three failed quenches. Theoretically quench could be attempted infinite amount of times – but practically the blade would burn away pretty soon.
Secondly I took my limited supply of stainless steel foil and wrapped it around the blade forming a little trough. Not wanting to perform more than one experiment in one project I did not experiment with the hardening material and I used powdered charcoal. Any organic material would do, really (sugar works extremely well I might add, and in the future I intend to experiment with bone dust and various mixtures from easy to get chemicals), but they give blotchy, coloured surfaces, and with charcoal I had a best shot to get evenly hardened and evenly coloured surface.
After filling the trough about halfway with the powder I crimped very tightly the edges, folding them twice and pressing the folds together in a vice. It is important to get the package as air tight as possible. But the tang was not packed in, because that was supposed to remain soft. Thus prepared package was now ready for heating. For this I have used my improvised setup build from fireclay bricks, but without forced air supply (a fancy way of saying I left the vacuum cleaner in the workshop). I filled the fireplace with charcoal, buried the package in it, lit it and left it to its own devices over night. Under an impromptu cover because it was raining.
The next day the package was burned through near the tip, a bad sign. But the charcoal dust was still all in there and it did cling to the blade very nicely – I had to scrape it off. A good sign. But I decided to repeat the process once more just to be sure.
Today, after I returned from work, the weather was again good so I could have another shot at quenching this cursed thing.
And it is a success. The blade is hard as glass on the surface and there are no cracks that I see after cleaning it with wire brush. I hope no hair thin cracks shows later on.
Tomorrow the blade goes into the baking oven for heat treatment 150°C half an hour. Maybe two courses.