Last time I was working on this project, I had some very bad results from quench. This week I have finally managed to test one idea of correcting the problem and maybe prevent it from ever happening again in the future. And I am glad to say that it did work. Not perfectly, but the new process is definitively worth to use instead of the old one.
Here is first the comparison of the three worst blades before and after. As you can see, there are still some curls in there, but they are noticeably less pronounced and one blade is almost completely straight. They will still come smaller than intended out of the polishing process, I will still have to remove some material from the edge until I get to the straight part, but I estimate it to be about 1/2-1/3 of what it was before. On the worst blade, the curls went about 10-15 mm from the edge towards the spine, whilst now it is about 3-5 mm. That is a significant improvement, and I think that had the blades been quenched from a straight form, they would never have curled in the first place.
As I alluded to previously, the process that I wanted to use for correcting the blades is called plate-quench. It cannot be used for simple carbon steels. Only so-called deep hardening steels can be thus quenched, and N690 is such steel, according to some articles I found on the internet. Nevertheless, it is better to not have the internet at all than to believe everything you can read on it – the manufacturer recommends oil quenching.
So I have tested the process first on one blade that I accidentally broke when correcting an ever so slight banana-bend. When the broken blade hardened properly – which I have confirmed not only by scratching with my gauges, but also by breaking off a tiny piece of it – I went on with the curly ones. On one of these, I confirmed the hardening too by breaking off a tiny piece of the tip, with the remaining two I was satisfied with the scratch test only.
For the plate-quench are used two flat plates from either alluminium or copper. These two metals have very high heat conductivity and thus can cool down some steels fast enough for them to turn into martensite. Luckily I got quite a few nice slabs of alluminium on hand. And because I wanted to make the process a bit faster (despite not making time-measurements this time), I have made a simple prototype quench-jig.
It consists of two identical pieces of alluminium with a small hinge, and locking pliers. The hot blade went out of the forge between the plates with the edge towards the hinge. Then it was firmly clamped by the pliers to hold it straight. When it stopped glowing near the tang – indicating a temperature well bellow 600 °C – I dunked the whole thing in a bucket of cold water just to be sure. And just as last time, because it costs nothing, I have put the blades into a freezer straightway for a few hours before tempering them. None of the three blades cracked.
It worked reasonably well and quick. I will definitively improve it and build a proper jig when the weather is nicer and I do not freeze my nuts off in my workshop. I will add a more stable hinge(s) and maybe even screw one of the plates to the pliers.
Another advantage of this process is no burnt oil gunk on the blade, no flames and no stinking oil fumes.