Different Hardening Methods for N690 – Experiment

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.

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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.

Quenching Jig

I hope you are not bored with my writing about knife making yet. It is not that I do not care about other things, but knife making is where most of my focus is right now. And this week I had barely the strength to do anything else at all.

I have started a new batch of blades, 18 pieces altogether – 4 small hunting knives, 4 badger knives, 5 chef knives (prototyping new design), and 5 universal kitchen knives. So this whole week I was drilling, cutting, and grinding steel every day. I have progressed reasonably fast, despite being also slow. Because I simply cannot handle more than 6 hours net a day at the grinder, and I have to make a substantial pause every two hours. My hands are doing reasonably OK and I have been pain-free for a few months by now (despite never having a diagnosis about what was wrong last year), but even so, the vibrations are a strain on the fingers. And after two hours not only the glasses start getting foggy – the mind does so as well and thus the risk of injury increases.

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Today was a big finale of that busy week and all knives went into quench and tempering. This time I have decided to use quenching foil on all blades, despite bad experience on some blades the last time. I have decided to do so because the protective coating that I have concocted did work well but also was a real pain in the ass to remove afterward. And that did cost me a lot of time and I destroyed two belts before I figured it out. But I have tried plate quench with on this steel with moderate success), and I have hoped, really, really hoped, that doing plate quench on newly ground and straight blanks will lead to straight blades without having to scrub off a hard crust.

And following the maxim of Scrooge McDuck “Work smarter, not harder”, I have done my best to make the plate quench easy and reliable – I have built myself a jig. The construction is very simple and it did only take me a few hours.  I have used the locking pliers and aluminum plates from last time, but I have connected it all into one piece that can be easily used in one hand. So instead of one hinge in the middle of the plates, I have added two hinges on the sides. Then I have drilled holes in the fixed jaw of the pliers (and re-ground it a bit) so I can screw one of the plates to it with two M4 screws. The movable jaw is not fixed to the second plate, but it does fit into a groove cut in it and it was also re-ground for a better fit at the angle where most blades will be gripped.

© Charly, all rights reserved. Click for full size

© Charly, all rights reserved. Click for full size

The idea was that I will hold this jig in my left hand, with my right hand I pull a blade out of the forge, insert it between the plates (still in the foil), lock the pliers and dunk the whole thing into a bucket of water. And it worked well! None of the knives came out of the quench with a perm or as a banana-imitation, none have cracked either, at least I did not notice it yet. All but one blade quenched properly (and that one I have re-quenched OK the second time) with hardness above 55 HRC, most even over 62.

© Charly, all rights reserved. Click for full size

© Charly, all rights reserved. Click for full size

Hardening all these blades took me only 15-16 minutes per blade, so even with all that foil wrapping, I was well within a reasonable time. There was only one small problem towards the end – my new burner worked really well and I have reached and held 1.050-1.080°C without problem with four blades in the forge, but after a few hours, it started to struggle. I thought at first that I am running out of propane, but that was not the case. As it turns out, the propane bottle got too cold (just 10°C) after that long continuous decompression, and the gas was evaporating less. I am not sure yet how to solve that problem for the future.

Now the blades are in the second tempering cycle (each cycle two hours at circa 180°C) and they did not seem to develop any bends or curls in the oven either. So far, so good, let us hope it stays that way since I am going to try some new techniques with this project again.

 

Mushroom Hunt Pictures – Roadside Desert

Death and destruction on the roadside. A parched strip of land, baked by the sun in the summer, destroyed by salt runoff in winter. But silverweed endures and sends its creeping stolons across it.

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I did not notice the dead moth until today. But it fits the scenery just fine.

Mushroom Hunt Pictures – A Bumblebee

I find it interesting that not only is the pollen on this flower pink, but that also the bumblebee apparently collected enough of it to have pink pollen sacks on her legs.

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© Charly, all rights reserved. Click for full size.

© Charly, all rights reserved. Click for full size.

Women Artists on YouTube – Violinist

In yesterday’s kerfuffle, I completely forgot this series. So, belatedly, a video from a Spanish violinist.

I have not seen a single whole episode of Star Trek, ever, and I fell absolutely no inclination to do so. I have only ever seen snippets of TNG, so I just about know it is a Sci-fi and I know who Jean-Luc Piccard and Data are.

But this music is beautiful to me all the same.

Project Badgermascus – Part 12 – The Final Badgering

The knife is finished, although not sharpened, and here are the final pictures. I am going to make more knives of this design because I think it is a good knife for forest walks, especially mushroom hunts. And I am naming the design “Badger”.

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The overall length in the sheath is 205 mm. The sheath is from vegetable-tanned leather, carved (badger paw print, frame edge), stamped (leather weave, edge trimmings) and dyed, thoroughly infused with beeswax, oiled with olive oil on the inside and buffed with dubbin outside.

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The sheath has a metal belt clip. This allows the knife to be worn not only on the belt but also clipped onto/into the pocket or boot. The pommel sticks out of the sheath far enough for a comfortable grip with index finger and thumb when pulling the knife out. The knife clicks into the hardened leather firmly enough to not fall out of it with its own weight, but can still be pulled out comfortably.

Stitching is made with a two-needle saddle stitch with artificial sinew. The end knots are melted together and hidden.

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Brass fittings have a black patina with clear brass badger paw-prints over the pins. Scales are from naturally aged cow bone, slightly larger than the tang and wider than the brass fittings. The right scale is adorned with scrimshaw engraving of a badger head – this is a new skill that I had to learn but kept secret since I had no guarantee it will work out. I think it did work out OK. I may add a bit more pigment once this hardens, but maybe not, I like it the way it is. The badger is colored black with coal dust, the patterns, and the frame, with ochre (rust).

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Handle length 100 mm, drop-point style blade 87 mm length, 3 mm thick at the ricasso. Point of balance just behind the first two pins in the handle scales, also between the index finger and middle finger. It feels a bit handle-heavy but nimble. The steel has dark-grey patina from an oak bark extract.

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The left side of the handle is unadorned except a few scratch grids around the pins. The left side of the blade has significantly different steel pattern due to how the san-mai was hammered at the Badger Forge.

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Back of the tang has a row-of-crumpled-leaves fileworks, the belly has a simple triangle pattern.

Overall, I am satisfied with my work here. There are things that I am critical of, there are things that I definitively might have done better – it is not a masterpiece yet, but I think I am getting there. I have learned a lot again and many of the things that took me several days this time should be just a day or even less the next time (the leatherwork for example).

Project Badgermascus – Part 11 – Stitch This!

The day before yesterday I had to make the leatherwork all anew, because I spilled some glue on the forefront of the sheath and it has proven to be irreparable damage, it completely ruined the leather. So I have made a new piece, this being, therefore, the fifth attempt. And when I was at it, I have improved my tools to make a better job.

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I have re-filed the basket-weave stamp to make it crisper and bite more deeply. That has allowed me to better feel the position of the stamp when moving it along the edge and positioning it in the next point on the pattern grid. I have also used a straight edge and I scribbled a faint line for each row before stamping along it.

To get the dye better into the tips of the claw marks I have needed a very fine brush, so I had to make one. And since I do not have any weasel hair, I have used a few strands from a goose feather. It has worked perfectly.

This is my first attempt at a sheath with a metal belt clip. I have first riveted the belt clip to a piece of leather and then glued/stitched that piece of leather onto the sheath. Only then have I glued and stitched the rest of the sheath together. This avoids any contact between the knife and the metal of the clip and secures the clip very firmly in place.

For making the holes I have used 2 mm drill bit in my drill press since I found that to be by far the most convenient method. For stitching, I have used saddle stitch, and I hope I have used it correctly. I had trouble finding a comprehensive tutorial on how it is done.

BTW, holding the work between my knees is really uncomfortable, I will have to make myself a proper tool for this. So many tools that I still need… Anyway, back to the stitching.

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Saddle stitching is made with two needles and here you can see how I do it. First I am pulling the right needle through the hole all the way to the end (top). Then I put the left needle in the hole whilst wrapping the thread from the right needle around it in a loop (middle). When carefully pulling the left needle through the hole (taking care to not stitch through the thread already inside) it creates a knot that gets tightened inside the hole when both threads are pulled to the limit (low). This kind of stitching is very strong and it does not unravel even when the thread is worn/cut/breaks in one spot.

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The almost last step in making the sheath was wet forming. I have put the whole sheath into lukewarm water and let it soak in there as long as bubbles were coming out of it. When it was all soft and pliable, I have wrapped the knife in food wrapping foil, put it in the sheath and formed it around it simply by finger pressure until it had a shape that I wanted. I have used clamps too, but only for a few minutes around the edge, I did not leave in clamps until it was fully dried, it was not necessary. In fact, to let it dry I have simply hung it on a window handle as you can see in the picture.

Today was the final day of this project. I did make some minor mistakes when making the sheath, so I had to glue a little wedge of leather inside with five-minute epoxy, but that is simply a lesson learned for the future and not something to worry about aesthetically or functionally. I must simply remember next time to make the blade portion shorter (and slimmer) and the handle portion longer. But it was working just fine, the knife holds in it strong enough to not fall out, but it can still be pulled out with just two fingers by the pommel. So I have infused it with beeswax and dubbin*, so it is hard almost like plastic, but still with a pleasantly natural and organic feel to it. That made it a lot darker and I applied some black patina too to make it look a bit aged, to fit the knife.

So, after a month of procrastination interspersed with bouts of hectic work and attempts at learning new skills, at least one piece of steel out of three has made it into a final product. I am going to try and make some nice pictures and tomorrow I will post them.

  • The dubbin that I have made almost two years ago and which I stored at room temperature the whole time did not go rancid or moldy or bad in any noticeable way, and the leather on the products I used it on did not degrade in that time either.

 

Mushroom Hunt Pictures: Jay Feather

In Czech folklore, Jays are the guardians of the forest due to their habit to make an awful lot of noise when they spot humans. They also used to be game birds and considered to be a pest, but they are protected by law now all year round. So when a gamekeeper wants to adorn his hat with a jay feather, as per custom, nowadays he has to find the feather in the forest and cannot just kill the bird for them.

They are beautiful but tiny and rare to find. This one is less than 5 cm long.

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