I need help with tempering

[Dan_the_DJ]

Hello everyone,

 

Its been very cold for the past month or so, and I was forced out of my workshop, probably all the way till spring 

I havent made a knife or some other edged tools in a while, mostly because of the weather, but also because Im getting something wrong during the HT process, my guess being the tempering stage.

I have to point out that Im using mystery steels for my projects, mostly leaf springs and old files. So I know the steel is more than adequate for what Im aiming for, but I fail somewhere along the way.

Im also using mostly hand tools, with the exception of one angle grinder, and one bench grinder...

I will now explain my process at going about to make a knife, feel free to correct me with anything that seems wrong to you.
 

The method I use is stock removal:

 

1) Annealing the selected piece

 

3) Cutting the shape with an angle grinder/hacksaw

 

4) FInal shaping and bevel cutting with a file, rarely grinding

     I need to point out that Im very careful not to overheat the steel doing these two steps

 

5) Heating to non magnetic in my charcoal forge, counting to about 20 and then quenching in some industrial oil that doesnt seem to flash what ever you put in it 

     I do not usually do normalization, because most of the time Im so excited and nervous about the quench that I simply forget to do it.

     Nevertheless, I have yet to pull out a warped or cracked piece out of the oil.

     I dont know, for the life of me, how have I managed to do this, but sure is great 

 

6) Checking for warps, cracks and hardness

     Again, I seem to have the fortune of not having any visible warps and cracks, and the blade always seems to get hard enough to make the file skip across it...

 

7) Cleaning the surface and tempering

     I do my tempering almost immediately after the quench, on the coals, waiting for some pale straw color to appear on the edge, when it does, I simply remove the piece and let it air cool. Sometimes I have to hit it with a sprinkle of water if I heated too much, but the color has yet to escape from me using this method. When I arrive home, which is normally at dusk, I do another tempering cycle in my kitchen owen, at 200c, or about 400f for an hour, hour and a half without preheating.

     I get that the oven is not accurate, but from the patches of straw and purple color that it produces its really difficult to judge what the temper did to the knife. Maybe its just the surface getting hotter than it should and the interior is getting a somewhat different temper, but I have no idea really...

Because of this, I have since stopped using the oven for tempering, until I get some kind of a oven thermometer or what not. Any recommendations on that front?

 

Anyways, I will show you some pictures of the 3 knives I made, the smallest one, tagged bird and trout knife, fared the best, though the temper from the oven left an almost black finish, I recon that has to do with the oil not being removed from the surface, but in strong light purple and straw hues were visible. The edge is not as hard as I would want it, but at least it doesnt bend or chip, its ok for the tasks it was intended, just needs a bit more regular sharpening.

The second knife, tagged mini survival  knife in the albums, also tempered in the oven, is by far my most used knife, it has gone through hell and back, was abused like there is no tomorrow, and when it comes to the outdoors, there is nothing the knife cant handle. Except for when it comes in contact with metal...
If that thing touches, god forbid, a stainless steel can, the edge is done for, this is even a problem with regular beer cans which are made of aluminum. The edge is so fragile then, I can not explain it. But when it comes to wood it can cut down a tree the size of ones leg and still shave after that...
Can someone please explain me what the heck is going on there?

I also managed to break the tip while throwing the knife into a board. My guess is, that the knife was thrown at a bad angle, and considering its weight and the immobility and durability of the target, the tip simply could not handle the forces. 
 

Lastly, there is the third knife, tagged as survival knife, the largest and newest of the bunch, and the only one made from leaf spring steel and tempered only on the coals, 1 and 2 were made out of files.

My guess is that this steel didnt get as hard as the file knives to begin with, and topping that with a loner quench, for some deeper straw, it turned out very, very soft...

This knife was also tempered on the coals, with some good pictures of the results. It is the softest knife of the bunch, the edge bent on one spot after some light chopping. And the sharpness was lost overall to the point of not being able to cut pretty much anything. The geometry of the edge is even done with more blunt angles to accommodate for it being softer, but with no avail...

I also need to say that I recently tried taking a file, and just tempering it at 200c, or 400f, to see how it would turn out, and I have to say, Im quite pleased with it, it can cut nails in half without suffering any deformation or chipping. The HT on that thing is truly beyond my capabilities... The only limiting thing about this method is that there is only so much space on the file, and the knives I can make with it are therefore limited in width, length and curvatures...

What Im asking, I guess, is to help me improve my HT for the better, without having to spend money on expensive equipment.

Sorry for the long rant, I hope that you can understand a newbies concern 

Here are those pictures:

https://imgur.com/a/erLW3

https://imgur.com/a/5Prj6

https://imgur.com/a/kyEro37

 

[ThomasPowers]

1 get an oven thermometer and a tray of clean sand

2 heat the oven till the thermometer is stable at the temperature you want and the sand tray has had time to come up to temperature as well

3 temper the knife in the sand tray

0;  TRY A LOWER TEMPERATURE THAN 400 degF

[Dan_the_DJ]

2 minutes ago, ThomasPowers said:

1 get an oven thermometer and a tray of clean sand

2 heat the oven till the thermometer is stable at the temperature you want and the sand tray has had time to come up to temperature as well

3 temper the knife in the sand tray

0;  TRY A LOWER TEMPERATURE THAN 400 degF

Is there a recommended brand, or a specific thermometer that I should get?
Will this do?
https://www.ebay.com/itm/CDN-ProAccurate-Oven-Thermometer-Model-DOT2/231833873789

Also, wouldnt a lower temperature make my already brittle knives even more brittle? 

[ThomasPowers]

I'm sorry I must have missed the part where you said you had problems with brittleness; all I saw was you wanted them harder.

As hardness is linked to brittleness if you have an issue with both you have a difficult problem to resolve.  However there is are ways!

The harder to do without problems is a differential hardening where you only quench the edge or coat the spine with furnace cement or clay to leave it unhardened.

The easier way----Do a differential temper on them!  Leave the edge straw for hardness and temper the spine/body to purple or even blue.  Many ways to do this---some common ways are to temper the entire blade to the straw temperature and then heat from the spine and tang to the softer colour leaving the edge straw.  You can do this using "tempering tongs"; or a hot block to run the spine along or even a propane torch ---usually helps to have the hard edge in a pan of water so it CAN'T overheat from the torch plume. if you have troubles using the tongs you can also use the hard edge in water method.

NOTE after getting the proper shades where you want them, quench in water before they run to the edge!

Note that leaving the file cuts in place increases the brittleness a lot in those areas as they act like stress concentrators.

[Dan_the_DJ]

I might have been a bit unclear.

The oven either leaves the blade too hard, hence too brittle, or just overtempers the heck out of them, and I end up with a softer edge.

What ever I do, I cant seem to get into that sweet spot.

I will try a differential temper next time, but about 1 cm from the business end of the blade, the steel aint that hard anyway, its because of the oil I use, I suppose. Its a bit of a slow quench for the steels Im using.

[ThomasPowers]

Ditch the oven!   "The tool I am using doesn't do what I want it too." => use a different tool!

Sounds like it has too great a temperature fluctuation built in.

[Buzzkill]

57 minutes ago, Dan_the_DJ said:

What ever I do, I cant seem to get into that sweet spot.

Sounds like a toaster oven I had.  If I set it to the maximum dialed in temperature it was a little too low, but if I put it at broil (always on) it was a bit too high.  There's not much we can do to help you there unless you want to get into rewiring the oven to use a PID controller and thermocouple.  However, the simple solution is the one Thomas has already given you:  Do a differential temper to keep the cutting edge hard while softening up the spine.  There will most likely be some trial and error involved while you're learning to get the results you want.

4 hours ago, Dan_the_DJ said:

also need to say that I recently tried taking a file, and just tempering it at 200c, or 400f, to see how it would turn out, and I have to say, Im quite pleased with it,

This is not surprising.  It's unlikely you will be able to do a better job of backyard hardening the file steel than the manufacturer could do.  All you have to do is soften it up to the point where it works for you.  Like you said, it limits the shapes and sizes of your knives, but as far as uniform hardness and grain size goes it's already about as good as it will get.  Of course you have to avoid overheating the steel during your stock removal process, and shaping the steel with unpowered tools is pretty much out the window too.  Still, it's not a bad way to start making knives IMHO.