Added a picture where the grain size and edge deformation is visible

When you say "hardened by eye" what exactly does that mean?

Posted by: @joshua-c-states

When you say "hardened by eye" what exactly does that mean?

It means that I judged the temperature by eye, I don´t have an electric kiln or controlled furnace, so I can´t really say what the temperature was.

Please keep in mind that there are MANY possible reasons that the blades failed, or any number of combinations of reasons.  That being said, and based solely on what I can see from the pic you posted...

-The overall geometry is too thin/light for the tasks.  

-Based on what I can see of the chip/break, it appears the grain is awfully large, indicating to me too high and/or too much duration of exposure to heat.  

-Based on the deformation, the tempering temp was possibly too high.

  Now... those are all POSSIBILITIES, and certainly not definitive. How do you get from where you are... to where you want to be?   It has to be ONE step at a time. Meaning that you must address/fix one issue at a time, and eliminate it, prior to moving on to the next step(s). 

   First, I would start with enlarging the overall geometry, making the blade somewhat heavier/thicker, and increasing the amount of convex on the edge.   That also must include correct forging with proper heat levels and not over exposing the blade to that heat.  In my shop student live by the saying... "you can always take away steel, but you can never put it back.  Meaning to ALWAYS leave a blade larger and thicker all the way around than you think it should be....then, if necessary, you can always take some steel away/off towards the end of a project.  But take too much off in the "rough grinding" stage (which is what my instincts tell me happened with you) and you have no place to go, and end with a blade that is too thin/light for it's intended purpose(s).

 Of course, then you must start over when considering the heat treat. You have to figure out the CORRECT hardness level, not only for the steel, but for the geometry in both the blade and edge, for the tasks required of the blade.  Sometimes that's not as easy as it sounds...but it ALWAYS involves properly treating the steel so it has a smooth, fine grain structure.  Maybe with a LOT of practice, you will be able to harden "by eye", but if you're doing that for an ABS test blade..... well, let's just say that can often be "bad juju".  There are a lot of ways to do things that are very accurate, without having to own a heat treat oven.  Unless you are well versed/experienced, ALWAYS leave your blades overly thick when heat treating.  I typically teach my students for a test blade sized knife, to leave the edge the thickness of a nickel, or more prior to heat treating.

  Next. tempering. I strongly suggest always erring on the "hard" side....meaning that if I am experimenting to find the proper working hardness for a new steel, or a new grind, or a new anything that I am doing, and  think I want to temper a blade at say 400F.... I will start at 350F.  Temper, finish grind and test....if the blade chips, all you have to do is go back into the tempering oven at 25F MORE than the previous temper, then test again, and repeat until you find the tempering temp which the blade does not chip, etc. BUT, remember that is the tempering temp for THAT STEEL, WITH THAT BLADE AND EDGE GEOMETRY....AND IS NOT UNIVERSAL FOR THAT STEEL TYPE....but rather a good starting point for that steel.  I am also heavily reliant on the brass rod test when I am searching for the correct working hardness in a given blade. 

  I know I'll likely stir up a hornet's nest with this, but I passed both my JS and my MS tests with blades that were heat treated with a torch, and edge quenched only.   It's about knowing how to build a blade for a given set of circumstances. 😉 

  Sometimes it might seem overwhelming to get from point A to point B.... but give yourself the necessary time and experience, and it will come.  

Posted by: @pieter-paul-derks

Posted by: @joshua-c-states

When you say "hardened by eye" what exactly does that mean?

It means that I judged the temperature by eye, I don´t have an electric kiln or controlled furnace, so I can´t really say what the temperature was.

You are watching for Decalescence/Recalescence?

Thanks for the extensive reply mr Caffrey,

The problem might very well be overheating and grain growth, I am working on a replacement blade and have paid extra care of not overheating my steel. You also are totally correct, the blade got a bit thinner with rough grinding than I wanted, I will leave the next one thicker and temper a bit higher to start. I will keep trying until I get it just right.

With the ´´brass rod test´´, do you mean flexing the edge on a piece of brass?

To Joshua, 

I am watching for the recalesence, I did harden this blade during the day though, maybe it would be better to do it at night so I can see the phase change better.

To Joshua, 

I am watching for the recalesence, I did harden this blade during the day though, maybe it would be better to do it at night so I can see the phase change better.

 

I have never been very good at spotting recalescence. 

Here is a much better way to get the right temp for hardeneing if you don't have a controlled oven or a way to measure temp in your forge. Go get some rock salt or coarse grain Kosher salt from the store. Put one or two large grains of salt on the blade. Now salt melts at 1474 degrees F, which is right about the right temp for most of the alloys we typically work with. 80crv2 wants a quench temp of 1500 (I think), so once you see a puddle of salt, you are desperately close.

If you put the salt on the thicker spine, your edge is likely a little hotter anyway, so the salt trick should work well.

Posted by: @joshua-c-states

To Joshua, 

I am watching for the recalesence, I did harden this blade during the day though, maybe it would be better to do it at night so I can see the phase change better.

 

I have never been very good at spotting recalescence. 

Here is a much better way to get the right temp for hardeneing if you don't have a controlled oven or a way to measure temp in your forge. Go get some rock salt or coarse grain Kosher salt from the store. Put one or two large grains of salt on the blade. Now salt melts at 1474 degrees F, which is right about the right temp for most of the alloys we typically work with. 80crv2 wants a quench temp of 1500 (I think), so once you see a puddle of salt, you are desperately close.

If you put the salt on the thicker spine, your edge is likely a little hotter anyway, so the salt trick should work well.

Now that is something I've not heard of before, I will give it a try, I have plenty of salt in my kitchen drawers.

Knife making hacks that work!

Sorry for taking so long to reply...... I've had family in town, and have been visiting with them.

With the ´´brass rod test´´, do you mean flexing the edge on a piece of brass?

  Yes, the "Brass Rod Test" in using a round brass rod, and involves flexing the SHARPENED EDGE over the brass rod.  Many misunderstand this "test" in that they try to over flex the edge, when really all you're looking for is seeing the blade's edge slightly flex over the brass rod, and then return to true/center.  If the edge flexes, and stays flexed (bends), the heat treat is too soft.  If the edge chips during the test, then it's too hard.  

Here is link about the Brass Rod test: http://www.caffreyknives.net/testing_art.html   The pic on this page is hand drawn, and some misinterpret because it makes them believe they have to "flex" the edge far too much.  Typically all you're looking for is a change in how light reflects, which is the edge flexing over the rod.  I start near the plunge cuts, with the blade as a very shallow angle, and apply enough pressure to see the edge flex...then draw the edge across the brass rod all the way to the point.   Once you learn/understand how to conduct this test, I think you will find it is perhaps, the most reliable way to determine the correct working hardness for most any blade made of commonly forged steels.   NOTE:  This test does not work with stainless steels!  Those are a totally different animal when it comes to hardening/tempering.  

  Keep us posted on your progress.....and best of luck.

I just got the second replacement blade through rigorous testing without any issues, including all the parts of a ´´performance test.´´ I think the problem was in overheating after all, I blinded the windows in my workshop and got way better visuals on temperature and recalesence, showing that I had been working and quenching too hot on the first knife.

I tried the brass rod test, but it was quite difficult to deflect the convex edge of this knife. The edges of kitchen knives I´ve made did flex beautifully.

Thank you for the help, this has given me more confidence in my heat treating

Glad things are working for you!   Something I mention to all my students..... these days "Temp Guns" are so darn cheap, you really can't afford not to have one.  They are VERY helpful for newer makers who are trying to figure out temps.  

  I actually have two of these.... one stays near the forge, and the other resides in my heat treat area:  

https://www.amazon.com/dp/B085H8BW81?ref=ppx_yo2_dt_b_product_details&th=1

  If you get one, just make sure it has a single point laser as the sighting device.... those with multi-point targeting can be really difficult to use on items the size we typically deal with. 😉  

That is interesting, I never knew those infrared thermometers could read temperatures as high as used in forging.

I was initially taught by blacksmiths who tend to look down on modern tools like these, but with bladesmithing a bit more precision is nice.

I have actually already ordered a probe type thermocouple for my heat treating forge, it is amazing what you can get from amazon for little money.

I can remember catching flak from then "old timers" who rode me hard for owning/using Little Giants power hammers, and forging presses.  Telling me... "That's not real blacksmithing!"  "That's cheating!"   and it bothered me... that is until I was visiting a 15th century castle in Germany.... and in the "Armory" there was a 1500lb water powered drop hammer.

  That's when I realized.... those folks where using the top "technology" that was available to them.... and if they'd have had Little Giant Hammer (or presses, or air hammers) they would have been using them!   

  I also had some other life long Blacksmiths & Bladesmiths who told me.... If you want to keep doing this when you're old and gray.... then get yourself a hammer and press NOW. 

  And I did.  I sold off my 25 & 50lb Little Giant hammers, and purchased a Say-Mak 110lb air hammer..... best tool money I have ever spent!   I also sold off my 25 ton forging press, and purchased "Orange Crush".... my 80 ton press, that was conceived and built by my good friend Jim Clow out of Alberta, Canada.   

  I have since been diagnosed with Lupus, Lung disease, and fibro... which is short for saying if I didn't have those "power" tools, I would not be physically capable of forging anything near that I can with them. 😉   

  As for the infrared temp guns, I use them to train my Bladesmithing students, and have had some of them balk.... but then I tell them... "Use it to train your eye for gauging what colors equal what temps in your shop." (one of the biggest variables in Bladesmithing is the difference in ambient light from shop to shop, which makes teaching    Most are very surprised when they realize that by "eyeballing" temps...they are typically WAY off one way or the other.  😉