Hello Bobby,

Your question is a good one and is often asked by folks. First it needs to be known that the edge quench is in no way an ABS requirement, many pass their test with a fully quenched blade that is zone tempered.  I personally no longer teach the edge quench method due to metallurgical and, more importantly, safety reasons.  I do however show students how to achieve the differentially hardened blade with an even more traditional approach by full quenching a clayed blade.   

The confusion comes from what the ABS test is really all about.  The heat treatment which facilitates the bending of the blade is not an indicator of a great blade, we are not testing the blade, we are testing the smith.  The bend shows us that the smith can control heat very precisely, overall, and in very distinct zones of the blade.

I am also a martial arts instructor and I liken this to things like form, kata or poomse in a belt testing.  Going into a rigid, pre-choreographed routine of basic moves while being mugged on the street will only get you a sound thrashing.  But judges need a standardized display of your skills in order to assess your mastery of basic techniques.  The test blade allows the ABS to see that you can control heat in many ways all demonstrated on one test blade, but this does not necessarily equate to the best blade in real life use.  

I also have my Intro to Bladesmithing Students make two test blades, one that will be differentially hardened, and one that will be full hardened and differentially tempered.  This is to allow them to see, and feel, the difference in performance between the two.   Differentially hardened blades will have as much as 2/3’s of the blade in a dead soft phase, it is not uncommon for such blades to bend a little in heavy use due to the lack of strength.  Differentially tempered blades tend to be much more springy and will resist taking a set in use, and even in the bend; it is not uncommon for such blades to almost return to true when the pressure is removed at the end of the 90° bend.   

Tempered martensite will give you the highest strength of any phase, other phases will produce ductility which, while not breaking, will not resist deformation.  A differentially hardened blade may reach its yield point somewhere around 75lbs. and then will take no more pressure to continue to bend.  But a fully hardened, and tempered, blade may take as much as 425lbs. to begin to yield and then break at 450lbs.  But that extra 350lbs is a pretty big deal in what a blade can do, and it could be argued that the human arm can only provide so much load, but it can certainly deal out 50-75lbs.   

This also leads to the discussion of whether to quench and draw back the tang. I know a lot of smiths who purposely do not quench and harden the tang area. Others, like myself, through quench the entire blade and tang, and then draw back the tang into the ricasso area. 

@joshua-c-states

Interesting question.

Hold on- lemme get the popcorn. 😆 

-Evan

@joshua-c-states

Making conversation: I would say your heat treat equipment and its capacity vs. the size of your blade affects if you heat the tang or not. If you're using your forge to heat treat a blade that is larger than the forge, it's easier to leave your tang soft. However, even then, you should still draw it back as you may heat a bit of the tang anyway. have a heat treatment furnace easier to heat treat the entire blade, then draw it back on the tang provided your furnace can handle the entire blade. Doing a torch quench or traditional edge quench. It's easier not to heat the tang. Then the size and shape of your quench tank can also factor into if you want to heat your tang.

Clarifying note: what I call a traditional edge quench is using the coals a charcoal or coal forge to heat the edge of your blade then going into your quench medium.

I personally harden at least 1/2 of the tang and then draw it back.  Martensite will give you the highest strength properties of any phase and when properly treated can yield very good ductility (what is spheroidizing, but very extreme tempering).  Avoid hardening the very end of tangs due to threads and the only reason I go for extreme ductility in a tang end is to facilitate peening on daggers and swords.  Of course damascus full tangs really need to be left the same as the blade otherwise the etch will look fairly silly.  

@evan-cihak Here have some of mine.

As Kevin said, I wouldn't harden the tang end if I were planning a threaded tang or a peened tang. I rarely do either one though. I will typically braze or pin on a piece of threaded rod for a take-down or braze on a piece of non-ferrous metal for use in a same material peen block.

I cannot explain why, but I have always been adverse to creating a phase transition in a blade the way an edge quench does, or at the blade/tang junction. Just superstitious I guess.

Thanks for everyone's reply, Kevin how do you zone quench a blade? I understand you use clay on the blade to achieve this. By putting clay on the upper part blade and not on the cutting edge , are you still edge quenching? Is the advantage to keep you from draw the spine soft with a torch with the blade edge in water ? Have you seen anyone pass the test with a fully quenched blade that has not been zone quenched? 

I have students draw the spine back (edge in water) on all blades, even the differentially hardened- insurance.  I go with the full clayed quench to maintain basic principles and mechanism that are essential for fully successful quenching, which leaving a huge uncooled mass above a mere 1/2" depth of oil works against. 

I have often said that one of the worst thigs I ever did was look at steel through a microscope.  I made perfect blades at one time. I was an innocent in the garden and then the serpent coiled up the side of an Olympus MG and told me how much I would like that apple if I took a bite.

What the metaphor illustrates is the difficulty I have had explaining why I avoid so many common bladesmithing practices, that others see as just fine; others who have never seen the mixed phases and results on the microstructure of the steel.  Normal people, who don't live in a metallurgy lab, see a blade pass the ABS test with flying colors and ask why that is not good enough, and that is hard to argue.  But the ABS test is actually just the bare minimum in performance, once you see what it possible. 

Nobody should ever get and ABS stamp and say that is the end of the journey, when it is actually just the beginning.  Even more than handing out stamps, the ABS embraces the constant advancement of the forged blade in pursuit of excellence.  I am have not always been the most popular guy in the ABS, because I will never give up on the idea of tirelessly pursuing accurate knowledge and education of the forged blade, even if that includes admitting some things could be done better.