I have seen bladesmiths fully harden a knife blade and then soften the spine of the blade while keeping the edge cool.

This has been called "bluebacking"

What are the pros and cons of this technique?

I usually call it drawing back the spine or differential tempering, but bluebacking works too. The idea is to make the blade tougher and/or more flexible by tempering part or most of it back further (usually to a blue) while maintaining the hard edge. For something like a hunting knife that you might baton through a tree or something along those lines you wouldn't want to strike hard steel since it may break/chip.

As far as the cons... I suppose it's one extra step that's not always necessary. I'm sure there are others.

This topic has certainly been discussed here before so I'm sure you can find a more detailed explanation if you try looking it up with other names.

When done correctly you can go with a harder edge as the tougher spine can support it in use.  Also good for blades that will see a lot of force applied normal to the flat of the blade.   

I actually found a set of tongs that had been modified for differential tempering at a fleamarket once. They had bars welded crosswise on the tips of the bit so you could heat them in the forge and then grab the spine of the knife and apply heat to it. (They are now on my tong rack...)

I have done that. Positive is that you can do it at the forge right after the quench and it is done quickly. Negative, only the edge is hard if you grind it beyond a certain point it is all soft.

How soft is soft depends on the temp you draw to and the alloy of the blade.

Other issues:

1. It is very difficult to accurately control the temperature at the edge of the blade if you are torch-tempering the spine with the blade in air (directly after quenching with no oven temper).  You do want it to be tempered to remove some brittleness, but you don't want it to go full soft either.  Some folks use the temper colors to determine what temperature the blade edge gets to, but these oxidizing colors can be deceiving (especially if there are any oil deposits on the blade).  It can certainly work as a technique, but is more an art than science IMHO.  Much easier to do with less critical, thicker, tools like chisels.
2. If you torch temper with the blade in water the edge will never get up to correct temperature to properly temper it.  If the blade was properly hardened it will remain brittle.
3. I was taught that "blue-blacking" the spine to make the overall blade more resilient is best done after at least one round of normal oven tempering (almost always done off-screen on shows like Forge in Fire as it is too boring for the audience).  You want any remaining austenite to fully convert to avoid later stress (two temper cycles are better for this).  If you plan on blue-blacking the spine you can likely do a snap temper at a relatively lower temperature to maintain more hardness on the edge, but that still depends on the eventual use planned for the blade.  A wet sand barrier works well for protecting that edge while torching

A snap temper at a fairly low temperature is always a good thing to do ASAP after the quench.  Never put off tempering till later!  I've seen blades that have broken into pieces sitting on a rag on a workbench overnight because the maker didn't want to stay up later and do a temper on them.  Pre-heating the kitchen oven before doing the quench and then letting it soak while you get ready for bed would have saved a LOT of time and bother and the blades!

Well I had some blades I needed to harden and temper. So, I bring it up to quench temperature (magnet helps), quench it in motor oil and then I heated a chunk of steel in the forge and place the blade on it and watch the colours. Blue or close to it is fine, on the back of the blade not on the edge or the tip. And cool it in water. Now my test if it went good is: File on the back, must be soft. File on the edge, must skate or be hard. Then I clamp it in the vise and it must be flexible, last step I drop it from waist heigh tip first on the concrete. If it survives it is fine. One didn't survive, but that is nothing to worry about. Just reforge the tip.

Edited February 19, 20224 yr by Mod30
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Picture of flexibility.

Edited February 19, 20224 yr by Mod30
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dropping on the cement is NOT a good idea, you are asking for shrapnel injuries