Most basic blade hardening steps

[grepper]

Mr. Clueless here…
 
Would one of you guys be so kind as to very simplistically explain the process of hardening a blade?  I’m talking about just the very basic process. 
 
Am I correct that it is something like, get the blade really hot and cool it quickly.  That makes it very hard but brittle.  Then get it hot again (do you cool it quickly or slowly this time?) to temper (correct term?) the blade to reduce the hardness/brittleness?
 
I know there is a LOT more to it, but is that basically the process?

[me2]

Yep that's pretty much it. Heat really hot, 1400 to 2200 F depending on steel. Cool fast enough to harden, in brine, water, oil, or air. Then reheat to temper, from 325 to 1200 F, depending on steel an use. Quenching after tempering is good, but not typically necessary for knives.

[grepper]

So, if we then reheat this blade during sharpening, does it further soften the steel or work harden the edge like heat treating it?

[Mark Reich]

If you get it hot enough to see ANY color (the least would be very slightly yellow- doesn't make a big difference-1-2 points) yes, that will soften the steel. If you "blue" the tip, (or anywhere else) that is a significant reduction in hardness.

These are worst case scenarios. Normally you burn your fingers at 150°F. Nowhere near hot enough to soften (temper) the edge.

I never overheat a hardened blade by sharpening, but if you're regrinding the bevels of a hardened blade, you do need to be very careful. Like one pretty quick pass, dunk it in the water bucket quickly.

Mass of the blade makes a big difference. Pressure against the belt- big difference. Dull belt- big difference. SFPM- depends on pressure and belt condition. It will magnify the effects.

You can work harden an edge by drawing the Edge across another piece of steel, pressing hard enough that you can see the edge flex as it passes over the other piece of steel (like the handle of a vice).

This is known as the "Edge Flex" test. It's used to see how chippy vs how tough an edge is. All steel that is maxed out for hardness via tempering regimen will chip. It makes a difference how hard you're stressing the edge. When I do it, I press hard. I can easily see a "wave" traveling along the edge as it crosses the other piece of steel.

Soft steel can take a lot of edge flexing, but hard steel will chip pretty easily.

[me2]

So, if we then reheat this blade during sharpening, does it further soften the steel or work harden the edge like heat treating it?

If the blade is reheated to a higher temperature than the original tempering temperature, then softening can occur. The heating of the edge does not work harden the blade. Work hardening requires the material to be permanently deformed (plastic deformation) via cold rolling, peening, cold forging, bending, or any number of other means. Technically, machining and grinding create a very thin layer of work hardened material just under the material that was removed. However, this is generally not considered in cutting tools, as the ductility of the material is generally very low in hardened steels, and some ductility, or ability to be plastically deformed, is required for work hardening.

This video shows machining in slow motion and you can see the cold worked layer just under the cut. It's very thin, even here. This is for machine cutting bits, but similar layers appear under ground surfaces. Also, note how ductile the material is and how little the hardened bit moves. No deformation, either plastic or elastic, is visible to me.

https://www.youtube.com/watch?v=2fDJ1Wk-y04

[grepper]

Thanks guys.

Yeah.  I didn’t mean to say work hardening.  I completely understand that is different.  We have considered if repeated bending of a burr in an attempt to remove it may cause work hardening, but that is a different subject.

What I was wondering and you guys have answered, was if the reheating during sharpening acted like the original heat treating / flash cooling to harden, or further tempered the edge.  

Blades never get hot when I sharpen.  However, I’m open to the idea that some flash/grain/background heating may very well be happening at the microscopic level as material is brutally torn from the edge.

That is a very cool video.  It really goes to show just how savage machining and sharpening is to an edge.  Guess I’ll have to start squirting carbon tet like holy water.  I wonder how modern synthetic coolants compare with, or if they contain cabon tet mentioned in that, what looks like, a 1950’s video.

Seeing the world on a microscopic level is always amazing.  It’s amazing how things that seem like a non-event can be cataclysmic on the micro level.

I’m guessing the reason that heating during sharpening tempers rather than hardens is because it’s less than the original HT temp, and that it is not quickly cooled.  Correct?

[me2]

It's generally not discussed much, but auto hardening is possible.  It was proposed as a reason for some broken tips on knives several years ago on another forum.  The tip had even less mass than the edge, and heats up even faster, and if the rest of the blade is relatively cool, it can self quench.  

Softening is assumed because overheated blades generally don't behave as though they were quenched and and not tempered, ie the edges don't just break.  

There is also a phenomenon in hardening of steel where the faster you heat it, the higher the hardening temperature becomes.  I do not know the limits of this.  There is a fair bit of work being done on flash hardened steels for automotive uses.  The steels are heated, "soaked" and quenched in something like 10 seconds.  

Also, that was the wrong video.  Try this one.

https://youtu.be/mRuSYQ5Npek

[wadebevan]

Wow! Cool video.
As a machinist, I am never going to be able to get that video out of my head when machining
I find that little piece of material preceding the cutting tool a surprise for sure.
Thanks for posting!

[me2]

I see once again I didn't actually answer Mr. Greppers question.  I'll have to do better in the future.  Yes, it is possible to heat the edge or tip hot enough to reharden it when grinding.  I don't think it's very likely it could be done by accident, but I'm sure it's happened a time or 2 to somebody.

Many modern steels are air hardening, and if heated high enough, the blades will harden on cooling in air.  Again, not likely, but possible.  

Steel that's been hardened and not tempered behaves a lot like glass.  Unless the edge is breaking visibly, I wouldn't worry about it.