...After the blade cooled while he was talking, he "accidentally" dropped the blade while holding it from the tang. It hit the concrete point down and made a hole in the slab....
I do this at most of my heat treating lectures, mostly because it is fun to watch the eyes of the crowd when a fully hardened, untempered blade gets tossed point first into concrete. The coolest is when it sticks in the concrete.
I promise you will not find a DVD playing at the Bladeshow of Kevin Cashen hacking up cinderblocks then standing on the handles to hawk his
Super Knives made using the “super secret”, “cutting edge” technology of
Martempering .
All I am doing is showing the effects of the even cooling and that auto-tempering thing I mentioned earlier, it is not magic, a secret, or even all that impressive once you understand how steel works and what it is capable of (and decide that good enough, is not good enough).
Auto-tempering: let’s go back to martensite for a second. Some reading this may realize, others may not, that there are two types of martensite (like this wasn’t already complicated enough
). It has lot to do with Ms temperatures, but for our conversation we will stick to carbon content. In steel that has .6% carbon or less hardening forms
lathe martensite. Lath martensite grows in fine little fern like packets, or laths, and thus the name. Tucked between these packets is ductile ferrite so this martensite is the tougher of the two, and when there is less than .45% carbon it can be quenched and not tempered and still be fairly tough.
In steel that has 1% carbon or more you get
plate martensite. Plate martensite forms on larger sheets with odd angles and tilts to it habit planes, often intersecting each other and causing points of high strain (resulting in plate micro fracturing in larger grains). This stuff is brittle and is one of the main reasons you want to temper higher carbon steels well and very soon.
In steels from .6% to 1% carbon you get a mixture of lath and plate with the ratio in accordance with the carbon level, so we get it on most of the steels we work with. Tempering takes the ‘edges” off all theses little needles and allows the steel to relax a bit, but by relaxing the stressed out body centered tetragonal configuration into a more stable body centered cubic configuration, and by rounding the ends of these needles and to take the pressure off their neighbors.
When you slam steel into Mf by quenching into something like cold water you subject the inside of it to all of these effects until you can get it back up to tempering temperature. But if you quench to 400f and then allow that blade to air cool, you may form as much as 40% of your martensite in a range that allows it to be quickly tempered by its own slow cooling thermal mass, thus taking the edge off before one even reaches Mf and before more needles can be slammed into it. This is what is known as the auto tempering effect, and what Tim Z. likes to call “happy martensite”.
No magic, or wacky theories, actually quite simple and reasonable when you understand it.
