Edge quenching

I’ve seen a couple of times on Forged in Fire where the smith just heats and quenches the very edge of the blade rather than the whole blade. This obviously results in a differential hardening where the edge is now harder than the spine; sort of a faux honyaki I guess.

I can understand why they are doing it on the show given that they are going to smash the knife into a tree or a thigh bone or something equally cringeworthy and they want the softer spine to absorb the shock. My knowledge of smithing is minimal so I’m curious as to the pros and cons of this method of differential hardening vs the more traditional method of clay along the spine that we’re more familiar with.

Can someone please enlighten me. Thanks.

How much detail do you want us to go into? I'm not sure how much knowledge you have on the topic, but I can give you the quick and dirty explanation.

You can think of hardening a knife as a three part process:

1. You heat the blade up, red hot (change it to austenite)
2. Cool it down quickly (quenching: changing the austenite to martensite)
3. Then heat it up again, but not as much as the first time (tempering: toughening the martensite)

Lets say you've worked out how to utilize each of these three steps to harden the whole blade. It's possible to tweak any one of those steps -without changing the other two- to get a differentially hardened blade (i.e. a softer, tougher spine when compared to the edge).
I'm not sure if you realize this, but the way you've described what you saw on Forged In Fire actually covers changing the first two simultaneously.

Some contestants on the show will change the first step and use a torch to heat the edge only, meaning that only the edge turns to austenite. In this case the spine (which hasn't changed to austenite) cannot be converted to hardened martensite, no matter how fast you quench it.

Honyaki blades only seem to mess around with the second step, controlling the speed of the quench. A layer of clay on the spine can keep the austenite underneath it from cooling fast enough to convert to martensite. Also, as you describe above, you can choose to only quench the edge. This leaves the spine of the knife outside of the coolant/quenchant, and so it doesn't cool fast enough to convert the austenite to hardened martensite.

The last one - tempering - gets played with on the show too. In general, the higher you heat martensite, the tougher and softer it becomes. You can take an all-martensite blade and toughen the spine of it by heating it up more than you do to the edge. You can sometimes see people doing this by torching the spine of the knife while holding the edge in water, keeping it cool.

I hope this helps to explain some of what's going on.

Thanks kip. The sum total of my knowledge could be written on the back of a postage stamp in big letters. I have a general knowledge of the process and why it’s done without any of the associated technical knowledge. I guess what I’m curious about is one process better than the other, ie: edge quenching vs clay on spine quenching and if so, why isn’t it done that way all the time.

sharp knives are sharp.

Claying requires clay, which also means drying time after application. Sometimes curse words can be heard for miles around if the clay pops off when you're heat treating but haven't quenched yet. :curse:

Edge quenching doesn't require any extra steps. But other than cases of autohamon, you usually just get a boring quench line and not the artistic shapes you can get from claying.

OliverNuther said:

I’m curious as to the pros and cons of this method of differential hardening vs the more traditional method of clay along the spine that we’re more familiar with.

Click to expand...

Edge quenching is also very traditional (and probably an older technique).

If you do edge quenching by only heating the edge, it can be difficult to make sure you have the right temperature, and uniformly, along the edge. If edge quenching by heating the blade uniformly, and then only quenching the very edge, it can be difficult to get a very rapid quench. If doing differential quenching by using a clay coating, (a) you need to learn how to do it, (b) the coating can fall off, and (c) you can harden the body/spine to a spring temper (because it's quenched, just more slowly than the edge).

Thanks for the replies guys. I’ve been offline the last week so I’m just catching up now. So edge quenching is basically a quick and dirty, less precise method than claying. That explains why it’s popular on the show given their time constraints and the fact that they’re going to abuse the hell out of the blade anyway. Thanks everyone.