Thread: Hardness of blade vs other parts?

Here's a quick question for those with access to a harness tester. Rockwell, Brinell, Vickers...whatever.

Has anybody found any difference in hardness between tang, spine. blade etc? I figure checking the blade on full hollows could be done if it was parked on some stiff clay or something.


Sigh, I wish I still worked in a machine shop!

It depends on the steel, the grind, and how it was heat treated. Say it was 1084, my expectation is that the blade would through-harden because that steel is designed to do that. So the edge would be the same hardness as the spine/tang. A low hardenability (e.g., simply alloy, low manganese) steel would likely harden at the edge and be more likely to have pearlite in the spine and different hardness numbers. The same differential could be had by a quick severe tempering of the tang and spine while protecting the hardened edge.

For production razors, the simplest easiest least expensive process is the most likely occurrence. Therefore, unless the owner wants some special process on special steel with multiple steps, and he/she can tell the accountant that is the way it will be, it will be heat/quench and temper and the whole blade will likely be hard to the same level.

My two p.

I think the edge will (and should) be harder than the spine. The edge is thinner and will cool (quench) faster. This would be good for the life of the razor, the edge is tough but thin, the spine is soft but thick so as the razor is honed the edge angle will remain the same… well… more so for a hollow ground than a wedge.

Razors are small enough that they harden through and through if heated and quenched properly. And as Mike said, it would be extra work to differentially soften the spine/tang.

Try taking a file to a junk razor at various points, I'll bet there isn't much difference in hardness.

Yeah, you guys are probably right. Based on the thickness and size of the material, it seems logical to assume uniform hardness. But you know what happens when you assume things.

If the razor was quenched, I bet the blade would be harder, but if they hardened and annealed it in an oven, it should be uniform...right?

Originally Posted by joke1176

If the razor was quenched, I bet the blade would be harder, but if they hardened and annealed it in an oven, it should be uniform...right?

All that matters for full hardening is that the blade gets below the "nose" of it's TTT curve.

For simple high carbon steels, if the entire blade drops from ~1475 degrees F to under ~900 degrees F in under one second, then it will harden the same at every point of the blade when it comes down to room temperature. It doesn't matter how the blade is cooled, just as long as it meets those requirements.

If for some reason the back/tang takes longer than 1 second it might be slightly softer than the edge of the blade, but for most quenching liquids it is not an issue until you get into much larger/thicker blades (in which case you might have issues with the inside of the blade not hardening to the same degree as the surface).

I've had a few blade harden through and through when I didn't want them too. Since I've been doing the clay coating to achieve a differential heat treat, I want the back to be softer than the edge, but on a few occasions I didn't have enough clay to properly insulate the back and the whole thing was a uniform hardness when it came out of the quench.

edit: there could be situations where the tang was held in a pair of tongs while the blade was quenched which could cause it to be softer because the quenching medium doesn't contact the steel at that point.

Originally Posted by smythe

I think the edge will (and should) be harder than the spine. The edge is thinner and will cool (quench) faster. This would be good for the life of the razor, the edge is tough but thin, the spine is soft but thick so as the razor is honed the edge angle will remain the same… well… more so for a hollow ground than a wedge.

Each steel has a maximum hardening depth. Since cooling begins on the outside of the blade, where the steel first hits the oil, the "skin" of the blade hardens first. Depending on the chemical makeup of the steel, there may or may not be some softer steel in the core of the blade, basically the middle of the spine.

To wind up with a softer spine, one of three things could happen. First, you could fail to heat the spine above the critical, or hardening, temperature. Since the edge is .030" thick or maybe less, it heats up much more quickly than the .250" thick spine. It's best to bring the whole blade up to temperature slowly so that the steel can absorb heat evenly.

Second, you could cool the edge more quickly than the spine. This is called edge-quenching, and I believe Russel does this with his Japanese-style blades.

Third, you could fully harden the blade, then heat the spine up to partially anneal it. This is done frequently by knifemakers who want a flexible blade.

I generally shoot for full hardness on my razors, unless I'm going for a decorative effect like a hamon, which is a cool-looking line between the hard and not-as-hard parts of the steel.

Josh