Saturday, November 22, 2014
Correcting a "chippy" blade...
I've got this great little kanna blade, 48mm wide and I use it all of the time. It has one significant problem, however. Try though I might, the edge develops little micro-serrations in some areas, and don't want to come out, no matter how much I sharpen.
There is a procedure called "Killing the edge" that often works to correct for this jagged edge tendency. When it is time to re-sharpen, you first drag the edge over something hard. A glass works nicely but the side of your sharpening stone works well, too. What you are looking for is a clear, unbroken line of "bright" that runs the full length of the edge. This line of clean metal ensures that you are into the fresh stuff, and not fighting some remnant scratch or blemish.
Some blades are difficult no matter what you do.
Right now at least, I prefer very hard blades. Sometimes the blade can be TOO hard, though. I suspect that the degree of hardness that the blade is capable of carrying is partially due to the underlying grain structure of the steel. A fine grain structure will support a very hard edge, where a more coarse grain will tend to chip or fracture. The grain of the steel is established by the blacksmith, so we are stuck with that, being what it is.
A coarse grain structure needs be softer, in order to maintain its integrity. Tempering is the act of softening a blade, and is something that is well within the abilities of anyone. Easy stuff.
Preheat the oven (I start at 325°F).
Preheat the oven, because your oven runs full-bore until it reaches operating temperature. The high heat would potentially harm the blade, so.... Preheat.
Place the blade on something large and heavy/dense.
Some ovens control their heat more accurately than others, and the cast iron skillet (in this case) acts as a buffer. A little extra protection (and I was seasoning the skillet anyways, so I got to kill two birds with one stone!).
I cook the blade for 30-60 minutes. The actual tempering occurs nearly instantaneously, but extra time won't hurt anything and may contribute to a more thorough temper.
After the blade is done cooking, let it cool, then back to the stones.
Not good enough. 325°F didn't seem to change a thing.
Magnification concurs. The jagged edge remains.
So, back into the oven, this time at 350°F for another 30 minutes, then sharpen.
Better, so at least I might be on the right track, but not good enough yet.
I try again at 375°F...
Much better, smooth and straight. I don't want the blade any softer than necessary, so I'll stop here, for now.
This blades jagged tendencies appear to have vanished, but I'll reserve judgment for now.
The blade seems to sharpen up much better now, both easier and "sharper" feeling.
The kanna works better. Nothing to write home about, but definitely good performance. The true test will be in the longevity and ease of re-sharpening.
I could possibly get more work done if my assistant would stop napping on the tools, though.
I was taking a break from all of the carpentry obligations of the last month, by spending a little time in front of the forge (Yay!!!! Hammer time!!!). I am trying to salvage a sloppy laminated blade that I forged a few months ago. After a lot of massaging, I've ended up with something that is getting close to being a proper blade.
The heavy lifting in the shaping department has been taken up by the $Store sharpening stone. This is proving to be a favorite of mine, being soft enough to function as a waterstone, but hard enough to hold its shape without immediately dishing. And it cuts fast! Amazing performance for $1!
I only tempered this blade to 325°F , and it's WAY too hard for a kitchen knife. Harder than anything that I've ever used before, anyway.
Even worse..... Chipping at the edge.
I tried a second temper at 350°F, and it might have been sightly better, but still chipped pretty bad.
So.... I skipped right to 400°F.
375°F would've been perfect, haha! 20/20...
400°F didn't ruin it by any means, I just like to be more thorough about these things. You don't learn as much if you skip the proper steps, you know?
Unfortunately, this blade is a junker.
This little crack will propagate. I could take in the edge a bit.... Regrind the bevel...
The tip would need some work too. See how the end looks all fuzzy?
That's because the hard steel runs out and gets mixed in with the soft iron. The tip is too soft. The "fuzziness" is due to the way that the iron finished on the natural waterstone. The hard steel finishes to a near-mirror state. I could grind the tip back until I get to the better steel......
Even though the steel is now soft enough to *barely* form a burr, it still tends to chip out.
I don't want to temper the blade any higher. It's soft enough already (actually it's a very practical, hard edge. I just want to see *how* hard I can make a knife!)....I am wondering if I could improve the steel structure through a series normalizing/grain reduction heats, at the forge, and do a new quench...... Re-shape the blade...... Sharpen again......
The new blade would only be, ohhhh.....a paring knife maybe?
I've already spent an unreasonable amount of time grinding out that uber-hard bastard, what's another hour?