Wednesday, September 17, 2014
A tale of two extremes--- heat treating woes.
Heat treating issues. Here are a couple of fails (Well, one fail, one partial), that may provide some insight into making a laminated blade tool..... And offer another opportunity for me to abuse some edges!
These blades were: normalized, clay slip coated, water quenched, then tempered at 325°F.
My forge black skin (kuro-kawa?) is looking better, at least.
A slender, curved marking knife.
I made this knife with a rather pronounced curve, to more easily rest in the hand.
The short, steep (around 45°) beveled edge rides nicely along any guide that you happen to use.
I favor wooden try-squares that won't harm bladed tools...... And you can make them whatever size that you need. The only issue is that you need to use a marking knife that won't cut into the blade of the gauge, ergo...
After the water quench the blade warped just a bit, towards the iron side of the blade. The bend at the right is intentional, the very slight gap underneath the tip, to the left, is not.
The warp is about 1/16", but is not something that I want.
This blade was dead flat, prior to quenching. Other, more learned folks can better explain how this works, but essentially, the hardened steel is locked into a crystalline structure that is larger than it is in its soft, annealed state. That means that laminated blades tend to warp away from the steel side, towards the soft iron side.
This blade was easily corrected by carefully hammering it flat, using a good sized hammer, and a stump as an anvil. I say "carefully", but I didn't use THAT much care, really. Just tried not to hit the edges, and checked it continually against a flat reference surface. The hard steel/soft iron combo can be remarkably durable. It didn't make for interesting photos, however.
This blade sharpened up nicely, and proved very hard. It easily resists my hardest file.
The lamination line is instructive. Notice the diffuse border?
I spent too much time working this blade, and it got too hot at least once. Not sparking hot, but definitely a bright orange, welding heat. The fuzzy edge is (I believe) due to carbon migration from the steel, into the iron. While still VERY hard, I would NOT buy a blade that looks like this. I have chisels that have indistinct lamination lines, and they are decidedly softer than my best ones.
Now for the fun one. In a prior post, I mentioned a kuri-kogatana blade that I broke into pieces because it had a failed forge weld. I made this blade in a paired state, meaning that I forged it double length, then cut the blank down the middle, forming two blades. This is the other half, and I thought that it might be salvageable.
Wrong. The blade warped significantly, nearly 1/8".
The fact that it warped 1/8" isn't overly concerning. The WAY that it warped is. This didn't photograph well, but almost the entirity of the warpage is in the last 1" of the blade.
If you zoom in, it's at the point furthest from the camera. My apologies.
After polishing, the cause is revealed.
Not only is the weld of poor quality, the steel also got WAY too thin in one spot. This thin area corresponds to the origin of the bend. It is a toss-up as to which is more deleterious, the poor weld or the too thin steel.
The steel/iron line shows poor definition (I want some of that good, dark, soft iron that they use in Japan!), so I etched the blade with phosphoric acid, to check for carbon migration.
Most of the blades lamination looks fine, with little evidence of carbon migration. It's still a junker, though, so I get to bash it up!
I set the blade, edge up, on the anvil, then smacked it with some mild steel rods that were lying around. I started out with the small stuff, then graduated to the bigger size, hoping for a dramatic failure.
Surprisingly little damage, considering how many (+100), and how hard I was smacking the edge. Look at the notches, hammered into the iron!
I struck the edge over the entire length, but you can see that the damage was most severe in 3 areas. The heel and the tip showed the most damage, but there was significant damage to the area of the bad weld, too.
The steel actually folded over at the heel and toe, indicative of a blade that wasn't fully at its transformation point temperature, before quench. The blade was hot at the center, but too cool at the ends. The edge failure at the weld is due to the steel not having the full support that is provided by the iron.
I want to check the edge deflection, so I pushed the edge (just the steel portion) against the anvil. I was expecting a chip, a crack, something, but got nothing dramatic. The knife easily shaves steel from the corner of my anvil (Fun, but probably shouldn't be a regular test criterion, haha!).
Although the knife is hard enough to shave steel from the anvil, it is not as hard as I was trying to achieve.
Both blades were subjected to the same file treatment. The marking knife was sufficiently hard, showing only a few trace scratches, but the kuri-kogatana tested far too soft. The hard file easily scratched the steel edge, scarcely harder than the Stanley chisels that I used to use.
Blade hardness shouldn't be the sole defining criteria of a good blade. Durability and ease of sharpening are important too. I want all three!
I was surprised that the two blades turned out so dramatically different in hardness. I performed the quench only minutes apart, so you would assume that my ability to gauge color/temperature would have been more consistent. It's not like I did these hours or days apart. I want to heat the tool to juuuuuust above the non-magnetic, to minimize the grain size in the steel. I used a magnet to verify the Curie point, but I obviously wasn't quick enough into the quench bucket.
I also learned that my anvil isn't that hard.