- Joined
- Nov 26, 2001
- Messages
- 1,375
Recently I tried my hand at a C70 carbon steel (roughly 1075) yari spearpoint but got it wrong, so I decided to use the blade for a destructive test.
I normalized it three times and then heated it.
While heating I happened to put it directly in front of a burner outlet and keep it there still for a few moments (I was doing this with a couple of friends around and was explaining one who'd like to make knives some basics).
I realized I had overheated that part of the blade but decided to just do a single normalization cycle and go on with it, as part of the experiment.
The blade was slightly too long for my forge, so I couldn't get it evenly heated in the tip and tang, but this didn't matter as the destructive test I had in mind didn't involve these two parts.
I quenched the blade in oil and then very gently tempered it to dark yellow/brown over about half an hour.
I then closed the first quarter of the blade in a vise and started working it back and forth.
The part nearest to the tang, which was thinner, as I had ground it too much, started to take a set at about 60° digrees bend.
I bent it to some more than 90° one side, then the other, then again one side and again the other. The blade was bending heavily near the tang, so that I had to make great force to make the rest of the blade bend at about 90° in its midst.
At this point the blade snapped hard at about halfway between the tang and the vise, not in the point of minimum thickness but just where I overheated it. I took the remaining piece and bent it again and again until it started to break and finally came off.
The first fracture was plainly a brittle fracture. The second was a plastic fracture and the metal was torn more than fractured.
Comparing the grain size of the first fracture with the second, the grain was much more coarse and, in the very centre, was quite visible.
In the second fracture where the steel had broken the metal had a greysh satin look, extremely fine and homogeneous.
So: grain growth can wreak havoc on a blade so much that a bad misshaping error comes second in importance for what concerns blade toughness.
I couldn't take pics of the parts, and anyway I doubt any of the effects discussed would show in a pic.
Hope the description was clear enough and this was useful.
I normalized it three times and then heated it.
While heating I happened to put it directly in front of a burner outlet and keep it there still for a few moments (I was doing this with a couple of friends around and was explaining one who'd like to make knives some basics).
I realized I had overheated that part of the blade but decided to just do a single normalization cycle and go on with it, as part of the experiment.
The blade was slightly too long for my forge, so I couldn't get it evenly heated in the tip and tang, but this didn't matter as the destructive test I had in mind didn't involve these two parts.
I quenched the blade in oil and then very gently tempered it to dark yellow/brown over about half an hour.
I then closed the first quarter of the blade in a vise and started working it back and forth.
The part nearest to the tang, which was thinner, as I had ground it too much, started to take a set at about 60° digrees bend.
I bent it to some more than 90° one side, then the other, then again one side and again the other. The blade was bending heavily near the tang, so that I had to make great force to make the rest of the blade bend at about 90° in its midst.
At this point the blade snapped hard at about halfway between the tang and the vise, not in the point of minimum thickness but just where I overheated it. I took the remaining piece and bent it again and again until it started to break and finally came off.
The first fracture was plainly a brittle fracture. The second was a plastic fracture and the metal was torn more than fractured.
Comparing the grain size of the first fracture with the second, the grain was much more coarse and, in the very centre, was quite visible.
In the second fracture where the steel had broken the metal had a greysh satin look, extremely fine and homogeneous.
So: grain growth can wreak havoc on a blade so much that a bad misshaping error comes second in importance for what concerns blade toughness.
I couldn't take pics of the parts, and anyway I doubt any of the effects discussed would show in a pic.
Hope the description was clear enough and this was useful.