So I probably already know the answer to this, but it can't hurt to ask. I hardened a blade today, and when I was starting to grind I found a hairline crack a little less than a half an inch long running parallel to the edge, a couple inches from the tip. The blade still had some meat so I tried to grind it out, but no luck. At this point I would just scrap the blade. Following my usual protocol I went to break the blade, if nothing else I can see how my grain refinement is. This is were I began wondering if a crack means I need to start over because the blade didn't break. I bent it just to were it took a bit of set and the crack didn't give. Then bent it back and still holding. It's only had an hour at 400f so it's still pretty hard, I'm sure a little more bending would have cracked the edge, but I don't think the hairline crack was going to go first.
Then I whacked the spine of the knife on an anvil/r.r.track numerous times and nothing, I even gave it a couple on the edge. I don't imagine that the typical knife is getting the kind of abuse that this blade has had, so at this point I'm almost thinking of the crack as a cosmetic flaw. When fully polished I imagine you would have to really look hard to spot it.
So like I said, I probably already know the answer and I need to start over. But I figured it can't hurt to ask.
Thanks for any input, -Justin
Hello Justin. You did not say what type of steel you were using, or quench medium. Needless to say, I think the consensus will be scrap bin. It might not be a crack, could be an inclusion in the steel. But still, I would test it to destruction. Think about it this way. Would you want a knife with your name on it out there in the world, with you wondering is it going to fail at some point. Make the best you can and chalk it up to learning.
Brion
Brion Tomberlin
Anvil Top Custom Knives
ABS Mastersmith
Thanks Brion, that is pretty much what I was thinking. -Justin
If it's cracked, it's really important to know WHAT happened and then understand WHY it happened so that it can be avoided in future. As for trying to save the blade, DONT! The reason you posted the issue is proof enough that sub consciously, you know the blade is no good - especially if you are intending to sell it.
Justin, Neels has some great advice. You need to find out where the crack came from. Was it the steel? Or the heat treating? Or something in the forging? Kind of like an investigation. Tell us the particulars maybe we can help.
Brion
Brion Tomberlin
Anvil Top Custom Knives
ABS Mastersmith
Thanks guys, I already knew not to use it, I just figured I'd ask. Maybe just wishful thinking on my part as the hamon was very nice.
The crack, and I've seen similar cracks a couple times before, is near the tip, semi-circular, not on the edge, going about halfway through the blade's thickness. When I've broken the blades, the grain is always very fine.
I water quench W-2, so that alone could be the cause. My other thoughts have been that it is caused when forging the tip section to shape, or that it is a flaw in the steel. Honestly I'm glad I noticed it now, in the past I've only seen this type of crack after polishing.
Thanks for any insights. -Justin
Based on my experience Justin, I would say the water quench had a lot to do with it. You pretty much have to have everything go right for a water quench. Even though W2 is supposed to be a water hardening steel, in the sections we as knifemakers use, a fast oil is much better. Think parks 50. I have had success with an interrupted quench using water and oil, but it is a scary process. I would get some parks50. It works well for hamons in W2, 1095, 1075, W1. You can get it from Maxim Oil in Texas. The water would be my culprit.
Brion
Brion Tomberlin
Anvil Top Custom Knives
ABS Mastersmith
Thanks Brion, overall I have pretty good luck with water. I'm at about 90% success on the first try, if I'm not happy with the hamon and try again it pretty much always cracks. I enjoy forging so it usually isn't that big a deal if I have to start over. The only issue I've had are these little "surface cracks" forming near the tip. I've only seen it four times in as many years so it isn't worrying me too much, but it is stressful when I don't notice them till after I polish.
It's weird, about two years ago, I had two blades in a row with this type of crack. I wasn't able to see either one till after polishing and etching. Following that, I began doing flex tests on my knives after tempering so I didn't waste time polishing cracked blades. Then I didn't see another for two years, and now I've just had it happen on two blades in a row again. On the knife I mentioned above, and on the bowie I forged just before it. On the bowie I didn't see it till after polishing (and making the guard and handle), fortunately it was about .25" from the tip so I was able to save the knife.
I'm thinking back and wondering what was the same then, as now. All I can think is either I'm causing it while forging out the tip, or that it is just from the stress of the quench.
Thanks again, -Justin
Justin,
It's very possible that the crack wasn't caused by the quench (though that may be when it occurred). Typically a quench crack will be perpendicular to the cutting edge. It may well have been caused during the forging of the tip. W2 can be finicky, especially if the temperature isn't just right while forging. You may have forged it a little too cold causing stress and thus the fracture.
Gary
Thank you Gary, that's kind of what I've been thinking given that I've only seen these cracks near the tip and that they don't go through. Also I'm self taught so there has been a lot of trial and error in bladesmithing for me, still a lot to learn.
Thanks to everyone for their advice, and for helping me finally(I hope) get to the bottom of these hairline cracks. -Justin
