Blades breaking after quench

B

bobfridaycustom

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Oct 2, 2013
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I am new to knife making and I have made 7 blades. Of those 7, two of them broke post quench while I was standing off the scale. I use a propane forge of sorts to do my heat treating.
After the first one broke I got a thermocouple to help me gauge the temperature better. I originally thought I had over heated the first one and gotten significant grain growth and that's why it broke.
The second one I know it was around 1500-1520 (the thermocouple has a couple percent error either way.)
So I know that wasn't and significant grain growth there.
I guess my question is: Is this just something I have to deal with? Is there anything I can do to avoid that? I'm still "finding my way" as far as heat treating goes, but it's frustrating putting hours of work into a blade and then having it break!
Please help me figure this out!
 
Go on over to the knife makers forum and introduce yourself:) Fill out your profile so they can relate to you a bit better. Read the stickies! Lots of good information there. You might ask to have your post moved.
 
Did you temper before grinding the scale? Another thought, what are you quenching in, what temp is the quenchant at? WhaThe steel?
 
It's 1095 steel, and the quenchant is canola oil at about 120 f. I didn't temper become I tried to grind off the scale. I was trying to get rid of it to see the temper colors. I did it that way on my other knives with no problems.
 
bobfridaycustom said:
It's 1095 steel, and the quenchant is canola oil at about 120 f. I didn't temper become I tried to grind off the scale. I was trying to get rid of it to see the temper colors. I did it that way on my other knives with no problems.
Click to expand...

I always quench then go directly to temper as soon as it hits room temperature. I also do two cycles of temper and the only thing I do during that time is on the final temper I treat with beeswax. Is there a reason why you have to remove scale before temper? If not than I suggest just going through your temper cycles then removing any scale present.

But I should note that I am not a professional, I am a noob as well. So I am sure there are some well seasoned knifesmithing vets that can point you in a better direction.

Edit: Have you considered normalizing before you HT? I've heard this helps a lot with cracking DURING heat treat, however I am not sure if it helps with post HT cracking.
 
The knife is brittle after quench and needs to be tempered as soon as possible before you do your grinding
 
Have your kitchen oven running 30 minutes before you harden your blade, have a cookie sheet above and below the blade to help prevent over heating from the elements. As soon as the blade has cooled to room temp, set it in spine down. Forget using color for tempering, the oven will work much better. It need to be tempered for hours, not seconds. Colors will not be accurate when it comes out of the oven. Any oils or fingerprints will be different colors.
 
1095 and similar hyper-eutectoid steels can crack just sitting on the bench top if not tempered very soon after the quench. ANY DELAY IS AN INVITE FOR A BROKEN BLADE. Never place a untempered blade on the grinder, as the shock and vibration can easily crack it.

If you are tempering by temper colors.....strop using that method. It is a very poor system for gauging the temperature of the blade.
Use the kitchen oven and set it at 400F for 1095. Two one hour tempers, with a water quench/cooling between them ( just hold the blade under the cold water tap to cool it off) will give you a very good balde.
 
I appreciate the answers. I am a little curious about what bladsmth had to say about running the blade under the tap in between temper cycles. You are saying not to let it air cool by itself? So after the first temper cycle run it under the water, and then right back into the oven for the second cycle? I haven't heard of that before.
 
Respectfully, that's why you should read the stickies. They will give you all the info you seek, as well as answers to questions you WILL have in the future. You really need to temper right away.
 
I've never had a blade break while grinding but I have had one that I ht and left over night because it was late then came back the next morning and it was broken. I heat my oil to 150 but that may be to high from what I hear. Normally I do one temper cycle at 440 for my I-1 blades but I plan to change after some more research. Definitely got to temper quick.
 
bobfridaycustom said:
I appreciate the answers. I am a little curious about what bladsmth had to say about running the blade under the tap in between temper cycles. You are saying not to let it air cool by itself? So after the first temper cycle run it under the water, and then right back into the oven for the second cycle? I haven't heard of that before.
Click to expand...

I use a bucket of water instead of tap water, but my understanding (and practice on the couple of blades I've done so far) is that as soon as your blade is cool enough to handle from the hardening oil quench, wash off the oil and temper twice back to back for 2 hours with a water bath between tempering cycles. The first temper cycle relieves internal stresses from the hardening and the second temper cycle takes care of any residual internal stresses that weren't completely relieved on the first go-around.
 
Well that is sort of close.

After quench let the blade cool to room temperature slowly ( I just let it sit in the quench tank).
As soon as possible after the quench, clean the blade gently and temper it at 400°F for an hour. Durring this first temper, the brittle martensite is converted into tempered martensite. Any Retained Austenite is converted into new brittle martensite.
Take out of the oven and cool rapidly in running water or a bucket of water. The blade should not cool down slowly in the air. Rapid cooling will help prevent some less desirable internal structures forming.
The second one hour temper will convert the new brittle martensite into tempered martensite. Again, cool rapidly in water.

There are battles to fight and battles to walk away from:
Any cooling event that does not involve a phase change should be cooled fairly quickly, not slowly.
Slow cooling isn't going to destroy the blade, or make a difference that you probably could even detect, but metallurgically it is the sound principle.
If you have always been air cooling your blades from temper or annealing go right ahead and keep on doing that if it is what you like to do. This isn't a big enough deal to matter.
 
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