Broken WIP kitchen knives. Who is to blame?

eyeeatingfish

eyeeatingfish

Joined
Dec 9, 2003
Messages
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I am trying my hand at kitchen knives and today was not a good day, I lost 3 out of my 4 blades. Luckily 1/6" AEB-L is quite inexpensive! Please let me know if I screwed up, the heat treat screwed up, or its just the nature of the beast?

The blades were all 1/6th" AEB-L 8" chef size knives. Blades were profiled but no bevels ground. Knives were heat treated by a forum member who runs a heat treating business and has a good reputation from what I can tell. Knives were marked as being tested around 61.5 HRC +/-. Blades did have a bunch of very shallow circular and line dents which I think was part of the straightening process?

First Problem
1st blade I was grinding and I noticed it had some flex. I grabbed the tip and the handle and flexed it a bit, curious how flexible it would be. The tang (1/2" wide) snapped where it reached the blade despite not having put much muscle behind it. Knowing that AEB-L is a very tough steel I was very surprised it snapped that easily. Maybe at that hardness no steel flexes well? Certainly nothing like the 90 degree torture tests you see. I wondered if it was a bad heat treat? I took a broken piece of the blade which hadn't been beveled enough to have a rough edge and used a hammer to test the steel further. I hammered the blade through a 1/16" nail with no significant edge damage. I then hammered it on the edge of 1/4 plate steel and it really dug in, only chipping once despite many hits digging into the steel plate quite a bit. So now I am bot amazed at the toughness at the edge and at how easily it snapped.

Second Problem
While beveling 2 blades at about the 120 grit stage I noticed hairline cracks at the edge of the blade. I confirmed by flexing the blade just a little and could feel the crack with my finger nail. One blade had one crack almost a centimeter inwards, the other I noticed one on the edge and 3 on the spine. I know I didn't grind the blade too hot so I am wondering is this a heat treat problem, a steel supplier problem, or just the nature of steel at that thickness?
Thanks in advance!

Edited to add photos but they are from instagram stories so I don't know how long they will stay up.
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The first problem was called a stress riser. The 90° bend at the tang/blade junction made a place where a break would propagate easily. The issue could also be heat treatment related. Trying to flex at the thin joint will easily break the tang off.
The second issue is more troubling. Some reasons would be:
1) The steel was not AEB-L
2) Improper HT
3) Improper straightening

I would talk to the HT person and get his input on the problem.
 
There was a 90 degree angle where the tang opened up to the blade, I didn't think about rounding the edge there before heat treat but it sounds like it might have helped. I did try snapping the blade where it was wider. It required a lot more effort but I was able to snap them with my hand.

The steel was from Alpha Knife Supply, they are very reputable right?
I will reach out to the heat treater and see what he says about the small cracks. I have seen from other knife makers where they found such cracks in the edges going in a few MM and they described them as unfortunate realities so I kind of assumed it just happens sometimes.

Added photos to original post
 
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Is it possible they forgot to temper them?
Do you have any unbroken ones left? Put in your oven at rated temp
 
It is quite possible that the stress cracks in the blade came from the HT and straightening of the blade. It may have had a lot of warp and possible a bacon edge after the quench. Was it oil quenched? The blade may have been flexed/bent and then heavily peened to straighten it. The little divots are from a carbide peening hammer that was used to straighten it. There was obviously a big warp at the tang junction, and it looks like a lot of peening was done along the edge. I would suggest talking to the HT provider. He may want to examine the blade.
 
Another thing that can happen. If you profile you blade with an 80 grit or courser belt and leave those scratches, every one of the can cause a stress fracture at h/t on 1/16" stock.
Go around your blanks with a 220 grit belts with lines running parallel to the blade.
 
Looking at the first pic, it looks like the carbide hammer was used all along the edge area as well as the narrow tang junction. I leave my tangs much wider and with a bigger radius and then trim them down after grinding. Wider tangs give me more to hold on to while grinding as well! It's too easy to put a lot of stress in that narrow area when grinding!

I am not sure if the deeper straight lines are from a carbide straightening hammer, or leftover deep scratches from a belt sander/blanchard grinding? Either way, those could be stress risers that lead to cracking. Did you do any grinding on the steel sides before heat treating? If they are from a hammer, that's a pretty sharp carbide tip on a hammer and could cause stress cracks. If they are left over coarser marks from blanchard grinding the steel, they could also be stress risers.
 
I was the heat treater of these blades. I am trying to figure out what happened as we processed these just like we have done thousands upon thousands of AEBL blades. What’s weird is we heat treated these blades with other AEBL blades at the same time and thy are all fine.

I have never seen AEBL ever crack like this in probably the 5-10k we have done. I have seen some carbon steels do this. It honestly I can count the number of times I have seen cracked blades of any alloy on one hand out of all the years we have been doing this.

I have had some AEBL crack when straightening but it’s been .040 ulus or the round leather knives when thy are ran up really hard in the 63-64rc. I had a few I was making loose a tip then surface peening. We do not attempt any straightening until all tempering and hardness testing is finished.

I am not sure what is going on. We use gas shielding an do not foil wrap unless a blade is finish ground and polished. We also noticed because there is no envelope involved we could drop the austonite temp down about 40° from high 1900°s to mid 1900°s abd get the same hardness. We only use the higher temp if someone requests MAX hardness possible and at that point oil quenching is on the table as well. Even with that we have not seen cracks when oil quenching AEBL that is almost 2000°.

As for soak time it’s also less as we can judge blade temp better and a blade that thin takes 2min to reach oven temp and then it gets a 10min soak and straight to the clamps. So I don’t know if 1/16 is to thin to plate quench without foil but this would be the first time it’s ever been an issue. I did a thin test blade last night the same way I did the OP blades and I’m going to torture test it and see if I can recreate the issue.

He did state when he messaged me that he hammered some ground sections into metal and thy held up perfectly like I would expect with our heat treated these blades in the 61-62 range with very little edge damage.

On a side note we have gotten a few AEBL blades which are from AKS that have been acting weird lately. Never cracks but thy get this weird pattern on the surface and thy refuse to harden over 61rc. I even surface ground 3 of them and (bumped the temp up to our standard max hardness and did an oil quench) retested and the hardness was still low. It’s just been recently that I have started to see this pattern in the steel and it was 2 separate orders both from AKS. But thy were heat treated with other AEBL blades and thy did not show this pattern. So I’m not sure whats up as I have NEVER had any issues with anything from AKS.

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Here is the OP’s blade befor straightening. Thy did have a rather aggressive bow after heat treating. I have straightened blades bowed worse than this before without any adverse reactions. But the sad reality is that thin AEBL can be crazy to keep under control during heat treating. IMG_4195.jpegIMG_4196.jpegIMG_4197.jpeg
 
Jarod has done over 500 blades for me, straightened them like this, and I grind post HT. Not a single blade had this happen.
 
OP, can you get some pictures of the grain structure at the cracks? Do you remember the batch letters on your bars of steel from AKS or when it was purchased?

Those short, vertical scratches look weird to me? They look like really coarse belt grinding scratches, I wonder if the blanchard grinder was acting up?

Jarod, that patterning/crazing in the steel is really strange!!! Looks almost like ice cracking, wonder what is causing it or if AKS got some mislabeled steel or something??

I had some 3/16" deep warps across a 24" x 2" piece of 1/16" Magnacut, carbide hammer took them out after tempering, no cracks. Those warps look deeper over a shorter length, yikes!
 
Taz said:
OP, can you get some pictures of the grain structure at the cracks? Do you remember the batch letters on your bars of steel from AKS or when it was purchased?

Those short, vertical scratches look weird to me? They look like really coarse belt grinding scratches, I wonder if the blanchard grinder was acting up?

Jarod, that patterning/crazing in the steel is really strange!!! Looks almost like ice cracking, wonder what is causing it or if AKS got some mislabeled steel or something??

I had some 3/16" deep warps across a 24" x 2" piece of 1/16" Magnacut, carbide hammer took them out after tempering, no cracks. Those warps look deeper over a shorter length, yikes!
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Yeah 1/16” AEBL can get wild and .040” is even worse. We don’t do .040 any more as it’s so stressful.
 
I have posted some more pictures below. Without a macro lens the photos might not help that much but the breaks seemed to be a pretty even gray color, the grain structure looks fairly fine except for the broken tang which looks a little rougher.

I took the front sections of three broken blades and hammered them into a nail a dozen times. One blade chipped a couple times but the other two suffered no damage. They weren't all profiled the same so the chipping may be due to one having a thinner profile. The one with the more obtuse edge had the least damage. I also hammered them into 1/4" plate steel and they dug in quite a bit without damage although they all chipped badly when I tried to twist the blade while dug in.

To make things worse, the one blade I thought escaped now is in question. I examined the edges closely and found no problems but then found a 2-3mm long crack looking thing on the tang. I am buying a loupe to look at it closer but I definitely can't sell it if it is a crack. There is a photo of this one too.

The steel came from AKS with a very smooth finish. Not rough at all like most bars I get, not precision ground either, just like a smooth matte finish which I had never seen before in knife bars I bought but I had never purchased AEB-L or 1/16th" bars either so I don't know if this is significant. I do remember now that these bars were marked as being "blemished" so who knows if the blemishes meant something had been compromised. Luckily each bar of these 4 lost knives only cost $7!

This is making me think I should go up from 1/16" to 3/32" for kitchen knives. JT does have two other 1/16" blades I sent so we will see how those turn out. JT has been very good at helping me figure out what went wrong. This is my 4th or 5th time using him and I have never had any problems before.

TAZ
Good idea on keeping the tang wider and then grinding it down. I had sized them at 1/2" so I could use the dowel method to place them in the handle. The two marks are from two different straightening processes used by the heat treater. I don't have any batch information. If it was on the labels when I bought it the labels are long gone. Not sure what short vertical scratches you are referring to. I was grinding these blades at two different angles after heat treat. Started perpendicular to the platen to grind to the center part way up the blade but then I switched to grinding the blades length wise on one of the contact wheels as I felt I could view the grind angle better and keep it more consistent. That is why you see two different grind lines.

SBUZEK
That is a good idea, I would guess it isn't a problem 99% of the time but it sure wouldn't hurt to take out the deeper scratches to prevent rare instances.

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eyeeatingfish said:
I have posted some more pictures below. Without a macro lens the photos might not help that much but the breaks seemed to be a pretty even gray color, the grain structure looks fairly fine except for the broken tang which looks a little rougher.

I took the front sections of three broken blades and hammered them into a nail a dozen times. One blade chipped a couple times but the other two suffered no damage. They weren't all profiled the same so the chipping may be due to one having a thinner profile. The one with the more obtuse edge had the least damage. I also hammered them into 1/4" plate steel and they dug in quite a bit without damage although they all chipped badly when I tried to twist the blade while dug in.

To make things worse, the one blade I thought escaped now is in question. I examined the edges closely and found no problems but then found a 2-3mm long crack looking thing on the tang. I am buying a loupe to look at it closer but I definitely can't sell it if it is a crack. There is a photo of this one too.

The steel came from AKS with a very smooth finish. Not rough at all like most bars I get, not precision ground either, just like a smooth matte finish which I had never seen before in knife bars I bought but I had never purchased AEB-L or 1/16th" bars either so I don't know if this is significant. I do remember now that these bars were marked as being "blemished" so who knows if the blemishes meant something had been compromised. Luckily each bar of these 4 lost knives only cost $7!

This is making me think I should go up from 1/16" to 3/32" for kitchen knives. JT does have two other 1/16" blades I sent so we will see how those turn out. JT has been very good at helping me figure out what went wrong. This is my 4th or 5th time using him and I have never had any problems before.

TAZ
Good idea on keeping the tang wider and then grinding it down. I had sized them at 1/2" so I could use the dowel method to place them in the handle. The two marks are from two different straightening processes used by the heat treater. I don't have any batch information. If it was on the labels when I bought it the labels are long gone. Not sure what short vertical scratches you are referring to. I was grinding these blades at two different angles after heat treat. Started perpendicular to the platen to grind to the center part way up the blade but then I switched to grinding the blades length wise on one of the contact wheels as I felt I could view the grind angle better and keep it more consistent. That is why you see two different grind lines.

SBUZEK
That is a good idea, I would guess it isn't a problem 99% of the time but it sure wouldn't hurt to take out the deeper scratches to prevent rare t
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To tell if that’s a scratch or a crack you need to magnetize that blade. Take a strong magnet and drag it along the blade from tip and put ff the tang. Once the blade is magnetized go into your shop and find the finest grinding dust you can and sprinkle it over tgat area. If it’s a crack it will create a N and S pole on each side of the crack and the grinding dust will stick to the crack. If it’s just a scratch you will not see the dust stick to the line.IMG_4206.jpegIMG_4205.webp
 
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A friend I work with personally ordered “aeb-l” from aks some time last year or the year before that was not hardening correctly and cracking without him even straightening and I believe we figured out it may have been A2 I think it was the same 1/16” thickness, they eventually sent him a refund for the steel, I’d suggest reaching out and see if they can track that bar just to confirm it is aebl and rule that out of the equation. I plate quench and straighten 1/16” stock quite regularly and haven’t had any issues with cracking either aebl or nitro-v JTknives JTknives I’m not sure if you already do this but on identical profiles I clamp them as a stack between two straight pieces of angle iron and leave them clamped for cryo and both temper cycles before removing the clamps and that has drastically reduced warping especially on thinner stock.
 
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