My fault: M4 blade @ 64Rc breaks

Larrin said:
Multiple quenches are not always good, they are often damaging. Doing multiple austenitizing/quench treatments to a blade of tool steel, especially high speed steel which requires high temperatures, gives a "duplex" grain structure with very large grains. To combat this you must anneal the blade before re-heat treating. Not all heat treaters know this, and there is an unhealthy number of heat treaters and knifemakers that are willing to re-heat treat blades without annealing.

This fact, when combined with the hole, leads to breaks.
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Excellent, thoughtful reply that I think gets to the core of the issue here. And agree that the stress risers around the hole are part of the issue here.

Also, the re-heat treat probably warped the blade slightly, which resulted in the attempt to straighten it out, and the snap. The multiple pieces in the original pic point to a very brittle blade.

Thanks to Twindog for submitting this interesting and educational post.
 
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I did get a nice reply from the knifemaker. The blade was annealed. Here's what he said (I didn't see any prior crack, but I didn't look that closely):

Wow thanks for showing me the picture. I assumed that it was CPM M4 not M4. Hope that was a good assumption, if not could explain a break like that. With side force on the blade it is concentrated at the hole and as you know that is why it broke there. Was there any evidence of a crack or flaw? If you look at the end grain if there was a crack it should be visible. It would be darker with the new break a nice silver color like a fresh bead blast finish. Yes it was annealed prior to re-treat. Also I did not go to full heat (2150) but ran it at 2100 to be on the conserative side. I did 3 tempers at 1000 to get the final hardness. Rc 64 is as hard as a carbon steel file and if you put enough side force on a file with a big hole in it, it will break like that. We were pushing the envelope for max hardness so that is always a risk, to me and also to you. I appreciate you letting me know what happened.


 
Twindog said:
I did get a nice reply from the knifemaker. The blade was annealed. Here's what he said (I didn't see any prior crack, but I didn't look that closely):

Wow thanks for showing me the picture. I assumed that it was CPM M4 not M4. Hope that was a good assumption, if not could explain a break like that. With side force on the blade it is concentrated at the hole and as you know that is why it broke there. Was there any evidence of a crack or flaw? If you look at the end grain if there was a crack it should be visible. It would be darker with the new break a nice silver color like a fresh bead blast finish. Yes it was annealed prior to re-treat. Also I did not go to full heat (2150) but ran it at 2100 to be on the conserative side. I did 3 tempers at 1000 to get the final hardness. Rc 64 is as hard as a carbon steel file and if you put enough side force on a file with a big hole in it, it will break like that. We were pushing the envelope for max hardness so that is always a risk, to me and also to you. I appreciate you letting me know what happened.
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Thanks for the blow-off dumba**. If I ever need a blade heat treated I'll PM you and find out where NOT to get it done! :rolleyes:
 
Twindog said:
I did get a nice reply from the knifemaker. The blade was annealed. Here's what he said (I didn't see any prior crack, but I didn't look that closely):

Wow thanks for showing me the picture. I assumed that it was CPM M4 not M4. Hope that was a good assumption, if not could explain a break like that. With side force on the blade it is concentrated at the hole and as you know that is why it broke there. Was there any evidence of a crack or flaw? If you look at the end grain if there was a crack it should be visible. It would be darker with the new break a nice silver color like a fresh bead blast finish. Yes it was annealed prior to re-treat. Also I did not go to full heat (2150) but ran it at 2100 to be on the conserative side. I did 3 tempers at 1000 to get the final hardness. Rc 64 is as hard as a carbon steel file and if you put enough side force on a file with a big hole in it, it will break like that. We were pushing the envelope for max hardness so that is always a risk, to me and also to you. I appreciate you letting me know what happened.
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I don't think I would be sending anymore knives his way to heat treat. LOL

The least he could do is offer to make you a new blade I think. :thumbup:
 
If he was knowledgeable, he would have annealed it prior to re-heat treating, so I don't doubt he did. He's got a good point to take a look for the existence of any cracks or nicks at the locations the cracks initiated. Is that tempering sufficient for this steel?

I'm still not sure that there has to be a problem with heat treating, given the mode of failure and that blade design being particularly weak with side loads. The only sure way to find out what caused failure would be to do a metallurgical inspection of the blade - which is something that is done by heat treaters through the process, if the price of failure is high...
 
Benchmade was pretty nice. They said send it in and they'd put in a new blade for $35 (coated non-stainless). I don't know if they have M4, but anything is better than my current situation: a quarter-inch long blade with a safety tip. A little lacking for self defense
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You'll be happy soon when it come back with a new blade. Your fingers, cord, cardboard, envelopes and such won't know the difference of blade steels and heat treatment.

I hope you do send it back, they'll put as close to the original blade (or maybe they have one exactly the same) as they have on that baby and it'll be up & running again! :D
 
Hmmmm well My data sheet for CPM M4 says that 2200 F is the max austenizing temp not 2150, and that 1000 F is the bare minimum temp for tempering, 1025-1050 F is recommended and that quench from 2100 F, and temper at 1000 F should give you 63.5 Rc which is close enough to 64 Rc
 
Hmmmm well My data sheet for CPM M4 says that 2200 F is the max austenizing temp not 2150, and that 1000 F is the bare minimum temp for tempering, 1025-1050 F is recommended and that quench from 2100 F, and temper at 1000 F should give you 63.5 Rc which is close enough to 64 Rc
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I assumed that it was CPM M4 not M4.
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Read here! :)
 
I didn't take this as a blow-off at all. While it's no fun, things like this do happen when you live on the edge. If too many people that want to push the limits demand new blades, we're not going to see many knife makers willing to work with us any more, and that would be a shame.
 
I'd guess CPM M4 would break in the 30 to 40 ft lb category. Believe it or not that's higher than a lot of steels used for cutlery.

If there are stress risers or a weak spot, that just makes it easier.

That kind of force could be applied by a couple of fingers on one hand. Cheater bars or extentions or anything else aren't even necessesary .

Depending on where on the blade the force was applied I could see doing it by accident. An honest accident. not even one where someone got a bit mad :)
 
Is it customary to heat treat a blade without determining exactly what steel the blade was made from??? :confused: :confused: :confused:


Twindog said:
I did get a nice reply from the knifemaker. The blade was annealed. Here's what he said (I didn't see any prior crack, but I didn't look that closely):

Wow thanks for showing me the picture. I assumed that it was CPM M4 not M4. Hope that was a good assumption, if not could explain a break like that. With side force on the blade it is concentrated at the hole and as you know that is why it broke there. Was there any evidence of a crack or flaw? If you look at the end grain if there was a crack it should be visible. It would be darker with the new break a nice silver color like a fresh bead blast finish. Yes it was annealed prior to re-treat. Also I did not go to full heat (2150) but ran it at 2100 to be on the conserative side. I did 3 tempers at 1000 to get the final hardness. Rc 64 is as hard as a carbon steel file and if you put enough side force on a file with a big hole in it, it will break like that. We were pushing the envelope for max hardness so that is always a risk, to me and also to you. I appreciate you letting me know what happened.
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The Mastiff said:
Mostho, CPM M4 lives and excels at this hardness. I'd rather sharpen at this hardness than at RC 58-59.

That's what diamonds are for in any event.

Joe
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Joe, I mean is not necessary to have 64HRC on general as previously mentioned. Yes I know that CPM M4 has to live in higher hardness but also there are the other sides to consider that it begin a little brittler.Just personal thoughts.

Cheers
:D
 
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JNewell said:
Is it customary to heat treat a blade without determining exactly what steel the blade was made from??? :confused: :confused: :confused:
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It's not the heat-treater's responsibility to determine the steel composition. I know that if I mark my blades wrong, it's not Paul Bos' fault if the heat treat is off.

I recently received a batch of CPM M4 blades back from heat treat (Paul Bos). These also happen to be at 64 Rc. While noticeably hard, they don't seem to be overly brittle. While grinding and sanding, I put a decent amount of force on the straight razor I did, and it's fine (and much thinner than the blade in question). It's very possible that something was wrong with the Benchmade.

Phillip
 
The Mastiff said:
I'd guess CPM M4 would break in the 30 to 40 ft lb category.
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Crucible lists 20 lbs @ 65, 28 lbs. @ 63.5, 32 lbs. @ 62 (impact testing). This is probably why the competition cutters like their choppers at around 61.

PS- you still looking for something in PD#1?
 
Thanks you, Phil. I assume in this case that the blade was still clearly marked as CPM-M4, as it was when it left Benchmade?

Phillip Dobson said:
It's not the heat-treater's responsibility to determine the steel composition. I know that if I mark my blades wrong, it's not Paul Bos' fault if the heat treat is off.

I recently received a batch of CPM M4 blades back from heat treat (Paul Bos). These also happen to be at 64 Rc. While noticeably hard, they don't seem to be overly brittle. While grinding and sanding, I put a decent amount of force on the straight razor I did, and it's fine (and much thinner than the blade in question). It's very possible that something was wrong with the Benchmade.

Phillip
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JNewell said:
Thanks you, Phil. I assume in this case that the blade was still clearly marked as CPM-M4, as it was when it left Benchmade?
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I got the steel in sheet form from Niagara Specialty Steels, so any markings were removed when I ground the blades. When sending blades in for heat treat, I specify the steel type on the spec sheet and scratch it into the tang. Alternately, I could trace the blade profile on a piece of paper and mark it there. You can't just write on the blade because the ink wont survive the ovens.
 
This particular blade was clearly marked CPM M4, so no assumption was needed. But it appears that even in the best of circumstances, this steel at that hardness has little lateral strength, even without considering the hole.

The knifemaker who rehardened it asked if he could get the blade back for testing. Unfortunately, I had already sent it in to Benchmade. He made this additional comment:

If there was no original flaw or crack then it looks like from the picture that the blade was not sufficiently tempered. Maybe it would have been better (considering the large hole that serves as a stress concentrator) to go higher on the temper and drop the hardness at least one more point.
 
Twindog said:
I did get a nice reply from the knifemaker. The blade was annealed. Here's what he said (I didn't see any prior crack, but I didn't look that closely):

Also I did not go to full heat (2150) but ran it at 2100 to be on the conserative side. I did 3 tempers at 1000 to get the final hardness. Rc 64
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not a metallurgist myself so i can be completely wrong but my understanding was that if you aim a hardness level, like here around 64, a blade quenched higher and tempered more would be tougher than a blade quenched a bit lower with less tempering. it seems that the heat treater did the exact opposite than what i've been told.

once again i could be totally wrong.
 
Twindog said:
This particular blade was clearly marked CPM M4, so no assumption was needed. But it appears that even in the best of circumstances, this steel at that hardness has little lateral strength, even without considering the hole.

The knifemaker who rehardened it asked if he could get the blade back for testing. Unfortunately, I had already sent it in to Benchmade. He made this additional comment:

If there was no original flaw or crack then it looks like from the picture that the blade was not sufficiently tempered. Maybe it would have been better (considering the large hole that serves as a stress concentrator) to go higher on the temper and drop the hardness at least one more point.
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He should have known better and warned you off. Send him the bill for $35 plus shipping. :grumpy: I know, it's not worth, it but I wouldn't be so calm and cool with him over it. I can't experts that "aren't" or if they've clearly been party to a catastrophic failure within their filed of expertise, act like nothing happened...too bad (reading between the lines).
 
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