My fault: M4 blade @ 64Rc breaks

T

Twindog

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First, it’s all my fault. No sour grapes. No complaints. No fault of Benchmade.
But.
My AFCK in M4 was never a great cutter. The blade profile was thick. The M4 didn’t sharpen well. I really noticed it when compared to the Ritter RSK M4, which gets very sharp, very easily and cuts like a champ.
I saw where a pro-sharpener felt Benchmade M4 is on the soft side. So I checked out the hardness on my AFCK and it came to 59Rc. I’d read several comments from forum members who recommended M4 at 64Rc. So I thought I’d try it.
I had the blade rehardened to 64Rc and was going to send it to Tom Krein for a thinner blade grind, like the Military. I thought M4 would be the steel to handle this combination, and I’d end up with a normal-use EDC that cut like crazy and held an edge forever. But, dang, this was a hard blade. It felt different. It sounded different. I tried to power buff off the ragged finish with green stropping compound. Nothing. I tried black stropping compound. Nothing. I tried hand sanding with 2000-grit sandpaper. Nothing. I tried 1,500-grit sandpaper. Nothing. I tried 600-grit sandpaper. Nothing. I gave up.
Then I reinstalled the blade in the handle so I could try sharpening it on diamond hones. The blade ended up off center, despite a lot of installation tinkering, so I put some lateral force on it, not too much, maybe 20 pounds, maybe less, the way I usually fix an off centered blade. Snap. Three pieces.:eek:
A learning experience. Have had worse.
 
sounds like the reheattreat was defective. all the cutting contests use m4 & those guys put the knives thru torture. there is a new post by Diaz in the testing section that shows what m4 can take.i think you would appreciate the bradley from spydie.
 
Hi,

I agree with Dennis, not a good heat treat. It happens. sometimes there isn't anything you can do about it. I've had most kinds of tool steel not come out well from heat treat. Do enough, often enough, and a bad day catches up to you.

dalee
 
The competition blades are 61-62 Rc, according to some old posts, but there may have been a HT issue. M4 isn't tough at such a hardness, just as tough as other steels at several points lower on the scale. It also broke at the hole, a weak spot for the blade. I snapped an 806D2 at the tip with some light prying.
 
Nice knife, and bummer. Did you find all the pieces? I think you just discovered why makers are afraid to sell a lot of m4 64 HRc blades! Depending on how and where you had the blade secured (did you clamp the blade on the flat?) and how big of a moment you had on it (clamped near the pivot and with a 6" vice-grip on the flats?), and your 20 lbs being possibly 40 lbs (I am a guy who has broke many things in my life), I could see it breaking pretty easy.

The opening hole is ny favorite opening design, but it made that blade weaker than it would have been without a hole.

Unless there were little stress risers (little nicks or scars in the steel) in the thumb holes, I think that double break is indicative of an extremely hard & brittle material. It may be the angles of the breaks also indicate that a torque was applied. Either that or I don't know what I'm talking about.
 
A lot of steels have a very precise heat treat and a couple dozen degrees difference can change the hardness a point or two. Was the blade verified to be 64 HRC, or that is just what the heat treat was supposed to put it at? I would be curious and have it tested again to double check it is 64. That sounds like it might be maxed out and for M4 that could be closer to 67, I think. The Gayle Bradley is supposedly coming in at 65 HRC and yours sounds harder from your description and how mine behaves cleaning and sharpening it.

I have thought about doing the same thing to a Rift and this makes me question actually doing it.
 
Though below is in reference to shafts, I think it would apply to this failure also.

THE EFFECT OF STRESS CONCENTRATIONS ON A FRACTURE FACE

If a part is relatively lightly stressed, the cracking will start at only one point and the result will look like one of the examples above. However, if a shaft is more heavily loaded, then cracks can start in several places and work their way across the part.

Realize that the "work their way" refers to something that occurs at near the speed of sound! Though no one would think of this as heavily loaded, the blade I think disagrees!

Above is an excerpt from:

http://www.plant-maintenance.com/articles/rcfa.shtml
 
It was tested at 64Rc, so that's all I know. It broke in my hands. No vice. No cheater pipe. Just my hands. It was a fair amount of torque, but I was going slow and being careful because I was worried about the pivot. I never thought the blade would break in my hands.
 
You can see the hardness testing dents on the blade near the handle.

If you look at the middle piece, the break split off in a lateral direction, leaving a sculpted, concave/convext surface. There were two vertical breaks, in addition to the break above the hole.

This was a very hard, very brittle piece of steel. Without tools and leverage, I should not have been able to break this blade.
 
I think the design with the large hole in the blade presents an area where stress would be likely to occur.
No matter what the steel is if the heat treat was done correctly with the proper tempering afterward it should not have broken with hand pressure. Not even at 64HRC
A blade must be hard, ductile and abrasion resistant to perform at its peak this can all be controlled by proper heat treat.

Fred
 
Sorry to hear that Twindog, yeah, it's always a bummer when things like this happen. They call it the bleeding edge for a reason, but still.

I know it's hard when you put a lot of time and money into a blade, then something like this happens.
 
How thick is the blade that you were going to send it off to thin it down?
Looks pretty thin already from the photo.

Where it broke is the only place I could see it breaking.
 
Twindog said:
It was tested at 64Rc, so that's all I know. It broke in my hands. No vice. No cheater pipe. Just my hands. It was a fair amount of torque, but I was going slow and being careful because I was worried about the pivot. I never thought the blade would break in my hands.
Click to expand...

Thanks - so not a lot of force did it, but yet the failure is indicative of a "heavy loading" failure. Obviously it could not take a lot of the loading you were giving it. And Mr. Rowe is right about the hole being the place of highest stress.

The one "Kreined" knife I have is a mini-AFCK, and it cuts like a demon. I'm sorry - should not have told you that! :foot:
 
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2brothers said:
How thick is the blade that you were going to send it off to thin it down?
Looks pretty thin already from the photo.

Where it broke is the only place I could see it breaking.
Click to expand...


It's actually just a bit thicker than the Military, but it has a sabre grind rather that a full flat grind. I was going to have just a full flat grind, so it would have been in the neighborhood of a standard Military.

There were two problems with it as a cutter: it had a thicker profile than better cutters, such as the Military or RSK, and the steel did not sharpen well. The blade grind was going to solve the first problem and the rehardening was going to solve the second.

It is M4 steel. Benchmade did a limited run. If I remember, it was 250 each of plain edge and partial serrations.
 
Sounds like the Heat treament/Tempering was done wrong.

It shouldn't have snapped that easy.
 
Hi Twindog -

That is too bad about your knife.

I would have thought it could handle what you did to it.

Thank you for sharing the story and posting your picture.

best regards

mqqn
 
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