My fault: M4 blade @ 64Rc breaks

FlaMtnBkr said:
I have thought about doing the same thing to a Rift and this makes me question actually doing it.
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Speaking of M4 rift blades, could I order a M4 replacement blade from BM for a Rift? I can;t find a complete knife anywhere. Sorry for the thread derail.
 
Twindog said:
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.
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I've spent about 9 hours with 220 and 400 grit sandpapers on 64HRC M2 steel, which is a less wear resistant than M4. Still, after all that time I gave up too, and the result was marginal at best. Lesson learned, polish or sandblast etc before rehardening.

Sorry about your knife, I went through the same route few times, reharden, regrind, resurface. Hard to see a blade like that broken.
I suspect the hole had big part in it. 710 M2 blades at 64 and 64.5 HRC are holding fine for me, and so is the 64HRC M4. Although, I can't recall putting any lateral forces on them on purpose. Those super hard blades are not meant for prying.
 
shelbysguns said:
Speaking of M4 rift blades, could I order a M4 replacement blade from BM for a Rift? I can;t find a complete knife anywhere. Sorry for the thread derail.
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No. They will only allow a replacement if you already have a M4 knife, and that is assuming they have any left which I think is doubtful. I have asked them that question and they don't do it because everyone would be trying to get a high end replacement blade for their regular knife and they would run out of spare parts very quick. I also heard that the last 50 piece M4 run was using spare replacement M4 blades from a previous run of Rift that had carbon fiber handles. That probably pretty much depleted their inventory if true.
 
that is not good, and strange too. you should tell the Heat treater and see what he thinks. If I were in the buissness of HT blades I would want to know when something went wrong, so I can improve my process
 
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.
 
A few minor comments.

First, thanks for bringing this here and subecting yourself to all the scrutiny and risk of second-guessing and all the rest. I usually learn better when things go wrong, but it's easier to learn when things go wrong for someone else - sorry and thank you again. :(

Second, the (ahem) Spyderhole itself shouldn't be the fatal flaw. It's probably a factor, but lots of blades have holes of various sizes and descriptions and we don't read about failues there with any frequency, so it seems logical that the primary factor was the heat treat.

ThomBrogan's point about the structure of the elements in the blade would be more persuasive to me if this were not one of the CPM-process blades. I guess that's a question, though it's worded like an assertion. (Sorry.) I may be wrong, but I thought the whole point of this process was to more evenly distribute the elements in the finished product and reduce grain structure?

Best luck and thanks again - sympathies about the knife.
 
JNewell said:
ThomBrogan's point about the structure of the elements in the blade would be more persuasive to me if this were not one of the CPM-process blades. I guess that's a question, though it's worded like an assertion. (Sorry.) I may be wrong, but I thought the whole point of this process was to more evenly distribute the elements in the finished product and reduce grain structure?
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i have seen broken test blades made in XC75 (probably close to 1080, iron, o,75% C and impurities that's all even the finer ht'd m4 is, i assume,way coarser than that.) with rice sized grain structure. even a very fine grained steel can have a coarse structure when poorly heat treated.
cpm process makes the steel finer than the equivalent melted steel with the same alloying elements with the same proper heat treatment.
 
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JNewell said:
ThomBrogan's point about the structure of the elements in the blade would be more persuasive to me if this were not one of the CPM-process blades. I guess that's a question, though it's worded like an assertion. (Sorry.) I may be wrong, but I thought the whole point of this process was to more evenly distribute the elements in the finished product and reduce grain structure?
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you can grow/reduce grain through thermal cycling. I think one of the main benefits of PM is carbide size, not grain.
 
The CPM process also contributes to a small grain size. However, as I already mentioned, heat treating a high speed steel twice can have disastrous consequences for the grain size, so it hardly matters which method was used to produce the steel.
 
Larrin said:
The CPM process also contributes to a small grain size. However, as I already mentioned, heat treating a high speed steel twice can have disastrous consequences for the grain size, so it hardly matters which method was used to produce the steel.
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Interesting - thanks.
 
Personally I think the hole had some contributory value to the failure. In my humble opinion I think putting a medium sized to large hole in a perfectly good blade utterly defies logic and common sense. I understand the whole spyderco thing, so don't start, I said in my humble opinion. I know Sal is a knife god around. This picture reenforces my belief that these large holes in the blade will just magnify any under lying problems which may otherwise gone unnoticed. With the drive to keep taking steels further and further in terms of hardness. Things like large holes in the blade are going to present more and more problems as lateral pressure is going to become more and more of an issue.

There is something to be said for toughness over hardness, I know right tool for the right job, etc., etc., but as the old saying goes in a perfect world. Under EDC and real world conditions a folder is going to be used where lateral pressure gets placed on the blade and not always as a direct result of misuse as it relates to prying.
 
Well something happened besides just the hardness of the blade and the hole. The Spyderco Gayle Bradley uses the same steel and has been reported to be harder (65 vs 64 for this one. 1 point doesn't sound like much but it is a log scale). It is also a full production knife and not a sprint run so there should be lots of them in use and I have not heard of the first failure. It also has the dreaded hole in the blade.

A correctly heat treated blade, even a very hard one, can take a lot more abuse than most think. M4 is also a tool steel that is probably more tough at 65 than some of the higher end stainless steels at 61. A metal that is designed to cut other metal in a high speed cutting machine has to be pretty tough to stand up to that kind of use.
 
Please, don't misunderstand what I said. I was not trying to say that this is going to start happening all the time, and after re reading my post can see where it came off as I was trying to imply that. Rather that I do think your going to start seeing more of this kinda thing, not all over place but that it is going to become more of issue taking stainless steels to these really high hardness levels. Florida I do agree with you that proper heat treat protocol solves all kinds of problems including the ability to derive great toughness even at high RC's and that tool steel is tougher than stainless goes without saying. I think the blades with large holes in them are going to be more suceptible to failure especially if their are other underlying problems such as improper heat treat, accidental improper heat treat, bad run of metal, etc.
 
Seems like something happening due to the heat treatment, or a problem with the heat treatment, is the most plausible major issue?
 
We also need to remember that this is not so much a toughness issue. The knife was not impacted, but flexed & broke. Depending on the load & dimensions, I would certainly expect a 65 Rc piece of steel to break instead of bend, but also depending on the load & dimensions, I would expect it to be perfectly fine.
 
As a starting point I would ask the heat treater if the blade went through an annealing cycle before it was rehardened and tempered. If he says no then I believe Larrin may have the answer to the most likely cause of the fracture.

If I was a heat treater I would want to know that because ultimately I am sure that most reputable heat treaters would always want to improve any potential deficiencies in their heat treating methods.
 
alex said:
As a starting point I would ask the heat treater if the blade went through an annealing cycle before it was rehardened and tempered. If he says no then I believe Larrin may have the answer to the most likely cause of the fracture.

If I was a heat treater I would want to know that because ultimately I am sure that most reputable heat treaters would always want to improve any potential deficiencies in their heat treating methods.
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OK, Larrin seems to have the most plausible explanation. I sent a photo of the blade to the knifemaker and asked him about the annealing. If I get an answer, I'll post it.
 
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