CPM-M4 as a blade material : materials science thesis

[Cliff Stamp]

I decided awhile ago to clean up a reference page I had on steels as some of it is almost 20 years old and quite dated. I am taking it bit by bit and reviewing the materials data on each steel and putting it in a semi-sensible state. I am working on M4 now and turned up this reference :

- Heat treatment effects on CPM-M4 tool steel performance as edged blade material : https://ir.library.oregonstate.edu/xmlu ... 1957/54906" target="_blank

There are some interesting results and they directly answer some questions asked here on M4. For example it is known that M4 was too brittle in the BladeSports competitions until they adjusted the standard hardening. However you could ask is it better to adjust the temper or the soak? In general, very general it is better to adjust the soak - but that doesn't mean it works for everything. Well this paper addresses that as it looks at both adjustments and confirms that the soak adjustment produces the greater toughness.

There is also a very interesting point as it is one of the few materials papers which not only talks about knives but actually uses an ISO standard developed for knives - CATRA and presents data on M4 under varying HT protocols. What is very interesting is to look at the range of results or the precision/uncertainty. Remember now that this is a CATRA result, an edge retention trial by a machine on specific graded material. The results they got had a precision as low as 20%.

This means even a CATRA type test is +/- 20% and again that is a machine on graded stock. Now if you take away the graded stock and make that random, and you replace the machine by a person who varies speed, force, angle, and then you add in a very loose/subjective sharpness measurement then what do you think happens to that 20%. If it doesn't explode much higher - well then that data is likely manipulated by an external bias.

Here is some real hand sharpened data in comparison :



This isn't mine, it is a guy attempting to duplicate or at least parallel investigate some claims I made about steeling. This is real hand data with a careful sharpness measurement, just look at how much it scatters. However you can still draw some interesting conclusions, you just have to be very careful to not let your conclusions go beyond the ability of the data to support :

-edge retention is strongly non-linear

-white steel looks to respond well to steeling

[senorsquare]

Did you discuss M4 at all in your phone conversation with Gayle Bradley? I only ask because he mentioned in the video that they tried several heat treat recipes when they were developing M4 for the bladesports competitions and he might have some relevant input on this topic.

[Cliff Stamp]

Did you discuss M4 at all in your phone conversation with Gayle Bradley?

I was going to, I had a list of questions however I never got much beyond the first one because he provided too much detail for me to skip ahead. I ended up discussing a lot of small things I never really meant to get into simply because of the detail / experience he had with even small things. I happened to mention I use DMT products and prefer the older ones (big plastic bases). This lead to a discussion of tips which can be problematic with the old dot style (he is right) and the new continuous ones are better. This lead to a discussion of how tips are the hardest working part of the blade, hence why he really like the new ones and so on. I could have went on however it was getting very late and I didn't want to keep rambling at him.

[KevinOubre]

Very nice reading Cliff, thanks for taking the time to go through all that material.

[Ankerson]

One of the best ones I have seen in awhile actually and very complete and with actual lab testing.

Read through it yesterday, was very well done.

[JD Spydo]

OK that's great overall info even for us "non-metallurgists" :D But it was extremely interesting to say the least. However I think it sort of popped my bubble in regards to whether or not that M-4 would be a good blade steel for Spyderedges? From what I'm gathering or at least what I'm gather from that study is that it's intended to be a hard tool steel and completely unlike S-7 which I've always heard is great for axes and other striking edged tools.

Or can M-4 be made into a rugged/tough steel by different heat treatment for use on all serrated edges? I think I already know the answer but I just wanted to know what you think based on this study you just read?

I remember about 10 years ago or so at one of the Blade Shows in Atlanta when CPM did a seminar on M-4. They really touted it for fixed blades in particular from what I got from the presentation. Also I'm now wondering how it will stack up against "3V" and 10V for fixed blades? But from what I read and if I interpreted right I guess it's a dead issue for Spyderedges ?

[edge-e]

Excellent information on CPM M4. The best study I've read! Thanks for posting.

[Cliff Stamp]

From what I'm gathering or at least what I'm gather from that study is that it's intended to be a hard tool steel and completely unlike S-7 which I've always heard is great for axes and other striking edged tools.

Yes, that would be a fairly noncontroversial statement.

Or can M-4 be made into a rugged/tough steel by different heat treatment for use on all serrated edges?

Lets say you and I are talking about typing and I mentioned to you that my typing speed increased by 50%, a huge amount, after taking an on line course. Now do you know my typing speed, or even if I can type fast or slow? I could have went from 6 words per minute to 9 for example, still extremely slow.

There is a large change in toughness in M4 from the adjustment of the austenization temperature - but it was very low to start with so it ended up being still very low even though it increased by about 50%. This is the problem with relative arguments, you need some kind of reference or base point to start from.

What do we know, that ZDP-189 used by Spyderco wasn't a successful steel for serrations. ZDP-189 has a very high hardness, a very high wear resistance - but there were too many issues with damage and so Spyderco recommends H1 over ZDP-189 for serrated edges.

But, what we don't now is where is the tipping point exactly. Spyderco has used VG-10 in serrations for example. If we drag a graph and put VG-10 on one side and it is good for serrations and we put ZDP-189 on the other side as it is bad for serrations due to issues with toughness then we can ask at what point between them is the critical point passed?

I have used a fair bit of M4 in both customs and production knives, it is difficult to make such comments but I would be comfortable to say at least it would be to the left of ZDP-189 on that graph meaning it would be better. Now is it tough/durable enough so that fracture would not be the common mode of failure - good question.

Take your VG-10 knife in serrations and use it, now when you sharpen it how did it fail :

-did the edge roll

-chip or fracture

-just wear

Based on these answers you can look for an upgrade. For example if the edge doesn't significantly fracture and rolls and wears then M4 seems like a likely candidate as it has higher hardness and wear resistance and the only question is then have you passed the tipping point for fracture and this will depend on the serration pattern and its usage.

[The Mastiff]

Thanks for posting this Cliff. Also, good post to JD about needing a reference point. It's vital however people often miss it when trying to make points or explain things.

"Which is tougher, 440C or 154cm?" "I want a large EDC knife for hunting, hiking and bushcrafting"

You can tell them which is tougher or explain that neither is particularly tough and if you make it tough enough to baton it won't cut very well at all, then tell them which is tougher. :)

It's amazing how many people I see recommending steels very unsuitable for the tasks required of it.

Joe

[Cliff Stamp]

It's amazing how many people I see recommending steels very unsuitable for the tasks required of it.

I think we have to realize two things :

-lots of people will talk about their passions without being informed

-not everyone has the same passions

You can see a question like "Hey, what about X for a camp knife?" . If X is a steel like S90V then that question is like asking "Hey, what about an axe for filleting fish?" If that is a good choice then it must be an odd kind of filleting on an extremely large fish, similar a S90V camp knife implies you are camping in a cardboard factory maybe.

But to recognize that means that you are not only passionate/interested, you take the next step and learn about the steels on a fundamental level, not simply repeating what is popular, but digging into texts to get a ground up level of knowledge. Not everyone is going to do that, but a lot of people will still give advice. We have to realize that a lot of people simply don't take questions seriously, they are almost idle conversation.

I drink tea for example. I enjoy tea. I buy different kids of tea. However, and this is a big however to some people, I brew all my teas at the same temperature and it is whatever comes out of a Keurig. To some people this is a sin as bad as punching a baby in the face because different teas require different soak times at different temperatures (just like steels ironically) .

But for me it is just tea, so I will make tea using a too hot/cold brew and at a base level it is no different than making a D2 machete. You can still use/enjoy it and if you are not really particular you might even ask what everyone is spazzing about as if by asking for a D2 machete you were suggesting that we cook a homeless person for dinner.

[Ankerson]

I think it depends on what the person wants to do with the knife in the end.

Do they want something that they can break rocks with and pry apart car parts or do they want to cut stuff.

Geometry can have a rather large influence on performance.

[shunsui]

That was an interesting read. Thanks for posting that Cliff.

[Cliff Stamp]

More information on M4, conventional vs salt bath hardening : http://ppunch.com/pdf/heat.pdf

Salt baths have the ability to produce very fine austenite grain size, this can be a dramatic improvement as in reducing the austenite size by a factor of three from conventional very fine grain (9-10) to ultra-fine grain (11-12).



Kevin Cashen is one of the few makers advocating salt bath heat treating and is very focused on heat treatment in general, and utilization of metallurgy in blade smithing. It would be interesting to see a collaboration with him.

[JD Spydo]

More information on M4, conventional vs salt bath hardening : http://ppunch.com/pdf/heat.pdf

Salt baths have the ability to produce very fine austenite grain size, this can be a dramatic improvement as in reducing the austenite size by a factor of three from conventional very fine grain (9-10) to ultra-fine grain (11-12).

This is all extremely interesting>> and the difference between the conventional versus salt bath hardening is huge when you look at it on that graph. But to get down to the bottom line let me ask you this??? How does properly heat treated M-4 stack up against let's say VG-10 ( the way Spyderco does VG-10 that is). Or is M-4 so brittle/hard that only a plain edged, fixed blade could be made to be workable?

BEcause from what I'm gathering M-4 for all intent and purposes wouldn't be a great blade steel for serrated edges at all?? But if it's a great steel for plain edged blades only then I guess that's not necessarily a bad thing at all except it would just limit it's viable uses?

[bluntcut]

OK, you can stop teasing us now Cliff. A 4 inch diameter test piece, 1hr soak at 2250F and gas quenched. It isn't very applicable to a blade, doesn't it?

A 1/8" blade will gets to 2125F in about 1 minutes, add a few more minutes soak -> pressure gas quench. would yield similar result as (D). Rather than massive grain growth(due to 2250F too long but necessary because of large diameter) and carbide aggregation(due to slow cooling) depicted in (C).

In knife perspective, (D) doesn't look that impressive with quite a few large clump of carbides (white blobs).

[Cliff Stamp]

OK, you can stop teasing us now Cliff. A 4 inch diameter test piece, 1hr soak at 2250F and gas quenched. It isn't very applicable to a blade, doesn't it?

That is the counter argument brought up immediately in regards to using salts on knives, they are so thin that time at heat isn't the critical problem which also applies to the pre-soaks for the same reason. The problem is that there isn't a lot of direct data showing the difference and it is a odd argument to make for a maker/manufacturer which is of the type "good enough".

The argument isn't that salt heat treating is worse for knives, or that it isn't better, is that it is "good enough" without it as the difference it makes isn't significant. The "good enough" argument can always be used and in fact practically it might even be true but it prevents a lot of manufacturing claims and who really likes to argue their product isn't the best but that it is "good enough".

In regards to the carbide volume, M4 is basically a high carbon/high carbide version of M2 but what exactly is it being compared to? I find the MBS-26 that Spyderco uses for example to have a very high ease of sharpening at a high sharpness but this is only because of the over abundance of knives in ATS-34 class steels. It doesn't have that perspective if all you have used is AEB-L and 52100, in fact it has the opposite.

If you look at M4 vs ATS-34, S110V it looks different than if you compare it to 13C26/Nitrobe 77.

[Cliff Stamp]

... M-4 for all intent and purposes wouldn't be a great blade steel for serrated edges at all?

This is both a very simple and very difficult question at the same time because while the question seems simple, and it is, it is fairly undefined. Consider this question :

-I have an application which requires a specific level of strength, wear resistance and toughness, is M4 a good choice?

Now your first response to that is - I don't know, how much strength, wear resistance and toughness are you talking about? This then is the question that you have to ask :

-how much strength, toughness and wear resistance is required for a "good" serrated knife

This is going to depend on :

-the type of material cut
-how it is cut
-the serration pattern
-the demands of the user

As these are not that easy to quantify, what we are left with is comparisons, look at how serrated knives you have used performed and then based on the relative properties of M4 make some inferences.

For example, have you used ZDP-189 in serrations and found that the tips were too easy to break, well that style of serration isn't like to lead to positive performance with M4. But if you did like the way ZDP-189 performed then M4 could be a sensible choice.

The main issue is going to be due to the high carbide volume, if you get a few of those carbides close together in the tips it can make a very easy fracture line. This can also be dealt with my making serrations which have much more rounded points.

[bluntcut]

LOL, "good enough" is just another way, there is a better way "trust me". Ironically there isn't much data to shows/supports "better way". Basically for knives - it come down time elapse to bring a blade up to aust temp. If I plop a 1/8" thick blade into oven pre-heated at 2150F, it will take ~ a minute to hit that temperature. Whereas with salt pot it takes half the time.

In turn point out the difference in soak time, will there be huge different between 4 vs 4.5 minutes (for 5 minutes soak) or 9 vs 9.5 minutes(for 10 minutes soak)? So aha, this is not the key advantage of salt pot. It's the fast temperature ramp to aust, avoiding M23C6 formation, which could reduce grain nucleation points(even when m23c6 dissolve and reform as m6c3 during soak)- right? If so, is 30 seconds ramp time difference the key point to "good enough" vs "better way"? Would powerful inductance even be a "better better way" :D

As for quench, interrupted oil will be faster than molten salt, hence not much to tout for either camp.

I am not saying salt pot doesn't has advantages over air/vacuum for many applications but mainly object to 4" diameter test piece as why salt pot is superior. When test piece is 1/64" or thinner, salt cocoon will be un-even and probably will distort the piece, while no such problem with vacuum.

OK, you can stop teasing us now Cliff. A 4 inch diameter test piece, 1hr soak at 2250F and gas quenched. It isn't very applicable to a blade, doesn't it?

That is the counter argument brought up immediately in regards to using salts on knives, they are so thin that time at heat isn't the critical problem which also applies to the pre-soaks for the same reason. The problem is that there isn't a lot of direct data showing the difference and it is a odd argument to make for a maker/manufacturer which is of the type "good enough".

The argument isn't that salt heat treating is worse for knives, or that it isn't better, is that it is "good enough" without it as the difference it makes isn't significant. The "good enough" argument can always be used and in fact practically it might even be true but it prevents a lot of manufacturing claims and who really likes to argue their product isn't the best but that it is "good enough".

In regards to the carbide volume, M4 is basically a high carbon/high carbide version of M2 but what exactly is it being compared to? I find the MBS-26 that Spyderco uses for example to have a very high ease of sharpening at a high sharpness but this is only because of the over abundance of knives in ATS-34 class steels. It doesn't have that perspective if all you have used is AEB-L and 52100, in fact it has the opposite.

If you look at M4 vs ATS-34, S110V it looks different than if you compare it to 13C26/Nitrobe 77.

[Cliff Stamp]

In turn point out the difference in soak time, will there be huge different between 4 vs 4.5 minutes (for 5 minutes soak) or 9 vs 9.5 minutes(for 10 minutes soak)?

That is the question, the same benefit is there, it is just reduced - thus the question is what is the gain? But again you are faced with an argument not that your knives could not be improved, but that they are "good enough" without it. For customers, that argument is a hard one to take especially when you face competition which argues (which is perfectly true) that he has a better process. Your response is that you use an inferior one sure, but it is "good enough".

Now to be frank, everyone does settle at some stage for "good enough" as there is always something further you could do. The question in reality always come down to how much does the improvement cost vs what is gained. Why don't you mirror polish blades for example, it is hard to argue from a performance perspective it is reduced but mirror polishing is very expensive compared to stain or even much more coarse finishes so it is common to leave blades "good enough".

It's the fast temperature ramp to aust...

Yes but you don't need salt for that, the blade can be equalized at temperatures below critical, hence why Landes for example has two equalizations in the ramp to the soak temperature which keeps it at the minimum time.

[The Mastiff]

Yes but you don't need salt for that, the blade can be equalized at temperatures below critical, hence why Landes for example has two equalizations in the ramp to the soak temperature which keeps it at the minimum time.

In reading over data sheets for steels over the years I've come across situations where that was recommended usually in thicker pieces in industrial uses. In practical speaking how does Landes do this? One oven and it just ramps up with the oven itself? I've seen Salt pots that were at different temps and they moved the blade from one to another but for the one electric oven knife maker it takes some time for the oven to hit those really high temps.I suppose the easier to heat treat steels will remain in the lead just as the easier to finish steels keep guys trying to eke out a living from straying too far.

By the way, this is another good thread. Hopefully I won't side track it. It's got me paying attention. Thanks Cliff and Bluntcut.

joe