Cobalt in the steel...

[me2]

Yes but not anything specifically on this testing. The variables listed on the chart, hardness and steels, are the least influential of the ones being discussed with respect to CATRA edge holding tests. It pays to be sure everything else is the same if it's not already stated.

 

[Larrin]

I agree the superior measured edge retention of N690 vs 154CM is somewhat unexpected. The test is largely the result of hardness of the steel and the hardness and volume of its carbides when the other factors are kept constant. It's weird that the chart lists both steels as 61+. Does that mean almost 62? Why the plus? While cobalt can strengthen the matrix, this difference would likely be irrelevant when the steels are tempered to the same hardness.

To look at the potential difference in carbide hardness and volume, I did thermodynamic calculations with JMatPro of the carbide content of N690, N690 without cobalt, and 154CM. Here are the results:

N690 without cobalt:
2000F - 12.47% M23C6
1960F - 13.3% M23C6
1920F - 14.28% M23C6

N690:
2000F - 12.72% M23C6
1960F - 13.54% M23C6
1920F - 14.28% M23C6

154CM:
2000F - 11.07% M23C6
1960F - 12.18% M23C6
1920F - 13.2% M23C6

A couple of caveats: 1) These are models not experimental measurements. There is no guarantee they correlate with reality. 2) These are calculations for thermodynamic equilibrium, something we never actually reach in reality. Often the carbides present in these high alloy steels are formed at high temperatures during casting and never dissolve. In other words, they are not the carbides that would be calculated thermodynamically.

With that out of the way, we can analyze the results. N690 has around 13% more carbide volume than 154CM at the same austenitizing temperature. Furthermore, since N690 datasheets recommend lower austenitizing temperatures than 154CM, it is possible that it has even more carbide volume. The 13% volume difference with 154CM is pretty close to the 17% increase found in the CATRA testing. The thermodynamic calculations predict very little difference in carbide volume with the cobalt addition. Therefore, we would expect little difference in CATRA testing if N690 was made without cobalt, and differences between N690 and 440C are probably primarily due to differences in Cr and Mo. To test this conclusion, I also ran the thermodynamic calculations for N695 (440C):

2000F - 6.16% M7C3
1960F - 6.92% M23C6, 3.37% M7C3
1920F - 11.65% M23C6, 0.88% M7C3

As expected, the carbide volume of 440C is significantly lower.

In conclusion, thermodynamic calculations support the result that N690 has superior edge retention to 154CM. There are always concerns with testing, of course, we don't know how much statistical variation there is in the CATRA test or if multiple runs were used to give us an average result. Superior edge retention does not always mean a superior steel, of course. And superior CATRA results do not always mean greater edge retention depending on the cutting application.

 

[DevinT]

Why does the m7c3 carbide volume go up with higher austenitizing in the 440c?

Hoss

 

[Alpha Knife Supply]

Larrin, thank you for the information. The carbide percentages are interesting. I wish I had access to JMatPro. :thumbup:

Regarding Rockwell hardness 61+, the test coupons were harder than 61 and less than 61.5 so the hardness was listed as 61+.

Chuck

 

[Larrin]

Why does the m7c3 carbide volume go up with higher austenitizing in the 440c?

Hoss

That carbide is thermodynamic ally preferable at that temperature, so with eno ugh time it predicts that one carbide would dissolve while the other precipitates and grows. On a practical level it would be unlikely to happen based on the time to austenitize and the kinetics of carbide dissolution and precipitation.

 

[Chad Michael]

Thanks guys for the great info. This is the stuff that sucked me into knifemaking more than anything else.

 

[stezann]

A stainless steel has amounts of RA, with Co has less RA, the rest follows unsurprisingly

 

[Larrin]

this is from the 'Cobalt in Steels' article referenced above:

Co is the only one alloying element that increases the critical cooling rate of steel and accelerates pearlitic transformation thus reducing hardenability.
Co is unique among alloying constituents in steel in that it is the only element that has negative effect on the hardenability of steel by accelerating the decomposition of austenite.

I forgot to mention that this statement is not true. Aluminum has the same effect. Aluminum also raises Ms. Of course, one is usually on shaky ground when stating that "only X" has one effect of another.

 

[jdm61]

I had a Hattori "forum knife" 270mm suji in VG-10 and it was quite nice, but I have heard that the quality of the heat treatment with VG-10 and I would assume N690 can vary from so-so to really good. The Hattori knives are supposedly shaded toward the really good end of the spectrum. So what is the "secret sauce" require to get the cobalt steels done right?

 

[mete]

I really don't have much to add! the term for 'enhances properties ' is "synergistic effect"
My early search for N690 was confusing as there are a number of types . The one with cobalt should be written N690Co. The knife I have with N690 [don't know which type ] is made for TOPS by Fox of Italy -a very satisfactory blade ,like VG-10. Hardness for the TOPS was higher than a similar knife fro military use .Secret sauce ? Maybe the 2 extra point hardness is it .
I wish some of the knife companies would give more technical info to show exactly why it's being used !!
Sorry I can't help you more .

 

[jdm61]

Yes, that extra couple of points of hardness has been the secret sauce for some other common steels.

I really don't have much to add! the term for 'enhances properties ' is "synergistic effect"
My early search for N690 was confusing as there are a number of types . The one with cobalt should be written N690Co. The knife I have with N690 [don't know which type ] is made for TOPS by Fox of Italy -a very satisfactory blade ,like VG-10. Hardness for the TOPS was higher than a similar knife fro military use .Secret sauce ? Maybe the 2 extra point hardness is it .
I wish some of the knife companies would give more technical info to show exactly why it's being used !!
Sorry I can't help you more .

 

[stezann]

I may sound silly, but i believe that given that Co rises Ms & Mf it will lower the RA, thus the hrc point higher and following claimed performance....i can't see any mistery

High alloy steels like stainless have to deal with the RA compromise. Anything, from cold treatment (cryo...) procedures to special ingredients (Co...) will only reduce the RA to reasonable percentages. The less RA in the final product wins

 

[Natlek]

M2 steel is 63 - 64 Hrc .Same composition + cobalt = M35 and 67 Hrc ? And M35 is much more resistant to rust then M2 .