Thanks to Phil Wilson for become a Knife Steel Nerds Patreon supporter. We have now reached our second goal of funding a toughness study! I will start putting together the plan and discussing it with the patrons. Next goal: funding an edge retention study.

Cobalt in Steel

There is a lot of misinformation out there about the effects of cobalt on steel, particularly when it comes to VG-10 and N690. Generally cobalt is added in certain high speed steels to improve “hot hardness” which is the ability to maintain hardness at high operating temperatures. Therefore its addition to N690 and VG-10 is a bit of a mystery since those are not high speed steels. Adding to the confusion, there are reports from some people that cobalt improves toughness, or, as Spyderco says [1] in their summary of cobalt: “Intensifies the individual effects of other elements in more complex steels.” What in the world does that mean?

Hot Hardness and Tempering Resistance

Since hot hardness is the main reason a cobalt addition is made to steel, I think it makes sense to start here. An introduction to the mechanisms of tempering can be read here: What Happens During Tempering of Steel? It was found that a cobalt addition led to higher hardness with high tempering temperatures in high speed steels [2]:

The cause of this effect was mysterious for some time, as cobalt does not form carbides, but since then several investigations have been conducted. The effect has been attributed to a few different mechanisms, but I will stick with the simplest and most often cited: suppression of recovery [3]. In the tempering article that I linked to above I showed this plot:

During tempering the martensite decomposes meaning that it recovers and finally recrystallizes which softens it. The precipitation of Mo, W, V, etc. carbides at the higher temperatures leads to precipitation strengthening and that leads to an increase in hardness despite the recovery of martensite. These small carbides precipitate preferentially at the dislocations that are recovering during tempering. The change in dislocation structure was also described in my tempering article and shown schematically where the black swirling lines represent dislocations which decrease in density with increasing temperature moving left to right:

Cobalt contributes to tempering resistance by slowing the recovery of martensite and therefore there are more dislocations present to act as the sites for nucleation of precipitates. With more sites for precipitation there is a greater density of precipitates for precipitation strengthening and also stronger martensite even without precipitates, as can be seen with a low-alloy 4340 steel that doesn’t have sufficient Mo or W for significant sencondary hardening [2]:

So when the cobalt has a significant effect on tempering even with the low-alloy 4340, I think it shows that the dominant effect is to slow down recovery of martensite. There are some other theories involving accelerating diffusion of molybdenum or tungsten, or suppressing growth of carbides by cobalt diffusing out of carbides into ferrite [2], etc. but I think in general things can be understood in terms of recovery. In the plots for both M2 and 4340 above, however, the hardness is the same without tempering and therefore I think we can rule out “solid solution strengthening,” in other words, cobalt doesn’t increase hardness directly, but through tempering resistance.

Toughness

There are some claims I have seen that cobalt increases toughness, so I spent some time investigating this effect. In high speed steels a cobalt addition does not increase toughness, even when tested at the same hardness, as can be seen in this plot where JYPS30 is nearly identical to JYPS23 but JYPS30 had 8.5% Co [4]:

But that is with high speed steels and in this case a relatively high cobalt addition. So I had to find other sources for using low temperature tempering and with lower alloy steels. Luckily someone already did a review of the literature on this for me [5]. Unfortunately he found that cobalt decreased toughness in almost all studies he investigated, and it seems to act in the opposite way that nickel increases toughness. In fact, the only reason that the high cobalt maraging steels have high toughness properties is because they have sufficient nickel to overcome the negative effects of cobalt.

Corrosion Resistance

The effect of cobalt on corrosion resistance doesn’t seem to have been investigated much, and the results are mixed. In steels that are given a high temperature temper, cobalt accelerates the precipitation of chromium and molybdenum carbides and therefore those elements are not available for contributing to corrosion resistance [6]. Therefore, cobalt decreases corrosion resistance with high temperature tempering. In low alloy steels (little to no chromium), however, cobalt has been found to decrease corrosion rates in sulfuric acid, but does so by forming a dense rust layer [7][8]. In conclusion, it’s possible that cobalt may increase corrosion resistance for certain types of corrosion but I wouldn’t count on it.

Effects on Austenite

Cobalt is one of the only elements that has been reported to raise the martensite start temperature [2], which also raises the martensite finish temperature. For high carbon and high alloy steels the martensite finish temperature is often below room temperature which is why low temperature treatments are required to eliminate retained austenite. Therefore cobalt additions may help to minimize retained austenite in steels that will not be given cryo treatments or high temperature tempering. However, the effect is relatively small with a 1.5% cobalt addition. Using a method that I described in a forum post [9], I estimate that VG-10 with a 1080°C austenitize would have 16.9% retained austenite without cobalt and 14.1% with the cobalt addition.

At high temperatures, however, cobalt stabilizes austenite [10]. In the PM steel S110V, it has a high amount of “ferrite stabilizing” elements like chromium, vanadium, and molybdenum, so ferrite is present even at very high temperatures. Full hardness cannot be achieved unless the steel transforms from ferrite to austenite so that the steel can be quenched to form martensite, see What Makes Quenched Steel so Hard? They found that with a cobalt addition full austenite was formed at high temperature [10]. Nickel and manganese have a similar effect but they also make annealing more difficult.

N690 and VG-10

So I think we’ve gotten enough background on cobalt to talk about N690 and VG-10, now. Here are the compositions of the steels I will be covering, along with some JMatPro calculations:



N690 has a similar composition to 440C/N695 but with an addition of Co and a small bump of Mo. It is a similar story with VG-10 and 1.4111, but they both have 15% Cr instead of the 17% Cr steel in the 440C grades. 19C27 is yet another ~1.0% carbon steel but with 13.5% Cr and no Mo or Co. In the chart above I also included JMatPro estimated carbon in solution (controls hardness), chromium in solution (controls corrosion resistance), and carbide volume (more means higher wear resistance but lower toughness). These estimates align well with reported hardness of 440C, VG-10, and 19C27 (in order of increasing hardness) and also their carbide volumes as seen in micrographs (440C, N690, and 19C27 from Sandvik and 1.4111 from Roman Landes [11]):

N690

440C

1.4111 [11]

19C27

You can see that with decreasing chromium content the amount of carbides (light grey or white particles) is less and the largest carbides are smaller. You can also see that the cobalt addition to N690 had little effect on the carbide structure when comparing with 440C, as we would expect based on everything you learned about cobalt in this article. Therefore, because these steels all have approximately the same carbon content, their properties are greatly controlled by their chromium content. Higher chromium means higher corrosion resistance and wear resistance, but also lower toughness and peak hardness.

Effect on Tempering

The addition of cobalt and the small bump in molybdenum means that that N690 has greater tempering resistance than N695/440C [12][13]:

N690 has a higher recommended austenitizing temperature than N695 which is the likely explanation for the higher initial hardness of N695; however, there is a clear secondary hardening bump with N690 that isn’t present with N695. That is due to the higher cobalt and molybdenum for the reasons that I covered before. Cobalt suppressed the recovery of martensite for higher hardness with tempering, and also increased the effect of molybdenum with secondary hardening. This is what the Spyderco article means by saying that cobalt increases the effect of other elements. The suppression of recovery leads to enhanced secondary hardening which is an effect that occurs due to other elements, especially Mo or W.

Why cobalt was added to VG-10

The Takefu website says the following about the effect of cobalt on the properties of VG-10 that they produce [14][15]:

Promising stainless blade steel with Co addition for enhancing substrate toughness and better grinding. Addition of 15% of Cr, 1% of Mo and 1,5% of Co makes the matrix (substrate) stronger and prevents carbides from dropping out. VG10 also has quadratic effect in high-tempereature (sic) tempering that it is ideal use in cutting tools which are given surface-coating at up to about 450℃.

The first sentence is on the list of grades page and the second two are on the page dedicated to VG-10. I believe that the first sentence is a mistranslation or a mistake, as that sentence does not exist in the Japanese language version of the site. It is very similar to the second sentence but “toughness” was replaced with “stronger” and “better grinding” was replaced with “prevents carbides from dropping out.” As I covered before, cobalt does not increase toughness, but “stronger” makes some sense based on tempering resistance. However, the “better grinding” and “prevents carbides from dropping out” doesn’t make much sense. Cobalt does help suppress the coarsening of tempering carbides therefore minimizing their size and reducing the possibility of their dropping out. However, it is the coarse primary carbides that are more likely to affect grinding and I don’t know how cobalt would affect that. I think that is marketing BS. The third point is true; the cobalt addition gives the steel higher hardness with high temperature tempering and therefore allows it to be used with surface coatings that require high temperature application. That is the most likely reason for adding cobalt.

VG-10 vs N690

N690 is sometimes promoted as an alternative to VG-10 because they both have a 1.5% Co addition and a small vanadium addition. However, as I showed in the micrographs above it is the chromium content that most greatly controls the microstructure of these steels. I would see N690 as an alternative to 440C that has some tempering resistance and secondary hardening. I would not see it as an alternative to VG-10 unless the property you want to match is secondary hardening. N690 and VG-10 are different in terms of hardness, corrosion resistance, wear resistance, toughness, etc. 1.4111 would be a closer match to VG-10 in terms of those properties though as far as I know it is not regularly available in the USA. 19C27 does have some availability and despite having somewhat lower corrosion resistance and wear resistance would offer better toughness, hardness, and edge stability than either N690 or VG-10.

[1] http://www.spyderco.com/pdfs/steelbrochure.pdf

[2] Chandhok, V. K., J. P. Hirth, and E. J. Dulis. “Effect of cobalt on tempering tool and alloy steels.” Trans. ASM 56 (1963): 677-692.

[3] Speich, G. R., D. S. Dabkowski, and L. F. Porter. “Strength and toughness of Fe-10Ni alloys containing C, Cr, Mo, and Co.” Metallurgical Transactions 4, no. 1 (1973): 303-315.

[4] Moon, H. K., K. B. Lee, and H. Kwon. “Influences of Co addition and austenitizing temperature on secondary hardening and impact fracture behavior in P/M high speed steels of W–Mo–Cr–V (–Co) system.” Materials Science and Engineering: A 474, no. 1-2 (2008): 328-334.

[5] Garrison, Warren M. “Cobalt and the Toughness of Steel.” In Materials Science Forum, vol. 710, pp. 3-10. Trans Tech Publications, 2012.

[6] Peissl, S., G. Mori, H. Leitner, R. Ebner, and S. Eglsäer. “Influence of chromium, molybdenum and cobalt on the corrosion behaviour of high carbon steels in dependence of heat treatment.” Materials and Corrosion 57, no. 10 (2006): 759-765.

[7] Ina, Katutoshi, Kazuo Yamamoto, Hirokichi Higashiyama, Norio Ishikawa, and Hiroshi Miyoshi. “High tensile steel and process for producing the same.” U.S. Patent 4,407,681, issued October 4, 1983.

[8] Kim, Keon Ha, Seung Hwan Lee, Nguyen Dang Nam, and Jung Gu Kim. “Effect of cobalt on the corrosion resistance of low alloy steel in sulfuric acid solution.” Corrosion science 53, no. 11 (2011): 3576-3587.

[9] https://www.bladeforums.com/threads/liquid-nitrogen-vs-dry-ice.1540810/

[10] Kajinic, Alojz, Andrzej L. Wojcieszynski, and Maria K. Sawford. “Corrosion and wear resistant alloy.” U.S. Patent 7,288,157, issued October 30, 2007.

[11] Landes, R. “Messerklingen und Stahl.” Aufl. Bad Aibling: Wieland Verlag (2006).

[12] http://www.bucorp.com/media/N695_Data_Sheet.pdf

[13] http://www.bucorp.com/media/BOHLER_N690.pdf

[14] http://www.e-tokko.com/eng_original_list.htm

[15] http://www.e-tokko.com/eng_vg10.htm

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