Cobalt in the steel...

timos-

timos-

Knifemaker / Craftsman / Service Provider
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Am I wrong in thinking that most steels commonly used for knives do not have any cobalt?


It seems to me that the newest darling steel in kitchen knives is Hitachi's HAP-40:

They are claiming working hardnesses of 65+ on the RC scale.

How does cobalt interact with the other elements and does it change anything about heat treating it?

It looks like Crucibles Equivalent is Rex 45. HAve any of you guys made knives out of this stuff?

hap%2040%20composition.png
 
To my knowledge, which is limited to the application of cobalt HSS alloys, it's mostly useful for raising heat resistance or hot hardness of steel. I don't know that it makes a tool (or knife) able to hold a higher working RC, or that it's room temperature wear resistance is any greater than other wear resistant alloys.

This is why I believe it's generally not found in knife steel.

IE: good in a drill bit where heat is created, just added expense in a knife.
 
I would have to look it up at home tonight where I have a whole book on cobalt. But, IIRC, the cobalt carbides are larger, and besides the high temperature resistance, have no gain over the other hard carbides of tungsten (wolfram when I was young), molybdenum, and vanadium. I also seem to remember that Co-C-O2 is much more brittle than the other carbides.

One of the metallurgists will probably chime in with better info.

If you look at that alloy graph, you see that there is 25% carbide elements and 1.5% carbon. With .85% going into the steel, that leaves very little to form the other carbides. It won't be stainless. The vanadium will grab that first (IIRC). This will put a lot of elemental cobalt and tungsten in the matrix. Elemental cobalt is just barely harder than iron. Tungsten is 500 times as hard as cobalt ( mohs 5 vs 7.5). Good for temperature resistance, but I am not sure about toughness/wear gain over other knife alloys. There may be some gain, but there is also a point where it really doesn't matter.

Hereare some charts that may help:
http://www.zknives.com/knives/steels/steelgraph.php?nm=W2 ,HAP40, S35VN&hrn=1&gm=0
http://www.simplytoolsteel.com/knife-steel-comparison-chart.html
 
Last edited:
N690 & VG10 both have cobalt.

The addition of cobalt gives both steels a significant increase in edge holding over steels like CPM154 & 440C.

Tbl--Elements.gif
Tbl-N690.gif
Tbl-VG10.gif
Tbl-CPM154.gif
Tbl-440C-Bohler.gif


Chuck
 
I'm no metallurgist, but I was amazed at how well VG-10 performed in my F1 when my buddy and I field dressed and skinned a black bear; it performed as well as Carbon V of my buddy's blade, held an edge beautifully throughout despite that gritty hide. Takes longer to sharpen than high carbon, though that's not a problem with diamond hones. Fallkniven claims that the addition of cobalt to the steel enhances the properties of other alloying elements. VG-10 was created in Japan, I believe for horticultural use, i.e., pruning tools.
 
I've also heard that it magnifies properties of other allowing elements. At higher amounts cobalt reduces the amount of retained austenite during ht.
It's used in hss to increase hot hardness. It also adds some corrosion resistance. Not sure about the affects on edge retention. Would like to know more if someone with more knowledge could chime in
 
To my knowledge, cobalt isn't a significant carbide former and will mostly be found dissolved in the iron matrix and will strengthen it. It is an austenite stabilizer and is partly used for that purpose. Many element in steels are ferrite stabilizers and push the temperature needed for austenite higher, sometimes too high. Cobalt helps keep these temperatures down to a reasonable level, at least as high speed steels go.
 
here is a pretty good description of using cobalt as an alloy http://ispatguru.com/cobalt-in-steels/. HAP-40 is drill bit and end mill steel, similar to M35 and M42. It was the "Steel of the Week" spring last year, Chef's Knives to Go still carries two knives in this steel. I think getting a piece to work with would be as easy as getting a piece of Blue#2.
 
me2 said:
To my knowledge, cobalt isn't a significant carbide former and will mostly be found dissolved in the iron matrix and will strengthen it. It is an austenite stabilizer and is partly used for that purpose. Many element in steels are ferrite stabilizers and push the temperature needed for austenite higher, sometimes too high. Cobalt helps keep these temperatures down to a reasonable level, at least as high speed steels go.
Click to expand...

I have several knives from M2 and M35 steel / basucly M2 HSS steel + cobalt/ .I do not know if it is up to hardness or added cobalt / M2 - 64Hrc and M35 - 67Hrc / but M35 is much better at keeping sharpness and resistance to chipping .And I have a feeling that it is easier to sharpen M35 on 67 hrc then M2 on 64 Hrc .............
 
Hey chuck,
It's kind of off topic but any chance we will see some maxamet in your inventory in the near future?
 
Ok, so cobalt acts like a glue in the matrix? soft but tough
 
Chad Michael said:
I've also heard that it magnifies properties of other allowing elements. At higher amounts cobalt reduces the amount of retained austenite during ht.
It's used in hss to increase hot hardness. It also adds some corrosion resistance. Not sure about the affects on edge retention. Would like to know more if someone with more knowledge could chime in
Click to expand...

^this

It raises Ms & Mf
 
Those CATRA results are missing a fair bit of information. It's not possible to say N690 is superior to 154CM based on those CATRA results alone.
 
All the test coupons were ground to the same thickness, grit, bevel angle and edge thickness. The only variable was hardness. In your opinion, what is missing?

Chuck
 
Is that chart from a larger article? No mention was made of angle, grit, number of tests, initial sharpness, etc. on the chart.
 
me2 said:
Is that chart from a larger article? No mention was made of angle, grit, number of tests, initial sharpness, etc. on the chart.
Click to expand...
I got a copy of the test results. The testing was done blind. CATRA did no know the steels being tested. Each coupon was marked with a letter.

As I wrote earlier:
All the test coupons were ground to the same thickness, grit, bevel angle and edge thickness. The only variable was hardness.
Click to expand...

CATRA sharpens the coupons to ensure the initial sharpness is the same.

Did you read any of the information from the CATRA website?

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

Influence of Co on steels
Solubility of Co in the ? iron is up to 75 % while in the ? iron it is unlimited. 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. It has a tendency to graphitization and is a very expensive component, hence it is not used as an alloying addition in normal steels. It is never used in the standard heat treatable steels. Co is, however, a constituent of maraging and 9Ni-4Co ultrahigh strength steels but here, its presence is overcome—from a hardenability standpoint—by the remaining alloying constituents.
Since Co has a negative effect on hardenability, it affects the hardening less than other elements.
The presence of Co in the steel improves its durability and hardness at higher temperatures, reduces the fall in hardness of austenite and ferrite under the influence of temperature increase, and therefore is used as a supplement to some grades of high speed steels and tool steels. It is also a component of creep resistant steels.
Co is a valuable alloying element for high speed tool steels. It has the effect of raising to softening temperature of ferrite so that tools made from Co bearing alloy steel can operate at high temperatures, maintaining their cutting capacity.
Co has few, but highly specialized, uses in alloy steels. Its behavior is similar to Ni, in that it forms a complete series of solid solutions with iron at elevated temperatures and is also extremely soluble in ferrite. It is a potent ferrite strengthener; this solid solution strengthening persists to quite high temperatures, and hence Co is used in several grades of high speed tool steels, among others.
Co is ferromagnetic like Ni and has magnetic permeability.
Co increases hardness and allows for higher quenching temperatures during heat treatment. It intensifies the individual effects of other elements in steels with complex compositions.
Co is not a carbide former. However addition of Co to alloy steels allows for higher attainable hardness and higher red hot hardness.
The Co matrix has a high-work hardening sensitivity, which combines with the carbide fraction and allows to achieve excellent wear resistance associated with a high degree of corrosion resistance.
The effects of Co additions on the mechanical properties and strengthening mechanisms of martensitic precipitation strengthening stainless steels, whose composition is 0.005 % C/12 % Cr/5 % Mo/1.5 % Ni with Co levels of 9 %, 12 %, 15 %, 18 %, and 21 %, shows that hardness, yield strength and ultimate tensile strength increases as the Co content increases, while the charpy impact energy decreases as tempering temperature increases

a good element to add if you are making a drill bit or end mill or broach, problematic if making a knife
 
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