Another perspective on Steel composition

[fujita yuji]

Powder steels are easier to predict as they follow theory more closely. In a powdered steel the alloys are finer and more evenly spaced, so they dissolve more readily. You typically have more carbon dissolved in a CPM version so you end up with a little higher hardness than the wrought version of the steel, and strong carbide forming elements like Mo/W/V prevent Cr from forming carbides by taking up the carbon (as theory predicts). Thus CPM-D2 ends up with much more free Cr than D2, and is basically a stainless steel, which you can see by looking at the composition (same for CPM154). Whereas regular D2 is less corrosion resistant than what the composition implies. In this way powder steels are much easier to interpret by looking at the composition.

Wow this is an informative post. Thanks as usual, cotdt.

 

[me2]

42Blades - Thanks.I'm trying to keep it easy to read/understand, not being metallurgist myself.

ME2, in your reactivity list, where does the Cobalt stand? I found Cobalt in another reactivity series chart, where it was between Ni and Cr. But I can't deduce where it will be in relation to Mn ans Si.

Ni
Co ???
Si
Mn
Cr
Mo, W
V
Ti
Nb

Also, I have carbide forming temperatures as following:
V - 1100 - 1200
Nb - 1300 - 1400
Cr - 1400 - 1800
Mo - 1200 - 1400
W - 1400 - 1600

Is that correct? I thought Nb carbides for at lower temperatures than V.

I'm not sure where cobalt stands. If its near nickel, then it basically doesn't form carbides, as Si and Ni don't (I know SiC exists, just not in steel). I've never read about cobalt carbides, so I'm guessing there aren't any. Mn doesnt form carbides either, but it occasionally replaces Fe atoms in the Fe3C cementite carbide.

I assume your temperatures are for the element's native carbides? My reference doesn't take much about NbC exept in the section on microalloyed sheet and structural steels. The precipitation temperature given there is about 1600 deg F. However, these steels have very low concentrations of Nb and C. With blade steel carbon contents and higher content of Nb, the temperatures may well come down into the range you have there.

 

[cotdt]

Wow this is an informative post. Thanks as usual, cotdt.

ME2, in your reactivity list, where does the Cobalt stand? I found Cobalt in another reactivity series chart, where it was between Ni and Cr. But I can't deduce where it will be in relation to Mn ans Si.

Ni
Co ???
Si
Mn
Cr
Mo, W
V
Ti
Nb

Also, I have carbide forming temperatures as following:
V - 1100 - 1200
Nb - 1300 - 1400
Cr - 1400 - 1800
Mo - 1200 - 1400
W - 1400 - 1600

Is that correct? I thought Nb carbides for at lower temperatures than V.

Reactivity with carbon increases from right to left on the periodic table:
Ti > V > Cr > Mn > Fe > Co > Ni > Cu

It also increases downward though there may be some exceptions.

The alloys are in the form of carbides before the heat treat but these carbides dissolve in the austenitic matrix at very high temperatures. As an example, here's an image from Verhoevan's book:

Nb carbides would dissolve even less readily than V carbides. They pin down the grain boundaries, so that the grain size remains very fine.

 

[Gator97]

I'm not sure where cobalt stands. If its near nickel, then it basically doesn't form carbides, as Si and Ni don't (I know SiC exists, just not in steel). I've never read about cobalt carbides, so I'm guessing there aren't any.

I've searched for cobalt carbide, most of the sources mention cobalt carbide nanostructures, which I am assuming is for now very far from knife blades, but in general it does exist - ref link. I guess we can leave it out for knives.

Mn doesnt form carbides either, but it occasionally replaces Fe atoms in the Fe3C cementite carbide.

Is that something desirable or the opposite.

I assume your temperatures are for the element's native carbides?

I think so. Can't find the link since yesterday, I was researching reactivity, copied materials into separate file, but no original link

Reactivity with carbon increases from right to left on the periodic table:

Thanks! That is pretty good rule

 

[Larrin]

Cobalt generally doesn't form carbides, either does nickel.

 

[me2]

I'm not aware of any additions to carbides that makes them softer, so Mn getting into the Fe3C is probably good. Mn mostly disolves in the austenite, or gets tied up with sulpher, so there's not much to get into the carbides anyway.

 

[Nilladaspilla]

I'm not sure if Mn can get into Fe3C (cementite) unlike austenite, ferrite, and I believe pearlite, it is stoichiometrically locked Fe3C. A phase diagram for CPM steels would be really interesting to see what percent of each phase is dependent upon the cooling and atomic composition.

 

[cotdt]

I'm not sure if Mn can get into Fe3C (cementite) unlike austenite, ferrite, and I believe pearlite, it is stoichiometrically locked Fe3C. A phase diagram for CPM steels would be really interesting to see what percent of each phase is dependent upon the cooling and atomic composition.

Not sure what you mean. Carbide forming elements can substitute for Fe in cementite, they are carbide formers after all. You can have Cr substituted cementite in 52100 or W subsituted cementite in Super Blue. These carbides are much harder than plain cementite, and finer than the Cr and W carbides in higher alloy steels. Only small amounts of alloys can substitute before you form other crystal structures and formulas though.