The Chromium/Carbon Ratio

Would the development of a chrome/carbon ratio in blade steel alloys be useful?

I wonder if extrapolations from it might be used to establish, perhaps, a "stainless" rating system or any of several other predictive comparisons

For example, let's begin with an assumed analysis of one heat of 154CM. If this heat has 14.50% Cr and 1.10% C, said ratio could be expressed as 14.50/1.10 or, more simply, 13.18.

A typical heat of 440A would have 18.00% Cr and .75% C, resulting in a ratio of 18.00/.75, or a rating of 24.

This seems to reflect the known relative "stainless" properties of each alloy. To wit: the higher the rating the more rust resistant, 13.18 vs 24. (440A is known to be exceedingly stainless, generally.)

I thought I was onto something, until I began to consider the highly alloyed steels.

A typical heat of CPM S90V would reflect 14.00 Cr, and 2.20 C resulting in a ratio of 14.00/2.20 or a rating of 6.36. Comparing that to 154 CM (6.36 to 13.18) suggests that 154 CM is more than twice as rust-resistant as S90V, which it is demonstrably not.

Where does this go wrong? I feel like a one-armed paper hanger. Once I nail down where the formation of Chromium Carbides ends, and the distribution of free Chromium begins, I lose my hold on any prediction of the behavior of other alloying elements, such as where the formation of Vanadium Carbides begins.

Is it possible to determine how much Carbon, by %wt., is typically used in the formation of Chromium Carbides, notwithstanding the Iron Carbides? Where does free chromium begin? How much does Carbon favor bonding with Chromium over Vanadium?

I worked on this for six months, but never got any further. I wanted to lob it over here, to see what you guys thought. It may be such a ratio isn't useful at all. I hope, rather, that I am overlooking a unifying line of reasoning which will be obvious to you.

I don't think that the ratio of Chromium to Carbon is relative to anything else unless only Chromium and Carbon are present.

As soon as other elements are present that will form carbides as well the whole hypothesis breaks down. Consider if you will Stellite 6K which has 30% Chromium and about 1.4% Carbon. The key to this metal's performance is the presence of tungsten, molybdenum, and vanadium which produce much more wear resistant carbides than chromium.

We come to now is determining what carbides are more wear resistant and how are they distributed in the matrix.

This could get complicated unless someone has a degree in physics with a minor in metallurgy.

All well received, Mr. T. and I considered Stellite 6K and Talonite for this ratio. It is still the preponderance of free Cr in the formula, which makes those "stainless," alloys, too.

I was interested in ranking all alloys, first, in terms of "Stainless-ness," since that is the one characteristic that always seems to be the primary thing customers want in a blade.

I guess the ratio would not be very useful after all, but I wanted to hear opinions from people who think about blade steel at least as much as I. Thanks!

A lot of what you're trying to determine is mitigated by other elements in the mix, as well as how the steel is made, and even how it is tempered. Vanadium, Tungsten, Molybdenum all form carbides and compete with Chromium for the available Carbon as does the Iron. What ultimately becomes carbide and what remains as free Chrome is an elusive number. That's why steel companies have developed tests (actually I think the government developed the tests) to determine corrosion resistance. Even though they design the steels to be corrosion resistant, they really don't know how corrosion resistant until they measure it.

A good example is S30V, which appears to be more corrosion resistant than even 440C in actual tests, with just 14% Chromium and 1.45% Chromium. Powder metallurgy, 4% Vanadium, and 2% Molybdenum change everything. In the end it has less Chromium Carbide than 440C which has 17% Chromium.

Right, but we have entire classes such as the 14-4's and the later CPM steels that haven't been submitted to empirical testing, that I can find anywhere. Testing for rust, and exposure to blood and animal fat, I mean.

Re: the mitigating forces of the secondary alloying elements. It seems clear that it is not likely one can NOW predict "stainless," properties, after HT, from the nominal chemistry. But do you think it might be done, eventually?

Supposing we developed an axiom for predicting behavior before any applied HT, and then adjusted for a standardized treatment later?

Each of you does this, subconsciously, even when you send out your blades for HT. You start with an assumption that your BG-42 or S90V will have certain minimum characteristics of corrosion resistance, and design the HT to to optimize the others around that. I hoped to find a way to make it conscious, or disseminated to everyone else.

So a reliance upon the Cr/C ratio is fallatious. Is there a way to reliably make these predictions at all?

It's really pretty tough, Sam. I've seen huge differences between various sources of ATS-34, 154CM, and RWL34 - all having nominally the same formulation and heat treating regimen. On top of it all, the way in which the steel takes a finish greatly influences the corrosion resistance.

At the end of the day, corrosion resistance is among the least of my concerns when I make a knife. I'd really like to better know how to make a sharper edge tougher...

Yeah, this is a snipe-hunt. Thanks for spending some time on it, though.

Gentlemen,

this is the very reason i got into metallurgy as my career field because i use to and still have all these questions to answer. i'm a metallurgy student currently and all of you are on the right track. if you were to hold all but two variables equal then mess with the alloy you would get pete results. if you add an additional variable then you no longer have pete results but fred results.

there are good properties that the powder metallurgy contain that simply in my humble opinion run cirlcles around the conventional steels as far as edge holding ability.

in regards to the Cr/C ratio there are many many variables and alot of it has to do with how much matensite you want to form, what organic percipitate you want to form, and things i'm sure i ain't gotten edumacated on yet.

nedozier@utep.edu

Nathan, as you keep learning I hope you'll keep sharing. I agree on the powder metallurgy issue, and I think Crucible's angle on that with cold precipitation and particle sizing lead the pack. The new S30V is a great steel.

Thanks for sharing.

What a great threads that discuss the science behind the blades! Hope to continue to see alot of this information posted in CKD!

This is what makes this the superior knife forum!