Which tool steel is most corrosion resistant? D2?

[Larrin]

Corrosion resistance of tool steels isn't studied much since they aren't designed for corrosive environments. However, knife makers are often interested in the relative corrosion resistance of these steels as there can be benefits to offering "some" corrosion resistance while also offering superior performance to stainless steels. Many knifemakers have advertised their D2 knives as being "semi-stainless." However, with the lack of information available, how does one know which tool steels are likely to offer better corrosion resistance? The "bulk" chromium content of the steel is not necessarily a good indicator because some of that chromium is usually tied up in chromium carbides.

Using thermodynamic calculations along with an austenitizing temperature, we can calculate a predicted amount of chromium in solution, which is a good indicator for relative corrosion resistance. The more chromium there is in solution, the more complete will be the passive film of chromium oxide that contributes to stain resistance, as shown below:

Another measure of corrosion resistance is pitting resistance, which is calculated with this equation:
PREN = Cr% + 3.3*(Mo%+0.5*W%) + 16*N%
However this equation can be somewhat misleading because molybdenum, for example, helps with pitting because it helps where the passive film breaks down, not by creating its own passive film. Crucible metallurgists used this equation after experience with 154CM (4% Mo) to design S30V and S110V with intentional additions of Mo. However, if the chromium oxide film isn't mostly complete no amount of molybdenum is going to make up for it. After all, we don't have any moly-only stainless steels. Therefore, in this case where these tool steels all have less than 10% Cr in solution, I think looking at chromium content in solution is most useful when predicting stain resistance.

Here is the chromium in solution I calculated, along with two stainless steels for comparison at the top:

As you can see, D2 does not have the highest chromium in solution! While A8 mod (chipper knife steel) is less commonly used, 3V is certainly pretty common, and it is predicted to have significantly more chromium in solution than D2. Another interesting note is that most of the chromium in 52100 is in solution rather than being tied by carbides, which is perhaps expected by its relatively low bulk chromium content. One potential caveat: if any of these steels are heat treated with an upper temper, a significant amount of chromium will be precipitated out as carbides reducing the corrosion resistance, which is why datasheets for stainless steels always say use the lower temper for best stain resistance. Therefore, 3V might only be superior to D2 if using a lower temper.

 

[Stacy E. Apelt - Bladesmith]

Thanks Larrin.

I have often said in trying to explain things like this that having an element in the mix, and having it available to do something you want are different. This is what the temperatures and times in HT can change.

I use the analogy of having a friend who is very rich, but a miser, and having a friend who is moderately financially comfortable, but very generous .... which one would you want to go to when you had a need?

 

[DevinT]

More good stuff Larrin, would love to see numbers on 154cm.

Hoss

 

[Larrin]

More good stuff Larrin, would love to see numbers on 154cm.

Hoss

For 2000°F austenitizing temperature, 154CM is 10.3% Cr and 3.33% Mo, and S30V is 10.8% Cr and 1.5% Mo.

 

[Willie71]

I’ve noticed that my low temper z-wear/cru-wear knives are basically stainless. After a year in the kitchen, no patina at all. I know they’re not truly stainless, and don’t have the chromium content to technically be called stainless, but functionally, they act as if they are stainless.

 

[Alpha Knife Supply]

Thank you for the information. The results are very interesting.

Chuck

 

[Justin Schmidt]

Love these threads. I hope they get added to the stickys.
Larrin can you touch a bit more on the chromium oxide film? This is the first I have heard of it. I've always heard the phrase "free chromium carbide is what relates to stainless" or something to that effect

 

[Larrin]

Love these threads. I hope they get added to the stickys.
Larrin can you touch a bit more on the chromium oxide film? This is the first I have heard of it. I've always heard the phrase "free chromium carbide is what relates to stainless" or something to that effect

With normal iron or steel, iron oxide forms (rust) which flakes off exposing more iron for more iron oxide to form. Other materials behave differently. Aluminum, for example, spontaneously forms a thin layer of aluminum oxide but unlike iron, this oxide layer is very strong and does not flake off. That oxide layer prevents further oxygen from penetrating the aluminum and therefore prevents further oxidation. Adding chromium to steel has a similar effect. If you add enough chromium to steel a strong chromium oxide layer is formed at the surface that prevents further oxygen from penetrating, just like aluminum with its aluminum oxide. It is called a passive film because it is not actively corroding after formation. The passive film is extremely thin and when removed will spontaneously reform. When you add carbon along with the chromium, carbides form with the chromium so that not as much chromium is present "in solution" or in the "matrix" so that the chromium is not available for forming the chromium oxide film at the surface. This can occur even with low carbon stainless steels at sources where carbon segregates such as grain boundaries leading to grain boundary carbides which is called sensitization, so great pains are taken to avoid this with many types of stainless steels.

Different heat treatments can lead to different amounts of chromium in solution, or can cause sensitization, so both heat treatment and steel selection are important for corrosion protection. Here are a few examples:
1) Higher austenitizing temperatures, in general, lead to higher corrosion resistance because more chromium carbides are dissolved so more chromium is in solution.
2) Upper tempers should be avoided since the increased hardness is achieved through the precipitation of small carbides including chromium carbides.
3) Different annealing treatments can lead to different sizes of carbides and therefore require different hold times to dissolve those carbides during austenitization. Smaller carbides are usually better though that also means harder annealed material.

 

[Justin Schmidt]

Ah ok that makes sense! So is there a direct correlation with amount of carbon and chromium content that we could predict stainless properties? For instance d2 has 1.5% C and 12%CR while 3v has about half those numbers at .8%c and 7.50%cr. However 3v has significantly more Mo at 1.3 vs d2 .8
Yet 3v is more stainless?

 

[Larrin]

Ah ok that makes sense! So is there a direct correlation with amount of carbon and chromium content that we could predict stainless properties? For instance d2 has 1.5% C and 12%CR while 3v has about half those numbers at .8%c and 7.50%cr. However 3v has significantly more Mo at 1.3 vs d2 .8
Yet 3v is more stainless?

See the table in the first post of this thread that calculates chromium content in solution for D2 and 3V at their mid-point austenitizing temperatures. Unfortunately it requires thermodynamic software to calculate it.

 

[Stacy E. Apelt - Bladesmith]

Any solution specs on BG-42?

 

[Willie71]

These are the best threads. I agree they should be stickies.

 

[Jrmysell]

A

These are the best threads. I agree they should be stickies.

Any way they could be combined into 1 and stickied?

 

[Justin Schmidt]

A

Any way they could be combined into 1 and stickied?

There's already a metallurgy sticky so I'm sure if decided upon they ould have their own larrin section within that sticky

 

[Larrin]

Any solution specs on BG-42?

With 2050F austenitize: 10.38% Cr and 3.38% Mo

 

[Willie71]

...snip...
3) Different annealing treatments can lead to different sizes of carbides and therefore require different hold times to dissolve those carbides during austenitization. Smaller carbides are usually better though that also means harder annealed material.

Is this why some steels are available in course spheroidized condition instead of fine? Ease of machinability?

 

[hugofeynman]

That’s why A8mod is one of my favorite steels, besides high toughness and some wear resistance, has good corrosion resistance. And some big players in the knifemaking world use it in their hard use Knives.

 

[Larrin]

Is this why some steels are available in course spheroidized condition instead of fine? Ease of machinability?

The point of annealing for commercial purposes is to make it as machinable as possible, which is achieved by coarsening the carbide size. I have never heard of a steel being offered in "fine spheroidized condition" until recently on this forum.

 

[Willie71]

The point of annealing for commercial purposes is to make it as machinable as possible, which is achieved by coarsening the carbide size. I have never heard of a steel being offered in "fine spheroidized condition" until recently on this forum.

I think chuck at AKS is the only supplier to offer fine spheroidized steel.

 

[kuraki]

How would you heat treat D2 for ultimate wear resistance? High end of the austenitizing temperature and low temper temp? There's no secondary hardening bump in high temp tempers with D2? So what would yield the largest carbide size?

Asking due to making radius platens from D2.