Knife Steels Special Request

Rob_Mob

Rob_Mob

Joined
Oct 16, 2013
Messages
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Fellow Hogs,

I'm working on a special project for a client. I wanted to know if any metallurgist or experienced knife owner would be able to compile 2 lists for me. The first list would be of blade steels that don't have issues rusting. The second list would be a list of blade steels that do have a tendency to rust. If possible, could someone please attempt to rank each list in order of more likely to rust/rusts quicker to less likely to rust/rusts slower. I would be immensely grateful for the assistance. Once again thanks in advance and hope you're all doing well.


-Rob Abyane
 
If you’ll allow me, had a few thoughts on this subject I thought I’d share while you wait for an expert reply. I’m certainly not one, and the following is just off the top of my head, so I welcome corrections from those in the know:

All carbon-iron alloy steels rust, including “stainless” steels, if immersed in salt water/salt spray for long enough. There are blade materials that do not rust, such as cobalt and titanium alloys, ceramics, etc., but the only ferrous-based alloy I know of that does not is H-1 steel used by Spyderco in its “Salt” series. Here’s what they say about it:

“H-1 … is a nitrogen-based blade alloy rather than carbon-based. The elimination of carbon in the steel renders it one-hundred percent rust proof…”

Mission Knives used to have photos on their website from test results they ran on several stainless blade steels alongside their titanium knives, showing that with salt spray and salt water immersion, all would rust and pit extensively--ATS34, 440C, and some other popular knife steels. Titanium does not rust. Neither does Stellite 6K or 6B or Talonite, all of which are 50+% cobalt, 25-30% chromium, along with other alloying elements.

I also know from talking to knife maker Phil Wilson, who has an engineering background, that heat treatment plays a significant role in determining corrosion resistance in blade steels, including “stainless” steels (the term “stainless” is a standard applied to any steel with a chromium content of higher than, I believe 10.5% –I’ve also seen the figure of 11% and 12% from different sources, and I don’t know which exactly is correct. But the term "stainless steel" is basically just a chromium content threshold). So listing one steel as more or less “stainless” than another would have to be qualified to a specific alloy and heat treat protocol.

In general, as we all know, higher chromium content steels are typically more rust resistant than lower chromium content steels, but it also depends on the carbon content of the alloy, since the more carbon in the mix, the more chromium carbides it will form, leaving less free chromium in suspension and thereby reducing corrosion resistance.

So corrosion/rust resistance in steel alloy blades may not be quite as easy to categorize as you might think.
 
Rob,

Will york is...mostly correct. I am not an expert nor do i claim to be a metallurgist.

Simply put, the higher the carbon content the more likely to corrode quicker. The magic number for carbon content (with regards to the iron carbon diagram) seems to be .08%. With stainless steels it is a combination of 23% (any amount of which but not lower than 8% of each from my recollection, might be as low as 6%) chromium and nickel content, but not lower than 12%. As Will said, stainless steels will corrode if given enough time. True, they only stain-LESS than a standard carbon steel. With this there are several types of stainless as well. Two common types are austenetic, martensitic and ferritic stainless. the latter is actually magnetic.

Heat treatment. This does change the characteristics of the material vastly. Steel manufacturers use a simple iron-carbon diagram to determine the iron to carbon ratio along with a temperature to essentially get the steel they are intending. It doesn't stop there though, with the addition of the above chromium, nickel, manganese, sulfur, and so on, you can also change the properties of the steel. So, again Will is right, there are so many factors to consider in determining a steels properties. One, could also nitride or heat treat the material in carbon (causing the material to absorb the carbon along the surface making the outer layer of the material harder than the core). Also to be considered is the cooling. Rate of cooling, quench medium, tempering, how many times tempered and so on.

Without getting into a full on lesson, the microstructure of the steel also has some say in properties. a body centered cubic, face center cubic and close packed hexagonal.

Most metals have weaknesses to corrosion; it all depends on the environment that the material is used in.
 
Thank you for posting that, mainaman. That's a very nice resource to have--appreciate that. :thumbup:
 
I don't post in the Busse forum very often, though I tend to follow Will around some. How's things Will? Haven't seen you in a while.

To the original question - those lists are quite a bit more complicated than one might think, as was explained above. Rather than make a list of specific steels, allow me to give some general trends.

Stainless steels don't have issues rusting under normal conditions. (I know, thank you Captain Obvious). That said, the trend is more chromium and less carbon will be more corrosion resistant. You can look at the ratio of chromium:carbon. The higher it is, generally speaking, the less subject to corrosion that steel is. Very corrosion resistant steels are things like 420, 420HC, 12C27M, 12C27, 440A, H1, and some others.

Non-stainless steels cannot be counted on to resist corrosion to any great degree. Everything from 1095 to M4 can rust within a day if conditions are right. The same can be said for the stainless steels, but those conditions are nothing you'd want your hand in to use the knife anyway.

To clarify a few points, the Iron-Carbon phase diagram doesn't really have anything to do with corrosion resistance. It's really just a starting point and any alloying elements added change it. The one to see for stainless steels is the Iron-Chromium diagram, which then will be changed by adding carbon and other elements, just like the Iron-Carbon diagram.

Nickel is a major alloying element in austenitic stainless steels, which are for the most part non-magnetic, though that changes with treatment and specific grade. Martensitic and ferritic stainless are both magnetic, though only martensitic is common for knife blades. Carbon is kept very low in the austenitic and ferritic steels, just as Will described. The carbon will bond with chromium and corrosion resistance is lost. Martensitic stainless by definition needs carbon, but not too much.

The 10.5% to 13% chromium range in stainless comes from a graph that was produced when researching stainless in it's early years. If one graphs corrosion resistance on the vertical and chromium content on the horizontal axis, one finds that corrosion resistance increases with chromium content (again, duh). However, around 10.5% the corrosion resistance hits a plateau and doesn't start improving again until (wait for it) around 13%. This is why it's not just a fixed value, but a range you will find if you start researching it at Google University.

Lastly, a point I've seen with H1. It does not form the orange/red/black rust we see that will pit and ruin a blade. It does oxidize even in a weak salt water solution (like a salt water pool). It turns a nice shade of blue and yellow around the laser etching that will almost wipe off with just a paper towel. Also, fresh coarsely sharpened areas will darken ever so slightly. I back bevel my Salt 1 with a 220 grit stone and apply a polished microbevel with the Sharpmaker.
 
Ah, there you have your expert, Rob. Great advice on looking at the chromium:carbon ratio. That's a very useful rule-of-thumb index for your purposes.

Thank you, me2--I always learn something memorable from your posts, and thanks this time especially for clarifying that 10.5-13% chromium threshold on stainless steels. Always wondered about that. Appreciate you sharing your own experience with H1, also--very illuminating.

Me2, you are too kind and much too modest. :) I'm fine and enjoying life, thanks for asking. How are things with you? Great to see you posting here! :thumbup:

Will
 
I've cut back on my knife forum time, though I tend to ramble when I do post (see above). That thing about 10.5-13 percent wasn't obvious until I started seeing it as a point of contention. I did a little digging to see if the 12-13 percent I'd always used was wrong. The graph is available in Verhoeven's book, and there it is, plain as day now that I'd bothered to look. The corrosion loss (measured as grams lost from a standard sample size over fixed time) steadily decreases as chromium goes up, then just stops in that range, then picks right back up again. There is another plateau at higher chromium, but that much in a knife blade is very rare, so I didn't figure it would apply.

I should mention that different stainless is suited to specific conditions. Rob_Mob, do you have specific environments in mind? If so, we might be able to tailor a selection based on that. Any other properties you think are applicable for material selection?
 
Gentlemen -- sorry it took me so long to respond. I was unfortunately locked into some other obligations. Will as always thanks for your insight and for responding. That was actually a great starting point for me.

Mainaman -- thanks for the link -- super valuable and quite possibly the right thing I was looking for.

Busse007 -- thank you sir for the input it was also a helpful starting point.

Me2 -- I genuinely appreciate you chiming in, especially since you don't comment on the forums as much nowadays. Yes the Iron - Chromium diagram and the Iron - Carbon diagram is what I'll focus on for now. As for environments, I'm looking for any environment other than salty ones (i.e salt water submersion). I'm currently formulating with another Chemical Engineer 2 solutions. 1 of them has been exclusively tested with INFI and some of the other more common steels (e.g S30v, 1095, 5160, S7, CPM-S30V, D2, 154CM, AUS8, SR-101, 8Cr13MoV, and others). 1 solution will be the ideal one for the steels that don't have a tendency to rust "quickly" and the other solution is intended for the steels that do. In short gentlemen, my love for Busse and the community has inspired me to create 2 products that are specifically familiar with INFI, SR-101, and some of our other favorite common blade materials. I'm also working with another very well known and highly regarded member/moderator on this sub-forum. He will remain anonymous for now :)
 
Heck yeah, man. Very interested to see what you come up with. Good luck with it. :thumbup:
 
Too late, I already have a special Infi Treating/cleaning product..........Tree sap and dirt.......
 
resinguy said:
This is all very interesting Rob_Mob; what do you do professionally?
Click to expand...

Well I work about 50 hours a week as a Behavioral Therapist working with children diagnosed with Autism. I also own a business that works with the Los Angeles County Probation Department providing Anger Management Services, Parenting Education Services, and Domestic Violence classes. My father is a Chief Chemical Engineer for a chemical manufacturing company. I have a bit of a background in chemical formulation myself, so we've been working together on formulating. I've been talking/working with a certain Moderator on this forum about formulating 2 solutions (1 that focuses on blade steels that rust quickly and 1 that focuses in blade steel's that don't). The solutions work to clean and preserve. I started the formulation to work specifically with INFI and SR-101. Then I started testing with other common steels and the results have been very satisfying. What can I say -- I love Busse and Busse kin products and find the need to pamper them after I BEAT them silly. :)
 
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