How do you tell if steel is stainless or not?

[eyeeatingfish]

I apologize if this has been covered but I couldn't find any threads on this. I got a box of scrap damascus steel from Vegas Forge and I have been making small knives with them that I want to send out to heat treat. I called Vegas forge and they said it is a mixed box so it might contain some stainless and some carbon. I asked them how to tell and he said use gun blue to see if it changes color.

Well I tried using gun blue and sometimes it would react, turning dark, and sometimes it wouldn't. But then I noticed I wasn't always getting consistent results, sometimes the same piece would turn dark with one drop but not with the next. I figured maybe it was dust contamination from the sanding belts or whatever so I took denatured alcohol and wiped the blades down then tried it again. I thought it was working but then I tested a drop on a bar of S30V and it turned dark too. Gun blue is only supposed to work on carbon steel per the package but now I am doubting the accuracy of using it to see if a steel is stainless.

Are there any other tricks? Magnet clearly doesn't work since knife steels are still magnetic. Obviously this is not something to send out to a lab to test either. I need to be sure whether they are SS or carbon steel so I can select the right heat treat option.
Thanks in advance!

 

[samuraistuart]

I watched a video online where a young bloke who is known for edge retention testing on rope used mustard to see how reactive different steels were. The carbon steels clearly had a mark on them, while the stainless steels did not. You might try just squirting a good sized dot on them and see what's what. Mustard is used to do forced patinas on carbon steel, so it should be a good test for you.

 

[eyeeatingfish]

I watched a video online where a young bloke who is known for edge retention testing on rope used mustard to see how reactive different steels were. The carbon steels clearly had a mark on them, while the stainless steels did not. You might try just squirting a good sized dot on them and see what's what. Mustard is used to do forced patinas on carbon steel, so it should be a good test for you.

I assume the mustard works because it has vinegar. I guess I could try vinegar, maybe the bluing is too strong an acid or something?

 

[Bill DeShivs]

Cold bluing is an electroless copper sulphate plating. It is sort of a good indicator, but it doesn't "rust" the steel.

 

[Richard338]

I blue things using oxpho cold blue.
If nicely cleaned, rubbing it on for 30 seconds with a cotton ball will significantly darken mild steel and tool steels like A2. Hardened 3V doesn't react, but can be darkened with ferric chloride. The cold blue will distinguish steels with very different stainlessness, but won't help with some intermediate ones.

 

[Hengelo_77]

Stainless knife steel is only stainless after hardening. That might be a problem

 

[Tyson A Wright]

I'm curious - how is knowing whether or not it's stainless enough to tell you how to heat treat it? There are lots of different HT recipes for both stainless and non-stainless, depending on exactly what kind of steel it is. Is there a generic, "good enough" HT recipe that just depends on stainless vs. not? (Not intended as a snarky question - I'm new to all of this and still learning.)

-Tyson

 

[Natlek]

I'm curious - how is knowing whether or not it's stainless enough to tell you how to heat treat it? There are lots of different HT recipes for both stainless and non-stainless, depending on exactly what kind of steel it is. Is there a generic, "good enough" HT recipe that just depends on stainless vs. not? (Not intended as a snarky question - I'm new to all of this and still learning.)

-Tyson

In this case he knows what he has..........he does not know which one is stainless Damascus which one is carbon steel Damascus

I got a box of scrap damascus steel from Vegas Forge

 

[tim37a]

Stainless steel is determined by the percentage of chrome in the steel. It has nothing to do with being hardened or not.

 

[Skiller1nc]

Stainless steel is determined by the percentage of chrome in the steel. It has nothing to do with being hardened or not.

yes but for many steels, the level of stainlessness goes up exponentially when hardened, ie when annealed they will rust like a non-stainless steel

 

[eyeeatingfish]

I'm curious - how is knowing whether or not it's stainless enough to tell you how to heat treat it? There are lots of different HT recipes for both stainless and non-stainless, depending on exactly what kind of steel it is. Is there a generic, "good enough" HT recipe that just depends on stainless vs. not? (Not intended as a snarky question - I'm new to all of this and still learning.)

-Tyson

Vegas Forge has heat treat instructions on their website. One for carbon and one for stainless. They use different stainless steals so I don't know how a single heat treat recipe can accommodate multiple steels but maybe they can get 95% of the way with a generic recipe? The stainless recipe was significantly different than than the carbon recipe.

 

[eyeeatingfish]

yes but for many steels, the level of stainlessness goes up exponentially when hardened, ie when annealed they will rust like a non-stainless steel

I have heard many people say that stainless isn't stainless till heat treated but I have also heard others say this is a myth. I have no idea who to believe on this one. I know my bar of 1095 rusted like crazy sitting on the shelf while my 440C bar looks unchanged.

 

[fitzo]

Grind a bar of 440C into a knife, cooling with water. It'll rust like mad.

As for why, it's about chromium carbides versus free chromium.

 

[David Mary]

My understanding is heat treating changes the microstructure, and "allocates" different amounts of chromium different roles in the matrix depending on heat treatment specifications. That's about as much as I can go into detail. But D DevinT Thomas has intimated that the the harder a steel is heat treated to, the more stainless it will be.

 

[samuraistuart]

Looks like Vegas Forge uses 440C/302 or AEBL/302. As I understand stainless steels that we commonly use in knives (440C/AEBL/154CM etc) those need to be hardened to get the stainless properties, as most of the chromium is tied up in carbides in the annealed state and is not "free" chromium to provide the corrosion resistance.

 

[Richard338]

Stainless steel is determined by the percentage of chrome in the steel. It has nothing to do with being hardened or not.

This article discusses stainlessness with respect to the nominal amount of chromium and the effect of heat treatment.

How Much More Chromium Does D2 Need to be Stainless? - Knife Steel Nerds

Thanks to Devon Craun for becoming a Knife Steel Nerds Patreon supporter! Your support is funding knife steel research. Some materials like aluminum form a passive oxide layer that prevents further corrosion. Steel is not one of those materials. Instead, steel forms iron oxide, or rust, that...

knifesteelnerds.com

 

[Pinoy Knife]

try a scotch brite then expose to moisture.. maybe a damp sponge?? the high carbon should start to change before the stainless . easy test to try...
un used Damascus can show a "Fingerprint "in minutes/hours. the acid in gun blue is Selinic if im not mistaken ,and depends on its strength.
i used "Nickle Black from a jewelry supply years ago to instantly blacken stainless.. so that might not help figure out which is which....

 

[Stacy E. Apelt - Bladesmith]

Pinoy has the test - a corrosion test., Sand them clean, wsh off well, and spray with a salt water solution. The difference will show clearly in a day or less. Using some comparison pieces of known steel in the test to compare with also helps.

 

[eyeeatingfish]

I tried a saltwater test. I mixed up hot water and a spoonful of salt, Then I dribbled on a few drops onto the pieces. Some had quite a bit of surface rust the next day while other pieces had minor reddish discoloring. Even the S30V blade had a little discoloring on it though it wiped off mostly with just my fingers. One of the unknown damascus pieces had some light rust coloring on one side and nothing on the other side even though both were exposed.

I think that some are obviously a carbon steel but a few pieces I can't tell whether it is stainless with minor corrosion or a carbon with minor corrosion.

 

[shqxk]

Even stainless has kinda low corrosion resistance in annealed state. I would try etching it with intense instant coffee (use the cheapest one you can find) only the carbon steel would react to this.