Weird Things Appear on Cold Steel Knives When Soaked in Citric Acid...

[stoffi]

I usually dip my carbon steel knives in citric acid to get a dark-grey finish, before doing the whole mustard/vinegar/coffee/apple/whatever-thing.

I always get pitch black knives from this oxidation process, which can be wiped off by up to 70% with just soap and water. What is left after that is a nice deep grey color.

However, when doing this to my Cold Steel SK-5 Gurkha Kukri, I got like thousands of spots on the blade (second pic). The lightning-pattern was done with mustard.

I believe they must be oxidized carbides (very large ones!!!), but this never happens to ANY of my other carbon steel blades, such as:

ESEE
MORA
Hultafors
Fiskars
Opinel
...and so on.

So, now that I recently bought a Cold Steel Norse Hawk, I stripped the paint off and left the axe head in a bath of citric acid,
which revealed what would seem to me as the "differentially tempered blade", which is claimed about it.
If so, one might have some concern about that heat treatment, judging from the pic below (same slap-dash dark oval spot on both sides!).

What gives?!
Is the SK-5 and 1055 carbon steel which Cold Steel uses (or the heat treatment) complete crap or what? Should I be concerned?
What has happened? Has it been overheated or something?

 

[stoffi]

Does anyone know what is up with this..? I'm just curious as to why these two CS products react this way, when none of my other blades (20-ish different kinds) have ever turned out like this.

 

[Jason B.]

The hawk looks like they only HT'ed half the head, hard and soft metals will color differently.

The kuk idk, I agree that it looks like the carbides but that's way too big. Usually you need a microscope to see that and the surface would need to be mirror polished before the etch for it to even work. I have never seen that though, maybe the steel got too hot?

 

[Blankwaffe]

The freckles on the kukri blade is probably transfered metal from the wire brush when they brushed it in the finishing process.Ive seen this happen even with steel wool.Personally I wouldn't worry about it as I think it adds even more character to the nicely done finish you have there.

As far as the Norse bit,I'd say the edge was only partially heat treated and maybe overly hardened in that area.Hard to tell in the pic,but looks like a slight temper line running down from the large dark area.When sharpening see if the edge wants to chip in the dark area.Actual use will say how robust the edge overall will be.But again you should be able to feel the differences(soft/hard) in the edge when you run it across a fine stone.

That said,the temper line on my Trench and Riflemans hawk is approx. 3/4" wide and evenly applied.Both take and hold an edge nicely.My new Pipe Hawk had a darker temper line and was brittle causing it to want to chip in the middle part of the bits edge when run on a stone.Almost felt/sounded like hard cast metal on a stone.With a little work on the stones I got to good metal pretty quickly though and has shown to be robust.

If the Norse bit has issues I'd contact CS and get a return.CS is quick to take care of issues in my experience with them.

 

[stoffi]

Thank you guys for your answers! CS replaced a Gurkha Kukri for me once,
because an inch-wide section of the factory edge bevel immediately got deformed after some regular, non-abusive woodchopping (from new),
but it was still as sharp as it was before in that spot. If the Norse Hawk truly has a poor heat treat, which time shall tell,
I'll probably send it somewhere for a custom heat treat instead.

The spots on the current replacement Gurkha Kukri I have now though, are not just on the surface, they are small CRATERS.
When I convexed the edge, they were still there on the edge and a needle catches in them. Could be a wire brush which as made small indentations, but I honestly dunno.
Could be impurities, since they are evenly spread out, but the wire brush theory seems plausible as well.

 

[stoffi]

Having read a bit more about citric acid rust removal on the net, I'm beginning to worry about a concept which was new to me: "hydrogen embrittlement".
As I oiled up my CS Gurkha Kukri immediately after having cleaned and dried the blade after the citric acid bath (of at least 12h +),
I'm starting to worry that any hydrogen atoms which could have entered the metal structure of the blade due to hydrogen embrittlement,
may have stayed there (potentially causing fracture to the steel during stress). I've cleaned and dried the blade again,
so as to leave it out in room temperature and to give the hydrogen atoms a chance to bail, if they indeed are present.
Does anyone know anything about hydrogen embrittlement and does it affect our high-RC carbon steel knives?
I've read that one can bake a blade at 400F within 4 hours to remedy this (otherwise the hydrogen may become alloyed with the steel),
but I did the citric acid bath several months ago, so go figure.
Should I be worried that the blade will snap when chopping out in the woods?
Have I screwed up my blade and if so, can it be saved?