What is this stain/mark in my blade?

[Glenn Dirado]

Hello everyone, new to the site and really like it.
I am a hobby maker. I recentley made my first batch of knives in stainless(440c) and had them profesionally heat treated. When I got to the finer grits of the finishing stage of my blade an odd stain began to appear in the steel. The best way I can describe it is it looks like a small drop of acid ran down the blade and etched the steel. It is not rough but is deep in the steel. It is still there even after removing several thousandths during the finish grind. It does not go through to the other side of the blade. Anyone know what this is? Will it compromise the strength of the blade? Thanks in advance,
Glenn

 

[Rock]

I haven't a clue, Glenn, but that is a great looking knife! Have you tried asking the heat treater yet? It definitely looks like there was a run of some sort.

 

[HELLGAP]

Ive heard that ATS 34 can have dirty spots with in the steel and maybee thats just what this is, a small flaw or dirty spot with in the steel .

 

[The Tourist]

I have also seen 'dirty spots.' I'll buff an edge, most of it will get mirror bright, and then I see about 1/8th of an inch that looks cloudy.

While I cannot prove it, I imagine it's alloy not made from a CPM process. There might be hunks of carbides in the mix, and I cut across one periodically.

Take heart, I've never seen such an area prove to be weaker or softer.

 

[James Terrio]

While I cannot prove it, I imagine it's alloy not made from a CPM process.

That's true. To oversimplify, 440C is a good steel; 154CM was an upgrade from that, and CPM154 is a further upgrade. 154CM was once among the best available but later, there was a lot of concern about QC. People reported finding inclusions and dirty spots and for a while ATS34 (Japanese version) kind of took over... but I digress.

I'll hazard a guess and agree it's just a weird spot in the steel itself. I'd try etching the whole blade with something mild like vinegar or cold blue. These should have almost no effect on 440C. If the "spot" reacts noticeably different than the rest of the blade, that might be a concern.

 

[The Tourist]

440C is a good steel; 154CM was an upgrade from that.

I agree. The problem is often a trade-off. The issue centers around chromium.

Most of my hard-use knives are 154CM, and I wouldn't trade them on a bet.

My only proviso is the working environment. If the owner is a working sailor or carries his knife in a damp, sweaty pocket, then I would advise an alloy with a higher chrome content.

But as Mr. Terrio points out, that alloy is not manufactured by a CPM style process. Once again, there's the possibility for a 'trade-off.'

My 154CM knives are not showing rust or corrosion. But then, I'd bet the guys here who care about such things are having similar luck.

(BTW, and not to hijack, but would MIM style 440C solve the 'carbide' problems?)

 

[BossDog]

that appears to be amalgamation banding. I haven't seen it for some time and never in 440c. For awhile some time ago it was an obnoxious issue in some cold rolled ATS34. (Like 10 years ago...)It caused me to switch away from ATS34 to 154CM that I never experienced that issue with. It's very frustrating to work hard on a knife and have that show up. It won't buff away and by the time you grind past it, you've lost the knife. The biggest problem is it doesn't show up until after heat treat and most of the finish work is done. If it shows up in another knife, consider scrapping the entire batch and sourcing from somewhere else entirely for that kind of steel for at least a year until that batch is sold through. Please do NOT post where you bought it. We don't know exactly the cause and trashing some one here isn't what we do.

 

[James Terrio]

(BTW, and not to hijack, but would MIM style 440C solve the 'carbide' problems?)

Metal injection molding? I'm not really familiar with it, but it sounds similar to the particle metallurgy process. If so then yes, that's why we have CPM154 now

that appears to be amalgamation banding... It caused me to switch away from ATS34 to 154CM that I never experienced that issue with....

Looks like I had my anecdotes backwards. Thanks for clearing that up, Boss.

 

[The Tourist]

Metal injection molding?

Yes, the Kershaw Offset used an alloy called MIM/HIP440C.

I'm not a metallurgist, but I assumed/inducted that if something was to be used in an injection-style process, then the alloy chosen would have to be 'pure' and ductile.

For me, knives using ZDP-189 polish up quite nicely and that alloy is 20% chromium. It appears that whatever process is needed to produce flaw-free high chromium knives, then some smelter found the trick that works.

 

[James Terrio]

I'm not a metallurgist either, so take this with a grain of salt. Chromium has a huge impact on corrosion resistance and other good things, but it's not the only factor. Several elements can contribute to it. Heck, simply heat-treating a steel increases its ability to resist rust, so does a higher degree of polish. S30V has plenty of chromium too (14%), but is widely regarded as very difficult to polish, due to it's vanadium-carbide content. You seldom see mirror-polished S30V.

For instance, 440C is listed as having up to 18% Cr as I recall; 154CM/CPM154 "only" 14%. Yet the 154 series are claimed by the manu to be MORE corrosion resistant, not less, with better toughness and edge-holding. The other most obvious difference between the two series is much more molybdenum in the 154's. The biggest trade-offs are higher cost and somewhat decreased machineability.

Fun topic isn't it? Once again, sorry for the thread-drift.

 

[The Tourist]

Fun topic isn't it? Once again, sorry for the thread-drift.

Gee, I hope no one thinks I'm a bad boy...

But the issue here could have many facets. It is reasonable to show that a mark or color that appears on a blade could be from a smelting process, the elements in the alloy itself, or just an impurity. Many of us have seen these marks.

I've been pretty lucky on this issue. If the knife is used often I get to sharpen it, and since most of my examples are shallow in nature, they buff out.

 

[Glenn Dirado]

that appears to be amalgamation banding. It's very frustrating to work hard on a knife and have that show up. It won't buff away and by the time you grind past it, you've lost the knife. The biggest problem is it doesn't show up until after heat treat and most of the finish work is done.

That was my exact experience. Mystery solved. Thank you!