How to clean the black crust on tool steel hardened parts, after hardening in oil ?

Hello,

I started a CNC business now 7 years ago, in the begining making just Policarbonate and textolite parts, but after a few years I purchased an Tormach Mill, to be able to make cutting dies for leather, I use 1.2379 tool steel for all my cutting dies.
For hardening , until now we worked with a local old man that used to "harden" by heating just the top of the blade using a welding torch, but now we want to fully harden the parts better, the entire part, not just the top of the blade.

We got for tests an home made oven, from a guy that makes blades for knives, we can set the temperature as we need but after cooling in oil, on the surface of the cutting dies appears an black crust, I want to find a way to clean that crust without doing it by had, and the first thing that I was thinking was un ULTRASONIC CLEANER, do you guys think that might work ?

Those are some cutting dies I made, the small one on top of the bigger one, is hardened in a factory, in pressure controlled chambers, hence the yellowish color, sometime we get them in blueish color, but it's complicated for us to harden in that factory as they have about 1 week lead time.



If I could remove easily / automatically the crust on the dies, my problem would be solved.

thank you

Glass bead blaster. Myself, I just use the damn things straight from heat treat.

alphonso said:

Glass bead blaster. Myself, I just use the damn things straight from heat treat.

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Yes, and the dark will wear off pretty quick, maybe run some scrap parts first.

Is the "dark crust" just a color in the metal? Or is it scale from being heated red in the presence of oxygen? If it is scale then you need to eliminate the oxy from your oven or protect your part from that oxygen by wrapping the part with stainless foil or similar.
As the guys above said above, dont worry if it is just colors, scale on the other hand....

alphonso said:

Glass bead blaster. Myself, I just use the damn things straight from heat treat.

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Won't that affect the thin / sharp section? I will try on a test part, as I have a cheap sandblaster, and a bottle of glass particles that came with the blaster. I never used them, just the blaster filled with fine sand

Rob F. said:

Is the "dark crust" just a color in the metal? Or is it scale from being heated red in the presence of oxygen? If it is scale then you need to eliminate the oxy from your oven or protect your part from that oxygen by wrapping the part with stainless foil or similar.
As the guys above said above, dont worry if it is just colors, scale on the other hand....

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I don't mind the black color, I like that, but the crust I'm talking about, I think is scale .. I just started to read about scale, I now heard this word for the first time

Rob F. said:

protect your part from that oxygen by wrapping the part with stainless foil or similar

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I will google about this method too

In a moment I will attach a photo with one cutting die we hardened, where the crust , hopefully is still visible, as my brother took it home to try clean it.

since its carbon steel and oil quench all you need is antiscale rated at the proper temp for your HT. i use a clay coating on the hand ful of carbon steel knives i do a year. it works but antiscale woudl be better. i dont worry muchh cause i always finish grinding post HT anyhow
edit to add black smiths will pickle parts over night in i believe vinegar and that loosens the scale that then just wire brushes off (least thats my understanding i dont do forge work )

When you wrap the part in stainless foil, throw in a small square of cardboard. As your temperature increases, it will ignite and burn off any oxygen trapped in the bag.

We find that air-hardening tool steels work better for leather dies. they are more durable and hold an edge better than oil-hardening.

We've glass-bead blasted parts that we wanted to be natural color. Unless you are using some exceptionally aggressive blasting media, the only places you'll have to watch for warpage is if you are blasting thin pieces of aluminum or sheet metal. If your parts were machined without warping issues, then you likely won't have any issue in the bead-blaster.

lr harner said:

since its carbon steel and oil quench all you need is antiscale rated at the proper temp for your HT. i use a clay coating on the hand ful of carbon steel knives i do a year. it works but antiscale woudl be better. i dont worry muchh cause i always finish grinding post HT anyhow)

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Antiscale compound seems to be like a paint, but adds another layer on my steel dies, which I have to clean after hardening? and I think the anti-scale compounds are good when like you said, you always grind the product after that process. In my case, I don't want to machine the cutting die again.

M.B. Naegle said:

We find that air-hardening tool steels work better for leather dies. they are more durable and hold an edge better than oil-hardening.

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Air-hardening slow or fast? Can you tell me your process of air-hardening?

The people from the steel hardening factory told us this "receipe":
1. heating and maintaining to 550ºC for 40 minutes
2. heating and maintaining to 850ºC for 50 minutes
3. heating and maintaining to 995ºC for 50 minutes
4. fast cooling to 50ºC with AZOTE ... we used heated oil to 50ºC
5. heating to 280ºC for 2 hours

Bellow is another photo with one of our cutting dies, hardened at the factory, don't know why it came out blueish, but it looks so cool

"Air Hardening" is a specific type of tool steel.

When you harden steels like your average low-carbon steel, you have to add carbon to it by quenching it in oil or packing it in carbon. Tool steels have carbon already present in them, but it is has to be activated in a process specific to the type of tool steel you are using. Over here, your three main groups are air-hardened, oil hardened, and water hardened. Each has properties better for certain tasks, and each has a different process to harden.

For the leather dies we make, we often use either A2 or D2 air-hardening tool steels. After the part is machined, it is folded into an air-tight stainless steel foil bag with a piece or cardboard (as I noted earlier, this burns off any remaining oxygen in the bag so it doesn't scale). The bags are then placed in an oven at 1750 degrees Fahrenheit for long enough for the heat to permeate the part evenly, also refereed to as a "soak". This typically means the part needs to sit in the oven for an hour for every inch of thickness. After that, the part is removed and can cool to room temperature. Air hardened steels don't require any special compounds or oils to harden, they just harden in the "air."

At this point the part is "hard", but it is often too hard and needs to be "drawn" to a softer hardness by annealing it at a lower temperature. This can be anywhere from 500 to 750 degrees depending on the target hardness. You'll need a hardness tester to confirm what the actual hardness of the part is and then determine the temperature.

Depending where you get your tool steels, the companies that sell them will often help you with literature for the process specific to the steel. We've never had much luck case-hardening low-carbon steels. It's popular because low carbon steels are cheap, but it can be tricky to get the desired results. Leather dies require sharp and hard edges that stay sharp but are not brittle, and air-hardened tool steels seem to do the best.

If you heat your dies without any coating in air, then quench in oil, any crust will be burned oil, carbonized. In my experience, that will come off with a wire brush. Ultrasonic cleaner very possibly will work...Alkaline solution should help. I would temper before exposing to the ultrasonics, because hardened but untempered steels can be very eager to crack

Unless you have some scale eating acid? an ultrasonic cleaner will do nothing to help. Wrong sort of process, I use the aqua blaster with some kind of grit for stuff like that, it’s light duty but does get under the skin.

all the antiscacle products i have seen pop off or clean right up leaving a great surface finish. and since you are dealing with scale im sure your kiln is not set up for argon purge. that woudld be the next step in clean heat treat shielding gas pumped into the kiln while its running. draw back is most kiln elements once used with shielding gas have to use it every time or they burn out

I'd try electrolysis. It's easy and cheap to set up to do and if it doesn't work you aren't out much. Electrolysis seems to drive most everything away but the parent metal in my experience.

I would NOT try electrolysis on a hardened part, certainly not until after temper, would temper again after elecrlysis. Electrolysis candrive hydrogen into part, causing cracks if there is residual stress.

M.B. Naegle said:

When you wrap the part in stainless foil, throw in a small square of cardboard. As your temperature increases, it will ignite and burn off any oxygen trapped in the bag.

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We had an old timer that used to wrap parts in newspaper in an attempt to do the same thing. I know he did it but I don’t know if it actually worked.

Nerdlinger said:

We had an old timer that used to wrap parts in newspaper in an attempt to do the same thing. I know he did it but I don’t know if it actually worked.

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Probably trying to impart some carbon from the burned paper to the steel.

I so happy to see so many people participating in this thread, THANK YOU.

I made a few photos of one part we hardened in oil, and the same part that we did not harden it yet. In the photo there are some rust spots, my brother tried to leave it overnight in vinegar, that's the reason the rust spots appeared.

sibianul said:

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That's some nice looking die making!

The hardening process make them look so nice, the blueish color is awesome, unfortunately we can't have them hardened in the same factory everytime, so we need to find a way, to hardening the parts ourselves. I will try to find an atiscale compount localy, in romania, and try to see if it solves my problem, as that seems to be the best way .. if it will be easy to clean after hardening.

Glass bead blast won't hurt D2 edges enough for you to notice, especially just cutting leather. As others have said, the tool steel needs stainless wrap with some paper combustible in it. Or...you could send it to a proper heat treat place with an atmospheric furnace, and scale is no problem after that.