Keeping 01 free of carbon buildup while hardening.

[SmithDoor]

Charcoal can be used instead of CO2. I used that a lot over years when had oven.
Now I just torch for few small parts.

Dave

The only way to prevent the black iron oxide from forming is to prevent oxygen from contacting the steel. This can be done by

• painting on of certain "mixes" such as those mentioned above (results are very variable)
• wrapping the steel airtight in stainless steel foil (my preferred method that works with 99% reliability)
• removing the oxygen from the kiln or oven where the steel is heated - this can be as simple as dropping a piece of wood into the kiln/oven a second or three before the part is placed inside this kiln/oven. The wood will then ignite, and burn up most of the oxygen in this oven (obviously the oven should be rather well sealing to prevent fresh air from moving in and replacing the O2...)
• if using a forge, one can place the part just inside the top layer of coal/coke/charcoal that is used to make the fire (in the region where the flames have used up most of the O2) - this sounds crude, but is how most tools were hardened in the years before electricity...
• using a reducing flame on a cutting torch and holding the part " just right" where the oxygen in the flame had been depleted, but not so far away that it is exposed to air - very difficult to get right...
• or by placing some inert gas (CO2 works well) inside a metal tube that is closed off air tight on it's bottom, and then heating this tube in a furnace/kiln/forge with the part hanging some way down inside this pipe. The heavier than air CO2 will stay inside the pipe even when heated, and this process works well if one uses a pipe that is (say) 200mm taller than the top of the part hanging in it, as this slows down the process of air diffusing down into the pipe, but one does have a practical time limit of only about 10 - 15 minutes to heat the part.

Apart from the wrapping in foil and the painting on of a protective coating processes, all of the other processes mentioned above will still result in some de-carbonation/forming of iron oxide when the part is removed from the heating container prior to quenching, and it then makes contact with oxygen in the air - but this process should only take a second or two before the part is quenched into water or oil, and the de-carbonation is usually not very severe.

Colin, if you don't want to use the foil method, I suggest you follow Tornitore45's advice and just block up the screw holes to prevent O2 from entering the holes. Also, keep in mind that any sharp corned (and threads...) could lead to cracking of the part when quenched, so use a slow oil for quenching, and don't be tempted to quench in water.

Good luck.
Hennie

 

[R Degen]

The first thing you need to do is use a fire Tap .

Randy

 

[tornitore45]

Home shop environment:
I am still trying to figure how to unwrap a red hot chunk of metal in less than 2 seconds before quenching, given that leaving the part under wrap slows the quenching down below critical speed.

Professional Heat Treatment Facility:
I dunnow only seen one on a 10 minutes tour 50 years ago.

 

[HennieL]

Home shop environment:
I am still trying to figure how to unwrap a red hot chunk of metal in less than 2 seconds before quenching, given that leaving the part under wrap slows the quenching down below critical speed.

This is "business as usual" when making knives.

• Attach a wire to the part to be hardened through a convenient hole or wrapped around a narrower "neck" piece, if possible
• Enclose the part with the foil, leaving a "bundled" elongated "***" of the foil below the part, directly opposite the attached wire.
• Wrap the wire as close to the part as possible, without bending it into holes or funny shaped nooks in the part
• Take the foil up over the wire for some distance (depending obviously on the size of the foil and the shape of the part - I would say 100mm / 4" would be minimum
• Just form this excess foil by squashing it by hand parallel to the wire, and do not twist it around the wire.
• Hang the part and foil in the oven (mine is a cylindrical vertical one, made to treat quite long knives) or place it such that you can easily reach the exposed wire.
• When the soaking is done, lift the part with the wire (I tie the top of the wire to my furnace lid, and just lift off the lid with wire and part...) clear from the oven
• Grab the "***" on the bottom of the part with a stout pair of pliers, and with one hard yank remove the foil
• In the same motion, turn to where your quenching tank is located (I keep mine very close to the oven, just a 90° turn to the side...) and dunk the part into the oil.
• Enjoy the smoke and flames, knowing your part is going to be perfectly hardened, but prepare to speak in French if you hear the dreaded "ting" faerie landing on your part

 

[HennieL]

I am still trying to figure how to unwrap a red hot chunk of metal in less than 2 seconds before quenching, given that leaving the part under wrap slows the quenching down below critical speed.

Actually, taking this a bit further, you do have more than two seconds for O1. Referring to the following TTT diagram for O1, you have approximately 8-9 seconds in which to drop the temperature to below about 600°C / 1100°F (the most critical period), and another 50 seconds to then get it to below 300°C / 600°F. (For anyone interested, the two lines in the diagram roughly looks like a nose, and one should never "cross the nose" by cooling too slow if one wants full hardness). Also important is to ensure that the final "cold" temperature of the steel is around 50°C / 120°F or less in order to achieve around 99% transformation into Martensite (the hard steel phase).

 

[Chiptosser]

This is a bit more expensive for most home use, we used nitrogen as the atmosphere. The oven was plumbed with a regulator to control gas volume into the chamber.
Nitrogen is an inert gas, not allowing surface oxidation. Argon could be used too, more expensive. I have wrapped many D2 steels and oil hard steels over the years.
Wrapping is a art to practice, along with the proper wiring method.
Also, soaking time at Proper temperature is very important to obtain the properties that you want. Don't discount the proper draw, temper, anneal temps.

 

[clockworkcheval]

According to a tale from an old machinist in his time there were travelling tradesmen specialized in heattreatment and grinding of cutting tools. He tells that they got the tools very hard by dunking them in a bath of mercury.

 

[goldstar31]

According to a tale from an old machinist in his time there were travelling tradesmen specialized in heattreatment and grinding of cutting tools. He tells that they got the tools very hard by dunking them in a bath of mercury.

I have a cpntainer full of dental amalgum of silver and mercury whilst my(late)mm wife's fillings wpuld not assay becaisr the gold from extracted diseased reet has planinum as a hardener. It will not assay

And shades of Gas Chmvers and ZykomeB

 

[stanstocker]

The mercury quench was a common gunsmiths trick as carbon steel taps like to break off in ordnance steels. By getting the tap crazy hard and not tempering it, when it broke you could shatter it completely with a punch and one swift tap. Dump out the granular bits formerly known as tap and have at it again. Haven't head of this being done in years, modern steels and carbides have made such things unnecessary.

For those dealing with thin section hardening like knives, plane irons, and such, look into quenching between plates. Fast quench, no smoke, and less risk or warping. I know a few folks doing things this way with good results.

Cheers,
Stan

 

[HennieL]

For those dealing with thin section hardening like knives, plane irons, and such, look into quenching between plates. Fast quench, no smoke, and less risk or warping. I know a few folks doing things this way with good results.

Stan yes, that's become rather standard practice for knife makers working with high-alloy stainless steels. I've been quenching between plates for the past 8-9 years with very good success. I attached two aluminium plates of 20mm thickness to a wide mouth vice grip (normally used by welders...). The vice grip is then clamped horizontally to a bench or table, with the plates then being vertical next to the table top. One simply moves the red-hot knive in between the plates, and then close the vice grip, One minute later the blade is cold and hard, and is then just dumped into water for a final cooling down to room temperature. From there, the blade is just wiped dry, and placed directly into a container with liquid nitrogen to complete the transformation of any retained austenite, and then tempered 2 or 3 times, depending on the type of steel. This process is obviously for high quality knives, and the plate quenching cannot be used for anything thicker than about 6mm, but if one needs to get the absolute best out a high-alloy steel then the cryo-quenching into liquid nitrogen or alcohol and dry ice will result in a smaller steel grain, and tougher and harder steel than without this treatment. Everyone hardening HSS tools should really consider doing this.

 

[Tim Wescott]

I know we're pulling the thread off topic from the OP's original post -- but how do you get the plates into intimate contact with the knife? It seems like you'd clamp onto the blade's backbone -- where you want the hardening to happen least -- and not rapidly cool the edge -- which you do want hard.

 

[HennieL]

I know we're pulling the thread off topic from the OP's original post -- but how do you get the plates into intimate contact with the knife? It seems like you'd clamp onto the blade's backbone -- where you want the hardening to happen least -- and not rapidly cool the edge -- which you do want hard.

Tim yes, apologies to the OP and everyone else not interested in this.

The plates can swivel around the bases of the vice grip - mostly in the "X" axis across the width of the knife, but due to sloppy construction and wear of the pivots of the hinges, also in the vertical "Z" axis along the length of the knife - see photos below. I harden my blades before grinding in the cutting edge, thus there are no "thin" and "thick" sides when I do the hardening, but I have hardened many ground (and even tapered lengthwise) knives for fellow knifemakers and have never had any problems with the thin ground edges not hardening fully (I do test every knife that I make with a proper industrial Rockwell tester that I bought some years ago).
This is the vice grip with plates:

And this is the hinge that allows it to rotate:

I trust that explains it for you
Hennie

 

[trlvn]

If you want a short summary of traditional high-carbon steel versus the fancy alloys that benefit from cryogenic treatment, try:

http://www.hocktools.com/tech-info/o1-vs-a2.html

... A2 is one of the steels that respond well to cryogenic treatment. This extreme cold treatment (-320°F) essentially finishes the original quench, increasing the steel’s toughness without any decrease in hardness.

Hock Tools is highly respected in the woodworking arena for making top quality edge tools.

Craig

 

[Tim Wescott]

@trlvn : I've always wondered why the "fancier" steels had the rep for not taking as keen an edge. Now I know!

 

[Zeb]

In addition to the stainless wrap, I used a small piece of paper that would consume any remaining oxygen in the wrap cavity. Never had issues as long as the cooking was done by the book. Tool steels will grow non-linearly a certain amount, so it was accounted for in the grinding process.

 

[HennieL]

@trlvn : I've always wondered why the "fancier" steels had the rep for not taking as keen an edge. Now I know!

Keenest edges are obtained with fine-grained steels that do not contain large carbides. Some modern powder metallurgy "super steels" actually have much finer grain, and smaller carbides than O1 or A2, and can achieve both a keener (i.e. thinner) edge AND be stronger (less likely to chip or "turn over") than the fine grained normal steels. I love working with O1, but e.g. Bohler Elmax, CPM S30V and S35VN and many others WHEN HEAT TREATED PROPERLY will beat it hands down - only problem is that these steels ideally require diamond or ceramic sharpening equipment, and getting a fine enough diamond sharpening stone can be the limiting factor in achieving ultimate keenness.