Heat Treating 01 Tool steel

[Tony Shea]

I need a bit of help heat treating my O1 molding plane blades. I have a setup of just a Bernzomatic 8250 torch with a can of Mapp gas and a few blocks of fire brick to hold in some heat. I'm sure I;m able to get the heat high enough with this setup but due to the lack of control I am not able to get a sufficient soak time at the right temperature. I've heard that this should be fine with such small blades in a molding plane (#8 Hollow and Round--1/2" radius). I've made my planes per Larry Williams DVD, which is great.

Anyways, my question is about knowing when I'm at the right temp. Larry suggests not using the color of the steel but watching for bubbles (which he calls flux bubbles) coming to the surface of the steel. I personally never saw any of these bubbles on my first and only attempt but didn't dare go any further as the steel color was into the orange range. I also had a magnet nearby but couldn't get that setup so the magnet wouldn't stick to my red hot blade, just got too awkward. What are these bubbles Larry is talking about and is it something truely reliable in determining the proper temp?

My blade does seem a bit soft so I'm not sure I really did get up to temp. But seems to hold an edge well enough to make the plane useable. In fact I'm very happy with the plane so far. But I want a better result on the matching hollow.

Also, I'm stuck on the tempering side of things as well. I have a reliable oven that I use but am not confident on the time and temp to temper the blade at. Larry suggests 375 deg and Ron Hock 325deg. But the time is not clear to me. Ron says 325 for 20mins per inch of cross section. With my half inch by 1/8" blade what does that equal? Might be a dumb question by I just can't wrap my head around that.

Last question is about re-hardening 01. So if Im not happy with the hardness of the blade is it possible to re-harden it? I've seen where it has to be annealed first before this is possible and that seems like it would be beyond what I can do with my setup. And I've also heard where there may possibly not be enough carbon left to re harden this blade.

Sorry for being so long winded. There is a lot more to this heat treating than I thought.

[Adam Neat]

Ive heat treated a few items using O-1 but I always use a small lab oven that I have. The big test after you quench is run a file over the part, If its "hard" the file will just slide over the surface and not bite into the part.
If the file bites in and cuts, The part didint heat treat right.

On tempering, its not the time thats critical in as much as getting the part to the correct temp. So you want your part in the oven long enough to reach the desired temp, A little longer wont hurt as long as the temp is the same.

Im not sure about the re-heatreating a what steps are needed. If you do need to anneal its just a matter of getting to the critical temp (1400-1500 in this case) and cooling very slowly instead of quenching

[Mark Baldwin III]

Orange is the right color for O-1, so you did good there. Color vs bubbles is probably a matter of personal preference, I've gone by color with the couple of irons that I've made. You may want to try a dual torch set up to get a bit more heat. I've rehardened one iron, with no noticeable ill effects, though it probably lost some carbon.
Tempering takes a little more touch than hardening. You will want to verify the temp of the oven you are using. A kitchen oven can be off by 50 degrees in some cases. That's enough to change the level of temper you're shooting for. The advice given to me was to start at the lower end of the tempering scale, and if the blade was too hard, then to retemper at a slightly higher temp.
While reading "The Tool Steel Guide" (Jim Szumera), he says that you must let the iron cool back to the 150 degree range before tempering. The hardening process is still working as it cools, and that process must be allowed to work. Once you can hold the iron in your hand, it is ready for temper. If you go too soon, it's actually possible that you might not obtain full hardness.
No...not trying to sound like an expert, I'm still learning this too! But hey...my irons work pretty darned good

[george wilson]

The Szumera book is an excellent book. I had one at work and need to buy one for home,though I memorized it pretty much. Just like to have one.

Orange is a good color for hardening. Use vegetable oil,automatic transmission fluid(we had a 5 gallon bucket at work),or any other CLEAN oil. Used motor oil is a big NO NO. It will coat your iron with a thick,black coating that is hard as blazes. The quench should be large enough that the quench is not noticeably heated by the blade being quenched.

If your steel "bubbles",you have burned it. Likely,you can't get a decent sized piece of steel that hot with a Mapp gas torch. Just heat to orange,no more.

Do not bevel the blades before hardening,or you run the LIKELY risk of the blade becoming cupped,as there is a different amount of surface area on either side of the blade. You could file the shape of the molding you want onto the blade before hardening,but leave the edge square from front to back,and grind the bevel later. If the blade is pretty narrow,like 1/2" wide,not much cupping will occur.

To temper,do like the above mentioned book says: Have a PRE HEATED toaster oven ready. As soon as you can BARELY hold the blade in your hand,put it into the hot toaster oven. I recommend about 400º. Too many people like to make their blades TOO HARD,and they don't hold a good edge because the thin cutting edge microscopically breaks off,and the blade seems dull. I KNOW THIS FROM LONG EXPERIENCE.

The toaster ovens and kitchen ovens do not have reliable temperature settings. The kitchen oven can really be off as much as 75º. I bought a long backed high temp.thermometer from Brownell's Gunsmith Supply,and inserted the long thermocouple into the toaster oven from the back.

If you want to keep things cheap and simple,for most of my life,I just tempered the blades to a medium brown(straw) color,varying as to the purpose of the steel. Blue for a spring. The FIRST thing you MUST do correctly,though,is get the steel so hard a NEW,fine toothed file,and NOT a Mexican Nicholson,will just skate across it. Use an older USA one,or buy a Grobet from LV. Polish off the crud from hardening the blade till clean steel is had. Heat the blade from the non cutting end,and let the colors SLOWLY creep to the cutting edge,then quench at once. You will make a perfectly satisfactory simple blade this way. The book(above) was really written concerning extremely expensive tool and die work,when you want to extract every bit of the performance from the tool steel that is possible. Your plane blade is not as critical as a $100,000 die.

I have used the more advanced methods for the last 20+ years,but in the 60's and 70's,things were done more simply with a torch OR 2,bricks,and a quench.

[Mel Fulks]

Thanks George. I have always been interested in this type of stuff,but no real expirence with it.Printing your info out as a permanent reference and I know many others will too.

[Trevor Walsh]

Tony,

I would agree with your assessment that the iron is not fully hard. I've done a lot of blades and cutters in 1095, W1 and some O1. I usually do this work in a dark room (not black but no overhead lights indoors around 6pm in the fall is good) or a basement with small ground level lights or a door open to the outside, heat to a medium/bright orange and quench. In the past I've tempered according to color in the straw to dark bronze range, but on the #6 round iron I made I used a toaster oven at 350F. The difference in temperature between Larry and Ron's suggestions is a few points on what's called the Rockwell Hardness Test scale C, It's like the Janka test but for hardened metals. The higher temperature of the temper means the metal is a little softer but a little tougher. Either would work fine.

As for re hardening, don't worry about burning out the carbon, you would have to hold it at very high heat for a while to burn the carbon out of it. Think of it like this, Steel is an alloy of Iron and Carbon, Iron makes neat organized lattice (cages) of atoms and the carbon sits around these, when you heat the metal to Critical (~1600F) the carbon atoms go inside the little iron cages, when cooled quickly the carbon doesn't have time to exit the cages and gets trapped inside, this makes the metal really hard but brittle, tempering takes some of the hardness out of the metal so it doesn't shatter when used.

To reharden the iron, just get your torch and a can of wood ash or sand or vermiculite, heat the iron up to critical (~1600F, med/bright orange in a dark room, until it's not magnetic) and bury in the can of material until cool. Then it's really soft again and you can try the hardening over again.

[Adam Neat]

Here is a good PDF with the basic parameters for heat treating O1

http://buffaloprecision.com/data_sheets/DSO1TSbpp.pdf

[David Posey]

Tony, to get the magnet to work, put it in a pair of vice grips. When you get ready to check the iron, pull it out of the fire and bring the magnet to it. When it's non-magnetic the iron is ready. I usually have to heat treat in full sunlight, so I tend to rely on this rather than color.

[george wilson]

Try to not get the magnet too hot,as that will demagnetize it. I'd prefer to take the Alnico V magnet out of an old speaker and use it. Alnico V looks like surface ground steel. Don't try using rubber based magnets. I guess ceramic based ones would be o.k.. I just prefer the old time Alnico V myself. I'm not sure how sensitive two heat the new "super magnets" like LV sells are. They aren't real large,so might heat up quicker when touching to red hot steel.

I don't use a magnet myself any more,as I've done hundreds of heat treatments,but if I did,the old Alnico V has some mass to it. One out of an old 4" radio speaker is fine.

[Ron Hock]

Hi Tony,

Sounds like you got pretty close. I find the magnet the most reliable temperature sign for the "shade tree" heat-treater. I use the magnetic end of a carbide scribe but any magnetic parts pick-up type tool will work. You can't just leave the magnet on the piece until if falls off, though. DAMHIKT. Kluge up a way to either stick the magnet into the "furnace" or remove the piece to test. I test when the steel makes a leap in color brightness. The time-temp curve flattens out while the steel is undergoing the transformation from ferrite to austenite, the high temp crystal structure. So the steel glows progressively brighter as it heats but the increase in brightness levels out for a while during the phase change then jumps to a brighter orange. That's when I test with the magnet to be sure. When you've done it a few times you'll start to trust your eyes and won't need the magnet except to show others the magic.

I like peanut oil to quench because of its high flash point and it smells way better than any petroleum oil in this app. It will still catch on fire though so use all safety gear and keep the extinguisher handy. And don't even think about doing this in the kitchen.

Temper as soon as you can handle the piece with your bare hands. For Rc62, shoot for 325F. This you can do in the kitchen, especially if you have an oven with a digital control. The rule is, as you mention, 20 minutes per inch of cross section but for things as thin as blades, you can pretty much just get it to temp and you're done. A short soak is recommended, however and if you trust your oven to not exceed the target temp, you can leave it in there while you have some lunch. And no need to quench from here, just let it cool and you're good to go.

There will be some degradation at the surface from carbon that was lost at the high hardening temp so plan on giving the cutting edge an aggressive, deep honing to expose the good metal that lurks below the soft surface. If you test with a file, it may bite that soft surface but if you cut a bit deeper you'll feel the hard steel beneath.

There's more here: http://www.hocktools.com/diyht.htm

Good luck. Be safe. Have fun!

[Jacob Reverb]

Tony, to get the magnet to work, put it in a pair of vice grips. When you get ready to check the iron, pull it out of the fire and bring the magnet to it.

Another trick is to hang a magnet from a wire or piece of string, and simply wave the hot steel past the magnet. When the magnet no longer moves when you wave the steel past it, you've hit the critical temperature.

[Tony Shea]

Thanks for all the advise. I've used the blade just a bit more without doing anything more to it and it seems to be working ok. I'm not 100% happy with it but haven't decided if it's the hardening or the temper. I'm leaning toward the temper as the edge of the blade is more chippy than folded over steel. Sharpening the blade also proves to act as though the steel is hard. Running a file over it is not really that convincing to me. The file marks it up but really doesn't remove much metal. The blade is small so I don't really want to file marks into it to test it out, more than I already have. But the file is certainly not biting into it like soft metal. I'm just going to use it a bit more, make my matched hollow and see how it performs.

I like the magnet hanging from a string idea so as not to have to put the steel on the magnet and struggle with getting it off. As long as I hang it close enough to my bricks.

Is there some kind of thermometer I can buy that I can throw inside my oven to show the accuracy for tempering?

So, not many responses on re-hardening the iron. If it does turn out the blade is actually soft what methods should I do to re-harden my blade. Trevor's advise does sound reasonable about annealing. I do understand that annealing is a very slow cool down time but there is no need to soak the steel at the proper temperature for an extended period of time?

Thanks again for all the help.

[Adam Neat]

Something that had slipped my mind that I used before I got my oven is temp-laq paint.

Its a temperature indicating paint, you put it on your part and when the specified temp is reached the paint melts away. Its pretty accurate stuff and available all the way from 175 to 1900 degrees.

I would say you could get by with using your torch and getting 1500 for the heat treat and 400 for the tempering.

McMaster sells the stuff http://www.mcmaster.com/#temperature-indicating-paint/=js2tcl

[Adam Neat]

Oh and the re-hardening, I would just try re-heating and quenching, and not worry about annealing

[Larry Williams]

Tony,

First, I suggest you avoid using a MAPP torch. MAPP only burns a little hotter than propane, maybe 100º if I remember right, but all the MAPP torches I've seen generate a pencil-point flame. Your goal should be to uniformly bring the bit of the iron to critical temperature and the concentrated pencil-point flame makes this very difficult. You'll most likely overheat areas of the surface which will result in pitting.

I realize the torch I use is expensive and maybe hard to justify for a few planes. If that's the case, I think the one at this Harbor Freight link would work okay:
http://www.harborfreight.com/propane...tm_source=1002
I haven't used this torch and can't say for sure but I believe it would do the job. I can forward a coupon for this if you're interested.

Another thing is that I greatly appreciated Ron's information and help when I was trying to figure this all out. We still use peanut oil today because of his suggestion. When heat treating small things like what you're doing, though, I found the magnet unworkable. By the time I knew what the magnet was supposedly showing me, I'd lost a lot of heat in the small piece of steel.

With Ron's information and encouragement, I did learn to judge color with pretty good success. But I was heat treating outside in varying light conditions. The time I realized how unsuitable judging color was is when I taught a workshop and planed to heat treat at dusk on the loading dock of the school. It was about this time of the year and by the time we were set up the mercury vapor lamps had come on. The color shift of those lights made judging color impossible.

I knew I needed better consistency, the kind of consistency I found in old tools. I started reading all the old texts containing heat treating information I could get my hands on. When they did mention judging the phase change to austenite it was always cryptic. I found descriptions like, "when the steel opens", "when it sweats", "when the flux rises." Never any mention of color or magnets. Then I found a better description in a 1938 Machinery's Handbook in a section on heat treating high speed steel. I knew I'd seen what was described as soon as I read it. I set about finding a way to consistently get the steel to show when the phase change happens then wrote to everyone I knew who could help explain what was happening. I even wrote Ron and he didn't seem interested so I moved on. I had a lot of help from some pretty educated and experienced people and at one point a PhD metallurgist in Australia had the information run past some of the metallurgy faculty at Oxford University which is considered the World authority on the phases of carbon steel.

When heat treating steel going beyond critical temperature will increase grain size so will keeping at critical temperature or above for any length of time. If you look at the carbon content of O-1 steel you'll find a 15% variance in tolerances. Look at a phase chart and you'll see that 15% makes a difference as to what temperature the phase change to austenite happens. There are six or seven other alloys in O-1 that also have variable tolerances and these also impact critical temperature. If you want to work accurately, work to the visual signs of the phase change.

We've made a significant effort to make sure the work we do in our shop is the best we can and that the information in that DVD was the best we could provide. If you actually use that information, I believe you'll get very good consistent results.