Accurately heat treating small tools

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Blog entry by lwllms posted 11-22-2012 09:16 PM 4570 reads 18 times favorited 18 comments Add to Favorites Watch

Let’s see if I can write this without getting real technical. It’d be a good thing because I don’t have a PhD in metallurgy or physics and I don’t fully understand a lot of what’s happening in heat treating tool steel.

Like water, tool steel exists in several states. Water can be vapor, water or ice. Tool steel is similar.

In its soft state tool steel is made up of ferrite and cementite, iron molecules and iron/carbon molecules. When heated to critical temperature, around 1450° F, the steel changes states or “phases” and becomes a different crystalline structure called austenite. If allowed to cool slowly, it returns to its ferrite and cementite phase. If cooled quickly, it changes again and becomes martensite. Martensite is the hard and stable state or phase we’re looking for.

I started out using methods I’d read about for determining when the phase change to austenite occurred. First I tried a magnet and found I really needed three hands to check the steel for loss of magnetic property. By the time I fumbled around with the magnet, the steel had cooled somewhat. I taught myself to work by color but I knew I was always guessing and color changed with ambient light conditions. I knew I needed something more dependable after teaching a workshop where we ended up heat treating on a loading dock after dusk and under mercury vapor lights. None of the plane irons ended up as hard as they should have been and we had to heat treat twice.

I started reading everything I could find on heat treating. Modern sources didn’t have anything to offer because they all dealt with using a furnace. We have a computer controlled heat treating furnace but using it is an all-day process. Hardening a batch small molding plane irons can be done with a torch can be done in less than an hour.

The old texts never mentioned judging color or using magnets. They were cryptic in their description but all mentioned some visual indication of a change in the steel. They said things like, “when the steel opens,” “when the steel sweats,” or “when the flux rises.” Finally, in a 1938 Machenery’s Handbook I found a description of “rising flux” in the section on heat treating high speed steel. “Oh, I’ve seen that,” I thought immediately. I set about being able to get the steel to show this with dependable repeatability.

When you harden small tools, the first thing you need is a good heat source. I use a Goss propane torch with a BP-5 head to get a flame pattern that’s about 2 ½” in diameter. Here’s the torch:

Stuff on the Internet often suggests using a MAPP torch. Don’t do that!! A MAPP torch has way to small a flame pattern. The flame pattern is a pencil point and the flame temperature is only about 100° hotter than propane. If you insist on using hand-held tank/torch units, use two propane torches with flame spreaders attached.

If you use a MAPP torch you’ll most likely overheat areas on the surface. If you do, you’ll end up with pitted steel; at the least, you’ll warp the steel.

We use O-1 steel and quench in peanut oil. Here’s the set-up in our shop but the vent hood isn’t visible in the photo. If you don’t have a vent hood, do this outdoors.

There are some important things that need to happen for this to be dependable. First the steel has to be clean. It it’s old steel it need to be lapped to make sure no rust or pitting remains. Secondly you want to preheat the steel. In the video, when the iron is moved to the quench, you can briefly see an iron behind the working area preheating.

A brief description of what you see in the video: When the steel makes the phase change there’s a volumetric change. It’s like water expanding as it freezes but tool steel shrinks slightly in the phase change to austenite. In the austenitic state the carbon has combined with the iron to form a crystalline matrix but the carbon is basically in solution and is free to flow through the steel. At this temperature the steel is above the flash point of carbon and the carbon works its way free to burn off. Each carbon atom that leaves the austenitic crystalline matrix leaves behind iron that’s no longer part of the matrix and is no longer in its shrunken state. It rises to the surface and forms small pools or puddles. A link to the video:

After hardening the steel is too hard and brittle for good use. You then need to temper the steel. I bake it in an oven at 350°F for 45 minutes to an hour.

It’s important to know when the phase change happens. The finest grain steels are created by heating only to the critical point for phase change and not kept at critical temperature for longer than necessary to get a uniform heat soak and complete phase change.

This is the most accurate method I’ve been able to find. Steel loses its magnetic properties below the actual critical temperature. Critical temperature is determined by the actual carbon content of the steel and specification tolerances for O-1 steel allow for a 15% variance in carbon content. When I use our furnace I have to assume carbon content is on the high end and end up heating to a higher temperature than I likely have to.

We heat treat tools that already have the bevel formed. In the video you can see the iron being moved in and out of the hottest part of the flame to keep from overheating the thin steel at the cutting edge.

The small pools of pure iron left on the steel are soft and about a molecule thick. They’re easily removed on honing stones.

You’ll also notice some honeycomb ceramic elements under the steel. I salvaged these a number of years ago from a propane barbecue grill put out for trash pickup. These are really helpful for helping heat the back of the tool as its face is heated. I didn’t pay any attention to the make of that grill. Here’s a photo, maybe someone will recognize it and know the make of the grill:

18 comments so far

View b2rtch's profile


4867 posts in 3250 days

#1 posted 11-22-2012 09:50 PM

Whoooaa! very informative.
Thank you.
Happy Thanks Giving Day.

-- Bert

View Don Johnson's profile

Don Johnson

702 posts in 2982 days

#2 posted 11-22-2012 10:06 PM

Mettalurgy was included in my college course in the UK – some 50 years ago – but I later spent over 30 years working for a company making springs – from both wire and strip. Although I mainly dealt with IT, I also produced electronic devices that assisted automatic production and I obviously became familiar with heat treatment processes as is was the most important part of making functional springs. However, throughout those years I did not read an explanation of the processes involved that was clearer than yours. I wish that it had been available both for my own clarification and when trying to explain heat treatment to operators.

When hardening individual items, I had never noticed/been aware of the small pools or puddles that you describe, which would have been a great indicator when hardening small tools – although obviously this would not have been visible during mass treatment in shaker hearth furnaces.

Despite the fact that I have been retired now for seven years or so, I found your video and explanation very interesting, and thank you for publishing it here. I may even find it helpful if I do have to harden any items, although I must admit to depending on power tools rather than hand versions.

-- Don, Somerset UK,

View tomd's profile


2175 posts in 3972 days

#3 posted 11-22-2012 11:14 PM

Many welding suppliers carry temperature crayons. They come in different temperature ranges, you write on the steel and while heating it you watch for the crayon to melt then you are at the temperature for that crayon. That is the system I use, but I’m not sure how accurate it is, but at least your in the ballpark.

-- Tom D

View Brian Strothcamp's profile

Brian Strothcamp

114 posts in 2895 days

#4 posted 11-23-2012 02:28 AM

If you see sparks coming off carbon steel its too hot… a neat way to note that.
It means you are burning off the carbon and basically ruining it.

View Planeman40's profile


1307 posts in 2962 days

#5 posted 11-23-2012 04:47 AM


This is one of the best explanations I have ever seen! In years of oxy-acetylene welding and heating of steel I have seen these “puddles” often and never knew their significance. You are doing a GREAT service to those of us who would like to make our own tools.

I know some other Lumberjocks have oxy-acetylene welding equipment so I would like to throw in the following. You can “flame harden” the surface of steel by heating it until it is red hot using a “carburising” flame of a torch. As those of us who weld with this method know, you have to balance your flame so there is neither an excess of oxygen or acetylene. Excess oxygen “burns” the steel (oxidizes the steel making a poor weld) and an excess of acetylene injects carbon into the steel making the weld brittle. But by heating the steel with a carburising flame (adding carbon the the hot steel) you can increase the carbon in the surface of the steel as a way of hardening it. Years ago while visiting at LeTourneau-Westinghouse where huge earth movers were being built, I watched a welder doing this to a huge gear that was about 4 ft in diameter. He sat on a stool playing the torch over each tooth, one at a time. It took a while but it got the job done. The advantage of this method is the carbon only penetrates a short way into the steel (about 1/32” to 1/16” as I understand) leaving the rest of the steel unhardened to take shock and stress.

I hope this thread has others with input.


-- Always remember: It is a mathematical certainty that half the people in this country are below average in intelligence!

View nobuckle's profile


1120 posts in 2962 days

#6 posted 11-23-2012 02:21 PM

That’s way to much technical information at this time of the morning. Seriuoslly though, I found this information to be very intersting. I’m curious to know the Rockwell of the O-1 after you’ve heat treated it.

-- Doug - Make an effort to live by the slogan "We try harder"

View Stephenw's profile


273 posts in 2587 days

#7 posted 11-23-2012 03:17 PM

“Like water, tool steel exists in several states. Water can be vapor, water or ice. Tool steel is similar.”

Water can be gas, liquid, or solid.

To be taken seriously, your opening statement needs to be accurate.

View Woodstock's profile


255 posts in 3489 days

#8 posted 11-23-2012 04:47 PM

Back in college my adviser/instructor/mentor/and eventually my boss was a retired metallurgists.

In my metallurgy course he told the story of one foundry he worked at who had a deal with a local leather manufacturer in that the foundry bought all of the scrap leather cut-offs. The foundry would pack the gears that were to be case hardened in a ceramic heat treating oven with the gears packed tight with leather cut-offs surrounding the gears on all sides. And then they would heat up the ovens in a specific time temp schedule. The leather would burn off creating a high carbon environment which at the right temp and state the gear surface would absorb the excess carbon down to aprox 1/32 – 1/16” while leaving the interior material in a softer more malleable state. So the gears would at the end have a hard wear surface while leaving the steel with a soft inner core to take the shocks and not become brittle and crack so easily. Different method but same principle as described here by Planeman40.

As for the use of magnets- I can remember some of the local blacksmith workers who used pneumatic power hammers to fold and forge custom steel knife blanks together, always had a small magnet with a long handle stuck to the side of the hammer. When the hot steel being forged cooled to the point that the magnet stuck to the blank he had to reheat the metal blank as the phase needed to weld and shape the metal was too cold to effectively forge the metal together. Most of them had done this enough that they could just go by the color of the hot steel to know when to reheat.


-- I'm not old. Just "well seasoned".

View Dennisgrosen's profile


10880 posts in 3317 days

#9 posted 11-23-2012 08:42 PM

thanks Larry
compliment the DVD of yours I have :-)


View lwllms's profile


555 posts in 3483 days

#10 posted 11-23-2012 11:26 PM

Stevenw wrote, “Water can be gas, liquid, or solid.

To be taken seriously, your opening statement needs to be accurate.”

I assume that comment represents an unfortunate lapse in judgment on your part and not a troll. The term “vapor” is an appropriate term to use when discussing the phases of water.

A screen shot from

A quote from Wikipedia:
”A vapour (British spelling) or vapor (see spelling differences) is a substance in the gas phase at a temperature lower than its critical point.[1] This means that the vapour can be condensed to a liquid or to a solid by increasing its pressure without reducing the temperature.”

View Stephenw's profile


273 posts in 2587 days

#11 posted 11-24-2012 12:15 AM

Sorry I hurt your feelings.

States of Matter

View lwllms's profile


555 posts in 3483 days

#12 posted 11-24-2012 12:37 AM

View lwllms's profile


555 posts in 3483 days

#13 posted 11-24-2012 01:45 AM


” I’m curious to know the Rockwell of the O-1 after you’ve heat treated it.”

O-1 should be about Rockwell C64 after hardening. The final hardness is determined by tempering.

We have a Rockwell hardness tester but it hasn’t been calibrated since 1974. I wouldn’t want to really trust the results from it. Thomas Lie-Nielsen didn’t think new ones are worth buying and my business partner doesn’t think it’d be worth the high cost of getting ours cleaned and calibrated just to confirm what we already know.

When I did the plane making video we were in Mane in Thomas’ shop and he had the irons I did for that video tested. The both tested at RC60. A young guy in Finland was getting a college degree in Architectural Historic Preservation and made some planes as part of his program. He had to make the irons under the supervision of the head of the university’s metallurgy department. The head of the department said he’d never heard of such a method and didn’t think it would work. In four different heat treating sessions, using what I described here and on the DVD, the student produced irons that tested a consistent RC61. The head of the metallurgy department was surprised and said the student got more consistent results than they normally get in a single batch using the furnace.

I didn’t just run with this after I was able to consistently get what I wanted. I felt I needed to understand the process. I ran it past a number of people. A PhD metallurgist in Australia used his contacts to run it past the faculty at Oxford University, which is considered the World authority on steel and its phases. It turned out that this is well known and considered a flaw in commercial heat treating. They use inert atmosphere furnaces or stainless foil wraps to avoid this. Most of what is commercially heat treated involves large runs of precision things like hardened gears. Small iron deposits on the surface add production steps to clean the parts up. We’ve now had a few generations of commercial heat treaters using the high volume methods. The techniques I show here are best suited to very limited production of things like plane irons where the iron deposits on the surface are irrelevant but fine grain and accuracy are most important.

One thing I should add here is that it’s natural to expect the phase change from austenite to martensite to be instantaneous on the quench. It’s not. When we made plane maker’s floats using O-1 the side floats would usually warp slightly in the quench because of the surface area differential of the two sides. I’d have about 10 seconds to straighten them after quenching. If I was too slow, I broke them. Highly alloyed steels A-2 and others take longer for the austenite to change to martensite. Some steels it can be up to six months. That’s what cryogenic tempering is for—change the retained austenite to martensite quickly so the item can be put into service. Cryogenics does nothing for normal high carbon steels like water or oil hardening steels.

View oldnovice's profile


7330 posts in 3569 days

#14 posted 11-24-2012 04:07 AM

OK, This good on heat treating, how about cryogenic treatment or does that only apply to cutting bits for metal?

Back about 30 years ago my company was investigating cryogenic treatment of all the milling/drilling tools for extended cutting life and I know that many companies use it today on a regular basis.

-- "I never met a board I didn't like!"

View lwllms's profile


555 posts in 3483 days

#15 posted 11-25-2012 12:58 AM

Cryogenic treatment is for highly alloyed steels that retain austenite for a length of time after “quenching.” While way too wordy, I did mention that in the last paragraph of my last post.

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