O1 tool steel

[Brian Madigan]

O1 is one of the most commonly available tool steels.

 

Don't be fooled by its simple chemistry though, it is not easy to get maximum performance from O1 without proper equipment.

 

Typical chemistry:

0.90% C, 1.0–1.4% Mn, 0.50% Cr, 0.50% Ni, 0.50% W

 

O1 is usually through-hardened to 61-65HRC and tempered back to the desired hardness which can be as high as 60 HRC for a knife blade.

 

Evenheat oven w/ Rampmaster controller. This formula works for my oven which is an 18" by 6" 120V model.

I do one blade at a time if they are longer than 6". I don't do batch processing with this oven as it generally results in uneven heating and warping.

Hardening:

Segment 1

Pre-heat to 1300F

Ramp cycle 50%

Soak 15 minutes.

 

Segment 2

Heat to 1500F

Ramp cycle 75%

Soak 30 minutes

 

Quench in Parks 50.

 

Tempering:

Minimum 2 cycles of this program:

 

Segment 1

450 F

Ramp cycle 100%

Soak 2 hours

air or water cool to ambient temperature.

 

You can cryo treat in between the tempering cycles.

 

Without soaking at the right temp for enough time, you won't get the best performance out of O1.

 

[Gary LT]

Brian, thank you. I am working with some O1 tool steel right now (1/2" round stock, finishing flat to 4mm, blade under 4"). Based on this formula what hardness would you arrive at? I ask as I am looking for 62 or minimum of 60.

Thank you, GT

[shawn patterson]

I learned with 01. I still love it. From what I remember reading back when is that 01 needs long soaks to get maximum performance. I don't know how that translates with pieces as thin as we use for blades but, for thick stukk at work they would soak the hell out of it.

[Isaac Humber]

Very imformative post, thank yuo for putting that information up. All this talk about soaking... can someone explain the science there? What exactly is the advantage ?

[Alan Longmire]

Soaking is for high-alloy steels only. In a hypereutectic (carbon greater than 0.84%) steel like O-1 that has strong carbide-forming additives like tungsten and chromium, when annealed the carbides tend to segregate along grain boundaries which takes them out of play for what we want them to do in service, i.e. to be spread evenly throughout the crystalline lattice and ideally within the iron crystals themselves. Soaking allows these slow-moving carbides to do this. If you just bring it to critical and quench without a soak you are robbing this particular alloy of some of its more important properties. It will still be deep-hardening and hold an edge, but the carbides will just break off since they are not part of the crystal matrix unless soaked. From the perspective of the end user of a blade, you'd never notice the carbides breaking off the edge as they are microscopic, but you would notice a difference in edge-holding ability compared to a similar blade that had been soaked properly.

 

Conversely, soaking a low-alloy or simple steel only provides opportunity for grain growth. The tungsten in O-1 acts to pin grain boundaries, thus preventing growth at heat. With a hypoeutectoid (less than 0.84% carbon) low-alloy steel like 5160, there is no excess carbon to form carbides with the low chromium content. In that alloy the chromium just provides some toughness, wear resistance, and deeper hardenability, which is why it's such a good spring steel (and sword steel!). 52100 on the other hand has excess carbon and enough extra chromium to form carbides, which is why it is harder, tougher, and able to hold an edge longer than 5160. It also tends to be more brittle, which is why it is an excellent ball-bearing (and knife) steel but not so great as a spring (or sword) steel.

 

Again, this is for O-1 and other high-alloy tool steels only. Do not soak 1095, 1084, or anything like that. you'll just grow the grain.

[Brian Madigan]

Brian, thank you. I am working with some O1 tool steel right now (1/2" round stock, finishing flat to 4mm, blade under 4"). Based on this formula what hardness would you arrive at? I ask as I am looking for 62 or minimum of 60.

Thank you, GT

I can only guess I'm getting around 58-61 HRC. There's no way to tell without testing the hardness with a proper testing rig, which I don't have. For thinner flexible blades, the 450-500 is probably a good bet. If you're making short, wood carving blades or chisels, 350-420 is fine. You want more toughness in a blade that's going to flex like a fillet or boning knife. I have a few unfinished O1 boning and fillet knives that are tempered to 450. They are a nice blue color after tempering, and can be bent 90 degrees without taking a set. They also tolerate a very fine edge (>15 degrees) so are very sharp!

 

Also, the long soak times will cause a lot of decarb, especially in an electric furnace with no control of the environment. I use ATP-641 coating to prevent decarb on knives that have been ground and polished before heat treating. I have not found anything short of foil wrap to prevent scale better than this stuff:

 

http://www.brownells.com/gunsmith-tools-supplies/metal-prep-coloring/heat-treating-accessories/anti-scale-coating-prod23076.aspx

[Gary LT]

I appreciate the answers Brian and everyone.

That leads me to another question in similar vein as Alan brought up some good points as well. That question is "shallow hardening" vs. "medium" or "deep hardening" steels. Pardon my ingorance but I judged this nomenclature as strictly related to carbon .%

What guidelines should someone follow for heat treating, hamon creation, edge quench, full quench? In the past I've over heated 5160 and 1095 and 1084 by thinking they required a longer soak time. I use a blown forge and haven't really tied a pyrometer to it although it;s time probably to do so.

(Lots of stray questions......sorry but I am off this week and busting at the seams to do some forging!)

 

Thank you all, GT

[Alan Longmire]

Gary, the carbon content has absolutely nothing to do with how deep or shallow hardening a steel is. Well, there has to be at least 0.3% carbon for it to show any hardening at all, and once you get over about 1.5% things get really weird, BUT: The two elements added to steel that have the greatest impact on hardenability (same thing as depth of hardening) are manganese and chromium. Manganese is the main offender ( or saviour if you like through-hardened blades) in steels that don't make good hamon. Anything over about 0.35 to 0.4% Mn will harden deeply enough that you can't get a really active hamon. That's why 1084 for all its qualities is not a good choice for hamon. Yes, it is considered a simple 10XX steel, but it has around 0.8% Mn content so it hardens too deep/fast to form much more than a simple line. Chromium is an even stronger hardenability-influencing element, which is why 5160, despite having only 0.6% carbon, will through-harden up to an inch thick.

 

The old standard demonstration of this concept was to take two chunks of 1" round drill rod and harden them, then snap off the ends to show the structure. A W-1 or 1095 drill rod will only fully harden to a depth of around 1/16" from the surface no matter if quenched in oil or water. An O-1 drill rod will harden right through to the center. It's that 1% Mn and the 0.5% chromium that does that.

 

Think about what this means for hamon production by any method. If we define hamon as a differential hardening of a blade that shows a wide band of mixed crystalline structure from the fully martensitic edge to the fully perlitic spine that reaveals a bunch of interesting shadowy and frosty stuff when polished, then by definition you want a steel with low hardenability, or a shallow-hardening steel. You simply cannot get that wide band of mixed structures if the steel is a type that wants to harden all the way through. That's why you can get great hamon with Aldo's 1075 and 1095/W-1/W-2, but not his 1084. The only purpose of the clay treatment to encourage certain types of hamon is to add a little extra control of how and where you want hardening to take place. With these alloys you have only A SECOND or LESS to drop the temperature of the steel from 1425 degrees F to less than 900 degrees F if you want them to harden at all, and the clay helps create turbulence in the quench that causes thicker parts of the blade to miss that curve from just enough not to harden all the way to enough not to harden at all.

 

If you know that you have a steel that will harden up to 1/16" from each side, that means you can through-harden a blade that's up to 1/8" thick. Anything thicker will not harden fully all the way through, which leaves a visible mixed structure of martensite, perlite, bainite, and ferrite on the junction between fully hardened and fully not hardened. We call that visible structure hamon.

 

Now then, if you clay up a blade made from O-1 or 5160, steels that will through-harden to a depth of an inch or more and with which you have something like TEN to FIFTEEN seconds to drop the temperature from 1550 degrees F to less than 900 degrees F, what does that tell you about the chances of that steel to produce a wide band of mixed structure of fully hardened through totally unhardened steel? If you answered "close to zero" you're right. You can edge-quench and get a line between fully hard and not-so-hard, but both of these steels are so deep-hardening that in knife-blade sections they can even harden somewhat in air. Clay it up and you'll still get some hardening under the clay even if edge-quenched, because that cool edge is going to be pulling heat out of the spine fast enough to allow it to harden. You can polish a blade like this all you want, but it's not going to ever show a lot of activity since that mixed structure is pretty much not there. You can soak it longer or hotter, but all that's going to do is cause alloy banding and grain growth. Well, if it's O-1 the tungsten and chromium work together to greatly slow down grain growth, as does the vanadium in W-2.

 

Gets complicated, eh? Keep reading everything you can find, it's all here on the forums.

[Gary LT]

I knew you would respond! Thank you Alan.

Yes it can be complicated/confusing when you try to extrapolate this and that from here and there. And just think, I could have held off another 61 years, never asked and never heard the truth!!

Merry Christmas!

Gary

[Simon W]

This was a surprisingly interesting read, and really cleared up my understanding of hamon formation in different steel alloys. Thanks Allan!

Edited December 25, 2013 by Simon W

[harry_r]

Very good info Alan, thanks a lot. Each time I think I begin to understand
this all a bit more. One area that I am having trouble wrapping my skull around is:

 

"the clay helps create turbulence in the quench that causes thicker parts of the blade to miss that curve from just enough not to harden all the way to enough not to harden at all."

I always understood the thicker coating at the spine to be insulating rather than water jacket affecting, and heard that one could leave the edge clay free (normally a thin wash coat to avoid scale is used, I thought) and still get a hamon.

 

I think I get the water jacket and turbulence, and how that can help in rapid heat dissipation, as opposed to an air/steel interface. I have heard that brine, as opposed to pure water, helps in this regard as well.

 

But again, it never struck that the wash coat of thin clay at the edge is helping with rapid cooling and I would have believed that any clay coating would result in the opposite, i.e., slower cooling.

 

Cheers

[Alan Longmire]

Yes, you do leave the edge clay-free, or a VERY thin coat at most, because you really need that to harden. The thicker clay does act as insulation to a degree, but it is my understanding (and I reserve the right to be wrong!) that it's more about disrupting the quench just enough to miss the nose of the hardening curve than it is about insulating. And brine is a faster quench than water because of its reduced turbulence during the vapor phase of the quench, but is less stressful than water because of a more stable nucleate boiling phase, which is where most of the heat transfer happens.

 

I will say again what I told Wes in one of his threads, that is, I am not an expert on hamon by any means. I know the theory, but they aren't really my thing so I've only done a couple of them.

 

Maybe we should split this thread into one about O-1 and one about the metallurgy of hamon?

[Freya W. Ward]

Not sure if it was mentioned yet, but it's been my experiance that O-1 can sometimes spiderweb like a piece of tempered glass if you wait too long to temper after the hardening. It's only done it to me once, but after that it goes straight into the oven to temper after the hardening

[Wes Detrick]

lol, did I hear my name?

 

Man Alan, you are a fount of information.

 

In the spirit of testing a knife the other day, I forged and ground one out of 1080, and then heat treated it. Stuck in my vice, and broke it in two. I was expecting to see very fine grain as I normalized 3 times in descending temps and finally quenched at 1500. Wrong. Grain was huge. And I had no idea why until now. I was letting the steel soak for 10-15 minutes before air cooling it. I had absolutely no idea that soaking at temp with simple non-alloy steels would cause grain growth. Thank you, thank you, thank you so much for this!

[Matt Todd]

My head just exploded

[DFogg]

Well done, I copied this thread

[jake cleland]

Alan, somewhere on the forum there is a Japanese paper that demonstrates that the thin wash of clay on the edge actually increases the speed of the quench dramatically, by disrupting the vapour phase of the quench, precipitating nucleate boiling, and by increasing the surface area for heat dissipation in the boiling and convection phases of the quench (this latter being a problem, as that is when most cracks happen, but also sori). of course this is only in water, as oil has little or no vapour phase.

 

It was also my understanding that in hypereutectic simple steels (1095, W1, W2) a degree of soak time was still desirable, provided you have the capacity to soak at the lowest possible austenitising temp, i.e. no hotter than 1425f, grain growth being more dependant on temperature than time at lower temps, and increasing more rapidly the hotter you go...

[Jerrod Miller]

Jake,

Something to keep in mind: Both grain growth rate and diffusion rate are temperature dependent.

 

Generally speaking temperature in diffusion and grain growth calculations is in the denominator of and exponent {for example: e^(1/T)}. Depending on if you are calculating based on grain boundary segregation, pure material gran boundary movement, or any number of other situations there are a lot of other factors that will go into the equation, both preceding the exponent and within the exponent.

Example: Diffusivity along grain boundaries. Ds = Dso e^(-Q/RT)

Ds = Diffusivity on the surface

Dso = Diffusivity constant (material specific)

e^ = constant exponential

Q = Activation energy for diffusion (material specific)

R = Universal gas constant (turns out it works for my than just gas, but it already had a name and apparently nobody wanted to change it)

T = Temperature

 

Again this is just a general example to show where temperature fits in. This shows that if keeping everything else constant then higher temps will allow things to move around more. Keeping everything else constant is not really possible because changing the temperature also changes the other factors, as could be supposed by looking at the phase diagram.

 

Moral of the story: Soaking at lower temps will slow down grain growth, but it will slow down diffusion too. If you knew everything possible about your given piece of steel then you could theoretically know a sweet spot for things to work out perfectly, but you would probably have to know grain size, carbide size, exact composition, and who knows what else. I did say "probably" though because it may be that all those things may add up to a small enough difference that it doesn't change that potential sweet spot (if it really even exists) very much.

[B. Norris]

 

It was also my understanding that in hypereutectic simple steels (1095, W1, W2) a degree of soak time was still desirable, provided you have the capacity to soak at the lowest possible austenitising temp, i.e. no hotter than 1425f, grain growth being more dependant on temperature than time at lower temps, and increasing more rapidly the hotter you go...

 

Provided I have understood everything I've read, correctly (a stretch, I know!) The soak (at lowest possible austenitization temperature) allows the extra carbon, in a hypereutectic steel, time to disperse more evenly throughout the blade and promotes formation of very fine, evenly dispersed carbides. So... Hypereutectic, simple steels such as 1095, W1, and W2 benefit from soaking. As long as you have good temperature controls. Information on 1095 specifically taken from the topic How the hell do you heat treat 1095?!, on BladeForums. More can be read about this under the topic Heat Treating in a Kiln and Grain Growth, on Kevin Cashen's Forum. There are links within the thread to other threads detailing a 5 hour soak on O1 and a 24 hour soak on 52100.

 

Here is an excerpt:

 

For austenite grain to grow the grain boundaries must become unstable enough to move, and there are two really good safety mechanisms in modern steels. The first and most powerful are carbide forming elements that will do their work in the grain boundaries and lend great stability to them until they are dissolved. With something like Vanadium for example you will need temperatures in excess of 1900F to break the bonds, otherwise your wait will be very long indeed if you plan on relying on time alone to grow grain. however even in the absence of carbide formers most modern steels are de-oxidized or "killed" using aluminum before going to the ingot, giving them the label of fined grained steel. These steels are separated from their forerunners which were killed with silicon alone are are referred to as coarse grained steels. Course grain steels have a very low grain coarsening temperature and will readily commence with a steady grain growth as soon as full austenite is achieved. Fine grained steels will resist grain growth to a higher grain coarsening temperature but will then rapidly grow in grain size once the drag effect of the aluminum nitrides is overcome.

 

All bets are off for those of you making your own steels!

 

~Bruce~

[Jeppe]

How much will O1 steel change it's shape when heat teated? I am planning to make some blades by stock removal

and the question is how much I will need to compensate/adjust the ground shape to get the final shape that I want.

[Alan Longmire]

 

How much will O1 steel change it's shape when heat teated?

 

Very little indeed. O-1 is meant to be used in applications that require greater dimensional stability than W-1. As in, you can quench a piece with drilled and tapped holes and (provided you didn't do anything to cause a warp) the part you fitted to it pre-HT will still fit.

 

Interestingly, one reason the air-hardening steels were developed because there are certain critical applications for which even O-1 was not dimensionally stable enough. Knifemaking is not one of these critical applications, luckily. We're talking about multistage dies and punches with radical section changes and lots of precise holes, slots, and tabs.

[Justin leonard]

ok so for a beginner is O-1 a decent steel for stock removal or should i jst continue with a 10XX steel?

[Wes Detrick]

Stick with the 10XX steels. To get the most performance out of it, you have to let it soak for a decent amount of time at critical(+10 minutes) during heat treatment. Lots of people don't have the temp control to do that without overheating the steel. It takes time for all of the carbides to get into solution. If you have the means to do long soaks or send out for heat treatment, then you can get the most for your money.

[Justin leonard]

ive got full aces to a HT oven