Purpose of "soak time"?

R

R.C.Reichert

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Jul 26, 2008
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Newbie question here....

I am curious about the benefits obtained by having a "soak time" for steel prior to hardening. I am using 01 steel and have been told that it benefits from a certain amount of soak time, like holding it for around 10mins at just above non-magnetic. In what way does it benefit the steel and could doing this cause unwanted grain growth? Some say just to heat the blade to non-magnetic and quench it. What is the best thing? Also, is this any different with a steel like 1095? Thanks.
 
Alloy steels have elements to retard grain growth, those same elements that slow grain growth slow the carbon from getting into solution. You need a soak time to get an even carbon distribution. Provided you are not overheating, you should not run into grain growth with a soak.

The simple steels have lower soak times. Holding just above non-magnetic may not really be a "soak" since you have no way of knowing where you are in the austentizing cycle. You could be causing grain growth, just sitting below austentizing levels or soaking properly. Or you could be moving between all three.

In my opinion, the best thing is to use a simpler steel than O1 if you can't control the temperature. 1084 or 1080 will both make exceptionally nice blades with very low-tech heat treating and they don't suffer from rapid grain growth as badly as low-maganese/high-carbon steels like 1095, do not have the stringent time/temperature window of 1095 nor do they require the soak times that O1 does.
 
Kevin Cashen recently had a good post on this subject.

The reason most steels need to soak is that there needs to be enough time for the alloy elements and the carbides to distribute equally throughout the steel. The only steel that can be heated to a little above non-magnetic and quenched immediately is 1084, the eutectiod. It will come into solution, and since there is no excess or shortage of carbides, or major alloy elements, it can be quenched as soon as it is fully at the austenitization point. A short soak of a minute or two is still a good idea, though.

Other steels, like O-1, need time to get all the alloy elements and such distributed. Five to ten minutes is more than enough for most carbon steels. If you are using an uncontrolled heat source ( torch, unregulated forge, etc.) then holding it at the exact temperature is nearly impossible, so just hold it as long as is practical and quench. The thing to avoid is overheating the blade ( which is very easy to do).

Stacy
 
It depends on the steel and the alloying elements in it. Take a stroll around the Crucible Steel website and look at the HT info they provide for individual steels. It seems that the simpler the steel, the less soak time it needs, although I'm sure this is a gross generality.

If you have time, search for threads and posts here by mete and Kevin Cashen and bladsmth and others. Those are some guys who know a bit about HT.

EDIT well, speak of the devil... :)
 
Thanks for the replies. That's about all I have is a torch...a JTH7 Bernzomatic to heat my blades. I would love to get some 1084, but not sure where to get it around where I live. I can get 1080 or 1075 though. Would those be just as good? I still have to manage with the O1 that I got for now.
 
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At his Blade show presentation, Kevin showed some micrographs of O1 held for 6 hours, still looked good. O1 held for just a brief period looked really bad. The thing is, all the hard pockets will still skate a file, but in use it won't hold a good edge because all the soft spots (on a really small scale). Kind of like playdoh with rocks in it.

Nothing bad happens if, while you're heating it, you happen to fall under the austenitizing temp, you just won't be dissolving anything or moving any carbon around while you're under temp. Some people heat treat it three times to achieve better distribution in a steel that needs a soak, but you can achieve the same thing with just an equivalent amount of soaking. Just keep heating it.
 
R.C.Reichert said:
Thanks for the replies. That's about all I have is a torch...a JTH7 Bernzomatic to heat my blades. I would love to get some 1084, but not sure where to get it around where I live. I can get 1080 or 1075 though. Would those be just as good? I still have to manage with the O1 that I got for now.
Click to expand...

Doing O1 with just a torch is going to be hard to pull off. If the blades are short it is possible. If you had a couple of insulating bricks (Wayne Goddard "one" or "two" brick forge) it is easier (not easy). The things you have to control are eveness of heating, and temperature range. If the steel is all the same temperature (color) a magnet will tell you when the O1 starts to austenitize. The phase change colors will tell you when the steel is about Accm. A touch more color, then held at that color for 10 minutes minimum... evenly colored, one end to the other... will soak the steel (move the alloys in the steel to the places they need to be to maximize the O1's potential).

Here is what is problematic. O1 phase change colors are not all that obvious and consistent, continuous color held through out the process. And then there is holding a blade for the heating time to Accm and the 10 minute minimum soak... it heats the holding implement.

I've done O1 in a "two brick" forge. I've come close to "right" based on after quench and after temper HRc testing. I'd try it with a soft torch like you have but I'd learn the techniques on scrap first. If you have a fire brick (a soft brick would be better than a hard brick), you might get help by heating the blade between the torch and brick. The brick would pick up some heat and maybe help with eveness as you turn the blade side to side in the torch flame.

Mike
 
Soak dissolves carbides and diffuses that carbon through the matrix. You should saturate the matrix to get maximum properties. Soak time depends on type of carbide. Mo, V, and W carbides are particularly difficult to dissolve and require longer times and higher temperatures.
 
Thanks for the suggestions. What I have been doing is basically using a couple fire-brick (hard type because I can't find the soft kind anywhere and I don't have a forge) as a backstop and then moving the torch flame very quickly up and down the blade. So far it seems to do a pretty decent job of keeping the color even. I suppose I just need to hold the color for a longer period of time. I was contemplating setting one of the fire-bricks, which are about an inch and a half thick on the stove burner to get it nice and hot and then using that for added heat to lay the blade on and torch it, but I don't know if that would help much or not. Guess I gotta get creative.:D Or maybe I should set it on the barbecue?

The blades I have ready are about 4-5 inches long max. on the cutting edge.
 
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Unless you can get the brick to 1500F on the stove top (you can't) it would make the HT impossible.

Set up the bricks in a "U" shape, and hold the blade in the "U", edge down. Use the torch to heat it as evenly as possible. Have a friend touch a magnet (one of those extension pick up wands for a buck at HF) to the blade every now and then as it gets red. When it stops being magnetic, notice the color. You want to heat it just a little higher than this color. Heat to the (admittedly guessed at) austenitization temp and hold for at least two minutes. Quench in ATF if no quench oil is available.

Not a proper HT, but it may work with O-1. 1084/1080/1075 will work with the same procedure with much more dependable results.

Stacy
 
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R.C.Reichert said:
Thanks for the suggestions. What I have been doing is basically using a couple fire-brick (hard type because I can't find the soft kind anywhere and I don't have a forge) as a backstop and then moving the torch flame very quickly up and down the blade. So far it seems to do a pretty decent job of keeping the color even. I suppose I just need to hold the color for a longer period of time. I was contemplating setting one of the fire-bricks, which are about an inch and a half thick on the stove burner to get it nice and hot and then using that for added heat to lay the blade on and torch it, but I don't know if that would help much or not. Guess I gotta get creative.:D Or maybe I should set it on the barbecue?

The blades I have ready are about 4-5 inches long max. on the cutting edge.
Click to expand...

Wayne Goddard has described using a brick back stop in straightening vehicle coil springs. He rounded out a depression in the brick and the intent is to have the flame wrap around to some extent.

My thought is, with a blade, this will help. But I would rotate the blade side to side (even heating) and have a space between the blade and the brick so some wrapping around can happen.

As far as preheating the brick(s), experimenting will show whether it is worth the effort. I would try without first and work with getting the flame to wrap around the blade as much as feasible.

One thing a person could mess with in your situation is coating the fire brick with a heat reflecting product... ITC-100, Larry Zoeller's Plistex-900, etc. If the flame can be made to wrap a little this might help with eveness. Again, I wouldn't do things like this without failing at simpler solutions first.

The blades do need to be heated evenly and not just end-to-end, top-to-bottom. You do have to see the phase change colors.

Mike
 
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