Wrong Soak?

[ckluftinger]

After reading ? perhaps too much ? about ?soaking? my steel before quenching I ran into the problem in the picture below: I soaked ? meaning I attempted to hold at temperature ? a blade of 5160 (bottom) for 12 minutes before quenching. My problem is, of course, that I can only go by color as I have no other means to measure the actual temperature. I may very well have over-heated the steel in the process. You can see the grainy texture of the ruined steel. It snapped easily when put in a vice. The other piece (top) was held at temp for about three minutes, then quenched. It took considerably more force to snap, and the grain is fine and tight, like fine bone china. Both blades cracked during quench, but that was probably for other reasons. So, my question is, if I cannot ?soak? at an accurate temperature, am I not better off keeping the soaking time short and sacrificing a few points of Rc hardness instead of risking extreme grain growth from a long soak at an inaccurate temperature? Or is there a better way to do this beside a temperature controlled salt pot or pyrometer? Or maybe this is the wrong soak time for 5160? I dunno...

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[Doug Lester]

With 5160 being a hypoeutectic steel I would only heat it to about 1500 degrees and hold just long enough to to make sure the steel is heated evenly all the way through. As you know, judging temperature by color is tricky; I can't spot red in the steel even on an overcast day at dusk. What I do is heat the steel until it goes non-magnetic and then get it just a little brighter. My plans for when I heat treat my next blades is to get appropriate heat crayons from a welding supply store and go by them.

Even when I harden 52100, a hypereutictic steel, I only soak for 5 minutes. I also try to keep my forge at about 1600 degrees by pyrometer by cutting the gas flow back and, if the steel starts to get much brighter than I want it, I pull it out for 2-3 seconds and stick it back in for a few seconds. A controled molten salt tank would really be nice but I just don't have the green to get one.

Doug

[WBE]

Grain enlargement is a product much more of heat than time. Under good temp control, 5160 benefits from a 12 minute soak, but in your case I would stick with a short soak.

[Kevin R. Cashen]

As Doug and WBE have pointed out, it is all about temperature at these temperatures. You will often encounter the statement that ?time = temperature? but what is not said is how much time for how much temperature. The fact is that while you can produce many results of a little temperature with a whole lot of time, at hardening temperatures the heat wins hands down over time every time. So as has been mentioned, grain growth is caused by overheating, not by oversoaking- unless your soak ended up being hours or days.

Be aware that the amount of soak necessary is mostly a factor of previous thermal operations done to the steel. If all you did was forge and normalize, the soak time can be very short. If you annealed it gets longer, if it was a very thorough traditional full anneal, longer yet, and if you spheroidized (a bit overkill for 5160) then you will need a full soak.

Two solutions without having to sacrifice full solution on the blade- I have seen some very accurate and controlled setups done with gas forges that can be choked way down and read with a thermocouple. If you can put a gate valve or needle valve on the gas and use refractory wool lining I have seen some forges hold temp really close to what a salt bath could. The second solution would be to precede the hardening heat with some preparatory heat that sets you up for quicker solution. Either cut back on the intensity of the annealing or following it with a normalizing type of heat to set the carbide up for quick solution. This is, after all, the real secret of the success of multiple quenching techniques, it sets up the blade for greater success in the final quench in the absence of any temperature controlled soaks.

And then there is the easiest option of all by just switching to a steel like 1084 that reaches solution almost as soon as you reach temperature, but you probably have at least some stock of 5160 that you would like to get some use out of.

[Eli Jensen]

As a newer knife maker, that's a really great picture. Thanks for posting it! Yeah I agree with a shorter soak.

[Eli Jensen]

Also its hard to tell, and someone else please verify this, but it looks like the top piece might have slight grain growth as well?

[ckluftinger]

Thanks, guys for the insightful information - although its a lot to take in all at once. I do have a lot of 5160 - and a cheap source for it (scrap pieces from a local spring manufacturer), and none for any of the 10-series yet. Cpt-Jens, I didn't know what grain structure looked like until I started breaking a few bars and comparing them. Ed Caffrey did a demo at a hammer-in last summer where he heat treated a bar both properly and wrongly, and then broke them for us to look at to see the difference. Very educational! His "properly" done bar looked like my top sample. It reminded me of a shard of broken porcellain. That's the best description I can give of a good heat treat. Does anyone have a better way of describing it, or a better picture? You know a picture is worth a thousand words...

[Doug Lester]

That top piece could have a little bit of grain growth but there have been a lot worse shown. Actually I've seen worse posted than the bottom piece. Soak times can be an item that can cause a lot of contriversy but I would still stick with only soaking long enough to heat the blade throughout with hypoeutectic steels, steels with less than 77 points of carbon. Also, try to cut the heat in your forge back so that the thin parts don't over heat as much while you're trying to get the thicker parts up to temperature. You might even have to resort to quenching the thin parts and then returning the blade to the forge to heat everything together.

Doug

[Eli Jensen]

Top piece still looks good just wanted to point that out. Could anyone post a similiar picture with no grain growth, super fine grain and one that is extreme opposite? That or even the one above is a great thing for HT beginners to see.

[WBE]

Originally Posted by Cpt-Jens

Top piece still looks good just wanted to point that out. Could anyone post a similiar picture with no grain growth, super fine grain and one that is extreme opposite? That or even the one above is a great thing for HT beginners to see.

If you want to see a truly beautiful grain size, break the tip off of a good file. It will make that top piece look slightly coarse.

[ckluftinger]

Originally Posted by WBE

If you want to see a truly beautiful grain size, break the tip off of a good file. It will make that top piece look slightly coarse.

Great idea. Why didn't I think of that? Here you go: grain growth in degrees...

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Thanks for the tip!

[Jason Fry]

That pic of three grain sizes in a stack is perfect! No better way to illustrate the concept than that.

[WBE]

A very obvious example, and good photography.

[ckwatson]

I am trying to make sure I under stand. I will be HT my first knives in a week or two.

I will not be able to do a soak I am working with a home made forge. To keep grain size down I should make sure I don't over heat the steel?

[WBE]

Yes, but if your forge and fuel will allow, you might want to do a few heat cycles before you acually quench and harden. I know you are doing stock removal, and chances are the grain in your steel is in at least fair condition, but to be sure, and possibly refine it more than it is, heat it just past non-magnetic and air cool a couple of times, followed by one or two cycles just below non-magnetic. That should reduce the grain size if it is not already in good shape, and it will reduce the chances of warp. Then heat a good color phase beyond non-magnetic, try and hold there as best you can until you are pretty sure it is evenly heated, and quench in the canola heated to 130?. Be very aware of the point area. It can over heat in a heart beat. Hold the blade edge up in your fire so that the spine can take the brunt of the heat first.