Liquid Nitrogen

[ryano]

Hi everyone!

I'm new into knife making - started off with buying blades online and making handles, and I just finished my first stock removal knife last week.

I'm working with stainless steel at the moment, some 316 and something else I can't remember... possibly 440. Anyway, I haven't been able to find much information on heat-treating these, but I've been reading a little bit on cold treating these steels.

Would these benefit from cold treating from room temperature straight into liquid nitrogen? Or do they have to be heated to a certain temperature first, and possibly soaked there for a given time before being dipped?

I've searched these forums and elsewhere and I can't find the info I'm looking for. Thanks in advance for any insight you can offer!

-ryan

[AUBE]

316 excels at being corrosion resistant but it is not suitable for blades. It simply does not harden.

440 (is it 440a, 440b, 440c?) can be heat treated but you definitely have to accurately heat the blade to a high temp, then quench it at a certain rate before going to a cryo treatment. If you do not have the tools to do this, I suggest sending it to a heat treater.

[ryano]

Thanks for the reply Aube!

I'm not sure if it's 440A, B, or C... I'll have to check the invoice when I get home.
When you're talking about heating the 440, then quenching it prior to cryo, are you saying that you bring it up to temperature, then over time bring it down to room temp, then cryo treat it? Or should I be dipping it in the nitrogen when it's still very hot?

The reason I ask is this: if it's the first way, then I can send the blades off to be heat treated and once they come back I can just put them in nitrogen and be done with it. But if the nitrogen has to be applied while the blade is still cooling, then I'll have to buy the heat treating equipment and do it all at once.

As for the 316, the cryo treatment won't change the structure at all? I understand that stainless steels are austenitic, and that heat treating them won't change that... I don't know why, but that's my understanding of it.

But won't cryo treatment change the structure of 316 from austenite to martensite? I guess it depends on the martensite start and finish temperatures, which I can't find anywhere online. But from what I've read, austenite steels like 316 will change structure to martensite when cooled to "sub-zero" temperatures. And my understanding is that this is a one-way change, unless heated back up to something quite a bit hotter than room temperature.

If only I had a Rockwell tester... I could test a piece of plain 316, then dip it in the nitrogen, let the boil stop, and test it again. Anyone in the Calgary are have access to such a device?

-ryano

[Ray Rogers]

Ryano,

You're on the verge of causing yourself a lot of trouble. Don't even think of plunging hot steel into LN. That would be fatal to the steel and most likely very dangerous to you as well.

With 440C you heat the steel to 1850 F and hold it at precisely that temperature for 30 minutes. Not close to 1850 F but exactly 1850 F (within say 15 degrees). Then allow the steel to cool in still air to room temperature. As soon as it gets to room temp it goes into cryo (if you plan to use cryo). This has to be done slowly so as not to fracture the steel. After a few hours at cry temp (I like to leave it over night) then the blade is warmed to room temperature and then tempered in an oven at 450 F or so according to what you want as an end result. This is my process and there are other variations to this process which work on 440C but they all require precise temp control and that, in turn, requires some fairly expensive equipment. A forge cannot do this.

If you are talking about 440A or B or 316 then forget it, these steels will not harden (316) or will not harden enough (440A and B) to make a knife worth making for a custom maker (440A will make a blade but they are inferior). So, with stainless steels you either send the blades out for HT or spend $3k or more getting set up to properly HT them.

And that brings us to what properly heat treating means. Get some books, read some articles, watch some videos, go to a hammer in, and/or use the Search button on this forum. There are plenty of discussions on heat treating, you're not the first to ask these questions. I'm being a little bit hard on you because the way you talked in your post makes me worry that you are likely to do something that would be very ill advised, mostly because you have a tiny bit of information and don't know what to do with it yet. We don't want anyone to injure themselves or even waste money doing things the wrong way while pursuing our hobby.

So, study a lot before you do anything at all concerning heat treating. Send out your stainless blades or spend a lot of money. Or, switch to a good carbon steel like 1080 where you will have a reasonable chance of HTing the blade yourself without all the expense ....

[AUBE]

Ray offers good advice.....do not even think of putting a hot piece of steel in liquid nitrogen. It would more than likely cause a violent reaction, which would throw nitrogen all over and more than likely shatter the blade you were working on.

Professional heat treaters that do blade steels often have their own cryo setup so that saves you the hassle of doing it yourself. You can send them a ground blade and they take care of every step of the heat treatment process.

As for the metallurgy of 316....its been nearly 2 decades since my metallurgy classes and as they say "if you don't use it, you lose it" so I don't know the effects of cryo on 316. Hopefully someone more into metallurgy chimes in on the subject. I *think* 316 can be hardened to around mid 40's on the rockwell scale but that is a far cry from most knives which are in the 55-60rc range. I've dealt with hardened 300 series blades(304 and 301), for industrial pickle slicing machines, and they dull pretty fast and the edge rolls when it contacts anything remotely hard.

In short stick to stainless intended for blades (440c, 154cm, ats34, etc etc etc) and if you can't afford a proper ht setup then use a reputable heat treater and save yourself a lot of frustration.

-Jason

[Doug Lester]

Hey, Ryan. It looks like you are just starting out and have a bunch of terms swimming around in your head and are not quite sure what they all mean. I don't know if 316 retains austinite or not upon quenching but there are alloying elements, including carbon, that can retard austinite from converting to martensite upon rapid cooling, with rapid being defined as cooling fast enough to avoid the nose of an IT diagram.

You do need some book learn'n. Two books, Metallurgy Fundamentals and Steel Metallurgy for the Non-Metallurgist are basically Cliff Notes on the subject. You can also see about getting some text books on metallurgy, which are readily available used (as in cheaper). Be forewarned, this is a study. You will not get it all the first, or probably not even the third, time through them. Fortunately, there are people here that really understand the subject, and that excludes me, though I'll help where I can.

But for now, you may just want to get some basic texts and learn how to make knives and send your blades out for heat treatment if you want to use air quenching steels. If you want to heat treat for yourself. I would start out with some plain carbon steel or maybe some of the simpler spring or tool steels, such as 1080, 1084, W1, W2, 5160, or 9260, if you can find the latter. These can be heat treated with a simple gas forge and a kitchen oven (or a toaster oven if she who is master of the kitchen says no way).

Doug

[ryano]

Thanks for the replies everyone. It looks like I'm going to put these steels aside for the time being and pick up some proper steels for blades until I make a few dozen and start to understand the processes involved.

Doug you are 100% right with your second sentence. I'm going to look for those books you suggested and try to arm myself with some knowledge.

Jason I know exactly what you're talking about with the violent reaction - the N2 boiled over very violently when I dipped a room temperature blade into it, let alone one that would be heated up. I was wearing proper safety gear however and I anticipated the reaction, so all I got was a cool fog effect and a cold cement floor in the garage for a few minutes.

Ray, I don't feel like you are being hard on me at all. I appreciate the advice and the words of caution. I never planned on heating a blade up and dipping it in N2 as I don't have the capabilities to heat it at this point - I was just curious what the method was for cryo treatment. Re-reading my post, I can see why it came across that way. I guess that's what I get for posting at 3:00am after a 12 hour shift.

-ryan

[Cthulhu]

If you'll accept a bit of advice from soemone not much further along than yourself:

You're getting WAY ahead of yourself. Not cart before the horse ahead, but wheel not invented to make a cart to put before the horse ahead.

Go back to basics, which are difficult enough, and start from there. There are agencies and individuals that have sub zero quenching down to a science, and it's best to leave such things to them.