First time HT 15n20- looking for advice

[Heikki]

So I just finished profiling 3 chef knives from .090" 15n20. This will be my first time working with this steel. I've done some research on HT, so the plan is to austenize at 1480*, hold for 10 minutes, then into Parks 50 and off to temper. Looking for 64-65 HRC as quenched. I also plan to do a couple coupons first and test hardness out of the quench.

My questions for those of you that have more experience with this steel:

-What hardness should I shoot for for kitchen knives with this steel? I've read anywhere from 60 to 64.
-Would it be better to grind pre or post HT? I normally grind pre heat treat.
-How prone to warping is it? I have 3/4" aluminum plates that I can use to help if needed.

Any advice is appreciated, just looking to cover all my bases so I'm prepared.

Thanks!

 

[billyO]

I've only done a couple of straight 15N20 blades, but use it a lot with 1080, so take my answer with whatever size grain of salt you feel is appropriate.

-What hardness should I shoot for for kitchen knives with this steel? I've read anywhere from 60 to 64.

This will depend on your tempering temp, but that's the range I think you'll get.

-Would it be better to grind pre or post HT? I normally grind pre heat treat.
-How prone to warping is it? I have 3/4" aluminum plates that I can use to help if needed.

Grinding pre-HT is easier than post HT. but it'll also be more prone to warp.
The last question depends kind of on the previous one. The plates definitely help (I dunk in my blades in Parks 50 for a 5 count, then straight into the plates).
It's also a good idea to do some grain refining/thermal cycling before HT even if you don't do any forging or grinding. This helps even out internal stresses and can help mitigate warping.

 

[EdCaffreyMS]

My input....

I'll avoid advice on the heat treat, because I personally think soaking for 10 mins is pretty absurd, and am sure that doing so produces far more harm then good.

15N20 essentially 1075, with 1.5% nickel. Treat it as you would 1075/1080. Personally I think 60 is a bit too hard. In my experience, thin edges on 15N20 at 60 will frequently chip. Personally, I I like it in the 57-59 range. If you want to go with 60, you'll need to thicken the edge(s) to prevent chips.....but then you're just building in cutting resistance by thickening edges.

In my shop, anything .125" of less, and often times thicker, gets ground post heat treat. Doing so just saves me a lot of heartache.
15N20 is pretty prone to warping, partly due to the nickel content, but when combined with a thin cross section....even more so.

 

[Kevin R. Cashen]

Like Ed said, 10 minutes is a bit overkill for such a simple steel. The only alloying here is the nickel and it does not engage the carbon so solution should be easy. If you are doing the coupons anyhow I would start at 1 minute and add minutes to coupons and see where the HRC levels out for you. You may find that a solid 65 might elude you with this steel. And be aware that with that Ni level oversoaking could reduce your HRC, even in such a simple steel.

Desired HRC depends on three factors- the alloy used, the geometry that the HRC has to support and the intended use of the knife. For large choppers you will have a stouter geometry that can handle lower Rockwells, and benefit from the higher Charpy impact numbers. But this is a kitchen knife, one of the few knives left where you can optimize the performance and put it in the hands of somebody who will appreciate using it for what it was meant for.

For such a knife I would want a very fine, keen, geometry for long edge retention, the opposite of a chopper/cleaver, so I would go for optimizing strength. The beauty of 15n20 is the inherent toughness gained from the Ni. If I were making a kitchen knife of this type I would not go below 60HRC, and if I were putting it in the hands of somebody with kitchen knife skills, I would push it to the high side. My favorite kitchen knife is 64HRC and that is only because I had to stabilize it from the 65.5, as-quenched, HRC. You can rest the edge, on its side, on your thumbnail and see it deflect under its own weight, so it needs that HRC strength. The knife is so sharp that I am the only one in the house that is not afraid to use it, but it only takes two or three strokes on an ultra fine stone to have it back to that level, and I rarely have to do that.

All this being said, if you go for that ultra fine, high strength edge I am describing, you will want to grind it after heat treatment, keeping it cool at every step. Have a dip bucket next to the grinder and use it on EVERY pass. If you feel the end user may decide to abuse it by whacking bones, or frozen food, then you will want to beef up that edge, lower the HRC and just grind it before heat treatment.

On a side note, one of my favorite things to do with my kitchen knife (O-1/L6 damascus by the way) is to thaw chicken breast just enough so that it is just able to be cut, but still quite frosty, and then shave slices so thin you can sort of see through them. Is great for making tender strips for quick stir frying.

 

[Heikki]

Thanks for all the input. I ran 3 test coupons tonight, all at 1470*F, all as quenched after grinding away any decarb.
1. 1 minute soak- averaged HRC 62
2. 3 minute soak- averaged HRC 64
3. 5 minute soak- averaged HRC 65

Broke one of them to see the grain...



Looks like 5 minutes at 1470* will do it. I have some more pieces I can use for testing. I may try lowering the temp to 1450* to see what happens. Having an oven sure does make testing stuff easy.

 

[tkroenlein]

*And a hardness tester. (I still gotta take mine to work.)

 

[Heikki]

*And a hardness tester. (I still gotta take mine to work.)

That too

 

[EdCaffreyMS]

Looks like 5 minutes at 1470* will do it. I have some more pieces I can use for testing. I may try lowering the temp to 1450* to see what happens. Having an oven sure does make testing stuff easy.

I can't help myself here, because I see a tunnel forming. So, before something goes very wrong for you, I'm going to interject here.....

Don't get tunnel vision on a specific Rc hardness number(s). There's a whole lot more to it then just a hardness number/level. I do/say this because I've seen it happen time and time again.... folks get zeroed in on a specific Rc number, and either forget, or don't realize that it impacts everything else about/with a knife blade.
Next, as is necessary with any given steel type, you have to discover/determine the correct geometry for that hardness level.... not only where it will perform, but also balance out the hardness level, with the geometry, to achieve a balance of cutting ability and durability (or whatever characteristic(s) you desire/choose).

I just see so many knifemakers who get wrapped up in specific hardness numbers, that they trip themselves up on the rest of the "overall package". The sad part is that it usually shows up AFTER they have either given away, or sold that particular knife, and the new owner is the one discovers the flaw(s)....and usually in a not so good way, that is very embarrassing to the knifemaker. Now, having said all that, it's in no way meant derogatory, I just have enough experience to see the signs, and know the direction it often takes.

 

[Kevin R. Cashen]

Thanks for all the input. I ran 3 test coupons tonight, all at 1470*F, all as quenched after grinding away any decarb.
1. 1 minute soak- averaged HRC 62
2. 3 minute soak- averaged HRC 64
3. 5 minute soak- averaged HRC 65

Broke one of them to see the grain...

View attachment 77113

Looks like 5 minutes at 1470* will do it. I have some more pieces I can use for testing. I may try lowering the temp to 1450* to see what happens. Having an oven sure does make testing stuff easy.

Sometimes it can take quite a few tests to dial it all in, but you may have found the timing with your batch. The time for solution should be the same for any steel of identical chemistry, but that is not the case. Prior thermal history plays an equal, if not more important role in the amount of time to put the carbon into solution. You could give me a bar of steel, I could cut it in half and do some normalizing, cycling and annealing magic on one piece and totally frustrate you on matching the Rockwell with the same heat treatment.

Steels like 1080 or 1075 (just ignore you nickel content and that is what you are dealing with) do not have the wider gradient of climbing Rockwell numbers from increasing soak temperatures like alloyed steels may. Instead, it is more black and white, like an on/off switch that is hit when you reach a certain temperature. Beyond that temperature there are no real gains from higher heat. Instead staying at the lower temperature, and giving it some more time, will yield inherently tougher conditions with finer grain size. I use ten minutes for my baseline, and adjust temperatures around it, but you may want to try the 5 minutes that is working. If you don’t establish a soak time, you end up chasing your tail when the time/temp combination variables skyrocket.

By establishing a soak time and then zeroing in on that key temperature, you can make the Rockwell readings much more reliable. Many makers do, indeed, put too much stock on HRC numbers alone. Rockwell hardness is not a statement about the individual knife, it is a tool for dialing in your heat treatment. I could name you about half a dozen things, off the top of my head, that could be very wrong with a knife blade that still has great looking HRC numbers. I can even name some things that a really bad which will result in higher HRC numbers. But, if you are testing to find that on/off switch temperature, at a consistent soak time, you are greatly eliminating many of those bad scenarios, making your HRC numbers the invaluable heat treating tool that they are.

 

[Heikki]

I can't help myself here, because I see a tunnel forming. So, before something goes very wrong for you, I'm going to interject here.....

Don't get tunnel vision on a specific Rc hardness number(s). There's a whole lot more to it then just a hardness number/level. I do/say this because I've seen it happen time and time again.... folks get zeroed in on a specific Rc number, and either forget, or don't realize that it impacts everything else about/with a knife blade.
Next, as is necessary with any given steel type, you have to discover/determine the correct geometry for that hardness level.... not only where it will perform, but also balance out the hardness level, with the geometry, to achieve a balance of cutting ability and durability (or whatever characteristic(s) you desire/choose).

I just see so many knifemakers who get wrapped up in specific hardness numbers, that they trip themselves up on the rest of the "overall package". The sad part is that it usually shows up AFTER they have either given away, or sold that particular knife, and the new owner is the one discovers the flaw(s)....and usually in a not so good way, that is very embarrassing to the knifemaker. Now, having said all that, it's in no way meant derogatory, I just have enough experience to see the signs, and know the direction it often takes.

Thanks Ed. I'll try to temper my enthusiasm. It's just that now I finally have the tools to use (oven and hardness tester) that will allow me to be able to test and adjust my heat treat with better consistency and confidence (compared to a forge and files). I wasn't necessarily shooting for a specific HRC, just the expected as quenched hardness of 65 that both AKS and NJSB show in their HT information. Then I'll temper for geometry and intended use of the blade. AKS says "It has enough toughness to be used at HRC 61-62 and resist chipping." so I thought I'd start there for a chef knife.

 

[Andre Grobler]

i have to say i absolutely love the edges you can get on 15n20... i may have soaked it a little long, but because the steel is so tough at high hardness, you can choose just about any angle from 7dps to 18dps, depending on the user and the task.... and it sharpens great no or very little burr unless you look for making a burr, and even at a thin 7 dps the edge rolls instead of chips, i don't do ultra thin anymore, as very little people can handle it, least of all my family, and i struggle to get good food release on ultra thin geometries, how do you go about that Kevin? if i do stainless with high carbides, you almost have to choose an edge based on what the steel will do, so you end up having to work very hard to get stable edges below 15dps...