15n20 filet knife

Evan Miner

Evan Miner

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Nov 24, 2011
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Earlier this week I decided to make a pair of filet knifes with my 9yr old son while I have him for the summer. The question I have is at what hardness does 15n20 need to be to flex and not have the bend set?

So a little detail in to where we are at the moment. So we have both ground and shaped our blades I've heat treated and tempered the blades the problem is I don't think they are hard enough after tempering.
They both flex and both take and hold a bend after even the smallest of flex.

Heat treat:

Normalize 3x 1600, 1550, 1500
Aus 1525 hold 15min
Quench 125* +/- 5 canola oil thermometer jumps around due for replacement.
Place between Quench plates to reduce warp.
Remove from plate after cool enough to touch.
Temper 350 one hr between Quench plate to assist in keeping straight

Hope every one has a wonder full 4th of July weekend
 
Isn't the "flex" of a blade, more a product of the thickness and geometry used? I didn't think it had anything to do with hardness of the blade.
 
Yes, Im sure Stacy will chime in shortly but that's what he has said before. I made a filet knife a few years ago and did so based on his comments. Flex is mostly about thinness of blade, width of blade and edge geometry. Whether the bend holds is about hardness of blade and acuteness of the bend. I made mine out of .065" 15n20, just under 3/4" wide x 6" long and would flex fairly well but had good hardness. My HT was similar to yours but I didn't use quench plates after and my temper was at 400. I would check the HT again to ensure that isn't the issue, then look at how much you are bending it for a test. Just place the first 1" on a table and push, no need to clamp and bend hard, a filet knife only needs to flex about 30 degrees max.
 
The flex yes is more a product geometry and thickness. I'm more then certain I will have more then enough flex to keep from shaping. But for to bend to stay I feel is more about the hardness then the geometry and thickness. I could be wrong and that would be why I'm asking. I've made a few filet knives a few yrs ago but they were with 5160, and I had to forge them down from 1/4in x 2in stock and I'll never do that again with out a power hammer or press. It was way to time consuming to move the much metal into a .075 x 1in section of steel. And at that time I tossed 5 blades to each one I had to test. And only made 2 that were finished out completely.
 
Right now the bend sets at even the most minimal of flex so I'm leaning towards it being the heat treat. I'll probably redo it today and tweak the Aus. Temp a few degrees or the hold time. The current thickness I'm using is .065 as well.
 
I've done a few knives in 15n20, and I'm no pro, but the charts I've used say the AUS should be around 1475F. 1525F with a 15min soak seems too hot and too long.
 
Ditch the quench plates and use oil for quenching. If it's taking a set after a 350deg temper then it's not hard at all. 15n20 heats treats very easy and I most of the time never have any warping problems whe quenching in oil. And if it does warp a tiny bit it's just a gental curve and can be easily straightened by hand befor the hardness sets.

You flex is a produc of the thickness and geometry not hardness, changing hardness just changes where it will take a set (plastic deformation point). Hardness does not in any way change a steels resistance to flex but it does change its resistance to staying bent. My fillet knives I make from 15n20 get a 400° temper and are still very hard but can be flexed to 90° and will pop right back straight.


EDIT* just noticed you did not use the quench plates for quenching.
 
The main problem was almost surely the quench in plates. 15N20 is a good steel, but it needs an oil quench. Assuming your times and temperatures are accurate, they would produce a hardened blade with an oil quench. The other thing I see is the austenitization is a good bit high. I would use 1480F. An approximate guess would be Rc 59-60 for those parameters.

As said, flex is a matter of thickness and geometry. It is the same for soft or hard steel of the same dimensions. What is a factor of the hardness is when the blade takes a set and when it will break. The harder the steel, the more it will return to straight up until the point where it breaks. Soft steel may take a set with very little bend.
 
I try and set right at 1475-1480 but never over 1500°.
 
Just to clarify I'm not Quench in plate I'm quenching in oil and placing between plates after to help with keeping straight. While it finishes cooling off is all. So I'll adjust my Temps down to 1475 and try again. Thank you all for the kind words of wisdom and support
 
Knife makers cannot read... :rolleyes: LMAO :D

I made a 15N20 fillet knife recently that was at 60C... it has quite a bit of taper from the ricasso out, which was a pita since it was only 0.065 to start with! LOL I quench it from 1475, into Park50... I soak it in my salt for about 10 minutes, just to be sure it's good to go. :)

I'd love to see how it turns out. :) :) :)
 
Yup, 1475-1480 is the sweet spot with 15n20. Leave it at Rc62. It's like a non stainless aeb-l. It has a lot of nickel, so it's pretty tough, allowing higher hardness than 1075/1084 without worrying about chippiness. I have quite a few skinners out there at Rc62, and zero have come back chipped, even going through rib cages. One was hammered through a pelvis. I use a 10 minute soak to make sure everything is in solution. You can get away with less, but I wouldn't go shorter than 5 min.

What is the source of the 15n20? If it's Aldo, you can forgo the cycling. It's ready for heat treat as is, unless you are forging, then you need to refine grain.
 
Willie71 said:
Yup, 1475-1480 is the sweet spot with 15n20. Leave it at Rc62. It's like a non stainless aeb-l. It has a lot of nickel, so it's pretty tough, allowing higher hardness than 1075/1084 without worrying about chippiness. I have quite a few skinners out there at Rc62, and zero have come back chipped, even going through rib cages. One was hammered through a pelvis. I use a 10 minute soak to make sure everything is in solution. You can get away with less, but I wouldn't go shorter than 5 min.

What is the source of the 15n20? If it's Aldo, you can forgo the cycling. It's ready for heat treat as is, unless you are forging, then you need to refine grain.
Click to expand...

What temp are you using to temper Warren?
 
I start at 300, and walk it up to the desired Rc#. I find the chart at AKS shows rc#'s a bit on the low side. 325 to 350 gives me Rc62, usually closer to the 325 number. I'm usually at Rc63/64 after the 300 initial temper. I use fast oil to quench, so that might give me a little more initial hardness too.
 
I really love 15n20 and not because I got so much but even befor that I was really coming around to it. I just recently did some skinners for donation to a local cancer auction. Thy where full flat ground out of .075 with about .005 behind the edge. Wicked sharp and I normally do them at 400° temper but might try bumping up the RC a tad and see how thy preform.
 
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