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5160 getting a few extra points of hardness...


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HI, I am hoping some people who have a bit more experience with 5160 can give me some advice on how to squeeze out a point or two more of hardness from my process.

 

So far I have mostly used 5160 and like it well enough.  I heat to 1525° F and quench in warmed vegetable oil (homemade electronically controlled oven).  I then do two cycles of tempering at 375°F.  I am not getting any noticeable decarb or warping that I am concerned about.

 

I was hoping to be around 58-59 Rc but I brought the knife to work and it tested at 56.5 Rc.  Not bad, but I was hoping for a couple points more.  Is there any big risk of using room temperature vegi oil to speed up the quench some?  What other options might be worth while to get a couple more points?  Or is 56-57 Rc a realistic number to be hitting and getting a bit more points isn't very practical?

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I personally would be ok with 56-57, that said, I doubt that it's the quench how accurate is that 375, maybe drop the temper by 50 or so. Unless it is 5160H (guaranteed hardenability) you might have some variation in your steel.

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I think the temper was reasonably accurate, I used a toaster oven, but had a thermocouple  in there as well to verify the thermostat setting.  The thermocouple was peaking around 375 +/- a bit. I am not sure if that thermocouple is accurate or not however, because the thermostat was set at 400 to get there.

 

Good to know about 5160H, I wasn't aware of that.  I might try to drop the temper some, and while this knife is okay and I will try and complete it without messing it up, I think I may move to some 1084 soon and try some knives with that.

 

I am not sure if thinking and hoping if a blade is hard enough is better or worse than knowing it isn't where I intended it to be.

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Using cooler oil will actually lower the hardness.  Warm oil is a faster quench.  I agree Rc56 is fine.  If you want it harder, you can quench from a slightly higher temperature, like 1600 F, as long as you don't soak it at heat.  I also agree with Jim, temper at 325 and see what you think.

 

Also, if you are testing on the bevels you won't get an accurate reading.  Same with surface finish, but if you have a Rockwell tester at work I assume you know all that.  

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20 minutes ago, Alan Longmire said:

Using cooler oil will actually lower the hardness.  Warm oil is a faster quench.  I agree Rc56 is fine.  If you want it harder, you can quench from a slightly higher temperature, like 1600 F, as long as you don't soak it at heat.  I also agree with Jim, temper at 325 and see what you think.

 

Also, if you are testing on the bevels you won't get an accurate reading.  Same with surface finish, but if you have a Rockwell tester at work I assume you know all that.  

 

 

Our 5160 will take 880 C  which is even slightly more than you suggest. I only temper once and dont care for time, just go for pretty collours. (For chopper I end up with first violet straks in the dark brownish surface). With choppers very reasonable combination of toughness and hardness can be achieved.  I had one of my friends disassemble old barn with such chopper, nails and everything and he didnt do any teeth.

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So it isn't my tester (a pity really), but the one at work in our tool and die shop.  The test was done on a smooth and flat surface of the blade, which I assumed was required but didn't actually know that for sure or from experience.  I imagine our T&D lead would have said something if we were doing it wrong.

 

I also wasn't aware that warm oil was a faster quench!  Obviously I was thinking the reverse of that.  My next 5160 knife I make I will warm the oil up a bit more, hold at 1525 for a few minutes and raise the temp to 1550° hold for maybe a minute and see where I land on the scale after those changes.

 

I am a bit hesitant to make major changes to the soak temperature, but if I get positive results from the above I may try another 25 degrees and see if it improves further.

 

Thanks for the info gentlemen!  I learned quite a bit.

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There is really no need to soak 5160 once it's up to heat.  All that does is grow grain.  There's not enough carbon in it to form a lot of chromium carbide, and carbide distribution is the only reason some steels need a soak.  The 1525 F is the minimum it needs to harden.  It can go to 1625 with no problems as long as you don't soak it. Like Jaro says, it's tough stuff.

 

Try it from 1550 and temper at 325 and see what you think.  You should end up right around Rc59. That's good for a slicer or small camp knife, a bit hard for a chopper.  If you want the high hardness with additional toughness, go to a richer alloy like O1  or 52100.  But for a chopper Rc59 in most steels is asking for an edge to chip.  

 

 

 

 

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Good deal Alan, again thank you for the additional information, I will try as you suggest for the next one.

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Adam, there are two ways to look at hardness in a knife blade.  One side of the coin is that the higher the hardness the better it will hold an edge.  The other side of the coin is that the lower the hardness, within reason, the easier it will re-sharpen.  So you have a choice, a blade that's in the high 50's HRc that you can sharpen on a flat piece of hard sandstone that you found out in the woods if you forget your sharpening stone at home or a blade in the low 60's that will hold an edge longer but will need that hard stone to dress up the edge when it goes dull.  Remember all blades will loose their edge with use.  Based on the knife's intended purpose and the steel that you choose, you're going to have to choose the compromise that you make.

 

Doug

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