Hi All,
First time posting so appologies if i have missed a thread that this question should be on, but:
I have been attempting to make a hunter style knife out of cpm 20cv. This is my first time using a fancy powder met. steel, and so I have (innnevitably i suppose) run into some trouble in the heat treat.
The short version of this story (long version below) is that I've roughly followed the suggested heat treat online, and have found my knife to be way harder than anticipated (66 HRc ish), and that it doesnt respond to tempering much if at all.
So essentially, I'd like to know if there's anything to do about this, and if the knife being that hard is going to be problem or not?
Any advice would be much appreciated!
Thanks,
Michael
-------------------------------------------------------------------------- Long story: -------------------------------------------------
My process thus far has been roughly following a mix of the data sheet and the knife steel nerds post on M390 (equivalent) steel, and is:
1) rough grind knife (including bevel), drill holes in handle.
2) heat knife in a nitrogen flushed furnace (02 at less than 1%), heat to 780c, hold for 10 mins, increase furnace temp to 1149C, hold for 30 mins once there.
3) remove knife which was intact but had scale on it - clearly my nitorgen purge didnt work, but I cannot find 309 tool wrap in the UK so had no choice - quench in rapeseed (canola) oil - interupt quench half way through and allow to cool in air after about 7s.
4) Cryo treat in LN2 - just until knife had equalised temp.
5) 225C for 2 hours temper.
6) grind off oxides on one side leaving some clean areas of steel for hardness testing.
7) hardness test
And this is where i ran into issues.
I did all the heat treating steps above (2-5) one after the other, but had to go home before doing the second temper cycle (I'm a phd student to im borrowing Uni kit). Before the second temper I decided to hardness test my knife, both on the handle and on the spine (which are both roughly flat). I got an Hv (vickers hardness) value of about 920, which (depending on where you look - wikipedia seems to be wrong on this one) is equivalent to about 67-68 HRc. i.e. too hard by my klnowlege.
8) I hoped the second tempering cycle would help, so i put it in for 2 1/4 hours at 260c (500f), air cooled and then hardness tested again. This time i got 900Hv, or about 66/7 HRc. Still too hard.
9) I then put it back in the oven at 275C for a further hour, and the hardness reduced a tiny bit (maybe) to about 890Hv on the average... i.e still too hard.
Any one have any advice for me here? I think My heat treatment should have yielded something like 63 Hrc in the first place ( i got 66), and it seems to not be responding to tempering at all. Is there anything I can do here to reduce the hardness to a more sensible value? I'm concerned that much more tempering will just lead to martinsite embrittlement, which will make things worse...
Or, if there is nothing to be done, does anyone have any experience with super hard (HRc = 66 ish) knives? Are they way too brittle or alright?
Thanks again!
Michael
First time posting so appologies if i have missed a thread that this question should be on, but:
I have been attempting to make a hunter style knife out of cpm 20cv. This is my first time using a fancy powder met. steel, and so I have (innnevitably i suppose) run into some trouble in the heat treat.
The short version of this story (long version below) is that I've roughly followed the suggested heat treat online, and have found my knife to be way harder than anticipated (66 HRc ish), and that it doesnt respond to tempering much if at all.
So essentially, I'd like to know if there's anything to do about this, and if the knife being that hard is going to be problem or not?
Any advice would be much appreciated!
Thanks,
Michael
-------------------------------------------------------------------------- Long story: -------------------------------------------------
My process thus far has been roughly following a mix of the data sheet and the knife steel nerds post on M390 (equivalent) steel, and is:
1) rough grind knife (including bevel), drill holes in handle.
2) heat knife in a nitrogen flushed furnace (02 at less than 1%), heat to 780c, hold for 10 mins, increase furnace temp to 1149C, hold for 30 mins once there.
3) remove knife which was intact but had scale on it - clearly my nitorgen purge didnt work, but I cannot find 309 tool wrap in the UK so had no choice - quench in rapeseed (canola) oil - interupt quench half way through and allow to cool in air after about 7s.
4) Cryo treat in LN2 - just until knife had equalised temp.
5) 225C for 2 hours temper.
6) grind off oxides on one side leaving some clean areas of steel for hardness testing.
7) hardness test
And this is where i ran into issues.
I did all the heat treating steps above (2-5) one after the other, but had to go home before doing the second temper cycle (I'm a phd student to im borrowing Uni kit). Before the second temper I decided to hardness test my knife, both on the handle and on the spine (which are both roughly flat). I got an Hv (vickers hardness) value of about 920, which (depending on where you look - wikipedia seems to be wrong on this one) is equivalent to about 67-68 HRc. i.e. too hard by my klnowlege.
8) I hoped the second tempering cycle would help, so i put it in for 2 1/4 hours at 260c (500f), air cooled and then hardness tested again. This time i got 900Hv, or about 66/7 HRc. Still too hard.
9) I then put it back in the oven at 275C for a further hour, and the hardness reduced a tiny bit (maybe) to about 890Hv on the average... i.e still too hard.
Any one have any advice for me here? I think My heat treatment should have yielded something like 63 Hrc in the first place ( i got 66), and it seems to not be responding to tempering at all. Is there anything I can do here to reduce the hardness to a more sensible value? I'm concerned that much more tempering will just lead to martinsite embrittlement, which will make things worse...
Or, if there is nothing to be done, does anyone have any experience with super hard (HRc = 66 ish) knives? Are they way too brittle or alright?
Thanks again!
Michael
