"A" and "B" are both 1084 steel,they were clay hardening, the quenching and tempering temperature is the same.
Their difference is "A" did the cryogenic heattreat,which cool the metal slowly to ï¼320F, hold 6 hour,and take 24hr to room temputure.
After cryogenic heattreat, I did another tempering(400F) for 1hr,then draw back.
"B" was flexible, I did 90 degree bending test for 5 times then the knife was break.
First time I bend 90 degree was fine,secend time I reverse it to another 90 degree,the edge started to break ,but spine was fine,
"B" was finally break on fifth time.
But "A" seems a little bit brittle,it was break when I bend it to 70 degree.
So I ask a lab took some metallographs on "A" and "B",trying to figure out what happened in the metal........
Maybe the cryogenic heattreat release more carbide in "A" ,makes "A" brittle than "B",
maybe I should do some XRD to know the % of martensite and carbide in these steels.
The metallographs between A3 and B3 look quite different,maybe it is another reason makes "A" brittle......... <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//unsure.gi f' class='bbc_emoticon' alt=':unsure:' />
A1 and B1 were the edge;A3 and B3 were the spin.
A1 500X
B1 500x
A1 1000X
B1 1000X
A3 500X
B3 500X
A3 1000x
B3 1000X
A & B
I also did some hardness test on 3 places .
But!These knives were finished before I findout my oven thermometer has some problem, so the hardness on the edge was tooo high.
A1 837HV(65HRC) B1 819HV(64HRC)
A2 362HV(36HRC) B2 300HV(30HRC)
A3 295HV(29HRC) B3 274HV(26HRC)
Very interesting Chunwei. It is surprising how much difference there is in the grain structure between the cryo treated sample and the one which was not subjected to cryo. Perhaps the cryo treated steel requires more tempering time to draw back the brittleness.
Thanks for sharing your observations, Chunwei.
As I read through your notes, I began to wonder if maybe the differential hardening may have played a role in how these two test blades performed. In fact I was quite surprised to read that a 1084 blade measuring 64 HRC at the edge could flex 90 degrees 5 times before breaking. My experience with 1084 suggests it should have behaved much more similarly to the other test blade, breaking before it ever reached 90 degrees, especially at that hardness. There's a reason 1084 is very rarely used on JS performance test blades...
I'm curious, how thick were your test blades? Is it possible that test piece B had a particularly narrow hardened section in the affected area below the hamon? It seems that this variable might be a bit difficult to control for in a testing environment.
Since cryo treatment of knife steels is generally meant to improve wear-resistance and edge holding by converting retained austenite - especially in more complex alloys - I would be curious to see how two fully hardened 1084 blades, each tempered back to, say 61 HRC (even if the cryo-treated blade required a slightly higher tempering temperature to achieve the same hardness), would perform in side by side cutting tests. I also wonder if a repeat test with fully hardened blades would see such a large differential in flex / ductility.
While we're at it, I'd love to hear other bladesmiths' experiences with deep cryo treatment of simple carbon steel blades as well. Is it common to see such a large decline in ductility or elasticity for just one more HRC point in hardness?
Hi Derrick,
I usually made performence knives this way,thickness on the ricasso is 0.19",and distal tapper to 1/3 length of the blade to 0.12",
then cover clay on spine 2/3 width of the blade.
After tempering 2 hr, I draw back 4 times.Every cycle I use belt grinder to remove the oxide color.
Actually, the 1084 "B" blade crack on the edge when I did the reverce scend 90 degree bending, but the spine still held.
Then the third nothing happened.
Then the fourth time reverce 90 degree bending, there comes another edge creak.
And it broke on the fifth time.
That is how it looks like.
I thought clay hardening really help a lot,I think the microstructure in the spin probly were pearlite, it is very flexable.
Here is another clay hardenind knife I made by W2(from NJ steel).
Here I want to share smothing more~~
This knife B first 90 degree bending.I only normalize it 1 time before I quench it.
It can pass the first time 90 degree bending,but it broke when I reverce it.
So I reheattreat it,normalize it 3 times before quench.This time it can survive 4 times until it totally break.
|quoted:
Very interesting Chunwei. It is surprising how much difference there is in the grain structure between the cryo treated sample and the one which was not subjected to cryo. Perhaps the cryo treated steel requires more tempering time to draw back the brittleness.
Hi Steve：
I think I did not heattreat them properly because the high hardness. Maybe next time I'll treat them properly and do another test, and do more draw back than I usually do.
Thanks for the advice. <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//smile.gi f' class='bbc_emoticon' alt=':)' />
