Too many tempering cycles...?

Jesse WW

Jesse WW

Joined
Jul 8, 2017
Messages
71
Hey everyone,

Iv been banging my head over this blade for the past week now, trying to eliminate some warp in the tip while tempering - slow going, but its getting there. The problem is... this is going to be the 6th tempering cycle I've put it through. What kind of reduction in hardness am I going to see each cycle? For specifics: Its W2 and tempering is done at 200c (392f). Blade was quenched at 798c-802c (~1472f) in Houghton fast quench, so as far as I know it should theoretically be close to full hardness. I know W2 gets awful hard so I'm figuring it can take quite a few tempering cycles before it goes bellow the desired 60HRC hardness?

I appreciate any responses!
 
If all tempering cycles is done at 200c , I don t see any reason to have reduction in hardness ?
 
I don't know if you can see these articles like I can. I work at a university and may have access due to institutional subscription.
https://www.sciencedirect.com/science/article/pii/S0924013698003513
https://www.sciencedirect.com/science/article/pii/S1665642316300013

From the article on 4340 steel comparison of 2h versus 48h tempering, there is a drop in hardness but looks like only in the range of about 2%? Not sure the pic will show up:
1-s2.0-S0924013698003513-gr2.gif


Both articles seem to say that at much longer tempering times, temper martensite embrittlement occurs due to the formation of interlath carbide films due to the decomposition of retained austenite (basically quoted from the first article). This lead to a drop in impact toughness for DMR 1700 steel as determined by Charpy impact test and also depth of penetration by armor piercing rounds. But... the test conditions were 2 hours, 24 hours, and 48 hours of tempering time which may not be applicable to most knife making.

I'm sure Larrin will have a lot more intelligent and better researched things to say than me tho...
 
In W2, I doubt there will be any measurable drop in hardness after six temper cycles at 200C/400F. If you gave it 50 cycles, there probably would be less than a 1 point drop.
Temper embrittlement will not be an issue in W2 at 200C. ( that chart is for 4340 steel)
 
Temperature controls the hardness much more strongly than time. Time controls hardness on a log scale. You can look up the Hollomon-Jaffe equation to learn more, or my article on tempering. Using that equation I estimate that for W2 it takes 33 hours of total tempering at 400°F to equal a 450°F temper for 4 total hours.
 
Hmm I've always figured tempering time does affect hardness, just not in my particular case or to the extent I initially thought. Seeing the actual numbers now gives me a bit of relief.
I did manage to get that warp out on the 6th cycle. What I find interesting is that; W2 and 1095 seem to be the most stubborn steels to bake warpage out of. The two need to be clamped/bent in the opposite direction quite excessively compared to 1075/85 and 5160.

I appreciate the responses. I'm looking forward to reading further into this.
 
That is because the tempering on those steels is more like 450F. At 400F you haven't released as much tension. The higher thee hardness, the harder to straighten the warp.
W2 tempered at 400F gives Rc 64, at 450F you will drop to Rc 62-63, at 500F it is still Rc 60-61.
By comparison, 5160 tempered at 400F is Rc 59, and 1084 at 400F temper is Rc 60. That is why you find it easier to straighten those.
 
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