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Heat Treating and Metallurgy Discussion of heat treatment and metallurgy in knife making.

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  #1  
Old 08-12-2004, 07:23 PM
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SteveS SteveS is offline
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My LN is broken

I repaired the door in my oven and I can't do a thing with it

Actually, it changed my results quite a bit. Before the steel (ATS-34 and S30V) acted like the data sheets. However, now I have to set the temperature 30 degrees higher to get closer results, but still things are different.

I thought it was that the tighter fit lost some kinda convection effect, so I slowed down the ramp time a lot and increased the soak. Still didn't behave the proper way. So I ran some test strips of metal thru. And discovered I had to increase the set temp by 30 degrees to get the previous results.

But then other interesting things showed up.


----------------------

On top of all that I've determined the Liquid Nitrogen is broken . Not one sample changed its hardness after a soak (4 different cycles and 3 different steels) I checked hardness after 1/2 hour, 1 hour and 15 hours in LN. No change. Oven temps (my settings) of 1950, 1970, and 2000.

The quench method was identical in all cases: In the foil, 1" aluminum plates, and compressed air.

So do I have the perfect quench method and achieve complete martensite conversion? (I doubt it, but I'm still suprised.) I'll still use LN, but I don't get a measurable change.


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On the brittle test, something else was odd. 2 pieces, both had over 16 hours of LN soak. One was heated to 1970 ramp slowly and as quenched 61HRC, the other 2000 ramp quickly and 63HRC. Both were tempered as follows:

Start -> #1=61; #2=63
400 1 hour -> #1=60; #2=62
500 1 hour -> #1=58; #2=60
600 1 hour -> #1=58; #2=59
1000 1 hour -> #1=58; #2=60

Here's the odd part: #2 was much tougher than #1. #1 was quite brittle; #2 bent to near 45 degrees before snapping.

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Moral of the the story: not all ovens are the same. Heck, just changing the brick means re-test your procedures.

Moral #2: HRC and toughness are not always in perfect inverse proportion; sometimes you can have a higher HRC and still be tougher.


Steve


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Last edited by SteveS; 08-12-2004 at 07:26 PM.
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Old 08-13-2004, 06:56 AM
RJ Martin RJ Martin is offline
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Steve: For the Austenitizing temps you used, those as quenched hardnesses sound fine. What were your soak times at temp?

Were you freezing before or after the first temper?

I see you tempered at 1000F. Is this the temperature you use for your knives, or was that just to check the furnace?

Please tell us more about the tempering cycle on #2, the one that bent.


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Old 08-13-2004, 10:35 AM
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Hi RJ,

Tell more? Sure love to. (I really like reading up on other people's tests.)

After I replaced the bricks in the door, I did a set of CM154 blades using the 'normal' procedure and soaked at 1950 for 20 minutes. However after tempering to 500 the hardness was 56! I knew something had changed.

All that to say, I'm listing my oven settings, but I think you should subtract 30 to 50 degrees.

Plus I just list the information for blocks #1 and #2 (there were many others).

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>> What were your soak times at temp?

#1 was 1970 for 45 minutes (blade in cool oven, slow ramp to temp to boot, 4 hours total oven time).

#2 was 2000 for 23 minutes (Blade in at 1500 and fast ramp)

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>> Were you freezing before or after the first temper?

These two samples went straight to the LN.

BTW there was another sample done exactly like #1, but had a snap temper for 1/2 hour at 300. It was not as brittle!!! Maybe had some of bainite?

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>> I see you tempered at 1000F. Is this the temperature you use for your knives, or was that just to check the furnace?

No not what I normally use on knives (usually 400-500). I was just testing with the temps on the Crucible data sheets.
I know Paul Bos uses the high temp tempering cycle. Maybe I'll start.

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>> Please tell us more about the tempering cycle on #2, the one that bent.

Not sure what to add. Let's see.

I did one test immediately after quench, while it was warm: 63.
One test after I chilled it in the freezer: 63
LN for 1/2 hour: 63
LN for 1 hour: 63
Ln for 15 hours: 63
400 for 1 hour: 62
500 for 1 hour: 60
600 for 1 hour: 59
1000 for 1 hour: 60

The first 2 tempers were in the toaster oven with a good themometer. The second 2 where in the heattreating oven, so maybe the temp was a little less?

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Results of all the testing showed slow ramp and extra soak times didn't really have an effect. Crucible lists equalizing at 1400 degrees. However, 1/8" steel probably doesn't need it.

Steve

PS I'm still wondering why the LN didn't raise the hardness.


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Old 08-13-2004, 11:29 AM
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Steve, for ATS-34 and its cousins and for S30V, increase the soak time to 25 - 27 minutes. For high alloys it is better to over soak a little than to under soak a little. It is also best to get to austenitizing temperature as quickly as possible after steel is equalized at preheat (preheat / about 1400 F for ATS-34, 1550 F for S30V).

RL


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Last edited by rlinger; 08-13-2004 at 11:33 AM.
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