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

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  #1  
Old 10-16-2013, 08:02 AM
Radar Radar is offline
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Heat treat quandry

Okay, newbie here.

Just a bit of background, this question is regarding my first knife made from scratch, material removal method, using 3/32" O1. I don't have a picture of the blade in its current condition, but I think I can express the situation well enough without a visual reference. It's a skinner with a gut hook, overall length just shy of 9", cutting edge length about 3", overall height ~1 1/2".

I ground the blade progressively from 60 -380 grit on my cheapie Harbor Freight grinder, leaving the edge ~.020 flat. I hand filed the gut hook and hand sanded heavy grinder marks out of the cutting edge so as not to leave any stress concentrators during hardening.

I was lucky enough to stumble upon a digital heat treat oven for a good price, a Ney Vulcan 3-550, FWIW.

After doing a ton of research, mostly from data supplied by Crucible, I finally got the guts to heat treat my first blade, using the following procedure.

Knife stood on edge, supported by Ti pins drilled into a strip of firebrick to promote even heating.
Programmed the oven to slowly ramp (22? /minute) to 1300? for stress relief, hold for 20 minutes.
Second step raised temp, again at 22? deg/min, to 1475?, hold for 20 minutes.
Quench was done in a gallon of 11 second quench oil preheated to ~130? from McMaster Carr. I got everything lined up so the blade wouldn't cool before quench. I grabbed the heel of the blade with a vice grips and dipped the blade, point first, straight down into the oil being careful not to go in at an angle and not to swish it around once submerged in the oil. I held it in the oil for about 30 seconds and then withdrew it. It was hot'ish, but I could touch it without burning myself.
There was very little scale on the blade, that wiped off with a paper towel. The color was uniform gray, and I let it hang until cool to room temperature.
After the oven had cooled down to about 275?, I programmed it to hold at 425?. After stabilizing for a half hour, tempered the blade (again, on edge as before) for ~2:20, cooled to room temp and then again for ~2:30.

Based on my research, the hardness should be right about 62 RC, but it tested out at 55-57 RC.

The color is still gray, but darker and maybe a little brown'ish.

My first inclination was that there's something wrong with the oven and I didn't achieve the temperatures indicated on the display, however, I don't really have a good method to verify with a high temperature thermocouple. I have access to an IR thermometer, but those are flaky depending on the emissivity of the surface being measured and I don't really trust the results.

I have not done a file test to see if I get bite, and I did not scuff the surface to brighten it up before doing the hardness test. I thought I read something about a decarburization layer forming on the surface that can be misleading, but it seems to me it was more in the context of scale, and that all either blew off in the oil or wiped off easily with a rag.

So, any suggestions as to what I should try next?

Thanks for any help and suggestions.

-Radar

Last edited by Radar; 10-16-2013 at 08:17 AM.
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  #2  
Old 10-17-2013, 05:49 AM
jmccustomknives jmccustomknives is offline
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Looks like you've got your procedure down. I would polish up a long piece and put it in the oven @ 425 to check the uniformity of the heat by the colors. You also didn't check the blade with a magnet before quenching. Starting with new equipment takes a little practice to get its personality down.
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  #3  
Old 10-17-2013, 09:32 AM
Radar Radar is offline
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Thanks for the input.

I was able to scrounge a thermocouple to check my oven. Placing the thermocouple right next to the oven's thermocouple measured right on at 425, so I relocated my thermocouple to the location in the oven where the blade was and proceeded to check the 3 temperatures I used, 425, 1300 and 1475. It measured roughly 25 deg low at 425 and 1300, but just 7 low at 1475. Provided that holds true with my HT session, I should be in good shape.

I did the file test, and once through to bright steel, the file skates, that's good.

I also ground off some material on either side of the handle, deep enough to where I can barely detect where the first hardness tests were. I'm planning on going back to the shop and retesting and hope to get better results. Machinist is out today, so may have to put it off 'til tomorrow.
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  #4  
Old 10-17-2013, 09:52 AM
Jason Fry Jason Fry is offline
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Your procedure looks spot on. I bet your re-test comes out great. I've had some signficant variation in RC values just from the tiny bit of scale/decarb left after HT, even if the majority of the scale pops off in the quench.
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  #5  
Old 10-18-2013, 03:37 PM
WBE WBE is offline
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How did you test for RC?
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  #6  
Old 10-18-2013, 05:56 PM
Radar Radar is offline
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So, after the first test for hardness, I temperature tested the oven, see previous post, things don't look too bad. I went back to the machine shop to do some more hardness testing and the results were really weird, sub 50.

From there, I cut a couple of scraps from my steel and made 2 test coupons, both following the original hardening protocol (did the magnet test before quench, passed, non-magnetic). One coupon I tempered again at 425, the other left as-quenched.

The values were weird again, still way soft. So I performed the test on a HRC-60 calibration sample and that tested 47, woah! Retested my blade, 47, tested the tempered coupon, same, tested the non-tempered coupon, 53. So, assuming the tester is out of whack, and that it's an offset error, my blade should be right around 60, and as-quenched should be about 64. Those results, at least, make some sense, although I was hoping for 62 on the blade.

There's still a shadow of doubt, but at this point I feel confident enough to move forward with completing my knife.

-Radar
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  #7  
Old 10-19-2013, 06:59 AM
WBE WBE is offline
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Quote:
Originally Posted by jmccustomknives View Post
Looks like you've got your procedure down. I would polish up a long piece and put it in the oven @ 425 to check the uniformity of the heat by the colors. You also didn't check the blade with a magnet before quenching. Starting with new equipment takes a little practice to get its personality down.
A magnet would only tell him that it is above or below 1414?. Why bother if the blade is in the red-orange zone, which it should be? I would not trust a single gallon of oil to do the cooling on 9" of steel, and it should be moved fore and aft in the quench. Just not sideways. If it is not moved in the quench, the vapor jacket is going to slow the cooling effect. He also needs decarb protection for a 20 minute soak time. ATP-641 does a very good job of protection in his heat range.
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  #8  
Old 10-20-2013, 10:00 AM
Radar Radar is offline
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Thanks for the quench technique tip and the ATP-641. I will give both a try.

You mentioned that a gallon of oil isn't enough to quench a 9" blade. When making my test coupons, the mass of the two coupons combined was maybe 2/3 of the actual blade itself. I measured the temperature of the oil before the first one, between them and after the second. they were ~135, ~150 and ~160, from memory, that is. What do you consider to be too warm for the oil bath prior to quench? I thought that the critical temperature to achieve was considerably higher than the oil bath temp.

Thanks again for the help, this has been a new learning experience and the information keeps flooding in.

-Radar
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  #9  
Old 10-20-2013, 12:50 PM
WBE WBE is offline
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I won't quench at more than 140?, but I'm sure you can go a little more with 01. Most of the producers of 01 recommend the oil quench to be 125? to 130?. The hotter the oil temp, the slower the cooling effect. If it goes too slow getting below 900?, you will have some pearlite formation which will not transform to martensite. If you do not have enough oil, it could over heat fast enough to cause this. I am not saying that is your problem, but why chance it? I run 3 blades at a time in succession and quench in 5 gallons of AAA starting at 130?, and watch the oil temp.
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  #10  
Old 10-21-2013, 11:59 PM
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GHEzell GHEzell is offline
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Quote:
Originally Posted by WBE View Post
The hotter the oil temp, the slower the cooling effect.
I'm having trouble with this statement, room temperature oil does not cool faster that 120F oil, in my experience. Just the opposite, in fact.


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Old 10-22-2013, 07:13 AM
WBE WBE is offline
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Originally Posted by GHEzell View Post
I'm having trouble with this statement, room temperature oil does not cool faster that 120F oil, in my experience. Just the opposite, in fact.
Very true, but if you get into much higher ranges of oil heat, you reach a point of retarding the speed of the cooling effect. The type of oil and the steel will have some bearing on this. Parks #50 is formulated to work best at between 70? and 100?. AAA is recommended at between 120? to 140?. If the cooling is too slow below 900?, the steel may begin producing bainite at the expense of martensite. Another reason to have plenty of oil to absorb the heat of the blade. I may be overstating the importance of keeping the oil within a given range, but I'd rather not chance it.
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Old 10-22-2013, 12:47 PM
Radar Radar is offline
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Out of curiosity, how can you tell if bainite or pearlite have formed following heat treatment? Are there specific tests that can reveal this, or is there a visual appearance of the steel, either before or after post-heat-treat grinding and polishing?

THX

-Radar
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  #13  
Old 10-22-2013, 01:34 PM
Kevin R. Cashen Kevin R. Cashen is offline
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A Rockwell test will tell you something is amiss but it will not tell you what that something is. To identify the structure responsible exactly you will need more in-depth methods of examining the steel, and just looking at the surface, regardless of the etchant will not identify the phase responsible. I use metallography, which is much more involved than many folks realize, to microscopically examine the internal structure of the steel and identify phases that result in mixed microstructures. Even microscopes have their limitations, such as quantifying bainite and retained austenite, and are still reliant upon the observer?s ability to identify the phase they are observing (a major part of the metallography skill set). To some extent there are even more advanced ways of spotting even more tricky phases, such as X-ray diffraction crystallography, or SEM work. There is much confusion about what can or cannot be seen by various methods, for example- spectral analysis is often cited but all that does is give you the chemical elements present, not what form they take, so I am not sure where some folks are getting more from it but I certainly hope they aren?t paying for it.

If you use the preheat I would cut your soak time at 1475F in half, your results could be retained austenite or decarb due to over-soaking for the size of part. Keep your oil around 130F and agitate as WBE suggests, forward and back in a cutting motion, entirely below the surface of the oil, but never side to side. Contrary to popular belief proper agitation will reduce the chances of distortion, not increase them.
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Old 10-22-2013, 04:21 PM
Radar Radar is offline
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Thanks for the clarification. As I've been sanding down the blade, I've noticed some surface anomalies that kind of look like fish-eyes in wood finish. They're kind of clustered together in regions and sanding through them is becoming a chore. I got to wondering if those are different crystal structure than the surrounding steel as a result of my rookie heat treat.

Thanks for the tip on soak time. I'll knock it back on the next one.

I just stumbled on your Mayo thread, hope you're making good progress.

-Radar.
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Old 10-22-2013, 06:47 PM
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GHEzell GHEzell is offline
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Quote:
Originally Posted by WBE View Post
Very true, but if you get into much higher ranges of oil heat, you reach a point of retarding the speed of the cooling effect. The type of oil and the steel will have some bearing on this. Parks #50 is formulated to work best at between 70? and 100?. AAA is recommended at between 120? to 140?. If the cooling is too slow below 900?, the steel may begin producing bainite at the expense of martensite. Another reason to have plenty of oil to absorb the heat of the blade. I may be overstating the importance of keeping the oil within a given range, but I'd rather not chance it.
Thank you for clarifying that.

I thought you might be interested in this, as it has to do with quench oil temperatures.... I would very much like to hear your opinion.


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