Tempering 0-1 (Blue color at 375 Degrees)

Chriszep

Chriszep

Member
Hello folks,

Id like to ask the internet what the heck is going on with my tempering colors. Ive made several knives out of 0-1 and I used to temper them in a toaster oven to around 375 and get a golden color. Recently I got myself a Rapid fire pro table top oven. Its great to use and seems much more accurate than what I was doing before with a torch and some fir brick. Here is my issue. I heat treated a new 0-1 blank, I used anti scale and the blade came out of the quench nice and hard. When I tempered it at 415 the blade came out a solid blue/purple. I thought it strange but did my two more cycles and continued on. (the blade was a test of the oven so I'm not too concerned about the hardness)

I then tried another blank, same thing, heated to 1500, soaked for a few minutes, quenched in war oil with anti scale. This time I tempered at 375, just to see what color it came out, AGAIN, blue/purple...

Now, is my kiln way off? or is it possible that a 1/8" thick blade tempering at 375 for 1 hour would come out blue?? Every chart I see online, shows blue/purple appearing around the 575 degree mark!

Any guidance on this would be very helpful. Ill attach a photo as well:

IMG_4622.jpg
 
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I'd double check your oven with an oven thermometer or pyrometer just to be sure but I doubt your oven is off.

Surface colors on the steel are not the best way to accurately judge temperature. It could be caused by a variety of things including residual oil from the quench or any cleaning solutions or even the oil on your skin.

If you have good equipment and/or good thermometers I'd trust those way more than surface colors.
 
I've seen things of this nature happen many times..... it's almost always the temper oven being used. DO NOT TRUST THE TEMP READING/SETTING ON ANY OVEN, until you prove it for yourself through the use of separate thermometer.

That being said, you have to eliminate as many variables as possible to nail down the exact issue(s)...... based on the photo, it appears that either the mill scale wasn't removed, or the anti-scale/quench residue wasn't removed prior the tempering.... unless you cleaned/sanded off the blade(s) between hardening and tempering, the "colors" are not going to show up in the same shades/hues you'd expect them to.
 
Thanks, ill do some digging online to find a thermometer that i could possibly leave inside the oven that will withstand the heat, or I wonder if I got an IR thermometer and just shot it into the kiln as soon as i open the door.. I clean my blades with a degreaser after I wire wheel them out of the quench oil... maybe thats whats causing the color...

Thanks!
 
IR thermometer is gong to be very inaccurate in this case..... I'd go with a standard oven type thermometer...or two. :) Also, make sure you allow the oven to "cycle" for 15-30 mins when you're trying to determine the temp..... many ovens overshoot on heat up cycles, and undershoot before the cycle begins again. That's usually the issue with ovens......especially "toaster" ovens.....they often fluctuate wildly.... sometimes to the tune of 100-200 degrees or more. That means a lot when you consider every 25F degrees after 350F, creates changes in the steel's tempering.

The wire wheel is certainly a contributor, and depending on the degreaser you're using, residue from it is likely have an impact too. If you want to see the "true" temper colors, a cleanly sanded finish of 220-400 grit, and IF you clean it, use something that will not leave any residue/film..... IE: Acetone.
 
Thank you! Ill skip the degreaser and wire wheel next time for sure. Id like to hope my rapid-fire oven is fairly accurate, during the 1 hour tempering cycle the Internal temperature reading barely moves 1 degree up or down. Ill get a thermometer and do a test this week. Thanks again!
 
Congratulations on upgrading from the toaster over, on my list of all time things that budding knifemakers do to handicap themselves the toaster oven temper is in the top five; wildly inaccurate with detrimental temperature swings, great for warming a bagel but terrible for tempering a knife. But now unfortunately you have a dedicated oven that seems to have it own temperature regulation issues. The photo is not clear as to the oxides that are on the blade. Be certain that the blade is clean, silver and shiny before the temper when assessing the colors which, as has been mentioned, can be deceptive as an accurate temperature indicator. If your blade was polished clean before going into the temper then your oven is decidedly off in its calibration, the oxide colors can be off but not that much. 375F, should barely show some light yellow coloring on a silver blade, 400F should be more a golden bronze or straw color with some odd spots of crimson-purple where you had oils or residues on the blade.

Now, as for your tempering temperatures themselves, for a blade that size in O-1, 375F is what you want. 400F and above will pull O-1 out of its highest performance range for a blade that size, and if you go above 400F or are making a much larger blade it would be better to switch to a different steel that will have a better strength to toughness ratio for the application. I can make just about anything from scalpels* to swords by keeping O-1 and L6 in my shop. Anything under 8" that needs to have a fine long lasting slicing edge- O-1, blades over 8" that require toughness for chopping or cleaving- L6. This same setup can de done with something like 1095 or W2 for smaller blades and 15n20, 5160 or 80CrV for larger blades, but each steel will have that tempering sweet spot for balanced properties in that application.


* if they weren't so picky about stainless for surgery :3:
 
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during the 1 hour tempering cycle
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Those few words made me think..... First, I personally don't feel 1 hour is enough for a tempering cycle. To explain, I've conducted many performance tests were blades have broken, where individuals only tempered for "1 hour", and based on the appearance of the interior of the breaks, it was obvious to me that there was a "tempered skin" about .020-.030" thick or so all around the exterior, but the cores of the blade were still in or very close to an as quenched hardness.

Also, it's very important HOW you temper a blade..... putting a blade into a cold oven, then, setting the temp and starting the oven with the blade already inside is not the way to go...... far too much fluctuation going on. I always encourage folks to turn the oven on, set the temp, then allow it to "cycle" for 15-30 mins AFTER the setpoint is reached. Then the blade goes in, and the clock starts for 2 hours. IF you are placing a blade in a "cold" oven, then starting it, there is a very good chance that until that oven settles to it's setpoint (after cycling several times), it's overshooting the desired tempering temp. That certainly isn't always the case, but it's very possible, and it very well could be that your blades are getting tempered softer then you're wanting because of it. The whole point being that in order to achieve the results you seek, it's common place to have to think it through, and eliminate as many variables as you possibly can.

I'm just trying to imagine your mindset, and picture how you might be conducting the process...... so there's a lot of educated guessing, going on in this response..... just take it for what it's worth to you.....hopefully it's helpful.
 
This is all useful information. I have tried a few different tempering methods, and I've read a few different books/ internet posts on tempering. The information is all over the place.... I will absolutely take your advice on this. At one point, I read to do 3 1 hour tempers at the desired temperature, and to keep the oven on in-between so that as soon as the blade is cool enough to handle by hand it goes back in. I am not at all married to that process and gladly will change it for a better method.

Do you recommend 2 - 2 hour tempering cycles?

(now I'm asking myself why I wasn't on here asking questions sooner!!)
Thanks!!!!
 
In my shop, due to spectrographing results, I believe that two, 2 hour cycles are necessary, and in some cases, depending on other factors, 3 times is valuable. Personally, I look at it, and explain it like this: I feel that two, 2 hour cycles is necessary, and I add a third for "insurance" purposes.

do 3 1 hour tempers at the desired temperature, and to keep the oven on in-between so that as soon as the blade is cool enough to handle by hand it goes back in.
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That is similar to how I do it, with the exceptions that I do 2 hour cycles. That's not to say my way is the only way, it's just my way, and works best for me.....at least to this point in my career. :) (always seek improvement)
 
Chriszep, you have good advice from Ed, I am glad he spotted the time factor because I completely agree that one hour is most likely not enough, especially in an oven or kiln. Air is a pretty good insulator and the driving forces behind tempering temperatures are not as intense as with other heat treatments so you will spend the good first part of that hour equalizing the blade at temp, you then need time for the tempering process to do its thing at the desired temperature. I prefer a 2 hour minimum at temperature, but spend less time beyond that due to the conductive nature of tempering salts, I also like multiple tempering cycles for homogenization of the heat treatment. I would also like to confirm what Ed is saying about letting the oven regulate at temperature before introducing the blades, that is a solid, sound practice to avoid the overshooting and undershooting of an oven regulating itself as well uncertain tempering times.

Ed, are you sure it was spectrometry work the lab did for checking tempering results? Spectral analysis is normally used to detect the chemical elements present in a sample, tempering shouldn’t change the chemistry, that I know of. For this reason, and since I normally insist on the chemical specs with my steel orders, I didn’t look into the possibility of getting a spectrometer until I started smelting my own steel. And then I found that any unit beyond a yard sorting gun (which wouldn’t read the carbon content like I needed) was beyond my finances to get and keep calibrated; so we knifemakers are still stuck going to labs for that work:(. I eventually would like to invest in the equipment to do spot elemental X-ray analysis with the SEM, but it still requires saving up for things when regular shop maintenance eats up every spare dollar. Anyhow, what was the type of information that came back from the tempering tests? That could help identify the type of analysis done at the lab.
 
You're right Kevin! I used the term because I was typing along and should have said "analysis", which might not be the correct word either, but it's what the individual/lab who did/does the testing for me uses. They also did the spectrometry work, but I was lumping the whole 9 yards into a single term, which I guess I shouldn't be doin, if nothing else for the sake of correctness. :)

Although for nearly two years I was dumping money like a dope addict on the stuff, it was worth ever cent, and it showed me just how misinformed, and misguided I had previously been. My whole foray into it was more a cause and affect thing.... I just "needed" to know/understand if I did XXX, what exactly was the affect. Prior to all that, I was one of the "one hour temper" crowd.....and after seeing the results of what time and temp could/does do, I realized just how much I really didn't know. I think it's that way for most.....generally all we see in knifemaking is a very rough "cause and affect", but when I started understanding what I couldn't physically see, that's when more and more things started making sense. Now if we could just overcome the many variables from shop to shop, and method to method...... :)
 
EdCaffreyMS said:
... Although for nearly two years I was dumping money like a dope addict on the stuff, it was worth ever cent, and it showed me just how misinformed, and misguided I had previously been. My whole foray into it was more a cause and affect thing.... I just "needed" to know/understand if I did XXX, what exactly was the affect...
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Amen to that, when I started outfitting my lab I was blown away by how much my own eyes were capable of lying to me. There is so much in the interpretation of what we see. We see “A” and “B” brought together and then witness “C”, so our brain logically deduces that A+B=C. But when we access means of really seeing all the variables it is shocking to discover that there were “A1, A2, A3…”, “B1, B2, B3…”, there was both “A” and “a”, and it turns out that “D, E and F” were present and we never even knew it! That knowledge drove me so batty that I knew it would be cheaper to just set up my own lab rather than pay for tests every time I had a question ( which is just about every waking minute).

One of my goals is to get some time with X-Ray diffraction crystallography equipment to answer some questions about some tempering effects I have observed. I need to schmooze some of the nearby Universities or see how good I am at tinkering with really out of date gadgets, but I don’t like doing the backyard mechanic thing with things that produce radiation.:nuke::scared:
 
Thanks. I wish I had read this earlier. I did two tempering cycles an hour each time at above 400 for my 01 blades. I only did two blades. When I get some more 01 I'll temper at a lower temperature.

Jake
 
Test them they might be fine. Take one of them and abuse the daylights out of it. You don't know if your heat treat worked until you test the blades.
 
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