Tempering Question

ScarFoot

ScarFoot

Knifemaker / Craftsman / Service Provider
Joined
Sep 16, 2021
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I am working on a batch of blades and have a question about tempering. I normally quench, straighten if needed, and try to get them in the toaster oven by the time the blade is cool enough to touch. The problem is my toaster temp varies quite a bit. Even with some cast iron in it. Would it hurt anything to quench them all and put them in the toaster at 250-300 degrees or possibly higher until I get them all quenched and then put them all in my heat treat oven (which is way more precise) for tempering? I can program it to run timed cycles and control ramp rate and everything so I’d really rather use it. I’m trying this now with some test pieces just to figure out what HRc I get at different tempering temps but I don’t want to put a bunch of stresses in my blades. Any thoughts?
 
What you're describing is often called a snap temper. I don't think it would cause any problems.

What steel are you using? And how many knives at a time?
I use simple carbon steels, mostly 26c3, 1080, and occasionally 5160, and I have no problem leaving the ~5 blades I do in a batch until i can put the entire batch in the oven.
 
Alex Topfer said:
What you're describing is often called a snap temper. I don't think it would cause any problems.

What steel are you using? And how many knives at a time?
I use simple carbon steels, mostly 26c3, 1080, and occasionally 5160, and I have no problem leaving the ~5 blades I do in a batch until i can put the entire batch in the oven.
Click to expand...
I’m using W2, 1095, and 80CrV2 at the moment. With some racks I could probably get 10-12 in the oven. Is there a temp threshold I should stay above before final temper? I figure I can set my toaster to the 300-325 range and hold them there until I run the tempering cycle.
 
It just needs to be lower than you're final temper. I assume we are talking fahrenheit?
If you're doing 400f final temper then 320f would be fine. If you leave them in there for 20 hours there might be issues.
You could also put them in there for a while to relax some of the stress then remove them once they have come up to temperature.
 
The effect of tempering is a function of both time and temperature. Temperature is linear, but time has a logarithmic component. This means that really long times at a temperature will result in a larger drop in hardness than you would expect from the temperature alone, but we are talking about increasing the time by an factor of 10.
The typical hour time is an easy compromise; it's more than enough time for the blade to get to a uniform temperature, long enough to not be a hassle but not so long it's annoying, and in our situations it's easier to vary the temperature.
So a snap temper should be long enough to get it heated through, but doesn't need to be longer than that
There's some more discussion here:
knifesteelnerds.com

What Happens During Tempering of Steel? - Knife Steel Nerds

Thanks to Shawn Houston and Tyler Christian for becoming Knife Steel Nerds Patreon supporters. We have reached our first goal and therefore I will be interviewing Devin Thomas and posting it as an article to the website. In a previous post I wrote about what happens during rapid quenching of...
knifesteelnerds.com knifesteelnerds.com
 
Alex Topfer said:
It just needs to be lower than you're final temper. I assume we are talking fahrenheit?
If you're doing 400f final temper then 320f would be fine. If you leave them in there for 20 hours there might be issues.
You could also put them in there for a while to relax some of the stress then remove them once they have come up to temperature.
Click to expand...
They’d only be in the toaster for a few hours while I get everything hardened and yes I‘m talking Fahrenheit. I’ve read what you said about temp over prolonged time but hadn’t studied on snap tempering. I normally just go straight to the toaster to temper but it bounces 20 degrees or more sometimes when the elements cycle so it’s hard to know what the actual temp is without shutting it off and watching the temp reading on the TC. My heat treat oven is way more precise.
 
ScarFoot said:
They’d only be in the toaster for a few hours while I get everything hardened and yes I‘m talking Fahrenheit. I’ve read what you said about temp over prolonged time but hadn’t studied on snap tempering. I normally just go straight to the toaster to temper but it bounces 20 degrees or more sometimes when the elements cycle so it’s hard to know what the actual temp is without shutting it off and watching the temp reading on the TC. My heat treat oven is way more precise.
Click to expand...
Install FAN from kitchen oven inside , problem solved :)
 
Yes, tempering is a factor of time am temperature, with temperature heavily weighted.

An increase of 20°F from 400°F to 420°F would make a change in an hour.

However, most steels don't change much at 320°F

At 320°F a W-2 blade would not have any significant issue unless it was left for 100's of hours. A day in the oven would not appreciably change things more than an hour in the oven. At higher temperatures (450-500°F) it might be more of an issue.

Point is - good temperature regulation is the most important thing. Time just needs to be "enough" One hour for carbon steels and two for stainless is usually sufficient for each temper cycle.
 
Stacy E. Apelt - Bladesmith said:
A day in the oven would not appreciably change things more than an hour in the oven. At higher temperatures (450-500°F) it might be more of an issue.
Click to expand...
That was my thinking, thanks for confirming.

You specifically referenced W2, but isn't this true for most all steels we use?
 
Yes, certainly for the carbon steels. I used W-2 because the OP said it was what he worked in.
Stainless steels have retained austenite and a very long hold may stabilize it to some degree.
 
No, stabilized austenite means the retained austenite won't convert to martensite. Austenite is soft martensite is hard. The reason you don't delay getting into cryo and then tempering stainless is any lengthy delay can make it end u softer than desired. This is not a concern with carbon steel because it has little or no retained austenite.
 
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weo weo ah i get what you're asking now. I should probably have been more precise; i was just trying to say that leaving things in there for a very long time might cause weird effects. I couldn't find any info on what effect this has at 150c

From what Stacy said it's not an issue
 
Thanks, Stacy E. Apelt - Bladesmith Stacy E. Apelt - Bladesmith
My memory doesn't seem to want to retain the austentite/martensite/bainite definitions, and I'm on vacation and don't have access to my copy of Larrin's book.
(i suppose I could have googled....🥴.)
 
Alright I‘m in the basic testing phase with the W2. I broke 2 pieces that were held at 1465F for 10 minutes each and quenched in Parks 50. After that they were snap tempered at 300F for a couple of hours. One piece is 1/8” and the other is 3/16”. I had to break the 3/16” piece with a sledge hammer so I definitely need to remember to cut in some notches before I heat treat next time. Anyway, the 1/8” piece looks okayish but the 3/16” piece has some cracks that run perpendicular to the surface. I assume they occurred when I broke it but am not sure. I sanded the pieces before breaking and didn’t notice anything on the surface at that time. Below are some pics under magnification. It was a little difficult to get everything decently lit and in focus. So, all the pieces look better in person than they do in pictures. Might see if my digital microscope still works and try to take a closer look. Let me know what y’all think as far as adjustments I might need to make. Wondering if I might need to cook them longer than 10 minutes at 1465F.?.?.? I‘m prepping some pieces for final temper and hardness testing and am hoping to heat treat blades the week after next.

1/8” at 45x magnification:
IMG_0037.jpeg

3/16th at 45x, notice cracks along right edge.
IMG_0034.jpeg

3/16“ at 10x. Note cracks on bottom edge by thumbnail. IMG_0043.jpeg

1/8” at 10x. I think the grain size looks decent.
IMG_0041.jpeg

BONUS PIC!!!! 80CrV2 at 10x. Held at 1525 for 10 minutes and quenched in Parks 50. Broken when it reached room temp after quench. Grain looks nice (all the black specs are the same stuff that’s on my fingers) but not sure about the center…
IMG_0025.jpeg
 
Got the microscope going and got some 250x images. Also did some hardness testing and ”for scienced” a few things. I wanted to see how much difference a little decarb made on hardness results. I sanded everything down on my disc sander but left spots on a few pieces that were slightly discolored looking. The hardness was on average 8-10 HRc lower in those spots. Definitely made a believer out of me. The 3/16th pieces of W2 didn’t fully harden. I got 61-62 out of one piece but the other pieces were lower and all over the place. Also the piece I broke to check grain structure definitely exhibited signs of not being fully hardened. On the 1/8th inch piece of W2 I got 64-65 on the pieces I tested which I felt was pretty good. I think I might need to let them soak longer at temp before quenching. I might try 15 minutes on the 1/8” and 20 minutes on the 3/16”. I’m going to see about getting them surface ground before I hardness test again.

The following 3 pictures show the cracking and grain of the 3/16th W2 piece at 250x magnification.
w2 1465 no temper 5 3-16.jpeg
W2 3-16 1465 no temper.jpeg
3-16 W2 1465 No temper2.jpeg
These pictures show the 1/8” piece of W2 at 250x:
w2 1-8 1465 no temper3.jpeg
w2 1-8 1465 no temper2.jpeg
w2 1-8 1465 not temper5.jpeg
Here is the piece of 80CrV2 at 250x:
80CrV2 3-16 1525 no temper.jpeg
 
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