Need Some Tempering Help...

B

Broomhead

Joined
Jan 12, 2012
Messages
335
I screwed up tempering two of my knives. I was aiming for a straw color and ended up with purple/peacock. They were hardened using my propane forge, heating them to non-magnetic and soaking for about 30secs, then quenching in canola oil. A sharp file skated off them like a needle on glass.
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I have two questions...
A. Do I need to re-harden these in my forge before re-tempering?
B. I did these at 450* for 1 hour, three times. What temp/time cycles would you recommend?
 
What kind of steel?
what did you use to temper them? An electric oven? toaster oven?
They can cycle quite high when they are ramping up to temp.
Shielding the blade from direct radiation from the elements can help prevent overheating - foil of some kind, piece of aluminum on the rack, etc.. Or get the oven up to temp before inserting the blade. There's a whole post (stickfied?) on improving the temperature control of electric ovens.

I've also wondered if tempering colors are cumulative. Stacy will probably check in soon.

-Daizee
 
Sorry, it's 1084. I used a gas kitchen oven, preheated to 450 before I put them in. They were sitting on a small thin aluminum baking sheet.
 
Looks like no obvious errors, but apparently the real experts are taking the holiday off. :)

Have you tested them against other things of known hardness?
 
You said 450f three times but I've heard that you should do your temper 25f lower every time you temper so 425 and 400 should have been the next too tempers. Has anyone else heard this and what is the reasoning behind it?
 
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daizee said:
Looks like no obvious errors, but apparently the real experts are taking the holiday off. :)

Have you tested them against other things of known hardness?
Click to expand...

I have no way of doing this, nor do I have anything of a known hardness, besides files.
 
I temper twice at 400f for about 2 hrs each time. I dont have a hardness tester though. I usually check after hardening with a file to see if it will skate off once thru the decarb layer. Then I temper and once thats done I finish it up and give it an edge then do cutting test. Thats just what I do. When I get more knives done and get a little farther along in my abilities will probably send a few off to see exactly where I am at on my HT to ensure my process is working.

One of these days would like to get a rockwell tester but they are expensive.
 
Broomhead said:
I have no way of doing this, nor do I have anything of a known hardness, besides files.
Click to expand...

I bet you've got a cheap knife somewhere. Or a not so cheap knife! See if you can shave metal off the edge of your blades (where you'll sharpen it anyway). And be prepared to sharpen the knife you use to test it...
 
daizee said:
I bet you've got a cheap knife somewhere. Or a not so cheap knife! See if you can shave metal off the edge of your blades (where you'll sharpen it anyway). And be prepared to sharpen the knife you use to test it...
Click to expand...

Didn't think of that. I'll see what I can do. Thanks.
 
The blades are probably fine,the purple colors are probably residual oil on the blade from quenching.Watch time and temp.not colors.

Stan
 
From what I've read tempering at 450 should give you an HRc @ 54 -56. Your purple color indicates that your temperature was about 100 degrees higher. Giving an HRc in the low 50's. Do you have a temperature gauge in oven? You can heat a pan of sand and place the blades in the sand to minimize radiative heating, although you might not get as strong of a color because of less free oxygen in the sand. What hardness are you trying for?

Oops, I was writing while Stan posted. Did you clean the blade before tempering? You need a clean surface for the tempering colors to mean anything.
 
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I sanded the blades back to 120 grit to remove the decarb, so they were fresh and shiny; the only oils would have been from my hands. I do have a thermometer that I can put in the oven, but it is usually on par whenever we cook anything, like I said it's a gas kitchen oven.

Is an HRC in the 50's too low for a decent knife? I was shooting for 55+.

If I want to re-temper do I need to re-harden it first in my forge? Or can I just throw it back in the oven at a lower temp?
 
No you can't temper at a lower temp now to raise Rc.Tell you what if you want mail it to me and I will Rc test it for you and send it back.That way you will know for sure where your at.If your interested shoot me an e-mail.
Stan
 
Take Stan up on his offer. It will tell you if there is nothing wrong, or a problem needs to be figured out.

The colors after temper have almost nothing to do with the "Straw" etc temper colors often referred to. Those are the colors used for reference when drawing a temper with a torch. If held in an oven at 450 for several hours, all sorts of colors can happen. Mine usually come out looking like a rainbow.

1084 should be around Rc 59-60 if everything was perfect in the quench and temper at 450F.

Tempering is done at the same temperature both times. You gain nothing by lowering it the second temper cycle.
 
If you get the hardness checked let us know what it is. I am learning a lot from these discussions.
 
1084 tempered at 450F-475F is where I usually end up. 58HRC is a good number for what I expect from the majority of my knives.

Don't worry about the colours. Raidiant heat is less of a worry in gas ovens and since you said that you let it heat up before puting the blade in, I think you probably nailed it. Take Stan up on his generous offer.
 
Thanks for all the advice guys.

Stan, I'll email you in the morning and take you up on your generous offer.

I think I did get it pretty good, I tried to file a rough spot along the spine and had trouble getting the file to bite, it kept skating right off.
 
I just heard back from Stan. He got the knife today and tested it. He was unsure of the results so he ran it three times. Turns out I nailed the process, two came back at 60 and one at 59.5. I can't thank him enough for doing this for me.

BTW, I sent him the larger of the two knives pictured, which was the first one out of the forge and quenched. That was the one I was really worried about as it is for my Dad.
 
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No,I wasn't unsure of the results,just wanted to string you along a bit before you saw the final numbers.The third test was to get you a shot of the dial. You did good.Glad to help.Next step is to make one and harden it,sharpen it,beat the snot out of it, then break it to check the grain size,and you will know you realy got it right.
Nice looking blade also.

Stan
 
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