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

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  #1  
Old 08-18-2017, 03:26 AM
Rasmus Kristens Rasmus Kristens is offline
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Colouring the knife by tempering

Hi all

I was wondering if it is possible to colourise your blade by tempering?
After finishing HT I rather quickly temper the blade before doing the finial finish.

Is it possible to "re temper" after you have done all finish and after the initial temperingcycle to reach something like a light straw colour on the blade for final finish?

I guess it would require a thorough cleaning to make sure no residues is on the blade.

This is just a general question, but for the currentblade i'm speaking of, the steel is 80CRV2.

Hope you can help me out :-)

Rasmus
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  #2  
Old 08-18-2017, 11:23 AM
samuraistuart samuraistuart is offline
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Sure you CAN. But the problem then lies in the fact that the straw color (or blue as you go hotter or longer) will easily come right off of the blade during use. It won't stay looking like that.

You're right, in order to get a nice even color, make sure it's de-greased thoroughly prior to the "temper". I recommend that if you want a light straw color, that your REAL tempers done previously be at a higher temp than what you want as a straw color. It would be OK to be at the SAME temp, but not above that, of course, or you're HRC will be lower than what you had planned.

But, again, with all that said, the oxide layer will just get rubbed off later.
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Old 08-18-2017, 12:49 PM
Rasmus Kristens Rasmus Kristens is offline
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Thank your fir the respons. I mostly just make show knife for the hobby of it, so maybe ill try it.

I have a few follow up questions about the temper process.

1. I cant meaure rc except for the file scate check. So If after HT the blade has a rc hardness of 57. If i follow temper chart for rc 60, because i dont know the start rc after ht. What will happen? Will it stay at 57 og go lower?

2. What happens if you keep running tempering cycles at the same temperature over and over?

Last edited by Rasmus Kristens; 08-18-2017 at 12:54 PM.
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Old 08-18-2017, 02:45 PM
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Ed Caffrey Ed Caffrey is offline
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Much depends on the steel, but theoretically the tempering temp for 60Rc will generally have a lighter color then the tempering temp for 57Rc.

If you do tempering cycles over and over, as long as the temp remains constant, and you allow the blade(s) to cool to room temp between cycles. you will get a buildup of the oxide "colors". When tempering the oxides that form are cumulative.... meaning that they will layer upon each other and give colors that would indicate a higher tempering temp then you actually subjected the blade(s) to.

The whole issue with "coloring" blades in this manner is that the color/oxides are VERY fragile, as Stu mentioned they will literally "rub" off.


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Old 08-22-2017, 02:29 PM
Rasmus Kristens Rasmus Kristens is offline
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Hi Ed

Thank you for your reply. My questions were concerning the mteallurgy of the steel not as much the colour. I'm just asking how multible temper cycles would impact the RC.
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Old 08-23-2017, 05:21 AM
WBE WBE is offline
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If all tempers are done exactly at the same temp, there will be no change in hardness.
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Old 08-25-2017, 07:41 AM
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  #8  
Old 08-26-2017, 02:20 AM
epicfail48 epicfail48 is offline
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I've actually tried this before, and its a pretty common practice in clock-making and gunsmithing. That said, its not my favorite process. The colors are formed by a very, very thin oxide layer, and that layer isn't durable at all. If the knife is a user, of wears off quick. You also have to make sure that the finish on the blade is perfectly uniform and absolutely positively clean. Anything on the surface will interfere with that oxide layer. You also have to have the temperature bang on to get the color you're shooting for, 20 degrees or so too hot and your blade is blue instead of gold. Getting the color you want can also affect the temper, if you want an eye catching lbue you have to temper up in the 500f range, which softens the steel a bit much for knives.

As far as multiple tempering cycles goes, you won't hurt the knife. Tempering multiple times at the same temperature in only going to further the conversion of martensite into austenite (think I have that the right way around), which is what you want the tempering process to do anyway. Only the highest temperature counts as well, so if you do a cycle at 400, then 450, then 400 again, the temper of the steel will make it as soft as 3 cycles at 450, if that makes sense
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Old 08-26-2017, 11:50 AM
samuraistuart samuraistuart is offline
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epicfail48:

Post quench but pre temper, the matrix is already "martensite", it becomes "tempered martensite" after the temper.

Depending on the steel and heat treatment, there will be some small (or large God forbid) amount of retained austenite after the quench. This retained austenite will get converted to martensite during the temper, usually only at temperatures at or above 400F. The higher the alloying, and the higher the austentizing temperature, the more retained austenite will be present. And it is a good idea to get rid of that retained austenite as much as possible before tempering, ie sub zero or cryo. Most of the low alloy carbon steels, using the low aus temps, have very very little RA to deal with.
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Old 08-26-2017, 02:34 PM
epicfail48 epicfail48 is offline
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Quote:
Originally Posted by samuraistuart View Post
epicfail48:

Post quench but pre temper, the matrix is already "martensite", it becomes "tempered martensite" after the temper.

Depending on the steel and heat treatment, there will be some small (or large God forbid) amount of retained austenite after the quench. This retained austenite will get converted to martensite during the temper, usually only at temperatures at or above 400F. The higher the alloying, and the higher the austentizing temperature, the more retained austenite will be present. And it is a good idea to get rid of that retained austenite as much as possible before tempering, ie sub zero or cryo. Most of the low alloy carbon steels, using the low aus temps, have very very little RA to deal with.
Dang it, I knew I had a few of those -ites mixed up!
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  #11  
Old 08-29-2017, 01:47 AM
Rasmus Kristens Rasmus Kristens is offline
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Thanks for your time and the answers.
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