Temper Help ....?

First off are you sure you want blue for a knife? Typically that is considered too soft...most makers d blades to a straw/gold color (400-450).

Definitely leave it in for longer times. Most makers leave it in for 2+ hours, for 2-3 cycles. At 20mins there is a very good chance you are only getting the surface hot, and not all the way through (might explain your splotchy colors).

No water quench is needed after tempering, just let it air cool. Quenching is done after hardening....did you harden the steel prior to tempering it?

 

If you want a blue/dark color on a knife blade use bluing, FC, or Parkerize it. You don't want to temper much above 450° for a knife blade. A hawk could take 500°.

Fire bluing ( heat bluing) is done for guards and pommels by heating them with a torch or oven to get the rich blue color. A knife blade would be ruined if done this way. The trick is to heat it to just the right temp and dunk it in water before the blue passes into gray.

As to the water quench from the temper oven, Roman Landes has done some pretty extensive studies on this and recommends that a hardened knife not be cooled slowly from the temper oven, but quenched in water to drop the temperature quickly. It has to do with the structures, but the long and short is that it is beneficial, and there is no risk of cracking the blade or warping it.

 

So tempering 1095 at 500 is bad? I read in Kevin Cashen's site that would put you at around 59-60 rc.

 

Well thats interesting...most people I know just allow it to air cool. I'm not familiar with Landes, but after a quick Google I saw Kevin Cashen talk highly about him so I am definitely interested in learning more. Most of what I am seeing from my search are people saying "Landes does it this way....anyone know why?" then others giving their best guess. Is there a repository of info straight from him or published testing? I'm curious if the fast cooling applies to simple carbon steels, higher alloys, if it refines grain structure further, etc etc.

 

Cold blue.

 

There was another thread similar to this one, www.bladeforums.com/forums/showthread.php/988992-Need-Some-Tempering-Help , you might read it.

From what I've read, tempering a blade at a given temperature for longer periods of time will result in a decrease in hardness.

 

PF,
You need to do a lot more reading in order to understand the tempering and hardening process well.
The colors on that photo link were just the oven colors from the post-quench heating. They have noting to do with temper colors. Those, and temper colors, are very thin and would rub off in assembly and clean-up. The color a blade comes out of quench, and then out of the oven can be a wide variety of colors. The golden/straw color is normal for very clean blades at temper, but any shade of the rainbow ( and often all of them at one time) are common. Getting a deep blue color requires a lot higher heat than a user blade should be at. A good rule of thumb is - If it gets hot enough to blue it - Re-do it!
If you want the deep blue/purple color you see in many knife books, look up Niter Bluing. It is a low temp (500-600) salt pot treatment with nitrate salts. The blade does not stay in the salts long enough to greatly affect the temper....and these knives are usually safe queens. Even this is only a very thin layer.

Shaw,
In a perfect world,......in a metallurgical lab,...... with an absolutely nailed hardening by water quench,...... a temper at 500F would give about Rc 59-60.
In the real world,...on a knife heated in a forge and quenched in oil..... a 500F temper would probably give you Rc 57-58.
450F would get about Rc59-60.

Generally most metallurgical bladesmiths advise to stay away from tempering much above 500F. Temper embrittlement is probably an over-rated problem in knives, but avoiding it completely is surely a good idea.

 

If you want the knife to have a blue color, then, as many others have said, don't try to do it with tempering. I have used Birchwood Casey's Perma Blue cold bluing and it turned out really good. Take everyone's advice and skip trying to temper to a certain color.

BTW, that was my thread that was linked above and I can tell you that the temper color came off way too easily for me to ever consider trying to use it as a patina.

 

Thanks Stacey. I guess the concern is the oven spiking in temp to come back up to 500 when it drops below? So that would at times put the blade over 500 degrees in the temper? I try to overcome that with a slotted fire brick holding the blade and a couple of firebricks in the oven to soak up radiant heat. 450 does sound like a safer bet though.

 

So am I reading this correctly or out of context, that it may be a good idea to quench after temper? I usually do all my worrying at the forge and quench bucket, but after tempering, I let my blades sit in the toaster oven until cool enough to touch. I haven't noticed any problems with HT of 1084, but if I can improve it with a simple step, I'm all for it. If this step is advised, should it be water or oil?

 

Stacy said water quench.. it won't shock the steel at such a low temp.

 

I'll throw out AN answer, not sure if it's the answer. I think that lots of axes of old were edge quenched, and with heat still in the body, let the colors run to the cutting edge, and then quenched again to stop everything. So, to recap... after the edge quench, quickly file/grind the cutting edge to clean off the decarb. When the desired color is reached quench again. This leaves the body soft, and the edge hard. That and most of the axes of old had iron bodies and tool steel cutting edges forge welded into them.

I think that hammer poll should be tempered softer then yellow, my guess would be blueish.

Just my take on that.... Many more knowledgeable then me on here.

 

I wouldn't want any any knife edge of mine to get over 450 for any length of time. 1/100 of a second or less. Tempering does have more effect over time, but stuff starts to happen right away and the edge is a very thin section. The reason a hatchet, ax, of hawk is usually tempered to a higher temp is the nature of its cutting with a high striking force means that it would be much more prone to chipping than a knife. The edge of an ax is also at a much wider angle than a knife and therefore it will not roll over at a softer temp. But, ax, knife or anything else will benefit much more from a long soak at the desired temp in a controlled oven than any torch type temper. You are reforming or refining the martensite formed during hardening when you temper and the process is far more complete with time. A second temper is also done because during the first temper the retained austentite from hardening is converted to martensite and it needs to be tempered, which occurs during the second temper. After those steps you can soften the body of the ax or a knife to a softer state with a torch while keeping the edge cool and hard. The smiths of old did not have even kitchen ovens with temp controls and they did not, have the means to achieve a better temper. You do. USE it.

There are lots of people here with far more knowledge than me. Some of them are highly educated in metallurgy. They are very willing to share their knowledge and it behooves you to listen to them. I have and still do. I thank them for the knowledge they have given me. I highly suggest you read the stickys and do some seaching if you want to understand what is going on. Your questions make it obvious that you do not.

There are several types of bluing processes and the temps necessary to achieve bluing with them vary from ambient air temps to over 600f. IF you Want to keep a carbon steel blade at an acceptable hardness you need to use one that works below 450f.

The colors that you achieve heating in an oven vary according to the steel, what contaminants are on the steel and the atmosphere in the oven. Heat coloration is not very durable at all.