Weird temper

[Cheese lord]

Hey guys! I'm having some tempering problems. I tempered in my oven at 325, I started this low because its an old oven and I doubt it is very precise. Im having some purple/blue colour at the edge and nice straw colour towards the spine. Do I have to redo the heat treat?

 

[unwisefool]

I'd probably check in the knifemakers section.

 

[catspa]

I agree with uwf, there’s good info on differential tempering there.

The short answer is, you want the edge harder than the spine. You’re describing colors that suggest the spine being harder than the edge. Probly the edge soaked up heat faster because it’s thinner.

There are a number of approaches to differential tempering depending on the equipment you have available.

Parker

 

[davek14]

Your edge is thinner and is absorbing the heat faster.

Purple/blue would mean the heat treat is ruined. How deep and or extensive the color is would be a judgement call. I've had a *little* blue on the edge which sharpened out.

On a home oven, you could try approaching that straw color much more slowly.
Dial the temp back even further and keep the blades in longer. That will tend to get the whole blade to an even temp more readily.

If you decide to redo the heat treat, buff those shiny again and play with them until you get it right before starting over.

Oven thermometers are still not perfect, but they will help you hit the mark better and are cheap.

 

[TC]

Oven thermometers are still not perfect, but they will help you hit the mark better and are cheap.

Thermocouples are cheap to make/buy and much more accurate. I figure everyone already owns a multimeter anyway.

 

[Stacy E. Apelt - Bladesmith]

The colors on a blade in an oven temper can be a rainbow of colors. A good oven thermometer will help with temp control.

I would sharpen the blade and test it. If it holds up, the bade is fine. A brass rod test will tell you a lot. If it chips, the edge is way too hard and needs a higher temper. If it rolls, then the edge may have been over tempered.

I would not normally temper any blade at 325°F. That is too low. Try 375-400°F.

You did not tell us what the steel is, BTW.

 

[weo]

The colors on a blade in an oven temper can be a rainbow of colors.

Colors can come from oils on your fingers when you put the blade in the oven....

 

[Drew Riley]

I've noticed that if I don't clean my quench oil off of the blades, that it can sometime take on a few different shades of colors.

While a thinner edge will absorb heat faster, your oven shouldn't be having a 150 to 200+ degree temperature swing that it would take to get both straw and blue colors. This should be pretty easily verifiable with an independent oven monitoring thermometer. You can find them on Amazon for less than 10 bucks all day. If your oven is in fact swinging that far out of range, I'd just go to your nearest thrift store and get a used toaster oven. The ovens with elements on both top and bottom tend to hold pretty steady temps in my experience. I've seen where quite a few makers have added thermal masses and insulation as well to help even things out. It also wouldn't be too hard to wire in a PID and thermocouple to get better control as well, though this will obviously add to the cost of your setup.

 

[Stacy E. Apelt - Bladesmith]

A really good trick for oven tempering (toaster oven, too) is to put the blade between two pieces of steel plate. 1/16" to 1/8" (1.5-3mm) is plenty thick enough. They shield the blade from direct radiation and even out the heat distribution. They don't need to be huge, a pair of plates 3"X12" (75x300mm) will work for most all knives.
To kick this up a notch, if you have/get a digital meat thermometer you can place the probe between the sheets alongside the blade. The readout will show the exact temperature of your blade. These thermometers cost less than $25.

 

[Natlek]

A really good trick for oven tempering (toaster oven, too) is to put the blade between two pieces of steel plate. 1/16" to 1/8" (1.5-3mm) is plenty thick enough. They shield the blade from direct radiation and even out the heat distribution. They don't need to be huge, a pair of plates 3"X12" (75x300mm) will work for most all knives.
To kick this up a notch, if you have/get a digital meat thermometer you can place the probe between the sheets alongside the blade. The readout will show the exact temperature of your blade. These thermometers cost less than $25.

A really , really good trick would be to build one . It will probably cost less than a new toaster oven .You have no idea how accurate it is and how much energy it saves.......

Building tempering Oven

Yes, the muffle is what protects the blades from direct exposure to the hot elements. The metal would replace what you had as glass.

bladeforums.com

 

[Natlek]

Colors can come from oils on your fingers when you put the blade in the oven....

It's just a myth I've heard many times here Of course, to know that I tried it many times , i mean on oils from my fingers .
Some tempering color i get on clean steel depend of temperature

 

[weo]

It's just a myth I've heard many times here

Ummm...how do you explain purple/blue/straw colors that are in the shape of a fingerprint then?

Of course, to know that I tried it many times , i mean on oils from my fingers

Still a very small sample size of 1 there. Perhaps it's your diet? Maybe I eat more garlic than you. Maybe you eat more garlic than me....

 

[Natlek]

Ummm...how do you explain purple/blue/straw colors that are in the shape of a fingerprint then?

Still a very small sample size of 1 there. Perhaps it's your diet? Maybe I eat more garlic than you. Maybe you eat more garlic than me....

I make on purpose fingerprint on blade , I mean i can see them before tempering but not after . Maybe there is something under fingerprint which then make them visible after tempering ? I have no idea , but look at that even color I get on steel EVERYTIME I temper steel ? That should mean something ? Or not ?

 

[weo]

That should mean something ? Or not ?

I'd say, or not.
While this may very well be true for you, that doesn't mean it's true for everybody.

Perhaps it's the steel, perhaps it's the quench medium used, perhaps it's individual body chemistry.

 

[Stacy E. Apelt - Bladesmith]

Lets just leave it at:
There are many colors that can form on the surface of a blade slowly heated for an hour or longer.