Tempering Trouble

[Samuel Goodhue]

Hey all,

Relatively new maker here looking for some advice. I heat treated a couple of 1084 kitchen knives yesterday and am seeing some strange colors on the blades after tempering.
In the past I’ve seen all sorts of colors in random places on my blades after tempering, so now I wipe ‘em down with acetone before temper to get off any oils or other surface impurities. These colors seem to be different, as they are only along the edge and follow a gradient from violet to blue... which is concerning. The rest of the blade is a nice straw color, see pics below. My process is as follows.

I hog out the profiles and rough grind in the bevels w/ a 36 grit belt. I leave the edge at .01”-.02”. Most of my grinding is done post HT, so this is all left very rough.
Harden with a MAPP gas torch and 2-brick forge. Quench in canola oil at 120-130 degrees Fahrenheit. Check for hardness with a file. Grind off the heat treat gunk with a knocked down 120 grit belt and wipe clean with acetone.

I then hang the blades from my oven rack for temper. Oven is solidly preheated to 400 degrees Fahrenheit, though my oven runs hot and the secondary thermometer I put in there reads 435 Fahrenheit. Temper twice for 2 hours with a room temp cool in between.

Did I ruin my temper at the edge? If so, how and why?

EDIT: can’t post pics apparently? Did Bladeforums changes this recently?

 

[weo]

I then hang the blades from my oven rack for temper.

My first thought on reading this is to ask, when doing this, how close do the blades get to the heating element? If I'm envisioning this correctly, I'd guess the tips are pretty close to the bottom of the oven, which is the element that heats up unless you are on the broil or grill setting. The element will get significantly hotter than the oven temp (ever notice the orange glow?) and I'd bet that, if what I described is what you are doing, you overheated the tips if not whole blade.

Others may correct me, but I just lay my blades on the rack in the middle of the oven when tempering mine.

 

[Samuel Goodhue]

Ooh, thanks for the reply, that makes a lot of sense. I think I’ll try again with the heat treat using that method.

i’ve always hung my blades, thinking that contact with the oven rack steel would cause them to heat unevenly... though i guess as long as the whole blade eventually comes up to temp, this shouldn’t matter?

The violet/blue color is about 1/2 cm deep on the edge and runs the whole way from tip to heel, which is odd. Also, it’s on both knives.

 

[Joshua Fisher]

I think you are grinding your edges too thin before heat treating, ideally you want to leave it .03-.05 to help prevent any warps and give room to grind out decarb. Also if you know your oven is running hot maybe set the temp lower to like 350-375.

 

[Stacy E. Apelt - Bladesmith]

The colors on an oven tempered blade don't mean anything. You can get orange, yellow, blue, and peacock colors. It is surface oxides from oils and such.

The difference between a 400°F and 435°F temper in 1084 is only one Rc point - Rc62 vs Rc61. I usually use 450°F for my 1084 blades - Rc60

 

[Alex Topfer]

The oxide responsible for the colour will keep forming over time, it just forms faster when hotter. Thicker oxides have different colours. I turned an entire o1 blade blue by doing 3 shimmed tempers at 200c.

A bit off topic, but you mentioned leaving the blades rough. I've found 36 grit scratches can act as stress risers and crack blades, so i do a quick 120 grit grind.

 

[Joshua Fisher]

The oxide responsible for the colour will keep forming over time, it just forms faster when hotter. Thicker oxides have different colours. I turned an entire o1 blade blue by doing 3 shimmed tempers at 200c.

A bit off topic, but you mentioned leaving the blades rough. I've found 36 grit scratches can act as stress risers and crack blades, so i do a quick 120 grit grind.

I’ve heat treated over 300 blades that were profiled with just 36 grit belts and have not had a single blade crack. In my opinion I think it’s better to focus on proper temperature and oil/quench medium.