Heat treatment question

S

syvmn

Joined
Apr 5, 2009
Messages
169
Hey everyone. I heat treated a piece of 1080 last night and did one hour in the toaster oven at 400F after quench. I'm wondering if I would benefit anything from giving it another hour soak in the toaster oven today before I mess with it more?

(side question...should the toaster oven be ON for an hour, or should the knife be at 400F for an hour?)
 
As to question two - The knife should be at 400F for one to two hours.

To answer question one it will help to understand what the temper cycles does. The steel after quench has had the austenite that formed at 1500F converted into martensite. The martensite is very brittle and needs tempering. You raise the temperature to a point around the Ms point (martensitic start point) and hold it for two hours. One hour will do, but two is the standard. That will temper the martensite, and would be enough if that was all that was happening. However, along with the martensite, there is a small percentage of unconverted austenite ( probably very little with 1080). The austenite gets converted to martensite when it is heated back to the Ms. This new martensite finishes its conversion when the blade cools to room temperature. The new martensite is untempered, and needs a second temper to make it tougher. Tempering is a slow speed conversion ( compared to the conversion at quench)and takes time. Two hours is a practical length of time to use.

So the short answer is that you always should do two tempers, cooling to room temperature between them. Two hours is the length of each cycle.
Stacy
 
I've been wondering for a while why when tempering in an oven, one to two hours is always the recommended time period. Whereas when tempering with a torch (or other heat source) obviously the blade is not held at temperature very long at all, at least the way I understand the process to be.

Am I missing something in the comparison, or is oven tempering vastly superior to the other method due to the soak time?
 
P J 234 said:
I've been wondering for a while why when tempering in an oven, one to two hours is always the recommended time period. Whereas when tempering with a torch (or other heat source) obviously the blade is not held at temperature very long at all, at least the way I understand the process to be.

Am I missing something in the comparison, or is oven tempering vastly superior to the other method due to the soak time?
Click to expand...

Perhaps with a torch I think the molecules move much quicker whereas the oven is a much slower process. I'm sure this is wrong somewhere and either Stacy or Kevin will correct me :D
 
So if the knife is supposed to be at 400F for an hour or two, then how long should it be in a toaster oven? I know this has to depends on blade thickness right?

The knife I'm working on right now is 1/8" x 3/4" or so. I figure I'm going to leave it in the toaster oven set at 400F for 2 hours, hopefully that is long enough to work the process through.
 
A torch draw is properly done after an oven temper.

Drawing a temper with a torch is applying a selective variable temper. It tempers the spine much softer than the edge. In the process the spine becomes a mix of pearlite and martensite, while the edge stays martensite. A torch draw should be repeated several times, sanding the colors off with a piece of 400 grit paper between each temper.

It will not replicate the results attained in an oven over an hour or two, but will work for taking the extreme brittleness out of the brittle martensite blade after quench. It will do nothing to convert retained austenite, which isn't usually a problem except with complex stainless and alloy steels.

If you had no accessibility to even a simple oven, then a torch drawn temper would do fine for most simple carbon steel blades. It is most useful ,however, when used after an oven temper. This way you get a fully tempered blade with a selectively toughened spine and tang.

Stacy

Yes, two hours in a toaster oven will be fine. Putting a reliable thermometer in the oven, or making it a controlled toaster oven will really help. Toaster ovens swing a lot in temperature.
Here is a thread I did on converting a toaster oven:
http://www.bladeforums.com/forums/showthread.php?t=599423&highlight=PID
Stacy
 
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