Tempering - Why is 2 better than 1?

[J. Keeton]

I've been wondering this... I'm sure there is a reason that you guys know! I may

When tempering a knife why are two tempering cycles generally recommended instead of one longer tempering cycle?

So for 1084, why not temper for 4 hours at 400F instead of two 2 hours cycles at 400F? Has anyone tried this?

Cheers,
JK

 

[J. Keeton]

EDIT:

I just found this.... I think that's my answer:

*2017

It is explained in detail in the metallurgy stickys, but the Cliff notes info is:

The newly hardened steel formed in the quench is brittle martensite, it also contains some retained austenite.
The first temper tempers the brittle martensite.
Upon cooling to room temperature, it also converts some of the retained austenite to new brittle martensite.
The second temper changes the new brittle martensite to tempered martensite, and further relaxes the martensite that was tempered in the first cycle.
The cooling to room temperature preserves these changes.

The cooling rate should be fairly quick, to avoid any martensite from slipping back into austenite. This really isn't a big deal, but metallurgically is better. In truth, it would not affect then blade any appreciable amount if you let it hang in the air to cool to below 100F for 30 minutes, or ran it under running cold tap water for 30 seconds. What is most important is that the blade get below 100F after any temper cycle.

AND

*2003

As far as tempering, the more complex the steel the more multiple tempering will help.A second or third tempering will help reduce the amount of retained austenite and stabilize any that remains.....Reading the other ongoing thread will explain why multiple quenching is not necessary.

 

[Natlek]

why not temper for 4 hours at 400F instead of two 2 hours cycles at 400F?

Cheers,
JK

It must be very hard work for you to pull blade from oven cool down and put back in oven

Well , you find all answers why ....I m just kidding

 

[J. Keeton]

N Natlek ... lol...

So I don't sound like a crazy person... this is why I was asking. My PID works up to my set temperature very well, however, when I open the toaster door and put in cold steel it overshoots my set temperature on the second ramp:

*Bottom Graph is just Zoomed in
*PID was set to 395
PID Testing

 

[DevinT]

Tempering transforms some retained austenite to fresh martensite. A 2nd or 3rd tempering cycle is needed to temper the newly formed martensite.

Hoss

 

[J. Keeton]

I figured out my PID issue today. (All I have to do is turn it off & on after placing the blade back in the oven; then it won't overshoot)

That being said, I'm happy to now know why 2+ temper cycles are necessary!