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Time between Hardening and tempering

Posted by Eee 
Eee
Time between Hardening and tempering
February 28, 2016 05:59AM
What effect does time between hardening and tempering (or retempering) have?

I was just reflecting on two things;
- recent forum posts elsewhere about re-tempering blades which had come back from HT harder than expected &
- file knives where the tempering can be done years after the original hardening with a view to increasing toughness. To be clear some don't anneal, grind and re-treat, they just re-temper and grind.

I'm assuming neither is as simple as it sounds?

Thanks
Re: Time between Hardening and tempering
February 28, 2016 10:13AM
Eee,

To clarify something, tempering essentially is defined as holding the blade at a temperature below the critical temperature (where it transforms to austenite). If you quenched the blade after soaking and then did nothing at all well then you basically tempered at room temperature.

The effect of tempering is dependent on the temperature the blade is held :

- < 250°C, precipitation of transition carbide; loss of tetragonality in martensite
- between 200 and 300°C, decomposition of retained austenite
- between 200 and 350°C, replacement of transition carbide by cementite; martensite loses tetragonality
- >350°C, cementite spheroidizes; recrystallization of ferrite
- > 450 C, secondary hardening (precipitation of alloy carbide, Cr/Mo)

Now some of these effects are not at all effected by the time between the exposure to temperature. For example if you left the blade sit at room temperature for a month and then tempered at 350F you would still get rapid softening as the martensite would turn into ferrite and the cementite would rapidly coarsen.

But some of those effects are strongly effected by the time between exposure. For example if you did the same thing but tempered at 300 C then you would get little to no decomposition of the austenite because it becomes very resistant to changing as time passes. If you want to do this then you have to do it right away or it basically won't happen.

The other thing to note is that you can only go ahead, you can't reverse. If for example you have a knife tempered at 450C then you can't just temper it at 250C and get a blade which behaves as if it was tempered like that. But the reverse is true to some extent. This is why makers will often temper up in ramping like try 250C then 275C then 300C, testing the knife each time.
Eee
Re: Time between Hardening and tempering
February 28, 2016 03:02PM
Thanks Cliff,
So in the case of the file knives, I'd assume a temper somewhere between 260 & 300 degrees C (assuming W1 or W2 aiming for 57/58 RockwellC) so that would imply limited decomposition of RA. However, the file would already have been tempered to a higher hardness when originally produced so would RA be an issue?
Re: Time between Hardening and tempering
February 28, 2016 07:58PM
Eee,

Generally, files should have very little RA, and will have a minimal temperature tempering to maintain a high hardness. The tempering by makers would generally just soften/weaken the steel .
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