Tempering question

[Tooln]

I've been wanting to ask this for a while now but just never have had the time. I'm talking about tempering blades made from 1095. Currently I temper them at 450 for a 2 hour cycle. Would I be better off tempering 2 cycles at 400?

 

[Alex Topfer]

what sort of knife are you making?
at 450 you're probably getting into tempered martensite embrittlement, so you're losing hardness and toughness. i use 1095 for fine slicing kitchen knives, so would temper around 350f.
there's a lot of info about the metallurgy on knife steel nerds that you might find useful (https://knifesteelnerds.com/)

 

[Tooln]

what sort of knife are you making?
at 450 you're probably getting into tempered martensite embrittlement, so you're losing hardness and toughness. i use 1095 for fine slicing kitchen knives, so would temper around 350f.
there's a lot of info about the metallurgy on knife steel nerds that you might find useful (https://knifesteelnerds.com/)

I'm making hunting knives. 4-5" blades.

 

[Stacy E. Apelt - Bladesmith]

First off, don't worry about TME, that requires much higher tempering.
Second, always do two tempers. It is needed to get a complete conversion from brittle martensite to tempered martensite.
Depending on the use and desired hardness, anywhere in the 400-450°F range is OK.
Do both tempers at the same temperature.
I do hunters in 1095 at 400°F for a final hardness of Rc63/62. 450°F will drop that down to Rc 61/60.

 

[Tooln]

First off, don't worry about TME, that requires much higher tempering.
Second, always do two tempers. It is needed to get a complete conversion from brittle martensite to tempered martensite.
Depending on the use and desired hardness, anywhere in the 400-450°F range is OK.
Do both tempers at the same temperature.
I do hunters in 1095 at 400°F for a final hardness of Rc63/62. 450°F will drop that down to Rc 61/60.

So if I understand right you do 2, 1 hour tempers and allow to cool to ambient temp between the 2 cycles. Making knives is a lot more science than I thought. Thanks for the help.

 

[Diego_B]

So if I understand right you do 2, 1 hour tempers and allow to cool to ambient temp between the 2 cycles. Making knives is a lot more science than I thought. Thanks for the help.

Welcome to Knifemaking.

 

[weo]

So if I understand right you do 2, 1 hour tempers and allow to cool to ambient temp between the 2 cycles.

Yes, but it's OK to cool the blade off under running water between the cycles so that you don't have to wait for it to air cool

 

[Stacy E. Apelt - Bladesmith]

Two tempers of at least one hour each.
It doesn't hurt, and in some steels is necessary to do two hour long tempers. Two hours is what I do on all stainless steels. Simple carbon steels don't have much issue with RA, so one hour long tempers are sufficient.

What happens during the temper is two things:
In the first temper the brittle martensite structure relaxes and becomes a less brittle form called tempered martensite. During this temper, if there is any Retained Austenite (RA), it gets converted to new brittle martensite.
The second temper relaxes this new martensite and finishes converting any remaining RA.
The amount of RA converted in the second temper isn't significant enough to require a third temper, unless the steel is a high alloy stainless steel hardened at the higher temperature rages. Some of those steels need a third temper to relax the new martensite.