Optimal soak time for 1084

cotdt

cotdt

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Oct 2, 2006
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I do 1500F for 2-3 minutes, but I found something interesting in the Steel Heat Treatment Manual, section on eutectic steel (1080):

"If an austenitizing temperature of 1382F is chosen, the transformation will begin in 10 s, and if a temperature of 1490F is selected, in about 1 s. The transformation of pearlite to austenite and cementite is in this case completed in about 6 s. If the steel is to be fully austenitic (all carbides dissolved), it must be held at this temperature for about 2 h."

Would there be any benefit to soak 1084 for 2 hours to dissolve all the carbides? Or just 6 seconds for smallest grain size?
 
I don't have that reference but you've taken something out of context ! Remember that these things are taken from tests under laboratory conditions and often don't include things that you experience in normal operations. For that steel 2 hours would give unacceptable grain growth !!!
 
Mete, would heat treating in a lab coat mitigate some of this grain growth? :p

I think something's missing from the quote also.
 
If the steel was subjected to a long slow anneal, a bit of extra soak time will allow the carbon (carbides) to dissolve and move back out and even out across the matrix. That isn't going to happen in one second. If you have performed some heat cycles on the steel already (perhaps for grain refinement) and the structure is a fine pearlite it will be good to go very quickly. However, that said, if you're using an oven I'd still let it set 10 min while the oven is at temp so the blade can equalize with the oven.
 
(temperatures rounded for clarity)
While the structure converts to austenite rapidly at 1350 upon heating the steel, there is a small percentage of carbide and ferrite in any steel above the critical temperature. It decreases with time and temperature. At 1500F and 2 hours, the Carbides would have completely converted to austenite. However, we are not making industrial bainite by austempering, where you completely dissolve everything into austenite, quench into 600F salt and hold for 1 to 2 hours to fully convert to bainite.
We actually want some free carbides in our final martensitic blade, to increase edge retention and hardness. The amounts we are talking about with 1080 are small, and as mete said, are more of a lab issue than a knifemakers issue. 5 minutes is plenty of time for our ourposes.

(again,temperatures rounded for clarity)
Upon heating above critical,1080, being eutectoid, will quickly equalize the C and F. Upon cooling from austenite, it will begin to start conversion rapidly at 1350F, and must get from there to below 1000F in about 0.7 second, or it will convert to pearlite. The level of Mn in the alloy can increase the hardenability a bit and allow a tad slower hardening ( which is a good thing), but for simple terms, you have less than one second to get it down below the pearlite nose.This requires a fast oil, or water/brine. The structure (if below 1000 fast enough) will remain austenite until it reaches 425F ,when it will rapidly convert to martensite. Once below 900F but above 400F the austentitic steel is stable, soft, and very plastic. Straightening and such done at this stage is easy and permanent. As long as the steel gets cooled below 200F in less than an hour, it will be fully martensitic ( fully in knifemakers terms), and hard ( Rc 62-65). A one hour temper at 400-450F will leave the blade with good blade qualities ( edge retention, sharpenability, proper hardness).

Stacy
 
bladsmth said:
(temperatures rounded for clarity)
While the structure converts to austenite rapidly at 1350 upon heating the steel, there is a small percentage of carbide and ferrite in any steel above the critical temperature. It decreases with time and temperature. At 1500F and 2 hours, the Carbides would have completely converted to austenite. However, we are not making industrial bainite by austempering, where you completely dissolve everything into austenite, quench into 600F salt and hold for 1 to 2 hours to fully convert to bainite.
We actually want some free carbides in our final martensitic blade, to increase edge retention and hardness. The amounts we are talking about with 1080 are small, and as mete said, are more of a lab issue than a knifemakers issue. 5 minutes is plenty of time for our ourposes.

(again,temperatures rounded for clarity)
Upon heating above critical,1080, being eutectoid, will quickly equalize the C and F. Upon cooling from austenite, it will begin to start conversion rapidly at 1350F, and must get from there to below 1000F in about 0.7 second, or it will convert to pearlite. The level of Mn in the alloy can increase the hardenability a bit and allow a tad slower hardening ( which is a good thing), but for simple terms, you have less than one second to get it down below the pearlite nose.This requires a fast oil, or water/brine. The structure (if below 1000 fast enough) will remain austenite until it reaches 425F ,when it will rapidly convert to martensite. Once below 900F but above 400F the austentitic steel is stable, soft, and very plastic. Straightening and such done at this stage is easy and permanent. As long as the steel gets cooled below 200F in less than an hour, it will be fully martensitic ( fully in knifemakers terms), and hard ( Rc 62-65). A one hour temper at 400-450F will leave the blade with good blade qualities ( edge retention, sharpenability, proper hardness).

Stacy
Click to expand...

That there's good stuff. Thanks Stacy,,, and cotdt for bringing it up this time,,,,
 
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