Stacy E. Apelt - Bladesmith
ilmarinen - MODERATOR
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Knifemaker / Craftsman / Service Provider
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- Aug 20, 2004
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In answering a request for info on a delayed temper, and some other requests, I though some here might appreciate the same info ( it comes up often). Here is an excerpt of the reply.
On a question about delaying temper overnight:
No, you didn't damage the blades by waiting to temper the next day.
Why temper should be done soon is different for carbon and stainless steels. In carbon steel, the blades can crack from the internal stress if left too long. In stainless, the RA can become stabilized with time, and won't convert to martensite if too much time has elapsed. Since you didn't do a sub-zero treatment ( putting the blades in the freezer does absolutely nothing ), your stainless blades will be about the same with a delay as without. The amount of RA is small anyway. More later on this.
On a question about getting blades into temper the soonest, and kitchen ovens:
Many makers use the kitchen oven for the first temper ( or all tempering).
Here is how to get the max from your Evenheat and also get the least delay into temper.
Set the Evenheat to drop to 375F (eg) and hold for four hours after the austenitization step. Set your kitchen oven to 350F and have it pre-heated about 20 minutes before the quench. Quench the blade (do any sub-zero treatments if applicable) and gently clean it off by washing with dish soap and water to remove the oil. Put it in the 350F oven for two hours, then rinse with running water to cool off. Slow cooling in air is actually not as good as quick cooling in water. By then, the Evenheat oven should be at 375F. Put the blade in for another 2 hours, cool off in water, and repeat for a third 375F temper if desired. If the Evenheat has not dropped enough, you can wait as long as needed, or until the next day, since the first temper is done.
The kitchen oven temper will get the blade's stress taken care of, and any worry about temperature fluctuations in the home oven are not a worry at the lower temp. Some folks use 325F , just to be extra cautious if their oven has a lot of temperature fluctuations.
My kitchen oven is semi-commercial gas fired unit, and keeps very good regulation ( I stick a digital monitor in it when tempering to make sure), so I usually just do both tempers at 375F in it. On high alloy blades with very strict temper regimens, I sometimes use two HT ovens. One to austenitize, and one to temper. That is not normally necessary.
About the sub-zero treatments.
In complex steels, like stainless and some high alloy steels, the austinization temperature is up around 1900F to allow all the large atoms of alloy material to form their carbides. In a simple carbon blade, the much smaller carbon atoms move around in the ferrite ( iron) and make cementite ( iron carbide) at 1500F or less. Because there are a limited number of carbon atoms ( less than 1%), and the rest is mainly iron ( 98+%), the austenite mix is very simple.
In the carbon steel blade, the austenite, if cooled quickly enough to pass the pearlite nose, will convert to martensite at around 400F. This ends at around 200F, and all conversion is done before it reaches room temperature.
Putting a carbon steel blade in the freezer, on dry ice, in LN, or between your gal's boobs ( highly recommended), will not change the situation ( but the boobs thing can lead to other fun activities if your gal is willing
) .
However, in the more complex stainless steel alloys, the carbide formers usually make up 15-25% of the mix. That requires a higher temperatures to form the carbides. Because of these larger carbides things change slower, and the austenite will have much more time to cool down as it forms martensite. Forming pearlite is not really possible under normal cooling rates. The temperature drops in air, between aluminum quench plates, or sometimes in oil, to around 400F where the conversion to martensite starts. The difference is that the austenite has only partially converted at 200F, and still may have up to 10%
at room temperature. This retained austenite ( RA) will not finish converting to martensite until it reaches about -90F. That can be easily done with dry ice and some sort of liquid to transfer the heat out of the blade ( I use alcohol, some use acetone, others use kerosene).
That is not all that is happening in these complex steels, though. The carbides also are changing at these super low temps. Something called eta-carbides form at -350F, which is reached by placing the blades in liquid nitrogen ( LN).
The sub-zero ( -100F) treatment will add a little hardness to the blade over a room temperature finished blade, because it converts all the austenite into martensite. The cryo (-350F or lower) treated blade will add even more hardness and increase the wear resistance, due to the harder and different structure of the carbides.
A blade done without these low temperature treatments is still a good blade. The ones done with it gain a little in performance and edge durability. The big plus for some steels is the final hardness. Take CPM-S35VN. It will reach about Rc 59-60 under a room temperature quench, and be Rc 57-58 after temper. That is going to be fine for a kitchen knife, but the edge will need re-sharpening after a bit of use. With the cryo treatment, the final hardness after temper can be Rc 59-60, which is going to last a lot longer between touch ups. Other steels can quenched with cryo, and be taken to Rc62-63 after temper.....and still not be chippy.
On a question about delaying temper overnight:
No, you didn't damage the blades by waiting to temper the next day.
Why temper should be done soon is different for carbon and stainless steels. In carbon steel, the blades can crack from the internal stress if left too long. In stainless, the RA can become stabilized with time, and won't convert to martensite if too much time has elapsed. Since you didn't do a sub-zero treatment ( putting the blades in the freezer does absolutely nothing ), your stainless blades will be about the same with a delay as without. The amount of RA is small anyway. More later on this.
On a question about getting blades into temper the soonest, and kitchen ovens:
Many makers use the kitchen oven for the first temper ( or all tempering).
Here is how to get the max from your Evenheat and also get the least delay into temper.
Set the Evenheat to drop to 375F (eg) and hold for four hours after the austenitization step. Set your kitchen oven to 350F and have it pre-heated about 20 minutes before the quench. Quench the blade (do any sub-zero treatments if applicable) and gently clean it off by washing with dish soap and water to remove the oil. Put it in the 350F oven for two hours, then rinse with running water to cool off. Slow cooling in air is actually not as good as quick cooling in water. By then, the Evenheat oven should be at 375F. Put the blade in for another 2 hours, cool off in water, and repeat for a third 375F temper if desired. If the Evenheat has not dropped enough, you can wait as long as needed, or until the next day, since the first temper is done.
The kitchen oven temper will get the blade's stress taken care of, and any worry about temperature fluctuations in the home oven are not a worry at the lower temp. Some folks use 325F , just to be extra cautious if their oven has a lot of temperature fluctuations.
My kitchen oven is semi-commercial gas fired unit, and keeps very good regulation ( I stick a digital monitor in it when tempering to make sure), so I usually just do both tempers at 375F in it. On high alloy blades with very strict temper regimens, I sometimes use two HT ovens. One to austenitize, and one to temper. That is not normally necessary.
About the sub-zero treatments.
In complex steels, like stainless and some high alloy steels, the austinization temperature is up around 1900F to allow all the large atoms of alloy material to form their carbides. In a simple carbon blade, the much smaller carbon atoms move around in the ferrite ( iron) and make cementite ( iron carbide) at 1500F or less. Because there are a limited number of carbon atoms ( less than 1%), and the rest is mainly iron ( 98+%), the austenite mix is very simple.
In the carbon steel blade, the austenite, if cooled quickly enough to pass the pearlite nose, will convert to martensite at around 400F. This ends at around 200F, and all conversion is done before it reaches room temperature.
Putting a carbon steel blade in the freezer, on dry ice, in LN, or between your gal's boobs ( highly recommended), will not change the situation ( but the boobs thing can lead to other fun activities if your gal is willing
) .However, in the more complex stainless steel alloys, the carbide formers usually make up 15-25% of the mix. That requires a higher temperatures to form the carbides. Because of these larger carbides things change slower, and the austenite will have much more time to cool down as it forms martensite. Forming pearlite is not really possible under normal cooling rates. The temperature drops in air, between aluminum quench plates, or sometimes in oil, to around 400F where the conversion to martensite starts. The difference is that the austenite has only partially converted at 200F, and still may have up to 10%
at room temperature. This retained austenite ( RA) will not finish converting to martensite until it reaches about -90F. That can be easily done with dry ice and some sort of liquid to transfer the heat out of the blade ( I use alcohol, some use acetone, others use kerosene).
That is not all that is happening in these complex steels, though. The carbides also are changing at these super low temps. Something called eta-carbides form at -350F, which is reached by placing the blades in liquid nitrogen ( LN).
The sub-zero ( -100F) treatment will add a little hardness to the blade over a room temperature finished blade, because it converts all the austenite into martensite. The cryo (-350F or lower) treated blade will add even more hardness and increase the wear resistance, due to the harder and different structure of the carbides.
A blade done without these low temperature treatments is still a good blade. The ones done with it gain a little in performance and edge durability. The big plus for some steels is the final hardness. Take CPM-S35VN. It will reach about Rc 59-60 under a room temperature quench, and be Rc 57-58 after temper. That is going to be fine for a kitchen knife, but the edge will need re-sharpening after a bit of use. With the cryo treatment, the final hardness after temper can be Rc 59-60, which is going to last a lot longer between touch ups. Other steels can quenched with cryo, and be taken to Rc62-63 after temper.....and still not be chippy.
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