Tempering times

[Dmitry5]

I understand how the tempering temperature effects final steel hardness but the tempering time itself - does that really matter? why do we do 2 hors and not one or three for example?

 

[A.McPherson]

I do 2 one hour tempers. Others do 2 two hour tempers. I don't think it REALLY matters all that much with simple steels like the ones I use...

 

[weo]

Hopefully Larrin, Devin or others smarter than me will chime in, but to quote Larrin's book: "... tempering is typically recommended for 1-2 hours, because at that point, the change in hardness with further time is relatively small." p284.

 

[Dmitry5]

Thanks for the replies, sounds like 1-2 hours would work. I have lardons book but have not gotten to page 284 yet.

 

[Stacy E. Apelt - Bladesmith]

Tempering allows changes in the structure to shift and remove the excess brittleness from the quench. This is a factor of temperature and time. Temperature is the factor that is most critical. You want to hold the blade within 15°F of the desired tempering temperature. You use the metallurgical charts and information to decide the tempering target. Larrin's book is an excellent choice for this information.
Time is less critical. You need enough time for the changes to happen. This is a least one hour. Two hours is a length of time that many use, but one hour is sufficient for most steels. Longer will do nothing , but unless you are talking over ten hours, the length of temper has little additional effect after one hour. Contrary to what you often read online, it will not make your blade soft by accidentally tempering longer (within reason). A cooling to room temperature ( approx. 70°F) and a second temper is needed to complete the process. The cooling can be done in running cool water. Letting the blade air cool for 15-30 minutes between the temper cycles won't hurt, but it wastes time. In some complex steels, a third temper is needed, but most knife steels only require two temper cycles.

So, to sum that up:
Temper twice at a target chosen to give the desired final hardness.
Use an oven that keeps reasonable regulation of the temperature. A home kitchen oven will usually work fine.
Temper for between one and two hours for most steels.
Cool to room temperature between the cycles.

 

[Natlek]

I understand how the tempering temperature effects final steel hardness but the tempering time itself - does that really matter? why do we do 2 hors and not one or three for example?

Because first temper cycle make some correction in steel from quenching .And second temper make some correction in steel from first temper cycle .You don t need to know what is going on inside steel , just follow instruction for HT protocol and you are good
If you want to know more , buy this book , i will get mine on New Year
Three cycles of one hour each is also good , maybe best to do that way.....with each temper cycle, more of the retained austenite will convert to martensite

https://knifesteelnerds.com/2020/07/22/new-book-knife-engineering-steel-heat-treating-and-geometry/
PS . you can decrease the temperature of the second tempering for 20 K , it is good thing to do .But in kitchen or toaster oven it is hard to do that .........

 

[Stacy E. Apelt - Bladesmith]

I have seen people suggest lowering the second temper, but there is no realistic advantage to it.
The first temper sets the hardness pretty much. The second only coverts the new martensite, which is a very small percentage. The second temper does not appreciably lower the hardness from what it was after the first unless it is done at a higher temperature. On that subject, I see people suggesting tempers at 350°F and such. A temper below 400°F for most normal steels and normal use knives is not needed. At 400°F the hardness is still in the Rc63-64 area for most simple carbon steel. Raising it to 425°F only lowers the hardness about 1 point. You have to get to 500°F to drop below Rc60.

All this is based on a proper and complete quench. If you are not getting the desired hardness results, the likely culprit is that you did not fully harden the blade.
This requires the right temperature and soak time for the austenitization, the right quenchant type t the right temperature, and the right rate of cooling after the quench. While we all love to see the guys on FIF pull the blade right out of the quench and have it burst into flames ... which they blow on to extinguish ... THIS IS NOTA GOOD QUENCH PROCEDURE!!!
Quench into the proper oil and hold in the oil ( moving up and down) for 8 to 10 seconds before removing. Then let cool at a normal rate hanging in the air until you can pick it up by hand ( a magnet bar is great for this). If using quench plates, place the blade in the plates and leave for at least a couple minutes. Doing a proper quench will give a hardness of Rc 66-65 on most simple carbon steels. Tempering at 400°F will drop that 2 to 3 points.

 

[imill3567]

I have seen people suggest lowering the second temper, but there is no realistic advantage to it.
The first temper sets the hardness pretty much. The second only coverts the new martensite, which is a very small percentage. The second temper does not appreciably lower the hardness from what it was after the first unless it is done at a higher temperature. On that subject, I see people suggesting tempers at 350°F and such. A temper below 400°F for most normal steels and normal use knives is not needed. At 400°F the hardness is still in the Rc63-64 area for most simple carbon steel. Raising it to 425°F only lowers the hardness about 1 point. You have to get to 500°F to drop below Rc60.

All this is based on a proper and complete quench. If you are not getting the desired hardness results, the likely culprit is that you did not fully harden the blade.
This requires the right temperature and soak time for the austenitization, the right quenchant type t the right temperature, and the right rate of cooling after the quench. While we all love to see the guys on FIF pull the blade right out of the quench and have it burst into flames ... which they blow on to extinguish ... THIS IS NOTA GOOD QUENCH PROCEDURE!!!
Quench into the proper oil and hold in the oil ( moving up and down) for 8 to 10 seconds before removing. Then let cool at a normal rate hanging in the air until you can pick it up by hand ( a magnet bar is great for this). If using quench plates, place the blade in the plates and leave for at least a couple minutes. Doing a proper quench will give a hardness of Rc 66-65 on most simple carbon steels. Tempering at 400°F will drop that 2 to 3 points.

I’d think most carbon steels are going to be closer to 60RC at a 400F temper. 26C3 is the only steel I’d be close to 63RC at a 400F temper. I quench for 10s in parks 50 followed by quench plates. So far my hardness results have matched up (+-1pt) with the appendix in knife engineering. For example all the 52100 I just did was at 62RC after a 350F temper. I think lower tempers than 400F have their place especially with kitchen knives.

 

[Bill DeShivs]

While this not a true comparison, I have made literally tens of thousands of switchblade leaf springs.
I torch heat treat, and I use 1095 almost exclusively, but I occasionally use 1084 or 0-1.
Heat to cherry red, evenly- and quench in room temperature canola oil.
I immediately torch temper to just past blue.
I think 4, maybe 5 have broken and they were from the same piece of steel over the last 20 years or so.
It makes me wonder if people overthink the heat treating process for simple steels.

 

[Seedy Lot]

I’d think most carbon steels are going to be closer to 60RC at a 400F temper. 26C3 is the only steel I’d be close to 63RC at a 400F temper. I quench for 10s in parks 50 followed by quench plates. So far my hardness results have matched up (+-1pt) with the appendix in knife engineering. For example all the 52100 I just did was at 62RC after a 350F temper. I think lower tempers than 400F have their place especially with kitchen knives.

Complelty agree with you. All my low alloy steels have matched up with Larrin's work and are softer than what Stacy is implying.

I also temper at lower than 400 degrees Fahrenheit to get the hardness I am looking for, not concerned with loosing a little impact toughness for a large inceres in yeild strength.

 

[Stacy E. Apelt - Bladesmith]

I was working out of memory.
I'll just took a look at Larrin's charts. I was thinking the other simple steels (1095, 1084, W2, Hitachi W/B, 23C3), were around Rc62-63 at 400F like 26C3was. I see that some drop to Rc61 and one closer to Rc60 at 400.

 

[Natlek]

I was working out of memory.
I'll just took a look at Larrin's charts. I was thinking the other simple steels (1095, 1084, W2, Hitachi W/B, 23C3), were around Rc62-63 at 400F like 26C3was. I see that some drop to Rc61 and one closer to Rc60 at 400.

I would take that result with grain of salt not as 100% accurate or as rule .... On other equipment someone will get other result ............

 

[Stacy E. Apelt - Bladesmith]

Deleted ........Merry Christmas Natlek.