Tempering - temperature/time Relationship

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Mar 6, 2017
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So I'm getting ready to heat treat my second knife. It's in 1084 from Aldo. When looking for information regarding tempering, I see quite a bit of variance. Most people I've seen recommend right at 400 degrees F but the amount of time and number of cycles is all over the place. some say 2 cycles of 2 hours each some say 3 cycles at 1 hour each some say just one cycle at 2 hours. so because I am a newbie, I really want to understand what is going on with this tempering process. I understand what tempering does just not the relationship between temperature and number of times and cycles. Is the temperature the only thing that really matters? Is a 1 hour temper at 400 degrees the same as a 4 hour temper at 100 degrees?

Cashen has this chart in the link below but doesn't mention how many cycles to get that RC. He does mention how many cycles; he does at 400 degrees in his other link.
400 degrees F is roughly RC 60-61 and 650 degrees F is about 50RC. Does that mean that only the temperature is what will get me to a set RC or would the number of tempers also get there? If I tempered at 400 for 10 cycles, would that also get me to a lower RC like 50? Even further, lets say I tempered a blade 100 times at the same temperature every time, what would be the affect? Would there be a point where changes stop happening altogether or would each successive temper have an effect?

I hope I'm making sense in what I am asking. I would love to hear the vast knowledge you all have to help better understand. Thanks

http://www.knivesby.com/knifemaking-Kevin-Cashen-treating-1084.html
http://www.cashenblades.com/steel/1084.html
 
time will not play a roll as long as you do the minimum time for temp. I do two 1 hour cycles. going longer won't change the RC. the higher the tempering temp, the lower the resultant RC.
 
Time and number of cycles does effect the hardness, but not nearly as much as temperature. If you temper two times at 400 and put it in a 3rd time, the 3rd temper will drop it again, but only perhaps 1/4 point. It's small enough that for most practical purposes it can be ignored, but it is there.

If you forget and leave it in temper for 6 hours instead of one, that will also drop it, but probably less than a point.

Two one hour tempers or three two hours tempers will yield different hardness, but the difference might only be one point (which is in of itself normal variation from piece to piece and hardness tester to hardness tester)
 
Is a 1 hour temper at 400 degrees the same as a 4 hour temper at 100 degrees?
I'm not sure if you got the answer to this, but these 2 things are NOT even closely related. It's my understanding that you have to get the steel up to at least a 'critical' (my word) temperature to start to effect the hardness (somewhere around 350?) and 100deg will not do anything to take the brittleness out of the steel.
~billyO
 
Thanks for you responses. That definitely helps. It seems clear that it is indeed the temperature that matters most. And no tempering will even begin until a minimum temperature has been reached, regardless of the amount of time. And it must be held at that the desired temperature for a certain (minimum) amount of time to get the desired RC. Any further cycles would be mostly to ensure you did in fact reach that minimum time and to convert a bit of retained austenite. Does that pretty much sum it up? It sure is nice having a bunch of people who know much more than I do. Thanks again
 
" And no tempering will even begin until a minimum temperature has been reached, regardless of the amount of time."

This is part wrong. It is tempering to some degree at any temperature above 300F. The time at temperatures approaching the temper target do affect things. It is the highest temperature that determines the target in a standard 1 or 2 hour tempering, but if you put it in for 6 hours at 390F, and then raised to 400F for an hour, the hardness would be lower than just a 1 hour temper at 400F. Maybe as much as a point.

For most steels being tempered in a good oven, two one hour tempers will do fine. Rapid cool in water between tempers and when done. For stainless, I like two or three two hour tempers. Cyro or sub-zero is done immediately after the quench when the blade is down to room temperature. There is no need to do a second cryo between tempers.
 
For those who passed math -- temperature is a linear scale , time is a logrithmic scale , so temperature is much more important than time !! We metallurgists normally use two hours as a standard . The more complex the steel the more cycles , that is for example " 2+2 at 450 F " meaning two hours at 450 F cool to room temp [cooling in water is a bit better ] then another 2 hours at 450 F , cool in water .
Cryo is done to form eta carbides . The cooling time tweaks the matrix to permit spaces .These spaces are where the carbides form on tempering normally at 300F ! :)
 
Would that be the same for all steels, such as AEB-L. 3 times at 400 for an hour to temper?
Do you quench after air cooling after each 400 degree cycle or just at the end of the 3rd cycle?
 
Would that be the same for all steels, such as AEB-L. 3 times at 400 for an hour to temper?
Do you quench after air cooling after each 400 degree cycle or just at the end of the 3rd cycle?
that would work ok, so would 2 times at 400 for an hour. you can quench in cool water after each cycle, you need the blade to cool to room temperature between cycles. Each steel has its own temperature to hardness chart. here is a link to Sandvik 13C26 which is almost identical to AEB-L, they recommend 2 hours at 400 to give a hardness of Rc58 http://smt.sandvik.com/en/products/...ening-programs/sandvik-13c26-piece-hardening/ if you used 350, you would get Rc59. For 1084 I like tempering at 350 which can yield a hardness of Rc62. many reference books recommend the second temper be 25 degrees cooler on than first temper.
 
It is explained in detail in the metallurgy stickys, but the Cliff notes info is:

The newly hardened steel formed in the quench is brittle martensite, it also contains some retained austenite.
The first temper tempers the brittle martensite.
Upon cooling to room temperature, it also converts some of the retained austenite to new brittle martensite.
The second temper changes the new brittle martensite to tempered martensite, and further relaxes the martensite that was tempered in the first cycle.
The cooling to room temperature preserves these changes.

The cooling rate should be fairly quick, to avoid any martensite from slipping back into austenite. This really isn't a big deal, but metallurgically is better. In truth, it would not affect then blade any appreciable amount if you let it hang in the air to cool to below 100F for 30 minutes, or ran it under running cold tap water for 30 seconds. What is most important is that the blade get below 100F after any temper cycle.
 
Was reading the survive! Instagram feed and they had a post about the delta 3v one with a high heat temper and one with a low heat temper. The low heat temper had even more impressive results for corrosion resistance.

How does that work?
 
The carbide formers will precipitate out during tempering. High alloy steels that have chromium, vanadium, and tungsten can be tempered in the high range for higher toughness. The hardness only falls of a small amount. The steel however has more free iron, so it will be slightly less resistant to corrosion. The upper range for tempering varies a lot with the steel, but is usually between 700F and 1050F.

Basic carbon steels will lose too much hardness in a high temper, so unless a specific toughness is the desired target ( like a spring), the temper is usually kept as low as needed for the desired target hardness. Normal tempering for basic carbon steels and low alloy steels is 350F-550F.

This balance between toughness and hardness is what tempering is all about. The lower range favors hardness over toughness ... and the higher range favors toughness over hardness. Since edge life is the main concern for knives, the lower range is normally the best choice.

However, there are some exceptions to the temperature/hardness curve. Steels with certain alloying do not drop in hardness as the temperature rises during tempering. Up to the maximum hardness, it actually rises from the as-quenched hardness during tempering. Most steels with reasonably large amount of vanadium don't reach full harness until near 700F.
 
We use the Holloman-Jaffe equation for expressing the effect of time and temperature on tempering, Hardness = T(C + log (t)) where T is temperature, t is time and C is a constant. A plot showing what this is calculating is on page 481:
http://mmm.sjtu.edu.cn/userfiles/1/files/2008-96_Canale.pdf

You can see some experiments confirming this effect below:
Page 227:
https://www.researchgate.net/profil...asurements/links/0a85e53a9e5f4a1fa3000000.pdf
Page 805:
https://www.researchgate.net/profil..._Parameter/links/575b73a408aec91374a62677.pdf
 
Was reading the survive! Instagram feed and they had a post about the delta 3v one with a high heat temper and one with a low heat temper. The low heat temper had even more impressive results for corrosion resistance.

How does that work?
The high temperature temper leads to high hardness through the precipitation of very small carbides. One of the carbide types that precipitates is chromium carbide, which takes chromium out of "solution" so that corrosion resistance is reduced.
 
It is explained in detail in the metallurgy stickys, but the Cliff notes info is:

The newly hardened steel formed in the quench is brittle martensite, it also contains some retained austenite.
The first temper tempers the brittle martensite.
Upon cooling to room temperature, it also converts some of the retained austenite to new brittle martensite.
The second temper changes the new brittle martensite to tempered martensite, and further relaxes the martensite that was tempered in the first cycle.
The cooling to room temperature preserves these changes.
That is essentially what happens.
The cooling rate should be fairly quick, to avoid any martensite from slipping back into austenite. This really isn't a big deal, but metallurgically is better. In truth, it would not affect then blade any appreciable amount if you let it hang in the air to cool to below 100F for 30 minutes, or ran it under running cold tap water for 30 seconds. What is most important is that the blade get below 100F after any temper cycle.
I have never heard this explanation before; I don't know what the mechanism would be for martensite "slipping back into austenite." Rapid cooling after tempering in the upper temper range is generally recommended to avoid the "temper embrittlement" temperature range. I haven't seen it recommended generally for low temperature tempering.
 
that would work ok, so would 2 times at 400 for an hour. you can quench in cool water after each cycle, you need the blade to cool to room temperature between cycles. Each steel has its own temperature to hardness chart. here is a link to Sandvik 13C26 which is almost identical to AEB-L, they recommend 2 hours at 400 to give a hardness of Rc58 http://smt.sandvik.com/en/products/...ening-programs/sandvik-13c26-piece-hardening/ if you used 350, you would get Rc59. For 1084 I like tempering at 350 which can yield a hardness of Rc62. many reference books recommend the second temper be 25 degrees cooler on than first temper.

1084 at 350 will still be above Rc63, even Rc64. It'll be chippy at that hardness. I like Rc60/61 for 1084 for general use.
 
Willie is bringing up something I often point out.

The old temper recommendations were often because the early makers were doing seat-of-the-pants HT and judging everything by eye. They determined if it was hard by running a file along the edge and pronouncing, "Yep, she's hard." They probably left a lot of hardness on the table that way. The recommendations of tempering at 300F, 325F, and 350F are generally too low for most all knife use. It worked for them, because they were starting with a less reliable HT. Tempering was also often done by heating the knife over the coals and watching the blade color change. This works for making tools and springs, buy is a poor method for a knife blade. The recommended temperature was often stated using a "color chart", and not by an actual reading. Home ovens were also used for tempering, and those ovens back then ( usually gas) swung wildly in each on-off cycle.

With modern HT ovens, PID control, and more metallurgically experienced makers giving advice today, you can easily get a reliable as-quenched hardness of Rc 65-66. A temper below 400F will barely drop this, and the edge will likely be too hard and tend to chip. Tempers of 425F and 450F are more the norm for general use knives made in carbon steel. Also,we have much better regulation in kitchen ovens today.
 
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