Tempering - temperature/time Relationship

scott.livesey said:
a reference for quench between tempers http://www.hypefreeblades.com/forum/viewtopic.php?f=3&t=762&p=6557#p6557. on tempering, if you temper at a lower temperature, say 350F, finish the blade and test before putting on handle. if steel chips too much, back in oven at 400F or 425F. you can always temper again at a higher temperature to soften the blade. but once done at 450F, you can't go back and make it harder.
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So the process has been recommended by knife makers but the mechanisms in that thread at least demonstrate some misunderstanding of tempering. Maybe another thread somewhere has a plausible mechanism by which rapid quenching would be beneficial, but it's more likely that slow cooling from low tempering temperatures is equivalent to rapid cooling.
 
Larrin said:
So the process has been recommended by knife makers but the mechanisms in that thread at least demonstrate some misunderstanding of tempering. Maybe another thread somewhere has a plausible mechanism by which rapid quenching would be beneficial, but it's more likely that slow cooling from low tempering temperatures is equivalent to rapid cooling.
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blade needs to get to room temp before next temper. quench in cold water is fastest. no bad effects. will save you 15 to 20 minutes each cycle. at the office, axle end parts were induction heated to final temper, allowed to stand for a minute, then quenched in room temperature cleaning solution. time is money, which justified the cost of induction heaters and quenching system.
 
Larrin said:
So the process has been recommended by knife makers but the mechanisms in that thread at least demonstrate some misunderstanding of tempering. Maybe another thread somewhere has a plausible mechanism by which rapid quenching would be beneficial, but it's more likely that slow cooling from low tempering temperatures is equivalent to rapid cooling.
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I asked a metallurgical guru about this a few years ago, and he told me there are no ill effects from rapid cooling between tempers, and some steels benefit. The benefit may be minimal, or non existent for many steels, but it does save time as Scott says.
 
scott.livesey said:
blade needs to get to room temp before next temper. quench in cold water is fastest. no bad effects. will save you 15 to 20 minutes each cycle. at the office, axle end parts were induction heated to final temper, allowed to stand for a minute, then quenched in room temperature cleaning solution. time is money, which justified the cost of induction heaters and quenching system.
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Induction heating is very time sensitive since typically they are tempered at higher temperature for shorter times. Quenching would make sense in that case.
 
Willie71 said:
I asked a metallurgical guru about this a few years ago, and he told me there are no ill effects from rapid cooling between tempers, and some steels benefit. The benefit may be minimal, or non existent for many steels, but it does save time as Scott says.
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That's the thing, I am not convinced that the steels are benefitting except for possibly the case of high temperature tempering.
 
Larrin said:
That's the thing, I am not convinced that the steels are benefitting except for possibly the case of high temperature tempering.
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That's the sense I got. For steels typically forged, probably no benefit for our use.
 
Larrin said:
Induction heating is very time sensitive since typically they are tempered at higher temperature for shorter times. Quenching would make sense in that case.
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you induction harden to the heat you want 325, 575, 632 dont matta, you set the machine up knowing 632 is full power for 33.25 seconds. steel heats from the inside out. once 632 is reached and power turned off, it cools, whether in air on belt waiting for next operation or quenched. the quenching allows the part to be handled or another operation started once quench is done.
 
Larrin,
You are much more trained and informed than I am, so I always deferr to your oppinions.

What I remenber was a discussion where I brought this up (with Roman landes, IIRC) and the metallurcical reason was that some structure or something can "slip" in the grain boundaries if given enough time in cooling from temper point to room temp. I believe we were discussing the secondary Ms and Mf of the newly formed martensite at the time ( that is why I made the austenite statement, which probably was inacurate).
I also seem to recall Verhooven making some comment on cooling rate. It surely isn't as critical as the cooling rate from Ar to Mf, but there was some very minor reason to do a faster rather than slower cooling.

In the long run, it would take a metallurgical lab to tell the difference, so time, safety, and convenience is the prime reason to do a cool water quench and then stick it back in the oven for the second temper.
 
Stacy E. Apelt - Bladesmith said:
What I remenber was a discussion where I brought this up (with Roman landes, IIRC) and the metallurcical reason was that some structure or something can "slip" in the grain boundaries if given enough time in cooling from temper point to room temp. I believe we were discussing the secondary Ms and Mf of the newly formed martensite at the time ( that is why I made the austenite statement, which probably was inacurate).
I also seem to recall Verhooven making some comment on cooling rate. It surely isn't as critical as the cooling rate from Ar to Mf, but there was some very minor reason to do a faster rather than slower cooling.
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We might need Roman to explain what it is you are referring to.
 
I vaguely remember talking to him on the phone and asking him about this topic among several others, but I have no memory of his answer.
 
I remember comments too but they would be second hand .It had to do with possibility of forming undesired microstructures in the slower cooling. Hardly a problem with low temperatures like 400F !
Nice thing about today's electron microscopes is that you can cut to the chase and actually see what's happening - no guessing ! I assume here that any problems would be alloy sensitive .
 
Spot on. I temper my Cpm154 at 525 twice for 3 hours and I'll consistently get 61-61.5rc.
. It was late one night in the second temper and I forgot about them. Ended up with an eight hour or so temper time and I checked them at 60-60.5rc.


Nathan the Machinist said:
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)[/QUOTE
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mete said:
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 ! :)
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I would like to know why the standard holding time is 2 hours. Is it so the steel all comes up to temp inside and out?
 
Welcome Tristan,
This is a year old thread, but the question is worth an answer.

The structure formed upon quenching is brittle martensite. There is also some retained austenite.
Tempering is a factor of time and temperature.
The brittle martensite is converted into tempered martensite slowly over an hour or two at a temperature close to the Ms ( around 400°F). Upon cooling to room temperature, the retained austenite is converted into brittle martensite. The second temper converts that into tempered martensite.
Both these conversions take at least an hour at temperatures around 400°F. The two hour time was partly because the harts were made for industrial heat treatment of much larger pieces of metal, and partly to assure full conversion.

For most simple carbon steels, one hour each temper is sufficient. High alloy and stainless steel need two hour tempers, and a few need three temper cycles.
 
To add to Stacy - some steels have problems when tempered @ 500 F and some steels were better when tempered above 300 F so 400 F became the industry standard !
Yes the 2 hours was to get a thorough tempering especially large pieces.
Special steels and their tempering get special tempering .Different carbides are formed as in "secondary hardening " at 900 F or cryogenic cooling to produce " eta carbides "
at 300-400 F [don't mix the two ]
 
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