Question on Time and Temp

[Ed Tipton]

I have frequently read in this forum where it is recommended that multiple tempering cycles be given for optimal effect. My question is there any advantage to giving three one hour tempering cycles at 400 degrees and allowing the blade to cool between cycles, and giving the blade one single three hour cycle at 400 degrees. On the surface, it seems to me that the time and temp are the same. Am I missing something?

[Doug Lester]

Yes, Ed, you are missing something. The repeat cycles trigger retained austinite to convert to untempered martensite and then temper it. That said, some of the simpler steels, read 10XX series, could possibly get by with only one or two cycles as they don't form much retained austinite. Some steel, like 9260 tend to form much more retained austinite and need all three cycles to be sure that the retained austinite is converted to tempered martinsite.

Another thing is that most knife makers can't afford to send a sample blade off to a laboratory to have the microstructure of the steel checked so we tend to rely on overkill. It is known that most of the effects of tempering take place within the first two hours. That's why some makers recommend that the cycles be two hours long and use three cycles on all steels. The extra time and cycles may not help anything but they're not going to hurt either.

Doug

[Ed Caffrey]

I concur with Doug. My viewpoint on tempering is that I feel two times is necessary, and consider the 3rd "insurance". I don't know if I'm lucky or not, but I have access to a person who does spectrographing for me (I'm not sure I'm lucky because it costs me every time I have it done)

Anyway, there is a marked difference in the conversion of retained austinite between 1 and 2 hour tempering times, and also from between 1 to 2 cycles. The conversion is less on the 3rd tempering cycle, but it is measurable....which is why I continue to utilize the 2 hour X 3 times tempering cycles methodology.

[Robert Dark]

I agree Ed. If 2 is good, then 3 just may be a bit better. I am sure there is a point of diminishing return, however, as long as you don't overshoot your tempering temps, what can it hurt?

Sort of like running a vacuum over a carpet........Once gets the big chunks, second time around gets a few smaller pieces, third time gets most all the left-overs, but maybe, just maybe the fourth round makes a bit of a difference.

Robert

[Kevin R. Cashen]

My experience has been that this can most easily be seen in its effects on Rockwell readings. I keep a record and database of every Rc tests performed in my shop over the years and the data has proved useful in areas like this. After the initial quench there will commonly be a variance of up to 1 point HRC when several readings are taken. This will drop to half a point after the second temper and then to almost nothing after the third. It is not that significant considering Rockwell is only good to around + or ? one point, but it does clearly show some sort of homogenizing effect on the hardened state, and the pattern seems to hold true on just about every steel I have tested.

I am currently working on access to x-ray diffraction crystallography equipment at a university to get a bit better idea of what is happening here, as that is really the only way to measure crystallographic changes such as retained austenite. This is what I assume Ed?s friend actually means instead of spectrographing, which can only indicate chemical elements present. I have also been looking into spectrographs for a couple of years now with the dream of owning one myself, but alas the calibration and maintenance, on top of initial cost, keep it out of the range of average guys like us; and in the end I always try to get the chemistry and certs from the mill so I manage to survive without it. But even access to a friend like Ed?s is handy to have should any mystery steel ever come your way.