Quenching, Tempering And Straightening During A Temper.
Posted 25 February 2012 - 11:15 AM
So my questions are:
Once we get past the nose of the cooling curve, we are on track to form martensite. Why then is the temperature we cooled to important other than being below the nose? Is it because of the clay? I feel like I am missing something here.
Since the martensite is going to form now, why do we need to cool the blade to RT? Why don't we pop it right into a tempering oven? Heck, why even wait for anything below the nose? Couldn't we let it cool in the tempering furnace?
Why the heck does holding the knife under stress in the temper straighten the bend out? Does this always work?
I suspect that some of this has to do with the kinetics of crystallization and that the formation of martensite has a long time constant - so it doesn't form instantly but need a finite time to complete the rearrangement? Isn't it endothermic and shouldn't it pull heat out of the blade when it happens? If it does, does that affect anything?
John Updike - The Dance of Solids
Posted 25 February 2012 - 04:58 PM
Proper marquenching relies on avoiding all phases above martensite. Immediately below pearlite there is a danger of forming upper bainite, which is not good and can form much more quickly than its lower temperature brother so it is rather important to also get below around 650F -700F as soon as possible. Lower bainite takes much more time to form so does not pose as many threats to martensite but the auto-tempering effect does, particularly with the thermal mass of the clay. This means the edge can begin to form martensite that could be over- tempered by the excessive heat bleeding down from the clay coated spine. Marquenching is always best if performed with an interrupt as close to Ms as possible without going under Ms. But the interrupted quench is not true marquenching/martempering which utilizes special mediums that are capable of extracting heat while held at Ms, thus allowing a total hold above Ms for the temperature to equalize throughout the part before proceeding with the air cooling.
Martensite is not like any of the other phase we deal with in the process in that it is not diffusive, but athermal and not time dependent. If we hold the steel at temp for austenite or pearlite for example, even if the temperature is a bit off, time will finish the job for us because these phases are a result of carbon moving through the steel. Martensite formation is the result of trapped carbon so no carbon diffusion is possible, the way martensite forms is via a shearing type deformation within the parent austenite, and cooling is the only driving force that can accomplish this. Ms is “martensite start” but all the other designations down to room temp are %, i.e. M40%, M90% etc… Let’s say Mf is at 400F cooling to this temp will initiate the strain based transformation from austenite to martensite but if we stop the cooling at 375f (M30% perhaps), we can hold it there until hades freezes over and no more martensite will form until cooling resumes since it is not time dependent. So it is very important to continue cooling the steel if we want the maximum martensite, and thus maximum hardness. Reheating steel that has not reached M90% will only stabilize the austenite present and result in less than full hardness.
Pretty much the tempering straightening thing does work real well, it may take a couple of cycles but yep it works. Tempering causes quite a few changes within the steel the steel tends to want to retain the shape it is in when these changes occur.
All of the morphologies of martensite we are dealing with here are body centered in their crystallographic configuration, body centered tetragonal for the alpha martensite (untempered) and bcc for the beta (tempered) so there is simply a relaxing of the stacking structure rather than an entire rearranging of the crystallography. Making austenite requires the massive shift from bcc to face centered cubic and so it is indeed endothermic (decalescence) while making pearlite is the opposite massive shift from fcc to bcc and is exothermic (recalescence), but tempering only involves the slight movement of carbon atoms, that don’t belong there to begin with, in order to allow the stacking to assume its natural bcc.
The class sounds like it was good who was your instructor?
Posted 25 February 2012 - 06:09 PM
Nick Rossi. It was his American Tanto class, so no forging but business had me in the area and my grinding skills suck, so I took it. The hand finishing part with stones was really enlightening.
I actually think I understand that except for why the deformation could be removed in tempering. Why did that work Is that a general trick 'cause it was a damned cool?
In theory i should be able to calorimetricly measure that energy and calculate how much needs to be pulled to figure heat loadings, I guess. Might make a nice experiment for a graduate lab...
John Updike - The Dance of Solids