Zack,

I'm certainly no Kevin Cashen but I will attempt a simplified answer. When the steel is heated to critical & beyond the carbon atoms go into solution (or start moving) and when quenched they become trapped in the matrix of iron atoms. Since carbon atoms are much larger they expand the atomic bonds of the iron and form martensite. After tempering you have tempered martensite as you have released part of the carbon atoms from inside the iron matrix forming a softer state of martensite (tempered).

I'm sure that Kevin can explain it better but that is how I understand it.

Gary

Good answer Gary! I always like to see as many folks that can jump into these conversations even when I am paged, life is so boring if I do all the talking- just ask my family! But essentially you nailed it, all I can do is give a few details.

Iron at room temperature is stacked in a body centered cubic configuration, this staking only allows a limited amount of carbon in between the iron atoms and so only so much can physically be in solution. When we harden the steel we heat it to a transformation temperature where the iron atoms rearrange themselves into another stacking that has a face centered cubic configuration. This stacking has many more spaces for carbon atoms to fit between the iron atoms so it can hold very much more carbon in solution. We then quench the steel so quickly that the carbon atoms cannot get out of their spaces fast enough to make pearlite at around 1000F.

At this point the carbon atoms are trapped in a stacking that really must return to a body centered state but can no longer do it by the simple carbon movement that is possible at higher temperatures. This all builds up to tremendous strain inside the steel that will reach a critical point at a certain temperature below around 550F that is determined by the amount of carbon that was in solution still holding things up, the more carbon the lower the temperature.

When that critical temperature is reached the whole stacking heavily distorts into a body centered tetragonal state that is very rigid and stressed, that phase is martensite. But for our conversation let’s call it “alpha” martensite, as it is the initial form that is so strained that it is ready to shatter like glass if stressed further, or perhaps even crack on its own if not relaxed a bit.

Tempering raises the temperature just enough to allow some carbon atoms to slip out of their trapped positions and form tempering carbides that are even too small to be seen with a traditional microscope. When these atoms slip out of the spaces it allows a relaxation of the body centered tetragonal stacking to the more stable and natural body centered cubic phase that we can call “beta” martensite. It is still martensite but it is much tougher and with the added benefit of countless little carbides. Interestingly enough, while we cannot see tempering carbides we can see the overall effect of countless numbers of them within the martensite as they turn the tempered martensite darker than its raw “alpha” martensite phase.

Many folks confuse how and when pearlite can form and it is not at these lower temperatures. Pearlite is the product of the carbon rapidly diffusing out of the face centered phase (austenite) to gather up into sheets called lamellae that alternate with similar sheets of iron. These lamellar structures require the diffusion rates possible at around 1000F.

Thank you Kevin. That makes things clear.

Brion

Great question and thanks for the information. Could someone explain a bit more about the time aspect? I've heard several times over that changes in temper have a great deal to do with time, as well as temperature. If tempering temps (say, 350-450) aren't high enough for the change to pearlite, then it makes sense to me that no amount of time at those temps will get the steel to pearlite-is that correct? If so, do the longer time periods just allow more of that carbon to migrate out of their trapped positions, thus lowering the Rc? And if that's correct, just how low will the hardness continue to go with several hours in the oven? Will the blade ever reach the same softness as is achieved from annealing-both soft, just not the same "form" (beta martensite vs pearlite)?

And one more thing-is it correct that tempering doesn't occur below 350 F? Is there a specific temp (and below) that is applicable for most carbon steels where there will be no tempering occurring, regardless of the time spent at that temp? Hope that makes some kind of sense...

Jeremy

please some one else chime in if I am wrong on this but if I remember right tempering range starts at around 200 deg and ends at around 1100degs after this temp is reached, you begin the subcritical annealing range. I the upper rage you start forming a spheroid structure rather than tempered martinsite. as far as time vs temp I understand that to be a soaking temper (of say 1 hour) at 350deg would give the same hardness as a flash temper (heated with a propane torch) some where in the 450-500 range.

|quoted:

Great question and thanks for the information. Could someone explain a bit more about the time aspect? I've heard several times over that changes in temper have a great deal to do with time, as well as temperature. If tempering temps (say, 350-450) aren't high enough for the change to pearlite, then it makes sense to me that no amount of time at those temps will get the steel to pearlite-is that correct?

If so, do the longer time periods just allow more of that carbon to migrate out of their trapped positions, thus lowering the Rc? And if that's correct, just how low will the hardness continue to go with several hours in the oven? Will the blade ever reach the same softness as is achieved from annealing-both soft, just not the same "form" (beta martensite vs pearlite)?

And one more thing-is it correct that tempering doesn't occur below 350 F? Is there a specific temp (and below) that is applicable for most carbon steels where there will be no tempering occurring, regardless of the time spent at that temp? Hope that makes some kind of sense...

Jeremy

All metallurgy texts will point out that in diffusion based transformations time equals temperature, but what they don't say is how much time equals how much temperature. I say time equals temperature but all things being equal temperature will trump time every time. The reason things like I-T diagrams are layed out in a logarithmic time scale is because relatively minor changes in temperature will effect time exponentially. At 1000F carbon moves through iron in fractions of a second, at 300F it takes hours. When it comes to diffusive based reaction/transformations one can get caught in a trap of compartmentalizing all of these things as entirely exclusive when really you can look at everything from the very first phases of tempering all the way up to normalizing as increasing levels of the same diffusion accelerated by increasing temperatures. This is not the same with something like martensite formation which being a diffusionless transformation is unequivocally a different beast altogether.

Diffusion can even occur at room temperature, but it is so slow that we can barely measure it. To put this in perspective some have studied phases in ancient steel artifacts to see if there have been diffusional changes over thousands of years. At around 250F the iron and carbon atoms get just enough energy to sluggishly crawl a bit; or more like roll over in their sleep. It is around this level that the carbon atoms move just enough to allow the relaxation from body centered tetragonal to BCC to begin, but there will be no significant changes in hardness. Once we get in the 350F range the carbon atoms are very slowly squeezing between iron and combing in in incredibly small groups to form tempering carbides, this is enough movement to relieve the interstitial distortion effect on the iron lattice so that there is a significant drop in hardness which will continue with every increase in temperature afterwards.

The full conversion from alpha martensite to beta martensite has perhaps the most profound effect on hardness for a given temperature, as confirmed with all of my measurements over the years, e.g., a piece of steel that is 65 HRC as quenched will rapidly drop to 62.5 to 63 HRC once you reach the 300F-350F tempering range, a rapid drop of at least 2 points Rockwell, but from 375F on up you will generally lose only about a point for every 25F.

From 375F to 430F you will have continually increasing amounts of tempering carbide formation. But you are still looking at minutes to hours for the movement, versus anything that could be measured in seconds. As you continue on up to the 700F to 900F degree range the tempering carbides will grow enough to become visible under the microscope and when you pass 1000F the diffusion is sufficient enough for the carbides to begin to become spheroidal cementite. From here the iron matrix will become about as soft as you can make steel below the austenite phase; yes even softer than pearlite. Once you reach the 1350F range the steady increase in diffusion rates suddenly goes off the scale when the iron atoms shift to face centered cubic and the carbon is free to move at will. At this point the atoms are so excited and the paths so open for travel that the carbon no longer balls up into carbides but spreads out and fills the iron matrix to create austenite. This state certainly takes diffusion to minutes or even seconds depending on how much heat you throw at it.

In all of this increasing temperature, nowhere did you see pearlite, because it isn't there. Even in its preferred range around 1000F, pearlite is an exothermic reaction that occurs on cooling, not heating. Pearlite is what diffusion does to austenite when it is destabilized by cooling. It will start with a point of nucleation at the austenite grain boundary; the excess carbon in solution will form a seed of iron carbide that will cascade out into the austenite grain pulling carbon out of solution to either side of it as it goes. This creates a band, or lamellae, of cementite (iron-carbide) with a band of carbon lean iron to either side. These iron based lamellae will in turn trigger bands of carbide, and on and on until the entire austenite grain is replaced with pearlite.

So to oversimplify- heating makes carbide spheres until austenite wipes the slate clean, cooling makes carbide sheets until diffusion is arrested, and then things have to find new ways to transform, e.g. martensite.

wow Kevin you just blew my mind....

That is one of the best most easily understood explanations of what is happening I have ever read, thank you! I knew what was happening in the steel in sort of a rough sense, but this go's a long way to helping me understand why these things are happening.

You mentioned that you had "have studied phases in ancient steel artifacts" did you find any evidence that over very long periods on time steel will revert to a softer state with in a time scale of say 500-1000 years. I have wondered if the little testing that has been done on very old blades has been affected by this. I know much of structure and hardness testing I have seen has been affected by a number of other factors, for example many of the pieces that were conserved in the past may have been heated to the point of destroying much of the original structure add to that blades that had been ritually "killed" with fire in period, or items that had survived a fire at some point in it's life. The understanding that swords and knives were not heat treated well or inconsistently heat treated makes little sense to me when you look at the design of the blades. The thin cutting swords of the mid Viking era for example would never have been able to function well if not heat treated, on the other hand some of the small to mid size seax's appear to be plain iron but are designed in such a way that they would function well with out heat treating (hammer hardened edges, thick spine etc)

Is it possible that this affect is messing with our understanding?

MP

Kevin,

Do the carbides formed during the tempering process effect the performance of the tempered martensite to any appreciable level? I'm sure that the amount of carbon is a factor so let's use 1084 as an example.

Gary

A lot of very useful information here, tempering is one of the things that I find most tricky, and this in depth explanation has increased my confidence with it. Thank you all very much.

Kevin - thank you again for sharing your clear descriptions of metallurgy - I've bookmarked this thread!