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- Sep 9, 2003
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DaQo'tah Forge said:
Sorry for the alpha iron thing, I get carried away. Most folks know that steel expands as it is heated (this is simple thermal expansion), but what some may not be aware of is that it contracts at the point of recrystalization. This is because of the way the atoms are stacked at room temperature versus higher temperatures. All metals have a specific arrangement to the way the atoms are stacked that contributes to its characteristics. We refer to this stacking by labels given to its unit cell or the smallest part of the repeating pattern. In iron, at room temperature, the unit cell is body centered cubic (BCC)like this:
This is called alpha iron.
When we heat the alpha iron to it critical temperature the atoms make a shift in their configuration to face centered cubic (FCC) like this:
This is called Gamma iron. This shift opens up new spaces for carbon atoms to occupy allowing gamma iron to hold much more carbon in solution. Carbon in a solution of gamma iron is what we call austenite.
Back to my original point, Gamma iron (FCC) also happens to be a more efficient stacking so it wastes less space and the steel will contract. Well the exact opposite happens when things are cooled. Austenite (FCC) is not stable at lower temperatures so when it is cooled it will decompose into a more stable structure (pearlite, martensite or bainite) that is BCC. Body centered cubic is less efficient in its stacking so it takes up more space and whenever this decomposition occurs there will be expansion. Pearlite is what is formed if austenite is allowed to decompose at temperatures above 900F to 1000F (this varies greatly, depending upon the steel). Bainite is what will form if the austenite is held between 450F and 900F (again this is dependant upon the steel). If one can cool things fast enough to halt the decomposition until below 450F. martensite will form. Martensite has a different makeup than other structures because you have forced FCC to hang around for a very unnatural amount of time. When the FCC structure finally gives up it has carbon trapped in it preventing the shift to BCC so martensite forms a new highly distorted unit cell known as body centered tetragonal. This is what leads to all the STRAIN and stored energy within hardened steel. It is what causes Japanese swords to curve in the quench, causes brittleness (cracks like stored energy) and other distortion.
So pearlite (air cooling 1084) will create fresh points of nucleation for the next heat, but martensite wins the prize for creating such points.
Another thing to remember, and it is very important for this thread, is that carbon going into solution is a diffusion based process, so the more bunched up and separated the carbon (cementite) is from iron (ferrite) the longer it will take to pull it into solution. In a speroidized condition the carbon is balled up in larger spheroidal cementite nodules. To address that was also mentioned earlier excessive spheroidal type treatments will cause the spheres to be even larger and pull more carbon out of solution and it will take that much longer to get it back into solution. Spheroidal carbides take some of the longest soak times to get proper solution. Coarse pearlite will take a little less time but still longer than fine pearlite (like what you get when you normalize). Making austenite from bainite or martensite takes no time at all because the carbon is already very finely dispersed.
Do you see how what I have outlined here can figure into this whole triple quenching thing?
. Then just go with the heat treat recommended for the given pre-heat treat.
I actually had a picture of a roadkill in mind as well when it wrote that. For some reason decomposition sounds so much more techinical than decompose :barf: Besides this wouldn't be the first time this thread smelled a little fishy 
)and then lock that carbon into a body centered tetragonal. And I will be doing it most of the day, if it still didn't all fascinate me it could be a boring afternoon- but is heat treating ever boring?
and why?