Difference in the crystal structure after normalizing and annealing?(5160)

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djomla

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Jan 22, 2012
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Can someone explain to me the difference between the following two processes: 1. when you warm up the 5160 steel above the critical temperature and cool it down in the air to room temperature? 2. same thing, but leave the knife in the ashes to slowly cool over night?

I assume that since it is a hypoeutectoid steel in both cases we will get a mixture of ferrite and pearlite, but will in both cases be lamellar perlite or in case of second process be globular shape?
 
In the second case the structure wiil have either coarser pearlite [than case 1] or spheroidized carbides , depending on exactly how fast it's cooled. The added time at higher temperatures lets things move around more !
 
Mete, just so I'm clear about this, the sphereodized carbides that you're referring to are chromium carbides, right? Not cementite, because 5160's a hypoeutectoid steel, right? (So, no excess carbon to form cementite.)
 
Pearlite is a mixture of ferrite and carbide [ cementite],the same holds for sheroidized structure .
Hypoeutectoid [less than about .70 carbon] when annealed will produce a structure of pearlite and typically ferrite in the grain boundaries.
Hypereutectoid [more than about .70 carbon] when annealed will give you pearlite and some extra carbides in the grain boundaries.
Carbide forming elements don't separate usually !! The carbides are then mixtures of iron & chromium [in this case ] or other carbide forming elements.
There are many chromium tool steels because the carbides are easier to machine and easier to HT.The iron to carbon and chromium to carbon are not too strong so these are softer carbides and dissolve easier when HT'ing. So "cementite" is carbide but not a separate carbide .
 
Thanks Mete:thumbup:

One more question. Does the coarse pearlite means lamellar structure only with thicker plates of ferrite?
 
Upper picture: hypoeutectoid steel, lower picture: hypereutectoid steel
Pearlite is structure composed of alternating layers of alpha-ferrite and cementite...

screenshot002yfc.jpg


screenshot003vx.jpg
 
I'm not Mete, but I'll take a stab at this one.

Yes, any steel with more than 0.02% carbon can have cementite in the matrix. That is the solubility limit for carbon in ferrite. Pearlite is a lamellar mixture of cementite and ferrite, so if you have any pearlite, there is cementite. You are correct in that they are talking about the extra (undissolved) carbides you get when heat treating hypereutectoid steels. The normal hardening procedure leaves undissolved carbides for wear resistance in hypereutectoid steels. In hypoeutectoid steels, the carbides dissolve completely by the time you get to the hardening temperature, for normal hardening procedures. After quenching, there are no "extra" carbides present.

djomla - yes the plates of ferrite will be thicker, and so will the plates of cementite. The distinction in made between coarse and fine pearlite, because as the plates/layers get thinner, the strength goes up. Normalizing is also used as a strengthening procedure, though the increase is not as much as quenching.
 
Me2, Thanks for the excellent post. That answers my questions. This is material that everyone needs to know.
 
Cementite is the chemical compound Fe3C.
"Only in hypereutectoid steels" is not correct. There is some confusion here . Perhaps you mean In a heat treated hypereutectoid steel there is martensite + cementite. A Eutectoid steel would have all martensite .
Good answer me2.
BTW the structure we usually see in two dimensions is actually three dimensional . Think of those artificial potato chips in a tube and color every other chip black. That's what you have . As you keep the pearlite at high temperature it wants to go to the lowest energy structure which is spheroidized .
 
Thanks, mete. The textbooks must mean in a heat treated hypereutectiod steel.
 
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