study note on Manufacturing property of engineering material
Tempering marten site produces pure ferrite? HUH? Or does it mean tempered marten site is pure ferrite? The marten site already is a Body centered structure (albeit tetragonal) what the hell do they mean here? Where do the carbon atoms go? Boil off to Al Gore's greenhouse gas love shack so they can hook up with Oxygen?? So does tempering reduce the wt % of carbon in the steel? Even in the center of mass? If not how does the center relax if the carbon atoms are still there?
Tempering is a low temperature thermal treatment, always below the lower critical point of steel that is employed to reduce the stress in a steel material. By this definition, tempering cannot result in "recrystallization to ferrite" as the WIKI article seems to state.
By heating steel below the lower critical point A1, Tempering relieves stresses that were induced by quenching, cooling, and reduces hardness and brittleness, without allowing the material to recrystallize.
In quench and tempered material, the untendered marten site is very brittle because the transformation from Austenite (Face Centered Cubic- Close Packed) to marten site creates a Body Centered Tetragonal structure which is not close packed. The creation of marten site expands the structure, since the unit volume per atom of iron in FCC is less than that in BCT.
Because the marten site phase expands on quenching oriented randomly in the prior austenite, it puts all material around it in compression, adding stress to the lattice. This stress inherent in the metal makes it unusable brittle. Adding heat to steel results in three things in this order:
The added energy provided by the temper allows the material to relax by allowing some of the marten site’s volumetric stresses to slip or relax; this reduces the volume of the marten site as it is tempered. This results in less brittleness and so greater toughness (TOUGHNESS which for now I will call the area under the stress strain curve.) Note, no carbon is lost by this process, unlike the WIKI claim.
Once the material has been heated been heated such that all stresses have been relaxed, the material can then recrystallize. If held at a high enough temperature for a long enough time, those new grains will grow, coalescing into their neighbors.
Normalizing of steels was more prevalent in the days of ingot casting, when there was greater segregation of and less control of as cast structures than on today’s modern continuous casters. By heating some temperature above A3, (usually 50 degrees F above that point) and then letting the material slow cool in still air, the effects of this micro structural segregation could be minimized. But because these steels had high harden ability, it was possible to have a small amount of hardness even following this normalizing procedure. Thus the temper to assure that the material was stress free as possible.