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This question is not easily answered since it depends on a myriad of factors. In general, crystal growth for small crystals does not take all that long (several hours under laboratory conditions for crystals 0.2 mm in size).First a little background: In order for crystal growth to occur, cooling must cross a critical temperature limit, called the “Liquidus”. For temperatures above the “Liquidus” limit, all is MELT and no crystals are present. Once dropping the temperature below the “Liquidus” limit, crystals start to form, but they are mixed with the melt. Once temperatures drop low enough, another critical point is reached, the so called “Solidus” limit. If the magma is cooled below the “Solidus”, NO melt exists anymore, and the magma has turned into a complete solid, a rock.It is this area between the “Liquidus” limit and the “Solidus” limit where crystal growth occurs. The rate of crystal growth often depends on how fast this area is crossed and if temperatures stay steady for a while during the phase change from a liquid to a solid.In addition, new studies show that the presence of a flux material within the melt, such as Cesium or Fluorine, can dramatically accelerate crystal growth and may cause very large tourmalines to from within a short time.In general, some crystals may grow out of a melt in less than a year, while others may take a decade or two or even centuries.

In searching through my physical metallurgy textbooks (Reed-Hill, Glicksman, and Porter & Easterling) I find no mention of the term “infinite mixing.” Nonetheless, from the context of this phrase one can infer it likely refers to the simplifying assumption of complete solute diffusion in the liquid phase of an alloy in the process of solidifying.To illustrate this, below I’ve sketched a eutectic phase diagram of an alloy with composition [math]C_0[/math]. I’ve also marked the liquid phase as L, solid phases [math]\alpha[/math] and [math]\beta[/math], and the liquidus and solidus curves.Now, what happens when this alloy in the liquid state starts freezing? If you take the above phase diagram at face value, then it’d have you believe that as the mixture cools, the composition of the evolving solid follows the solidus line right up until all the liquid has solidified. However, the above figure represents an equilibrium phase diagram, which presumes the phases as shown have solidified infinitely slowly so as to allow complete diffusion in the solid state.Given the lethargy of solid state diffusion at room temperature, in real life as-solidified structures won’t have compositions reflecting the above ideal phase diagram. One approach that better replicates real conditions is to assume that in the solid state there’s no solute diffusion but that in the liquid phase perfect mixing, that is, infinite mixing, occurs. The justification for this is that in the solid state there’s very little diffusion at the lower temperatures whereas in the liquid itself the diffusion coefficient for the solute will be orders of magnitude greater.So employing the perfect liquid mixing condition plus the constraint of zero solid state diffusion enables a qualitative description of alloy composition throughout solidification. The main departure from the unrealistic equilibrium solidification scenario is that the concentration of solute in the solid phase doesn’t obey the solidus curve but instead follows the dotted line. And why is that?The first solid to nucleate will have the composition dictated by the solidus. The subsequent solid phases that form as temperatures drop have their compositions given by the equilibrium solidus line. However, since diffusional mixing of these discrete solid sections is essentially nonexistent, the average composition of the overall mass will be less than predicted by the solidus line; that is, it’ll be given rather by the dotted line. This is known as coring and can be observed metallographically. In practice, homogenizing heat treatments can remove much of the coring if desired.The above is a qualitative description of alloy freezing within the framework of perfect liquid mixing with no solid state diffusion. A few additional assumptions can be invoked, however, to derive the corresponding equation for the solute concentration in the solid as it freezes. These extra restrictions include requiring the liquidus and solidus curves to be straight lines and presuming equilibrium at the solid-liquid interface. The ensuing relation is known as the Scheil equation and provides a fair estimate of experimentally observed solute segregation.

Sometimes, in order to clock more on the productivity chart, we tend to do a lot of deviation while casting the steel in CCM. Everything is fine, until you find that the billets you have produced are rhomboidal. More than a defect, it is something of an operational issue most of the times.Rhomboidal billets are a pain for the rolling and finishing mill personnel. You have to rework the billet, or reheat the billet for more time to attain the plastic deformation stage, resulting in production delay and increase in heating costs. However, if you want to reduce the rhomboidal issue in the caster itself, you should check the following parameters.Superheat of the steel - When the heat is lifted from the LRF or VD, you need a certain temperature above the liquidus of that particular grade of steel to account for the temperature drop during transferring of the heat to the tundish and last till the end of casting. Usually, the liquidus of the high carbon/silicon/aluminium/alloy steel and the rate of temperature drop is lower than that of mild/low-carbon/alloy steels. Casting the steel at an optimum temperature might help you in reducing rhombodity.Casting speed - Again, depending on the steel grade and the amount of liquid discharge, the casting speed should be determined. Casting the heat faster would result in rhombodity, while casting it slower would cause bleeding at the caster.Mould Alignment - The copper mould is the place where the solidification of the steel begins. Check your mould dimensions and alignment to control rhombodity.Mould Cooling - Since the solidication of steel begins at the mould, you should always monitor the cooling rate of the copper mould. Even a slightly lesser rate of water flow to the mould can distort the shape of the billet.Secondary cooling - The rhombodity can also occur if the cooling rate is less in the roller section of the caster. Monitor the water flow, and maintain the required level as per the casting speed, steel grade, and zones of roller.These are the primary and generalized cause of rhombodity in billets. Hope it solves the issue for you.