Solidification structure
5.3 Segregation
5.3.1 Planar front segregation
Solidification structure 139
cell count’, giving a measure of something called a ‘cell size’. In these difficult circumstances it is perhaps the only practical quantity that can be measured, whatever it really is!
I
Figure 5.25 Micrograph of AI-0.2Cu alloy showing porosity and interdendritic segregation. Some grain boundary migration during cooling is clear.
(Electropolished in perchloric and acetic acid solution and etched in ferric chloride. Dark areas are etch pitted.)
1. A cell can be a general growth form of the solidification front, as used in this book.
2. Cell is the term used to denote graphite ‘rosettes’
in grey cast irons. Strictly, these are graphite grains; crystals of graphite which have grown from a single nucleation event. They grow within, and appear crystallographically unrelated to, the austenite grains that form the large dendritic rafts of the grey iron structure. Analogous structures, again called cells, are seen in spheroidal graphite irons.
3. The term ‘cell count’ or ‘cell size’ is sometimes used as a measure of the fineness of the microstructure, particularly in aluminium alloys.
Here the distinction between primary arm, secondary arm and grain is genuinely difficult to make in randomly oriented grains, where primary and secondary arms are not clearly differentiated (see Figure 5.20). To avoid the problem of having to make any distinction, a count is made of the number of features (whether primary or secondary arms or grains) in a measured length. This is arbitrarily called ‘the
Rather pure solid Solid approaching average Segregated liquid
uid of average composition C,,
Directional freezing apparatus
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builds up ahead of the front if the liquid is still.
The initial build-up to the steady-state situation is called the initial transient. This is shown rather spread out for clarity. Flemings (1974) shows that f o r small k the initial transient length is approximately DN,k where D is the coefficient of diffusion of the solute in the liquid and Vs is the velocity of the solidification front. In most cases the transient length is only of the order of 0.1-
1 mm or so.
After the initial build-up of solute ahead of the front, the subsequent freezing to solid of composition Co takes place in a steady, continuous fashion until the final transient is reached, at which the liquid and solid phases both increase in segregate. The length of the final transient is even smaller than that of the initial transient since it results simply from the impingement of the solute boundary layer on the end wall of the container. Thus its length is of the same order as the thickness of the solute
Figure 5.26 Directional solidification on a planar front giving rise to two different patterns of segregation depending on whether solute is allowed to build up at the advancing ,front or is swept away by stirring.
boundary layer D/R. For many solutes this is therefore between 5 and 50 times thinner than the initial transient.
For the case where the liquid is stirred, moving past the front at such a rate to sweep away any build-up of solute, Figure 5.26 shows that the solid continues to freeze at its original low composition kCo. The slow rise in concentration of solute in the solid is, of course, only the result of the bulk liquid becoming progressively more concentrated.
The important example of the effect of normal segregation, building up as an initial transient, is that of subsurface porosity in castings. The phenomenon of porosity being concentrated in a layer approximately 1 mm beneath the surface of the casting is a clear case of the build-up of solutes.
The nucleation and growth of gas pores is discussed in Chapter 6.
Moving on now to consider an example of segregation where the liquid is rapidly stirred, the
Solidification structure 1 1 1
With the development of continuously cast steel, the casting of steel into ingots has now become part of steelmaking history. Even so, as an interesting diversion it is worth including at this point the other major classes of steel that were produced as ingots, since these still have lessons for us as producers of shaped castings. The two other types of ingots were produced as balanced and killed steels (Figure 5.27).
The balanced, or semi-killed, steel was one that, after partially deoxidizing, contained 0.0 1-0.02 per cent oxygen. This was just enough to cause some evolution of carbon monoxide towards the end of freezing, to counter the effect of solidification shrinkage. The deep shrinkage pipe that would normally have been expected in the head of the ingot, requiring to be cropped off and remelted, was replaced with a substantially level top. The whole ingot could be utilized. The great advantage of this quality of steel was the high yield on rolling, because the dispersed cavities in the ingot tended to weld up. For this reason bulk constructional steel could be produced economically. However. a difficult balancing act was required to maintain such precise control of the chemistry of the metal.
It was only because balanced steels were so economical that such feats were routinely attempted.
Some steelmakers were declared foolhardy for attempting such tasks!
In contrast, killed steels were easy t o manufacture. They included the high carbon steels and most alloy steels. They contain low levels of free oxygen, normally less than 0.003 per cent.
Consequently there was no evolution of carbon monoxide on freezing, and a considerable shrinkage cavity was formed as seen in Figure 5.27. If allowed to form in this way, the cavity opened up on rolling classic case was that of the rimming steel ingot
seen in Figure 5.27. During the early part of freezing, the high temperature gradient favoured a planar front. The rejection of carbon and oxygen resulted in bubbles of carbon monoxide. These detached from the planar front and rose to the surface, driving a fast upward current of liquid, effectively scouring the interface clean of any solute that attempted to build up. Thus the solid continued to freeze with its original low impurity content, forming the pure iron rim. At lower levels in the ingot there was a lower density of bubbles to scour the front, so some bubbles succeeded in remaining attached, explaining the array of wormhole-like cavities in the lower part of the ingot. During this period the incandescent spray from the tops of the ingots as the bubbles emerge at the surface was one of the great spectacles of the old steelworks, almost ranking in impressiveness with the blowing of the Bessemer converters. A good spray was said to indicate a good rimming action.
As the rim thickened, the temperature gradient fell so that the front started to become dendritic, retaining both bubbles and solute. Thus the composition then adjusted sharply to the average value characteristic of the remaining liquid, which was then concentrated in carbon, sulphur and phosphorus.
Rimming steel was widely used for rolling into strip, and for such purposes as deep drawing, where the softness and ductility of the rim assisted the production of products with high surface finish.
The oxygen levels in rimming steels were in excess of 0.02 per cent, and were strongly dependent on the carbon and manganese contents. Typically these were 0.05-0.20 C and 0.1-0.6 Mn, giving a useful range of hardness, ductility and strength.
Figure 5.27 Ingot structures: ( r i ) n killed .steel:
(b) a balanced steel; and ( c ) a rirnrning steel.
as a fishtail, and had to be cropped and discarded.
Alternatively the top of the ingot was maintained hot by special hot-topping techniques. Either way, the shrinkage problem involved expense above that required for balanced or rimming steels. Fully killed steels were generally therefore reserved for higher priced, low and medium alloy applications.
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