Primary inclusions

Solidification structure

5.6 Steels

5.6.3 Primary inclusions

When the liquid alloy is cooling, new phases may appear in the liquid that precede the appearance of the bulk alloy. We have already dealt with the formation of the primary phase in section 5 . 2 . 2 . Whether any newly forming dense phase gets called

a phase or an inclusion largely depends on whether it is wanted or not: keen gardeners will appreciate the similar distinction between plants and weeds!

New phases that precede the appearance of the bulk alloy are especially likely following the additions to the melt of such materials as deoxidizers or grain refiners, but may also occur because of the presence of other impurities or dilute alloying elements.

For instance, in the case of steel that has a sufficiently high content of vanadium and nitrogen, vanadium nitride, VN, may be precipitated according to the simple equation:

Whether the VN phase will be able to exist or not depends on whether the concentrations of V and N exceed the solubility product for the formation of VN. To a reasonable approximation the solubility product is defined as:

K = [%V].[%N]

where the concentrations of V and N are written as their weight per cent, More accurately, a general relation is given by using, instead of weight per cent, the activities av and uN, in the form of a product of activities:

V + N w V N

K' ⁼ aV.uN

It is clear then that VN may be precipitated when V and N are present, where sometimes V is high and N low, and vice versa, providing that the product

%V x

%N

(or more accurately, av x u N ) exceeds the critical value K (or K'). It is interesting to speculate that [N] may be high very close to the surface where the melt may be dissolving air. Thus the formation of a surface film of VN may be more likely.

In the case of the deoxidation of steel with aluminium, the reaction i s somewhat more complicated:

2A1+ 30

+

A1203

and the solubility product now takes the form:

K" = [uA1l2

.

^[aoI3

where the value of K" increases with temperature.

Again, the surface conditions are likely to be different from those in the bulk, with the result that a surface film of AlN or A1203 is to be expected, even if concentrations for precipitation in the bulk are not met.

These examples only relate to the case where the newly formed phase is in equilibrium with the melt. In practice higher concentrations of the individual constituents of the phases will be required to overcome the problem of nucleation of the new phase.

Solidification structure 17 1

This simple equation becomes even more simplified in its solubility product form, because the concentration of iron is very closely 100 per cent (Le. unity in the above equation). Thus the FeO can exist in equilibrium in an iron melt only if the oxygen concentration is high enough (since the iron concentration is already fixed at its maximum).

Thus in Figure 5.53 the threshold for the formation of FeO is very nearly a vertical line. The parallel line denoting the threshold t o overcome the resistance to the nucleation of FeO is quite close:

this is because the surface energy of the interface is low, in the region of only 0.25 Nm-’.

Turpin and Elliott take their analysis further to show that a melt that has been allowed to come into equilibrium at a high temperature may reach a sufficient supersaturation to cause nucleation as the melt is cooled. They effectively work their analysis backwards, aiming for a nucleation at the freezing point of iron, 1536”C, and calculating what equilibrating temperature would have been required to achieve this. Their results are summarized in Figure 5.54.

These results demonstrate that it is possible, in principle, to predict the arrival and stability of particles in melts, as a function of temperature and composition. Turpin and Elliot were not able to confirm their theoretical predictions for this system because of experimental limitations. However, much work on the grain refinement of metals would surely benefit from a careful, formal approach of this kind.

All this work so far has neglected the problem of the nucleation of the inclusion. We have considered examples of nucleation at various points in the book, especially in section 5.2.2. At this stage we shall simply note that any primary Turpin and Elliot (1966) were among the first

to study the problem of the nucleation of new dense phases from the melt. Using the approach of classical nucleation theory as illustrated in Equation 5.15, these authors used the standard free energy changes for the formation of oxides, which they took from the literature on thermodynamics, to find the energy for formation of a nucleus of the new material. We shall not follow their argument in detail, but merely quote their result in Figure 5.53 for the Fe-0-Si system. In this example two oxides are considered.

The first is from the reaction:

Si

+

2 0 SiO,

so that the equilibrium constant is now approximately:

Figure 5.53 shows this equilibrium threshold with its slope of 2 (i.e. an increase of a factor of 10 in oxygen concentration together with a decrease of a factor of 100 in silicon concentration will still result in the nucleation condition being satisfied). The higher threshold shown in Figure 5.53 corresponds to the concentrations required for nucleation, assuminp a surface energy of the interface of 1.3 Nm- . (In fact, the threshold required to nucleate silica can be shown to lie at increasing concentrations as the assumed value for the surface energy is raised.) (We shall continue to use Nm-’ in uniformity with the rest of this book. Otherwise, it would have been logiFal to quote surface energy in the identical units Jm--.)

Turning now to the possibility of forming FeO i n this system, the equation is:

Fe

+

0

e

FeO

- composition where

e

%;\, inclusions are observedg4\, to change from liquid

%,

^,\

FeO to solid Si02 ^I

8 \ \ r

1 --m-

0.0001 0.001 0.01 0.1

Oxygen (wt per cent)

3 Figure 5.53 Equilibriuni and nuclearioii tlire.sho1d.c 2 f o r silica and iron oxide iizc1lr.sion.s in .solid$i-ing

iron. Data on threslzo1d.r ,from Tiirpiii c i i i d Ellior ( 1 966).

1

I I I

0.01 0.1 1

Silicon (wt per cent)

Figure 5.54 The cooling required, from a temperature where the system was allowed to come into equilibrium, down to the freezing point

of

iron (1536“C), to nucleate oxides in the Fe-0-Si system (from Turpin and Elliot 1966).

inclusions form prior to the arrival of the matrix primary phase. Thus they appear in a sea of liquid.

During this ‘free-swimming’ phase, primary inclusions are thought to grow by collision and agglomeration (Iyengar and Philbrook 1972).

For liquid inclusions this is expected to result in large spherical inclusions whose compact shape will enable them to float rapidly to the surface and become incorporated into a slag or dross layer which can be removed by mechanically raking off, or can be diverted from incorporation into the casting by the use of bottom-pouring ladles, or teapot spout ladles.

For solid inclusions, the agglomeration process may form loosely adhering aggregates or clouds.

For instance, alumina inclusions in aluminium-killed and rolled steel appear to be fine clouds of dispersed particles, arranged in stringers, on a polished section.

There seems to b e more than one potential explanation of this appearance: (i) when revealed by deep etching the inclusion is sometimes seen to have a three-dimensional dendritic shape (Figure 5.55) - it is easy to see how the spindly dendrite arms of these alumina inclusions could align, elongate and fracture to form the long stringers observed in longitudinal sections of rolled steels, (ii) alternatively, the entrained and ravelled alumina films may condense into arrays of compact particles, analogous to the way in which sheets of liquid metal break up into droplets (a spectacular example is given in Figure 2.13), an effect driven by the

(4

(b)

Figure 5.55 Alumina inclusion in an aluminium-killed low-carbon steel, showing: ( a ) a two-dimensional section; and (b) a three-dimensional view (from Rege et al. 1970).

reduction of surface energy. The rolling out of these clouds of discrete particles will again explain the observed stringers. Work to clarify these possibilities would be welcome.

Hutchinson and Sutherland ( 1965) have studied the formation of open-structured solids. They find that flocs can form by the random addition of particles. If these particles are spherical and adhere precisely at the point at which they first happen to encounter the floc, then the floc builds up as a roughly spherical assembly, with maximum radius R, and about half the number of spheres within a region W2 from the centroid. The central core has an almost constant density of 64 per cent by volume of spheres. Occasional added spheres will penetrate right into the heart of the floc. Graphite spheres in ductile iron appear to be a good example of this kind of flocculation. Melts of hypereutectic ductile irons suffer a loss of graphite by the floating out of loose flocs of spherulites (Rauch et al. 1959).

We have only touched on examples of oxides and nitrides as inclusions in cast metals. Other inclusions are expected to follow similar rules and include borides, carbides, sulphides and many complex mixtures of many of these materials. Carbo- nitrides are common, as are oxy-sulphides. In C- Mn steels the oxide inclusions are typically mixtures of MnO, Si02, and A1203 (Franklin et al. 1969) and in more complex steels deoxidized with ever more complex deoxidizers the inclusions similarly grow more complex (Kiessling 1978).

Kiessling points out that steel that contains only as little as 1 ppm oxygen and sulphur will contain over 1000 inclusions/g. Thus it is necessary to keep in mind that steel is a composite product, and probably better named ‘steel with inclusions’. Even so, steels are often much cleaner than light alloy castings, that might contain 10 or 100 times more inclusions, partly helping to explain the relatively poor ductility of Al-based casting alloys compared to steel casting alloys.

Not all of these inclusions will be formed during the liquid phase. Many, if not most, will be formed later as the metal freezes. These are termed secondary inclusions, or second phases, and are dealt with in the following section.

Solidification structure 173

5.6.4 Secondary inclusions and second phases