SOME QUESTIONS IN THE THEORY OF MODIFICATION

V. T. SVISHCHENKO and N. D. TYUTEVA

Introduction

In recent years a large number of works have appeared in the technical literature devoted to investigating the influence of small additions of modifiers (boron, titanium, vanadium, and others) on the properties of structural and tool steels.

In these works contradictory opinions are expressed about the causes of the influence of modification. Many investigators suppose that small additions improve the properties of steel only in the case when they enter into solid solution, changing the alloying of austenite; that is, these investigators see no difference between modification and alloying.

The majority of investigators believe that small additions cannot change the properties of steel throughout the entire volume of the metal grains, acting as alloying elements. These additions are distributed nonuniformly, and only on the surfaces of the grains, changing (reducing) the value of the surface energy of the grains, as a result of which the hardenability, microstructure, and mechanical properties of the steel change. The distribution of the addition on the surfaces of the grains is associated with adsorption of surface-active particles (atoms) of the modifier by the growing faces of crystals; that is, the action of modifiers is explained from the point of view of the adsorption theory of modification [1;2]. In our opinion, experimental data on the investigation of the influence of small additions prove the correctness of the adsorption theory of modification.

This theory explains the influence of modification chiefly by the fact that the modifier, being surface-active with respect to the crystallizing substance, can be adsorbed by the faces of crystals growing from the melt, reducing the surface energy of the faces and of the crystals as a whole. A decrease in surface energy leads to an increase in the stability of crystallization nuclei and slows the rate of their growth, owing to which, in each given volume, a much greater number of centers than usual has the opportunity to develop, and the structure of the metal becomes finer-grained. This ability of modifiers to improve the structure and mechanical properties of an alloy is not always manifested, but only when very small quantities of the modifier are introduced and up to a certain optimal amount.

The addition of large quantities of modifier (more than the optimal amount) leads to so-called “overmodification,” when the structure and properties of the alloy are not only not improved, but deteriorate. As Semenchenko [2] points out, the phenomenon of “overmodification” still remains unexplained.

Some experimental data make it possible to conclude that the influence of modification depends to a very great extent on the cooling rate of the casting, and also on the regime of subsequent heat treatment. However, this question has been insufficiently investigated experimentally and has not been substantiated theoretically.

It should also be noted that in all experimental as well as theoretical works on modification, the effect of modification on the properties of an alloy is associated only with changes in the microstructure and, sometimes, in the hardenability of the alloy. In the present work it is shown that, in complex multiphase alloys, modification brings about a change in the composition of the phases, and a theoretical substantiation of this change is given. An attempt is made to explain the phenomenon of “overmodification” and to substantiate theoretically the dependence of the effect of modification on the cooling rate of the casting and on the regime of subsequent heat treatment.

Redistribution of Alloying Elements between the Phases of an Alloy under the Influence of Modification

The change in the composition of the phases of an alloy under the influence of modification, from the standpoint of the adsorption theory of modification, may be substantiated by a nonuniform change in the stability of the growing crystals (crystallization centers) of the individual phases, which is associated with a nonuniform change in their surface energy as a result of selective adsorption by the faces of the crystals of a surface-active element (modifier).

During the crystallization of steel according to the equilibrium phase diagram, crystals of the solid solution with the lowest potential energy grow (crystals of equilibrium chemical composition). In any given volume, crystallization centers of nonequilibrium chemical composition may arise, but these centers are less stable and therefore are more readily destroyed by thermal energy.

The potential energy of a crystal consists of lattice energy and surface energy; moreover, the value of the latter increases with an increase in the degree of dispersion of the system, since in this case the ratio of the number of surface atoms to the total number of atoms increases.

A number of theoretical works on modification [2; 3] indicate that the state and behavior of a highly dispersed system are determined by its free surface energy. The surface energy of crystals of different chemical composition, under the influence of a modifier, decreases nonuniformly owing to the selective adsorption of a surface-active element by particular faces (apparently, by the faces with the greatest surface energy [1]); consequently, the stability of crystals of different chemical composition should also change unequally under the influence of modification. Therefore, in the presence of a surface-active element, crystals of a solid solution whose concentration of alloying elements differs from that in the crystals of the solid solution of unmodified steel may prove to be the most stable. Thus, a surface-active addition may cause a change in the ratio of the amount of alloying elements in the growing crystals of the solid solution to that part of the alloy which is still in the molten state (as compared with the same ratio in unmodified steel).

Upon cooling from the molten state of a complex alloy, for example high-speed steel, several phases crystallize. In high-speed steel, first the \(\delta(\alpha)\)-phase [4] crystallizes, with an increased concentration of tungsten, vanadium, and chromium and with an insignificant concentration of carbon. As the crystals of the \(\delta(\alpha)\)-phase grow, in the adjacent volumes of liquid there occurs a gradual increase in the concentration of carbon, which at some point becomes so significant that crystallization of the \(\delta(\alpha)\)-phase ceases and precipitation of crystals of the \(\gamma\)-solid solution begins, since the crystals of the \(\delta(\alpha)\)-phase, at an increased carbon concentration, become less stable than the crystals of the \(\gamma\)-solid solution. In view of the fact that the average carbon content in the crystals of the \(\gamma\)-solid…

of the solution will be less than the specified one; in the remaining liquid the percentage of carbon will continually increase until its concentration reaches the eutectic concentration, after which crystallization of the eutectic (carbides) will begin. Apparently, crystallization of carbides cannot occur at a carbon concentration below the eutectic concentration, since in that case sufficiently large and stable centers of carbide crystallization cannot appear.

Since, under the influence of modification, the stability of the crystals of the individual phases changes unequally, modification must alter not only the composition but also the quantity of the individual phases in comparison with the unmodified alloy. In the presence of a surface-active element (modifier) in high-speed steel, crystallization of the $\delta(\alpha)$ phase may end, and crystallization of the eutectic may begin, at some other carbon concentrations as compared with unmodified steel. Thus, under the influence of modification, an increase or decrease of the $\delta(\alpha)$ phase and of the carbide phase is possible, which may also cause a change in the chemical composition of the austenite.

It should be noted that, for the same reasons—the nonuniform change in the stability of crystals of different chemical composition under the influence of the modifier—the composition of the carbide phase of modified steel should differ from the composition of the carbide phase of unmodified steel.

The influence of the surface energy of a polycrystalline system on the change in the chemical composition of the phases should be especially great at the moment when crystallization centers are formed, when the system is in an extremely dispersed state. This influence should increase with an increase in the cooling rate of the casting, since during accelerated cooling a greater number of crystallization centers is formed, i.e., the degree of dispersion increases. It follows from this that the most substantial redistribution of alloying elements among the phases of cast high-speed steel can be achieved when the casting is cooled at a high rate, is quenched in the process of cooling (when high-speed steel is cast into a chill mold), and does not subsequently undergo annealing and quenching operations. Annealing and quenching of high-speed steel to a considerable extent eliminate the effect of modification; this is confirmed by the experimental data of Nevzorova [3], who found no change in the cutting properties of a cast modified tool made of high-speed steel after annealing and quenching as compared with a cast unmodified tool.

Apparently, the possibility of changes in the chemical composition of the phases of modified steel increases with increasing content of alloying elements in the steel.

The question of the redistribution of alloying elements among the phases of an alloy under the influence of modification is raised here for the first time; therefore, the technical literature contains no works devoted specifically to the study of this question. However, some experimental data indicate changes in the composition of the phases of modified steel as compared with unmodified steel. Let us give several examples.

1. It is known [5] that modification of highly alloyed forged structural steel does not change its hardenability, whereas the hardenability of low-alloy steel is sharply improved under the influence of modification. The improvement in the hardenability of low-alloy steel is explained by a decrease in the surface energy of the austenite grains under the influence of adsorption of a surface-active addition. This decrease in surface energy leads to an increase in the thermodynamic stability of austenite, as a result of which the transformation of austenite begins later than without the influence of a surface-active addition.

A similar phenomenon should also occur in modified high-

alloyed steel. The absence of improved hardenability in high-alloy steel under the influence of modification can be explained only by the difference in the chemical composition of the austenite of the modified and unmodified high-alloy steel after the completion of heating of the steel for quenching. Apparently, the presence of a surface-active element increases the stability of carbides, as a result of which a smaller amount of them passes into solid solution when the steel is heated for quenching, and the austenite of the modified steel proves to be less alloyed than the austenite of the unmodified steel. It is possible that the overall alloying of the austenite does not change, but only the distribution of individual elements between the solid solution and the carbide phase changes. Thus, the hardenability of high-alloy steel depends on two factors: 1) a change in the stability of austenite under the influence of a decrease in the surface energy of its grains as a result of adsorption of a surface-active element, and 2) a change in the stability of austenite under the influence of a change in its chemical composition. These two factors may exert approximately equal, but opposite, effects on hardenability.

1. The redistribution of alloying elements between the phases of the alloy under the influence of modification is also evidenced by the appearance of temper brittleness in modified structural heat-treatable steel, as well as by the change in mechanical properties, which is not always connected with changes in the hardenability and microstructure of the steel.

2. At the Department of Metallography of the Tomsk Polytechnic Institute, work was carried out [6] on the modification with boron of cast high-speed steel cast in a chill mold. It was established that modification with boron leads to a change in the amount and chemical composition of the phases of cast steel. In the steel modified with boron (as compared with the unmodified steel), the amount of the dark constituent (the $\delta(\alpha)$ phase) decreases, while the amount of the carbide phase increases. The concentration of tungsten in the $\gamma$—solid solution increases, while in the carbide phase it decreases. The concentration of carbon and chromium in the $\gamma$—solid solution decreases, while in the carbide phase it increases.

The most favorable redistribution of alloying elements is achieved with the addition of $0.015 + 0.025\%$ boron in the form of ferroboron, provided that the steel is first deoxidized with aluminum, with an addition of $0.09\%$ aluminum.

The addition of larger or smaller amounts of boron than indicated leads to a less noticeable change in the composition of the phases.

As a result of the change in the chemical composition of the phases, the microhardness of the structural constituents of the boron-modified cast high-speed steel changes. The hardness of the carbide eutectic of the modified steel is higher than the hardness of the carbide eutectic of the unmodified steel, which can be explained only by an increase in the amount of carbides in the carbide eutectic under the influence of modification.

The microhardness of the light constituent—the $\gamma$—solid solution (Fig. 1)—decreases with an increase in the boron content up to the optimum content $(0.015 + 0.025\%)$ and increases again with a further increase in the boron content.

Modification with boron improves the microstructure of cast high-speed steel—a very fine network of carbide eutectic and a fine grain are obtained. As a result of the improvement of the microstructure and the favorable redistribution of alloying elements between the phases, there is a sharp increase in the toughness and durability of the boron-modified cast tool. The durability of the boron-modified tool increases by a factor of 3–4 in comparison with the durability of the unmodified tool when working under impact loading (milling).

On “Overmodification”

As has already been noted, modification has a positive effect on the properties of an alloy only when small amounts of a modifier are introduced, up to a certain optimum amount. A further increase in the addition of the modifier leads to “overmodification,” i.e., to a weakening of the modifying action of the surface-active element. When a modifier is added in an amount considerably exceeding the optimum amount, the properties of the alloy (hardenability) and the microstructure may even deteriorate in comparison with an unmodified alloy. The causes of “overmodification” have not yet been clarified.

In the present work an attempt is made to explain the phenomenon of “overmodification” from the standpoint of the adsorption theory of modification, on the basis of studies of the influence of modification on the growth of austenite grains during heating of steel.

Fig. 1. Change in the microhardness of the light constituent of melts with an average tungsten content \((13.01\text{–}15.35\%\,\omega)\) as a function of boron content

Fig. 2. Growth of austenite grain (Vinarov)

1—unmodified steel
2—modified steel

In the technical literature there are indications of nonuniform growth of the austenite grain during heating of modified steel. This is shown especially well by Vinarov [7], who investigated the influence of modification with boron on the size of the austenite grain during heating of steel of the following chemical composition: \(0.35\text{–}0.45\%\ \mathrm{C}\); \(0.25\text{–}0.30\%\ \mathrm{Si}\), and \(0.45\text{–}0.70\%\ \mathrm{Mn}\).

Vinarov [7] gives the following picture of the growth of austenite grains in modified and unmodified steel (Fig. 2) during holding at a temperature of \(900\text{–}925^\circ\). The grain of the unmodified steel (curve 1) grows more or less uniformly. The grain of the modified steel (curve 2) at first grows more slowly than that of the unmodified steel. This slowing is associated with the gradual filling of the grain surface with surface-active particles (atoms) of the modifier, as a result of which the surface energy of the grains decreases and their thermodynamic stability increases.

As the grains grow, their total surface gradually decreases to values at which the amount of modifier introduced into the steel proves sufficient to cover the entire surface of the austenite grains with a continuous monatomic (or nearly so) film of the modifier. At this mo—

growth of the austenite grain is very strongly slowed, or a brief cessation of grain growth occurs.

“However, the energetic impulse created by the nonequilibrium state of the grain does not cease to act and, with time, reaches a magnitude sufficient to break through the adsorption layer. From this moment the grain again grows at a rate greater than in the ordinary process, without the intervention of a surface-active element. From this same moment stabilization of the film itself also proceeds, i.e., its gathering, by stronger chemical-bond forces in separate regions, into peculiar ‘clots’—a process similar to coagulation, occurring inside the grain in its surface zone. Subsequently, during cooling of the steel, after the transformation \(\gamma \to \alpha\), these ‘clots’ give rise to the formation of a boron-rich phase (iron borides and carboborides), precipitated along grain boundaries and observed in microscopic examination” [7].

The greater the amount of surface-active element contained in the steel, the earlier the cessation of grain growth should occur. Vinarov [7] indicates that in steel with a content of \(0.003 \div 0.004\%\) boron the grain does not increase during holding for \(10 \div 20\) minutes at a temperature of \(900—920^\circ\); in steel with \(0.01\%\) boron, at the same temperature, the grain (which is finer) does not grow for \(4 \div 6\) minutes. In steel with \(0.05\%\) boron it is not possible to detect a cessation of grain growth, since, apparently, it occurs at very small grain sizes and with holding times that are practically impossible to capture.

It seems possible to us to connect the phenomenon of “overmodification” with the accelerated growth of the grain of modified steel. With accelerated grain growth, surface-active particles (modifier atoms) do not have time to distribute themselves uniformly over the contracting surface of the grains; conditions are created for the accumulation of surface-active particles in separate regions. In this case the surface-active particles (atoms) form a crystal lattice (of the modifier or of a chemical compound of the modifier with elements entering into the composition of the alloy) or enter into solid solution. In this case the modifier acts no longer as a surface-active element, but as an ordinary alloying element.

Modification of structural low-alloy steel can most noticeably improve the hardenability of the steel in the case when the amount of modifier is necessary and sufficient for the creation of a continuous adsorption layer (monoatomic, or close to it) of surface-active particles by the moment when heating of the steel for quenching ends (when the austenite grain has a definite size). With a smaller amount, the influence of the modifier will be less significant. With a larger amount of modifier, the austenite grain will be covered with a continuous film of surface-active particles before heating of the steel for quenching ends (when the austenite grain is finer than in the first case). Since heating of the steel will continue, the cessation of grain growth will be followed by its accelerated growth, which will cause the accumulation of surface-active particles in separate regions and the formation of the crystal lattice of the modifier or of a chemical compound, whereby the number of forced crystallization centers will increase. The surface energy of austenite grains deprived of the film of surface-active particles will not decrease, while the number of forced crystallization centers (of pearlite) will increase. As a result, the hardenability of the steel will not only fail to improve, but may even deteriorate.

An analogous process of accumulation of surface-active particles of the modifier in separate regions must also occur during crystallization of the alloy from the molten state. In this case the addition of a modifier may exert the most significant modifying action if the amount of the addition is necessary and sufficient to cover the grain surface with a continuous film of surface-active particles by the mo-

by the moment crystallization ends. With a decrease in the amount of the additive, the modification effect will decrease. When a larger amount of modifier is added, the surface of the grain will be covered with a continuous monoatomic film of the modifier before crystallization is completed. In this case, the accelerated growth of the grain, which follows the cessation of its growth, may lead to coarsening of the grain instead of its refinement.

Dependence of the influence of the modifier on the cooling rate of the casting and on the regime of subsequent heat treatment

From what was set forth in the preceding section it follows that one or another effect of the action of modifiers depends on the ratio of the number of surface-active particles (modifier atoms) to the magnitude of the adsorbing surface during the cooling of the casting and the heating of the steel for quenching. The greatest modification effect is achieved when the entire surface of the crystals is covered with a continuous monoatomic film of surface-active particles (atoms) of the modifier by the time crystallization of the steel from the molten state is completed and by the time the heating of the steel for quenching is completed. In this case the surface energy of the crystals will decrease most strongly; consequently, the influence of the modifier should be most significant. In the event that the number of surface-active particles is insufficient to create a continuous film on the surface of the crystals by the time crystallization ends, the modification effect will be less significant, since in this case the surface energy of the crystals will decrease less significantly.

In the case of an excessive amount of modifier, “overmodification” will occur, and the modifier will act not as a surface-active element, but as an ordinary alloying element.

The ratio of the number of surface-active particles to the magnitude of the adsorbing surface depends not only on the concentration of the modifier, but also on the degree of dispersion of the alloy.

In a casting cooling at a higher rate, a larger number of crystallization centers will arise than in a casting cooling at a lower rate. The degree of dispersion increases with increasing cooling rate. Therefore, with an equal amount of modifier, the ratio between the adsorbing surface and the number of surface-active particles in castings cooling at different rates will not be the same. Consequently, at different cooling rates of the casting the influence of one and the same amount of modifier should also be unequal. At a lower cooling rate of the casting, the optimum amount of modifier should be less than when the casting is cooled at a higher rate.

The heating time for quenching of specimens (parts) made of modified steel that differ considerably in size is not the same; therefore the size of the austenite grain after completion of heating for quenching of specimens of different sizes cannot be the same. Consequently, in this case as well, with an equal amount of modifier in the specimens, the ratio between the magnitude of the adsorbing surface and the number of surface-active particles will also be unequal. Therefore the influence of one and the same amount of modifier on the hardenability and mechanical properties of specimens of different sizes made of modified structural steel will also be different.

The optimum amount of modifier for large specimens (parts) should be less than for small parts.

Thus, the influence of modification and the optimum amount of modifier additive may vary depending on the conditions and dimen-

casting dimensions, as well as on the regime of subsequent heat treatment.

The question of the dependence of the effect of modification on the cooling rate of the casting and on the regime of subsequent heat treatment, as well as the question of changes in the phase composition of the alloy under the influence of modification, is raised here for the first time. There are no special studies on this question. But there are experimental data confirming this dependence. We shall give several examples.

1. It is known [5] that the addition of boron in an amount of \(0.0025 \div 0.003\%\) very effectively improves the hardenability and mechanical properties of low-alloy forged or rolled hardened steel after low tempering in comparatively small sections. At the same time, in steel articles of large thickness the same amounts of boron do not give a stable positive effect. This can be explained only by an increase in the holding time at the quenching temperature for articles of large size, as a result of which the austenite grain grows, and the total magnitude of the adsorbing surface decreases so significantly that overmodification occurs.

2. In Nezorova’s work [8] it is shown that the addition of one and the same amount of modifier (titanium) improved the cutting properties of cutters (size \(16 \times 25\)) made of high-speed steel cast in sand molds, as compared with unmodified cutters, and practically did not change the cutting properties of plates (size \(7 \times 20\)) cast in a chill mold (as compared with unmodified plates).

3. In the metallurgy laboratory of the TPI, a study was carried out of the influence of small additions of boron on the hardenability of specimens of different sections of cast chromium steel. It was established that the same amounts of boron do not have the same effect on the hardenability of specimens of different size.

Conclusions

1. Modification causes changes in the phase composition of a complex alloy. This change is especially significant at a high cooling rate of the casting and in the case when the alloy, after casting, is not subjected to annealing and quenching.

2. The phenomenon of “overmodification” is associated with the accumulation of surface-active particles of the modifier in individual regions and with the formation of the crystal lattice of the modifier or of a chemical compound. The accumulation of surface-active particles of the modifier is associated with the nonuniform growth of the grain of the modified steel and occurs during accelerated grain growth.

3. The effect of modification depends not only on the amount of modifier introduced, but also on the casting conditions (cooling rate) and on the subsequent heat treatment of the alloy.

LITERATURE

1. P. A. Rehbinder et al. Collection “Research in the Field of Surface Phenomena,” “Physicochemical Foundations of the Modification of Metals and Alloys by Small Surface-Active Impurities.” ONTI, p. 225, 1936.

2. V. K. Semenchenko. “Nonferrous Metals,” No. 6, 92, 1936.

3. S. M. Vinarov. Hardenability of Steel with a Small Boron Content. Journal of Technical Physics, vol. XIX, issue 2, 1949.

4. T. A. Lebedev, I. A. Revis. Structure and Properties of a Cast Tool Made of High-Speed Steel. Mashgiz, 1949.

5. Proceedings of the Central Scientific Research Institute of the People’s Commissariat of the Tank Industry, No. 1 (21), 1945.

6. V. T. Svishchenko. Abstract of a dissertation on the topic: “Modification of Cast High-Speed Steel by Boron.” TPI, Tomsk, 1952.

7. S. M. Vinarov. Kinetics of Austenite Grain Growth in Steel with a Small Boron Content. Reports of the Academy of Sciences of the USSR, Division of Technical Sciences, No. 6, 1948.

8. Z. D. Nezorova. Modification of High-Speed Steel. Collection “Cast and Surfaced Tool.” Mashgiz, 1951.