MODELING OF THE MODIFICATION PROCESS

N. D. TYUTEVA

Introduction

As has already been established by preliminary experiments, the effect of modification on steel is very complex. Apparently, according to P. A. Rebinder, as a result of adsorption the surface energy changes and, as a consequence of this, a different distribution of alloying elements between the phases of the alloy is observed, i.e., the composition of the austenite during crystallization does not correspond to the equilibrium diagram, both as a result of accelerated cooling and as a result of modification. When casting in a chill mold, these two factors act jointly and in parallel. It is extremely difficult to clarify these circumstances when dealing with such a complex system as high-speed steel. Therefore we decided to model the process, considering separately the effect of modification on alloys

\[ \mathrm{Fe — Cr — C;\ Fe — W — C;\ Fe — V — C.} \]

In the present work, the effect of modification with boron on alloys of the Fe—Cr—C system has been partially investigated.

If one accepts that on the state diagram of a complex system with 5 components (high-speed steel) there is a point \(E\), then under the influence of the modifier it should shift.

In high-speed steel one should speak not of a point \(E\), but of a region corresponding to point \(E\); therefore, in order to detect the displacement of point \(E\) and thereby prove the redistribution of alloying elements, we decided to model the process.

The work was intended to clarify three questions:

1. The influence of the modifier on the redistribution of alloying elements between phases.

2. The influence of the modifier on hardenability and grain size, and the dependence of the effect of modification on the cooling rate.

To study the influence of the modifier on alloys cooled at different rates, wedges were cast in a metal mold (Fig. 1) (Fig. 2). The wedge cools nonuniformly. The thinnest part of the wedge cools faster, and the thicker part more slowly. The heats were obtained in a high-frequency-current unit. The crucible was a rammed crucible with a capacity of 2 kg. Melting conditions: grid current 0.2 ampere, anode current 1.0 ampere. The alloy was deoxidized with aluminum in an amount of 0.1%. The modifier—boron—was introduced in the form of ferroboron immediately after deoxidation with aluminum. Forty-five to sixty seconds after introduction of the modifier, the metal was poured into metal wedge-shaped molds. Boron was added to the heats in amounts from 0.002 to 0.08%. The chemical composition of the heats is given in Table 1. In the heats obtained, the following were investigated: hardenability (by the change in hardness value), microstructure, and microhardness of the structural constituents. The heats were studied in the as-cast condition.

Table 1

Fig. 2

Fig. 1

Hardenability

It is known that, under the influence of modification, significant changes occur in the hardenability of low-alloy forged steel. The hardenability of forged steel under the influence of modification improves up to a certain (optimal) addition of the modifier and again worsens with a further increase in the amount of modifier. The hardenability of steel with ad-

The introduction of an excessively large amount of modifier may result in hardenability of the steel worse than without the addition. The improvement in the hardenability of modified steel is explained, from the standpoint of the adsorption theory of modification, by the fact that the modifier is surface-active with respect to the \(\gamma\)-solid solution (austenite). Surface-active particles of the modifier may be adsorbed by the surface of the austenite grains (held on the surface of the grains by the molecular-force field of the surface atoms), as a result of which the surface energy of the grains is reduced.

A reduction in the surface energy of the grains leads to greater thermodynamic stability of austenite, as a result of which the decomposition of austenite during cooling begins later than usual. Modification improves the hardenability of steel only when very small amounts of modifier are introduced, amounts necessary for the formation, on the grain surface, of a monatomic or nearly monatomic adsorption film of surface-active particles (atoms) of the modifier by the time heating of the steel for hardening is completed. When larger amounts of modifier are added, so-called “overmodification” occurs, i.e., a deterioration in the hardenability of the steel instead of its improvement. Apparently, at an increased concentration of the modifier, conditions are created for the transition of the surface-active particles into a solid solution, when the modifier atoms enter the lattice of the \(\gamma\)-solid solution.

It is possible that, at an increased concentration, the surface-active particles form an independent crystal lattice. In either case, the modifier atoms are held in the lattice by stronger bonding forces and can no longer be surface-active, i.e., be distributed on the surface of the grains and reduce their surface energy. The formation of a crystal lattice of the modifier, or of chemical compounds of the modifier with the elements making up the steel, may increase the number of inoculants (centers of pearlite crystallization); therefore the hardenability of steel with an excessively high concentration of modifier may even deteriorate in comparison with the hardenability of unmodified steel. In the technical literature there are a large number of works on the study of the influence of boron on the hardenability of steel in the forged state. On the basis of a whole series of studies it may be regarded as established that the optimum boron content in steel, increasing the hardenability of forged steel, is \(0.0025\)—\(0.003\%\). At the same time, there are almost no data whatever on the influence of boron on the hardenability of cast steel that is not subjected to forging and heating for hardening. Meanwhile, from the change in the hardenability of cast steel not subjected to heating for hardening, one may judge the influence of modification on the course of primary crystallization. It was precisely for this purpose that a study was carried out of the influence of modification with boron on the hardenability of cast steel.

Fig. 3

The change in hardenability was determined from the change in hardness \(R_c\) over the section of the wedge of the modified and unmodified casting.

Hardness was measured along the axis of the wedge at points 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 (Fig. 3). Since the modified and unmodified castings cooled at the hardness-measurement points at the same rate, the chang—

the hardness at the corresponding points of the modified and unmodified wedge can be explained only by a change in hardenability under the influence of modification.

Table 2

Hardness across the wedge section

Table 2 gives the results of measuring the hardness of wedges in the as-cast, untempered state at various points. The results of the hardness measurements show that a noticeable improvement in hardenability is observed only in heat No. 6в with an addition of 0.003% boron. The hardenability of this heat is higher than that of heat No. 5 without boron, despite the fact that heat No. 5 contains more carbon and chromium. The hardness at the first points of heat No. 6в is lower than the hardness at the corresponding points of heat No. 5, which indicates greater stability of the retained austenite in heat No. 6в as compared with heat No. 5. A larger amount of retained austenite in heat No. 6в is preserved up to the eighth point, since the hardness increases continuously up to this point. In heat No. 5, significant amounts of retained austenite are preserved only up to the third point, after which a continuous decrease in hardness begins. This conclusion is confirmed by the fact that the wedge of heat No. 6в is attracted by a magnet much more weakly at the first points than at the corresponding points of the wedge of heat No. 5.

In all the other modified heats the hardenability does not change appreciably, and at the first points it even worsens, which is especially noticeable in heat No. 6 with an addition of 0.014% boron.

It may be concluded that boron most noticeably improves the hardenability of cast low-alloy steel when introduced in the same amounts (0.003%) at which the most noticeable improvement is observed in the hardenability of low-alloy steel subjected to forging and heating for quenching. If the influence of boron on the properties of cast steel were explained only by a reduction in the surface energy of the austenite grains and an increase in the stability of austenite, then such a coincidence of the optimum amount of boron added to cast and forged steel could be explained by an equal magnitude of the adsorbing surface (an equal degree of dispersion) both at the end of heating of forged steel for quenching and at the end of crystallization of cast steel at the given cooling rate. However, it follows from the adsorption theory of modification that modification, in addition to changing hardenability, should cause a change in the distribution of alloying elements between the phases of a complex alloy. This change is associated with the nonuniform adsorption of surface-active particles by the crystallization nuclei of individual phases, as a result of which also nonuniformly

the surface energy of the nuclei and their stability changes unevenly. Therefore, an increase in the amount of one phase at the expense of another is possible.

A change in the composition of the phases should occur during crystallization from the molten state, since it is associated with a change in the number of crystallization centers of the individual phases as a result of the nonuniform change in the surface energy of the crystallization centers and their stability under the influence of modification.

The greatest change in the composition of the phases should occur under accelerated cooling, since with an increase in the degree of dispersion the role of surface energy in the formation of crystallization nuclei increases.

Apparently, the more complex the alloy, the more noticeable the change in the phase composition under the influence of modification should be.

When a modified alloy is heated (during forging and hardening), this change in phase composition may partially or completely disappear.

A change in the composition of the austenite of cast steel under the influence of modification may, in turn, cause a change in the hardenability of the steel.

Thus, the hardenability of cast steel, in contrast to the hardenability of forged steel, depends on two factors:

1. A change in the stability of austenite as a result of a decrease in the surface energy of the austenite grains under the influence of adsorption of the modifier.
2. A change in the stability of austenite as a result of a change in its composition.

In the steel under study, a change in the phase composition should occur, although a less significant one than in steels more complex in chemical composition; therefore, the change in the hardenability of the steel with an addition of 0.003% boron should be explained by the combined action of the two factors indicated above.

It should be noted that, as follows from a comparison of the change in hardenability of heats No. 6v and 5, the hardenability changes differently at different points cooling at different rates.

The greatest change in hardenability is observed at point 3. The unequal change in hardenability under the influence of modification at the locations of the wedge cooling at different rates shows that the effect of the modifier depends on the cooling rate of the casting. This dependence can be explained only by the fact that, at different cooling rates, a different number of crystallization centers arises; consequently, the grain size and the total magnitude of the surface adsorbing the surface-active modifier particles change. Since the magnitude of the adsorbing surface at different cooling rates cannot be the same, the degree of saturation of the surface with surface-active particles also cannot be the same for one and the same amount of modifier.

Consequently, the effect of one and the same amount of modifier on the hardenability of castings cooling at different rates cannot be the same.

On the grain size in the heats

In heats No. 5, 6, 7, 8, and 11, the effect of boron on the grain size was determined. There are very few data in the literature on the grain size of primary crystallization. It is known that boron reduces the grain size of primary crystallization. There are many published works on the study of the effect of boron on the grain size of forged steel. It has been established that small additions of boron contribute to a reduction in the actual austenite grain size. It is also known that the grain in steel with a boron addition grows nonuniformly. The grain size obtained by prolonged holding at high temperatures is greater in boron steel than in unmodified steels. This makes it possible to suppose that, in some interval of time at constant temperature, the grain of modified steel grows at an accelerated rate.

Therefore, the finer grain of the modified steel (with short holding times at \(T = 925^\circ\)) grows to a larger size in comparison with unmodified steel at longer holding times.

In this work, the grain size of cast steel was taken to be the intervals between the light network obtained on a polished section when etched with nitric acid. The light network may be a network of carbide eutectic or a solid solution richer in chromium and carbon. The grain size was determined from micrographs obtained at a magnification of 200. The structure was photographed in all wedges at one and the same point (point 1, Fig. 3). The finest grain was in the heats:

No. 11 with a boron content of 0.002%,
No. 6 with a boron content of 0.003%,
No. 7 with a boron content of 0.004%.

In comparison with the unmodified heats, in these heats the grain size decreases by a factor of 2–3. In heats without boron (heat No. 8) and in heats with a boron content greater than 0.004% (heats No. 5, 6v, 11v), the grain is coarser.

Conclusion: the optimum addition, which most strongly affects the refinement of the microstructure of cast steel, is in the range of 0.002–0.004% boron.

Microstructure of the heats

In studying the microstructure, the heats were divided into 2 groups. Group I included heats No. 5, 5v, 5vv, 6, 6v, 7, 7v (Table 1) with a chromium content within the range 3.95–4.47%, and carbon 1.26–1.42%. Group II included heats less alloyed with chromium and with a lower carbon content—heats No. 8, 8v, 11, 11v, 12, 12v, 13, 13v, 14, 14v, 15, 15v, 16v (Table 1).

The structure was examined at points A, B, C, D of the heats (Fig. 4).

Microstructure in heats of the first group without modifier (heats No. 5, 7).

Point A consists of three structural constituents: a light constituent, martensite needles, and a light network, which may be regarded as a network of primary carbides, or a network of solid solution enriched with carbon and chromium.

At point B the light constituent (austenite), the dark constituent (products of austenite decomposition), then martensite needles and separate fragments of the network of typical primary ledeburitic carbides are present,

Fig. 4

At point C there are only two structural constituents: a dark one (products of austenite decomposition) and a network of primary carbides.

Microstructure in modified heats of the first group (heats 5a, 5vv, 6, 6v, 7v).

The introduction of boron, especially the optimum addition, leads to an increase in the amount of the light constituent (austenite) and to a relative decrease in the dark constituent (products of austenite decomposition). From literature data it is known that boron makes austenite more stable. It was difficult to establish the change in the amount of the carbide phase from the microstructure.

Microhardness

The microhardness of individual structural constituents was determined on a PMT-3 instrument. Mainly, the microhardness of the light

component (the light component, according to our assumptions, is highly alloyed austenite). In determining the microhardness of the light component, the load on the diamond pyramid was 20 g. The impressions were measured at a magnification of 487. Table 3 gives the microhardness values of the light component measured in the same wedge section for all heats (point A, Fig. 4).

Table 3

Microhardness of the light component, kg/mm²

The microhardness values (data) were compared in heats close in chemical composition (heats Nos. 5, 5v, 5vv, 6, 6v, 7, 7v).

It was established that modification with boron leads to a decrease in the microhardness of the light component. A particularly marked decrease in microhardness is observed in the heat with 0.003% boron (heat No. 6v); consequently, the greatest change in the phase composition of cast steel is caused by an addition of 0.003% boron, since the change (decrease) in the microhardness of the light component can be explained only by a change in the chemical composition of the light component.

An addition of 0.003% boron, simultaneously with the most noticeable change in hardenability, has the strongest effect on the microstructure (grain size) and on the change in phase composition.

It should be noted that in more complex alloys the optimum amount of modifier that most noticeably changes the phase composition and grain size may not coincide with the optimum amount of modifier that most noticeably changes hardenability.

As already noted, this is connected with the fact that two factors affect the hardenability of modified steel: a decrease in the surface energy of austenite grains as a result of adsorption of the surface-active modifier, and a change in the chemical composition of the austenite.

Conclusions

1. Modification with boron leads to a change in the hardenability, microstructure, and grain size of cast steel.

2. Under the influence of modification, a change occurs in the phase composition of cast steel.

3. The effect of the modifier and its optimum addition depend on the cooling rate of the casting.