On the Question of Hardenability and Its Mechanism in Steels with Boron

A. I. Fal’kov

(Presented by Professor, Doctor A. N. Dobrovidov)

In the specialized literature, much attention has been devoted to questions of hardenability and its mechanism in steels with boron. However, despite the large number of studies, especially on hypoeutectoid steels, no unified opinion on this question has yet been established.

Regarding investigations of steels with boron abroad, it may be noted that they are being carried out on a large scale.

In the work of Lick [1], in which the author, setting forth his views, gives summarized material from well-known researchers, it is indicated that the chief significance of boron in steels is its influence on hardenability, and that this influence on increasing the stability of austenite differs from that of the alloying elements usually employed. However, as Lick further indicates, “...the mechanism of this is still not understood..., and in the published data much that is speculative has been proposed.”

In the domestic literature we also find many different and even contradictory views: on the questions of whether boron forms a solid solution of substitution or interstitial type, what the solubility of boron is in $\alpha$- and $\gamma$-iron, and so on. As for the question of the mechanism of hardenability of steels with boron additions, the contradiction in its explanation is most sharply expressed in the works of S. M. Vinarov [2] and V. I. Arkharov [3].

S. M. Vinarov’s conception [2, 4] of the slowing of the kinetics of austenite decomposition under the influence of small additions of boron is based on adsorption phenomena. According to this conception, the formation at grain boundaries of a monatomic layer of the adsorbed substance—boron—leads to a decrease in surface energy, which causes an increase in the stability of austenite.

V. I. Arkharov has shown [3, 5] that the conclusion that the hardenability of steel increases in connection with a decrease only in surface energy under the influence of small additions of boron is erroneous: with a decrease in surface energy, the critical size of the nucleus decreases, i.e., the nucleation of stable centers of crystallization of the new phase is facilitated, and in this case the hardenability of the steel should decrease.

Without denying that small boron impurities lower the surface energy, V. I. Arkharov indicates that, in order to increase the hardenability of steel, boron impurities, along with lowering the surface energy, must also reduce the amount released during the trans—

grouping of atoms, the volumetric free energy \((E_\gamma - E_\alpha)\), at the expense of which the work of forming the interface between the nucleus and the parent phase is performed.

If a number of other literature data are brought into the discussion of the question at hand, this makes it possible to form an understanding of the mechanism of hardenability that supports the viewpoint of V. I. Arkharov and, moreover, makes it possible to explain why boron additions should reduce the amount of volumetric free energy \((E_\gamma - E_\alpha)\) released during the rearrangement of atoms.

Let us note first of all that most investigations agree on one point, namely: on the preferential location of boron at grain boundaries, or more precisely—in intercrystalline transition zones [1, 4, 5, 7]. It has also been clearly established that, in terms of its beneficial effect on hardenability, boron is most effective in low-carbon steels. Therefore the view arose that boron hinders the formation of ferrite nuclei and does not exert a noticeable retarding effect on the nucleation of cementite in high-carbon steels.

Another general conclusion based on experimental data is that boron additions lead to a decrease in the melting temperature and the overheating temperature of steels.

K. M. Gelfand and Z. M. Gomolskaya [6], studying U10 steel with a boron content from 0.21 to 1.3%, found that the ES line is strongly shifted to the left under the influence of boron additions and that, for example, at a boron content of 0.5%, grain melting begins at a temperature of \(1200^\circ\).

In our investigation of the influence of boron on the properties of ShKh15 steel by heating at intervals of \(20^\circ\) in the range \(950\text{—}1250^\circ\), it was found that grain melting begins at \(1140^\circ\) when boron additions exceed 0.05% (by synthesis). Similar phenomena were established by Field’s investigations [11] with respect to overheating of steel, and with very small boron additions. Field found that with 0.003% boron the overheating temperature of structural steel decreases from \(1232\) to \(1204^\circ\).

As we shall see below, the phenomena associated with a decrease in the overheating temperature and the temperature of grain melting in boron-containing steel have a common origin. Therefore the mechanism of hardenability in boron-containing steels can be understood if, in addition to the general points mentioned above concerning the preferential location of boron in grain-boundary zones and the retarding effect of boron on the nucleation of ferrite nuclei, one also takes into account the ability of boron additions to lower the melting and overheating temperature of the metal.

It is known [8] that, in their structure, intercrystalline zones are regions of grains with a distorted lattice to a depth ranging from several hundred to tens of thousands of atomic layers, depending on how much the lattices and the principal crystallographic directions of adjacent grains differ from one another. The distorted crystal lattices of the intercrystalline interlayers are accompanied by a stressed state. The distortions are stress concentrators, and in the energetic sense these sites are less stable than the grain itself. Naturally, ordering, even partial, of the lattice of the intercrystalline interlayer, leading to the elimination of distortions and to a reduction of the stressed state, will increase the stability of this zone; i.e., the formation of ferrite nuclei will be hindered.

During ordering of the lattice in the intercrystalline interlayers, the same process takes place and the same driving forces act as are manifested during the recovery of cold-worked metal.

V. D. Kuznetsov [9] notes: “... if one imagines tempering as the result of atoms exchanging places in distorted regions of the lattice, leading to the elimination of these distortions, then it is clear that, for the possibility of exchanging places, atoms must possess kinetic energy at least equal to the height of the potential barrier separating one atom from another in the crystal lattice.”

Further, V. D. Kuznetsov says that, in order to exchange places with a neighbor, an atom must perform a definite amount of work overcoming the forces of interaction of the lattice. In turn, the forces of interaction of the lattice depend on its structure and on the structure of the elements entering into this lattice (molecules, atoms, ions). Taken together, these factors determine at the same time the melting temperature and, consequently, the height of the potential barrier, the magnitude of which is directly dependent on the melting point.

From these propositions of V. D. Kuznetsov, the important conclusion for us is that a lowering of the melting temperature of a metal is the result of a decrease in the bonding forces of the lattice, and that the energy of the potential barrier of the atoms (ions), the magnitude of which is directly dependent on the melting point, is also reduced.

If the metal of the grain boundaries, for some reason, acquires an even lower melting temperature, this means that the height of the potential barrier of the metal atoms of the boundary transition zone is correspondingly reduced as well. In this case it is logical to conclude that the exchange of atoms in the distorted regions of the lattice of the intercrystalline interlayer, and the elimination of distortions that are stress concentrators, will proceed more actively at austenitizing temperatures.

As applied to steels with boron additions, it must be assumed that the placement of boron impurities in the intercrystalline interlayers leads, as experimental data show, to a lowering of the melting point of the metal of these interlayers and, along with this, to a decrease in the energy of the potential barrier of the atoms (ions) of the metal of the intercrystalline zone.

The possibility of a change in the bonding energy in the lattice of iron alloys under the influence of boron is also indicated in the work of M. E. Blanter and N. P. Besedin [14].

The relation between the average kinetic energy of the thermal motion of particles (molecules, atoms, ions) and the absolute temperature is established by Boltzmann’s constant ($k$). As indicated earlier, the exchange of atoms and ordering of the lattice take place when the energy of the thermal motion of the atoms exceeds or is equal to the energy of the potential barrier. This condition is described by the equation $3kT \geq U$ (1), where $T$ is the absolute temperature, and $U$ is the energy of the potential barrier.

Suppose we have two specimens of steel of identical composition, with and without an addition of boron. In the steel without boron the energy of the potential barrier of the metal atoms of the intercrystalline zone is equal to $U_1$, and in the steel with boron—to $U_2$. From what has been said above, we may write that $U_1 > U_2$. At the same heating temperature of both specimens, $3kT - U_2 > 3kT - U_1$. (2).

From inequality 2 it is evident that the requirements following from equation 1 for the occurrence of the process of ordering of the lattice in the intercrystalline interlayers in boron steel are satisfied more fully than in steel without boron.

On the basis of the same considerations, it becomes clear why, in the presence of boron, at the heating temperature for quenching, austenit-

the grain is usually coarser [10, 11, 12] than the austenite grain in the same steel without boron.

Let us turn again to the equation \(3kT > U\). It is evident that the smaller the energy of the potential barrier \((U)\), the lower must be the temperature \((T)\) at which the process of atomic site-exchange activity and ordering of the lattice does not decrease. This indicates that, at identical austenitizing temperatures in steels with boron, the boundary zones of austenite grains are capable of assuming an ordered structure more rapidly than in steels without boron, and therefore in the former case more active grain growth occurs. This, in fact, explains the lowering of the overheating temperature of steel with small additions of boron, which was found in Field’s experiments.

As can be seen, the proposed mechanism of hardenability does not deny certain phenomena connected with the adsorption of boron impurities by crystallites. However, while explaining the cause of the increased concentration of boron in the intergranular layer, the adsorption theory does not reveal the mechanism itself of boron’s influence on hardenability, unless the change in the bonding forces of the crystal lattice of the transition zone is taken into account.

Incidentally, Adair, Spretnak, and Speiser [13] experimentally showed, using structural steels with a boron content of 0.0011—0.0013%, that the presence of the indicated modifying additions of boron, which produced a significant effect on hardenability, led to a decrease in surface energy by only 1.9 to 3.5% of the overall level. This, as the authors state, is far from sufficient to explain the influence of boron on the hardenability of steels solely by the phenomenon of a decrease in surface energy.

Conclusions

1. On the basis of data from a number of studies, it may be considered established that the preferential location of boron in intercrystalline layers causes a lowering of the melting point of this layer, as well as of the overheating temperature of steel, including with very small boron additions.

2. The lowering of the melting temperature of the metal of the transition zone indicates a decrease in this zone of the bonding forces of the crystal lattice and of the energy of the potential barrier of the atoms; therefore, at austenitizing temperatures in steels with boron, one should expect greater ordering of the lattice of the transition zone and elimination of defects than occurs in the same steels without boron.

3. The decrease in the energy of the potential barrier of atoms in the transition zone of austenite grains means that, during the rearrangement of atoms at the initial stage of crystallization and formation of ferrite nuclei, the amount of liberated free volume energy required for the work of forming the interfaces of the nucleus is reduced. This effect, in combination with the greater ordering of the crystal lattice of the transition zone, provides increased stability of austenite in steels with a boron addition.

LITERATURE

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3. Akharov V. I. Journal of Technical Physics, vol. XVII, issue 2, 332—335, 337—339, 1952.
4. Vinarov S. M. Journal of Technical Physics, vol. XIX, issue 2, 1949.

1. Arkharov V. I. Proceedings of the Central Scientific-Research Institute of the Ministry of Transport Machine Building of the USSR, No. 5, 25—27, 1946.

2. Gelfand K. M. and Gomolskaya Z. M. Cast and Surfaced Tool. VNITOMMASH, Moscow Branch, MASHGIZ, 1951.

3. Prosvirin V. I., Kreshchanovskii N. S., Ginzburg E. S. TsNIITMASH. Questions of the Metal Behavior of Austenitic Steels. MASHGIZ, 1952.

4. Bernshtein. Proceedings of NTO of Ferrous Metallurgy, Vol. VI, 1955.

5. Kuznetsov V. D. Physics of the Solid State, Vol. II, Tomsk, Krasnoe Znamya Publishing House, 479, 486, 1941.

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7. J. Field, Metal Progress 64, No. 2, 1953, 78—83.

8. L. Rohl, Iron a Steel Instit., 176, Part. 2, 1954, 173—187.

9. Ader, Spretnak, Speiser. Abstract Journal “Physics,” No. 2, 1956.

10. Blanter M. E. and Besedin N. P., Metallurgy and Heat Treatment of Metals. 6, Mashgiz, 1955, 3—10.