On Critical Points of Steel

I. V. Slyusman

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

Introduction

Heat treatment of steel is becoming ever more widespread in our industry, since it makes it possible to change the properties of steel within very broad limits. Although the practice of such treatment has been known for many centuries, a scientific explanation of the processes that occur in it became possible only as a result of the discovery of the critical points of steel.

This discovery was made 90 years ago by our outstanding compatriot Dmitrii Konstantinovich Chernov.

The critical points of steel discovered by D. K. Chernov

In 1866, in order to improve the quality of steel guns, the engineer-technologist D. K. Chernov was invited to the Obukhov Plant in Petersburg. After two years of intensive work, he not only successfully solved the specific production problem set before him, but also made, in the process, a discovery of exceptional theoretical significance, on which he reported in 1868 at a meeting of the Russian Technical Society [1]. This date—1868—should justly be regarded as the date of the beginning of the existence of the science of the laws governing changes in the structure and properties of steel during its heating and cooling, the science of heat treatment of steel.

D. K. Chernov began to solve the problem set before him by carefully examining the fractures of steel in guns that had failed prematurely and in guns that had produced a large number of shots. He established thereby that in the first case the fracture was coarse-crystalline in character, and in the second—fine-crystalline. Continuing observations of the heating, forging, and cooling of steel billets, D. K. Chernov came to the conclusion that the opinion, current among metallurgists of that time, that the character of the fracture depended only on the degree of deformation of the metal, was erroneous; that, by means of proper heat treatment, without any deformation, it was possible to obtain in steel products an extremely fine-grained fracture and the best mechanical properties.

In his report D. K. Chernov indicated that when steel is heated, when it passes through certain definite temperatures,

internal transformations take place in it, changing the structure and properties of the steel. There are two such temperatures. D. K. Chernov called them “special points” \(a\) and \(b\). Later they came to be called, in his honor, Chernov’s critical points.

Characterizing the significance of the critical points \(a\) and \(b\), D. K. Chernov said:

“The significance of point \(a\) is as follows: steel, however hard it may be, when heated below point \(a\), does not take a quench, however rapidly it is cooled; on the contrary, it becomes considerably softer and is more easily worked with a file.”

“The significance of point \(b\). Steel, when heated below point \(b\), does not change its structure—whether it is subsequently cooled slowly or rapidly.”

Critical points of steel discovered after D. K. Chernov

The discovery of the critical points \(a\) and \(b\) quickly attracted the attention of metallurgists throughout the world. It showed the way to other scientists, who devoted their work to the study of the transformations that occur in steel during its heat treatment.

The first to follow this path was the French metallurgist F. Osmond. If, however, D. K. Chernov had to determine the position of the critical points by eye, from the color of the incandescent surface of a steel ingot, then F. Osmond, who carried out his experiments in the 1880s, was already able to use Le Chatelier’s thermoelectric pyrometer.

Using this pyrometer, F. Osmond found that steel in the solid state has three critical points. He proposed designating them as the critical points \(A_1\), \(A_2\), and \(A_3\).

After the publication of F. Osmond’s work, the critical point \(A_4\) was found in Germany, and somewhat later in Japan—the critical point \(A_0\) [2]. In addition to the critical points listed, in steels of hypereutectoid composition another critical point, \(A_{cm}\), is also noted. Researchers in various countries worked on establishing the temperature dependence of \(A_{cm}\) on the carbon content in steel, including Professor N. V. Gutovskii of the Tomsk Polytechnic Institute [3].

On Chernov’s point \(b\)

We have already cited the definitions that D. K. Chernov himself gave to points \(a\) and \(b\). As for point \(a\), D. K. Chernov indicated its nature with sufficient definiteness, and no disputes arose in the literature in this connection.

The fate of point \(b\) turned out differently. Familiarity with all of D. K. Chernov’s works relating to this question shows that D. K. Chernov indicated the nature of point \(b\) with less definiteness. This gave rise in the metallurgical literature to a discussion which began during D. K. Chernov’s lifetime and continues to this day.

In view of the great importance of point \(b\) for the theory and practice of the heat treatment of steel, it seems very interesting to compare certain considerations expressed with respect to this point in the most recent years with what was said about it earlier.

Among the various opinions on the nature of point \(b\), the opinion most often expressed, then and now, has been that it is identical with Osmond’s point \(A_3\). D. K. Chernov himself showed a definite inclination toward such an identification, at least in some of his articles.

and reports [4, 5]. However, both during D. K. Chernov’s lifetime and after his death, other views were also expressed concerning the nature of point \(b\). In 1911–1916, in the pages of the journal of the Russian Metallurgical Society, a discussion on the nature of point \(b\) took place between two well-known metallographers, A. L. Baboshin and P. Ya. Saldau [6–9]. The discussion began with the appearance, in the first issue of the journal for 1911, of an article by A. L. Baboshin, in which he argued against the general opinion that points \(b\) and \(A_3\) are identical.

A. L. Baboshin experimentally established the dependence of the size of the pearlite grain on the temperature to which steel is heated in the austenitic region. It turned out that the curve expressing this dependence has a break: after a certain temperature is reached (about \(1000^\circ\)), grain growth accelerates (Fig. 1). In connection with these experimental data, A. L. Baboshin wrote that point \(b\) is not Osmond’s point \(A_3\), that it has independent significance, namely—that point \(b\) is the boundary beyond which, with increasing temperature, grain growth accelerates [6].

Fig. 1. Dependence of pearlite grain size on the heating temperature of steel in the austenitic region.

P. Ya. Saldau objected to this interpretation of point \(b\), asserting that point \(b\) is nothing other than Osmond’s point \(A_3\) [7].

In response to this discussion, in the November issue of the journal for 1916, a letter from D. K. Chernov to the editor of the journal, M. A. Pavlov, was printed. In his letter D. K. Chernov wrote:

“Point \(a\) was determined by me from the appearance of hardening; point \(b\)—from the transformation of a coarse-crystalline (physical) structure into a barely perceptible fine-crystalline one (by fracture).”

Having further expressed the supposition that point \(b\) represents the temperature at which steel during heating passes from the crystalline to the amorphous state, and drawing an analogy between the process of reverse transition from the amorphous to the crystalline state and the process of crystallization from a liquid mass, D. K. Chernov wrote:

“Here I consider it possible to establish a certain conventional reconciliation between Chernov’s point \(b\) and Osmond’s point \(A_3\): if the direct transition (on heating) through point \(b\) is denoted by \(+b\), and the reverse transition (on cooling) by \(-b\), then \(A_{c3}=+b\); \(A_{r3}=-b\); \(A_3=b\).

Having established an external equality between both designations, I shall dwell on their internal difference. As is known, in thermal analysis the designation of critical points on the thermometric scale can be reproduced automatically by a self-recording pyrometer, quite independently of exactly what process is occurring in the specimen under test, provided only that it is accompanied by thermal phenomena; the essence of the process, however, is controlled by chemical, physical, or mechanical investigation of the specimen. In the particular case, the structural transformations corresponding to the first and third waves of Osmond’s diagram are controlled chiefly, one may even say almost exclusively, by metallographic …”

by this method. The latter gives direct indications of the degree of homogeneity of the chemical composition of the metallic mass of the specimen, the degree of its heterogeneity, the form and character of the distribution of its constituent parts (components) in the case of segregation having occurred—in a word, it reveals the chemical structure of the metal.

It is necessary to dwell somewhat on the distinction between the chemical and physical structure of any crystalline mass of complex composition.” And further, in the same letter: “... transformations of the chemical structure of iron-carbon alloys, segregation and diffusion occurring in them, may take place with complete preservation of the crystalline (physical) structure of the metallic mass. Hence it follows that the pattern of an etched section, for example, of a given piece of steel, cannot serve as an indicator of its physical structure. On the other hand, the appearance of a steel fracture, directly showing its more or less developed coarse- or fine-crystalline constitution, cannot serve as an indicator of chemical structure. Here, too, the internal distinction between Osmond’s point \(A_3\) and Chernov’s \(b\) appears: since the chemical transformation (segregation) proceeds slowly and successively, the phenomenon of the \(A_3\) wave and its meaning denote the transformation inherent in Chernov’s point \(b\), and not Osmond’s \(A_3\).”

The cited letter makes it possible to draw the following conclusions about the views of its author on the nature of point \(b\).

The author of the letter definitely contrasts his point \(b\) with Osmond’s point \(A_3\). It is quite obvious that anyone who after this continues to identify points \(b\) and \(A_3\) shows an unwillingness to take into account, in this matter, the opinion of D. K. Chernov himself.

The author of the letter distinguishes the chemical and physical nature of steel. He indicates that the thermal method of investigation used by F. Osmond makes it possible to detect structural transformations controlled by metallographic means. The fracture method, however, by which D. K. Chernov found the critical point \(b\), makes it possible to establish only changes in the crystalline (physical) structure.

In his reports under the general title “Aphorisms from the Field of Steel Metallurgy,” D. K. Chernov also indicates that “... the process of structural transformation at point \(b\) is not connected with chemical transformations between iron and carbon, but represents a purely physical phenomenon [5].

What, then, does this “purely physical phenomenon” consist in? What causes the change in the structure of steel as seen in fracture, without a change in “chemical structure” (without phase transformations)?

Recently, works have appeared in the literature that provide an answer to these questions. We have in mind the articles of the Ural metallographers [11, 12].

In these articles, not only is the distinction between points \(b\) and \(A_3\) established, but a new interpretation is also given of what precisely should be understood by point \(b\). The authors report that in recent years a number of works have been carried out at the Ural Branch of the Academy of Sciences in which the method of comparing steel fractures was widely applied, i.e., the same research method that D. K. Chernov used and which, in connection with the successes of microscopic metallography, came very often to be underestimated.

The authors established that if strongly overheated steel with a coarse-crystalline fracture is heated to various temperatures, then correction of the structure (by fracture) occurs not after passing through the temperature \(A_{c3}\), but at a higher temperature, which is Chernov’s point \(b\). The fine grains of austenite that formed

immediately following the \(\alpha \to \gamma\) transformation, within a certain temperature interval lying above \(A_{c3}\), retain their general orientation (texture) within the limits of individual large grains of the initial structure. Only heating to a higher temperature (to point \(b\)) causes the elimination of the intragranular texture and the transition to a fine-crystalline fracture. This transition is a purely physical process—the process of recrystallization of small austenite grains, which acquire strain hardening in connection with the volume changes accompanying the \(\alpha \to \gamma\) transformation.

The materials obtained by the Ural metallurgists did indeed allow them to interpret the significance of point \(b\) in a new way. At the same time, in this new interpretation the validity is preserved of those definitions of point \(b\) that D. K. Chernov himself gave in the “aphorisms” and the “letter” (if one excludes his assumption about the amorphous state of steel above point \(b\)).

The significance assigned by the authors to point \(b\) leaves workers in metallurgical science the possibility of further studying questions concerning the influence, on the position of this point, of the carbon content in steel and of other elements, the rate of heating and, possibly, still other factors on which the recrystallization temperature of strain-hardened austenite grains depends.

The works of the Ural metallurgists are unquestionably a very interesting study of the nature of point \(b\). However, the question of the nature of this point will probably be resolved definitively only when researchers succeed, simultaneously with the performance of other measurements, in obtaining also the external signs of the passage of steel through point \(b\) that D. K. Chernov observed: wrinkling of the surface of a cooling ingot and the transformation of this surface from shiny, marble-like, to dull, gypsum-like [13].

Conclusion

The study of the critical points of steel, begun by D. K. Chernov 90 years ago, led ultimately to the development of the constitution diagram of the iron–carbon system. It is used daily by the broadest circles of workers in scientific and practical metallurgy. The modern form of this diagram has been obtained as a result of numerous investigations by scientists of various countries. But it is enough to compare the relationships established by D. K. Chernov with the “steel” region of the diagram, and we become convinced that D. K. Chernov said all the most important things about steel. Having made such a comparison in the already mentioned letter to the editor of the ZhRMO (Fig. 2), D. K. Chernov had every reason to write:

“In essence there is no difference; in particular, two lines have been added: the path of Osmond’s critical point \(A_2\) and the hypothetical line of separation of cementite (hypereutectoid).”

Fig. 2. Chernov’s critical points [1] and the “steel” portion of the diagram of state of the iron–carbon system.

Metallurgists throughout the world recall with deep gratitude, and for a long time yet will recall, the name of our great metallurgist Dmitrii Konstantinovich Chernov, to whom they owe knowledge of the fundamental laws involved in the heat treatment of the most important industrial alloy—steel.

REFERENCES

1. D. K. Chernov. Journal of the Russian Metallurgical Society, 1, 1915.
2. N. T. Gudtsov. Fundamentals of the Physical Metallography of Steel, 1939.
3. N. V. Gutovskii. Metallurgie, 22, 1909.
4. D. K. Chernov. Steelmaking. Collection “D. K. Chernov,” Mashgiz, 1950.
5. D. K. Chernov. On Chernov’s point “b.” Collection “D. K. Chernov and the Science of Metals,” Metallurgizdat, 1950.
6. A. L. Baboshin. Journal of the Russian Metallurgical Society, 2, 1911.
7. P. Ya. Saldau. Journal of the Russian Metallurgical Society, 5, 1915.
8. A. L. Baboshin. Journal of the Russian Metallurgical Society, 1–2, 1916.
9. P. Ya. Saldau. Journal of the Russian Metallurgical Society, 1–2, 1916.
10. D. K. Chernov. Journal of the Russian Metallurgical Society, 3–4, 1916.
11. L. V. Smirnov and V. D. Sadovskii. Proceedings of the Institute of Metal Physics, Ural Branch of the Academy of Sciences, 18, 1956.
12. V. D. Sadovskii, K. A. Malyshev, and B. G. Sazonov. Proceedings of the Academy of Sciences of the USSR, Division of Technical Sciences, 1953.
13. A. A. Baikov. Steel, 10–11, 1939.