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100 was formerly at about 900 deg. G. has been lowered to about 840 deg. C. Also, these two points appear at somewhat lower temperatures on the cooling curve than the}- did on the heating curve. The intermediate critical point remains at about 768 deg. C. It is therefore evident that the presence of carbon has a decided influence in determining the critical points of steel. Since the critical points are closely connected with the changes of structure and properties FIG. 65—COLD WORKING, (a) Annealed steel, carbon about 0.40 per cent, (b) Same after severe cold rolling. (c) Steel similar to that in (a), but heat treated to sorbitic state and then cold drawn into wire. All longitudinal sections (500x.) that take place in hardening, and since these points are influenced by the amount of carbon present and also by other elements, as will be discussed later, a careful study of the nature and occurrence' of these points throws a great deal of light on the behavior of steel in hardening. Symbols for Critical Points. For convenience in referring to the critical points of steel, they are designated by certain symbols. These were originated by French investigators and Fbrging-Stamping - Heat Treating March, 1925 are abbreviations of French terms. As the critical points are manifested by an arrest of temperature in the heating or cooling curves they are indicated by the letter "A", from the French word "arret", meaning arrest or stop. The arrests on heating are referred to by the symbol "Ac", in which "c" stands for the French word "chauffage", heating. The arrests on the cooling curve are known as "Ar" points, "r" standing for the French word "refroidissement", or cooling. The points are numbered 1,2, 3, in the order in which they take place on heating. The Ar3 point, therefore, which occurs on the cooling curve, is the reverse of the Ac3 point, which occurs on heating, and so on. Critical Point Diagram. Suppose that a chart is made in which the carbon content is laved off horizontally and temperature vertically, as in Fig. 69. Each vertical line on the diagram will represent steel of a certain carbon content, and on this line we may mark the critical points of that particular steel as found by heating or cooling curves. The points for steels of various carbon contents would be marked similarly on the vertical line corresponding to the carbon content and at a height corresponding to the temperature at which they occur. Lines would be drawn connecting the corresponding points for the various steels. This is called a "critical point" diagram. The Ar points generally occur at about 20 deg. to 40 deg. G lower than the corresponding Ac points, in a piece which is cooled at a moderate rate of speed. This is due to a "lag" in the transformation of the metal, a certain amount of time being necessary to complete the change. The lag also occurs on heating, so that the Ac points will appear higher if heating is more rapid. If heating and cooling were extremely slow, the Ac points and their corresponding Ar points would (theoretically, at least) take place at the same temperature. The presence of certain alloying elements greatly increases the lag or gap between the Ac and Ar temperatures. For simplicity, only the Ac points are plotted in the chart, Fig. 69, at the temperatures at which they occur in plain carbon steel on very slow heating. Since the presence of carbon is necessary to produce the Acl point, carbonless iron has no such point. The point Ac3 occurs at lower temperatures with increasing carbon, up to a carbon content of about 0.90 per cent, where it joins or merges with the Acl point. The latter point (Acl) remains practically constant with increasing carbon content and follows a horizontal line at about 725 deg. G (1337 deg. F.). The critical point Ac2 remains constant at about /68 deg. G (1414 deg. F.) with increasing carbon content until it coincides with the Ac3 point, which has come down to this temperature for a carbon content of approximately 0.35 per cent. From here the Ac2 point follows the Ac3 point, until both run into Acl at 0.90 per cent carbon. All three points then occur together in steels having a carbon content higher than 0J0 per cent giving a horizontal line on the diagram which is known as Acl, 2, 3. This point is often so pronounced that, under favorable conditions, a piece of slowly cooled tool steel may be seen to glow a brighter red as it passes through Arl, 2, 3. Heating and cooling curves taken with very sensitive instruments on steels whose carbon content is over 0.90 per cent will reveal another critical point just above the Acl, 2, 3 point for steel having some-
March, 1925 F<strong>org</strong>ing-Stamping - Heat Treating 101 what more than 0.90 per cent carbon and which rises rapidly with increasing carbon content, as shown in the chart. This is known as the Acm point, and will be discussed later. The range of temperature which lies between the upper and lower critical points, namely, between the lines Acl and Ac3—Ac 2, 3 or between Acl, 2, 3 and g eoo ^ 7°o
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January, 1925 BEAUDRY UTILITY HAMME
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