A. N. Dobrovidov.

Quenching of Tool Steel in Warm Water.

(From the metallographic laboratory of T. T. I.).

(The article contains 4 drawings).

Tool carbon steel is an alloy of iron with carbon, with the latter present in an amount of 0.6–1.7%, or, correspondingly, 8–18% carbide \(Fe_3C\). A distinction is usually made between hypoeutectic and hypereutectic steel; the boundary is purely eutectic steel with a carbon content of 0.95%. The degree of quenching of steel depends directly on the cooling rate. Instantaneous, or sufficiently rapid, cooling gives hardened steel, consisting of a solid solution of carbide in iron. This solution is called austenite. If the cooling of the steel is slowed, decomposition of the austenite and the formation of new structural elements are possible, corresponding to various degrees of tempering. In technology this method of tempering is sometimes used, but more common is quenching without tempering followed by tempering, characterized by the temperature of the secondary heating. If the heating of the steel before quenching was not excessively high and did not exceed by much the temperature of the critical point (during heating), then austenite is absent and is replaced by a supersaturated martensitic pseudo-solution, which is a product of the degeneration of austenite. Complete tempering—annealing—converts martensite and austenite, in the case of hypoeutectic steel, into ferrite and pearlite, and in the case of hypereutectic steel into cementite and pearlite. The austenite of eutectic steel passes during tempering into pearlite. The intermediate structural elements corresponding to definite tempering temperatures are troostite, osmondite, and sorbite. The natural order of transformation is thus: austenite—troostite—osmondite—sorbite—pearlite (or pearlite + ferrite or cementite). In this connection osmondite is considered to be a pseudo-solution of carbide in iron \(\alpha\) and \(\beta\), obtained under conditions that allow the phenomena to proceed freely. Troostite is a transitional form from martensite to osmondite, while sorbite is a fully decomposed solid solution, i.e., a mixture of carbide and iron with a vaguely expressed differentiation. It must be noted that, for practice, the tempering of steel is of interest chiefly in the interval from \(200^\circ\) to \(350^\circ\); the structures obtained thereby do not give an entirely clear picture under the microscope. It is not possible to determine the degree of tempering within these limits by using only the microscope. The purpose of the work was to find the dependence between the temperature of the warm water in which quenching was carried out and the degree of quenching. For comparison, experiments were first conducted with the quenching of steel and its tempering at various temperatures. A scale of differently tempered specimens was obtained and their physical properties, altered by heat treatment, were determined. Quenching very significantly changes the density of steel, the resistance to fracture \((K_b)\), and the “hardness,” determined by the Shore scleroscope. The concept of hardness is not altogether aptly applied in this case; therefore the reading of the apparatus, determined as the average of 10 observations, will be called the “Shore number.” For the work, carbon tool steel from the Zlatoust plant of the “medium-hard” grade was taken. Analysis carried out by the author showed a content of \(C = 0.95\%\), \(Mn = 0.35\%\), \(Cr = 0.10\%\), \(Si = 0.07\), with traces of \(S\) and \(P\). In order to conduct the experiments with one and the same initial material, the steel rods (round section; \(d = 3/4''\)) were carefully annealed in a Tereus furnace at a temperature of \(760^\circ C\). The critical points of this steel lie at temperatures of \(740^\circ\) and \(710^\circ\). From the annealed pieces, on

prisms of square cross-section, with dimensions \(5.00 \times 50\) mm, were made on a milling machine. The dimensions of the square were obtained with an accuracy of \(\pm 0.03\) mm. Slightly ground specimens were used for the tests. To heat them in a horizontal Heraeus furnace, a solid cylinder of the same steel was placed, into one end of which was inserted the junction of a previously verified platinum–rhodium Le Chatelier pyrometer, and into the other—a steel tube with the specimen. This tube was made of the same steel as the specimen and had a long handle. These arrangements made it possible to hope that the specimen was heated quite uniformly and could not undergo surface decarburization. The furnace was heated to \(760^\circ\), and with the aid of a rheostat this temperature was maintained uniformly throughout the experiments; fluctuations did not exceed \(\pm 5^\circ\). The specimen was held at this temperature for 12 minutes, removed together with the tube, grasped with flat-nosed pliers whose jaws were wrapped with asbestos, and quenched. In this operation its temperature did not fall below \(750^\circ\), as calorimetric measurements showed. In tap water, taken in an amount of 10 kg at a temperature of \(10^\circ \pm 0.5^\circ\), 30 specimens were quenched; several of these developed longitudinal cracks, apparently owing to improper immersion. The quenched specimens were lightly ground by hand on No. 000 carborundum paper. From 10 observations the Shore number and the specific gravity were determined. The experiments showed that the Shore number is affected not only by heat treatment, but also by the dimensions, especially the cross-section, of the specimens. More or less comparable results were obtained with specimens of the same cross-section but of different length. The specific gravity was determined on Sartorius balances with an accuracy of 0.0001 and was reduced to water at \(t = 4^\circ\) and to the specimen at \(t = 0^\circ\). This was the most painstaking part of the work. Tempering was carried out for 10 minutes in baths of various composition. Temperatures of \(160^\circ\) and \(200^\circ\) were obtained in a bath of 5 kg of American cylinder oil; \(220\)–\(550^\circ\) in a bath of 10 kg of mercury. After tempering, the Shore number, the specific gravity, and the temporary resistance in bending \(K_b\) were determined. The temporary resistance was determined on an Amsler machine with Gagarin’s kg apparatus. The results are presented in Table 1 and Fig. 2.

In the manufacture of tempered steel articles, negative quenching has certain advantages over the ordinary method of work, i.e., quenching and tempering to a straw-yellow temper color. Many articles cannot be tempered during quenching by making use of the heat remaining inside the part (tempering from the inside outward), and it becomes necessary to resort to subsequent tempering in heated media. This complication naturally makes the process more expensive. The smallest change in volume corresponds to the strongest quench, i.e., to a martensitic structure. Intermediate structures differ less in density from unquenched steel. The greatest change in volume also causes a maximum of internal stresses and, consequently, a greater amount of quenching scrap. With negative quenching, as an incomplete one, the changes in volume are naturally smaller. Twenty specimens were quenched in the same tap water, heated to various temperatures. The results are presented in Table 2 and Fig. 3.

A comparison of the curves shows that the physical properties of quenched and tempered and negatively quenched steel are very different. Portions of the curves (in Fig. 2) from \(280^\circ\) to \(400^\circ\) correspond well with portions of the curves (in Fig. 3) from \(55^\circ\) to \(75^\circ\). The transformed curves are presented in Fig. 4. The curves almost merge in the part corresponding, during tempering, to the formation of osmondite. The noncoincidence of the curves at low temperatures should be explained by the possibility of obtaining a structure, one of whose constituents is brittle and light martensite. This once again emphasizes the different sequence of transformation of martensite into pearlite during tempering than during cooling.

Tomsk.—1923.

Table 1.

Quenching in cold water and subsequent tempering.

Note to the table. The temperature of the specimen before quenching was \(t_0 = 760^\circ \pm 5^\circ\); the water temperature \(t_8 = 10^\circ + 0.5^\circ\).

Table 2.

Quenching in warm water.

To A. N. Dobrovidov’s article, “Quenching of Tool Steel in Warm Water.”

Drawing 1.

To A. N. Dobrokhotov’s article: “Quenching of Tool Steel in Warm Water”

Tempering of Hardened Steel

Fig. 2

Specific gravity.

\(\dfrac{V}{V_{\max}}\)

\(K_n\).

Hardness number.

Temperature, °C: 20, 100°, 200°, 300°, 400°, 500°, 600°, 700°.

To the article by A. N. Dobrokhotov: “Quenching of Tool Steel in Warm Water.”

Fig. 3.

Quenching in warm water.

Specific gravity.

\(K\), steel.

\(K\), kg/mm².

\(h_c\).

Shore number.

Temperature scale: \(10^\circ,\ 20^\circ,\ 30^\circ,\ 40^\circ,\ 50^\circ,\ 60^\circ,\ 70^\circ\).

To the article by A. N. Dobrovidov, “Quenching of Instrument Steel in Warm Water.”

Tempering of steel and quenching in warm water

Fig. 4.

Visible graph labels:

• \(K\), kg/mm²
• Tempering.
• Water temperature.

Visible scale markings and annotations:

• \(60,\ 70,\ 80\)
• \(100,\ 200,\ 300\)
• \(7800,\ 7900,\ 8000,\ 8100,\ 8200,\ 8300\)
• \(200^\circ,\ 300^\circ,\ 320^\circ,\ 350^\circ,\ 400^\circ\)
• \(55^\circ,\ 60^\circ,\ 65^\circ,\ 70^\circ\)