On the Hardenability of Gray Cast Iron

L. M. Sedokov

(Presented by the scientific seminar of the Department of Strength of Materials)

Computational equations for determining the cutting force on the basis of the physical essence of the phenomenon require reliable physicomechanical characteristics of the metals being machined, including their hardenability in the process of plastic deformation.

Obtaining the hardening characteristic for ductile metals presents no difficulties. However, the experimental determination of the increase in strength of brittle materials in the process of deformation is very difficult, since brittle materials, under ordinary testing methods, fail without significant plastic deformation.

Taking into account the large volume of machining of gray-cast-iron castings on metal-cutting machine tools, it is necessary to examine specifically the question of its hardenability in the processes of mechanical testing and cutting.

The authors of a textbook for machine-building institutes on the theory of metal cutting [1], referring to the investigations of N. F. Kiselev and V. A. Krivoukhov, indicate that, in the case of compression of specimens of gray cast iron, as also in the deformation of steel, there is a polytropic relationship. If, for example, a cylindrical specimen is compressed by an increasing force \(P\), while the remaining height \(h\) is measured, then the compression process will obey the polytropic law [2]

\[ Ph^{m}=P_{0}h_{0}^{m}. \]

Here \(h_{0}\) is the initial height of the cylinder;

\(P_{0}\) is the conditional compressive force, i.e., the force beginning with which the first residual deformations would appear if the polytrope of compression were valid from the very beginning of compression.

In this case, for gray cast iron the exponent of the compression polytrope—\(m\)—has the following values (Table 1).

Unfortunately, examples are not given for calculating the cutting force from the cited characteristics of the hardening of gray cast iron, nor are the results indicated of comparing calculated and experimental values of the cutting force in machining brittle materials.

The authors of the textbook [1] do not give values of the conditional compressive force \(P_{0}\) for cast irons for which the indicated values of the exponent are given.

Table 1

of the compression polytrope, which makes it impossible to use the obtained hardening characteristics of gray cast iron for practical calculations.

At the same time, it should be noted that the cited characteristics of the hardening of gray cast iron were evidently obtained in the range of very small degrees of plastic deformation, which occurs in ordinary mechanical tests of brittle metals; and at small degrees of plastic deformation there is no polytropic dependence even for ductile materials, which led to the introduction of the conventional compressive force $P_0$ [2].

This explains the obtaining of very high values of the exponent of the compression polytrope for gray cast iron, which turned out to be higher than for ductile metals known for their great tendency to hardening. Thus, for example, according to the experiments of M. A. Bolshanina [2], this exponent for copper is only 1.3.

In view of the foregoing, it became necessary to determine experimentally what the magnitude of the hardening of gray cast iron is under its considerable deformation, such as occurs in cutting.

In the Strength of Materials Laboratory of the Tomsk Polytechnic Institute, an experimental study of the question of the hardening of gray cast iron was carried out according to the method developed by Prof. B. D. Grozin [3], and also with the aid of conical collars.

Cast-iron specimens were compressed in copper collars, which made it possible to obtain considerable degrees of plastic deformation of the cast-iron specimens. On the basis of the calculations recommended by Prof. B. D. Grozin [3] in processing experimental data, a diagram of compression of cast iron under conditions of a complex stress state was obtained (Fig. 1).

From the graph shown it is evident that a cast-iron specimen compressed in a copper collar gives a twofold increase in the degree of plastic deformation corresponding to the maximum value of the true compressive stress, as compared with ordinary compression tests (without a collar—8%, and in a collar—15–18%).

Fig. 1.

However, the increase in the true maximum compressive stress of the cast-iron specimen in the copper collar, in comparison with ordinary tests, amounted to only 10–12% ($54\ \mathrm{kg/mm^2}$ without a collar and $61\ \mathrm{kg/mm^2}$ in a collar).

Consequently, the increase in the strength of gray cast iron in the course of plastic deformation is very small.

To verify this conclusion further, a series of experiments was carried out with conical crushers made of various metals. The conical crusher, the drawing of which is shown in Fig. 2, is compressed in a press; at each loading stage the force—\(P\), the imprint area on the deformed part of the cone—\(F\), and the Brinell hardness on the resulting surface of the truncated cone are recorded.

For comparison, conical crushers were taken from ÉYa1T stainless steel, which in the process of plastic deformation strain-hardens very strongly (the compression polytropy index for it is 1.45), from steel 3, and from gray cast iron with hardness \(HB = 150 \div 160\). The results of these experiments are shown in Figs. 3, 4, and 5.

Fig. 2.

From the graphs presented it is evident that the contact stress during compression of a conical crusher made from a given metal remains constant. Likewise, at all loading stages the hardness on the deformed part of the crusher also remains constant, although the value of this hardness will differ from the initial one.

The ability of a material to strain-harden during plastic deformation, in our opinion, can be characterized by the relative increase in hardness during deformation of conical crushers made from these materials.

Table 2

Taking into account that the hardness of the deformed specimen is linearly related to the compressive stress [4], one can calculate the degree of plastic deformation occurring when a cone with an apex angle of \(90^\circ\) is compressed. According to these calculations, the relative shear in compressing such a cone is approximately equal to 2 units.

At such a considerable degree of plastic deformation, cast iron showed strengthening on the order of 14%, steel 3—36%, and ÉYa1T steel—120%.

Consequently, specimens of gray cast iron, under special test conditions that make it possible to attain high degrees of plastic deformation, showed a very weak increase in the initial strength, considerably smaller than for ÉYa1T steel and steel 3.

This conclusion, confirmed by the results of special investigations using two different methods, has very important practi-

Fig. 3.

Fig. 4.

Fig. 5.

... practical significance, since it makes it possible to calculate the cutting force in the machining of gray cast iron from its initial hardness, i.e., without taking into account the degrees of plastic deformation, which is also confirmed by the results of experiments on turning and milling gray cast iron [5].

Since determining the hardenability characteristics of brittle materials in the process of plastic deformation by the usual methods of mechanical testing is very difficult, the method of deforming conical indenters with subsequent measurement of the hardness of the deformed part of the indenter should be recommended for this purpose as a very simple and reliable method. The proposed method can also be used to determine the hardenability of ductile metals. The hardenability characteristic of a metal during plastic deformation may be taken as the relative increase in the initial hardness of the metal when deforming an indenter with an apex angle equal to \(90^\circ\).

REFERENCES

1. Granovskii G. I., Grudov P. P., Krivoukhov V. A., Larin M. N., Malkin A. Ya., Cutting of Metals. Mashgiz, 1954.

2. Kuznetsov V. D., Bolshanina M. A., Physics of the Solid State. Vol. II. Tomsk Publishing House “Krasnoe znamya,” 1941.

3. Grozin B. D. Mechanical Properties of Hardened Steel. Mashgiz, 1951.

4. Rozenberg A. M. and Khvorostukhin L. A. Hardness and stress in a plastically deformed body. Journal of Technical Physics, Vol. XXV, No. 2, 1955.

5. Rozenberg Yu. A. and Sedokov L. M. On the calculation of cutting force in turning and milling gray cast iron. Vestnik mashinostroeniya, No. 12, 1957.