REPORT ON THE RESULTS OF STUDIES TO DETERMINE THE IMPACT HARDNESS OF STEEL 45

V. P. Shubin

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

Steel 45 is widely used in the structures of various machines, including in the manufacture of impact-action machines. The durability and reliability of parts of impact-action machines depend in many respects on hardness. In the Strength of Materials Laboratory of the Tomsk Polytechnic Institute, mass tests of the impact hardness of steel 45 were carried out on a vertical Amsler impact tester by indenting a cone with a point angle of 90°, falling from a specified height.

The magnitude of the indentation obtained when the cone is driven in was related only to that impact energy which is obtained as the difference between the energies of the falling and rebounding striker, since the rebound energy has no relation to the measured indentation. The energy difference was determined using the diagram apparatus of the impact tester, set in rotation by a special motor.

Fig. 1.

The impact diagrams have the form shown in Fig. 1, where \(mn\) is the axis of the circumferential velocities of rotation of the drum; curve \(AB\) is the line of fall of the striker; curve \(BC\) is the line of rebound of the striker after impact.

Knowing the number of revolutions of the drum \(n\) and its diameter \(D\), one can determine the speed of the drum in a given test:

\[ V_{\text{B}}=\frac{\pi \cdot D \cdot n}{60 \cdot 1000}\ \text{m/sec}. \tag{1} \]

To determine the velocity of motion of the striker corresponding to any point of the rebound curve \(BC\), it is necessary to multiply the rotational speed of the drum \(V_{\mathrm{b}}\) by the tangent of the angle between the tangent at the given point of the curve and the axis \(mn\).

Drawing a tangent at the beginning of the rebound of the curve \(BC\), we lay off from point \(D\) along the line \(mn\) 50 mm to point \(E\), at which we erect a perpendicular to the line \(mn\). Having measured the length of the line \(EF\), we obtain

\[ \operatorname{tg}\alpha=\frac{EF}{50}. \]

The initial rebound velocity of the striker will be

\[ V_{2}=\frac{V_{\mathrm{b}}\cdot EF}{50}. \tag{2} \]

The height of rebound of the striker, according to the law of free fall, will be

\[ H=\frac{V_{2}^{2}}{2g}. \tag{3} \]

Multiplying the weight of the striker by the rebound height \(H\), one can obtain the magnitude of the rebound energy.

The conical impressions formed after impact on the tested specimens were measured in the direction of two mutually perpendicular diameters using a microscope with a reading accuracy of 0.01 mm.

For the investigation, 28 specimens were made from normalized steel 45, and only one impact was made on each specimen. The specimens had the following dimensions: diameter \(d=80\) mm; specimen thickness—40 mm. Both surfaces of the specimens were ground and brought to a class 7 finish. The height to which the striker was raised was varied within the range from 10 to 40 cm with an interval of 5 cm. At each lifting height, 4 specimens were tested. The weight of the striker with the cone was \(Q=25.80\) kg.

Table 1 gives the determinations of impact hardness \(T\). Analysis of the data in this table permits the following conclusion: “When the difference between the energies of the falling and rebounding striker with the cone is divided by the volume of the cone-shaped impression formed, a value is obtained which is constant for the given metal and characterizes its impact hardness.”

The greatest deviation of the impact-hardness coefficient

\[ T=\frac{A_{1}-A_{2}}{V} \]

from its mean value does not exceed \(\pm 3.8\%\), which is quite permissible.

To determine the impact hardness at higher impact velocities, in a special tube of the institute’s physics laboratory an apparatus was arranged for dropping a cone weighing \(Q=540\) g from a height of up to 16 m. The rebound height of the cone after impact was recorded visually on millimeter paper by the shadow of the cone. For the test, 16 specimens of diameter \(d=80\) mm and thickness—40 mm were made. Only one impact of the cone was made on each specimen (in the central part of the specimen).

The results of these investigations are given in Table 2.

As follows from the data of Table 2, the conclusion we had drawn earlier was confirmed also at higher impact velocities.

Table 2

Average: \(T = 16000\)

The relationship between the depth of penetration of the cone \(h\), the impact energy expended \([A_1 - A_2]\), and the volume of the impression \(V\) obeys the following regularities:

\[ h \cdot 10^{-3} = 51.42\, V^{0.312} \cdot 10^{-4}, \]
\[ h \cdot 10^{-3} = 41.60\, [A_1 - A_2]^{0.324}. \tag{4} \]

Fig. 2.

The theoretical determination that we carried out of the depth of penetration of the cone into the metal under impact according to the scheme in Fig. 2 made it possible to establish that the greatest depth \(z_{\max}\) obeys the equation

\[ z_{\max} = \frac{1}{20.1}\sqrt[3]{\frac{2g \cdot H}{\tau_{\mathrm{v}}}}, \tag{5} \]

where \(H\) is the height of fall of the cone, cm;
\(\tau_{\mathrm{v}}\) is the ultimate shear strength of the metal, kg/cm².

The deviation of the theoretical values of \(z_{\max}\) from the experimental values in our tests did not exceed \((10 \div 11)\)%.