STRENGTHENING OF THE SURFACE LAYER OF METALS AFTER VIBRATION TREATMENT

N. T. LYALIKOVA, V. A. OGORODNIKOV

(Presented by the scientific seminar of the Department of Applied Mechanics)

In the process of vibration treatment, particles of the working medium enter into dynamic contact with the part being treated and deliver numerous impacts to its surface. As a result of these impacts, microcutting and plastic deformation occur, which makes it possible to carry out cleaning, finishing, and strengthening operations. Plastic deformation of the surface layer leads to an increase in the initial hardness, the formation of residual compressive stresses, and a reduction in surface roughness [1]. This improvement in the quality of the surface layer leads to increased wear resistance and fatigue strength of the metal and, consequently, to increased reliability and durability of machine parts.

The literature still contains insufficient information on how strongly the mechanical properties of the surface layers of treated parts change; therefore, the authors of the present article carried out experiments to investigate the intensity and depth of strengthening that arises during vibration treatment. The results of these investigations are presented below.

The experiments were carried out on specimens of copper and ShKh15 steel, which were treated in a vibration drum for 120, 60, and 15 minutes in a medium of hardened steel balls with diameters of 30, 15, and 5 millimeters, at an oscillation frequency of 1900 per minute and an amplitude of 4 mm. Treatment in these working media was carried out with flushing by running water for cooling and for removal of waste products after vibration treatment. Specimens for calibration and testing were made from the same annealed bar, after which measurements were made of the initial hardness of the test specimens on the end face and on the cylindrical part. After vibration treatment, the specimens were cut in a meridional section, polished to the 10th class of finish, and a measuring grid with a base of 1 mm was rolled onto this surface. At the nodes of the grid, hardness was measured on a Vickers hardness tester: for specimens of nonferrous metal under a load of 10 kg, and for ShKh15 steel under a load of 30 kg on a diamond pyramid. The arithmetic mean of these measurements was taken as the hardness at the given point. The residual deformations and the intensity of stresses characterizing the deformed zone of the specimens after vibration treatment were determined from the hardness distribution in the meridional section using the method of G. D. Delya [2].

Based on the test results, graphs were plotted of the distribution of strain intensity and the magnitude of work hardening as functions of the vibration-treatment time and the particle size of the working medium; a pattern was constructed of the distribution of hardness isolines over the entire surface of the specimen in a meridional section, which makes it possible to represent visually the distribution and depth of deformation throughout the entire volume of the specimen.

Figure 1 shows a calibration graph for copper, obtained by upsetting specimens in a press, from which the magnitudes of strains and the intensity of stresses are determined from the hardness distribution in the meridional section of specimens subjected to vibration treatment.

Fig. 1. Calibration graph for copper

Figure 2 presents graphs of the distribution of hardness, strain intensity, and stresses over the depth of the deformed layer of copper specimens subjected to vibration treatment for: a) 120 min, b) 60 min, and c) 15 min in a medium of hardened steel balls 30 mm in diameter, in a section along the cylindrical part. Exactly the same graphs were constructed for specimens of ShKh15 steel subjected to treatment in the same working media for the same time intervals.

The tests showed that in all cases there is a noticeable increase in hardness in the direction from the specimen axis toward the surface layer. The magnitude of the strain intensity and the stress intensity increases sharply toward the hardened surface, especially in copper specimens. This stress intensity in the plastic region may be regarded as an increased yield strength of the deformed metal. As can be seen from Fig. 2, in copper specimens the yield strength increases already after 15 minutes of treatment in the vibration bunker; with treatment for 60 and 120 minutes the specimens are deformed through their entire depth. Hence it may be concluded that, in order to obtain work hardening to a small depth, a copper specimen need only be treated in a medium of hardened steel balls 30 mm in diameter under the given conditions for 5–10 minutes. With treatment for 120 minutes, the yield strength on the surface of copper specimens increases by approximately 4.5 times, and on the surface of specimens made of ShKh15 steel, by 2.5 times. The hardness increases correspondingly, and the strain intensity increases over the depth of the plastically deformed layer. It was established that the particle size of the working medium also affects the magnitude of the strain intensity and the stress intensity. As the particle size of the working medium increases, the magnitude of work hardening and the depth of the deformed layer increase.

Fig. 2. Distribution of hardness, stresses, and the deformed layer of copper specimens after vibration treatment for:
a) 120 min; b) 60 min; c) 15 min.

Curve labels: $HV$, $\sigma_i$, $\varepsilon_i$.
Axes: $\varepsilon_i$; $HV$; $\sigma_i$; $h$, $\mu\text{m}$.

With vibrocleaning one may expect substantial strengthening of the surface layer; moreover, as experiments have shown, the deformations penetrate to a relatively great depth and may ultimately lead to warping of parts during their operation.

Thus, when using vibrocleaning, it should be taken into account that the part may be substantially strengthened, with deformation penetrating to a relatively great depth.

REFERENCES

1. V. P. Ustinov. Strengthening of the surface layers of machine parts in the process of vibratory bulk treatment. Vibratory Technology in Mechanical Engineering, Lviv, 1967.

2. V. M. Pimenov, K. N. Tsukublina, G. D. Del, V. A. Ogorodnikov. Method for determining deformation and work hardening during roller treatment. TsNIITMASH, No. 112, 1968.