HARDENING OF ShKh15 STEEL BY VIBRATION TREATMENT

N. T. Lyadikova, V. A. Ogorodnikov

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

In [1] data are given on the hardening of the surface layer of copper specimens after treatment in a working medium consisting of hardened steel balls in a vibrating bunker. As a result of studies of the degree of plastic deformation and the intensity of work hardening of copper specimens, data were obtained that make it possible to recommend this treatment method for improving the quality of surface and internal layers. It was found that by this method it is possible to achieve a significant increase in wear resistance, fatigue strength, and, consequently, an increase in the reliability and durability of machine parts.

The purpose of the present work is to study the effect of different vibration-treatment regimes on the degree of plastic deformation and the distribution of the yield strength in specimens of annealed ShKh15 steel. The experiments were carried out according to the same procedure and under the same technological regimes as the treatment of copper specimens [1]. Specimens of ShKh15 steel 30 mm in diameter and 40 mm long were subjected to vibration treatment in a working medium consisting of hardened steel balls of 30, 15, and 5 mm diameter for 120 minutes. After treatment the specimens were cut along a meridional section, polished to cleanliness class 10, and a rectangular dividing grid with a base of 1 mm was rolled onto the polished surface. At the nodes of the dividing grid, hardness measurements were made on a Vickers hardness tester at approximately 300 points under a load of 30 kg. The calibration graph for ShKh15 steel in the coordinates yield strength—hardness—strain intensity \(\sigma_i — HV — l_i\) was constructed from the results of testing 10 cylindrical specimens 10 mm in diameter and 15 mm high in axial compression, with the ends lubricated with colloidal graphite paste and with the use of lead-foil gaskets. From the hardness measurements a pattern of the distribution of isolines in the meridional section of the specimen was constructed.

As a result of the tests carried out, it was found that the specimen undergoes the greatest deformation at the corners and in the middle section of the cylindrical part of the specimen, but the total depth of the deformed layer is considerably smaller than in copper specimens under the same treatment conditions. Figure 1 (a, b, c) gives graphs of the distribution of hardness, strain intensity, and yield strength over the depth of the deformed layer for a specimen treated in a working medium of hardened steel balls 30 mm in diameter for 120 min. (curve 1—in the section along the cylindrical part, curve 2—along the end face).

part of the specimen). From the graphs obtained in Fig. 1(c) it follows that the yield strength over the depth of the deformed layer in the cylindrical part changes by up to 33%, and over the end part by up to 25%, compared with the initial value, in the direction toward the outer surface of the specimen. Plastic de-

Fig. 1. Distribution of: a) hardness; b) strain intensity; c) yield strength over the depth of the deformed layer of a specimen of ShKh15 steel after vibration treatment

formation, Fig. 1(b), extends to a depth of up to 23% in the cylindrical part and up to 8.7% at the end.

As a result of the tests carried out, it was also found that the depth of the deformed layer depends to a significant degree on the size of the particles of the working medium. In Fig. 2 (a, b, c, d) curves are given for the change in hardness, yield strength, and strain intensity as functions of the size of hardened steel balls in which vibration treatment was carried out for 120 min. From Fig. 2(a) it is seen that the yield strength on the surface of the specimen during treatment in steel balls of diameter 5 mm increased only by 5.4%, while at a depth of 1 mm, Fig. 2(b), it was already equal to the initial value. During treatment in a medium of balls of diameter 30 mm, the yield strength at the surface increased by up to 36.3%, at a depth of 1 mm—by 18.7%, and only at depths of 2 and 3 mm did it approach the initial value. Correspondingly, the strain intensity also increases with an increase in the particles of the working medium. Thus, for example, during treatment in a medium of balls of diameter 5 mm the strain intensity increases only to 0.1% at the surface, while at a depth of 1 mm it is already equal to zero. During treatment in balls of diameter 30 mm, the strain intensity at the periphery increases to 1.2%, at a depth of 1 mm is about 1%, and only at distances of 2 and 3 mm from the surface does its value differ little from the initial value.

Thus, from an analysis of the data obtained, the following conclusions can be drawn:

1. The yield strength of ShKh15 steel specimens after vibration treatment, through the depth of the deformed layer respectively along the end face and along the cylindrical part, increases by 25–33%, which is considerably lower under the same test conditions than for copper specimens, for which the yield strength on the cylindrical surface increases by a factor of 4.6, amounting to about 80%.

Fig. 2. Dependence of hardness, yield strength, and strain intensity on the particle size of the working medium during vibration treatment of ShKh15 steel specimens for 2 hours:
a) on the surface; b) at a depth of 1 mm; c) 2 mm; d) 3 mm.

1. The depth of the plastically deformed region and the yield strength depend to a considerable degree on the particle size of the working medium during vibration treatment. Under the given test conditions, the greatest amount of work hardening on the surface and through the depth of the deformed layer can be obtained when treating ShKh15 steel specimens in working media consisting of hardened steel balls 30 mm in diameter. If it is necessary to obtain only surface work hardening, then parts may be treated in working media with particles of medium size, for example, in steel balls 15 mm in diameter.

REFERENCES

1. N. G. Lyalikova, V. A. Ogorodnikov. Strengthening of the surface layer of metals after vibration treatment. Proceedings of TPI, vol. 224, 1969.