CHANGE IN THE MICROHARDNESS OF THE SURFACE LAYER OF METALS AFTER VIBROABRASIVE TREATMENT

N. T. LYALIKOVA, N. A. EROFEEV

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

As is known [1, 2], when parts are treated in various vibrating abrasive media, the state of the surface layers of metals undergoes significant changes; in particular, the hardness changes.

Experiments were carried out to study the change in the microhardness of the surface layer of metals after treatment of specimens in an abrasive medium consisting of a mixture of high-speed-steel chips, cast-iron grit, and crushed scrap from grinding wheels. The tests were conducted on cylindrical specimens 20 mm in diameter and 20 mm long, made of steels 45 and R18 in the annealed and hardened states. The vibration frequency of the vibratory drum was 1500 and 1900 oscillations per minute, with a constant throw amplitude of 4 mm. The specimens were treated for 45 minutes, since at this duration of treatment a good surface finish was achieved. The charge volume of the vibratory drum (abrasive and specimens) was taken to be 15 kg. Measurements of the microhardness of the surface layer of the polished sections were made using a PMT-3 instrument under a load of 100 grams on specimens before and after vibro-treatment.

As a result of the experiments carried out, it was found that the surface layers of the specimens are plastically deformed (work-hardened) during vibro-treatment. For annealed steel, the depth of the work-hardened layers was from 50 to 200 microns, and for hardened steels, from 50 to 150 microns. However, the work-hardened layer on the surface of the specimens is not a continuous band, and the depth of work hardening differs in different areas. This can be explained by the fact that, during vibro-treatment in an abrasive medium, the surface layer is subjected to wear as a result of microcutting and chipping of pieces of metal, including work-hardened ones. Deformation of the surface layers is especially clearly noticeable along the edges of craters formed by abrasive particles that have penetrated into the metal, and along the edges of pits after pieces of metal have chipped off from the surface. Subsequently, in the open pits, the work-hardened layer is removed when the surface is abraded by the sharp edges of abrasive particles. Measurements of microhardness were made at distances of 0.05–0.5 mm from the edge of the polished section. As a result of the measurements it was found that, for annealed steels 45 before vibro-treatment, the microhardness of the surface layer in the direction from the middle of the polished section to the edge is 287.8—

297 HV, and after vibrotreatment at 1500 oscillations per minute at the same distances—287.8–344.4 HV; at 1900 oscillations/min—290–343.4 HV.

For annealed R18 steels before vibrotreatment—260–270.9 HV, and after vibrotreatment at 1500 oscillations/min, at the corresponding distances from the edge of the ground section, the microhardness was 290.1–297.4 HV; at 1900 oscillations/min—from 284.7 to 312 HV.

As can be seen from the measurements, the microhardness of the surface layer after vibrotreatment increases sharply both for steel 45 and for steel R18. With an increase in the oscillation frequency, the microhardness also increases.

From an analysis of the results obtained, it follows that vibrotreatment of parts in an abrasive medium can be used not only for cleaning and finishing operations, but also as a strengthening treatment operation in the manufacture of a number of critical machine parts.

REFERENCES

1. A. P. Babichev, B. B. Khodosh. Residual stresses and work hardening during vibro-burnishing. Stanki i instrument, No. 10, 1966.

2. B. B. Khodosh. Effect of vibro-burnishing on the physicomechanical properties of the surface layers of parts. Progressive Methods of Finishing Machining of Machine Parts, issue 12, Rostov-on-Don, 1968.