RESTORATION OF FATIGUE STRENGTH UNDER THE ACTION OF ELECTRIC PULSES
Gromov V.E., Zuev L.B., Tsellermaer V.Ya., Kuchumova E.S., Sosnin O.V., Gromova A.V., Gorlova S.N.
SSMMA,654007 Russia, Novokuznetsk, str.Kirova,42
Many details of machine and of different designs are working in regime of cyclic loads. Therefore, the problem of increating the resource of details reliability is very actual.
The ultrasound velocity is used in this work as a method of structure change evaluation during fatigue tests (VUS).
The possible bonds of experimental dependences in spreading the ultrasound velocity (n) from the number of cycles of loading with the model "durability-load" known in the theory of reliability are being examined. Three stages (quick changes of velocity during the small number of loading; the slow change of velocity during the very large number of cycles; quick , being finished during the specimen rupture) have been analyzed on dependences of (from the number of cycles). The possibility of correlation of these three stages with three ways in use of items: the extra working, normal use, intensive ageing and rupture are being discussed. It is shown, that the third stage of dependence corresponds to appearing the cracks in material of dangerous size and signalizes about exhausting the resource of product reliability.
For increasing the durability of steels in fatinge tests the treatment by strong current impulses of optimum parameters to freguency and duration is used.
The tests on small cyclis fatigue of steels X18H10T, 70XGSA, 40X, 38X2MYUA, P6M5 and welded joints made of them after different regimes of heat treatment with the registration of number of cycles up to rupture, change of sub- and microstructure, internal stresses, velocities of ultrasound in all stages of tests have been carred out. The dependences of velocity changes of ultrasound from the number of cycles and their correlations with the data of optical and electron scanning and transmission microscopy have been stated.
The evaluation of stress level of I and II
kinds, the data about the evolution of substructure and character
of rupture gives the possibility to state originally the physical
nature of electrostimulated increase of resouce. It is connected
with treating the microcracks being formed in the process of
fatigue loading on accound of the local heats of material in a
region of concentraition of current lines at their ends,
obsculing the latters due to the relaxation of stresses. The
samples from steel X18H10T were demagedafter 1300 - 1500 cycles
of loading. In such a state the samples were being undergone by
electrostimulation with the frequencyof current pulses 20 Hz and
by amplitude with ~ 20 kA during 30 minutes. The
electrostimulation practically has recovered the initial value of
SUZ after 1000 cycles of loading and allowed the samples
additionally to withstand about 3000 cycles. So, the combinaition
of developed techniques, namely, the looking for thefor the state
of metal in time of fatigue tests means of measurement of and
treatment bu the electrical impulses creates the principal
possibility of recovery of tems durability and makes a
contribution to the increase of machines and mechanisms safety.
This method is using on metallurgical and machine making plants.