INFLUENCE OF SHOCK VOLTAGE FROM THE ELECTRIC DISCHARGE ON THE FATIGUE ENDURANCE OF CARBON STEEL IN WATER

Dep. «Technology of Materials», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryana, Lazaryan St., 2, Dnipropetrovsk, Ukraine, 49010, tel. +38 (056) 373 15 56, e-mail dnuzt_texmat@ukr.net, ORCID 0000-0002-7353-1916 Dep. «Technology of Mining Machinery», Dnipropetrovsk National Mining University, K. Marks Av., 19, Dnipropetrovsk, Ukraine, 49027, tel.+38 (0562) 46 99 81, e-mail aleklisn@gmail.com, ORCID 0000-0001-6701-5504 Iron and Steel Institute named after Z. I. Nekrasova, NAN Ukraine, Starodubov Sq., 1, Dnipropetrovsk, Ukraine, 49107, +38 (056) 373 15 56, e-mail dnuzt_texmat@ukr.net, ORCID 0000-0002-8101-1654 Chinese Machine-Building Investment Group of Ltd, Anli St., 60, Beijing, Chinese Folk Republic, 100101, tel. 86 10 64827530, e-mail xxhai2004@163.com, ORCID 0000-0002-0338-5976


Introduction
Regardless of the circuit loading, the observed changes of the internal structure of metal materials are determined largely by the conditions of deformation.At a constant temperature and the appearance of the loading cycle, depending on the magnitude of amplitude exceeding the endurance limit, the qualitatively different structural changes in metallic materials are observed [2].For the fields of little or a high cycle fatigue the increase in the dislocation density and their redistribution, the number keeping of dislocations in a non-blocking state are among the major factors that determine stock endurance metal [14].On this basis, the use of external influences such as the introduction of additional harmonics in the loading cycle, or intermediate treatments, for example by passing an impulse of electric current [6,10], can significantly change the processes course of structure formation development under the cyclic loading.

The state of the question
The technology of processing metal materials using a shock wave, with relative ease of implementation, was widely adopted.[4].This technology allows to solve difficult tasks, such as deformation in the manufacture of large-size products, their hardening or welding of the individual elements construction.The achieved effect has an explicit dependence on the threshold voltage with the passage of the shock wave in the metal, depending on the level of disposable energy input, or pulse resulting voltage [7,13], their amount, the achieved effect has an explicit dependence on the threshold voltage with the passage of the shock wave in the metal.Moreover, as the experience of use said the impulse treatment to harden metal, the achieved result in several times may exceed the effect of equivalent plastic strain [4,12].
Known experimental results [4,5] indicate that the process of hydraulic shock encountered when forming an electric discharge in the liquid the achieved effect for most of metal materials is adequate hardening.On this basis it is quite natural to expect the increase in the number of crystalline structure defects.The results of studies that indicate a qualitatively different influence of the parameters of the specified impulsive loading have a particular scientific and practical interest.Thus, in [4] it is shown that the increase of the amplitude of the emerging pressure wave largely determines the increase in the number of dislocations and increase the pulse duration at constant amplitude mainly affects on the process of displacement.
There is almost no information on the influence of energy and number of pulses on several properties, including enduring quality of metal under the cyclic loading, except the known restrictions on the use of the technology shock pulse processing in the manufacture of certain products [5].

Purpose
Explanation of the effect of the resulting voltage pulses from electric discharge in water on the magnitude of the limited endurance of thermally hardened carbon steels under cyclic loading.

Methodology
The research material was steel 45 (0.45% of carbon) with the concentration of chemical elements within the grade composition.The test specimen produced in the form of plates thickness 1, width 15 and a length 120-180 mm.Structural state of steel corresponded to quenching on martensite from the normal heating temperatures and annealing at 300 C, with duration of 1 h.The microstructure was investigated using the electron microscopy; the dislocation density was evaluated by the methods of x-ray structural analysis [1].The hardness was measured with Rockwell method (scale «C»).Cyclic loading was performed under conditions of symmetrical bending on a testing machine «Saturn-10» at a temperature of +20 C. Treatment of shock voltage (SV) from the electric discharge in water was performed with the installation of bath type «Iskra-23» used for cleaning of foundry products.The electrical pulses formed at a voltage of 15 to 18 kV with energy of 10-kJ and the amplitude of 1-2GPa.Metal processing consisted in the set of pulses number up to 15 thousand, at a frequency of 2-3Hz.

Findings
In the present research when choosing a structural condition of carbon steel as a result of tempering on martensite and annealing 300 C the authors were guided primarily by the need to achieve the simultaneously a high density of crystal structure defects and sufficient endurance under the cy- clic loading metal.The analysis of the microstructure showed that as a result of the treatment (Fig. 1) the signs of the initial stages of emission of finely dispersed carbide particles on the dislocations, situated in the middle of the martensite strips and at their boundaries were found.Moreover, as it shown in [9], the evidence of the dislocations recombination beginning, which can result in a decrease of the dislocations density and formation of dislocation nonmonotonicity in the distribution, is the reduction of the contrast image of the microstructure.The observed loss of contrast of an image in separate places of the wide walls of dislocations (Fig. 1) with simultaneous decoration of carbon atoms can be regarded as evidence of the almost total absence of mobile dislocations.On this basis, it can be assumed that the majority of dislocations introduced into the metal during the formation of martensite crystal, after the following holidays are not able to move under cyclic loading.To analyze the degree of compliance of the obtained results with known experimental data of the investigated steel after annealing and tempering without cyclic loading was exposed by SV.In state after annealing and tempering the steel 45 had a hardness HRC of 46.6.The obtained values are in fairly good agreement with the known data [3,11].After treatment of SV it was found that the hardness increase of 11% (to 51.8 HRC), that does not contradict to data [10,12].Thus, the impact impulse from the pressure wave by the nature of its influence on the heat-treated carbon steel should be attributed to the hardening effect.Firstly, the curve of cyclic loading steel 45 after tempering and annealing of 300 (curve 1, Fig. 2) was received.Further, knowing the number of cycles that a metal can withstand before failure at a certain value of amplitude ( a σ ), the samples loading to about 0,6-0,65 limited endurance was carried out, they were exposed by SV and after that the samples were brought to failure.The value of the limited endurance was estimated as the sum of the number of cycles before the SV and after the final decay at a particular amplitude (curve 2).Comparative analysis of progress curves indicates the possible qualitative differences in the internal structure of the metal with different structural condition under the cyclic loading.For example, for a field of low-cycle fatigue the increases of the loading amplitude is accompanied by a decrease of differences in values of finite life (before and after SV), when a σ =100 2 kg mm is practically absent.Further extrapolation of the curves of cyclic loading in the field of amplitudes exceeding 100 2 kg mm indicates the achieving of the opposite effect: the impulse of the shock wave treatment helps to reduce the limited endurance (Fig. 2).On the other hand, the reduction a σ regardless of previous processing, is accompanied by a corresponding increase in the number of cycles before failure, and the observed more gently sloping curve run of metal fatigue after SV indicates a fairly effective impact on endurance.Indeed, exposing the carbon steel to the action of pulses of a pressure wave in water, the amplitude of the cyclic loading while achieving the same endurance increases significantly.This is especially noticeable for the transition curves of fatigue.On the other hand, for the same amplitude of the cyclic loading, for example, at 56 2 kg mm , the increase in endurance may exceed 30% (Fig. 2).
The analysis of changes in the dislocation density after reaching a certain level of limited endurance of metal was conducted with the aim to explain the mechanism influencing the metal of the voltage pulse from the passage of electrical discharge in water.In Fig. 3 the change in the density of dislocations depending on the number of loading cycles and previous treatment is presented.Regardless of the structural state of the metal (without or after SV) a decrease in the amplitude of cyclic loading is accompanied by increase of the accumulated dislocation density in the studied interferences ( ( ) hkl ρ ).Considering that the dislocation density was determined on the fracture surface of the samples, the nature of the change and absolute values should correspond to flat deformed state of the metal formed under the conditions of accelerated fatigue crack growth [2].
On the other hand, given the increasing role of the static component with increase the degree of overload under the cyclic loading (adequate growth a σ ) [8], the number of dislocations on the fracture surface should decrease.One explanation of given position is increased with the increase a σ in the proportion of the metal under the conditions of volumetric stress state near the surface of the fracture.On the basis of this the greater extent enrichment of near-surface dislocations of specified amounts of metal should be occurring and, as a consequence, there is a decrease in the dislocation density determined on the fracture surface.
Given the dislocation propagation mechanism of plastic deformation, the assessment of its value during the loading cycle will provide the additional evidence regarding the nature of structural changes with increasing toughness of the metal as a result of SV effect.The ratio of the magnitude of plastic strain per cycle of loading ( i ε ) and the achieved number of cycles to failure ( i N ) is subject to the famous Coffin-Manson Equation [2]: where a and b -are taken as constant and equal to carbon steel of 0.5 and 1 respectively [2].After conversion of (1), it was received for the dependence assessment of i ε : − a Substituting in (2), for the same a σ corresponding values i N , it was found the decrease of deformation per cycle of loading by approximately 20% as a result of processing metal by SV.On this basis, it can be assumed that the process of introducing of additional dislocations in thermally hardened steel from pulse pressure wave is not accompanied by the annihilation events apparently.Moreover, the implementation of the deformation per cycle for the region of high-cycle fatigue is possible and it is ensured by the participation of a smaller number of dislocations.Thus, either the process of SV has the vast number of dislocations that remain mobile and able to interact only at the subsequent cyclic loading, or there is an additional unlock of previously immobile dislocations after the heat strengthening.In general, the detected on the fracture surface after pulsed effect the increased number of dislocations, located in different crystallographic systems can be regarded as evidence of the development rather complicated dislocation reactions, which has provided the increase in endurance of the metal during the cyclic loading process of thermally hardened carbon steel.

Originality and practical value
1.For the transition field to high-cycle fatigue, the result of machining voltage pulses of carbon steel with structure improve the limited endurance increase is accompanied by a decrease in deformation per cycle.
2. With the increase of the amplitude of the voltage cycle, the effect of metal endurance increase from processing of SV reduced.
Metal working by pulses of pressure waves from an electric discharge in water can be used to extend the lifetime of rolling stock parts, which are subjected to cyclic loading.

Conclusions
1.As a result of processing by pulses of the pressure wave of heat-strengthened steel 45 it was found the increase of endurance under cyclic loading corresponds to the increased amount of accumulated dislocations on the fracture surface.
2. The increase of steel endurance under the cyclic loading after pulse treatment is largely associated with the change in the number of mobile dislocations.
In Fig.2the areas graphs of cyclic loading that correspond to low cycle and the transition region to high-cycle fatigue are shown.

Fig. 2 .
Fig. 2. The diagrams of cyclic loading steel 45 after tempering and annealing at 300 C (1) and after treatment of SV (2).