INFLUENCE OF STRUCTURAL PARAMETERS OF LOW-CARBON STEEL ON ELECTRIC ARC BURNING

Dep. «Applied Mechanics and Materials Science», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipro, Ukraine, 49010, tel. +38 (056) 373 15 56, e-mail dnuzt_texmat@ukr.net, ORCID 0000-0002-7353-1916 Dep. «Applied Mechanics and Materials Science», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipro, Ukraine, 49010, tel. +38 (056) 373 15 56, e-mail plit4enko@ukr.net, ORCID 0000-0002-0613-2544 «Design and Technological Bureau», Dnipropetrovsk National University of Railway Transport Named After Academician V. Lazaryan, Lazaryan St., 2, Dnipro, Ukraine, 49010, tel. +38 (056) 373 15 56, e-mail dnuzt_texmat@ukr.net, ORCID 0000-0003-2758-0749


Introduction
In conditions of electric arc welding the process of arcing is sensitive to the influence of a certain number of factors [7,11,12,18].They include maintaining the optimal ratio between the rate of the electrode metal melting and its feeding into reaction zone, conditions of metal transfer through the interelectrode space [8, 15-17, 19, 20], etc. Taking into account that metal transfer between the electrodes is carried out in the form of a gasdroplet mixture, the very process of liquid metal droplet formation at the end of the electrode, its size and shape should to some extent influence the technological characteristics of electric arc welding.Analysis of the conditions for formation of liquid metal droplet indicates existence of certain relationship between the surface tension of metal and gravitational component [9].Taking into account possible dependence of the surface tension forces on the structural state of electrode metal, the size of structural elements may have certain influence on the conditions of formation and burning of electric arc.

Purpose
The article is aimed to evaluate the influence of structural parameters of low-carbon steel on arcing process.

Material and methodology of study
A wire of 1 mm diameter of low-carbon steel with a carbon content of 0.2% was used as a material for electrode.The values of the micro-and substructure characteristics of the electrode wire metal were varied by varying the parameters of heat treatment and cold deformation by drawing.The values of the micro-and substructure characteristics of the electrode wire metal were changed by varying the parameters of heat treatment and cold deformation by drawing.The degree of plastic deformation was obtained by drawing blanks from different initial diameter to final dimension of 1 mm.Thermal treatment was carried out in electric chamber furnace of the SNOL-1,6.2,5.1/11-IZtype.To prevent the formation of oxide film on the metal surface, the samples were placed in quartz glass ampoules with preliminary deairing to the level of forvacuum.The temperature was measured by chromel-alumel thermocouple and the electromotive force was determined using the DC potentiometer.In order to obtain the substructure of different dispersion degree the steel after quenching from temperatures 3 Ac and tempering at 650°C for 1 hour was subjected to cold drawing to reduction 17-80%.To form structure with different ferrite grain size the steel after drawing was annealed at 680°C for 1 hour.The microstructure was exam-ined under a light and electron transmission microscope UEMV-100K at the accelerating voltage 100 kV.The grain and subgrain sizes were evaluated using the methodologies of quantitative metallography [5].A welding converter of the PSG-500 type was used to study the arc welding process of direct and reverse polarities.The welding current value was estimated as the average of 10 measurements.

Findings
Analysis of the results of investigations [7][8][9] indicates that when metal is melting, the surface tension forces form a drop at the end of the electrode.The moment of droplet detachment corresponds to the condition that the gravitational component exceeds the surface tension force.Taking into account that as the molten metal temperature rises, the surface tension force decreases, the welding current increase should lead to the dispersion of the emerging droplets.Condition of balance between the hydrostatic pressure from pinch effect and the surface tension (  ) makes it possible to estimate the critical value of welding current (( k I B d    , where В -is a constant equal to 32.7 А/ 0,5 dynes , d -the electrode diameter) upon the detachment of liquid metal droplet [9].Substituting the constant В and  of molten metal (1220 dynes/cm [9]) in the ratio for k I , the value k I for low-carbon steel should be about 360 А, which is confirmed by the data [8,9].The experimentally observed value of the welding current ( 1 I ) on the degree of deformation during wire drawing (  ) under conditions of stable arcing of direct polarity, is approximately an order of magnitude lower than the calculated value (Fig. 1).A similar difference was found for the arc of reverse polarity: 1 I less than the calculated one in 5-6 times. 1 -reverse polarity arc; 2 -direct polarity arc The extreme nature of 1 I dependence on  , is apparently due to the peculiarities of the substructural metallic structure.Considering the nature of dependence for k I , one can formally assume that either the value В depends on the chemical composition or structure of the steel, or  should be replaced by another characteristic.To explain the correlation ratio 1 I on  (Fig. 1), it was made an attempt to replace  by the metal surface tension coefficient in the solid state.Taking into account the presence of volume fraction of ferrite in the investigated steel around 97-98%, the ferrite surface tension coefficient ( F  ) can be taken as  .Using the experimental data [2] and the transformations carried out [14], for a cold-deformed state one can write: where G ferrite shear modulus (0.82 dynes/cm 2 ), b -Burgers vector 8 2,3 10 sm   [1], D -size of dislocation cell.
Refinement of the dislocation cell structure obeys a proportional dependence on the degree of cold plastic deformation.
Taking into account that 20-30% of reduction is enough to start the formation of dislocation cellular structure of various degrees of perfection (Fig. 2), determination of the dependence of D on  made it possible to calculate the welding current value ( D I ).As a ratio the dependence for k I after an appropriate substitution of  for F  was used: A joint analysis of absolute values D I and 1 I (Fig. 3) for direct polarity arc indicates a fairly good correlation only up to 60%reduction.For deformation degrees of more than 60%, the observed differences can only be explained by qualitative changes in the dislocation cell structure.
Indeed, at reductions more than 60-70% in carbon steels the processes of perfecting the formed dislocation cells start to develop.
At high degrees of plastic deformation, a progressive increase in the dislocation density is accompanied by intensive cleansing of the cell body from unbound dislocations, changes in the form of cells, decrease in thickness of subboundaries, and so on.[1,4,13].All this, apparently leads to violation of the ratio for F  and is inherited by calculations D I .The results obtained for reverse polarity arc are similar to the data for direct polarity arc.With constant sub-structural parameters of cold-deformed steel, for the arc of direct and reverse polarity, the nature of relationships 1 ( ) I f   (Fig. 1) indicates possible change in the coefficient В depending on the arc polarity.Taking into account existence of certain difficulties in burning an arc of reverse polarity, a change of value В can be fully justified.Indeed, when the polarity reverses, the conditions for stable arc burning should become more complicated [7,8].The evaluation showed that in order to increase the degree of coincidence between the calculated and experimental values of the welding current, for the reverse polarity arc, the value В should be about twice as large as for the direct polarity arc.The result of electric current calculating for the reverse polarity arc with respect to relation (2) (will be denoted as D I  ) is shown in the Fig. 3. Dependence analysis shows that, regardless of the welding arc polarity, a good enough agreement between the calculated and experimental values of the welding current is limited to deformations of 60%.For large reductions, the degree of mismatch is proportional to the level of welding current values.For the direct polarity arc, when the welding current level is 20-35, the deviation of calculated values ( D I ) from 1 I is 12-14%.For the reverse polarity arc at 1 I  45-57 A , the value of differ- ence between D I  and 1 I reaches 20%.Results of the study indicate the existence of a certain influ-ence of substructural metallic structure of the electrode metal on the welding arc burning processes.The observed dependence of the electric current value when burning the arc of different polarity is sufficiently well explained by the parameters of the substructure of cold-deformed low-carbon steel before the appearance of qualitative changes in the internal structure of metal.
In order to explain the nature of the observed welding current dependence on the substructure parameters of cold-drawn metal, investigations were carried out on the influence of ferrite grain boundaries of with large disorientation angles ( f dis the ferrite grain size after annealing the colddrawn metal).Such necessity is due to differences in the degree of accumulation and distribution of crystal structure defects from the reduction value when drawing and after development of recrystallization processes when annealing.Taking into account the inverse proportional relationship between the ferrite subgrain size (D) on the reduction degree when drawing [6] and the observed correlation relationship 1 ( ) I f   (Fig. 3), it can be assumed that to describe the dependence 1 ( ) I f d  the relationship of the following type can be used: The analysis of the ratios indicates a fairly good correlation when using the dependence: where i I -is the welding current at 0 In general, it should be noted that regardless of the type of interface, increase in their total length is accompanied by increase in the welding current value.Taking into account qualitative differences in the distribution nature of crystal structure defects during the formation of grain boundaries with large disorientation angles and sub-grains as a result of cold drawing [4,6], the existence of sepa-  А/mm ).On the other hand, the electric resistance value for the ferrite structure of low-carbon steels is proportional to the accumulated defect density of the crystal structure [3] and inversely proportional to the ferrite grain size [10], can to some extent exert its influence on the process of welding arc burning through the surface tension of the metal.

Conclusions
1.In the conditions of stable burning of the arc of different polarity for the low-carbon steel electrode, the extreme dependence of the welding current on the degree of cold plastic deformation was observed.
2. Influence of the ferrite grain size of the electrode wire on the welding current value is much greater than the effect of the substructure presence.

Fig. 3 .
Fig. 3.The influence of deformation degree by drawing the steel 20 on the welding current when burning the arc of direct (3, 4) and reverse (1, 2) polarity: 2, 4 -current curves 1 I obtained experimentally; 1, 3 -curves of currents, D I  and D I respectively calculated according to the relation (2) where A, k and n -are the constants.Results of the construction 1 I on ( 0 d ) 0,5  , where 0 d -ferrite grain size of the steel are shown in the Fig. 4.

Fig. 4 . 2 1, 5 А
Fig. 4. Influence of the grain sizes (1) and the ferrite subgrain (2) of steel 20 on the welding current when burning the arc of reverse polarity Analysis of the absolute values of the parameters of equation (4) indicates that the influence of the presence of ferrite grain boundaries with large disorientation angles ( i I =30 А, k = 2 1,5 А/mm ) in the steel structure is much greater than the influence of the subgrain structure ( i I = 8 А, k = 0,75