INFLUENCE OF CHEMICAL COMPOUNDS ON THE FORMING OF WELDING ARC

Dep. «Materials Technology», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipropetrovsk, Ukraine, 49010, tel. +38 (056) 373 15 56, e-mail dnuzt_texmat@ukr.net, ORCID 0000-0002-7353-1916 Dep. «Materials Technology», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipropetrovsk, Ukraine, 49010, tel. +38 (056) 373 15 56, e-mail dnuzt_texmat@ukr.net, ORCID 0000-0002-0613-2544 Dep. «Materials Technology», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipropetrovsk, Ukraine, 49010, tel. +38 (056) 373 15 56, e-mail dnuzt_texmat@ukr.net, ORCID 0000-0003-4327-9809


Introduction
For the technology of electric arc welding, the initial conditions of electric arc forming to a large extent determine the peculiarities of further process of arcing.Taking into account the array of factors, which according to quality indicators in different ways influence the conditions of electric arc ignition the state of the interelectrode space environment can be attributed to one of the most important effects [5].
On the other hand, the environment state will become no less important at the stage of stable arcing, defining the conditions of metal transfer through the electrode space.Furthermore, the nature of the molten metal transfer from the electrode to the welding pool itself will stipulate not only the technological characteristics of the electric arc and energy balance, but also the stage of development completion of metallurgical processes when forming the weld connection as a whole [3,14].Taking into account that the influence of interelectrode environment on the arc ignition process is determined by the ionizing power, the location of different substances between electrodes will change the initial conditions of the electric discharge forming [4,11].On this basis, the purposeful influence on the conditions of metal transfer at the moment of electric discharge forming is very important for the technologies of electric arc welding.

Current condition of the problem
The process of metal transfer through the interelectrode space is to a large extent determined by the initial conditions of the electric arc ignition.The above mentioned metal transfer process can be roughly divided into separate stages.The characteristic conditions of change of the metal aggregate state and its properties are inherent to these stages [1,6].Thus, after formation of liquid, the growth of its volume on the cathode surface takes place until the separation of droplets from the surface and transferring them to the anode.
Taking into account the dependence of the process of droplets forming, their size and shape on the chemical reactions behavior between the substances filling interelectrode space, the combined influence on the ionizing power of environment is difficult to predict [8,10].On the other hand, the formation process of droplets can be confidently considered as dependent on the ratio of the gravitational forces influencing the fluid and force of surface tension of the metal.
The force value of the surface tension of molten metal or alloy is determined by the characteristics of their internal structure, which include the type of crystal lattice, the chemical composition of the alloy.The temperature of the fluid heating and its fluidity has certain influence on the specified characteristic.
Thus, for the iron the surface tension coefficient equals to 1.22 N/m [6].Admixtures of alloying chemical elements to the iron lead to its change.For example, the addition to the iron of 18% Cr, 8% Ni, 1% Ti (H18N9T) leads to a decrease of the surface tension (σ) at the level of 8-10%.
As compared to the influence of chemical composition of the alloy, a mixture of gases of interelectrode space can also influence the level of values σ, although their influence in most cases is qualitatively different.
Considering the formation of metal droplets of the electrode it can be noted that the main influence is determined by the ratio between the axial force, which arises as a result of so-called pinch effect and the force of surface tension [4,6].The growth of heating temperature of the molten metal is accompanied by a decrease of the surface tension coefficient [6,9].
Accounting the fact that during the first approximation the heating temperature of molten metal is proportional to the strength of welding voltage (I), when the growth I , changing the influence of gravitational component reduces the critical size of the droplet (conditions of droplet detachment from the electrode) [6].At the same time the influence of electromagnetic force [2] including its compressing effect on the molten metal of the weld pool is increasing [2,12].After the detachment of droplet from the electrode surface, the process of its movement is starting to depend on the joint action of electrostatic and electrodynamic forces, reactive action of the electric discharge [10].
On the basis of the presented joint influence, the strength increase of electric current is accompanied by breakage of molten metal droplets, which are transferred in the interelectrode space [6,15].The presented compound influence on the process of transferring the molten metal during the arc welding can be corrected using various chemical compounds.

Purpose
Purpose of the article is the comparative analysis of the chemical compounds influence on the process of electric arc forming and the conditions of its burning.

Methodology
The law carbon wire with the diameter 3 mm, of the steel with 0.15% carbon was used as a material for electrode.As the substances determining the conditions of electric arc formation were used the kaolin; The studies were conducted with the use of electric current of constant direction and the arc of reverse polarity.As a source of electric current the welding transformer of the type PSO-500 was used.On the special stand the initial gap between the electrode and metal plate was 1-1.5 mm.Interelectrode space was filled with the studied substance and the electrical discharge was formed.
At the moment of the electric arc ignition the values of electric current and arc voltage were determined.After natural break of electric arc the final size of the space between electrodes is taken as the maximum arc length.

Findings
Except the dependency of metal heating temperature on the electric current force [7,10] the formation process of molten metal droplets is determined by the joint action of several factors.First of all, the existence of inverse proportion between the heating temperature and the force of metal surface tension shows that the growth I would en- hance the fluidity of liquid.
At the same time the rise of electrodynamic influence on the process of droplets formation is observed.The joint influence of the mentioned factors leads to increase in the dispersion degree of molten metal droplets [6].
The process of transferring the molten metal through interelectrode space can be decomposed into successive stages: detachment of the formed droplet from the surface of one electrode and its transferring to another one.Thus, the size of droplets that are detached from the electrode surface will decrease proportionally to the increase of I.At the same time the number of droplets that are being transferred through the space between the electrodes per time unit will increase.
When the electric current force is so that the droplets form a continuous chain it is considered reaching the critical conditions that determine the qualitative changes in the nature of metal transfer (from the drop to the spray one).Thus, the definition of metal transfer mechanism is of practical importance to assess the influence of the studied substances on the process of electric discharge forming.
The critical value of electric current ( c I ) meets the conditions when the diameter of molten metal column reaches about 0.7 of the electrode diameter [6].On this basis, the value c I is estimated by the ratio: where k and σ -are the coefficients of proportionality and surface tension of the metal respectively, d -is the electrode diameter.Taking into account the change range of σ for low-carbon steel can be taken σ = 1.2 n/m and k = 18.5 A/ dyne 0.5 .According to ratio (1) it has been determined that for low-carbon steel of electrode with d = 3 mm, c I should be equal 375-380 A. Comparing with the installation current 200 A it should be assumed that upon the experimental conditions the metal transfer in the interelectrode space corresponded to the droplet mechanism.
Under conditions of electric current of constant direction with reverse polarity the search for correlations between the maximum arc length ( д l ) separately for the strength of electric current or arc voltage (U ) have shown the need to considerate the influences of I and U at the same time.The results of the constructed dependences for the components under study are presented in the Fig. 1.
According to external characteristics one can talk of inverse relationship between д l and electric current capacity (W U I = ).The deviation of dependences ( ) from the rectilinear appearance indicates the need to reconstruct them in logarithmic coordinates (Fig. 2).
In the first approximation, the specified dependences are clearly subordinated to relation of the type: ln( ) ln( ) where A and K are the constant characteristics.
After transformation the expression (2) takes the form: where B -is a proportionality coefficient.
For each component under study from the graphic solutions of dependences (Fig. 2) the values A and K , for the dimension д l [mm], W [VA] were determined.Thus, in the order of decreasing the values A (14.75; 13.7; 10.5 and 8.8), the studied components are placed in the corresponding order:  for the studied components in the logarithmic coordinates.The identifications are similar to the Fig. 1 For more detailed explanation of the reasons leading to the influence on the formation process of electric arc components in the presented order from 2 SiO to kaolin, the additional analysis of the obtained relations was made.Thus, in a first approximation, accounting the known decrease in surface tension force of the iron from the oxygen concentration [6], the presented influence nature (Fig. 1) may be stipulated by the presence of oxygen atoms of different concentration as a result of the dissociation reaction of the studied components.On the other hand, at the initial stages of the arc forming, the increase of electric current strength is accompanied by proportional increase in the metal mass transfer through the interelectrode space [5,10].At the same time, due to the joint action of electric and magnetic fields that appear in the space between electrodes, the axial force determining the mechanical nature of the influence on the molten metal is being formed.The voltage that is appearing according to its action represents the pressure voltage.Under conditions of permanency of the most of factors influencing the formation process of electric arc the specified voltage can be estimated using the power that is appearing ( ρ ) with the use of dependence [6]: where д k -is a coefficient that determines the electrode diameter influence and the electric arc polarity.At the same time, according to the presented dependence the influence of ionizing ability of interelectrode environment is absent.Although using the form of the ratios ( ) 1) one can still identify the influence peculiarities of the studied substances on the environment of interelectrode space.
Using the electric arc power at the moment of arc forming as the characteristics determining the conditions for its further combustion, the maximum arc length should reflect the state of the interelectrode space environment.Indeed, the analysis of dependences ( ) shows that one can quite often observe the change д l in a certain range of values for the same levels W .In order to try to determine the index, which may allow assessing the influence of the studied substances on the process of arc formation, we use the dependence ( ) д l f W = (Fig. 1).But as an argument we take the value д l .The next step, after approximation of dependence ( ) with the use of straight lines, the slope coefficient determination allows assessing the degree of influence of the substances under study.
For the range of values W and д l (Fig. 1), let us assess the slope coefficient ( C ) using the ratio: where -W ∆ and д l ∆ are the corresponding change intervals W and д l .Taking into account the fact that to each chemical element corresponds its own value of ionization potentials (the first ionization potential, the second and so on [16]) one can make an attempt to assess the specified characteristics of the studied substance ( P ) with the defined simplification degree.The presented position is stipulated by the fact that during the electric arc formation mainly the extraction of outer (valence) electrons takes place, when the extraction of the electrons closest to the nucleus requires several times more energy [13,16].
In order to take into account the complicated joint influence of different chemical elements presented in the area of electric arc discharge at the same time, it is necessary to make two assumptions.
The first assumption -is an assessment of the P values of the studied substances excluding the energy spent on complete dissociation of molecules or chemical compounds to atomic state.The second assumption -the additive nature of ionization potentials influence of chemical elements forming the molecule or chemical compound in proportion to their number is accepted.The influence efficiency of the studied substances on the process of electrical discharge forming that is assessed according to the values C (Fig. 1), in fact, should be connected with the peculiarities of the atomic structure of the chemical elements that form the substances.On the basis of this, one can expect the existence of correlation between the values C and P .The result of pair plotting of the specified characteristics is shown in Fig. 4.
Considering the nature of the presented ration one can talk of the proportional relationship between the angular coefficient ( C ) and the value P .If we assume that under certain conditions the formation of electric arc discharge the value C characterizes the substance influence on the process of arc ignition, it is reasonable to expect the existence of relation with the values characterizing the state of interelectrode space.According to the presented dependence as compared to the other substances under study, filling the interelectrode space of Fe Si − is accompanied by the electric arc formation at the lowest values of specific power.Around 20% more of electric current power is necessary to have to enable the arc discharge excitation while filling the interelectrode space and even greater one for kaolin.

Originality and practical value
1. Based on the influence analysis of the studied substances on the formation processes of electric arc, the inverse relationship between electric current power and the maximum arc length until its natural break was determined.
2. The ratio between the maximum arc length and the electric current power, with a sufficiently high correlation coefficient are subordinated to the exponential dependence.The influence of the studied substances on the electric arc formation is determined using the degree indicators of the proposed relationship.
3. The value of specific power of electric current at the moment of electric arc formation, which is attributed to the arc length, can be taken as a parameter characterizing the state of the interelectrode environment.
Under conditions of the identical adjusting force of electric current the gradation of the studied substances in the order of increase of their influence on the arcing process is determined.Thus, the minimal influence is observed from kaolin, and the maximum one -for Fe Si − .Using the value C allows assessing the influence degree of chemical mixture on the formation of electric arc in practice.

Conclusions
1.Under conditions of the research the metal transfer in the interelectrode space corresponds to the droplet mechanism.
2. The ratio between the maximum arc length and electric current at the moment of forming the electric discharge for the conditions of inverse polarity, has the form of exponential dependence.
3. Substance influence on the state of environment of the interelectrode space can be estimated using the ratio of the change speed of electric discharge power at the moment of arc ignition per unit of length.

2 SiO , Fe Si − , 3 CaCO 2 SiO , Fe Si − , 3
and kaolin.The similar pattern of change is also observed for the value K in the specified order ( CaCO and kaolin): 1.5; 1.4; 1.03 and 0.817.

First
of all, to determine P let us use only the first ionization potential of chemical elements whose values are shown in the diagram (Fig.3).

Fig. 3 .
Fig. 3. Value of the first ionization potential of the chemical elements depending on the serial number of the periodic table.

Fig. 4 .
Fig. 4. The ratios between the values P and C for the studied substances ( -is a kaolin; -is