The book discusses the technological properties of electric welding arcs when welding with low carbon electrodes with various coatings. The influence of energetically: processes at the cathode, anode and in the arc column on the melting performance and the melting effect of electrodes, as well as on the transfer of metal in the arc and the stability of its combustion is shown. The nature of the change in the energy state of individual zones of the arc is established when various substances are introduced into it.
On the basis of the theory of heat propagation during welding, methods have been developed for calculating some technological characteristics of electrodes.
The book is intended for engineers, researchers and graduate students interested in the application of the arc discharge and its energy characteristics.
The properties of the electric arc should have a decisive influence on the features of the electrode welding process. This is due to the fact that the arc is the main source of thermal energy. Other possible energy sources (heating the electrode by current and the heat of chemical reactions during melting of the coating) are of secondary importance. This is confirmed by the following data. When heating rods with a diameter of 4-5 mm made of low-carbon steel at a current density of up to 20 ajmm2, only about 20% of the heat required for melting is released in them by welding current, and the main amount of heat is released at the end of the melting of the electrode, when its ohmic resistance from for warming up. The thermal effect of chemical reactions for the most common industrial electrodes, determined in operation using a special calorimetric technique, does not exceed ± 8-9% of the arc power.
The energy characteristics of welding arcs depend on the type of electrode coating. This dependence can be established at the same current I by the difference in the arc voltage Yes, since the arc power is / Da * It is advisable to compare the values of the so-called nominal arc voltage (arc voltage characteristic of a given electrode at optimal mode welding).
Below are the values of the nominal arc voltage obtained by A.A.
Without cover................................................ ............eighteen
Thin layer of liquid glass ...................................... 17
Chalk and liquid glass .............................................. . 15
Quartz sand and liquid glass ............................. 24
Kaolin and liquid glass ........................................... 28
Obviously, welding arcs with a higher rated voltage, all other things being equal, will be more powerful. The reason for the change in the power of the welding arc during the application of certain coatings lies in the change in the physical conditions for the existence of an arc discharge caused by the coatings.
At present, the characteristics of specific electric arcs when welding with various electrodes are extremely poorly studied. To a certain extent, only phenomena in the arc column are known. At the same time, the processes in the near-electrode regions, which are of great importance for understanding the technological role of the electric arc in the welding process, have hardly been studied. The results of the study of non-welded electric arcs give some idea of the phenomena in the near-electrode regions of welding arcs. Thus, in connection with the variety of types of electric arcs, physicists made attempts to approximately classify them according to the phenomena on the cathode.
A. Engel believes that it is advisable to divide self-sustaining electric arcs into two groups: arcs in which the cathodes noticeably evaporate at temperatures when thermionic emission is still absent (arcs with a "cold" cathode), and arcs in which the cathodes have a temperature sufficient for significant thermionic emission (arcs with a hot cathode).
Low-carbon welding electrodes are based on iron, the boiling point of which is approximately 2740 ° C. Impurities in steel can lead to a decrease in the boiling point of the electrode or to selective boiling at temperatures below the boiling point of iron. For example, manganese evaporates already at 1900 ° C, its losses during welding due to evaporation can be significant. The surface of the droplets at the end of the electrode is almost always covered with slags and oxides, the boiling point of which can also be lower than the boiling point of iron (А! 203-2250е С, SiO2 - 2230 ° С, etc.). The temperature of iron cathodes covered with slags and oxides, due to their evaporation in the arc and significant energy consumption for such evaporation, may not reach the boiling point of iron.
At a relatively low boiling point of iron and possible impurities and slags, noticeable thermionic emission from the surface of drops at atmospheric pressure is theoretically impossible and therefore welding arcs with consumable electrodes should be classified according to Engel's classification as arcs with a "cold" cathode. It should be noted that the separation of arcs proposed by Engel is not strict. Studies have shown that due to local increases in pressure and temperature in the cathode region in arcs with a "cold" cathode, thermionic emission is also possible.
Recently, more subtle phenomenological gradations of arcs have appeared. Thus, V. Finkelnburg and G. Mekker believe that there are arcs without a cathode spot, arcs with a highly compressed and stationary cathode spot, and nonstationary tugs with a cathode spot in rapid and chaotic motion. In nonstationary arcs, the lifetime of the cathode spot is very short, which, when it disappears, is replaced by a newly formed similar spot (or several spots). These arcs in terms of their parameters (current, pressure, state of the cathode surface) sweat most closely to welding arcs with a consumable electrode.
The work indicates that the intensity of the spike movement is significantly influenced by the cathode material. A relationship is found between the rate of evaporation of the cathode and the movement of the spot. With poorly evaporating cathodes, the spot moves more intensively.
An arc with a cathode spot under some conditions can turn into an arc without a spot. According to V. Veitsel, thermal emission of electrons from the cathode plays a significant role in an arc without a cathode spot. In an arc with a cathode spot in the contracted plasma, a cloud of positive ions is formed at the cathode, which tears out electrons from it.
An arc without a spot on an alternating current should burn without voltage peaks in every half-cycle due to the large thermal inertia of the electrodes. In an arc with a cathode spot, there is always a voltage peak at the beginning of each half-cycle. The energy spent on this peak is spent on reorienting the cloud of positive ions and creating the necessary emission conditions at the cathode.
The study of the phenomena in the cathode region, undoubtedly, would be of great importance for welding arcs, but for consumable-electrode arcs this is difficult, since the small length of the showerheads, the presence of a sleeve from the coating and the transfer of metal droplets are thrown by direct observations in the cathode region.
Despite this, some data can be obtained that convinces of a significant difference in the processes on the cathode in welding arcs of different electrodes. For example, analyzing AC welding from voltage oscillograms, it can be established that arcs of different electrodes differ from each other by the nature of excitation in each half-cycle and, therefore, by the characteristics of the cathodes. In the case of electrodes TsM7, OMM5 and TsTs1, voltage peaks during arc excitation exist in each half-period, and according to V. Weinel, such arcs can be attributed to arcs with a cathode heel. The largest voltage peaks are observed at the TsTs1 electrodes. Basic coated electrodes (UONI13, SMI,> 112) under the same conditions form an arc with a voltage peak in only one half-period (Fig. 1).
There are also differences in the intensity of the spot wandering. For example, as shown by high-speed filming, the cathode spot moves slowly on the chalk-coated electrodes, while on the fluorspar-coated electrodes it moves rapidly over the droplet surface.
The movement of the spot is not constant. It can be relatively quiet for a while and then suddenly start moving. The spot can make fast rotational movements around the drop. It is difficult to judge from motion pictures shot at 5000 frames per second whether the movement of the spots is continuous or intermittent. In the case of a very fast movement of the spot, the impression is created that it goes out and instantly reappears in a new, more favorable place, which can even be on the other side of the drop. The anode spot, like the cathode spot, can also wander intensively. Thus, the behavior of the active spots of the welding arc corresponds, according to the classification of V. Finkelnburg and G. Mecker, to the third type of arcs with a non-stationary cathode spot. 
It is very likely that the nature of the movement of the spot on the liquid cathode during welding is close to the nature of the spot wandering on the mercury cathode, which also belongs to the "cold" type cathodes. The cathode spot on mercury consists of separate cells. The rearrangement of these cells (the appearance of new ones and the disappearance of old ones) leads to a rapid chaotic movement of the entire spot. The cell sizes are very small. The current density in one cell is about 106 A / cm2. Arcs from mercury cathodes due to the cellular structure of the cathode can burn simultaneously from several cathode spots. A similar phenomenon is observed in a number of cases during high-speed filming of welding with a low-carbon wire at a current density of more than 18 A / mm2 pa of straight polarity.
Thus, even a purely phenomenological consideration shows that electric arcs when welding with various electrodes have significant differences in the physical processes occurring in them. These differences are the reasons for the change in both the arc power and its stability when applying various coatings.
Differences in the physical and energy characteristics of meadows must inevitably lead to different technological characteristics of the electrodes. Observations show that welding arcs that consume more power are characterized by more intense wandering of active spots. For the first time, G. M. Tikhodeev drew attention to the connection between the nominal arc voltage and its stability. The nominal voltage is also related to the melting rate of the electrode. This was established by I. D. Davydenko and A. A. Erokhin.
Despite the practical importance of these facts, relatively few works have been devoted to the relationship between the technological characteristics of electrodes and the features of electric welding arcs. Only a few works in this direction can be pointed out.
So, K-K-Khrenov showed that substances with a low ionization potential, introduced into the arc, even in small quantities, contribute to an increase in its stability and allow welding on alternating current. In this work, an increase in arc stability was associated with an increase in the degree of plasma ionization.
A. A. Erokhin found that the melting coefficient at straight polarity increases with an increase in the nominal arc voltage. With reverse polarity, the melting rate is less dependent on the rated voltage. This result of the study by A. L. Erokhin, as will be shown below, is of fundamental importance.
In a number of works it was shown that the properties of consumable-electrode welding arcs and the technological characteristics of the process depend on the polarity during welding, the material of the electrodes, the state of their surface and the arc atmosphere. However, in these works, in most cases, no attempts are made to link the arc energy and the technological characteristics of the electrodes.
Research is mainly devoted to the consideration of phenomena from the arc column. One can point, for example, to the typical monographs in this respect by KK Khrenov, A. Ya. Brown and GI Pogodin-Alekseev, GM Tikhodeev. However, the arc column usually consumes a negligible fraction of energy and cannot significantly affect the interaction of the arc and electrodes. The poorly studied near-electrode regions of the arc should have a much greater influence on this interaction.
B.E. thermal energy used to heat and melt the electrode is released in the near-electrode region. "
Of the works devoted to the welding arc, it is possible to name only a few in which the melting of the electrode is investigated in connection with the characteristics of the near-electrode regions. D. M. Babkin considered the action of the near-electrode regions of a powerful submerged arc welding arc on the melting of the electrode wire. Although some provisions of the work of D.M. Babkin (equal value of the electron and ion current at the cathode) meet with objections, he was the first to express important idea on the need for a separate consideration of the action of the near-electrode regions on the melting of the electrode and the corresponding calculations were performed. The Japanese researcher S. Ozawa made a similar attempt to consider the melting of various electrodes in connection with the energy in the near-electrode regions of the arc.
A certain negative influence on the development of studies of the near-electrode zones of the welding arc had the wrong position of K. Compton that for arcs high pressure the cathode voltage drop is numerically equal to the ionization potential of the arc gas. This created the illusion of the possibility of calculating the voltage drop in the cathode region of the welding arc by the value of the ionization potential of the metal vapor of the electrode without taking special measurements. Based on this point of view, for example, an attempt was made to create a model of a welding arc, in which the cathodic voltage drop of various low-carbon steel consumable-electrode arcs in all cases was 8 V, which approximately corresponded to the ionization potential of iron vapor.In reality, the cathodic voltage drop of the welding arc may vary greatly depending on the state of the electrode surface, the type of coating or flux, the welding mode, and this model is not justified.
The obvious connection between the phenomena in the arc and technological characteristics welding electrodes creates certain possibilities for regulating the technological properties of welding electrodes, which can be carried out in several ways. It is possible, within certain limits, to stabilize the processes in the arc (to improve the stability of combustion and reduce spatter) due to the appropriate selection of the electrical parameters of the power sources and the welding circuit. The principle of such regulation consists in the selection of the correct feedbacks in the arc - welding circuit - current source system, which is mainly associated with the establishment of a certain form of the current-voltage characteristic of the current source and its shnamichesky properties.
These phenomena have been investigated in detail by BE Paton. VP Nikitin, I. Ya-Rabinovich, VK Lebedev and MN Sidorenko, DB Keita and others. This method can be called an external way of regulating sinological properties.
Another, much less studied method of regulating the technological properties of electrodes is to actively influence the energy processes in the arc itself by introducing various substances into the arc, sometimes in very small quantities.
This book is devoted to the results of studying the possibility of such regulation of the technological properties of electrodes.
The nature of the melting and transfer of the electrode metal has a great influence on the productivity of welding, the interaction of metal with slag and gases; the stability of arc burning, metal loss, weld formation and other technological factors depend on it.
Melting the electrode. Melting of the electrode occurs mainly due to the thermal energy of the arc. The main characteristic of the melting of an electrode is the linear or mass melting rate, measured by the length or mass of the molten electrode (wire) per unit time. The melting rate depends on the composition of the welding wire, coating, flux, shielding gas, welding mode, current density and polarity, electrode stickout, and a number of other factors. But even for the same welding conditions, the melting rate of the electrode does not remain constant, but can gradually change. Therefore, in practice, the average melting rate of the electrode is used as a characteristic, which is usually determined over a certain arbitrary period of time, but significantly exceeding the duration of the drop transition period.
Since the average melting rate strongly depends on the welding mode, when assessing the influence of various factors on the melting of the electrode, it is sometimes more convenient to use the specific (per unit current) value of this characteristic, which is called the melting coefficient. The electrode melting rate Gp is related to the melting coefficient ap by the expression
where k is a coefficient depending on the choice of units of measurement.
The most important indicators characterizing the melting process of the electrode are also the deposition coefficient csn and the loss coefficient i |). The deposition rate, like the melting rate, is the specific value of the rate
yapkins. The speed of finding bn is related to the coefficient ^ bm d9-
swimming trunks
where gp and gn are masses of molten and deposited metal, respectively.
Expression (2-14) is valid only for electrodes that do not contain metal additives (iron powder or ferroalloys) in the coating.
In the presence of metal additives in the coating, the coefficient "f" can get negative values. In such cases, it is the difference between the amount of lost metal and the amount of metal that passed from the coating. For electrodes of this type, the total loss factor can be determined from the expression
metal additives from the coating.
Using the considered indicators, it is possible to determine such characteristics as the yield of the deposited metal kc and the yield of suitable metal k3.
For electrodes with metal additives in the coating, this indicator can be much more than unity (or more than 100%).
The yield of suitable metal k3 is the ratio of the mass of the deposited metal to the mass of the molten part of the electrode:
parts of the electrode; kn is the coefficient of mass of the coating, which is the ratio of the mass of the coating to the mass of the coated part of the electrode rod.
The melting rate of the electrode for all methods arc welding melting electrode increases with increasing current (Figure 2-23). In a wide range of modes, there is a proportionality between the melting rate of the electrode and the strength of the welding current. However, in the region of low and high currents, the proportionality is violated, which is associated with a change in the energy characteristics of the arc, the size of active spots and current densities in them, and the heating of the electrode with a current. An increase in the melting rate of the electrode at high current densities is also caused by the heating of the electrode rod by the passing current. The heating of the electrode at the overhang is proportional to the square of the current strength, the resistance of the wire and the length of the overhang.
The melting rate of the electrode is mainly determined by the conditions for the release and transfer of heat in the anodic and cathodic regions and depends on the polarity of the current. When welding with reverse polarity, the melting coefficient is practically independent of the composition of the wire, coating, flux or shielding gas. When welding on straight polarity, the melting coefficient varies widely depending on the composition and condition of the wire surface, the composition of the coating, flux or shielding gas (Figure 2-24). The arc voltage also changes accordingly. In practice, the value of the rated arc voltage UH is usually used - the voltage characteristic of a given brand of electrode, wire, flux or shielding gas at the working arc length.
The melting rate of the electrode can be controlled by changing the current strength or the magnitude of the cathodic voltage drop. The ability to increase the melting rate of coated electrodes for
by increasing the current strength is limited due to overheating of the electrode rod. With automatic and semi-automatic welding methods, this
limitation is less significant due to small wire overhangs.
The introduction into a wire, coating or flux of substances that increase the cathodic voltage drop (and, consequently, the nominal arc voltage), increases the rate of wire melting on straight polarity. Changing the composition of the shielding gas has a relatively small effect on the wire melting rate. The application of small amounts of alkali or alkaline earth metal salts to the welding wire drastically reduces the cathode melting rate. This phenomenon is sometimes used for the so-called activation of the wire in order to slow down the melting rate and obtain a fine-droplet transfer of the metal on straight polarity.
When welding with coated electrodes, the melting rate of the electrode also depends on the thickness of the coating. Thickening of the coating leads to additional consumption of heat for its melting, as well as to an increase in the power released in the arc column. For electrodes without metallic additives in the coating, the increase in coating thickness leads to a waste of melting costs. By introducing metal additives or iron powder into the coating, the deposition rate can be significantly increased. An increase in the thickness of the coating and an increase in the content of iron powder in it can significantly increase the current density without fear of overheating of the electrode rod. All of these factors contribute to an increase in welding productivity.
Direct current welding of metals can be carried out in two modes: with direct polarity and reverse. Forward polarity in welding is when a minus is connected to the electrode, plus to a metal workpiece. When welding with a current of reverse polarity, the opposite is true, that is, a plus is connected to the rod, a minus to the product.
When welding with direct current, a thermal spot is formed at the tip of the electrode, which has a high temperature. Depending on which pole is connected to the electrode, the temperature at its tip will also depend, and accordingly the mode of the welding process will depend. For example, if a plus is connected to a consumable, then an anode spot is formed at its end, the temperature of which is 3900C. If negative, then a cathode spot with a temperature of 3200C is obtained. The difference is significant.
What does it give.
- When welding with a current of direct polarity, the main temperature load falls on the metal workpiece. That is, it heats up more, which makes it possible to deepen the root of the weld.
- When welding with reverse polarity, the temperature concentration occurs at the tip of the electrode. That is, the base metal heats up less. Therefore, this mode is mainly used when joining workpieces with a small thickness.
It should be added that the reverse polarity mode is also used when joining high-carbon and alloy steels, stainless steel. That is, those types of metals that are sensitive to overheating.
Attention! Since the temperature is different at the anode and cathode spot, then from the correct connection welding machine the consumption of the electrode itself will depend. That is, the reverse polarity when welding with an inverter is an overexpenditure of electrodes.
In the process of direct current welding, it is necessary to ensure that the metal of the workpieces warms up well, almost to the state of molten. That is, a weld pool should form. It is the direct and reverse polarity of the welding mode that affects the quality of the bath.
- If the current strength is large, which means that the heating temperature will also be high, then the metal will heat up to such a state that the electric arc will simply repel it. There is no need to talk about any compound here.
- If the current is, on the contrary, too small, then the metal will not heat up to the required state. And this is also a minus.
With straight polarity, an environment will be created inside the bath that is easy to guide the electrode. It spreads, so one movement of the rod creates directionality of the weld. At the same time, the welding depth is easily controlled.
By the way, the speed of movement of the electrode directly affects the quality of the final result. The higher the speed, the less heat enters the welding zone, the less the base metal of the workpieces heats up. Decreasing the speed increases the temperature inside the weld pool. That is, the metal heats up well. Therefore, experienced welders set more current on the inverter than necessary. But the quality of the weld is controlled by the speed of movement of the electrode.
As for the electrodes themselves, the choice of polarity is due to the material from which it is made, or the type of coating. For example, the use of reverse polarity in DC welding, which uses a carbon electrode, results in rapid consumption of the weld rods. Because at high temperatures, the carbon electrode begins to break down. Therefore, this view is only used in direct polarity mode. Conversely, a clean uncoated metal rod fills well in the reverse polarity weld.
The depth and width of the weld also depends on the mode used. The higher the current, the more penetration increases. That is, the depth of the weld seam increases. It's all about the heat input in the arc. Basically, it is the amount of heat energy passing through the unit of length of the weld. But it is impossible to increase the current to infinity, even regardless of the thickness of the welded metal workpieces. Because heat energy creates pressure on the molten metal, which causes it to be displaced. The end result of such electric welding at an increased current is a burn-through of the weld pool. If we talk about the influence of forward and reverse polarity when welding with an inverter, then a greater depth of penetration can be provided by the mode of reverse polarity.
Some features of welding with straight polarity
What is straight polarity is defined. Some qualities of welded joints are indicated during the process of joining in the direct polarity mode. But there are still some subtle points.
- Large droplets of metal are transferred to the weld pool from electrodes or filler materials. This is, firstly, a large spatter of metal. Secondly, an increase in the penetration coefficient.
- In this mode, the arc is unstable.
- On the one hand, a decrease in the penetration depth, on the opposite, a decrease in the introduction of carbon into the mass of the metal of the workpiece.
- Correct heating of the metal.
- Less heating of the electrode rod or filler wire, which allows the welder to use higher currents.
- With some welding consumables, an increase in the deposition rate is observed. For example, when using consumable electrodes in inert and some active gases. Or when using filler materials that are applied under certain types of fluxes, for example, the OCTs-45 brand.
- By the way, direct polarity also affects the composition of the material trapped in the seam between two metal workpieces. Usually, there is practically no carbon in the metal, but silicon and manganese are present in large quantities.
Features of welding with current of reverse polarity
Welding of thin workpieces is a process with increased difficulty, because there is a constant risk of burn-through. Therefore, they are connected in a reverse polarity mode. But there are other methods to reduce the danger.
- Decrease the current potential to reduce the temperature at the workpiece.
- It is better to weld with an intermittent seam. For example, make a small section at the beginning, then move to the center, then start joining from the opposite side, then start cooking the intermediate sections. In general, the scheme can be changed. In this way, metal warpage can be avoided, especially if the length of the joint is more than 20 cm. The more welded sections, the shorter each section, the less the percentage of metal warpage.
- Very thin metal workpieces are welded with periodic interruption of the electric arc. That is, the electrode is pulled out of the welding zone, then immediately quickly ignited again, and the process continues.
- If overlapping welding is carried out, then the two workpieces must be hermetically pressed against each other. A small air gap will burn through the upper part. To create a snug fit, you need to use clamps or any weight.
- When joining workpieces, it is better to minimize the gap between the parts, and ideally, there would be no gap at all.
- For welding very thin workpieces with uneven edges, it is necessary to lay a material under the joint that would take up the heat of the process well. Usually a copper plate is used for this. You can also steel. In this case, the thicker the auxiliary layer, the better.
- It is possible to carry out flanging of the edges of the welded products. Flanging angle - 180 °.
Process performance. Many researchers have studied the performance of some straight polarity consumable electrode welding methods. I.I. Zaruba showed that when welding on straight polarity under fluxes OSTs-45, AN-348, AN-3, the surfacing coefficients are higher than when welding on reverse polarity. An increase in the coefficients of surfacing on straight polarity was also found in consumable electrode welding in inert gases and some active gases (hydrogen, argon-nitrogen mixture, Moscow heating gas).
A detailed study of the effect of polarity on the surfacing coefficients when welding in carbon dioxide at currents of 200-500 A (figure on the right) showed that the surfacing coefficients on straight polarity are 1.6-1.8 times higher than when welding on reverse polarity.
A significant increase in the deposition rate, and, consequently, in the melting rate of the electrode wire when welding on straight polarity indicates that much more heat is released on the electrode than when welding on reverse polarity, when the electrode is the anode. The calculation shows that when welding on straight polarity, the amount of heat spent on melting the electrode metal is almost 1/3 more than when welding on. reverse polarity (table below).
The amount of heat spent on melting the electrode metal when welding in carbon dioxide on direct and reverse polarity:
Seam geometry. In straight polarity welding, the amount of weld metal in the weld is much higher than in reverse polarity welding (picture below left). The penetration depth, on the other hand, decreases sharply when welding in straight polarity (figure below on the right).
Chemical composition weld metal. The chemical composition of metal deposited in carbon dioxide in forward and reverse polarity is shown in the table below.
High coefficients of carbon assimilation by the weld metal are noteworthy. This may be due to the extremely low carbon burnout from the weld pool when welding in carbon dioxide, as well as from the electrode wire when the carbon content in it is low. The latter confirms the data on the absence of carbon burnout at its concentration less than 0.1%.
Arc stability. Most in a simple way for evaluating the stability of arc burning is, as is known, its breaking length. Given in table. 37 the results of measurements of the breaking length of the arc when welding in carbon dioxide on the direct reverse polarity and for comparison when welding under the OSTs-45 submerged arc (reverse polarity) show that the breaking length of the arc on the forward polarity is much less than on the reverse one.
It is interesting to note the fact that the breaking length of an arc burning in an atmosphere of carbon dioxide at straight polarity is not less than the breaking length of the arc when welding with reverse polarity under the OSTs-45 flux.
Experiments have shown that welding with a wire with a diameter of 2 mm on straight polarity at relatively low currents (200-300 A) is characterized by reduced arc stability, large spatter (15-18%) and worse weld formation compared to welding on reverse polarity. In this regard, it is inappropriate to weld at these currents at straight polarity. For more high currents(over 400 A), the arc burns much more steadily, the spattering is noticeably reduced, the formation of the seam is improved. For example, when welding on straight polarity with a current of 400 A, metal losses for spattering, waste and evaporation are reduced to 8%, and at a current of 500 A - to 3-5%.
The reason for the formation of flakes is, as you know, hydrogen dissolved in the weld metal. Hydrogen can also reduce the plastic properties of the metal. It was found that from seams welded on straight polarity, 3-5 times more hydrogen is released than from seams welded under the same conditions on reverse polarity (table below).
The amount of hydrogen released from the metal deposited under the protection of carbon dioxide:
In reverse polarity welding, the excess of electrons that can be expected near the surface of the weld pool shifts the reaction equilibrium to the left and prevents hydrogen dissolution. When welding on straight polarity, the conditions for the absorption of hydrogen by the weld metal are more favorable.
Another mechanism of increasing the hydrogen content in the weld during welding at straight polarity is also possible. The number of droplets transported through the arc per unit time in welding with straight polarity is much greater (figure on the left) than when welding with reverse polarity. In this regard, the surface of their contact with gases increases, and, consequently, the hydrogen content in the liquid metal can also increase.
Increasing the degree of drying of carbon dioxide (table above) reduces the hydrogen content in the weld.
5.1 Purpose of work
Study of the influence of welding mode parameters on the electrode melting process, familiarization with the method of experimental determination of electrode melting characteristics.
Theoretical introduction
The heat introduced by the welding arc into the electrode is spent on heating and melting the electrode rod and electrode coating. The melting process of the electrode rod and the transition of the molten metal into the weld pool depends on a number of factors: the magnitude, type and polarity of the current, the composition of the electrode coating and the rod, the position of the weld in space, etc. The properties of the electrode, characterizing the productivity of its melting, are estimated by the melting coefficient α p, determined by the formula
where g p is the mass of molten metal, g;
I is the welding current, A;
t is the electrode melting time.
During welding, losses of liquid metal are observed due to its oxidation by air and through the slag, as well as as a result of evaporation and spatter outside the weld pool. Waste and spatter losses are estimated by the loss factor
Waste and spatter losses vary widely depending on various factors. For manual arc welding, the melting coefficient, depending on the specific brand of the electrode, is 8-15 g / Ah, the loss coefficient is 5-30%; for automatic welding under a layer of flux - α p = 13-23 g / A · h, ψ = 2-4%.
An increase in the welding current leads to an increase in the temperature of the arc column and the intensity of electrode melting and, as a consequence, to an increase in α p. At high current densities, the transition of metal droplets from the electrode to the seam can be of a jet character, which reduces spatter losses.
When welding with reverse polarity, the melting performance is significantly higher than when welding with alternating current and with direct polarity. This is explained by the fact that 2-3 times more heat is released at the anode than at the cathode, due to the bombardment of the anode with fast electrons, while at the cathode energy is spent on their emission.
The values of α p and ψ are influenced by the type of electrode and the composition of the rod, which determines the composition of the atmosphere of the arc column and, as a consequence, the effective ionization potential. In turn, a change in the effective ionization potential leads to a change in the temperature of the arc column in accordance with the empirical formula applicable for manual arc welding
T = 800U eff (5.3)
An increase in the temperature of the arc column leads to an increase in the amount of formed gases, increases their pressure in the drop of electrode metal and, ultimately, can lead to increased spattering.
The coefficient α p significantly depends on the heating temperature of the electrode rod. Heating the electrode rod by Joule heat accelerates its melting in an arc discharge and α p increases, while the value of ψ remains practically unchanged. In automatic and semi-automatic welding, welding with an increased wire overhang (the distance between the current-supplying nozzle and the product) is widely used to increase α p. An increase in overhang leads to an increase in the resistance of the wire and, as a consequence, an increase in the temperature of its heating. In manual arc welding, the inconstancy of α p during the combustion of the electrode rod can lead to a violation of the seam formation mode, therefore, the maximum current strength for each electrode diameter of a particular brand is strictly limited. The uniformity of electrode melting is facilitated by an increase in the thickness of the electrode coating, since it does not conduct current, is not heated by Joule heat and cools the electrode shaft.
Equipment and materials
1. Posts of manual arc welding on direct and alternating currents, complete with devices for measuring the welding current.
2. Technical scales with weights.
3. Stopwatch.
4. Vernier caliper and ruler.
5. Welding electrodes MR-3 Æ4 mm.
6. Mild steel plates.
The order of work
1. Clean, mark and weigh the plates to be welded.
2. Prepare the electrodes, mark, determine the diameter and initial length of the electrode rod.
3. For each brand of electrode, determine the mass l linear centimeter of the electrode rod, which is equal to the mass of the electrode rod cleaned from plaster, divided by its length.
4. Surfacing the bead onto the plate with an electrode with a direct current of reverse polarity. In the process of surfacing, record the arc burning time and amperage (the recommended amperage for all variants of experiments is 120-200 A) with subsequent entry into table 5.1.
5. After surfacing, cool, dry, remove slag and weigh the plate. Determine the mass of the deposited metal and enter the result in Table 5.1.
6. Measure the length of the electrode part remaining after surfacing and calculate the mass of molten metal, followed by entering it into table 5.1.
7. Calculate the characteristics of the electrode melting with subsequent entry into table 5.1.
8. Repeat the experiment according to claim 4 with the changed values of the current strength 2 times.
9. Repeat the experiment according to claim 4 for direct polarity and alternating current.
