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Section 5. Machinery construction
Dunyashin Nikolai Sergeevich, Associate Professor, Candidate of Science (Engineering) Tashkent State Technical University, Head of the Department of Process Machines and Equipment E-mail: dunjashin-nikolajj@rambler.ru Galperin Leonid Vladimirovich, Deputy Director, Po'lat-quvurservis LLC JV E-mail: nikitin.spttz@gmail.com Ermatov Ziiadulla Dosmatovich, Associate Professor of the Department of Process Machines and Equipment, Tashkent State Technical University E-mail: ermatov-ziyadulla@rambler.ru
DEVELOPMENT OF THE GAS-FORMING COMPOSITION OF ELECTRODE COATING FOR A HIGH-QUALITY CAST WELD STRUCTURE
Abstract. This article proposes the mathematical model of the gas-forming part of the electrode coating that ensures a high-quality cast metal weld with manual arc covered-electrode welding.
Keywords: Electrode, manual arc welding, gas-forming components, marble, potash, magnesite, dissociation, carbonate.
The gaseous protection of the reaction zone of welding and molten metal of the main electrodes is provided by carbonates of alkaline and alkaline earth metals, which oxide participates in the formation of slag [1-3] after the carbonate decomposition.
As carbonates in electrode coatings, marble CaCO3, magnesite MgCO3, potash K2CO3, sodium carbonate Na2CO3, carbonates of other metals, as well as combined materials are used.
The presence of CaO, MgO, K2O, Na2O oxides upgrades the arc stability, removes harmful impurities of sulfur and phosphorus from the weld metal and affects the physico-chemical properties of slags.
The dissociation of carbonates proceeds according to the scheme:
MeCO3-MeO + CO2 CO2 -CO + 1/2O2
Dissociation of carbonates of different metals is accompanied by the release of different amounts of carbon dioxide. The volume of gases formed during the dissociation of carbonates make up the electrode coating charge. We shall consider the
calculation of the amount of CO2 formed during the carbonates dissociation on the example of the dissociation of CaCO3.
The coating mass ratio makes RCM = 0.30.. .0.32, the share of carbonate CaCO3, which provides CO2 during the dissociation, makes 5.6% in the electrode charge. Then 1 g of the molten electrode accounts for 0.37 g of the charge, and in terms of the amount of CaCO3 in grams
mCaCOz = 0.37 • 0.048 = 0,0178^ To determine the mass of CO2 emitted, we have to know the molecular mass of all substances of the chemical reaction of carbonate decomposition. With regard to the dissociation of CaCO3, the molecular weights of the substances are MCaCOz = 100, Mco = 56 and MC03 = 44.
Calculation of the amount of carbon dioxide per 1 g of molten electrode metal gave the following results
M.
mCO mCaCO,
CO2
M
CaCO3
Calculation of the volume of CO2 produced by melting 1 g of an electrode shall take into account that the volume of a gram molecule of gas is equal to 22.400 cm3. Then
Section 5. Machinery construction
V^ = 22400
m,
CO2
Mr
The calculated gas volume is specific since it corresponds to a temperature of 273 K. As temperature increases, the gas volume increases too and is determined by the formula
Table 1.- The results of calculating the emitted amount of CO2 for different carbonates
Va = VT +a(T — 273) where a is the gas formation volume factor, a = 0.00366 K-1, T - gas temperature, K.
The results of calculating the emitted amount of CO2 by the above method for various carbonates are given in (Table 1).
Carbonate T , °c dis/ mco2, g T/273 3 VCO2 > cm3 T /-1700 3 Vco2 > cm3 T 7-2500 3 Vco2 > cm3 t 7-2700 3 VCO2 > cm3
CaCO3 880-1200 0.0078 4.0 26.2 35.3 40.6
MgCC>3 350-650 0.0093 4.7 31.3 42.0 48.2
Na2CO3 1000 0.0074 3.8 24.9 33.0 38.3
K2CO3 1200 0.0057 2.9 19.1 25.6 29.3
Calcium and sodium carbonates contain a sufficiently large amount of carbon dioxide, however, they dissociate incompletely and continue to dissociate in the weld pool, contributing to porosity in the weld metal due to the relatively high dissociation temperature and high movement speed in the arc gap.
Magnesium carbonate reveals high protective properties and is characterized by a lower dissociation temperature. It completely dissociates in the arc gap, increasing the resistance of the weld metal against pores. Potassium carbonates emit a smaller amount of CO2, have a high dissociation temperature, which degrades the protective properties and contributes to porosity.
The process of developing an electrode coating that provides an integrally alloyed cast metal is faced with the problem of reducing the number of components of the electrode coating. The content of alloying components (ferroalloys) in the electrode coating in the required amount allows optimizing the gas-slag component of the coating. This is achieved by reducing the slag-forming and gas-forming components in the coating composition and may cause deterioration in the protection of the cast metal, leading to the appearance of internal defects and a decrease in the strength properties. Therefore, the relevant task is to optimize the content of gas-forming and slag-forming components in the composition of the electrode coating to provide reliable protection against the interaction with air nitrogen.
The analysis of these data served as the basis for creating a rational composition of the gas-slag-forming part of the coating. Varying the content of the gas-slag-forming composition allows influencing the kinetics of gas generation, the
Table 2.- Experimental results
uniformity, and completeness of decomposition of the gas-forming components of the electrode coating. Based on the results of calculating the emitted volume of CO2 during the dissociation of carbonates and the data on their dissociation temperature, the composition of carbonates MgCO3, CaCO3, K2CO3, Na2CO3 was used as the gas-forming part of the electrode coating.
The relationship of nitrogen content in the cast metal of the weld to the percentage composition of carbonates of sodium, potassium, magnesium, and calcium was studied. To develop a mathematical model of the relationship of nitrogen content in the cast weld metal to the percentage composition of sodium, potassium, magnesium, and calcium carbonates, 15 electrode coating compositions for manual arc welding were studied which had a varying content of carbonates of alkaline and alkaline-earth metal (Table 2). The content of the main slag-forming components CaF2 and SiO2 in the charge of the electrode coating is 80%.
The nitrogen content in the cast weld metal, obtained by eight-layer welding, was determined using special cut samples with a diameter of 4 mm with the help of a TS-136 (TC-136) gas analyzer no later than 36 hours after the build-up. The following surface build-up mode was used: IW = 160...180 A, UD = 29.30 B, VW = 19.3 m/h.
A mathematical model was created based on a simplex-centroid four-factor experiment plan, a randomized order of experiments, and a special cubic model.
The results of the study showing the effect of the carbonate composition components ratio on the nitrogen content are shown in (Table 2).
№ Carbonate content, relative units Average nitrogen content in the build-up [N],%
CaCO3 MgCO3 Na2C°3 K2CO3
1 2 3 4 5 6
1. 1 0 0 0 0.0362
2. 0 1 0 0 0.0326
2
1 2 3 4 5 6
3. 0 0 1 0 0.0378
4. 0 0 0 1 0.029
5. 0.5 0.5 0 0 0.0333
6. 0.5 0 0.5 0 0.0376
7. 0.5 0 0 0.5 0.0325
8. 0 0.5 0.5 0 0.0338
9. 0 0.5 0 0.5 0.0297
10. 0 0 0.5 0.5 0.0336
11. 0.334 0.333 0.333 0 0.0354
12. 0.334 0.333 0 0.333 0.0319
13. 0.334 0 0.333 0.333 0.0341
14. 0 0.334 0.333 0.333 0.0328
15. 0.25 0.25 0.25 0.25 0.0337
As (Table 2) shows, the adequacy of the results obtained according to the mathematical model is checked by the Fisher factor F. The calculated statistics F is compared with the tabular value of the Fisher factor. If the model is inadequate, then transition to a more complex model is required. The significance of this model during testing the adequacy is p = 0.011, therefore the resulting model is adequate with a confidence level of 98.9%.
The significance of the regression factors is assessed using Student's t-test. Coefficients of the model for various factors are considered statistically significant at p = 0.05. If p = 0.05, then the coefficient is considered insignificant and should be excluded from the model.
Given the significance of the model coefficients for all factors, the mathematical description of the response surface is following:
[N]=0.0362-CaCO3 + 0.0326-MgCO3 + 0.0378 ■
■ Na2CO3 + 0.029 ■ K2CO3 - 0.004785 CaCO3 ■
■ MgCO3 - 0.004785 MgCO3 ■ Na2CO3 + 0.002617 ■ Na2CO3 ■ K2CO3 + 0.022295 ■ CaCO3 ■ MgCO3 ■
■ Na2CO3 - 0.01(973 ■ CaCO3 ■ Na2CO3 ■ K2CO3 +
(1)
+ 0.01609 ■ MgCO3 ■ Na2CO3 • K2CO3
Magnesium carbonate has the lowest dissociation temperature; it dissociates completely when approaching the arc
References:
gap and provides gas protection at relatively low temperatures. Sodium carbonate has the highest temperature of dissociation; it dissociates incompletely in the arc gap with excessive content in the composition and continues to dissociate in the weld pool, contributing to porosity in the weld metal. Calcium carbonate begins to dissociate at lower temperatures than sodium carbonate, thereby ensuring their more complete dissociation directly in the arc gap, which prevents the formation of pores in the weld metal. The established influence of the content of the carbonate composition is due to a decrease in the CO2 content at the early stage of the arc process, which is compensated by an increase in the volume of CO2 directly in the zone of the arc and molten metal. Calcium oxide also favorably affects the efficiency of sulfur and phosphorus harmful impurities removal from the weld metal.
Thus, based on the analysis of graphical relationships and the literature data presented, the following composition ratio of carbonates CaCO3 = 96%, K2CO3 = 1...2%, MgCO3 = = 2.3%, Na2CO3 = 1.2%. is considered as the most effective composition of carbonates, providing a low nitrogen content in the weld metal. This ratio provides reliable protection of metal from nitrogen due to the uniform release of protective gases in a wide temperature range increases the resistance of the weld metal against pores and improves both physical and technological properties of slags.
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Dunyashin N. S., Galperin L. V., Ermatov Z. D. On the development of a physical simulation of the cast metal weld chemical composition formation during manual arc welding on the basis of the electrode coating mixture components classification // European science review 2019.- No. 1-2.- Vol. 1.- P. 40-41.
