Metal-Gas Interaction at Arc Metallization

METAL-GAS INTERACTION AT ARC METALLIZATION

V. Boronenkov , M. Shalimov, Yu. Korobov

Urals State Technical University - UPI, Ekaterinburg, Russia

Abstract

The offered scheme of an oxygen input into a drop at electro arc metallization (EAM) takes into account processes both external, and internal diffusion. The separate stage of a drop shattering in an arc-burning zone is allocated. It appreciably influences over the total oxygen input. The oxygen input into metal is separately considered at two steps. At first the oxygen is dissolved in liquid metal of a drop up to a limit of saturation, and then the formation of slag begins accompanied by metal – slag interaction. These processes were modeled in case of 1.1 C, 1.9 Mn, 0.8 Si steel, which was metallized by propane - air mixture. Good similarity with experiment was achieved. Is shown, that the bulk of oxygen input occurs in the arc-burning zone. The modeling results are useful at metallization gun designing and at sprayed wires developing as well.

At EAM process a metal of feed wires is melted in electric arc and atomized by transporting gas flow, fig. 1. Process was divided to three consecutive stages, which are stood out from each other by melted metal - gas interaction features.

1 stage. Metal at an electrode face

Metal is continuously fed to interaction zone and is removed from it to drops. Oxygen content at gas phase is constant. According to [1, 2], temperature of metal at the end face makes 2400 – 2700 K. We accepted T = 2500 K. Evaluation of mass-transfer coefficient of oxygen in the melt (Ме) was done according to data of near-by processes of arc welding and plasma-arc remelting [2, 3]. At the electrode face zone it is by an order of magnitude greater then at metallurgical practice. Obviously, it is explained by the increased value of diffusion coefficient at temperature of a drop and by thinning of a boundary layer owing to drop’s intensive stirring. According to Erohin’s experimental data m/s at arc welding on drops [1]. We accepted .

Fig. 1. Scheme of electro arc metallization process

2 stages. Metal in an arc-burning zone

According to our mesh analysis, drops diameter is within an interval of (10 - 237)10-6 m. Formation of metal drops was considered according to a “tongue” scheme [4]. Metal is drifted from electrode face on exposure to gas head. It is stretched forth in a “tongue” form of a thickness ~ 1010-6 m, but it isn’t teared away while equilibrium between surface tension force and gas head takes place. Further breakage by necking way takes place at balance disturbing. Breakage is sharply amplified at the expense of joint processes of necks explosion and gas strike by means of necks current heating. It is accepted, that joule heat results in increase of temperature in necks up to 2900 K. Mass-transfer coefficient was accepted as at the first stage.

The form of language constantly changes under joint influence of hydrodynamic and electrodynamics’ forces, and it is represented inconvenient to describe it analytically. For calculation of a drop surface size the performance of the specified portion of metal as "tongue" (fig. 2a) is replaced by the mode submitted in a fig. 2b. Under this mode the drops of the spherical form are formed in the arc-burning zone. Their diameter is equal to the minimal size of drops dp = 1010-6 m. Upon termination of this site the distribution of the drop’s sizes corresponds to the mesh analysis. The physical picture of process has more complex character [5], however this simplification allows executing calculations. Further according to experimental data by oxygen input it is possible to modify initial drop diameter din.

Fig. 2. Drop motion scheme in the arc-burning zone, which is taken

at calculations.

3 stages. Flight on a spraying distance

Our earlier modeling [6] showed the following. Approximately after 15 - 25 mm of a spraying distance the temperature of drops is reduced up to melting point Тm = 1809 K. It remains constant up to the end of the distance, about (100-150)10-3 m, at the expense of allocation of melting enthalpy. Here an interval of speeds of drops is (120 – 250) м/с. At reduction of a fraction from dmax = 23710-6 m to dmin = 1010-6 m speeds of drops on the distance are increased in 2 - 3 times. Time of interaction of drops with oxygen accordingly differs. Temperature of gas falls from 2300 K up to 600... 800 K. Owing to intensive stirring of the gas jet with an atmosphere after termination of the initial site there are no other components, except for the air, in the gas jet. A physical model of oxygen input into the drops at EAM was offered by us, see fig. 3. When formation of independent oxide phase is impossible (G > 0), oxygen is dissolved at liquid metal. The temperature dependence of a saturation limit is expressed by the equation [7]:

.(1)

Owing to high temperature at the end face and in the arc-burning zone the saturation limit of oxygen in metal is much higher, than that one at spraying distance. At the first and the second stages all acting oxygen, or, at least, its basic part, is dissolved in the liquid metal. Thus the specific surface of drops in the arc-burning zone, according to the “tongue" scheme, is much more in comparison with the area of the end face. It results in the most favorable conditions for oxygen input in metal in the arc-burning zone.

Fig. 3. Physical model of oxygen input in various stages of EAM process.

At the third stage the temperature of drops is reduced up to melting point. It results in decrease of the saturation limit and, accordingly, to extraction of the oxygen from the oversaturated solution. The oxygen, extracted from the solution, forms a slag film on a surface of a drop. At the expense of influence of the transporting gas flow this film is pushed aside from a stern part of the drop, leaving open for oxygen input a nose part of a surface (fig. 3, view А). Outside oxygen input is expended on formation of an additional slag film, which also is pushed aside into nose part. Here there is a redistribution of elements between slag and metal. The arrangement of slag in the nose part is explained by smaller speed of drops concerning gas.

It is accepted, that the share of a surface, free from slag (L), remains constant all time of a drop life. Such scheme of process is rather conditional. A change of a drop speed respectively a gas flow; presence of rotary and transitional drop movement take place. It results in constant change of size of a share of a surface, free from slag. However, comparing calculations with measured oxygen content in the coating, final point of flight of a drop, it is possible to accept average value of L.

Such performance concerning slag distribution on the surface is well coordinated to the data on fusion of metal in an electromagnetic field of inductor [8] and with metallographic researches of EAM process [9, 10]. Here an iron oxide presence was specified on a surface of liquid metal and in the EAM coating.

We examined process for concrete example. Wire Св08Г2С (0.110 C, 1.92 Mn, 0.79 Si, 0.020 P, 0.021 S) was taken as initial metal. Total of the oxygen input which has receipted in a drop of i - fraction, will be equal to the sum of oxygen dissolved in liquid metal and which is taking place in slag:

. (2)

The degree of oxidation of drops in the given fraction (ri) will make:

. (3)

The common degree of oxidation R is summarized according to distribution of drops according to a mass share:

. (4)

Here for drops of the i - fraction: mMe - mass of liquid metal; mш - mass of slag; dmi - mass share of drops of the i - fraction ; М - molecular weight of a component; (FeO), (MnO), (SiO2) - concentration of components of slag, mas. %.

On the [1, 11] basis we have accepted, that the process of absorption of oxygen passes in diffusion mode, and the influence of reactions with a gas phase as a first approximation can be neglected. The rating of a limiting stage by us was carried out on possible limiting diffusion flows in gas and in metal [4]:

(5)

(6)

Here: - mass-transfer coefficients of oxygen in gas and in metal; c- concentration of oxygen; - partial pressure of oxygen; T - temperature.

The calculation has shown, that at an end face of an electrode an internal task of mass-transfer takes place, for the second and the third stages an external task takes place.

The calculation has shown, that at EAM in comparison with welding a mass-transfer intensity is about 30 % as large for the arc - burning zone, and it becomes less in flight for EAM drops of the basic range of diameters. At all stages at EAM and at welding this parameter is of the one order of magnitude. It confirms legitimacy of use the values of welding parameters with reference to EAM process.

At an interaction kinetics process between melted metal with an environmental atmosphere at the above-mentioned stages were considered as in [1], but with the account of diffusion both in metal and in gas. Allowing for only one of flows deforms a picture of occurring processes.

For open system, when reagents supply and remove take place, process is described by first order equation, i. e. process rate is in proportion to concentrations at the first order:

(7)

here: - - quantity of oxygen in melted metal, г; - density, square, volume of melted metal; gпл, gотв - mass speeds of melting and плавления and removal of metal; [O]0, [O], [O]' - initial, current and surface concentration of oxygen, mas. %, correspondingly.

The additional equation for finding [O]' and [O] we shall receive from a condition of equality of diffusion flows through the phase separation surface:

j г = j Me (8)

These diffusion flows can be expressed through partial pressures and concentrations of oxygen in gas and in liquid metal:

(9)

(10)

With the account of (9, 10) the equation (8) will get a kind:

; (11)

where: ,,

here: k – reaction equilibrium constant of an iron with a gas phase, [11].

We shall accept, that in the first stage, at the end face of the electrode, quasi-stationary regime is established. Here speeds of feeding and removal of the reagents are identical: gпл = gотв = g; and volume of a liquid layer at an end face of an electrode is constant: . In this case equation (7) will accept a following form:

(12)

From the joint decision of the equations (11) and (12):

;

, (13)

where: ;

On the second and the third stages, in the arc - burning zone and in flight, we consider, that the drop represents the closed system, in which feeding and the removal of metal does not occur: gпл = gотв = 0

The third stage differs from the second one that the temperatures of gas and metal are reduced, partial pressure of oxygen is increased, and mass-transfer coefficient in gas is decreased. The equation (7) for the second and the third stages will accept a following form:

(14)

If dividing a duration of process into such small intervals of time, that , for a (n+1) interval of time it is possible to write down:

;

where

(15)

From gas/metal diffusion flows equilibrium condition Iгаз = IМе for (n+1)'s step equation (11) looks like:

. (16)

Oxygen concentrations [O]n+1 и [O]' are determined from joint solution of (15) and (16) equations:

, (17)

. (18)

Only slag formation takes place, if . In that case, equation (15) is used to determine oxygen content in metal, here is specified. Diffusion flow from gas to metal is the following:

. (19)

In addition oxygen can be allocated from oversaturated solution when decrease of drop temperature upon termination of the arc - burning zone takes place. Additional diffusion flow is determined from the following equation:

. (20)

In the nose part of the surface on a step, the following quantity of slag is formed:

. (21)

In comparison with the previous step mass of liquid metal will be decrease at the expense of transition iron into slag:

. (22)

The slag on the equation [21] will be added to already formed one. The contents of the slag components, (FeO) and the other components of type, on a step after events in the stern part will make:

, (23)

. (24)

We assume, that diffusion mode of metal - slag interaction takes place in the nose part of the drop. Approach, which was put by V.Boronenkov with co-authors [12], was used. Following reactions describe the interaction process:

lgK1 = - 5152/T + 4.8 (25)

lgK2 = - 6320/T +2.734 (26)

[Mn] + (FeO) = [Fe] +(MnO) ;

lgK3 = 5550/T - 2.34 (27)

;;

lgK4 = 7825/T - 3.06. (28)

At (25 – 28) constants Ki include reaction equilibrium constants, coefficients of activity, and coefficients of a concentration conversion from mole magnitudes to mass ones. The temperature dependences of Ki are accepted according to [11]. Sulfur and phosphorus influence at slag content was ignored because of small concentrations. On a condition that each reaction proceeding is accompanied by definite demand of iron oxide, summary diffusion flow of additions equals to FeO diffusion flow:

IFeO - IC - IO - IMn - 2ISi = 0 (29)

For taken components, that equation looks like:

(30)

here .

Here the following assumptions were taken:

- at phase separation border;

- iron diffusion as the braking part of process can be excluded in connection with its high concentration. In that case x = (FeO).

Limiting diffusion flows of the components on the metal - slag border are the following:

. (31)

Here: Сi, ci - molecular and mass concentration; - convection constant,  - density of metal or slag; Mi - molecular mass of components; - mass transfer constant; I - thickness of a diffusion layer;  = 0.5 [11].

From area of the most probable values  = (0.5 – 2.0) c-1/2 [13], by us it is accepted that.. The values of diffusion coefficients Di of components in the melt are accepted according to [11].

After determination (х) from the equation (30) the diffusion flows on the metal-slag border are calculated (for FeO - IFeO, for components - IЭ). The contents of oxides and elements in a drop after events on the step in the nose part are the following:

MnO, SiO2 in slag:

(32)

FeO in slag:

(33)

C, Mn, Si in drop:

(34)

Oxygen in drop:

(35)

Mass of slag:

(36)

Mass of drop:

(37)

Concentration of slag components:

(38)

(39)

The degree of drop’s oxidation of i - fraction (ri) is determined by the equation (3), and total degree of oxidation (R) - by the equation (4).

Truth of calculations was tested by a measurement of oxygen quantity in the coating by methods of melting in inert gas atmosphere and in vacuum (instruments "RO - 116" и "EAN - 220"). Samples were cut out from the coating, which was sprayed at the following modes: current - 180 A, voltage - 30 V, spraying distance - 100 mm, coating thickness - 1 mm. Two types of metallizing guns were used:

- in the first one the products of combustion of propane - air mix were applied as transporting gas, input pressure of gases: air 3.4 Atm, propane 2.8 – 3.2 Atm;

- in the second one the compressed air was applied, input pressure 6.0 Atm.

Comparison of calculations with experimental data is shown at fig. 4. It shows, that at restricted quantity of oxygen in the arc - burning zone (ок = 0.8 – 1.2, corresponds to = 0.07-0.11 Atm) the calculated magnitudes hit the experimental data interval at size of the free from slag surface L = 0.4 - 0.5. When only compressed air is applied as transporting gas ( Atm) the meeting area lays at L = (0.3 - 0.35), here velocity of the oxygen input is slowed down. Obviously, the reduction of that share occurs because of decrease of a superficial tension of liquid metal at the presence of oxygen.

Fig. 4. Conformity of calculations with experimental data of the oxygen input.

The contribution of drops of small fractions to the oxygen input exceeds their mass share, and for drops of the large fractions reverse proportion takes place. The contribution of drops of fractions of (27 – 75) microns in the total oxygen input is (1.5 - 2.5) times as small, than that one of drops of other sizes, fig. 5. Obviously, it is connected with a ratio of a specific surface and a speed of the drops on the distance. A well of oxygen input at (27 – 75) microns fraction interval is favorable for coating quality, so it is necessary to aspire to such fraction structure of a drop jet at designing the equipment and development of technologies.

Fig. 5. Relation between a fraction contribution (dR) and the oxygen input.

The basic part of the oxygen input, 85-95 %, occurs in the arc-burning zone. It is caused by extremely high value of mass-transfer coefficient in gas and large specific surface of the drop. The oxidation degree is strongly depended on the content of oxygen in gas at this stage (fig. 6).

On the first and the second stages all acting oxygen is dissolved in the metal of the drop. The flight stage is marked by a sharp fall of the saturation limit and by the oxygen allocation from oversaturated solution as a result. Owing to joint influence of that process and the oxygen input from an atmosphere in the stern part only slag formation takes place at that stage. On a working distance the mass share of slag constitutes 6 - 17 %. That share is increased according to growth of partial pressure of oxygen and decreasing of the drop’s diameter (fig. 7).

Fig. 6. Oxygen input distribution on stages at various partial pressure

of oxygen (Р).

Fig. 7. Mass share of slag (N) at the drops of boundary fractions at various partial pressure of oxygen (P). L = 0.4.

Fig. 8. AAM gun without combustion chamber. Steel wire is sprayed

Fig. 9. AAM gun equipped with combustion chamber. Steel wire is sprayed

The specified features were taken into account at development a row of gun models for activated arc metallization (AAM). Reduced transportation gases, original gas feed through profiled nozzles, combustion chamber to form gas jet of supersonic velocity and of high temperature, special relative position of nozzle and current tips are used at AAM guns. Their technical characteristics are approximately identical: wire diameter - 1.5-2.2 mm; wire output, kg /h: 18.4 (steel), 6.5 (aluminum), 16.8 (zinc); mass - 3.1 kg; wire utilization factor – 0.85.

The gun shown in fig. 8, is suitable enough for wear-resistant recovery of average loaded parts. Necks of journals and axles, brake barrels, pistons are examples of stable successful results.

The main distinctive feature of the other model is presence of compact high efficiency propane-air mix combustion chamber (fig. 9). Supersonic jet of the mix has the speed of 1500 m/sec and the temperature of 2200 K at the outlet.

AAM process is resulted in wear-resistant coatings at high loads, up to100 MPa. Diesel engine crankshafts are recovered well, for example. Besides, anticorrosion coatings were sprayed at water purification tanks, heating boilers, digesters.

A wire used as a material for EAM spraying can be made of any metal (zinc, aluminum, copper, brass, bronze, carbon and stainless steel, nickel-chrome alloy etc.) as well as powder wire. Owing to the developed metal – slag interaction model a prediction of a chemical composition of the coating is possible in accordance with an initial wire analysis. A decision of a reverse problem can be done also.

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