Features of Cored Wire Heating

FEATURES OF CORED WIRE HEATING

AT ELECTRO ARC METALLIZATION

Yu. Korobov, M. Shalimov, V. Shymiakov

Ural StateTechnicalUniversity - УПИ, Russia

Abstract

Specific features of Electro Arc Metallization prevent to ensure quality of coatings at usage Cored Wires, designed with reference to welding and surfacing. It is connected with heightened oxidation of drops of sputtered metal and with uncompleted fusion of a charge core. A model of heat propagation in the cored wire from exposure of electric arc is introduced. Analysis of computation has shown, that pattern of cored wire fusion is strongly influenced by thermal and geometrical parameters of cored wire and modes of a spraying as well.

At usage of Cored Wires (CW) essential structure and properties of coatings are allowed by variation of charge components. Specific features of Electro Arc Metallization (EAM) prevent to ensure quality of coatings at usage CW, designed with reference to welding and surfacing. In comparison with the latter one specific surface of drops is by an order of magnitude greater, and the dwell-time of metal in liquid state is an order of magnitude less. The first feature easies receipt of oxygen into metal, and second – reduces efficiency of measures on deoxidation because of restricted time of interaction in a system "gas-CW components". Besides, incomplete fusion of a charge core is possible [1]. It results in lowering of adhesive and cohesive strength of coatings, increase of their porosity, decrease of alloying charge components influence.

With reference to solid wires we describe basic processes at EAM: fields of speeds, temperatures in a diphase flow [2], fusion and separation of a sputtered stuff from electrode end face [3], kinetics of interphase interaction with reference to a system "gas - drop of sputtered metal" [4]. CW differs by irregularity of thermal properties on cross-section from shell to charge. It changes nature of thermal and physic-chemical processes at electrode end face sharply.

For analysis of the indicated processes with reference to CW model of propagation of heat, which is caused by electric arc heating at EAM, was offered [5]. Heat propagation in area of electrode end face was considered as axially symmetric problem and it is depicted by differential equation of heat transferring:

, (1)

Where Т - temperature, K; t - time, s;  - thermal conductivity, W / (mK); c - specific heat, J / (kgK);  - density, kg / m3 ; x, y - coordinates on cross-section and through CW generating, accordingly, m.

Equation [1] was numerically solved by explicit difference method. The considered area was divided on radius (r) by coordinate lines i = const with step r = r/10 and on depth (s) by coordinate lines j = const with a step s = s/10 (fig. 1). Within borders of element temperature is considered as constant and is related to its middle.

As electrical conductivity of shell is by a factor of 102, than charge one; it is accepted, that active arc spots are localized at ends faces of shells. So, the surface temperature of a shell at end face is adopted 2500 K. Such overheating is typical for arc processes [6]. The surface of charge receives heat from arc column by radiation and is heated thus up to 1300 K.

At calculation the heat propagation during 410-4 s is reviewed, according to data of oscillography measurement of drops dwell-time at end face before separation in case of solid wire metallization [7].

The propagation of heat from end face surface deep into electrode is influenced by thermo exchange "shell-gas" and "shell-charge": shell, heated up by arc, give away heat to transporting gas stream and to charge.

At calculation of thermo exchange " shell-gas " the factor of convective surface heat exchange is determined by equation:

Fig. 1. a) Sketch of electrode end face at Electro Arc Metallization.

b) Scheme for heat propaganda computation.

(2)

Nusselt criterion is calculated by equation used for cross-flow of tubes [8]:

(3)

According to the conducted calculations, at usage of air and propane-air mixture as transporting gases, the value of (1) in interval (2000 … 20000) W / (m2K), is depended on change of transfer characteristics and speed of gas. Heat-transfer coefficient on border "shell-charge" constitutes 2 = 1400 W / (m2K) [9].

The temperature variation on outer surface of electrode shell owing to heat exchange was calculated by equation:

(4)

The temperature variation on border "shell - charge core" owing to heat exchange was calculated by equation:

(5)

Where "1", "2" - indexes conforming to shell and charge, accordingly; k - number of step on time.

Row of parameters was adopted simplistically at [5]. The conducted analysis has shown what it can strongly influence on accuracy of calculations. Values of some thermal parameters were updated to make the calculations more exact. It is shown below.

Low-carbon steel (0,08 % С) is adopted as material of the shell. The thermal characteristics, indispensable for calculation, thermal conductivity (), density (), specific heat (с), are approximated (table 1) by polynomials like:

Y = A1T3 + A2T2 + A3T + A4(6)

The charge thermal properties are adopted in interval of compositions, which are typical for these CW components.

Table 1. Equations of approximating of thermal parameters [10, 11]

Parameter / Temperature interval, К / Polynomial's coefficients
A1 / A2 / A3 / A4
, W/(mК) / 300..2500 / -210-8 / 110-4 / 0,1742 / 123,89
2500..3500 / 0 / -310-6 / 0,0175 / 22,42
, kg/m3 / 300..3500 / 0 / 810-5 / 0,463 / 7971,1
с, J/(kgК) / 323-1073 / 0 / 310-4 / 0,0969 / 409,12
1073-1273 / 0 / 0 / -1,045 / 1987,8
1273-2473 / 0 / 0 / 0,0658 / 575,03

In calculation by equation (1) fusion enthalpy was taken into account as follows. On each step temperature of calculation cell was compared with melting point (Tm) of shell or charge, accordingly. At achieving (Tm) temperature of cell stays constant on time at subsequent points. Here the heat, stored in a cell, is summarized:

(7)

This heat on each step is compared to specific fusion enthalpy of cell material (hпл). At fulfillment of condition on further steps of calculation the temperature rise of cell is allowed.

The calculation of depth of liquid layer at end face, which is fused by heat of arc spot, is executed according to the solution of problem of unheated body of restricted size melting at continuous removal of generating melted metal. Here fusion enthalpy is allowed [12]. In this case heat balance equation is as follows:

q1Fdt = dQ (8)

The left-hand part of an equation (q1Fd) represents input of heat into end face, which one is done at the expense of arc spot heat (q1 - specific heat flow from arc spot, W / m2 ; F - square of end face, m2). The right member of equation (dQ) introduces heat consumption on fusion of layer at end face and on warm-up of solid part of shell.

Portion of heat capacity of arc, which is passed on each arc spot, is adopted approximately equal. Such value of portion falls in interval of portion values of arc heat power income on anodic and cathode spots, presented by Erohin for a case of welding arc burning of normal polarity in air [6]. After permutation and integrating according to [12] the equation obtains a form, in which depth of liquid layer (s), heated by arc spot is unknown:

, (9)

Where  - function which is looking like:

, (10)

- melting criteria, (11)

, (12)

, (13)

, (14)

Where t = 4·10-4 s, drops dwell-time at end face before separation; Tc = 300 K, temperature of non-heated metal; Тп = 2500 K, surface temperature of melt at end face; h - extension of heating zone, from a surface of end deep into shell, m; hпл = 1319000 J/kg, fusion enthalpy of liquid iron [6]; a - thermal diffusivity, м2/s.

The calculation has shown (fig. 2), that a few times increase the depth of separated layer of liquid metal takes place in case of CW comparing with solid wire. Besides increase of the layer depth is inversely proportional to growth of shell thickness in case of CW. Computer algebra system Maple 7 was used at equation (9) solving. The correctness of the obtained data was tested for a solid wire by calculation from a condition of equality of liquid layer volume and separated drop volume according to the EAM oscillography data (the latter one value is marked at fig. 2 as solid, test).

Fig. 2. Output computation of depth of separated layer of liquid metal at EAM for solid wire and CW with various shell thickness (б, mm)

Inverse problem concerning thermal parameters of charge and blowing gas, geometrical sizes of CW cross-section was solved at heat exchange calculations. Favorable conditions for formation of sprayed drops were imposed as constraints at searching intervals of values of these parameters. In tab. 2 the CW parameters, adopted for calculations on different versions distinguished from base version 1 by charge heat conduction (version 2), depth of liquid shell (version 3) and conditions of heat transfer on border "gas-wire" (version 4) are listed. Calculations results are shown at fig. 3…6[1].

Table 2. CW parameters and modes adopted for different versions of calculation.

version № / r, mm / r1, mm / t, s / a, W/(m2К) / Т, К / , J/(kgК) / , W/(mК) / s,
mkm
shell / charge / gas / shell / charge
1 / 1 / 0,5 / 410-4 / 20070 / 1400 / 300 / 2500 / 1300 / 1600 / 20 / 22
2 / + / + / + / + / + / + / + / + / + / 5 / 22
3 / + / 0,3 / + / + / + / + / + / + / + / + / 33
4 / + / + / + / 2000 / + / + / + / + / + / + / 22

'+' – corresponds to version 1

Conclusions

1. Shell at all depth of liquid layer is strongly overheated above melting point (fig. 3).

2.Thinning-down of shell, alongside with increase of liquid layer depth, results in decrease of its temperature (fig. 3)

3.Temperature of shell surface does not exceed melting point owing to thermo exchange with colder gas. It promotes deceleration of drop separation (fig. 4).

4. Temperature of shell surface is strongly depended on conditions of heat transfer in "gas - shell" system (fig. 5).

5. Incomplete fusion of charge can be caused by decrease of charge heat conduction and shell depth thinning-down (fig. 6).

Fig. 3. Distribution of material temperature along cross-section of Cored Wire

Fig. 4. Distribution of temperatures at shell depth. Calculation by version 1

Fig. 5. Temperature variation of shell surface depending on conditions of heat transfer in "gas - wire" system

Fig. 6. Temperature variation in center of the core for different combinations of heat conduction of charge and shell depth

References

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[1] Curve's numeration corresponds to version's numbers, see table 2