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Exothermic welding of electrolytic cell cathode steel bar

Guodong Zhang1 , Fei Zheng1 , Longjian Xue1 , Jingdang Fei2 ,

Hongwei Yang3

( 1. School of Power and Mechanic of Wuhan University, Wuhan 430072, China;

2. Wuhan Billion Science & Technology Development Co., Ltd, Wuhan, 430071,

China; 3. China Power Investment Corporation Ningxia Energy Aluminum

Engineering and Overhaul Co., Ltd, Wuzhong 750002, China )

Abstract: The use of exothermic welding technology to achieve the molten

cathode steel bar welding of the electrolytic cell under the strong magnetic field

was introduced. The microstructures, microhardness and several other properties

of welding joints were analyzed from OM, SEM, EDS, etc. The results showed that

the exothermic welding was not impacted by the strong magnetic field of the

electrolytic cell, the welding interface was well fused, and there were no obvious

welding defects. The intermetallic metallurgical bonding was realized between the

base metal and the weld metal. The mechanical properties of steel bar after the

exothermic welding could meet the safety requirements of the repair or overhaul

of the electrolytic cell.

Key words: electrolytic cell; cathode steel bar; exothermic welding; mechanical

properties

DOI: 10.7512/ j.issn.1001-2303.2017.13.11

0 IntroductionIn order to improve the current efficiency and aluminum production

amount in the produce of electrolytic aluminum industry, the current

intensity of the aluminum electrolytic cell is increasing, the maximum

current in the world has reached 600 kA [1]. Such a large DC current

result in a strong magnetic field around the cell and its diameter reaches

several meters which can be calculated up to hundreds of Gaussian,

magnetic field distributes along the three-dimensional disorderly. An

important part of the electrolytic cell overhaul is the welding of the

cathode steel bar. In the implementation process, the cathode steel bar

and the cathode busbar are separated at first with the old steel bar, and

then connected with the new steel bar. The connection between the

new cathode steel bar and the small busbar is a plurality of parallel steel

plates, and the welding process is carried out by manual welding. Under

the effect of electromagnetic field, the arc shape and metal movement

change, resulting in some welding defects subsequent to magnetic bias

and welding pool asymmetry, in this situation, the welding quality will

be decreased. The uneven of the cathode steel bar current when started

Prof. Guodong ZhangEmail: gdzhang@whu.edu.cn

or operated seriously restrict the further extend of electrolytic cell life. So

the cathode steel bar welding under strong magnetic field is an urgent

and important technical problem to be solved [2--5].

Exothermic welding technology, which by means of reducing agent

in the redox reaction to restore the metal in the oxide, which produces

high-temperature liquid metal in a short time and releases large amount

of heat meanwhile, heating of the workpiece to form a certain shape

directly or indirectly, and the size of welding joint is also certain. Because

there is no arc, the interference of strong magnetic field could be

avoided, which ensures the quality of welding, reduces labor intensity

and makes operation simple and quick. This paper attempted to use

the exothermic welding technology to melt the welding of the cathode

steel bar and researched on the contains and components of welding

materials. The design of the welding mold and the optimization of the

welding process were also researched in the paper. The microstructures,

microhardness, chemical composition and mechanical properties of

welding joints were measured to see whether it meets the practicality in

the engineering field [6--7].

Guodong Zhang , an associate professor in School of Power and Mechanic of Wuhan University, Ph.D. One of the members of the Committee China Brazing and Special Joining, the executive director of Hubei Welding Commitee. The main researches are advanced material joining technology, simulation of materials' forming, compu-tational materials science, surface engin-eering and nanomaterials' fabrication.

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1 Experimental details1.1 Welding Materials and Molds

For the cathode steel bar, several major indicators of the exothermic

flux are: (1) the heat of the self-propagating reaction is sufficient to melt

the metal. (2) the surface of the base material is wetted in advance

when it exists the oxide layers and is sufficiently spread on the surface

of the base material, which means the infiltration is good. (3) the

mechanical properties ensure the safety of welding joints.

The CuO-Fe2O3-CaSO4-Al system is mainly used in the exothermic

flux, the CaF2-SiO2-CaO system is the slag-forming agent, the Cu-Mn-Si

system is the alloy agent, and the Cu powder has the effect of adjusting

the temperature and the speed of the reaction, as well as alloying effect;

other components for alloying, refining the grain and improving the role

of weld metal properties. According to the chemical reaction formula,

the components were weighed and mixed in a feeder, and then placed

in an electrothermal oven for 120 °C / 30 min to dry [8].

Graphite has high anti-heat impact performance, it’s less likely to be

broken in the welding process. It has less melt penetration, less adhesion

to other metals and better slag removal, it’s used to be the mold

material. The upper mold is a crucible with a lower flow guide hole

fixed to the upper part of the groove, and its shape is shown in Fig.1

The lower mold consists of two graphite plates, which are attached to

the left and right sides of the groove to be welded, together with the

upper plate to form a sealed cavity with a vent hole, and the outside is

clamped with a steel fixture.

Fig. 1 Schematic of the upper welding mold

1.2 Welding Process and Joint Properties Test

Before welding, the size of the groove of the cathode steel bar

should be measured, the filling volume and quality should be calculated,

and the amount of powder should also be calculated according to the

rate and requirement. Clean the surface first. Use a graphite U-shaped

plate with a groove to block both sides of the aluminum busbar and

then clamp the graphite plates on both sides with a steel gripper. Put

some high temperature glue on the graphite plate to prevent the

molten metal outflow. Place the upper mold crucible on top of the

base metal groove and align the groove with the liquid hole. Use the

blowtorch to preheat the mold so as to remove the water vapor. Place

the copper self-fusing plug at the bottom of the crucible and cover the

trough. The configured flux is loaded into the crucible and compacted.

The ignition powder should be spread evenly on the sfigureuppurface

of the flux, insert the magnesium bar. With the ignition the magnesium,

magnesium ignited the ignition powder, then the flux will also be

ignited, which release a lot of heat, get high temperature overheat

metal flowing from the liquid hole into the groove, melting the steel

bar to weld. After all these steps, remove the mold, remove the fixture

and graphite plate. Clean the weld metal surface of the welding slag to

expose the metallic luster [8].

After all the welding process, make some samples to measure its

properties, use the optical microscope to observe the metallographic,

use the scanning electron microscopy with EDAX to analyze the

microstructure and get the EDS result, use microhardness tester to

measure the hardness. Make some standard tensile specimen of

the cathode steel bar welding joint to measure the tensile strength

by a tensile tester in order to check whether it’s capable to meet the

requirement of long-term service.

2 Results and Discussion2.1 Fusion Performance

Fusion property directly affects the quality and of welding joints.

Fig.2 shows the manual arc welding macro samples of the cathode

( a ) Manual welding joint

( b ) Exothermic welding joint

Fig. 2 Macroscopic morphology welding joints

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steel bar and the exothermic welding one which were grinded and

etched. It can be seen that under the effect of the magnetic field,

various defects appeared such as pores, non- penetration, non-fusion,

etc. in the manual process. Except for some parts of the slag and non-

fusion defects, there was no continuous pores, cracks, non-penetration

and other defects in the inside and the interface of the welding metal.

Obviously, because of the high heat output of the exothermic welding,

sufficient metallurgical combination was achieved. Because there was

no arc and ferromagnetic material in the beam metals, the whole

welding process weren’t influenced by the magnetic field, which

resulted in the good quality of the welding.

2.2 Microstructure and Composition Analysis

The joint microstructure of the cathode steel bar shown in Fig.3. It

can be seen that a typical ribbon structure existed in the base material

Q235 steel, which consists of pearlite and ferrite (Fig.3(a )), light-

colored matrix in the weld metal is the copper-based solid solution, and

the black particles is for the precipitation of the second phase (Fig.3(b)).

The weld is well bonded to the substrate without the presence of various

microscopic defects, and there is an apparent transition zone between

the weld and the base material (Fig.3(c )) with a thickness of about

50µm, because of the overheated weld liquid metal melted and wetted

the base material of the interface. The column crystals are grown in

one direction close to the fusion zone and are larger than those of the

column in the transition zone. This is due to the fact that in the heat

affected zone, the heat received by the substrate near the weld is much

higher and the temperature rises quickly, and grains are combined at

the high temperature , grain boundary migration led to the increase of

the grain size (Fig.3(d )).

Electron probe microanalyzer was used to analyze the distribution of

interfacial elements in the exothermic welding joint of cathode steel bar.

The results showed that the weld zone mainly consisted of Cu which

varied slightly according to the different kinds of alloying elements

added; the interface fusion zone mainly consisted of Fe and Cu, and

a small amount of Mn, Al, etc., it indicated that the fusion zone is

produced by the process in which high temperature liquid metal melted

the base material and mixed together. The EPMA face distribution

near the fusion zone is shown in Fig. 4. Fe and Cu on both sides were

fully diffused, indicating that metallurgical bonding was achieved

between the weld metal and the base metal, and there were a small

amount of Mn, Fe, Al and other elements, which had strengthened the

alloy because of the solid solution strengthening in some aspects, the

mechanical properties of the weld metal was improved.

2.3 Mechanical Performance Test and Analysis

The microhardness distribution of the welding joint is shown in Fig.5,

and the microhardness fluctuates greatly. Because the middle weld

alloy area is mainly the α-Cu phase, the hardness is lower, about HV130

or so. The highest mictohardness appeared in the fused area, because

the heating process heating temperature exceeded Ac1 ( 735℃) ,

and a part of the tempered sorbite transformed into austenite, then

( a ) the microstructure of base metal

( b ) the microstructure of the welding seam center

( c ) the microstructure of the interface

( d ) the microstructure of large grain zone

Fig. 3 Microstructure of welding joint

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changed into martensite when cooling, and copper diffusion occurred

simultaneously. These changes resulted in Fe lattice elastic distortion,

which increased lattice distortion energy, so the hardness increased

greatly. The microhardness of the base metal is about HV 200.

The tensile test showed that the average tensile strength of the

cathode joint is 440MPa, which can meet the safety requirements

of electrolytic cell repair or overhaul under strong magnetic field. The

elongation is 16.5%, slightly lower than the plastic index of manual

arc welding process we use now, which is related to the weld structure

which was as-cast.

3 Conclusion( 1 ) Welding process can be carried out without cutting the power

supply when we use exothermic welding technology. The welding

quality will not be influenced by the strong magnetic field, and the

production will not be influenced either.

(2) Good interface fusion was acquired, in which no obvious non-

penetration, non-fusion and other welding defects were found.

(3) Weld metal consists mainly of Cu, and the good metallurgical

combination was achieved between the base metal and weld metals.

(4) The average tensile strength of the exothermic welding joint of

the cathode steel bar was about 440MPa, which could meet the safety

requirements of the repair and overhaul.

Acknowledgment:This research was supported financially by the Suzhou nanometer

special project (No.ZXG201447) and the Natural Science Foundation of

Hubei Province (No.2014CFB707).

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Fig. 4 EPMA analysis of welding joint

Fig. 5 Micro-hardness distribution of welding joint