Research on seam welding process of galvanized sheet

Text / Liu Xiqing

I. Discussion on weldability of seam welding of galvanized sheet

1. Characteristics of seam welding of galvanized sheet

Since the surface of the galvanized sheet has a zinc layer, the surface finish is improved and the surface conductivity is increased. However, during the seam welding process, the surface zinc of the workpiece is oxidized, and white zinc oxide is formed to cover the surface of the welding wheel, and under the action of cooling water, the basic zinc carbonate Ze2(OH)2CO3 is reacted, and the pressure and temperature are changed. It is a wear-resistant intermediate layer, which makes seam-welded galvanized sheets much more difficult than seam-welding ordinary low-carbon steel sheets.

When the galvanized sheet is seam welded, the contact resistance is reduced due to the zinc layer between the plates, and the oxide coating on the surface of the welding wheel increases the transition resistance, so that the welding heat of the welding wheel and the working contact is relatively increased. The welding heat at the contact of the two workpieces is relatively reduced, which also brings certain difficulties to the seam welding.

2, weldability analysis

(1) Short welding

When the galvanized sheet is seam welded, the heat distribution along the cross section of the weldment changes correspondingly due to the increase of the transition resistance. When the relative heat of the two workpieces is relatively reduced, there is a problem that congenital heating is insufficient and it is difficult to form a nugget. Welded galvanized sheet with seam welding ordinary low carbon steel plate, the weldment will often only be slightly bonded without fusion, the surface will not be seen, and the chisel will be lightly opened and will appear when the leak is detected. Severe leakage, there is a virtual weld. In order to avoid solder joints, the welding current must be appropriately increased during seam welding of galvanized sheets.

(2) Surface burnt, crack

When the galvanized sheet is seam welded, the heat at the contact between the welding wheel and the workpiece is relatively increased. In addition, in order to overcome the virtual welding, the welding current is increased during the seam welding, and the oxide on the surface of the welding wheel hinders the heat loss, thereby making the working surface When the heating is intense, it often causes over-burning, grain boundary cracks, and the overheating of the workpiece generates a large thermal stress under the intense cooling of the cooling water, which can cause longitudinal cracks in the weld. As the welding current density increases, the over-burning becomes more serious. It can even make the material crisp.

(3) Copper

Also due to the relative increase in heat at the point where the welding wheel is in contact with the workpiece, the copper is mainly caused by a low welding speed and a long welding time, and in addition, the zinc on the surface of the galvanized sheet is at a high temperature and the surface of the welding wheel is copper alloy. The melting point of the welding wheel is lowered, the electrical resistivity is increased, and the oxide coating layer reduces the heat dissipation effect of the welding wheel. The above-mentioned comprehensive factor causes the copper surface of the welding wheel to be melted on the workpiece weld seam. "Copper", the main phenomenon of "dip copper" is the lower welding wheel. The phenomenon of "dip copper" accelerates the loss of the electrode. In severe cases, the edge of the electrode can be detached in a granular shape, so that the welding cannot continue.

Second, the test argument

In terms of weldability of galvanized sheet seam welding, we have different views with foreign journals and scholars, such as: "On the reasons for increasing the welding current for galvanized seam welding", foreign scholars believe that it is caused by the shunting of zinc. "On the problem of seam welding cracks", foreign publications believe that the solid solution of zinc causes the material to be embrittled. In order to clarify these problems, we test the following:

1, static resistance test

The static resistance test is to directly compare the R-touch and R-pass of the ordinary low-carbon steel plate. The test record is shown in the attached table. According to the test record, we classify the four test points and obtain the average value. Listed in Table 1.

Schedule 1 Static Resistance Test Record

Test point

Welding wheel condition

Workpiece material

A×0.1
(mA)

V×0.3
(millivolt)

Ω×100
(micro Europe)

Remarks

1

Contaminated welding wheel

Galvanized sheet

87

15.5

530

Low board

87.5

20.3

696

Clean welding wheel

Galvanized sheet

88.9

3.9

131

Low board

89

7.2

242

2

Contaminated welding wheel

Galvanized sheet

87.5

15.5

531

Low board

88.5

17.5

593

Clean welding wheel

Galvanized sheet

86.5

6.3

218

Low board

89

6.9

232

3

Contaminated welding wheel

Galvanized sheet

88

16.9

576

Low board

86.5

19.2

666

Clean welding wheel

Galvanized sheet

90

3.3

110

Low board

89

10.3

347

4

Contaminated welding wheel

Galvanized sheet

89

twenty one

708

Low board

90

12.9

430

This value is abnormal

Clean welding wheel

Galvanized sheet

90

12.2

406

Low board

90.1

17.6

586

Note: 1 The test point is at different points on the circumference of the welding wheel. The galvanized plate and the low plate used are fixed and the thickness is 1.5+1.5 mm. 2 After measuring the value of the contaminated welding wheel, remove the cover layer and measure again. The value of the welding wheel is clean, so the welding wheel has not changed.

Table I

Serial number

Test status

resistance

1

Pollution welding wheel + galvanized sheet resistance

586 microohms

2

Pollution welding wheel + ordinary low carbon steel plate resistance

651 microohms

3

Clean welding wheel + galvanized sheet resistance

216 micro ohms

4

Clean welding wheel + ordinary low carbon steel plate resistance

352 micro ohms

The resistance R=2R in the table is +2R workpiece + R touch. The so-called pollution welding wheel is a welding wheel whose surface is covered with a zinc oxide layer, and a clean welding wheel without a zinc oxide layer.

The data for Table 1 is analyzed as follows:

(1) Compare Table 1 with 1 group, 2 groups, 3 groups and 4 groups, where R over and R pieces are unchanged, R is completely dependent on R touch, and the values ​​in the table show that the static contact resistance of galvanized sheet is lower than ordinary low carbon. The steel plate has a small static contact resistance.

(2) Comparing Table 1 with Group 3, Group 2 and Group 4, where R and R are not changed, and R is completely dependent on R. It can be seen from the values ​​in the table that the contaminated welding wheel causes an increase in transition resistance.

From the static resistance test, it can be concluded that the galvanized seam welding is reduced compared with the ordinary low carbon steel seam welding, the R contact is reduced, and the R is increased; the increase of the R is due to the contamination of the welding wheel.

2, metallographic analysis

We not only observe the metallographic phase of the weld fusion zone, but also observe the metallographic condition of the contact surface between the workpiece and the welding wheel. The metallographic observation is recorded in the second table, and the analysis of the second table is as follows:

(1) Comparing the welds of the first and third groups and the metallographic surfaces of the upper and lower surfaces, it can be known that the heat of the lower plate (the abbreviation of the ordinary low carbon steel plate) is mainly used to form the weld nugget (except for the loss). Galvanized sheet seam welding is used to form a weld nugget with relatively little heat, but has considerable heat consumption on the contact surface between the weld wheel and the workpiece.

(2) Comparing the two cases of group 1 and group 2, the common point is that the welding wheel is polluted and the result of seam metallization is similar, so that the main factor affecting the heat distribution of welding is transition resistance.
The above two tests are completely consistent. They reflect the characteristics of the welding start and end states, and basically reflect the characteristics of the welding process. The test results are a strong argument for our previous arguments and a refutation of different viewpoints.

It can be seen from the second table that when the low-plate is welded with the contaminated welding wheel under the general low-slot welding specification, the unfused state is also generated. As with the seam-welded galvanized sheet, the welding current must also be increased. Obviously this is the zinc shunt. The current theory cannot be explained.

It can be seen from the second table that when the lower plate is welded with the contaminated welding wheel under the specification of the large seam welding, there is no problem of embrittlement caused by zinc solid solution at this time, but the same phenomenon like seam welding of galvanized sheet is also appeared. The kind of crack, the zinc solid solution embrittlement theory is also unexplained.

Schedule 2 sample metallographic record

Welding primary current (A)

Grouping

1# seam welded galvanized sheet

2# pollution welding wheel seam welding low board

3# clean welding wheel seam welding low board

Weld metallography

Upper and lower surface metallography

Weld metallography

Upper and lower surface metallography

Weld metallography

Upper and lower surface metallography

170

Unwelded incomplete phase change

Unwelded incomplete phase change

Melt depth 2.1mm without defects

No over burning
No crack

200

Melt depth 2.3mm without defects

Crack with vertical surface

Melt depth 2.3mm without defects

Vertical surface crack depth 0.12mm

Infiltrated
flawless

No over burning
No crack

240

Depth of 2.4mm crack length of 0.41mm

Over-burning, the crack on the vertical surface is 0.54mm deep, with macroscopic longitudinal cracks

Melt depth 2.3mm
flawless

Infiltrated
flawless

No over burning

Note: 1 The welded plates in the above table are 1.5+1.5mm; 2 other welding specifications are p=1.5, t welding=3 weeks, t rest=2 weeks, V welding=1.5 m/min, b=6mm.

3. Spectral analysis

We performed spectral analysis on the nugget area of ​​the seam welded galvanized sheet on the WPG-100 grating spectrometer. The results show that there are only traces of zinc, and the amount of zinc is extremely small regardless of the welding current is large or small (about 10 -7 order), it can be seen that the change of the seam welding specification is not much change in the zinc dissolved in the nugget, which is a trace amount, but the welding quality is significantly changed, that is, the trace zinc has nothing to do with the welding quality, which further excludes The argument that the metallurgical factors affect the quality of the seam welded galvanized sheet still boils down to: the welding heat process, that is, the transition resistance affects the seam welding of the galvanized sheet.

Third, the process characteristics of seam welding galvanized sheet

1. Narrow range of process parameters

The process parameters of seam welded galvanized sheet are mainly: welding speed, welding wheel width, welding current, welding time and electrode pressure, especially the change of welding current and welding wheel width (ie, the change of current density) is more strict, to ensure To form a nugget to avoid solder joints, it is necessary to increase the current density or the heat of welding, but to avoid over-burning, cracking, and to reduce the current density or the heat of welding, these two contradictory factors determine that only a narrow range of process parameters is determined. It is suitable, it is best to require the welding current (once) to fluctuate less than 7%, the width of the welding wheel to vary by less than 1.0 mm, and other parameters to have a fixed value.

2, requires knurling transmission

The domestic FN1-150-1 seam welding machine has a welding wheel transmission mode of single-arm gear transmission. The performance of the seam-welding low-plate is good. Although the welding wheel size will increase during the seam welding process, only a few sets of welding are required. If the wheel is changed in time, the problem can be solved. However, when welding galvanized sheets, the ever-widening electrode size and the thickening of the zinc oxide coating layer become a great obstacle to stable welding. For this reason, the gear transmission is changed to a knurling transmission, and the knurling wheel is used to drive the welding wheel. The process realizes the correction and scraping of the surface of the welding wheel (three faces), one guarantees the width of the welding wheel, and the other, the scraping of the zinc oxide covering the surface of the welding wheel with a constant transition resistance, after our seam welding of FN1-150-1 In the practice of machine transformation, we believe that the knurling drive can achieve the intended purpose, namely: reliable driving of the welding wheel, correction of the welding wheel, continuous cutting, coordinated action, can create good conditions for seam welding galvanized sheet.

3, water cooling is an important guarantee for seam welding quality

The heat distribution of the seam welding of galvanized sheet causes the surface of the workpiece to be over-fired and even penetrate cracks, which affects the seam sealing performance. In order to reduce the surface of the workpiece and the superheat of the welding wheel, sufficient water cooling must be performed on the surface of the workpiece, especially under welding. At the contact between the wheel and the workpiece, the practice shows that the water cooling is insufficient or the seam welding leakage can reach 80% or more. In order to make the cooling more sufficient, the water spray direction should face the direction in which the seam welding workpiece advances.

4. According to the practice of our seam-welding steel drum, it is recommended to use small welding wheel width, small current and small welding time as well. This can reduce the burning loss of zinc, reduce air pollution and save energy. The point of welding is to maintain a small weld wheel width.

Fourth, the seam welding process and quality inspection of steel drum

1. Seam welding process

The steel drum we produce is made of 08 steel plate with a thickness of 1.5mm and a galvanized layer thickness of 0.007~0.13mm, which is passivated.

Seam welding specifications: welding wheel width b = 6mm, I once = 200A, V welding = 1.5 ~ 1.55 m / min, welding pressure P = 1.5 gauge pressure, t welding = 3 weeks, t rest = 2 weeks.

(1) Other parameters are unchanged. When I is >230A, the longitudinal cracks appear in the weld. When I <180A, the weld is unstable and there is intermittent solder joint phenomenon. Both of the above cases cause the steel drum to leak.

(2) In order to avoid the increase of pressure on the formation of the weld nugget, and cause deep indentation to increase the stress in the heat affected zone, the low seam welding pressure is still used.

(3) In the parameter adjustment, if the transformer number and the heat adjustment knob are unchanged, although only the welding and rest time are changed, the actual value of the welding current has changed, and the original current value needs to be re-adjusted, so the welding is adjusted. Time will inevitably cause a corresponding adjustment of the welding current. The overall adjustment of welding time and current is heat regulation. Knowing this allows finer adjustments to the welding specification.

(4) The over-burning of the surface of the workpiece and the creation of vertical surfaces and micro-cracks do not affect the sealing performance of the steel drum, so the specification can be regarded as a qualified process specification.

(5) The galvanized sheet seam welding uses a small welding wheel width, a small current, and a short welding time. High-current seam welding makes zinc evaporation large, pollutes the air, affects sewing, and small gauge welding heat affected zone is small, which is beneficial to maintain the corrosion resistance of the workpiece, and can save power surge and stable quality. The penetration depth is 2.3mm.

The fundamental point of using small gauge seam welding is to maintain a small weld wheel width.

2, quality inspection

(1) Chisel peel test

Seam welding was carried out on the test panel, and then flattened on a vise with a chisel along the knot surface, as a result of which the base material was torn and the joint surface was not peeled off.

(2) Test leak

30 kPa of compressed air was introduced into the steel drum, and the bubbling was checked in the water tank, and the pass rate was 98.5%.

(3) Tensile test

The tensile test of the weld was carried out, and the result was broken in the base metal, which proved that the joint quality was reliable.

(4) Metallographic examination

The galvanized seam-welded metallographic structure is the same as the low-profile seam-welded metallographic structure, which is bainite + a small amount of low-carbon martensite.

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