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High frequency induction welding flaws analysis

1. High frequency induction welding process

Steel strip after forming into pipe shape, it goes through an electromagnetic field which is caused by induction coils, the inducted current is produced at the edges of steel strip, and the resistance of steel make a lot of heat which melt the edges of steel strip, under the pressure of squeeze rolls, the edges welded together. This welding method have no additional metal, it is a kind of forging welding. Under exactly right setting, there is no melted metal and oxide squeezed out.

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When we cut a piece of welding seam from steel pipe, after polishing and clean, we can see a metallography under a microscope as below:

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In the center, it is just place get squeezed and welded. The color of heat-affected zone is lighter than other area because the carbon in steel gethers to steel edges during welding and get oxidized, the other place is less dark than heat-affected zone.

2. Welding flaws found in production

a. Black peroxide impurity

This kind of flow is made because the oxide is not squeezed out with melted metal, instead stays on the surface. If the metal melting speed is higher than the speed that steel strip edges getting close, it will cause the melting faster than metal being squeezed out, there would be a peroxide impurity inclusion zone before the top point of V-shape mouth as below photo showing, these oxide and liquid metal area is peroxide impurity inclustion zone that cause the flaw. If this kind of flaw appears, it can be separate or looks like continous chain.

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When the angle of V shape is getting smaller such as less than 4 degree or Mn/Si is less than 8:1(Mn and Si are element in steel material), this kind of flaw is more likely happen.

How to prevent this:

Control the angle of V between 4 to 6 degree;

Make sure the length of whole V is the right length during installing machine;

Try to control the welding temperature not that high, both make sure the metal can be melt and not too high;

When sourcing steel material, better the Mn/Si is less than 8:1.

b. Bridge short circuit flaw

If the metal burr(occationally appears during cutting) or rust drop on the top point of V, it will cause short circuit, the current change its way and cause the efficiency of heat at the normal heating length, like below phote shows:

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Beside steel burr and rust, other factors may cause short circuit such as guide rolls, narrow V angle and high voltage should all be settled due to the same reason.

How to prevent this?

Keep the V angle between 4 to 6 degree;

Reduce edge burr during cutting;

Keep the cooling water clean and prevent water to V mouth.

c. Inadequate fusion

Steel edge get heat but not well fused. Main reason about this is inadequate of heat, others such as inadequate squeeze, V mouth angle, heating length of V mouth, magnetic bar installation place, cooling condition and induction coil size, these factors may cause this flaw.

How to prevent this?

Welding input heat should match metal specification and welding speed;

The magnetic bar position should be 3.18mm(1/8 inch) in front of squeez rolls;

The length of V mouth should be less than pipe OD;

ID of industion coil should be more than pipe OD plus 6.35mm(1/4 inch);

Width of steel strip for producing pipe is the right width.

d. Peroxide and inadequate fusion

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The temperature outside is lower than inside temperature, it cause a bulge on the welding line. When this part get pressed, the bulge would open, this flaw is mainly caused by peroxide and inadequate fusion.

How to prevent this?

Make sure the two edges of steel strip straight and level, and get contact parallel;

More squeez strength is needed;

If the color of the bulge is silver gray, more welding heat is needed;

e. Inside of welding seam inadequate fusion

When the power requried by welding speed more than rated power of welder; also when the melted fusion metal doesn’t get full discharged, this flaw may appear. There is no enough time for the heating up tto the right temperature and depth.

How to prevent it?

Enhance welder’s power;

Increase the strength of squeez;

Reduce welding speed or increase the length of V mouth.

f. Insecure welding

This flaw is not like have some hollow or crack inside of welding seam, the ultrasonic detector can not find any difference, but when flattening test, the welding part is insecure and would crack.

How to prevent it?

The right welding power is needed for different pipe size and steel grade;

Full squeeze is needed;

The width of steel strip should be more than previous.

g. Brittle flaw

This is caused by impurity of melting area, oxide inside. During flattening test, this part will be cracked. The melting is good but the liquid metal discharging is not enough.

How to prevent it?

Make the width of steel strip more than previous.

h. Porosis

The porosis is caused by over heat temperature and inadequate discharging of liquid metal. The porosis inside of seam, it is white while it is black when it is on the surface of seam.

How to prevent this flaw?

Reduce the input heat for welding;

Enhance the strength of squeez.

 

Sometimes the welding seam area flaws are not caused by device on welding area. When the flaws are founded, we should analyze the reason and test again, search as many parameters as you can.


Post time: May-22-2018

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