TPT September 2014

Article

Caparo

3 Effects of the mismatch defect: • Variation in the vee length and vee angle • Variation in the current circulation and non-uniform heating of the edges • Lap seams and thinning of the edges 3.1 Variation in the vee length and vee angle – Vee angle is the angle between slit edges and the squeeze out rolls, and vee length is distance between weld roll centre line and HF coil.

Skin effect

such that the current density is largest near the surface of the conductor, and decreases with greater depths in the conductor. The electric current flows mainly at the “skin” of the conductor, between the outer surface and a level called the skin depth. (b) Proximity effect – In a conductor carrying alternating current, if currents are flowing through one or more other nearby conductors, such as within a closely wound coil of wire, the distribution of current within the first conductor will be constrained to smaller regions.

Vee angle and vee length

The vee angle and vee length affects the weld quality in high frequency welding of tube and pipe. We need to understand how the vee length and vee angle changes if there is a mismatch defect present in the tubes. Suppose at x = 0 is the centre point of the induction coil, and at x = 100mm both the edges are joining; so the vee length is 100mm. If there is some waviness present in the skelp, it will effectively change the position of the point where the two edges are joining. This, as a matter of fact, changes the vee length. Likewise, because of the change in length, the vee angle also gets changed. Other than the slit waviness, other factors causing mismatch defect may also be responsible for this change in the vee length and vee angle. (a) Entrapments (black penetrators). This type of defect is usually a metal oxide that has been trapped on the bond plane instead of being squeezed out with the molten metal. (b) Open Seam. When the vee angle is too big, the strip edges might not get sufficient weld heat. The edges usually show a blue heat tint suggesting that some heat was input. However, the edge face remains flat and smooth, showing no signs of having been molten. 3.2 Variation in the current circulation and non-uniform heating of the edges. (a) Skin effect – This is the tendency of an alternating electric current (AC) to become distributed within a conductor The following weld defects can be attributed to it:

Proximity effect

Heating and current flow

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September 2014