Distortion in welding operation

Distortion in welding operation

Distortion is counted as a type of weld imperfection, in which the object is said to deform when it is put out of shape or it becomes unshapely.

Distortion in welding operation

It was an old-time when if the completed work became distorted, it was assumed that it could not be avoided but today the welding work is being completed with minimal or no distortion. In today's era, the welding work is being completed with minimum or no deformation of metals, because it can be minimized in various ways.

DISTORTION

When weld metal is deposited, the base metal is heated, and thus it expands, since the temperature is different at different places in the parent metal experiences nonuniform expansion and when after the welding it cools, the base metal and weld metal shrinks. It is therefore obvious that the shrinkage of welded joints far greater than the expansion.

During welding, the parent metal near the arc is heated to the melting point, while a few centimeters away, the parent metal has a significantly lower temperature. This sharp temperature difference causes non-uniform expansion. This is followed by a base metal movement or metal replacement if the joint begins to be joined due to structural stiffness in that part. Also, the expansion of the hotter base metal that is near the welding arc is subject to restraint due to the resistance of the cold metal away from the welding arc. The metal extends farther than the nearby arc.

As the arc joint passes down thus removing the heat source the base metal begins to cool and shrink.If the surrounding metals prevent the adjacent base metal joining normally then the internal stresses buildup. So these combinations combine with the stresses developed in the weld metal and increase the tendency to deform.

Type of Distortion in welding operation

Distortion may be classified as follows:

Longitudinal shrinkage that occurs parallel to the weld line

Transverse shrinkage that occurs perpendicular to the weld line

Angular change that consists of rotation around the weld line

Longitudinal type Distortion

When the weld is deposited lengthwise on a light narrow and the perfectly flat strip of metal that is neither clamped nor geld in any way, the strip will tent to bow upward in the direction of the bead. This due to the longitudinal contraction of the weld metal as it cools.

Transverse type distortion

When two plates being butt welded together are neither too heavy nor geld together and are thus free to move they will be drawn closer together by the contraction of the weld metal. This is called transverse contraction

Angular type Distortion

When two beveled plates are welded it is found that the plates are pulled out of line with each other

Since the opening at the top of the single vee groove is greater than at the bottom a greater portion of the weld metal is deposited there and thus the bottom a greater portion of the weld metal is deposited there and thus the drawing of pulling is greatest on that side of the joint. Angular contraction is related to the shape and size of the cooling weld metal zone and the stiffness of the remaining unfused part. Double grove joints tend to minimize angular distortions because the contraction effects fo the two sides like the top and bottom of the plate get canceled with each other.

Control of distortion in welding operation 

If distortion is to be prevented or minimized in weldment strategies must be used in the design and in shop practice to overcome the effects of the getting and colling cycles shrinkage or contraction can not be prevented but it can be controlled.

- By keeping the contraction forces as low as possible by using only that amount of weld metal as is required by the joint. Another way to state this does not over weld.

By using as few welds passes as possible

By placing welds near the neutral axis.

Balance weld around the weld axis

Using backstep welding

Making shrinkage forces work in a certain direction

By balancing the contraction forces with opposing forces.

Other ways

Welding sequences, Removal of shrinkage forces during or after welding, By reducing the welding time, Breaking the large weldments.