What cause distortion in welding
During welding, the weldment and surrounding areas undergoes localized heating and cooling cycles; therefore, the temperature distribution in the weldment is not uniform, and structural and metallurgical changes take place as the welding progresses along a joint. As the weld pool solidifies and shrinks, it begins to exert stress on the surrounding weld metal and heat-affected zones. Depending upon the amount of welding heat input, these stresses can be high enough to cause dimensional changes into the part, known as Welding Distortion. So, the stresses caused by the welding is the main reason for producing distortion in welding.
The distortion found in welded parts or structures is mainly caused by three fundamental dimensional changes that occur during welding:
- Transverse shrinkage that occurs perpendicular to the weld line,
- longitudinal shrinkage that occurs parallel to the weld line, and
- an angular change that consists of rotation around the weld line.
Why is weld sequencing important?
Distortion in some form or another cannot be avoided. Therefore, appropriate steps must be taken to minimize it. Distortion can best be controlled by using a well planned welding sequence. A well-planned welding sequence involves placing weld metal at different points about the assembly so that, as the structure shrinks in one place, it counteracts the shrinkage forces of welds already made. An example of this is welding alternately on both sides of the neutral axis in making a butt weld.
An example of this is welding alternately on both sides of the neutral axis in making a butt weld as shown in the below figure. Number here represents the welding bead sequence.
How to fix welding distortion
While distortion due to welding cannot be avoided totally, steps can be taken to minimize the magnitude and effects of distortion. Control of distortion includes consideration during the design stage and the selection of the welding and assembly procedures, as well as the use of techniques such as elastic pre-straining and preheating.
Ways of controlling welding distortion
- Do not overweld: The more metal placed in a joint, the greater the shrinkage forces. Correctly sizing a weld for the service requirements of the joint not only minimizes distortion, it saves weld metal and time.
- Use Intermittent welding: Another way to minimize weld met.al is to use intermittent rather than continuous welds where possible.
- Use as few weld passes as possible: Fewer passes with large electrodes, are preferable to a greater number of passes with small electrodes when transverse distortion could be a problem. Shrinkage caused by each pass tends to be cumulative, thereby increasing total shrinkage when many passes are used.
- Place welds near the neutral axis: Distortion is minimized by providing a smaller leverage for the shrinkage forces to pull the plates out of alignment.
- Use backstep welding: In the backstep technique, the general progression of welding may be, say, from left to right, but each bead segment is deposited from right to left.
- Anticipate the shrinkage forces: Placing parts out of position before welding can make shrinkage perform constructive work.
- Plan the welding sequence: A well-planned welding sequence involves placing weld metal at different points about the assembly so that, as the structure shrinks in one place, it counteracts the shrinkage forces of welds already made. An example of this is welding alternately on both sides of the neutral axis in making a butt weld.
- Remove shrinkage forces after welding: Peening is one way to counteract the shrinkage forces of a weld bead as it cools. Essentially, peening the bead stretches it and makes it thinner, thus relieving (by plastic deformation) the stresses induced by contraction as the metal cools.
- Minimize welding tie: Since complex cycles of heating and cooling take place during welding, and since time is required for heat transmission, the time factor affects distortion. In general, it is desirable to finish the weld quickly, before a large volume of surrounding metal heats up and expands.
Types of welding Sequences
Welding Sequence or Welding Technique: Welding Sequence is the order of making the welds in a weldment. If we talk about welding technique, Welding Technique is the details of a welding operation which, within the limitations of the prescribed joint welding procedure, are controlled by the welder or welding operator.
When welding a long butt joint, various types of welding sequences—including the backstep, block, build-up, and cascade sequences—can be used in an attempt to reduce residual stress and distortion. The selection of the proper welding sequence is important, especially when welding joints with high restraint, such as repair welds.
A longitudinal sequence where the weld bead increments are deposited in the direction opposite to the progress of welding the joint. As you can observe in the below sketch, weld bead 2 is placed after the weld bead 1 and weld bead 3 is placed after the weld bead 2. By this sequence, the heat accumulation due to the welding direction is controlled. In usual welding practice, we weld from one direction to other, leaving behind the stresses accumulation only in one direction. With backstep welding, we can control the welding heat input and resultant weld stresses that cause the distortion.
A combined longitudinal and buildup sequence for a continuous multiple-pass & weld wherein separated lengths are completely or partially built up in cross-section before intervening lengths are deposited. An example of block sequence is shown in the below figure.
The order in which the weld beads of a multiple-pass well are deposited with respect to the cross-section of the joint.
A combined longitudinal and buildup sequence wherein weld beads are deposited in overlapping layers. (In manual shielded metal arc welding a backstep sequence is normally used.)
The order in which the increments of weld metal are deposited. This is similar to buildup sequence.
Wandering or Skip or Random Sequence
A longitudinal sequence wherein the weld bead increments are deposited at random. Whereas, in the Wandering Block Sequence: A block sequence g wherein successive blocks are completed at random after several starting blocks have been completed.