weldment design 02 - distortion

There was a time when the welding operator used to pick up his shield and electrode holder and commenced welding a job, beginning and finishing at any place. If the completed work became distorted, it was taken for granted that it could not be avoided. The impression was that all welding caused distortion, so there would be no point in worrying.
This was purely ignorance, because distortion can be controlled and minimized by approaching the job in a correct manner. Today, welded work is being completed with minimum or no distortion. For example, large machine beds are being fabricated out of rolled steel sections and plates and welded within a tolerance of 1.5 mm.

During welding, the base metal near the arc is heated to the melting point. A few centimeters away, the temperature of the base metal is substantially lower.
This sharp temperature differential causes non uniform expansion followed by base metal movement, or metal displacement if the parts being joined are restrained. Also, the expansion of the hotter base metal (i.e., which is nearer the welding arc) is subject to restraint, due to the resistance of comparatively colder metal away from the welding arc. The metal nearer the arc expands more than that away from the arc.

As the arc passes down the joint, thus removing the source of heat, the base metal begins to cool and shrink. If the surrounding metal restrains the adjacent base metal from contracting normally, internal stresses build up. These internal stress combined with the stresses developed in the weld metal will increase the tendency to distort. Even when there's no distortion, the stress would be kept as residual stress.

The volume of this adjacent base metal which contributes to distortion can be controlled by welding procedures. Achieving higher welding speeds through the use of powdered iron type manual electrodes and semiautomatic or fully automatic equipment using submerged arc or self-shielded welding reduces the amount of adjacent material that is affected by the heat of the arc and progressively decreases distortion.

CONTROLLING WELDMENT DISTORTION
Shrinkage or contraction cannot be prevented, but it can be controlled. There are various practical ways for minimizing the distortion caused by contraction:
>> No overweld. 
Keep the contraction forces as low as possible by using only that amount of weld metal as is required by the joint. The more the metal placed in a joint, the greater the contraction forces will be.
Correctly sizing the weld for the service requirements of the joint helps to control distortion. The amount of weld metal can be minimized in a fillet joint by use of a flat or slightly convex bead, and in a butt joint by proper edge preparation, fit up and reinforcement.
A bevel not exceeding 30 degrees on each side will give proper fusion at the root of the weld, yet require minimal weld metal. J or U preparations further reduce weld metal for thicker plates. A double joint requires about one half the weld metal of a single joint.
When attaching stiffeners to plate, intermittent welds (in place of continuous welds) will enable reduction of weld metal to one fourth, yet give all the strength needed.To summarize, keep weld as small as possible.

>> Use as few weld passes as possible
The more the number of passes, the more is resulting shrink age (because shrinkage of each pass tends to be cumulative), and hence the distortion. Apply fewer passes with large electrodes. Select electrodes for highest deposition efficiency.

>> Place welds near the neutral axis
This reduces distortion by providing a smaller leverage for the shrink age (contraction) forces to pull the plates out of alignment.

>> Balance welds around the neutral axis
This will balance one shrinkage force against another. Design and welding sequence can be used to effectively control distortion.

>> Use of backs step welding or skip method of welding
With this welding technique, weld bead increments are deposited in the direction opposite to the progress of welding the joint e.g., each bead is deposited from right to left, but the welding progresses from left to right.
As each bead is placed, die heat from the weld along the edges causes expansion there, which temporarily separates the plates; but as the heat moves out across the plate, the expansion along the outer edges brings the plate back together.
The expansion of the plate is most pronounced when the first bead is laid. With successive beads, the plates expand less and less because of the locking effect of prior welds. Back stepping may have less effect in some cases and cannot be economically used in fully automatic welding.

>> Make shrinkage forces work in the desired direction
Several assemblies can be preset out of position before welding so that the shrinkage forces will pull the plates into alignment. Prebending or prespringing the parts to be welded is a simple example of the use of mechanically induced opposing forces to counteract weld shrinkage.

>> Balance shrinkage (contraction) forces with opposing forces.
The opposing forces may be
(i) Other shrinkage forces.
(ii) Restraining forces imposed by clamps, jigs and fixtures.
(iii) Restraining forces arising from the arrangement of members in the assembly.
(iv)The counterforce from the sag in a member produced by the force of gravity.
A common practice to balance shrinkage forces is to position identical weldments back to back and clamp them tightly together. The welds are completed on both assemblies and allowed to cool before the clamps are released.
Clamps, jigs and fixtures, that lock parts into a desired position and hold them until welding is finished, are probably the most widely used means of controlling distortion in small assemblies or component parts.

>> Welding sequences
Welding sequence implies the order of making the welds in a weldment. The weld metal is placed at different points about the structure so that as it shrinks at one place it will counteract the shrinkage forces of weld already made. Also, weld down hand whenever possible. Weld outward, from a central point. Restrict heat affected zone by keeping metal adjacent to joint as cool as possible.

>> Removal of shrinkage forces during or after welding
Peening is one method, in which force is applied to the weld (with the help of a hammer) to make it thinner thereby making it longer and relieving residual stresses.
Stress relief by controlled heating of the weldment to an elevated temperature followed by controlled cooling is another way to remove contraction forces.

>> Reduce the welding time
It is desirable to finish the weld quickly before too great a volume of surrounding metal becomes expanded by the heat. Welding should be carried out as fast as possible.

>> Breakdown large weldments into subassemblies.
In this manner, distortion errors can be rectified on each subassembly before final erection.

SHRINKAGE CALCULATION 1. Transverse shrinkage of butt welds S = 5.16 x Aw / t + 1.27 d where S = transverse shrinkage Aw = cross-sectional area of weld t = thickness of plates d = root opening
2. Longitudinal shrinkage of butt welds: ∆ L/L = 3.17 x I x L/100,000 x t where ∆ L is the longitudinal shrinkage (mm) L is length of weld (mm) t is the plate thickness(mm) I is welding current(Amp).

Practice makes perfect. But a good planing and strategy can make you flawless