Saturday, November 7, 2015

Master the Art of Welding Distortion Control with These 13 Practical Ways (Part 2)

When you weld on stainless steel or thin sheet metal, the parts are often warped eventually. You tried clamping down all the pieces before welding. But when you remove the clamps, the parts get warped. What can you do for welding distortion control?

See also:
Master the Art of Welding Distortion Control with These 13 Practical Ways (Part 1)


Welding distortion control

8. Plan welding sequence

A nicely planned welding sequence involves placing the weld metal at different points of assembly so that, when the structure shrinks in one place, it counteracts the shrinkage forces of welds made. A good example is welding alternately on both sides of neutral axis to make a complete joint penetration groove weld in the butt joint (Figure 2 (k)). Another example, in the fillet weld, consists of making intermittent welds by the sequences shown in Figure 2 (l). In those examples, the shrinkage in weld 1 is balanced by the shrinkage in weld 2. 

Figure 2: Distortion can be averted or minimized by techniques that defeat - or use constructively - the effects of heating and cooling cycle.
Jigs, fixtures and clamps locking parts into a desired position and holding them until welding is finished can be the most widely used means for distortion control in small assemblies or components. As mentioned earlier, the restraining force given by clamps raises internal stresses in the weldment until the yield point of weld metal is reached. For common welds on low-carbon plate, this stress level will be around 45,000 psi. One may expect this stress to induce considerable movement or distortion after the welded part is removed from the clamps or jig. Still, this doesn’t occur because the strain from this stress is really low as opposed to the amount of movement that would occur if there were no restraint during welding.

9. Remove shrinkage forces after welding

Peening is one way to counteract shrinkage forces of weld bead when it cools. Basically, bead peening stretches it and makes it thinner, thereby relieving the stresses caused by contraction when the metal cools. Still, this method must be taken with care. For instance, a root bead should be peened never, due to the danger of either concealing or causing a crack. Generally, peening isn’t allowed on the final pass owing to the possibility of covering a crack and interfering with the inspection, and due to the unwanted work-hardening effect. Hence, the utility of this technique is limited, though there have been instances that between-pass peening proved to be the sole solution for the distortion or cracking problem. Before peening is applied on a job, engineering approval should be got. 

Another method availed to remove shrinkage forces is by thermal stress relieving (i.e. controlled heating of weldment to an elevated temperature, followed by controlled cooling). Sometimes 2 identical weldments are clamped back to back, welded and stress-relieved whilst being held in the straight condition. The residual stresses that would tend to distort weldments are, accordingly, minimized. 

For your information:

10. Minimize welding time

Because complex cycles of heating and cooling occur during welding, and because time is required for the heat transmission, the time factor impacts distortion. Generally, the weld is desirably finished quickly, before a large volume of surround metal heats up and expands. The welding process, type and size of electrode, welding current and travel speed are associated with the degree of shrinkage and distortion. Using mechanized welding equipment reduces welding time and amount of metal influenced by heat and, consequently, distortion. For instance, depositing a given-size weld on the thick plate with a process that operates at 175 amp, 25 volts and 3 ipm asks for 87,500 joules of energy each linear inch of weld (also, heat input). A weld with just about the same size produced with a process that operates at 310 amp, 35 volts and 8 ipm asks for 81,400 joules each linear inch. The weld created with the higher heat input generally induces a greater amount of distortion. Generally, the fillet weld size (in inches) is the square root of quantity of heat input (kJ/in) divided by 500. That’s why those two welds are most likely not of the same size. 

In case you want to have a look at:

Other techniques for welding distortion control:

11. Water-cooled jig 

Many techniques have been developed for controlling distortion on specific weldments. For instance, in sheet-metal welding, a water-cooled jig helps carry heat away from the welded components. Copper tubes are soldered or brazed to copper holding clamps, and water is circulated through tubes during welding. The restraint of clamps also helps reduce distortion. 

12. Strongback

Strongback is another helpful technique for distortion control during butt welding. Clips are welded to the edge of one plate, and wedges are driven under clips to force the edges into alignment and hold them during welding.

13. Thermal stress relieving

Except in special situations, thermal stress relieving isn’t used for correcting distortion. Still, there are occasions that stress relief is needed to avert further distortion from occurring before the weld is finished. 

Wrap up

A checklist for welding distortion control:

Don’t overweld
Control fit-up
Use intermittent welds where possible and in line with design requirements
Use smallest leg size as permissible when fillet welding
For groove welds, use joints which will minimize the volume of weld metal. Instead of single-sided joints, consider double-sided joints
Weld alternately on either joint side when possible with multiple-pass welds
Use minimum number of weld passes
Use low heat input procedures. That means high deposition rate and higher speeds of travel
Use welding positioners to attain the maximum amount of flat welding. The flat position allows the use of large-diameter electrodes and high-deposition-rate procedures of welding
Balance welds around the neutral axis
Distribute welding heat as evenly as possible through a well-planned welding sequence and weldment positioning
Weld towards the unrestrained part of member
Use fixtures, clamps and strongbacks to maintain fit-up and alignment
Pre-bend the members, or pre-set the joints to allow shrinkage to pull them back into alignment
Sequence sub-assemblies and final assembles for the welds to be made continually balance each other around the neutral axis of section. 

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