Friday, November 6, 2015

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

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 2)


Indeed, all welders have faced welding distortion problem at one time or another. At first, the parts are straight and square, and after welding. Thinner materials are more susceptible because they have less stiffness. Also, stainless steel is more susceptible because it has higher thermal expansion and lower thermal conductivity than carbon steel. 

What is welding distortion?

Distortion in welding eventuates due to the expansion and contraction of weld metal and adjacent base metal during the heating and cooling cycle of welding process. Welding on one side of a part will result in more distortion than if the welds are alternated from one side to the other. During the heating and cooling cycle, many factors impact shrinkage of the metal and cause distortion – for example, physical and mechanical properties that change when heat is applied. When the temperature of weld area rises, yield strength, elasticity and thermal conductivity of steel plate reduce, whilst thermal expansion and specific heat rises. Those changes, in turn, influence the heat flow and heat distribution uniformity. 

Distortion causes

Why does distortion occur? The weld metal is deposited at high temperature, above the melting point of material. For steel, it is about 2,500°F (1,370°C). When the weld cools to the room temperature, it shrinks but the adjacent cold base metal restrains it from doing so, inducing high-residual tensile stress. The weld becomes like a stretched rubber band with the work-piece holding the ends. That’s why the base metal moves or springs back when the clamps that holds the work-piece are removed, distorting the part. 

The weld shrinks across its width, causing groove welds to wing up, or fillet welds to close up. When the welding shrinks along its length, it makes base metal twist around the weld. 

For weld distortion prevention or minimization, methods must be taken both in design and welding to overcome the effects of heating and cooling cycle. Weld shrinkage can’t be prevented though, it can be controlled. Below are some practical ways that welders can used to minimize welding distortion.

Welding distortion control

1. Don’t overweld

The more metal is placed in a joint, the greater the shrinkage is. Correctly sizing the weld not just minimizes distortion but saves weld metal and time. You can use a flat or slight convex bead to minimize the amount of welding metal in a fillet weld; proper edge preparation and fit-up can help minimize the amount of weld metal in a butt joint. The excess weld metal in a highly convex bead doesn’t raise the allowable strength in the code work though, it does raise shrinkage forces.  

When welding the heavy plate (more than 1 inch thick), beveling or double beveling can save a considerable amount of weld metal that translates into less distortion automatically.

Generally, if welding distortion isn’t a problem, choose the most economical joint. If the distortion is a problem, choose either a joint that the weld stresses balance each other or a joint that requires the least amount of weld metal.

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2. Use intermittent welding

Another way to minimize the amount of weld metal is using intermittent welds rather than continuous welds where possible. For example, for attaching the stiffeners to the plate, intermittent welds can reduce the amount of weld metal by up to 75% while still providing the needed strength.

3. Use fewer weld passes 

Fewer passes with large electrodes result in less welding distortion than a greater number of passes with small electrodes. Shrinkage due to each pass tends to be cumulative, thus raising total shrinkage when many passes are used. 

4. Place welds near the neutral axis or the center of the part

Distortion is minimized by giving less leverage for the shrinkage forces to pull the plates out of alignment. Both design of weldment and welding sequence can be effectively used for welding distortion control.

5. Balance welds around the neutral axis

Welding on both plate sides offsets one shrinkage force with another to reduce effectively distortion. Here, design of assembly and proper welding sequence are important factors, too. 

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6. Use backstep welding technique

In the backstep technique, the general welding progression may be from left to right though, each bead segment is deposited from right to left (Figure 1 (g)). When each bead segment is placed, the heated edges expand, temporarily separating the plates at B. Still, when the heat moves out to C, expansion along the outer edges CD would bring the plates back together. This separation is most pronounced when the first bead is laid. With successive beads, the plates get less and less expansion due to the restraint of prior welds. The backstep technique mayn’t be effective in all applications, and it can’t be economically in automatic welding.

Figure 1: Distortion can be averted or minimized by techniques that defeat - or use constructively - the effects of heating and cooling cycle.

7. Anticipate shrinkage forces

Presetting parts before welding can make shrinkage do constructive work. Some assemblies preset in this fashion are shown in Figure 1 (h). The amount of preset required for shrinkage to pull plates into alignment can be decided from some trial welds. 

Pre-bending, pre-setting or pre-springing the parts to be welded is simple example of using the opposing mechanical forces to counteract welding distortion. The top of weld groove that will contain the bulk of weld metal is lengthened as the plates are present. Hence, the completed weld, if having been made on the flat plate, is slightly longer than it would be. When the clamps are released after welding, the plates come back to the flat shape, enabling the weld to relieve its longitudinal shrinkage stresses by shortening to the straight line. The 2 actions coincide and the welded plates would assume the desired flatness. 

Another common practice to balance shrinkage forces is to position the identical weldments back to back, clamping them tightly together. The welds are completed on both assemblies and cooled before the clamps are released. Pre-bending can be combined with this method by inserting the wedges at suitable positions between the parts prior to clamping. 

In heavy weldments, the rigidity of members and their arrangement related to each other may give the balancing forces needed. If there don’t present those natural balancing forces, using other means to counteract shrinkage forces in the weld metal is needed. This can be made by balancing one shrinkage force against another, or by creating an opposing force through fixturing. The opposing forces may be other shrinkage forces, restraining forces imposed by jigs, clamps or fixtures, restraining forces originated from the arrangement of members in the assembly, or the force from the member sag due to gravity. 

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