Distortion is a potential problem when metals are joined by arc welding processes. The intense heat of welding causes localized expansion of both the weld metal and the base metal, while the surrounding base material remains relatively cool. The latter area acts to restrict the localized expansion and, eventually, the metal heated in the welding process contracts, causing distortion.
While no one can change the laws of physics, we can control distortion — both in the shop and on the drawing board. This is the first of a three-part series on controlling distortion and will address what we can do at the design stage to minimize distortion, starting with the easier concepts. Later, we'll look at some advanced design-related topics and, finally, we'll consider what we can do in the shop to control distortion.
Before I dive into the details, it is good to remember that practices that are useful to control distortion often simultaneously reduce welding costs. If some of these principles of distortion control look familiar, it may be that you saw the concept in an article on cost control. Many such ideas are equally applicable to both objectives.
♦ Specify welds of the smallest size, consistent with design requirements.
Large welds cause more distortion than smaller welds, all things being equal. Too often, weld designers increase weld sizes "just to be safe." While welds must always be large enough to transfer the applied loads, larger-than-necessary welds will naturally result in more distortion.
Figure 1 demonstrates how small differences in weld size significantly affect weld volume, which is directly related to distortion tendencies. The values contained in the table represent the percentage increase in weld metal volume caused by the various increases in weld size in excess of the ideal size.
For example, if a 1/4 in. fillet is sufficient, but the designer specifies a weld that is 1/16 in. larger than ideal (a 5/16 in. fillet), the amount of weld metal and the amount of distortion both increase by approximately 56 percent.
♦ Make fillet welds longer and smaller in size.
Consider two fillet weld options as shown in Figure 2: The first has a leg size of 5/16 in., and is 8 in. long. The second has a 1/4 in. leg, and is 10 in. long. Since both have the same weld throat area, they have the same strength. However, the larger, shorter weld requires approximately 25 percent more weld metal, resulting in more distortion
♦ Use partial joint penetration (PJP) groove welds instead of fillet welds.
To minimize distortion, using partial joint penetration groove welds rather than fillet welds will result in about a 50 percent reduction in weld metal volume and distortion. This may or may not be the least costly weld detail from a production point of view, but distortion will be decreased. Here's why: As shown in Figure 3, for equal weld throat dimensions, a PJP groove weld requires 50 percent less weld metal. Although the PJP requires beveling the material, if distortion control is the goal, using it will be worthwhile.
♦ Use groove weld details that minimize the required amount of weld metal.
The root opening (typically abbreviated as "R") and the included angle (often designated as "α") for a groove weld must be sufficient to provide adequate access to the root of the joint. Furthermore, the root configuration must be one that encourages fusion and will not lead to cracking. Theoretically, there are an infinite number of combinations of root openings and included angles that could be used. AWS D1.1 Structural Welding Code—Steel provides three acceptable options for prequalified, single vee groove weld details, as shown in Figure 4.
If 1/2 in. plate is involved, the use of a 45-degree, 1/4 in. root opening, versus a 30-degree, 3/8 in. root opening, will reduce the required weld metal and distortion by about 10 percent. Lest you conclude that larger included angles and smaller root openings are always best, consider 4 in. thick material: In this case, the smaller included angle and the larger root opening are the best choice.
♦ Use intermittent fillet welds where appropriate.
Intermittent fillet welds may not be appropriate for all applications but, when they are, intermittent fillets can reduce distortion. For economy and distortion control, fillet welds first should be made as long as possible, with the smallest possible and practical leg size. If the capacity of the connection is still greater than required, then intermittent fillet welds should be considered.
As is the case with many of the concepts I address in Blodgett's Basics, distortion control starts on the drawing board.
|Omer W. Blodgett, Sc.D., P.E., senior design consultant with The Lincoln Electric Co., struck his first arc on his grandfather's welder at the age of ten. He is the author of Design of Welded Structures and Design of Weldments, and an internationally recognized expert in the field of weld design. In 1999, Blodgett was named one of the "Top 125 People of the Past 125 Years" by Engineering News Record. Blodgett may be reached at (216) 383-2225.|