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Butt Joints

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Figure 3-23.—Butt joints. Whether the load will be in tension or compres- sion and whether bending, fatigue, or impact stresses will be applied How a load will be applied; that is, whether the load will be steady, sudden, or variable The direction of the load as applied to the joint The cost of preparing the joint Another consideration that must be made is the ratio of the strength of the joint compared to the strength of the base metal. This ratio is called  joint efficiency.  An efficient joint is one that is just as strong as the base metal. Normally, the joint design is determined by a de- signer or engineer and is included in the project plans and specifications. Even so, understanding the joint design for a weld enables you to produce better welds. Earlier in this chapter, we discussed the five basic types of welded joints—butt, corner, tee, lap, and edge. While  there  are  many  variations,  every  joint  you  weld will be one of these basic types. Now, we will consider some of the variations of the welded joint designs and the efficiency of the joints. BUTT JOINTS The square butt joint is used primarily for metals that are 3/16 inch or less in thickness. The joint is reasonably strong, but its use is not recommended when the metals are subject to fatigue or impact loads. Prepa- ration of the joint is simple, since it only requires match- ing  the  edges  of  the  plates  together;  however,  as  with any other joint, it is important that it is fitted together correctly for the entire length of the joint. It is also important  that  you  allow  enough  root  opening  for  the joint. Figure 3-23 shows an example of this type of joint. When you are welding metals greater than 3/16 inch in thickness, it is often necessary to use a grooved butt joint. The purpose of grooving is to give the joint the required strength. When you are using a grooved joint, it is important that the groove angle is sufficient to allow the electrode into the joint; otherwise, the weld will lack penetration  and  may  crack.  However,  you  also  should avoid excess beveling because this wastes both weld metal and time. Depending on the thickness of the base metal,  the  joint  is  either  single-grooved  (grooved  on  one side only) or double-grooved (grooved on both sides). As a welder, you primarily use the single-V and double- V  grooved  joints. The single-V butt joint (fig. 3-23, view B) is for use on plates 1/4 inch through 3/4 inch in thickness. Each member should be beveled so the included angle for the joint is approximately 60 degrees for plate and 75 degrees for pipe. Preparation of the joint requires a special  beveling  machine  (or  cutting  torch),  which makes it more costly than a square butt joint. It also requires more filler material than the square joint; how- ever, the joint is stronger than the square butt joint. But, as  with  the  square  joint,  it  is  not  recommended  when subjected to bending at the root of the weld. The double-V butt joint  (fig. 3-23, view C) is an excellent joint for all load conditions. Its primary use is on metals thicker than 3/4 inch but can be used on thinner plate where strength is critical. Compared to the single-V joint, preparation time is greater, but you use less filler metal because of the narrower included angle. Because of the heat produced by welding, you should alternate  weld  deposits,  welding  first  on  one  side  and then  on  the  other  side.  This  practice  produces  a  more symmetrical weld and minimizes warpage. Remember, to produce good quality welds using the groove joint, you should ensure the fit-up is consistent for the entire length of the joint, use the correct groove 3-13



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