weldment design 01 - welding joints

Before an arc can be struck on metal, the product must be designed to serve its purpose, the material chosen and the method of welding determined in more or less detail. The weldment design engineer must understand the principle of welding design :
1. Approach the redesign of previously cast, forged or riveted products as a new design, on the basis of the functions to be performed.
2. Use materials, where possible, which require the least in welding precautions and least skill.
3. Welding is a means to an end, but to the designer not an end in itself. Avoid extra and unnecessary joints by flanging, bending or rolling, and use of standard sections, stampings, small castings or forgings, wher ever necessary or advisable.
4. At least mentally review all the welding processes available and applicable to various parts of the design on the basis of material, thickness, form and quantity.

5. Estimate the stress pattern under load with respect to stress concentration and examine the design for the possibility of notches. Notches give rise to stress concentration.
6. Welds should always be placed in regions of lower stress.
7. Secondary members should not be loaded up as main members.
8. Welding more than necessary should not be specified.
9. Welding should not be done where it offers no benefits.
10. Design should not demand a consistency in welder's performance as it is difficult to maintain.
11. Design should specify critical weld location.
12. Design should not generate unnecessary problems in the production department. The weld designers must consider the practicalities in production.

Proper joint design is a vital part of a welding procedure because it helps to
(i) Control distortion.
(ii) Minimize residual stresses,
(iii) Facilitate good workmanship,
(iv)Achieve proper weld strength,
(v) Reduce welding costs, and
(vi) Result in greater reliability.
The weld joint design should be such that the welds can be tested nondestructively for necessary quality control, especially if the welds are in pressurized or contaminated and inaccessible areas.

Here is the types of joints and its common characteristics :

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 angle, use the correct root opening, and use the correct root face for the joint. When you follow these principles, you produce better welds every time. Other standard grooved butt joint designs include the bevel groove, J-groove, and U-groove, as shown in figure 3-24.


T JOINTS

The square tee joint (fig. 3-26, view A) requires a fillet weld that can be made on one or both sides. It can be used for light or fairly thick materials. For maximum strength, considerable weld metal should be placed on each side of the vertical plate.

The single-bevel tee joint (fig. 3-26, view B) can withstand more severe loadings than the square tee joint, because of better distribution of stresses. It is generally used on plates of 1/2 inch or less in thickness and where welding can only be done from one side.

The double-bevel tee joint (fig. 3-26, view C) is for use where heavy loads are applied and the welding can be done on both sides of the vertical plate.


LAP JOINTS

The single-fillet lap joint (fig. 3-27, view A) is easy to weld, since the filler metal is simply deposited along the seam. The strength of the weld depends on the size of the fillet. Metal up to 1/2 inch in thickness and not subject to heavy loads can be welded using this joint.

When the joint will be subjected to heavy loads, you should use the double-fillet lap joint (fig. 3-27, view B). When welded properly, the strength of this joint is very close to the strength of the base metal.

>> Plug weld may be made without or with a hole (tapered or parallel) in the upper member. This joint is used where bottom/second plate is not easily accessible for fillet welding. Plug weld can be employed to impart added strength to the structure.


CORNER JOINTS

The flush corner joint (fig. 3-25, view A) is designed primarily for welding sheet metal that is 12 gauge or thinner. It is restricted to lighter materials, because deep penetration is sometimes difficult and the design can support only moderate loads.

The half-open corner joint (fig. 3-25, view B) is used for welding materials heavier than 12 gauge. Pene-tration is better than in the flush corner joint, but its use is only recommended for moderate loads.

The full-open corner joint (fig. 3-25, view C) produces a strong joint, especially when welded on both sides. It is useful for welding plates of all thicknesses.

EDGE JOINTS

The flanged edge joint (fig. 3-28, view A) is suitable for plate 1/4 inch or less in thickness and can only sustain light loads. Edge preparation for this joint may be done, as shown in either views B or C.



To summarize, choose joint types with the following in mind:
1. A type adequate or suitable for the nature of load.
2. Minimum use of deposited metal.
3. Least cost of joint preparation.
4. Ease in fitup and maximum tolerance, without undue increase in deposited metal.