Читать книгу Weld Like a Pro - Jerry Uttrachi - Страница 9
ОглавлениеHundreds of joint types are used in welded fabrication. Butt joints, tubular structural joints, and fillet welds are the most common. Complex joint designs are used for welding thick sections, and for these complex joints, J- and U-grooves are used to reduce the amount of filler metal required to complete a weld. Also, many joint types are used to weld sheet metal and tubular structures, which are employed in various industries. Fabricators developed many of these weld joints as an efficient method of achieving the fit-up needed to make consistent quality welds.
Various fabrication specifications exist that define specific welding procedures and detailed welding amps, volts, and travel speed ranges for these joints. This allows a fabricator to use specific joint types without the need to prove the joint can produce the required weld quality. A number of these weld joints are shown in this chapter and may provide ideas for their use in a specific street rod or race car project.
Fig. 2.1. Hundreds of joint types are used in welded fabrication. A number of more complex joint designs relate to welding thick sections, where J-grooves and U-grooves are employed to reduce the amount of weld metal needed. There are also many joints defined for use in sheet metal, such as for ductwork, that could be used in street rod applications.
Structural tubular joints are often used for race car chassis and roll bars. In addition, exhaust systems use thin-wall tubes that must be joined. A number of industries use tubular members for the fabrication of structures, such as building members and highway sign supports. These designs are subjected to varying loads. Designers use sophisticated stress analysis techniques to optimize the use of materials. Some of this design and welding information can be useful in race car and street rod fabrication.
Simple square butt welds are often used in automotive-type welding. Variations may be useful to provide increased strength. Welding from one side only can leave some of the root area unwelded. This leaves a stress concentration that can cause a crack to form in the weld. Where maximum strength is required, a full-penetration weld should be used. TIG can produce full-penetration joint welding from one side, but you need to carefully control the penetration and be sure the full joint is melted.
Fig. 2.2. Fabricators have developed many weld joint designs as efficient methods for achieving the fit-up needed to make consistent quality welds. A number of industries that use tubular structural components have developed design criteria and weld procedures, and these can be adapted to race car and street rod fabrication.
Fig. 2.3. A full-penetration weld should be used to achieve maximum strength. In the bottom left panel, the joint shown is very useful for somewhat thicker material. First, a weld is made in a single V-joint to achieve good penetration. Then the back side is gouged or ground into sound weld metal, making a U-groove. A second weld is then made in the U-groove to fully penetrate into the first deposit.
When you can weld from both sides of the joint, a full-penetration weld is easier to accomplish. For thin material, the edges can be butted together, a weld made on one side, and a weld made on the back side that fully penetrates into the first.
A single V-preparation for butt joints is a proven method accepted for a number of design specifications and can ensure a full-penetration weld is achieved. V-preparation used for a full-penetration joint is particularly useful for thicker materials, such as 3/16- and 1/4-inch plate. By first using a single V-preparation, leaving half the plate thickness as a land accomplishes two things. It ensures good penetration on the first weld and leaves a land under the V that prevents excess penetration where the fit-up is not perfectly tight.
The V can often be made with a grinder, but you must be careful to leave half of the surface as the land. The first weld is placed into the V-groove. It should be made with sufficient current and speed to penetrate about three-quarters of the plate thickness.
Fig. 2.4. These joints are suited for welding sheet metal. The upper left joint is commonly called a joggle joint or flange joint. The official AWS Sheet Metal Code name is an offset lap joint. Whatever it’s called, this is an excellent joint when welding a patch panel. Simple locking-type pliers are available that can progressively form the edges providing a backing for the subsequent weld.
Fig. 2.5. Fabricators, including those making air handling ducts and tractor cabs, weld sheet metal. They have developed a number of joints that make it easier to weld specific sections. Some are useful for specific automotive applications. Flange joints make welding easier and may require less heat input. Backing a weld, such as the corner weld shown, adds strength and allows less precise fit-up.
Then the back side is gouged or ground into sound weld metal by using a grinder held on its side or an air-powered chipper with the proper groove should go sufficiently deep, so the bottom reaches defect-free weld metal in the first-side weld, and it should result in a U-shaped joint.
A second weld is then placed in the U-groove with sufficient current, so it fuses into the groove on the first pass. The resulting weld should overlap about 20 percent of the joint thickness. This overlap eliminates any root defects that may have been created in the first weld.
A J-groove is essentially half a U-groove and can be employed where the edge of a thick plate butts to a vertical member, as might be encountered in a cross-brace attachment to a side frame rail. As with a U-groove, a J-groove minimizes the amount of weld metal and weld heat while still ensuring adequate penetration.
Square butt welds made in sheet metal require very close fitment. Several techniques are employed to make welding these joints easier. One approach is making a joggle or flange joint used to fabricate propane tanks, fire extinguishers, and other thin sheet metal vessel end-cap welds. With this approach, one edge of the joint is formed so the joining plate fits over the bent area. This provides back support for the weld, and it’s more tolerant of slight fit-up variations.
Offset lap joint is the official AWS Sheet Metal Code name for this type of joint. The weld itself is referred to as a flare-bevel weld. Whatever you call it, this is an excellent joint when welding a sheet-metal patch panel. Simple locking pliers are available with dies welded to the grip faces, from companies such as Eastwood, that can progressively form the edges, providing a backing for the subsequent weld. There are air-powered devices that provide the same progressive crimping and make the task for preparing the panel faster.
Fig. 2.6. Tubular joints are used to fabricate chassis and roll cages. The joint names shown are from the AWS Structural Welding Code for Steel. This joint type is a partial-penetration weld because there is an unwelded area at the root of the fillet weld. That creates a stress riser that can cause a crack in the fillet weld when subjected to high-stress, cyclic loading.
A number of industries fabricate sheet-metal parts such as air handling ducts and tractor cabs. A number of joints have been developed to make it easier to weld specific types of sections. Some of these designs, which include flange joints, may be useful for specific street rod applications. These flange joints, as they are referred to, make welding easier and may require less heat input. Melting the edges of a flange butt weld, as shown in Figure 2.5, is easier than making a square butt weld in sheet metal. In addition, the edges can be easily clamped together and the joint tack welded prior to final welding of the seam.
The same fit-up and welding benefits exist for the flange corner weld. Backing a weld with another part, for example in a corner weld, adds strength and is more tolerant of less-than-precise fit-up.
Tubular intersection joints are typically used in race car chassis and roll cage welding, and a number of non-automotive industries use tubular members in construction. They have developed standards that define allowable loads for various intersecting tubular joints. The AWS Structural Welding Code for Steel defines the official names of these intersections. Several of these commonly used for race car fabrication are shown in Figure 2.6.
This type of joint is considered a partial-penetration weld because there is an unwelded notch at the root of the fillet, and this unwelded area creates a stress riser at the weld root. Depending on the loads involved, this stress concentration can cause a crack to propagate into the fillet welds on thinner-wall tubes, such as those used in chassis and roll cage constriction. This is a problem with high-stress and cyclic loading. The allowable stress calculations can reduce the amount some of these joints can be safely loaded by a factor of 70 percent or more. Fatigue is a failure mode in which loads vary in a cyclic manner.
The stress riser, such as the unwelded root of a fillet, can cause a crack to form. Over time, with increasing loading cycles, these small cracks grow bigger and can lead to failure. An advantage of steel is that at a low-enough load level, the crack tip blunts and stops propagating. At that load level, the fatigue life of the structure is said to be infinite. With a fully penetrated weld or base material free from significant defects, that load or stress, to have infinite fatigue life, is about half the material’s ultimate strength. However, with high-stress concentrations, the load to achieve infinite life may be only 20 percent or less of the ultimate strength.
This infinite life characteristic is not applicable to all metals. Aluminum, for example, has no load that eliminates the growth of highly stressed cracks. For highly cyclic loading, such as a rotating member, aluminum is not a good choice.
Race cars often use many complex tube intersections for a lightweight, ridged structure. A NASCAR chassis is shown in the upper left of Figure 2.2 that has six tubes coming into one common point from various angles. To achieve the required welded-joint quality it is essential to have very good fit-up with minimum gaps. Time spent in joint preparation saves time in welding and produces the best quality structure. In Chapter 4, examples are shown of both proper and improper joint fit-up. In some instances small grinding wheels or abrasive cartridge rolls may be employed to achieve the desired maximum gaps of about .010 inch for thin tube walls such as .040 inch. For .062 and thicker wall tubes, .020-inch maximum gaps should produce satisfactory welds.
Fig. 2.7. The AWS Specification for Automotive Weld Quality—Arc Welding of Steel, defines the names for this series of sheet-metal weld joints. Arc and plug welds are commonly used for street rod fabrication. A plug weld is made through a premade drilled or punched hole. For thicker materials, a fillet weld can be made in an elongated slot.
Gussets can be used on tube joint intersections to stiffen the assembly. They are particularly useful for some high-strength materials such as 4130 chrome-moly, where a somewhat lower-strength, more ductile welding rod and smaller weld size can be offset with the added strength supplied by a gusset. An example of the use of a gusset is shown on a NASCAR roll cage in the lower right of Figure 2.2.
A fillet weld is a triangular-shaped deposit commonly used for many joints where two materials to be joined intersect at angles. In instances where two flat plates are joined there is little joint fit-up required. However, for fillet welding intersecting tubes, the complex joint geometry must be properly cut and matched to achieve the needed maximum gap of .010 to .020 inch. It is also important to ensure the bottom of the fillet weld, at the intersection of the shapes being joined, is melted and fused. It is possible to make a fillet weld having a good surface appearance that is not properly fused in this bottom area, called the root of the fillet.
Fillet welds are often partial-penetration welds and require a reduction in allowable loads because of the gap left at the weld root. The reduction factors depend on the exact joint, the amount of penetration, the type of loads involved, and design specification requirements.
Fig. 2.8. Fillet welds are considered partial-penetration welds and require a reduction in allowable loads because of the unwelded area at the root. The amount of reduction depends on specification requirements. A single fillet has the highest stress concentration because of the loading. The double fillet is better to use, and the full-penetration double-fillet weld in which the root gap is eliminated is best.
A single fillet has the highest stress concentration because of the loading. If loaded so that the joint is bent toward the weld, the stress at the root of the weld is significantly increased.
A double fillet weld is better because, although an unwelded area exists, when a side load is applied the stresses are shared by the two fillet welds and the root stress concentration is not as high as with a single fillet.
A full-penetration double fillet is the best joint because there is no unwelded area.
The commercial automotive industry has developed its own standards for the types of welded joints. These sheet-metal weld joints are defined in the AWS Specification for Automotive Weld Quality—Arc Welding of Steel. They include plug and spot welds, which are commonly used for street rod welding.
The difference between these two welds is that a plug weld is made through a premade drilled or punched hole while a spot weld relies on the arc melting through the top sheet and into the bottom sheet. This works well for thin sheet metal and quality can be ensured by having accurate times along with control of amps and volts.
The timing for a spot weld should start after an arc is established by having the welding machine start the timing sequence only when voltage and amperage are detected. If more strength is needed, for example on heavier top sheet materials, welds can be made in an elongated slot.
