Weld joints-Gas Metal Arc Welding Handbook

Chapter 6 Weld Joints and Weld Types 59

^Butt Fillet J-groove

Butt Bevel V-groove

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Figure 6-8. Joggle-type joint.

Figure 6-9. Tubular butt joint with a built-in backing bar.

Figure 6-11. Plate butt weld with a fabricated backing bar.

Figure 6-12. Controlled weld penetration joint.

Butt Bevel-groove J-groove V-groove

Weldment Configurations

The basic joint often is changed to assist in a component’s assembly. A weld joint might be modified to gain access to the weld joint or to change a weld’s metallurgical properties. Some common weldment configuration designs are described here. Joggle-type joints are used in cylinder and head assemblies where backing bars or tooling cannot be used. See Figure 6-8. Another application of joggle joints is in the repair of unibody automobiles where skin panels are placed together and welded. A built-in backing bar is used when enough material is available for machining the required backing or when tooling cannot be inserted (as in some tubular applications). An example in which tubing is being joined to heavy wall tube is shown in Figure 6-9. Pipe joints often use special backing rings or are machined to fit specially designed mated parts. Types of backing rings are shown in Figure 6-10. Figure 6-11

Figure 6-13. Buttered weld joint face.

Figure  6-14.  Overlaid  welds,  called  surfacing  or  cladding,

Figure 6-10. Various types of backing rings for pipe joints. protect the base metal from wear or contamination.

U-groove Edge flange Corner flange

Figure 6-6. Types of welds that may be made with a basic edge joint.

shows a fabricated backing bar. These bars must fit tightly or problems will be encountered in heat flow and penetration. Weld joints specially designed for controlled penetration are used where excessive weld penetration would cause a problem with assembly or liquid flow. This type of joint is shown in Figure 6-12.

A series of bead welds overlaid on the face of a joint is called buttering, Figure 6-13. Buttered welds are often used to join dissimilar metals. A series of overlaid welds on the surface of a part to protect the base material is called surfacing or cladding. Refer to Figure 6-14.

Welding Terms and Symbols

Communication from the weld designer to the welder is essential to proper completion of most weldments. Some of the common terms used to describe parts of the weld joint are found in Figure 6-15. Other

terms used to describe welds are given in Figure 6-16. The AWS welding symbol shown in Figure 6-17 was developed as a standard by the American Welding Society. This symbol is used on drawings to indicate the type of joint, placement, and the type of weld to be made.

Chapter 6 Weld Joints and Weld Types 61

The symbol may also include other information, such as finish and contour of the completed weld.

It is important to study and understand each part of the welding symbol. Figure 6-18 is a table showing basic weld symbols that are used with the AWS welding

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Figure 6-18. Basic weld symbols. (Printed with permission of the American Welding Society)

symbol to direct the welder in producing the proper weld joint. The arrow of the welding symbol indicates the point at which the weld is to be made. The line connecting the arrow to the reference line is always at an angle. Whenever the basic weld symbol is placed below the reference line, as shown in Figure 6-19, the weld is

made on the side where the arrow points (referred to as the arrow side). Whenever the basic symbol is placed above the reference line, the weld is to be made on the other side of the joint, as shown in Figure 6-20. By placing dimensions on the symbol and drawings, the exact size of the weld may be indicated. Study the

 Elements in this area remain as shown  when tail and arrow are reversed

 Weld symbols shall be contained  within the length of the reference line

Figure 6-17. The AWS welding symbol conveys specific and complete information to the welder. (Printed with permission of the American Welding Society)

Fillet weld symbol

Fillet weld symbol

Other side

Arrow side

examples of typical weld symbols and weldments shown in Figure 6-21.

The complete weld symbol gives the welder instructions on how to prepare the base metal, the welding process to use, and the finish for the completed weld. Through careful use of these symbols, the weld designer can convey all the information needed to complete a weldment.

Classes are offered that provide advanced study in the area of print reading for welders. By taking such classes, the welder can improve his or her ability to read and interpret welding drawings. Studying texts on print reading is another method of gaining ability to read prints.

Chapter 6 Weld Joints and Weld Types 63

U-groove weld Desired weld

symbol

V-groove weld Desired weld

symbol

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Overhead 4G

Overhead 4F

Vertical 3G

Fixed 6G

Weld Positions

For a welder, it is important to be able to weld in different positions. The American Welding Society has defined the positions of welding to include:

• Flat.

• Horizontal.

• Vertical.

• Overhead.

Figure 6-22 demonstrates the four positions for fillet welds, grooved butt welds, and pipe welds. While practicing welding in these positions, you should note how gravity affects the molten weld pools. In addition to this, heat distribution also varies with each position. These factors make the skills needed for each position distinct. Practice is required to produce good welds in all positions.

Design Considerations

Design of the weld type and weld joint to be used is of prime importance if the weldment is to do the intended job. The weld should be made at reasonable cost. Several factors concerning the weld design must be considered:

• Material type and condition (annealed, hardened,

tempered).

• Service conditions (pressure, chemical, vibration,

shock, wear).

• Physical and mechanical properties of the

completed weld and heat-affected zone.

• Preparation and welding cost.

Two-sided fillet weld symbol

Fillet weld-all-around symbol

Bevel-groove weld symbol

Desired weld

Desired weld

Desired weld

Vertical 3F

Horizontal 2G

Horizontal 2F

Flat 1G

Flat 1F

Fillet Welds Grooved Butt Welds

Figure 6-22. American Welding Society definitions of welding positions.

Vertical fixed 5G

Horizontal 2G

Rotated flat rolled 1G

Pipe Welds

• Assembly configuration and weld access.

• Equipment and tooling.

Entire surface built up weld symbol

Desired weld

• They conflict with penetration of the final weld

Whenever possible, butt joints should mate at the

Butt Joints and Welds

Butt joints are used where high strength is required. They are reliable and can withstand stress better than any other type of weld joint. To achieve full stress value, the weld must have 100 percent penetration through the joint. This can be done by welding completely through from one side. The alternative is working from both sides, with the welds joining in the center.

Figure 6-21. Typical weld symbols and weld applications.

Thinner-gauge metals are more difficult to fit up for welding. Thin metals also require more costly tooling to maintain the proper joint configuration. Tack welding may be used as a method of holding the components during assembly. However, tack welds present many problems:

Expansion of the base metal during welding often will cause a condition known as mismatch, Figure 6-23. When mismatch occurs, the weld generally will not penetrate completely through the joint. Many specifica-tions limit highly stressed butt joints to a 10 percent maximum mismatch of the joint thickness.

bottom, Figure 6-24. Joints of unequal thickness should be tapered in the weld area to prevent incomplete or inadequate fusion. This is shown in Figure 6-25. When this cannot be done, the heavier piece may be tapered on the upper part of the joint as well.

Weld shrinkage. Butt welds always shrink across the joint (transversely) during welding. For this reason, a shrinkage allowance must be made if the “after welding” overall dimensions have a small tolerance. Butt welds in pipe, tubing, and cylinders also shrink on the diameter of

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Correct

Figure 6-23. Welds made on mismatched joints often will fail below the rated load when placed in stress conditions.

Figure 6-27. Weld joint shrinkage can be determined in four steps. 1. Tack weld the test joint together. 2. Scribe parallel lines, as shown, on approximately 2” centers. Record this dimension. 3. Weld joint with test weld procedure. 4. Measure linear distance and compare with original dimension.

Single fillet

Correct

Section A Correct design

 Open to

corrosion

Section B

Incorrect design

Figure 6-24. Mating the joint at the bottom equalizes the load during stress when the weld is made from the top and pene-trates completely through the joint.

T-Joints and Welds

Various T-joint designs are used to join parts at an angle to each other. Depending on the intended use of the weldment, the joint may be made with a single fillet, double fillet, or a groove and fillet weld combination. Figure 6-28 shows these designs.

Double fillet

Figure 6-30. Corrosive liquids must not be allowed to enter the penetration side of the weld joint. In Section A, the back of the weld is closed to corrosion. In Section B, the back of the weld is open to corrosion.

Figure 6-25. Joints of unequal thickness absorb different amounts of heat and expand at different ratios. Equalize the heat flow by tapering the heavier material to the thickness of the thinner material.

Figure 6-26 Butt welds shrink during welding in both transverse and circumferential directions.

the material. This shrinkage is shown in Figure 6-26. In areas where these dimensions must be maintained, a shrinkage test must be done to establish the amount of shrinkage. Figure 6-27 shows how such a test is made. Heavier materials will shrink more than thinner materials. Double-groove welds will shrink less than single-groove welds. This is because less welding is involved and less filler material is used.

Fillet welds are made to specific sizes that are deter-mined by the allowable design load. They are measured as shown in Figure 6-29. Where design loads are not known, a “rule of thumb” may be used for determining the fillet size. In these cases, the fillet weld leg lengths must equal the thickness of the thinner material.

The main problem in making fillet welds is lack of penetration at the joint intersection. To prevent this condition, always make stringer beads at the intersection. Weave beads do not provide the desired penetration on fillet welds.

Lap Joints and Welds

Lap joints may be either single fillet, double fillet, plug slot, or spot-welded. They require very little joint preparation. They are generally used in static load appli-cations or in the repair of unibody automobiles. Where corrosive liquids are involved, both edges of the joint must be welded. See Figure 6-30. One of the major prob-lems with lap joint design is shown in Figure 6-31. Where the component parts are not in close contact, a bridging fillet weld must then be made. This leads to incomplete fusion at the root of the weld and oversize fillet weld dimensions. When using this type of design in sheet or plate material, clamps or tooling must be used to main-tain adequate contact of the material at the weld joint.

An interference fit eliminates this problem in assembly of cylindrical parts, Figure 6-32. The inside diameter of the outer part is made several thousandths of an inch smaller than the outside diameter of the inner