NAME 345 Welding Technology Lecture 03 (Welding Joint Design)

Transcription

1 NAME 345 Welding Technology Lecture 03 (Welding Joint Design) Md. Habibur Rahman Lecturer Department of Naval Architecture & Marine Engineering Bangladesh University of Engineering & Technology Dhaka-1000, Bangladesh

2 Types of Weld There are eight separate and distinct types of weld: 1. Fillet weld 2. Groove weld 3. Back or backing weld 4. Plug or slot welds 5. Spot or projection welds 6. Seam welds 7. Stud weld 8. Surfacing weld 2

3 Types of Weld (Fillet Weld) This is the most commonly used weld. The fillet weld is so named because of its cross-sectional shape. It is simple and cheap to assemble and weld. The fillet is regarded as being on the joint and is defined as a weld of approximately triangular cross-section joining two surfaces approximately at right angles to each other. Types of Weld (Groove Weld) This is the second most popular weld. It is defined as a weld made in the groove between two members to be joined. The groove weld is regarded as being in the joint. There are 7 main basic groove weld designs, and they can be used as single or double welds. 3

4 Types of Weld (Back or backing weld) This is a special weld made on the back side or root side of a previous weld. The root of the original weld is gouged, chipped, or ground to sound metal before backing weld is made. This will improve the quality of the weld joint by assuring complete penetration. Types of Weld (Plug or slot welds) These welds are made through holes or slots in one member of a lap joint. These welds are used to join that member to the surface of another member that has been exposed through the hole. It the hole is round, it is plug weld; if it is elongated, it is a slot weld. Hole/slot is drilled on the top plate only, then it is filled with filler metal. The hole may or may not be completely filled with weld metal. 4

5 Types of Weld (Spot or Projection welds) These welds can be applied by different welding processes, which change the actual weld. Resistance welding, electron beam welding, laser welding etc. are examples of these types of weld. Types of Weld (Seam welds) This weld in cross-section looks similar to a spot weld. Resistance welding, electron beam welding, laser welding etc. are examples of these types of weld. In this weld, section between the surfaces of two sheets are fused. It is mostly associated with resistance welding. 5

6 Types of Weld (Stud weld) This is a special type of a weld produced by a stud welding process. It is used for joining a metal stud or similar part to a workpiece. Types of Weld (Flange or Surfacing welds) These welds is composed of one or more stringer or weave beads deposited on base metal as an unbroken surface. It is not used to make a joint. It is used to increase the thickness of the plate, to build up surface dimensions, or provide protection of the base metal from a hostile environment. 6

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8 Basic Types of Joints and Term 8

9 Weld Joints The weld joint is where two or more metal parts are joined by welding. The five basic types of weld joints are shown in Figure 3.6. Parts of Joint The root of a joint is that portion of the joint where the metals are closest to each other. may be a point, a line, or an area, when viewed in cross section. A groove is an opening or space provided between the edges of the metal parts to be welded. The groove face is that surface of a metal part included in the groove. The bevel angle is the angle formed between the prepared edge of a member and a plane perpendicular to the surface of the member. The groove angle is the total angle of the groove between the parts to be joined. The groove radius is the radius used to form the shape of a J- or U-groove weld joint. The root opening refers to the separation between the parts to be joined at the root of the joint. Bevel angle, groove angle, and root opening for a joint depend on the thickness of the weld material, the type of joint to be made, and the welding process to be used. As a general rule, gas welding requires a larger groove angle than manual metal-arc welding. The root opening is usually governed by the diameter of the thickness filler material. This, in turn, depends on the of the base metal and the welding position. 9

10 Parts of Joint (Contd.) Root penetration refers to the depth that a weld extends into the root of the joint. Root penetration is measured on the center line of the root cross section. Joint penetration refers to the minimum depth that a groove (or a flange) weld extends from its face into a joint, exclusive of weld reinforcement. Weld reinforcement is a term used to describe weld metal in excess of the metal necessary to fill a joint. 10

11 Parts of Joint A weld bead is a weld deposit produced by a single pass with one of the welding processes. If two or more beads are deposited in the groove, the weld is made with multiple-pass layers. Buildup sequence refers to the order in which the beads of a multiple-pass weld are deposited in the joint. Tack weld is a weld made to hold parts of an assembly in proper alignment temporarily until the final welds are made. Although the sizes of tack welds are not specified, they are normally between ½ inch to ¾ inch in length, but never more that 1 inch in length. 11

12 Types of joints 12

13 Welded Joint Design The details of a joint, which includes both the geometry and the required dimensions, are called the joint design. Although welded joints are designed primarily to meet strength and safety requirements, there are other factors that must be considered. A few of these factors areas follows: Whether the load will be in tension or compression 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. Five basic joints are used for bringing two members together for welding: i. Butt Joint: a joint between two members aligned approximately in the same plane. ii. Corner Joint: a joint between two members located approximately at right angles to each other in the form of an L. iii. T-Joint: a joint between two members located approximately at right angles to each other in the form of a T. iv. Lap Joint: a joint between two overlapping members located in parallel. v. Edge Joint: a joint between the edges of two or more parallel or nearly parallel members When more than two members are brought together, the joint is a combination of one of the five basic joint. The most popular joint is cross or cruciform. 13

14 Square Butt Joint The square butt joint is used to weld sheet metal. 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. Beveled butt joint It is used to butt weld heavier pieces of metal together. 3/8 to ½ inch metal can be welded using a single V or U joint. ½ inch metal and up can be welded using a double V or U joint. 14

15 Grooved butt joint Welding Joint Design For metals greater than 3/16 inch in thickness. The purpose of grooving is to give the joint the required strength. Groove angle must be sufficient to allow the electrode into the joint; otherwise, the weld will lack penetration and may crack. Excess beveling wastes both weld metal and time. Depending on the thickness of the base metal, the joint is either or double-grooved. Single-V and double- V grooved joints are primarily used. The single-v butt joint is for use on plates 1/4 inch through 3/4 inch in thickness. Groove angles are 60 degrees for plate and 75 degrees for pipe. The double-v butt joint 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. Other standard grooved butt joint designs include the bevel groove, J-groove, and U-groove. 15

16 Grooved butt joint There are more types of grooved butt joint: i. Single beveled Butt joint ii. Double beveled Butt joint iii. Single J Butt joint iv. Double J Butt joint v. Single U Butt joint vi. Double U Butt joint 16

17 Corner Joints Flush or Closed Corner Joint For welding sheet metal that is 12 gauge or thinner. Restricted to lighter materials, because deep penetration is sometimes difficult and the design can support only moderate loads. Half-open Corner Joint For welding materials heavier than 12 gauge. Penetration is better than in the flush corner joint, but its use is only recommended for moderate loads. Full-open corner joint Produces a strong joint, especially when welded on both sides. It is useful for welding plates of all thicknesses. Corner joints on heavy material are welded on both sides. The outside first, then reinforced on the inside. 17

18 Tee Joints Tee joints are used to join two pieces of metal that are approximately 90 to each other, but the surface of one piece of metal is not in the same plain as the other metal. Square T-joint Requires a fillet weld that can be made on one or both sides. It can be used for light or fairly thick materials. For max. strength, considerable weld metal should be placed on each side of the vertical plate. Single-bevel T-joint Can withstand more severe loadings than the square T-joint, because of better distribution of stresses. It is generally used on plates of ½ inch or less in thickness and where welding can only be done from one side. Double-bevel T-joint It is used where heavy loads are applied and the welding can be done on both sides of the vertical plate. 18

19 Tee Joints There are more types of T-joint: i. A plain T joint ii. B single beveled T joint iii. C double beveled T joint iv. D single J T joint v. E Double J T joint 19

20 Lap Joints Tee joints are used to join two overlapping pieces of metal. Single-fillet Lap joint It 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. It is used to weld metal up to ½ inch in thickness and not subject to heavy loads can be welded using this joints. Double-fillet Lap joint It is used when the joint will be subjected to heavy loads. When welded properly, the strength of this joint is very close to the strength of the base metal. Offset Lap joint It is used when two pieces of metal need to be joined in the same plain. 20

21 Edge Joints Tee joints are used to join two parallel or nearly parallel pieces of metal. These joints are not very strong. These joints are used mainly to join edges of sheet metal, reinforce flanges of I beams, and mufflers. Flanged edge joint is suitable for plate ¼ inch or less in thickness and can only sustain light loads. Edge preparation for this joint may be groove welds can be made in all of these positions. 21

22 Welding Positions All welding is done in one of four positions: i. Flat ii. iii. iv. Horizontal Vertical Overhead Welding Joint Design The American Welding Society (AWS) identifies these positions by a number/ letter designation: For instance, the 1G position refers to a groove weld that is to be made in the flat position. Here the 1 is used to indicate the flat position and the G indicates a groove weld. For a fillet weld made in the flat position, the number/letter designation is 1F (F for fillet). 22

23 Welding Positions (Contd.) 23

24 Expansion and Contraction When a piece of metal is heated, the metal expands. Upon cooling, the metal contracts and tries to resume its original shape. When two pieces of metal are welded together, expansion and contraction may not be uniform throughout all parts of the metal. This is due to the difference in the temperature from the actual weld joint out to the edges of the joint. This difference in temperature leads to internal stresses, distortion, and warpage. When you are welding a single-v butt joint the highest temperature is at the surface of the molten puddle. The temperature decreases as you move toward the root of the weld and away from the weld. Because of the high temperature of the molten metal, this is where expansion and contraction are greatest. When the weld begins to cool, the surface of the weld joint contracts (or shrinks) the most, thus causing warpage or distortion. 24

25 Controlling Distortion Proper Edge Preparation and Fit-up By making certain the edges are properly beveled and spacing is adequate, you can restrict the effects of distortion. Additionally, you should use tack welds, especially on long joints. Control the Heat Input The faster a weld is made, the less head is absorbed by the base metal. It is possible to weld a seam with the minimum amount of heat by simple speeding up the welding process. It is often necessary to use a welding technique designed to control heat input. An intermittent welding or Back-step welding is often used instead of one continuous weld. 25

26 Preheat the Metal Welding Joint Design Controlling Distortion (Contd.) To control the forces of expansion and contraction, you preheat the entire structure before welding. After the welding is complete, you allow the structure to cool slowly Limit the Number of Weld Passes You can keep distortion to a min. by using as few weld passes as possible. You should limit the number of weld passes to the number necessary to meet the requirements of the job. Use Jigs and Fixtures Holding the metal in a fixed position prevents excessive movements. A jig or fixture is simply a device used to hold the metal rigidly in position during the welding operation Allowance for Distortion To reduce distortion, you angle the parts to be welded slightly in the opposite direction in which the contraction takes place. When the metal cools, contraction forces pull the pieces back into position. 26

27 Welding Errors and Failures Lack of Fusion Definition: Lack of fusion occurs when there is no fusion between the weld metal and the surfaces of the base metal. Cause: Lack of fusion is caused by: i. Too low current ii. iii. iv. Too high welding speed Incorrect torch/gun angle Incorrect angle preparation Prevention: Eliminate potential causes. Repair: Remove and re-weld, being careful to completely remove the defective area. This is sometimes extremely difficult to find. There is no efficient non-destructive method. Visual inspection can be used during welding and with bend test. Also ultrasonic testing, liquid penetrant test or magnetic particle testing methods can be applicable. 27

28 Welding Errors and Failures Incomplete Penetration This type of defect is found in any of three ways: i. When the weld bead does not penetrate the entire thickness of the base plate ii. iii. iv. When two opposing weld beads do not interpenetrate When the weld bead does not penetrate the toe of a fillet weld but only bridges across it. Incorrect angle preparation Welding current has the greatest effect on penetration. Incomplete penetration is usually caused by the use of too low welding current and can be eliminated by simply increasing the amperage. Other causes can be the use of too slow travel speed and an incorrect torch angle. Both will allow the molten weld metal to roll in front of the arc, acting as a cushion to prevent penetration. The arc must be kept on the leading edge of the weld puddle. 28

29 Welding Errors and Failures Undercutting Definition: Undercutting is a defect that appears as a groove in the parent metal directly at the toe of the weld and left unfilled. It is most common in lap fillet welds, but can also be encountered in fillet and butt joints. Cause: Undercut is caused by: i. High amperage ii. Electrode angle iii. Long arc length iv. Rust Prevention: Set machine on scrap metal. Clean metal before weld. Repair: Weld with smaller electrode, sometimes mush be low hydrogen with preheat. Sometimes must gouge first. 29

30 Welding Errors and Failures Porosity Definition: Porosity is the result of gas entrapment in the solidifying metal. Porosity can take many shapes on a radiograph but often appears as dark round or irregular spots or specks appearing singularly, in clusters, or in rows. Sometimes, porosity is elongated and may appear to have a tail. This is the result of gas attempting to escape while the metal is still in a liquid state and is called wormhole porosity. All porosity is a void in the material and it will have a higher radiographic density than the surrounding area. 30

31 Welding Errors and Failures Inclusion Solid inclusions are normally expected to be a subsurface type of defect and would include any foreign material entrapped in the deposited weld metal. The most common type of solid inclusion is a slag inclusion defined as nonmetallic sloid material entrapped in weld metal or between weld metal and base metal Porosity is caused by the absorption of nitrogen, oxygen and hydrogen in the molten weld pool which is then released on solidification to become trapped in the weld metal. Prevention The gas source should be identified and removed as follows: i. Air entrainment - seal any air leak - avoid weld pool turbulence - use filler with adequate level of deoxidants - reduce excessively high gas flow - avoid draughts ii. Hydrogen - dry the electrode and flux - clean and degrease the workpiece surface iii. Surface coatings - clean the joint edges immediately before welding - check that the weldable primer is below the recommended maximum thickness 31

32 Welding Errors and Failures Weld Cracks A crack is a fracture-type discontinuity characterized by a sharp tip and high ratio of length and width to opening displacement. Cracks are perhaps the most serious of the defects that occur in the welds or weld joints in weldments. Cracks are considered dangerous because they create a serious reduction in strength. The can propagate and cause sudden failure. They are the most serious when impact loading and coldtemperature service are involved. There are many types of cracks: i. Surface cracks; ii. Transverse cracks; iii. Longitudinal cracks; iv. Creater cracks; v. Toe cracks; vi. Underbead cracks 32

33 Welding Errors and Failures Distortion Most of the welding process involve heat. High temperature heat is largely responsible for welding distortion, warpage, or stresses. When metal is heated it expands, and it expands in all directions. Welding-distortion or deformation or warping of weldments during welding is a natural outcome of intrinsic non-uniform heating and cooling of the joint. Distortion is the result of the action of internal stresses which are produced while welding, and remain in the part after heating is removed, as briefly explained in the following. 33

34 Welding Errors and Failures Other Welding Errors 34