Welding and Fillet Welds Essay Example For Students

Welding and Fillet Welds Essay Design of Steel Structures Prof. S. R. Satish Kumar and Prof. A. R. Santha Kumar 3. 3 Welding and welded connections Welding is the process of joining two pieces of metal by creating a strong metallurgical bond between them by heating or pressure or both. It is distinguished from other forms of mechanical connections, such as riveting or bolting, which are formed by friction or mechanical interlocking. It is one of the oldest and reliable methods of joining. Welding offers many advantages over bolting and riveting. Welding enables direct transfer of stress between members eliminating gusset and splice plates necessary for bolted structures. Hence, the weight of the joint is minimum. In the case of tension members, the absence of holes improves the efficiency of the section. It involves less fabrication cost compared to other methods due to handling of fewer parts and elimination of operations like drilling, punching etc. and consequently less labour leading to economy. Welding offers air tight and water tight joining and hence is ideal for oil storage tanks, ships etc. Welded structures also have a neat appearance and enable the connection of complicated shapes. Welded structures are more rigid compared to structures with riveted and bolted connections. A truly continuous structure is formed by the process of fusing the members together. Generally welded joints are as strong or stronger than the base metal, thereby placing no restriction on the joints. Stress concentration effect is also considerably less in a welded connection. Some of the disadvantages of welding are that it requires skilled manpower for welding as well as inspection. Also, non-destructive evaluation may have to be carried out to detect defects in welds. Welding in the field may be difficult due to the location or environment. Welded joints are highly prone to cracking under fatigue loading. Large residual stresses and distortion are developed in welded connections. 3. 3. 1 Fundamentals of welding A welded joint is obtained when two clean surfaces are brought into contact with each other and either pressure or heat, or both are applied to obtain a bond. The tendency of atoms to bond is the fundamental basis of welding. The inter-diffusion Indian Institute of Technology Madras Design of Steel Structures Prof. S. R. Satish Kumar and Prof. A. R. Santha Kumar between the materials that are joined is the underlying principle in all welding processes. The diffusion may take place in the liquid, solid or mixed state. In welding the metallic materials are joined by the formation of metallic bonds and a perfect connection is formed. In practice however, it is very difficult to achieve a perfect joint; for, real surfaces are never smooth. When welding, contact is established only at a few points in the surface, joins irregular surfaces where atomic bonding occurs. Therefore the strength attained will be only a fraction of the full strength. Also, the irregular surface may not be very clean, being contaminated with adsorbed moisture, oxide film, grease layer etc. In the welding of such surfaces, the contaminants have to be removed for the bonding of the surface atoms to take place. This can be accomplished by applying either heat or pressure. In practical welding, both heat and pressure are applied to get a good joint. As pointed out earlier, any welding process needs some form of energy, often heat, to connect the two materials. The relative amount of heat and pressure required to join two materials may vary considerably between two extreme cases in which either heat or pressure alone is applied. When heat alone is applied to make the joint, pressure is used merely to keep the joining members together. Examples of such a process are Gas Tungsten Arc Welding (GTAW), Shielded Metal Arc Welding (SMAW), Submerged Arc Welding (SAW) etc. On the other hand pressure alone is used to make the bonding by plastic deformation, examples being cold welding, roll welding, ultrasonic welding etc. There are other welding methods where both pressure and heat are employed, such as resistance welding, friction welding etc. A flame, an arc or resistance to an electric current, produces the required heat. Electric arc is by far the most popular source of heat used in commercial welding practice. 3. 3. 2 Welding process In general, gas and arc welding are employed; but, almost all structural welding is arc welding. Indian Institute of Technology Madras Design of Steel Structures Prof. S. R. Satish Kumar and Prof. A. R. Santha Kumar In gas welding a mixture of oxygen and some suitable gas is burned at the tip of a torch held in the welder’s hand or by an automatic machine. Acetylene is the gas used in structural welding and the process is called oxyacetylene welding. The flame produced can be used both for cutting and welding of metals. Gas welding is a simple and inexpensive process. But, the process is slow compared to other means of welding. It is generally used for repair and maintenance work. The most common welding processes, especially for structural steel, use electric energy as the heat source produced by the electric arc. IS:816 in this process, the base metal and the welding rod are heated to the fusion temperature by an electric arc. The arc is a continuous spark formed when a large current at a low voltage is discharged between the electrode and the base metal through a thermally ionised gaseous column, called plasma. The resistance of the air or gas between the electrode and the objects being welded changes the electric energy into heat. A temperature of 33000 C to 55000 C is produced in the arc. The welding rod is connected to one terminal of the current source and the object to be welded to the other. In arc welding, fusion takes place by the flow of material from the welding rod across the arc without pressure being applied. The Shielded Metal Arc Welding process is explained in the following paragraph. In Shielded Metal Arc Welding or SMAW (Fig. 3. 12), heating is done by means of electric arc between a coated electrode and the material being joined. In case bare wire electrode (without coating) is employed, the molten metal gets exposed to atmosphere and combines chemically with oxygen and nitrogen forming defective welds. The electrode coating on the welding rod forms a gaseous shield that helps to exclude oxygen and stabilise the arc. The coated electrode also deposits a slag in the molten metal, which because of its lesser density compared to the base metal, floats on the surface of the molten metal pool, shields it from atmosphere, and slows cooling. After cooling, the slag can be easily removed by hammering and wire brushing. Indian Institute of Technology Madras Design of Steel Structures Prof. S. R. Satish Kumar and Prof. A. R. Santha Kumar The coating on the electrode thus: shields the arc from atmosphere; coats the molten metal pool against oxidation; stabilises the arc; shapes the molten metal by surface tension and provides alloying element to weld metal. Fig. 3. 12 Shielded metal arc welding (SMAW) process Fig. 3. 12 Shielded metal arc welding (SMAW) process The type of welding electrode used would decide the weld properties such as strength, ductility and corrosion resistance. The type to be used for a particular job depends upon the type of metal being welded, the amount of material to be added and the position of the work. The two general classes of electrodes are lightly coated and heavily coated. The heavily coated electrodes are normally used in structural welding. The resulting welds are stronger, more corrosion resistant and more ductile compared to welds produced by lightly coated electrodes. Usually the SMAW process is either automatic or semi-automatic. The term weldability is defined as the ability to obtain economic welds, which are good, crack-free and would meet all the requirements. Of great importance are the chemistry and the structure of the base metal and the weld metal. The effects of heating and cooling associated with fusion welding are experienced by the weld metal and the Heat Affected Zone (HAZ) of the base metal. The cracks in HAZ are mainly caused by high carbon content, hydrogen enbrittlement and rate of cooling. For most steels, weld cracks become a problem as the thickness of the plates increases. Indian Institute of Technology Madras Design of Steel Structures Prof. S. R. Satish Kumar and Prof. A. R. Santha Kumar 3. 3. 3 Types of joints and welds By means of welding, it is possible to make continuous, load bearing joints between the members of a structure. A variety of joints is used in structural steel work and they can be classified into four basic configurations namely, Lap joint, Tee joint, Butt joint and Corner joint. For lap joints, the ends of two members are overlapped and for butt joints, the two members are placed end to end. The T- joints form a Tee and in Corner joints, the ends are joined like the letter L. Most common joints are made up of fillet weld or the butt (also calling groove) weld. Plug and slot welds are not generally used in structural steel work. Fig. . 14 Fillet welds are suitable for lap joints and Tee joints and groove welds for butt and corner joints. Butt welds can be of complete penetration or incomplete penetration depending upon whether the penetration is complete through the thickness or partial. Generally a description of welded joints requires an indication of the type of both the joint an d the weld. Though fillet welds are weaker than butt welds, about 80% of the connections are made with fillet welds. The reason for the wider use of fillet welds is that in the case of fillet welds, when members are lapped over each other, large tolerances are allowed in erection. For butt welds, the members to be connected have to fit perfectly when they are lined up for welding. Further butt welding requires the shaping of the surfaces to be joined as shown in Fig. 3. 15. To ensure full penetration and a sound weld, a backup plate is temporarily provided as shown in Fig. 3. 15 Butt welds: Full penetration butt welds are formed when the parts are connected together within the thickness of the parent metal. For thin parts, it is possible to achieve full penetration of the weld. For thicker parts, edge preparation may have to be done to achieve the welding. There are nine different types of butt joints: square, single V, Indian Institute of Technology Madras Design of Steel Structures Prof. S. R. Satish Kumar and Prof. A. R. Santha Kumar double V, single U, double U, single J, double J, single bevel and double bevel. They are shown in Fig. 3. 13 In order to qualify for a full penetration weld; there are certain conditions to be satisfied while making the welds. Welds are also classified according to their position into flat, horizontal, vertical and overhead. Flat welds are the most economical to make while overhead welds are the most difficult and expensive. True Romance EssayDesign of Steel Structures Prof. S. R. Satish Kumar and Prof. A. R. Santha Kumar improved. For intermediate weld positions, the value of strength and ductility show intermediate values. Fig. 3. 20 Butt welding of members with (a) (b) unequal thickness (c) unequal width In many cases, it is possible to use the simplified approach of average stresses in the weld throat (Fig. 3. 22). In order to apply this method, it is important to establish equilibrium with the applied load. Studies conducted on fillet welds have shown that the fillet weld shape is very important for end fillet welds. For equal leg lengths, making the direction of applied tension nearly parallel to the throat leads to a large reduction in strength. The optimum weld shape recommended is to provide shear leg ? 3 times the tension leg. A small variation in the side fillet connections has negligible effect on strength. In general, fillet welds are stronger in compression than in tension. Fig. 3. 21 Fillet (a) side welds and (b) end welds Indian Institute of Technology Madras Design of Steel Structures Prof. S. R. Satish Kumar and Prof. A. R. Santha Kumar Fig. 3. 2 Average stress in the weld throat A simple approach to design is to assume uniform fillet weld strength in all directions and to specify a certain throat stress value. The average throat thickness is obtained by dividing the applied loads summed up in vectorial form per unit length by the throat size. This method is limited in usage to cases of pure shear, tension or compression (Fig. 3. 23). It cannot be used in cases where the load vector direction varies around weld group. For the simple method, the stress is taken as the vector sum of the force components acting in the weld divided by the throat area. Fig. . 23 (a) connections with simple weld design, (b) connections with Direction- dependent weld design Stresses Due to Individual forces When subjected to either compressive or tensile or shear force alone, the stress in the weld is given by: Indian Institute of Technology Madras Design of Steel Structures Prof. S. R. Satish Kumar and Prof. A. R. Santha Kumar fa or q = Where P t t lw fa = calculated normal stress due to axial force in N/mm2 q = shear stress in N/mm2 P = force transmitted (axial force N or the shear force Q) tt = effective throat thickness of weld in mm lw= effective length of weld in mm Fig. 3. 4 End fillet weld normal to direction of force The design strength of a fillet weld, fwd, shall be based on its throat area (Cl. 10. 5. 7). fwd = fwn / ? mw in which fwn = f u / 3 Where fu = smaller of the u ltimate stress of the weld and the parent metal and ?mw = partial safety factor (=1. 25 for shop welds and = 1. 5 for field welds) The design strength shall be reduced appropriately for long joints as prescribed in the code. The size of a normal fillet should be taken as the minimum leg size (Fig. 3. 25) Indian Institute of Technology Madras Design of Steel Structures Prof. S. R. Satish Kumar and Prof. A. R. Santha Kumar Fig. 3. 5 Sizes of fillet welds For a deep penetration weld, the depth of penetration should be a minimum of 2. 4 mm. Then the size of the weld is minimum leg length plus 2. 4 mm. The size of a fillet weld should not be less than 3 mm or more than the thickness of the thinner part joined. Minimum size requirement of fillet welds is given below in Table 3. 4 (Cl. 10. 5. 2. 3). Effective throat thickness should not be less than 3 mm and should not exceed 0. 7 t and 1. 0 t under special circumstances, where’t’ is the thickness of thinner part. Table 3. 4 Minimum size of first run or of a single run fillet weld Thickness of thicker part (mm) t ? 0 10 t ? 20 20 t ? 32 32 t ? 50 Minimum size (mm) 3 5 6 8 (First run)10 (Minimum size of fillet) For stress calculations, the effective throat thickness should be taken as K times fillet size, where K is a constant. Values of K for different angles between tension fusion faces are given in Table 3. 5 (Cl. 10. 5. 3. 2). Fillet welds are normally used for connecting parts whose fusion faces form angles between 60Â ° and 120Â °. The actual length is taken as the length having the effective length plus twice the weld size. Minimum effective length should not be less than four times the weld size. When a fillet weld is provided to square edge of a part, the weld size should be at least 1. 5 mm less than the edge Indian Institute of Technology Madras Design of Steel Structures Prof. S. R. Satish Kumar and Prof. A. R. Santha Kumar thickness . For the rounded toe of a rolled section, the weld size should not exceed 3/4 thickness of the section at the toe (Cl. 10. 5. 8. 1). Fig. 3. 26 (a) Fillet welds on square edge of plate, (b) Fillet Welds on round toe of rolled section Table 3. 5. Value of K for different angles between fusion faces Angle between fusion faces Constant K 60Â ° 90Â ° 0. 0 91Â °-100Â ° 0. 65 101Â °-106Â ° 0. 60 107Â °-113Â ° 0. 55 114Â °-120Â ° 0. 50 Intermittent fillet welds may be provided where the strength required is less than that can be developed by a continuous fillet weld of the smallest allowable size for the parts joined. The length of intermediate welds should not be less than 4 times the weld size with a minimum of 40 mm. The clear spacin g between the effective lengths of the intermittent welds should be less than or equal to 12 times the thickness of the thinner member in compression and 16 times in tension; in no case the length should exceed 20 cm. Chain intermittent welding is better than staggered intermittent welding. Intermittent fillet welds are not used in main members exposed to weather. For lap joints, the overlap should not be less than five times the thickness of the thinner part. For fillet welds to be used in slots and holes, the dimension of the slot or hole should comply with the following limits: a) The width or diameter should not be less than three times the thickness or 25 mm whichever is greater b) Corners at the enclosed ends or slots should be rounded with a radius not less than 1. times the thickness or 12 mm whichever is greater, and Indian Institute of Technology Madras Design of Steel Structures Prof. S. R. Satish Kumar and Prof. A. R. Santha Kumar c) The distance between the edge of the part and the edge of the slot or hole, or between adjacent slots or holes, should be not less than twice the thickness and not less than 25 mm for the holes. Fig. 3. 27 End returns for side welds The effective area of a plug weld is assumed as the nominal area of the whole in the plane of the faying surface. Plug welds are not designed to carry stresses. If two or more of the general types of weld (butt, fillet, plug or slots) are combined in a single joint, the effective capacity of each has to be calculated separately with reference to the axis of the group to determine the capacity of the welds. The high stress concentration at ends of welds is minimised by providing welds around the ends as shown in Fig. 3. 27. These are called end returns. Most designers neglect end returns in the effective length calculation of the weld. End returns are invariably provided for welded joints that are subject to eccentricity, impact or stress reversals. The end returns are provided for a distance not less than twice the size of the weld. Design of plug and slot welds: In certain instances, the lengths available for the normal longitudinal fillet welds may not be sufficient to resist the loads. In such a situation, the required strength may be built up by welding along the back of the channel at the edge of the plate if sufficient space is available. This is shown in Fig. . 28 (a). Another way of developing the required strength is by providing slot or plug welds. Slot and plug welds are generally used along with fillet welds in lap joints. On certain occasions, plug welds are used to fill the holes that are temporarily made for erection bolts for beam and Indian Institute of Technology Madras Design of Steel Structures Prof. S. R. Satish Kumar and Prof. A. R. Santha Kumar column connections. However, their strength ma y not be considered in the overall strength of the joint. The limitations given in specifications for the maximum sizes of plug and slot welds are necessary to avoid large shrinkage, which might be caused around these welds when they exceed the specified sizes. The strength of a plug or slot weld is calculated by considering the allowable stress and its nominal area in the shearing plane. This area is usually referred to as the faying surface and is equal to the area of contact at the base of the slot or plug. The length of the slot weld can be obtained from the following relationship: Load ( Width ) allowable stress L= (3. 15) Fig. 3. 28 Slot and plug welds Indian Institute of Technology Madras