Basic requirements All the standards have the same requirements in relation to the following items: Arrow line and arrow head Reference line The arrow line can be at any angle (except 180 degrees) and can point up or down. The arrow head must touch the surfaces of the components to be joined and the location of the weld. Any intended edge preparation or weldment is not shown as an actual cross sectional representation, but is replaced by a line. The arrow also points to the component to be prepared with single prepared components. See Figs. 1-4. Fig. 1. Fig. 2. Fig. 3. Fig. 4. Symbol types To the basic set-up of the arrow and reference line, the design draughtsperson can apply the appropriate symbol, or symbols for more complex situations. The symbols, in particular for arc and gas welding, are often shown as cross sectional representations of either a joint design or a completed weld. Simple, single edge preparations are shown in Fig. 5. For resistance welding, a spot weld and seam weld are shown in Fig. 6: Fig. 5. Fig. 6. Joint and/or weld shape The above examples can be interpreted as either the joint details alone or the completed weld, however, for a finished weld it is normal to find that an appropriate weld shape is specified. Using the examples above, there are a number of options and methods to specify an appropriate weld shape or finish. Butt welded configurations would normally be shown as a convex profile (Fig.7 'a', 'd' and 'f') or as a dressed-off weld as shown in 'b' and 'c'. Fillet weld symbols are always shown as a 'mitre' fillet weld (a right angled triangle) and a convex or concave profile can be superimposed over the original symbol's mitre shape. See Fig. 7. Fig. 7. Weld sizing In order that the correct size of weld can be applied, it is common to find numbers to either the left or to the right of the symbol. For fillet welds, numbers to the left of the symbol indicate the design throat thickness, leg length, or both design throat thickness and leg length requirements. Figure 1 gives examples of symbols used in different Standards. Fig.1 For fillet welds: Superseded BS499 Pt 2 gives a = design throat thickness b = leg length ISO 2553/EN 22553 requirements a = design throat thickness z = leg length s = penetration throat thickness For butt joints and welds, an S with a number to the left of a symbol refers to the depth of penetration as shown in Fig.2. Fig.2 When there are no specific dimensional requirements specified for butt welds on a drawing using weld symbols, it would normally be assumed that the requirement is for a full penetration butt weld (Fig.3). Fig.3 Numbers to the right of a symbol or symbols relate to the longitudinal dimension of welds, eg for fillets, the number of welds, weld length and weld spacing for non-continuous welds, as Fig.4. Fig.4 On fillet welded joints made from both sides, a staggered weld can be shown by placing a 'Z' through the reference line (Fig.5). Fig.5 Supplemetary symbols Weld symbols indicate the type of preparation to use or the weld type. However, there may still be occasions where other information is required. The basic information can therefore be added to in order to provide further details as shown in Figs.6, 7 and 8. Fig.6 Fig.7 Fig.8 Weld all round For a Rectangular Hollow Section (RHS) welded to a plate, for example: Weld in the field or on site The box attached to the arrow can be used to contain, or point to, other information. Welding process type ISO 4063 gives welding processes specific reference numbers. As shown in Fig.9 the appropriate process number is placed in the tail of the arrow. Other processes are given a unique number. In this example, 135 refers to MAG welding. Fig.9 There are a number of additional symbols given in the Standards (eg ISO 22553) which refer to additional welding or joint requirements. Figure 10 shows the requirement for a sealing run. Fig.10 Compound joints/welds A compound weld could be a 'T' butt weld which requires fillet welds to be added to increase the throat thickness as shown in Fig.11. Fig.11 The broken reference line The main feature that distinguishes weld symbol Fig.12 standards is that for ISO 2553 and BS EN 22553, there is an additional feature of a broken reference line. This method is used when a weldment or weld preparation needs to be specified on the 'other side' of the arrow as shown in Fig.12. Any symbol that is used to show a joint or weld type feature on the other side of the arrow line is always placed on a dotted line. BS 499 and AWS require symbols to be placed above the reference line (which indicate the other side) or below the reference line (indicating the arrow side). Summary Weld symbols are a very useful way of communicating welding requirements from the design office to the shop floor. It is essential that the 'rules' of the standard used are correctly applied by drawing office personnel. However, it is also important that shop floor personnel are able to read and understand the details of weld symbols. Much of this requirement can be met by reference to the standard being used within the organisation and by the drawing office personnel considering the needs of the end user such as the welders, welding supervisors, welding inspection personnel and welding engineers in order to minimise costly mistakes due to misinterpretation. Training of all personnel in the correct use of weld symbol specifications also plays an important role in ensuring that weld symbols are both correctly applied and correctly read. Fillet welded joints - a review of the practicalities Fillet welded joints such as tee, lap and corner joints are the most common connection in welded fabrication. In total they probably account for around 80% of all joints made by arc welding. It is likely that a high percentage of other joining techniques also use some form of a fillet welded joint including non-fusion processes such as brazing, braze welding and soldering. The latter techniques are outside the scope of this article. Although the fillet weld is so common, there are a number of aspects to be considered before producing such a weld. This article will review a number of topics that relate to fillet welded joints and it is hoped that even the most seasoned fabricator or welding person will gain from this article in some way. Common joint designs for fillet welds are shown below in Fig.1. Fig.1 Fillet weld features ISO 2553 (EN 22553) uses the following notation as Figs.2 and 3 show. a = throat thickness z = leg length s = deep penetration throat thickness l = length of intermittent fillet Fig.2 Fig.3 Fillet weld shapes Over specified fillet welds or oversized fillet welds Fig.4 One of the greatest problems associated with fillet welded joints is achieving the correct weld size in relation to the required leg lengths or throat thickness (Fig.4). The designer may calculate the size and allow a 'safety factor' so that the weld specified on the fabrication drawing is larger than is required by design considerations. The weld size is communicated by using an appropriate weld symbol. In the UK the weld size is frequently specified by referring to the leg length 'z' in ISO 2553 where the number gives the weld size in millimetres as shown in Fig.5. Fig.5 In Europe, it is more common to find the design throat thickness, 'a' specified (Fig.6). Fig.6 Once the drawing has been issued to the shop floor, it is usual to find an additional safety factor also being applied on by the welder or inspector. It is also common to hear 'add a bit more it will make it stronger'. The outcome is an oversized weld with perhaps an 8mm leg length rather than the 6mm specified by the designer. This extra 2mm constitutes an increase in weld volume of over 80%. This coupled with the already over specified weld size from the designer's 'safety factor' may lead to a weld that is twice the volume of a correctly sized fillet weld. By keeping the weld to the size specified by the drawing office, faster welding speeds can be achieved, therefore increasing productivity, reducing overall product weight, consumable consumption and consumable cost. The other benefit is that, in the case of most arc welding processes, a slight increase in travel speed would in most cases see an increase in root penetration so that the actual throat thickness is increased: An oversized weld is therefore very costly to produce, may not have 'better strength' and is wasteful of welding consumables and may see other fabrication problems including excessive distortion. Lap joints welded with fillet welds. As discussed earlier, oversized welds are commonplace and the lap joint is no exception. The designer may specify a leg length that is equal to the material thickness as in Fig.7. Fig.7 Strength considerations may mean that the fillet weld size need not be anywhere near the plate thickness. In practice the weld may also be deficient in other ways for example: Fig.8 Due to melting away of the corner of the upper plate (Fig.8), the vertical leg length is reduced meaning that the design throat has also been reduced; therefore an undersized weld has been created. Care is therefore needed to ensure that the corner of the upper plate is not melted away. Ideally the weld should be some 0.5-1mm clear of the top corner (Fig.9). Fig.9 It may be the designer may therefore specify a slightly smaller leg length compared to the thickness of the component. To compensate for this reduction in throat thickness it may be necessary to specify a deep penetration fillet weld. This amount of additional penetration would need to be confirmed by suitable weld tests. Additional controls may also be needed during production welding to ensure that this additional penetration is being achieved consistently. In addition to the reduction in throat thickness there is the potential for additional problems such as overlap at the weld toe due to the larger weld pool size (Fig.10) or an excessively convex weldface and consequential sharp notches at the weld toe (Fig.11). Fig.10 Fig.11 Both the potential problems shown in Figs.10 and 11 could adversely influence the fatigue life of the welded joint due to the increased toe angle, which acts as a greater stress concentration. Poor fit-up can also reduce the throat thickness as in Fig.12. The corner of the vertical component has been bevelled in the sketch in an exaggerated manner to illustrate the point. Fig.12 Summary Fillet welded joints are not only the most frequently used weld joints but are also one of the most difficult to weld with any real degree of consistency. Fillet welds require a higher heat input than a butt joint of the same thickness and, with less skilled welders this can lead to lack of penetration and/or fusion defects that cannot be detected by visual examination and other NDT techniques. Fillet welded joints are not always open to NDT or are indeed time consuming to many nondestructively testing techniques such as radiography or ultrasonic testing and the results are often difficult to interpret. Inspection methods such as visual inspection, magnetic particle inspection and penetrant inspection are surface examination techniques only and with visual inspection, much of the effort is expended in measuring the size of the weld rather than identifying other quality aspects. Fillet welded joints are therefore much more difficult to weld and inspect. Often the welds that are produced are larger than they need to be or they may be of a poor shape which can adversely influence their service performance. To overcome these difficulties, designers need to specify accurately the most appropriate throat size and welding personnel should strive to achieve the specified design size. Welders also need to be adequately trained and sufficiently skilled to be capable of maintaining an acceptable weld quality.
