Design Part 5
Job Knowledge 94
The previous Job Knowledge articles looked at fillet and partial/full penetration butt welds. The final three weld types to be dealt with in this series on weld design are the edge weld, the spot weld and the plug weld.
The edge weld is a specialised weld that has limited fields of application and is mostly used for the joining of sheet metal components although it may be used for the fabrication of tube to tubesheet welds. The edge weld is frequently used as an alternative to a corner weld where achieving an accurate fit may be difficult, particularly on thin section components. Instead, by raising a flange on one of the components and clamping the two components together a weld can be made along the edge. Sealing the lid on a can is one ideal application as the lid can be pushed in to the can, resulting in a minimal gap and a self jigged joint (Fig.1). The weld size and penetration is limited so this weld type is generally only possible on thin components using methods such as TIG, plasma TIG or the power beam welding processes.
Fig.1. Edge weld used to seal container lid
This type of edge weld may also be used for tube to tubesheet welding where, by machining a pintle onto the tubesheet, the tube can be inserted through the tube hole and an edge weld made, (Fig.2) This has the advantage that the heat sink is more evenly balanced when attempting to weld a thin tube to a thick tubesheet. In tubesheets of limited weldability or where postweld heat treatment is essential it is possible to deposit a ring of weld metal round the tube hole. This ring may then be machined to provide the pintle so that the residual stresses are reduced and the tube/tubesheet weld is made in good weldability weld metal. This results in a reduction in residual stress in the tubesheet and a reduction in the risk of cracking.
Fig.2. Edge weld used to weld tube to tubesheet joints
Alternatively, if PWHT is required the tubesheet and its weld rings can be PWHT'd, the pintles machined on and non-destructively examined (NDE) and the tube/tubesheet welds made in the thin section, removing the need for a second PWHT cycle. Because of the accuracy of these machined joints the welding process, generally TIG, is frequently mechanised or fully automated.
The spot weld, Fig.3, is normally associated with resistance welding where two thin sheets are overlapped and held in close contact by pressure from the welding electrodes during the welding cycle. The resistance spot weld could therefore be regarded as self jigging. Spot welding with the arc welding processes also uses a lap type joint but presents a more difficult problem in that the joint must be firmly clamped together such that there is no gap between the two surfaces. Failure to do this means that the weld metal may spill into the gap and full fusion to the underlying plate may not be achieved. Good jigging and fixturing is therefore essential.
Applications of this joining method include sheet metal work and the lining ('wallpapering') of ducts, tanks etc with thin, corrosion-resistant sheets. The greatest strength of the welds is developed when the welds are in shear parallel to the plate surfaces.
As mentioned earlier, penetration into the parent metal from the various arc welding processes is limited, around 4mm with TIG (perhaps as much as 10mm with activated flux TIG), 10mm with plasma-TIG and 6mm with MAG welding. The thickness of the upper plate that must be fully penetrated to provide a sound weld is therefore similarly limited. An additional problem with MAG welding is that the filler wire is fed continuously into the weld pool so that a large lump of excess weld metal may be deposited on the plate surface. Autogenous TIG or plasma-TIG will give a weld flush with or slightly below the plate surface. The process can be partially mechanized. Special torches are available that, when held against the plate surface, give the correct electrode/work piece distance and timers on the welding power source that may be set to give the desired arc time.
To enable thicker plate to be joined by 'spot welding' a circular or elongated hole may be machined through the top plate, enabling either a plug or a slot weld to be made by filling the hole with weld metal. Whilst this may seem tobe a simple and easy process the strength of this type of joint depends upon full fusion of the weld metal with the vertical wall of the hole cut into the upper plate, see Fig.4. As with a fillet weld, lack of fusion in this area will result in a reduction in the throat thickness of the joint. It is therefore essential that the welder directs the welding arc into the bottom corner of the joint and does not simply puddle the weld metal into the hole. With small diameter plug welds this can be a difficult and skilled operation and welders need to be adequately trained to ensure that they can achieve full fusion.
Fig.4. Plug and slot welds
Since the strength of the plug or slot weld is determined by the throat it may not be necessary to fill the hole completely unless the weld must be flush with the surface of the plate for cosmetic reasons. Besides being unnecessary from the point of view of joint strength, a completely filled hole will have high residual stresses. These may cause unacceptable distortion and will increase the risk of cold cracking in carbon and low alloy steels.
This brief series of Job Knowledge articles has concentrated on the design of joints for welding. The designer also needs to remember that, not only must the joints be suitable for welding, they must in addition enable any non-destructive testing required by the contract or specification to be carried out. Provision therefore needs to be made to allow adequate access for the positioning of radiographic film and the radiation source, or to enable the correct scanning patterns to be used if the joint is to be ultrasonically tested.
Whilst NDE of butt welds is reasonably straightforward, radiography or ultrasonic examination of fillet welds is not generally regarded as being possible. The designer must therefore take into account the possibility of undetected defects in this type of joint.
This article was written by Gene Mathers.