Welding Technology

• butt joints.
• tee joints.
• lap joints.
• corner joints.

As illustrated in Figure 2, a welded joint is made by fusing (melting) the steel plates or sections (the parent metal) along the line of the joint. The metal melted from each member at the joint unites in a pool of molten metal which bridges the interface. As the pool cools, molten metal at the fusion boundary solidifies, forming a solid bond with the parent metal, see Figure 3. When the solidification is complete, there is continuity of metal through the joint.


2. METHODS OF MAKING A WELDED JOINT

Two types of weld are in common use: butt welds and fillet welds. In the former the weld metal is generally contained within the profiles of the welded elements; in the latter, deposited weld metal is external to the profile of the welded elements.
Obviously the complete length of joint cannot be melted simultaneously. In practice a heat source is used to melt a small area and is then moved along the joint line, progressively fusing the parent metal at the leading edge of the weld pool, as shown in Figure 4. At the same time, the metal at the trailing edge of the pool solidifies. The most commonly used heat source, in structural work, is a low voltage (15 to 35 volt), high current (50 to 1000 amp) arc. As shown diagrammatically in Figure 5, the arc operates between the end of a steel electrode (rod) and the work piece. It melts both the parent metal and the electrode; molten metal from the electrode is thereby added to the weld pool.


The molten steel in the pool will readily absorb oxygen and nitrogen from the air, which could lead to porosity in the solidified weld and possibly to metallurgical problems. Figure 6 shows how this is avoided by covering the pool with a molten flux, as in Manual Metal Arc (MMA) and Submerged Arc Welding (SAW), or by replacing the air around the arc by a non-reactive gas, as in Metal Active Gas (MAG) Welding or cored wire welding.

3. STRUCTURE AND PROPERTIES OF WELDS

The solidified weld metal has a cast structure and has properties characteristic of cast steel, i.e. higher ratio of yield to ultimate strength than structural steel. The weld metal is a mixture of parent metal and steel melted from the electrode. In structural work the composition of the electrode is usually chosen so that the resultant weld metal is stronger than the connected elements. Occasionally, specific conditions may override this chocie. For example, when joining stainless steel to carbon-manganese steel, a highly alloyed electrode must be used to avoid cracking in the weld metal.
When the weld pool is cooling and solidifying, the majority of the heat flows through the parent metal alongside the joint. The steel is thus subjected to heating and cooling cycles similar to those experienced in heat treatment practice. As shown in Figure 7, the structure of the steel will be changed in this region (called the heat affected zone, HAZ). This must be taken into account in the design in terms of notch toughness (Charpy value), etc.

The structure of the HAZ will be controlled by:

• the composition of the steel (carbon equivalent).
• the cooling rate in the HAZ.

• arc energy, i.e. heat input to the joint.
• type of joint.
• thickness of steel.
• temperature of steel plate or section prior to welding, e.g. preheat.

A method of determining the interaction of these factors in relation to the avoidance of cracks in the HAZ is given in the sample chart shown in Figure 8.

In addition to its effect on the cooling rate, preheat is used to:

• Disperse hyrodgen from the weld pool and HAZ. Hydrogen in the HAZ increases the risk of cracking if hardening has occurred. The hydrogen comes principally from the flux. An appropriate electrode, correctly stored, will reduce the risk of hydrogen pick-up.
• Remove surface moisture in high humidity conditions or on site.
• Bring the steel up to 'normal' ambient conditions (20°C).

4. EDGE PREPARATION FOR BUTT WELDS

For square edge preparations the depth of melting into the plate is called the Depth of Penetration, see Figure 9a. As a very rough guide, the penetration is about 1mm per 100 amp. In manual welding the current is usually not more than 350 amp; more commonly 150-200 amp. This means that the edges of the plate must be cut back along the joint line for continuity through the thickness to be achieved (Figure 9b). The groove so formed is then filled with metal melted from the electrode (Figure 9c). Various edge profiles are used and are illustrated in Figure 10; the edges may be planed, sawn, guillotined or flame cut.


The first run to be deposited in the bottom of the groove is called the root run. The root faces must be melted to ensure good penetration, but at the same time the weld pool must be controlled to avoid collapse, as seen in Figure 11. This task requires considerable skill. The difficulties can be eased by using a backing strip.

The choice of edge preparation depends on:

• type of process.
• position of welding (Figure 12).
• access for arc and electrode.
• volume of deposited weld metal which should be kept to a minimum.
• cost of preparing edges.
• shrinkage and distortion (Figure 13).

• in cold weather the steel may need to be heated to bring it up to 20°C.
• overnight condensation and high humidity can lead to porosity.
• care must be taken to ensure the electrodes are kept dry in the stores.
• it is often difficult to achieve accurate fitting of the joint; variable and/or large gaps may result in defective welds, distortion and increased costs.

6. SHRINKAGE

During cooling, the hot metal in the weld zone contracts, causing the joint to shrink. The contraction is restrained by the cold metal surrounding the joint; stresses are set up which, being in excess of the yield stress, produce plastic deformation. This can lead to the distortion or buckling shown in Figure 13. Distortion can be reduced by choice of edge preparation and weld procedure; examples are shown in Figure 14.

When the plastic deformation has ceased, the joint is left with the residual stress pattern of Figure 15 with tension in the weld metal and HAZ, and compression in the surrounding steel. The significance of these residual stresses is discussed in other lectures.

7. CONCLUDING SUMMARY

• A welded joint is made by fusing parent metal from both components being joined, usually with added weld metal.
• The properties of both the weld metal, which has melted and solidified, and the surrounding heat affected zone may differ from those of the parent metal.
• Welding procedures should be properly specified to give a satisfactory welded joint. The major parameters are: welding position, electrode type, edge preparation, preheat, voltage, current, travel speed, number of runs and post-weld heat treatments.
• Hot metal in the weld zone contracts during cooling causing residual stresses. Distortion will occur if appropriate control is not exercised.

• it will be necessary to provide for temporary alignment of the adjacent components which are to be welded together, and to hold them in position until they are welded. The methods adopted for alignment may have to be able to carry the weight of the components and in some cases a substantial load from the structure.
• safe means of access and a secure working platform must be provided for the welder and his equipment. The working platform may also have to incorporate weather protection, since wind, rain and cold can all adversely effect the quality of the weld.
• the design of the weld and the preparation of the components to be joined must take into account the position of those components in the structure; the method statement for the erection and the welding procedure for each joint must take all these factors into account.
• all the welding must be carried out by qualified welders in accordance with approved procedures.
• a detailed welding plan must be made for the more important structural joints as well as for structures to which special specifications apply.
• the earth return of the electric current must never be made through the steel frame of buildings, cranes, or through metallic parts of installations but must be directly connected to the construction part being welded.
• the surface of the zone to be welded must be clean and dry.

• the Quality Assurance Manual.
• the General Contract for the Project.
• the general standards, applicable to the Project.
• the manufacturer's standards, procedures and specification.

• the site-organisation (in relation to Quality Control).
• the (written) procedures.
• the inspection programme.

• Reference numbers for the work procedures.
• Revision numbers.
• Written procedures for:

welding

heat-treatment

non-destructive testing

alignment and plumbing

tolerances

HSFG bolting.

1. The safety procedure should be communicated to all concerned with its implementation by, for example, issuing abstracts or running courses. In practice, awareness of safety aspects can best be maintained by continually monitoring hazardous areas of the site (restricted areas, scaffolding, plant, etc.) to check that the appropriate restrictions are in force, and by informing the person in authority if potential hazards exist.
2. The necessary equipment should be made available on the site and maintained in good condition. This equipment ranges from safety helmets and belts, to ladders, working platforms and properly selected tools.
3. 
   The work should be organised so that as little as possible is done at height. The danger can be minimised as follows:
   
   

   × by the use of sub-assembly techniques.

   × by fixing ladders and working platforms onto the steelwork before it is lifted into place.

   × by the early provision of horizontal access walkways.

   × by the provision of temporary staircases or hoists, where appropriate.

4. It should be ensured that all portable equipment such as gas bottles and welding plant, is firmly anchored while it is being used. Care should be taken to ensure that there are no flammable materials below on which sparks could fall.
5. Finally, and fundamentally, the design should be done with safety in mind as follows:

× Splices should be positioned to give simple site connections, bearing in mind that these may have to be connected at height.

× Lifting cleats and connections for heavy and complex components should be incorporated, as far as possible, in the fabricated elements to be connected.

× Consideration should be given, at design stage, to incorporating cleats, brackets or holes in the fabrication to facilitate fixing of safety belts, safety nets and working platforms.

• Careful pre-planning is required if site welding is necessary.
• A Quality Control Programme, involving inspections, should always be implemented.
• All site personnel should be made aware of the safety procedures.
• Safety procedures should be strictly enforced.

• oxygen-acetylene flame.
• resistance to the passage of a current.
• electric arc.

Each method produces a pool of molten steel which must be protected against atmospheric contamination. The method used to achieve this, i.e. the shielding technique, has a major influence on the characteristics of the process. For constructional steelwork, the processes used are usually based upon the electric arc.
In arc welding, a flux or a non-reactive (inert) gas can be used to 'blanket' the weld pool and thus exclude air. This lecture is particularly concerned with the four arc welding processes commonly used in structural work.
2. MANUAL METAL ARC WELDING

This manual method is one of the most widely used arc welding processes (see Figure 1). It requires considerable skill to produce good quality welds. The electrode consists of a steel core wire and a covering flux containing alloying elements, e.g. manganese and silicon. The arc melts the parent metal and the electrode. As metal is transferred from the end of the core wire to the weld pool, the welder moves the electrode to keep the arc length constant. This is essential as the width of the weld run is largely governed by the arc length. The flux melts with the core wire and flows over the surface of the pool to form a slag, which must be removed after solidification.

MMA has many advantages as follows:

• Low capital cost.
• Freedom of movement; it can be used up to 20m from the power supply (useful on site).
• It can be used in all positions.
• It is suitable for structural and stainless steels (but not aluminium).

Its main drawback is a low duty cycle, i.e. only a small volume of metal is deposited before the welder has to stop and insert another electrode. This is not a problem on short welds but becomes a consideration on long welds, especially when labour costs are high.
The operating characteristics of the electrode are controlled by the composition of the flux covering. A variety of electrodes are available to suit different applications. The current used is chosen to match the diameter of wire being used. When low hydrogen *******s in the weld pool are necessary to avoid cracks in the heat-affected zone (HAZ) on cooling, MMA electrodes must be baked and stored at temperatures and times recommended by the manufacturer. These procedures ensure that the electrodes deposit weld metal with appropriate low levels of diffusible hydrogen.
3. METAL ACTIVE GAS (MAG) WELDING

This process is sometimes referred to as Metal Inert Gas (MIG) Welding, although strictly speaking the term MIG should be limited to the use of pure argon as a shielding gas, which is not used for carbon steel.
MAG is a semi-automatic process where the welding gun at the end of a flexible conduit can be hand held and manipulated, but all other operations are automatic (see Figure 2).


The arc and weld pool are shielded by a gas which does not react with molten steel; in current practice the shielding gas is carbon dioxide, or a mixture of argon and carbon dioxide. No flux is necessary to shield the pool since the alloying elements are in the electrode wire, but sometimes a flux-cored electrode is used to produce a slag which controls the weld profile and reduces the liability of lack of fusion defects and the incidence of porosity. The arc length is controlled by the power supply unit. Although MAG welding is somewhat easier to use than MMA, skill is required to set up the correct welding conditions.
The way in which metal is transferred from the electrode wire to the molten pool depends upon current, voltage and shielding gas composition. As the current is increased the form of the transfer changes abruptly to a stream of fine drops which are propelled across the arc gap by the electro-magnetic forces in the arc. This is called spray transfer and it enables welding to be carried out against gravity. Changing the shielding gas to carbon dioxide (assuming steel electrodes) causes the transfer to become more globular and less well directed; however, the situation can be reversed by using a mixture of inert gas and carbon dioxide.
When using steel electrodes, decreasing the arc voltage markedly and also reducing current (by reducing the wire feed rate) results in a form of transfer known as dip transfer or short-circuit transfer. In this mode of transfer metal is fused directly into the pool without passing freely across the arc gap. At slightly higher voltages the transfer is across a gap but is in larger globules without the pronounced directionality of the spray transfer. The globular to spray change is less marked with steel than with certain other metals. Welds in steel are sometimes made in which this type of transfer predominates. It is also possible to control the type of metal transfer at low to medium currents by using a special power source which delivers pulsed current to the arc.
For 'positional' welding, i.e. vertical and overhead, the current must be kept below 180 amp (so that welding takes place in the 'Dip Transfer' mode) and welding speeds are comparable with MMA. Overall times for a joint, and hence productivity, are better since there is no need to deslag or change electrode. In the flat position, currents up to 400 amp ('Spray Transfer') can be used to give high welding speeds. MAG welding is especially suitable for fillet welded joints, e.g. beam to column and stiffener to panel connections. It is not easy to use on site because of problems of equipment movement and the need to provide screens to avoid loss of the gas shield in draughty conditions.
4. SUBMERGED ARC WELDING (SAW)

This is a fully mechanised process in which the welding head travels along the joint automatically (Figure 3). The electrode is a bare wire which is advanced by a governed motor. The voltage and current are selected at the beginning of the weld and are maintained at the pre-selected values by feed-back systems which, in practice, vary in sophistication. The flux is in the form of particles and is placed on the surface of the joint. The arc operates below the surface of the flux, melting a proportion of it to form a slag. Unfused flux is collected and may be re-used for the next weld.

Submerged arc welding is generally operated at currents of between 400 and 1000 amps. This means that weld pools are large and can only be controlled in the flat position, although fillets can be deposited in the horizontal-vertical position up to 10mm leg length in one run. Where it is difficult to control penetration in a root run a backing strip may be used; alternatively, the root run can be made by MMA or MAG and the groove filled with SAW. SAW offers considerable advantages when welding long joints (i.e. those in excess of one metre in length). The high welding speeds and continuous operation lead to high productivity. An accurate joint fit-up is, however, a prime requirement.
5. STUD WELDING

This is a variation of arc welding in which studs are welded to plane surfaces automatically (Figure 4). The stud, which may be a plain or threaded bar (if plain it will have a head) is the electrode and it is held in the chuck of a welding gun which is connected to the power supply. The stud is first touched onto the surface of the steel plate or section. As soon as the current is switched on, the stud is moved away automatically to establish an arc. When a weld pool has formed and the end of the stud is molten, the latter is automatically forced into the steel plate and the current is switched off. The molten metal which is expelled from the interface is formed into a fillet by a ceramic collar which is placed around the stud arc at the beginning of the operation. This ferrule also provides sufficient protection against atmospheric contamination.

Stud welding offers an accurate and fast method of attaching shear connectors, etc., with the minimum of distortion. Whilst it requires some skill to set up the weld parameters (voltage, current, arc time and force), the operation of the equipment is relatively straightforward.
6. CHOICE OF PROCESS

When choosing a welding process a number of factors must be taken into account:

• Thickness of the material to be welded.
• Where the welding is to be carried out. SAW and MAG are best carried out in the protected environment of the fabrication shop. MMA may more readily be used on site.
• Accuracy of fit-up and possibility of misalignment. SAW and Spray Transfer MAG require good fit-up; they are particularly sensitive to variation in root gap and/or root face dimensions.
• Access to joint. It is necessary to ensure that both the welding plant and the welding torch or head can be properly positioned.
• Position of welding. SAW and Spray Transfer MAG are not suitable for vertical or overhead positions. Dip transfer MAG is acceptable for vertical and overhead welding, but MMA is probably best for overhead work, especially on site.
• Steel composition. Steels with lower carbon equivalent values are more readily welded and require lower preheat levels.
• Comparative cost. The cost per unit length of weld can be calculated, but depends upon the burn-off rate of the process and must allow for differences in duty cycle (idle time between electrodes for MMA, etc.), Figure 5.

7. CONCLUDING SUMMARY

• The welding processes commonly used in constructional steelwork are: Manual Metal Arc Welding, Dip and Spray Transfer Metal Active Gas Welding, Submerged Arc Welding and Stud Welding.
• Stud welding is used for attaching shear connectors and other studs to structural steelwork.
• The correct choice of process depends on: situation, fit-up, access, position, steel composition and economic factors.

In considering possible structural options, an overall compromise has to be achieved which recognises the links between related cost areas. Unless this consideration extends from material specification to site erection, cost perceptions may become distorted. Details are largely dictated by the basic design concept which is the key factor in determining how the structure will be made, how it will be transported and ultimately assembled on site.
Whilst it is not possible to lay down hard and fast rules, the following examples are intended to be illustrative.
Column Bases (Figure 4): Detail (a) uses no fewer than eleven separate plate components with extensive welding. Not only has this work to be conducted during primary assembly but considerable manipulation will also be necessary not only for access but to control weld distortion.

By comparison, the base detail (b) using channels would probably be longer with thicker base plates but the number of components is reduced to six and workmanship is drastically cut down. Note also that the inner edge of the two base plates is welded to the column flange eliminating any need for separate stiffeners.
Multi-storey Columns (Figure 5): Based on the philosophy of lowest weight, the columns involve three changes of section profile with two splices. It will be noted that the latter require packing pieces either laminated or solid machined to accommodate the difference in depth.

The saving in material costs by reducing the shaft above the 4th floor will be overtaken by the cost of the splice and, if the total material requirement is less than 20 tonnes, further costs will be incurred by quantity premiums on the basic rate.
The change in section depth also varies the geometry and therefore the lengths of the bracing members will vary with consequent adjustment to the skew of the end connections.
Consider the column shaft from ground to 2nd floor. Clearly the loads will be greatest here. A possibility is to investigate the use of high strength steel to match the upper section in low strength steel. Although high strength steel is more expensive, the result will be consistency of details, beam lengths and connections throughout.
Finally, it should be noted that the bracing connection is attached to the bracing member rather than the column. The benefits are as follows:

1. Primary production is faster because operations on the column are kept to a minimum.
2. The column lengths are less obstructed and therefore easier to nest for transport.
3. Welded projections are vulnerable to transit damage and costly to rectify on site.

Lattice Trusses (Figure 6): Whilst there may be sound reasons for adopting bolted joints, a variety of differently shaped gusset plates are required. They all have to be punched or drilled in addition to the framing members and then individually bolted up.

Because the inside faces of the boom members will become permanently inaccessible, these components and the gussets would need to be painted individually in advance of assembly. This work is expensive and disruptive. Therefore, although the material cost of T-sections is up to 20% higher than angles, the welded truss may still prove to be a cheaper proposition, except for girders with fairly short spans.

• Hoisting
• Temporary Connections
• Plumbing, lining and levelling
• Permanent connections.

Under normal circumstances, single-storey buildings are quickly and easily erected. A high proportion of industrial buildings are rigid jointed. It is common practice to bolt, assemble or weld these joints on the ground and then lift the complete frame upright using a mobile crane.
Lattice girders and trusses are also erected in a similar manner but temporary stiffening may be required to prevent lateral buckling. Care should also be taken, by provision of lifting eyes or similar at specific positions, to ensure that slender members are not subjected to undue compressive stresses.
Ideally, erection should commence at an end which is permanently braced. When this is not possible, temporary bracings should be provided at regular intervals as a safeguard against collapse or deformation (Figure 7).

Space frames are designed to span in two directions. Because of the number of connections required, it is much more economical to assemble the modules at ground level where the joints are readily accessible and then hoist the complete framework. Two or possibly four cranes may be needed depending on the size of the building. Meticulous co-ordination is essential.

In most cases, multi-storey buildings are erected storey by storey enabling the lower floors to be completed earlier, offering access, overhead safety and weather protection. Depending upon the site, a single tower crane may be the sole lifting facility. In this case use of the crane has to be shared between a number of sub-contractors, thereby limiting available "hook" time for any given trade.
Since the position of a tower crane is fixed (Figure , it is completely independent of any obstructions, such as basements or ground slabs, which could deny access to a mobile crane. This independence allows useful freedom in overall planning. However, the fixed location also means a fixed arc of lifting capacity where the load will be minimum at the greatest reach. As a result the steelwork may have to be provided with site splices simply to keep the weight of the components within such limits.

One of the major virtues of a mobile crane (Figure 9) is its flexibility and independence which enables it to keep moving with the flow of the work. These cranes are generally fitted with telescopic jibs which allow then to become operational very quickly. The vehicles are stabilised during lifting by extended outriggers equipped with levelling jacks.

Whilst permanent stability in the completed building may be introduced, in a number of ways, including braced bays, rigid joints and stiff service cores (Figure 10) and via diaphragm action of the floors, stability must also be ensured throughout the entire construction programme. It may therefore be necessary to install temporary bracings solely for this purpose, which must not be removed until the permanent system has been provided and has become effective.

• type of end connections.
• extent/type of bolting or welding.
• number of separate pieces.

The erection of a building framework is potentially hazardous. Many serious and fatal accidents occur each year on construction sites and most of these are caused by falling from, or whilst gaining access to, heights; handling, lifting and moving materials, however, are also hazardous.
Risks can be minimised considerably by measures such as adequate provision for stability throughout construction, accessibility of splices and connections, guard rails and attachments for safety harnesses and so on.
In addition, safety, need not be compromised on grounds of cost. For example, it will prove cheaper to assemble frames at ground level (Figure 11) rather than bolt them together in mid-air. Metal decked floor systems are not only economical but offer rapid access for all trades whilst providing overhead protection. Safer access is also promoted by the immediate provision of steel stair flights at each floor level as steelwork erection proceeds.

Current and future legislation may place greater responsibilities upon the design engineer because of the influence of design and details on the method and sequence of erection.

• Steelwork erection normally occupies a relatively short period in the construction programme, but considerable activity occurs during this time which is vital to the performance of the contract as a whole.
• The steel framework should not be seen in isolation but as a key link in the construction chain where the time saved can have considerable impact in lowering overall costs.
• Early consideration should be given to erection during design and detailing so that the full benefits of steel construction may be realised and, the need for late changes and subsequent compromise can be substantially reduced.

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