Intro4u2u

Formal procedures are adopted for control of welding when the risk of failure must be minimised. These are based on the selection of suitable materials, use of a qualified procedure, and qualification of the welder.
The Welding Procedure Specification (WPS) is a written qualified welding procedure, prepared to provide direction for making production welds.
The WPS references and is supported by the Procedure Qualification Record (PQR) which reports variables recorded during the welding of test coupons and also contains the test results. Strength, ductility and toughness are
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commonly tested together with any other properties (e.g. hardness, corrosion resistance, creep strength ) required for the application.
Together, the WPS and PQR provide control of the structure and properties of the welded joint by ensuring that the essential welding variables do not differ significantly between the WPS and PQR. Essential welding variables are those which influence the joint properties and are specific to the welding process and other circumstances (such as application). For example, changing the heat treatment condition of the weld will affect properties, and thus be a change in the essential variables.
The Welder Performance Qualification (WPQ) determines the ability of welders to make sound welds. Here the essential welding variables are those influencing the difficulty in making the joint with the declared process.
The WPS provides the instructions, in all relevant detail, to make the joint in the structure. Some variables are non-essential, they do not significantly affect the properties of the joint but still need to be specified. The joint geometry including angle, root gap and root face is an example. All variables are thus specified and available to the fitter, inspector and surveyor. In some instances, particularly involving simple manual processes, there are fewer variables specified and greater reliance is placed upon the skills of the welder.
Inspection is an activity, focused upon the WPS, which occurs before, during and after welding.
Before production commences a Review of Welding Procedures should be carried out. The fabrication drawing shows where the WPS is to be used and the application standard or code (e.g. LR Rules for Ships; ASME Boiler and Pressure Vessel Code) plus requirements such as post weld heat treatment, corrosion allowance or design temperature. The WPS must conform with the drawing and be supported by a PQR reporting test results which satisfy the requirements.
Before and during welding, checks should be made that both parent materials and consumables are in accordance with the WPS. Is the joint gap being maintained? Are the heat input and interpass temperature within the limits?
Inspection carried out after welding should always include visual inspection (e.g.has the joint been welded at all? Particularly relevant for fillet welds in remote locations!). Non-Destructive Examination (NDE) carried out after welding, particularly when only required on a low percentage sample basis, does not enhance the quality of the welded joint. It provides assurance that the quality control has been successful. Conversely, if significant (potentially damaging) defects are detected, it indicates that the quality control has not
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been effective. It is then essential that the cause of the faults is established and all suspect work is examined and repaired where necessary.

The welding process is selected (in conjunction with the weld preparation) to provide the desired joint quality and properties as economically as possible. The welding engineer will consider the material and thickness, welding position, access and environment such as exposure to draughts. He will then choose between manual, mechanised or automatic (robotic) welding based upon the availability of equipment and the skills of the welders or operators.
Summary of common welding processes, together with abbreviations
AWS (American Welding Society)
ISO (International Standards Organisation)
Abbreviation
Process Name
Process Name
SMAW
Shielded Metal Arc Welding
Metal-arc welding with covered electrode
SAW
Submerged Arc Welding
Submerged-arc welding with wire electrode
GMAW
Gas Metal Arc Welding
Metal-arc inert gas welding (MIG)
Metal-arc active gas welding (MAG)
FCAW
Flux Cored Arc Welding
Flux-cored wire metal-arc welding with active gas shield
SSAW
Self Shielded Arc Welding
Self Shielded Arc Welding
GTAW
Gas Tungsten Arc Welding
Tungsten inert gas arc welding (TIG)
ESW
Electroslag Welding
Electro-slag Welding
EGW
Electrogas Welding
Electro-gas Welding
LBW
Laser Beam Welding
Laser Beam Welding
EBW
Electron Beam Welding
Electron Beam Welding
The welding consumables should be selected so that the deposited metal has adequate ductility and satisfies the toughness requirements of the parent material. In theory, the weld strength should equal that of the parent material. However, in practice it is beneficial for the weld strength to moderately
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exceed (ideally by 10 to 20%) or overmatch that of the parent plate. Obviously, the composition and properties need to be suitable at the levels of dilution which occur in all parts of the joint.

Steels for welding are commonly produced as rolled plate or related sections but it must not be forgotten that castings and forgings are also used in ship construction.
The properties of the material (strength and toughness grade) are determined by the chemical composition and manufacturing condition. For ships, steels generally conform to the following strengths:
Yield Strength
(N/mm2)
Tensile Strength
(N/mm2)
Normal Strength
235 minimum
400-520
Higher Strength(H32)
315 minimum
440-590
Higher Strength(H36)
355 minimum
490-620
Normal strength steels contain a minimum manganese composition of 2.5 or 3 times the carbon content, typically in the range 0.6-1.0%. The response to welding of these carbon steels can be improved by raising the manganese and decreasing the carbon content to achieve the desired strength. Carbon-manganese steels are used to achieve higher strength materials and typically contain 1.0-1.6% manganese. The introduction of microalloying elements such as niobium (Nb, also known as columbium in the USA), vanadium (V) and titanium (Ti) also allows higher strengths to be achieved.
The toughness grade of a steel is superior when impact energy requirements can be met at a lower temperature. Strengthening by manganese rather than carbon is beneficial. A steel of superior toughness can be obtained by producing a fine grained steel using aluminium and by either using a normalising heat treatment or by controlling the temperatures during the final stages of rolling. Thermomechanically controlled rolling of microalloyed steel (TMCP) has become the most common practice of producing fine grained structures with enhanced strength and toughness, allowing a reduction in the carbon and/or manganese content.
The strength and toughness of the parent steel affect the requirements likely to be specified for the completed joint. The strength level of the steel also influences the magnitude of the residual stresses generated by welding (which are often considered to be of yield strength in magnitude). However, composition is generally the dominant factor in welding a steel. It affects the weld metal composition by dilution, and the microstructure close to the fusion line (the Heat Affected Zone, HAZ). Here, exposure to high temperatures during welding largely eliminates the original structure of the
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steel, and a local structure with a high hardness is often generated during cooling.
The maximum hardness of the heat affected zone, and hence the susceptibility to embrittlement and hydrogen induced delayed cracking, can be related to the Carbon Equivalent (CE). The CE formula used by the International Institute of Welding (IIW) is defined as:
CECMnCrMoVCuNi=++++++6515
(wt.%)
Practical experience has shown that when the CE<0.41 (or C+Mn/6<0.40), steel plates up to 25mm thick can be readily welded without special precautions. If low hydrogen electrodes (H15: less than 15ml of hydrogen per 100g deposited metal) are employed, steels with CE<0.45 are considered weldable without further precautions.
With recent developments in thermomechanical controlled rolled microalloyed steels, especially for pipelines, it has been found that the standard CE formula is not truly representative of the susceptibility to cracking of these low carbon steels. A second formula is sometimes quoted for these materials, known as the PCM, which is defined as:
PCSiMnCrCuNiMoVBCM=++++++++30206015105 (wt.%)
There is no well-defined limit for the PCM, and the acceptable value is usually subject to agreement between the steelmaker and fabricator.
The HAZ microstructure is also affected by the plate thickness. Thicker plates conduct heat away quickly, resulting in accelerated cooling and an increase in hardness. Enhanced cooling can also occur in other situations, such as depositing a bead on a thick plate.
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The concept of combined thickness (t) is useful for heat flow. This is particularly important when considering hydrogen cracking, where the cooling rate of the joint is critical. For T-joints, there are three sections to conduct heat away from the weld, whereas for butt joints there are only two.

Heat is removed from a weld at a rate depending on the size and conductivity of the material surrounding the joint. Therefore, the flow of heat is influenced by joint design.
For a square butt weld, where a large area of material is adjacent to the arc, there is a high heat flow from the joint. Care must be taken using this joint design that a large heat input is used, otherwise the joint will cool too rapidly. 12
If a plate edge is prepared with a through-thickness bevel (leaving a feather edge), there is only a limited amount of material through which heat can be conducted, i.e. the heat flow is low. This can result in the arc burning through the material and the weld pool escaping.
Therefore, it is usual practice to leave a small root face during edge preparation.
Another problem with heat flow can occur in a T-butt joint, where the capacity for absorbing heat is much greater on one side of the joint than the other. This can lead to burning away of one side, or lack of fusion on the other. The solution is to direct the arc towards the member having the largest heat capacity.

Situations often arise where it is necessary to weld together plates of differing thickness. This can cause problems for a butt joint, where the thickness gradient between the sections will create a stress concentration at the weld. To reduce this local stress, the transition in thickness between the two sections should have a gradient no greater than 1 in 4.
Good thickness change joint
(maximum 1:4 gradient)
Note that if the centre planes of the two sections do not coincide, local bending will be induced when the joint is loaded in service.
Thickness change joint with centred axis to eliminate local bending stresses
2.2.3 Edge Preparation
The single-V preparation is defined by the V-angle, the root face and the gap. The V-angle is a compromise between ease of welder access for the electrode, penetration, and a need for a minimum volume of added filler metal (for economy and to minimise distortion). The V-angle is typically 60° for SMAW.
By using a double bevelled groove, access of the welding electrode to the root face may be improved during welding. However, this requires additional preparation, which can be expensive. 11
An extension of this is the U-preparation, which allows good access to the root area. This is a common edge preparation for pipe joints, in which the root run is GTAW without backing.
Where access to the back of the weld is not possible, a skilled welder can usually make the joint without backing. Alternatively, a backing strip may be attached at the weld root. However, the fatigue characteristics of such a weld are poor compared to a full penetration butt weld, and defects are often present at the intersection of the parent plate and backing strip.

A welded structure, however intricate its shape, is usually composed from a number of fundamental joint types. The following is a summary of common joint designs. A more extensive list can be found in National Standards.
A butt joint is an end connection between two parallel sections of metal. Full penetration butt welding is preferable, especially for a joint that is subjected to fatigue loading. Partial penetration is not generally accepted, as the root gap may initiate fatigue crack growth.
A T-joint is a connection between the end of one section and the face of another section, usually at an angle of 90°. The joint can either be butt welded (full penetration) or fillet welded (partial penetration). 9
A lap joint is a connection between two overlapping sections. This joint is always fillet welded (partial penetration).
A corner joint is an end connection between two sections that are at an angle to each other, usually 90°. The joint is normally butt welded (full penetration).
A cruciform joint is a connection where two sections are welded to opposite faces of a flat section in the same axis, at an angle of 90°. This joint is usually (but not necessarily) fillet welded.

Arc welding involves an electrode, workpiece and electrical power source. Heat generated by the arc melts the parent plate and the filler material (if any is used).
The simplest weld preparation is the square butt. Close butted square edges rely on the ability of the process and welding parameters (e.g. arc current and voltage) to achieve adequate penetration.
For thin plates, welding conditions can be selected which will give full penetration in a single pass of the welding process.
If the plate thickness is increased without changing the welding conditions, the resulting weld will achieve only partial penetration. This is understandable because as the thickness is increased, the area of parent metal (which acts as a heat sink) surrounding the weld increases. Thus more heat is conducted away from the molten weld pool, making it more difficult to achieve penetration. 4
To obtain full penetration in this thicker plate, it is necessary to increase the welding current or to widen the gap between the edges of the plates or both. Widening the gap between the parent materials effectively allows the heat source to penetrate further between the edges.
If the thickness is increased and the conditions changed correspondingly, eventually a thickness will be reached where a satisfactory single pass weld will be impossible to produce. The molten metal will simply fall though the gap, leaving solidified globules hanging underneath.
It is still possible to make a full penetration weld with square edges using the two-run technique. This requires a weld bead to be made with more than 50% penetration from one side (A). Then the assembly is turned over and a similar bead made from the second side (B).
The penetration can be increased by using a single V-edge preparation. The effect of the V-preparation is to reduce the area available for heat flow by conduction away from the weld. This concentrates the heat in the weld pool, thus maximising penetration. Unless the welder is particularly skilled, this preparation is arranged so that the first (root) run does not penetrate completely, hence there is no risk of the molten metal falling through the gap.
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Having made the root run and added further runs of weld metal to fill the joint on the first side, the root gap contains trapped slag, and often there is lack of penetration at the edges of the underside of the root bead.
These imperfections in the root region are then best removed by back-gouging to sound metal, using either mechanical cutting, grinding, thermal gouging, or a combination of these techniques.
The object of this back-gouging operation is to end up with a surface which is free from defects and has a rounded groove-like shape, thus allowing the welder to deposit further runs of weld metal at the root of the joint.
These root or sealing runs complete the full penetration joint. 7
Note that if special care is taken with the joint preparation and welding, it is possible to produce a sound root run. In this case, the need for back-gouging is eliminated, thus allowing a single-sided complete penetration weld to be made.
When attempting to use the two-run technique in plates of greater thickness, V-grooves of limited depth can be incorporated in the joint preparation to enable greater penetration from each side, in order to achieve overlapping beads.
When hot weld metal cools it contracts. The overall contraction at the face of a butt weld is greater than that at the root of the weld because of the difference in weld width. If the parts being joined are not restrained, the differential contraction results in relative movement of the two parts (transverse angular distortion). The amount of distortion increases with thickness.
A better balance of contraction between face and root can be obtained using a machined U-preparation. However, the machining operation significantly increases preparation costs. 8
A cheaper alternative is the double-V, which can be prepared by thermal cutting. By controlling the weld bead sequence, it is possible to balance the contraction occurring in one V by that in the other V.

The objective of fusion welding is to produce sound joints of the correct profile with adequate properties. The requirements depend upon the application, which (in marine terms) is primarily the ship hull and structure. There are other important applications such as pressure plant, for which different materials and criteria apply, particularly if operated at elevated temperatures.
The benefits of welded fabrication can be illustrated by comparison between a welded and a riveted structure.
The riveted joint requires extra material for the overlap and there is a risk of leakage, but fracture is generally limited to a single plate. The fusion welded joint can provide continuity of structure with a smooth profile, such that design can be efficient and high levels of structural integrity achieved. However, the continuity of the structure means that fracture, once initiated, might propagate very rapidly with little chance of crack arrest.
It is therefore very important that the factors conducive to fracture:
•tensile (including residual) stress
•a weld defect
•low toughness
all of which can potentially coexist at a weld, are adequately controlled to prevent structural failure.
It is the Welding Engineer’s task, on behalf of the Shipbuilder or Fabricator, to devise and prove welding procedures. The Surveyor may be called to witness testing and to ensure that the data is recorded correctly. Therefore, he requires an appreciation of the technicalities of welding rather than an expert working knowledge.

Storage And Care Of Consumables
In storage, the main enemies of electrodes and fluxes are mechanical damage and moisture. Careless handling of covered electrodes, such as those used for manual metal-arc welding, can lead to removal of areas of the flux cover and such affected materials should not be used for welding. Similarly, exposure to excessive amounts of moisture can lead to rusting of the core wire with a lifting of the flux coating. This also requires the electrodes to be discarded.
The flux covering on modern electrodes tends to be porous and will absorb moisture to some extent depending on the atmospheric humidity. Electrode coverings of the cellulose type can absorb an appreciable quantity of moisture with little effect on their properties. They should not be over dried or charring of the coating may result. Mineral coated electrodes do not naturally absorb so much moisture and can be dried out if damp. The electrodes should be well spaced out in an oven and subjected to a temperature of about 110°C for 10-60 minutes depending on their size. Cellulose type electrodes will require only about 15 minutes to dry.
Low hydrogen electrodes are specially designed to contain relatively little in the way of hydrogen containing compounds including moisture. They need to be kept in a dry, heated, well ventilated store at about 12°C above the external air temperature. Where necessary they should be oven-dried before use, at temperatures ranging from 150-450°C, depending on the permissible hydrogen content of the weld and the manufacturer’s recommendations. Some modern electrodes are vacuum packed and generally need no further drying if used within a specified time of opening.
Care must also be given to fluxes supplied for submerged arc welding which, although they may be dry when packaged, may be exposed to high humidity in store. In such cases they should be dried in accordance with the manufacturer’s recommendations before use, or porosity or cracking may result.
Ferrous wire coils supplied as continuous feeding electrodes are usually copper coated. This provides some corrosion resistance, ensures good electrical contacts and helps in smooth feeding.
Rust and mechanical damage should be avoided in such products as they will both interrupt smooth feeding of the electrode. Rust will be detrimental to weld quality generally, and to equipment condition in the case of GMAW.
Contamination by carbon containing materials such as oil, grease, paint and drawing lubricants is especially harmful with ferrous metals. Here carbon pick-up in the weld metal can cause a marked and usually undesirable change in properties. Such contaminants may also result in hydrogen being absorbed in the weld pool.
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In general it is a wise welder who studies and follows the manufacturer’s recommendations for consumables.

Consumable guide welding is a simplified version of the electroslag process for welding thick plate in the vertical or near vertical position, for joints of limited length: usually up to 2 m. The gap between plates is 25-30 mm, but when welding thicknesses less than 20 mm the restriction on the minimum gap being so as to ensure that the guide tube does not touch the plate edges and there is sufficient space for insulating wedges if these are needed to position the guide tube. Water-cooled copper shoes act as dams to confine the molten metal, and give it the required weld profile. As with electroslag welding the current passes through molten slag and generates enough heat to melt the electrode end, the guide tube and edges of the parts being joined ensuring a good fusion weld (see Figure 15).
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Figure 15 Consumable guide layout showing water-cooled dams
If a plain uncoated guide tube is used, flux is added to cover the electrode and guide end before welding commences. Otherwise, the process is started and operated in a similar manner to normal electroslag welding. Although there is no arc present after the starting phase of the process, the slag surface of the molten pool should be viewed through dark glasses (as in gas cutting) because of its brightness.
The equipment for welding is considerably simpler than that for normal electroslag welding, chiefly because the welding head and wire feed mechanism do not need to be moved up the joint as the weld is made. It is possible to weld where there is access from one side only, or indeed where there is a permanent backing bar on both sides of the joint. It is cheaper and more adaptable than other similar processes, faster than metal-arc welding of thick plate, joint preparation is cheaper, uniform heat distribution through the joint reduces distortion problems, and there are no spatter losses.