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Saturday, 13 February 2016

Health risks from fume and gases during welding Health, safety and accident prevention
























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Thursday, 11 February 2016

Fabrication Requirements of BS EN 1090 Part 2

Job Knowledge > Fabrication Requirements of BS EN 1090 Part 2
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Fabrication Requirements of BS EN 1090 Part 2


Now that CE marking of structural steelwork and aluminium structures is mandatory it may be an appropriate time to look at some of the fabrication requirements within the specification, BS EN 1090 Part 2, that must be complied with in order to be able to apply a CE mark. It is suggested that Job Knowledge No. 120 – Structural Steel, CE Marking and ISO 3834 should be read in conjunction with this article. It should also be noted that no brief article such as this Job Knowledge article can cover every aspect of a specification requirement - the specification itself must be referred to for accuracy.
Part 1 of the BS EN 1090 series specifies the requirements for conformity assessment of what are termed the performance characteristics of the structure – essentially those criteria such as toughness, fire resistance, fatigue performance etc – and requires the implementation of a factory production control (FPC) system – see Job Knowledge No. 120 for further information. BS EN 1090 Part 2 is entitled “Technical requirements for the execution of steel structures” and specifies the requirements for the manufacturing of structural steelwork - this includes both bolting and welding. The steels covered by the specification comprise not only the conventional carbon manganese steels such as EN 10025 S275 but also high strength steels up to grade S960, ferritic, austenitic-ferritic and austenitic steels.
This article will not cover what may be termed the conventional requirements for welding such as procedure qualification, welder qualification, welding plan requirements etc but will attempt to highlight those requirements that may not be normal practice within the fabrication shop. There are several clauses within the specification that may cause some difficulties if the requirements are strictly enforced and no concessions are permitted. For example clause 5.5 states that all welding consumables shallcomply with EN 13479, the general product standard for filler metals and fluxes – the term “shall” makes this mandatory and consumables complying with other specifications only, the AWS standards for example, cannot be used. The same does not apply to parent materials where specifications other than the European material specifications can be used “....if otherwise specified...”
The quality of cut edges is dealt with in clause 6.4. This requires that any ‘free’ cut edge, but not weld preparations, complies with limits on tolerance, hardness and smoothness. A footnote states that hand thermal cutting should only be used if it is not practical to use machine thermal cutting. If specified in the contract documentation thermally cut edge hardness shall not exceed 380HV10 for hot finished or normalised S235 to S460 steels and 450HV10 for cold forming or quenched and tempered S260 to S700 steels. Although this requirement may not be mandatory for all contracts it would be advisable to produce a qualification test record to demonstrate compliance. The clause also requires that the “...capability of thermal cutting processes shall be periodically checked....”. The period between checks is not specified. The checks comprise the production of four samples, one each from the thickest and thinnest product and one each from a sharp corner and a curved cut from products of representative thickness.  Records of such periodic checks should be maintained for audit purposes.
Clause 6.5 is entitled ‘Shaping’ and permits hot and cold forming and flame straightening  “....provided the properties are not reduced below those specified for the worked material...” and calls up the guidance given in TR 10347 ‘Guidance for forming of structural steels in processing’. For many structural steel fabrication companies, hot and cold forming is sub-contracted to specialist benders; these must supply the fabricator with documentary evidence that the properties have not been reduced below those specified for inclusion in the as-built QA documentation pack. Flame straightening, however, may be carried out by the fabricator and, for EXC3 and EXC4, a written procedure is required that details the maximum temperature, method of heating, workers permitted to use the process and the results of mechanical tests carried out for the process approval. A procedure qualification record (PQR), similar to that of a welding PQR, is therefore required.
Clause 7 covers the welding requirements, most of which are relatively conventional but there are some additional requirements that need to be taken into account as follows:
  1. Cl 7.4.1.2.b(3) requires procedure qualification tests to be carried out on test pieces with the maximum prefabrication primer thickness if welding is carried out over shop primers. This would need recording on the PQ certificate.
  2. Cl 7.4.1.2.c. Transverse stressed fillet welds in steels of a higher grade than S275 and with a fillet weld throat less than half the component thickness need an additional cruciform test piece to be welded and three tensile tests performed. 
  3. Table 12 permits EXC 2 welding procedures to be qualified in accordance with ISO 15614-1, ISO 15613, ISO 15612 (standard welding procedure), ISO 15611 (previous welding experience) and ISO 15610 ( tested welding consumables). EXC 3 and EXC 4 must be qualified to either ISO 15614-1 or ISO 15613.
  4. Cl 7.4.1.4 requires some additional testing to be performed if a welding process with a relevant PQR has not been used for a period of time –i) between one and three years a production weld test is required to weld steel grades greater than S355 ii) over three years for steels ≤S355 a macro-section is to be taken and for steels>S355 a new procedure qualification test is required.
  5. Cl 7.4.2 Welders must be specifically qualified to weld hollow section branches with an angle less than 60O.
  6. Table 16 requires electrode quivers to be heated to 100°C minimum rather than the 75°C that is generally used.
  7. Cl 7.5.5 requires tack welds and temporary attachments to be preheated in accordance with the relevant welding procedure specification.
  8. Cl 7.5.6 covers the requirements for temporary attachments which may be attached in accordance with a written welding procedure. Cutting or chipping of temporary attachments is not permitted for EXC3 and EXC4 components unless otherwise specified. It is not clear if cutting refers to thermal cutting and if this is permitted if the cut surface is clear of the component surface, say by a minimum of 3mm which is normal practice. It is recommended that an attachment removal method statement is written and implemented on the shop floor to remove doubt.
  9. Cl 7.5.7 covers the requirements for the qualification of tack welds,  a topic that is being interpreted differently by various companies.  Tack welds on EXC2, EXC3 and EXC4 components are required to be welded in accordance with a qualified WPS. Tack welds are to be a maximum length of 4x the thickness of the thicker part or 50mm “...unless a shorter length can be demonstrated as satisfactory...”. Unfortunately the clause does not define what is “satisfactory” so this is left to individual interpretation. Tack welds that are to be incorporated into the completed weld can be qualified by ensuring that the lengths and positions of the tack welds on the qualification test piece are marked and at least one macro-section is taken through the tack and hardness tested. This would need to be noted in the procedure qualification record. The major issue concerns the qualification of small temporary tack welds that may be used, for example, to tack weld a thin wall small diameter bracing to a large thick beam. How such a tack weld is to be qualified is not clear and has yet to be resolved.
  10. Cl 7.5.9 deals with butt welds and Cl 7.5.9.1 specifies that the ends of butt welds shall be terminated in a manner that ensures sound welds. For EXC3 and EXC4 and EXC2, if specified, this shall be achieved by the use of run on/off plates that will be later removed.
  11. Cl 7.5.9.2 covers single sided butt welds. Tack welds shall be included in the final weld and permanent or removable backing may be used. For EXC3 and EXC4 this backing shall be continuous for the full length of the joint, if necessary by joining the backing lengths together by full penetration butt welds. Note, however, that many contract specifications are prohibiting the use of backing for EXC3 and EXC4 components and specifying that the welds shall be full penetration unbacked joints.
  12.  Cl 12.4.2.2. This clause is entitled scope of inspection and is relatively straightforward – there is just one requirement that needs to be mentioned. This is that the first five joints made to the same new WPS require inspecting to twice the extent specified in Table 24 and to an EN 5817 acceptance level of Quality Level B. The reason for this additional testing is to establish that the new WPS can provide the required quality in production – as distinct from that which can be provided during procedure qualification testing.
Specific industry standards are being developed which, although based on BS EN 1090 Part 2, will contain additional requirements that may mean further qualification testing will be needed. Once such specifications are available then these may form part of a Job Knowledge article covering the key differences and additional requirements.
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Thursday, 4 February 2016

Plant Inspector/ NDE Engineer Job Location: Singapore and world wide offered by Cutech Process Services Pte Ltd, Singapore

Plant Inspector/ NDE Engineer
Job Location: Singapore and world wide
offered by Cutech Process Services Pte Ltd, Singapore


Job description / Skill Requirement:

Perform Inspection on various types of Plant components (Piping / Storage tank / Pressure vessel / RBI)
Able to perform Visual Inspection, UTG and adhoc site NDT activities as and when required
Shall coordinate with other NDT/ Advanced NDT team. Interpret the results and prepare final Inspection reports.

Liaise with the clients daily

Qualification and Experience Requirements:

Minimum O Level / ITE / Diploma / Degree
Preferably Certified in API ( 510/ 570 / 653 / 580) or having experience in Plant inspection
Preferably a Certified Welding Inspector
Preferably NDT Level II in UTG
At least 2 year experience in Plant Inspection

Best Regards,

P.Pugalendhi (Pugal)
Director – NDT & Projects,
Cutech Process Services Pte Ltd,
Singapore
Email : pugal@cutechgroup.com
Mobile: 65- 91052231
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Distortion - Types and causes


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Distortion Control - Prevention by fabrication techniques

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Distortion Control - Prevention by fabrication techniques


Distortion caused by welding a plate at the centre of a thin plate before welding into a bridge girder section. Courtesy John Allen
Distortion caused by welding a plate at the centre of a thin plate before welding into a bridge girder section. Courtesy John Allen

Assembly techniques

In general, the welder has little influence on the choice of welding procedure but assembly techniques can often be crucial in minimising distortion. The principal assembly techniques are:
  • tack welding
  • back-to-back assembly
  • stiffening

Tack welding

Tack welds are ideal for setting and maintaining the joint gap but can also be used to resist transverse shrinkage. To be effective, thought should be given to the number of tack welds, their length and the distance between them. With too few, there is the risk of the joint progressively closing up as welding proceeds. In a long seam, using MMA or MIG, the joint edges may even overlap. It should be noted that when using the submerged arc process, the joint might open up if not adequately tacked.
The tack welding sequence is important to maintain a uniform root gap along the length of the joint. Three alternative tack welding sequences are shown in Fig. 1:
a) tack weld straight through to the end of the joint (Fig 1a). It is necessary to clamp the plates or to use wedges to maintain the joint gap during tacking
b) tack weld one end and then use a back stepping technique for tacking the rest of the joint (Fig 1b)
c) tack weld the centre and complete the tack welding by back stepping (Fig 1c).
Fig. 1. Alternative procedures used for tack welding to prevent transverse shrinkage
Fig. 1. Alternative procedures used for tack welding to prevent transverse shrinkage
a) tack weld straight through to end of joint
b) tack weld one end, then use back-step technique for tacking the rest of the joint
c) tack weld the centre, then complete the tack welding by the back-step technique
Directional tacking is a useful technique for controlling the joint gap, for example closing a joint gap which is (or has become) too wide.
When tack welding, it is important that tacks which are to be fused into the main weld are produced to an approved procedure using appropriately qualified welders. The procedure may require preheat and an approved consumable as specified for the main weld. Removal of the tacks also needs careful control to avoid causing defects in the component surface.

Back-to-back assembly

By tack welding or clamping two identical components back-to-back, welding of both components can be balanced around the neutral axis of the combined assembly (Fig. 2a). It is recommended that the assembly is stress relieved before separating the components. If stress relieving is not done, it may be necessary to insert wedges between the components (Fig. 2b) so when the wedges are removed, the parts will move back to the correct shape or alignment.
Fig. 2. Back-to-back assembly to control distortion when welding two identical components
Fig. 2. Back-to-back assembly to control distortion when welding two identical components
a) assemblies tacked together before welding
b) use of wedges for components that distort on separation after welding

Stiffening

Fig. 3. Longitudinal stiffeners prevent bowing in butt welded thin plate joints
Fig. 3. Longitudinal stiffeners prevent bowing in butt welded thin plate joints
Longitudinal shrinkage in butt welded seams often results in bowing, especially when fabricating thin plate structures. Longitudinal stiffeners in the form of flats or angles, welded along each side of the seam (Fig. 3) are effective in preventing longitudinal bowing. Stiffener location is important: they must be placed at a sufficient distance from the joint so they do not interfere with welding, unless located on the reverse side of a joint welded from one side.

Welding procedure

A suitable welding procedure is usually determined by productivity and quality requirements rather than the need to control distortion. Nevertheless, the welding process, technique and sequence do influence the distortion level.

Welding process

General rules for selecting a welding process to prevent angular distortion are:
  • deposit the weld metal as quickly as possible
  • use the least number of runs to fill the joint
Unfortunately, selecting a suitable welding process based on these rules may increase longitudinal shrinkage resulting in bowing and buckling.
In manual welding, MIG, a high deposition rate process, is preferred to MMA. Weld metal should be deposited using the largest diameter electrode (MMA), or the highest current level (MIG), without causing lack-of-fusion imperfections. As heating is much slower and more diffuse, gas welding normally produces more angular distortion than the arc processes.
Mechanised techniques combining high deposition rates and high welding speeds have the greatest potential for preventing distortion. As the distortion is more consistent, simple techniques such as presetting are more effective in controlling angular distortion.

Welding technique

General rules for preventing distortion are:
  • keep the weld (fillet) to the minimum specified size
  • use balanced welding about the neutral axis
  • keep the time between runs to a minimum
Fig. 4. Angular distortion of the joint as determined by the number of runs in the fillet weld
Fig. 4. Angular distortion of the joint as determined by the number of runs in the fillet weld
In the absence of restraint, angular distortion in both fillet and butt joints will be a function of the joint geometry, weld size and the number of runs for a given cross section. Angular distortion (measured in degrees) as a function of the number of runs for a 10mm leg length fillet weld is shown in Fig. 4.

If possible, balanced welding around the neutral axis should be done, for example on double sided fillet joints, by two people welding simultaneously. In butt joints, the run order may be crucial in that balanced welding can be used to correct angular distortion as it develops.
Fig. 5. Use of welding direction to control distortion
Fig. 5. Use of welding direction to control distortion
a) Back-step welding
b) Skip welding

Welding sequence

The sequence, or direction, of welding is important and should be towards the free end of the joint. For long welds, the whole of the weld is not completed in one direction. Short runs, for example using the back-step or skip welding technique, are very effective in distortion control (Fig. 5).
  • Back-step welding involves depositing short adjacent weld lengths in the opposite direction to the general progression (Fig. 5a).
  • Skip welding is laying short weld lengths in a predetermined, evenly spaced, sequence along the seam (Fig. 5b). Weld lengths and the spaces between them are generally equal to the natural run-out length of one electrode. The direction of deposit for each electrode is the same, but it is not necessary for the welding direction to be opposite to the direction of general progression.

Best practice

The following fabrication techniques are used to control distortion:
  • using tack welds to set up and maintain the joint gap
  • identical components welded back to back so welding can be balanced about the neutral axis
  • attachment of longitudinal stiffeners to prevent longitudinal bowing in butt welds of thin plate structures
  • where there is choice of welding procedure, process and technique should aim to deposit the weld metal as quickly as possible; MIG in preference to MMA or gas welding and mechanised rather than manual welding
  • in long runs, the whole weld should not be completed in one direction; back-step or skip welding techniques should be used.
Bill Lucas prepared this article in collaboration with Geert Verhaeghe, Rick Leggatt and Gene Mathers.
For more information please contact us.
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