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Connect - March/April 1998

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The copper firebox was repaired in TWI's state-of-the-art electron beam chamber

To repair the steel back-head, a traditional arc process was used. The worn material was ground away and restored to its original dimensions by manual metal arc welding. However, to attach a new flange to the copper firebox, it was decided to employ the much newer electron beam (EB) process. This meant the repairs could be completed much more rapidly compared to conventional welding techniques. This steam locomotive now contains the first EB-welded component anywhere in the world.

To find out more about either traditional or innovative methods of welding, contact Derek Patten in the Arc Welding section or Chris Punshon in the Electron Beam Department.



 

Making light work of manufacturing

Connect, March/April 1998

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The copper firebox was repaired in TWI's state-of-the-art electron beam chamber

To repair the steel back-head, a traditional arc process was used. The worn material was ground away and restored to its original dimensions by manual metal arc welding. However, to attach a new flange to the copper firebox, it was decided to employ the much newer electron beam (EB) process. This meant the repairs could be completed much more rapidly compared to conventional welding techniques. This steam locomotive now contains the first EB-welded component anywhere in the world.

To find out more about either traditional or innovative methods of welding, contact Derek Patten in the Arc Welding section or Chris Punshon in the Electron Beam Department.



Making light work of manufacturing

Connect, March/April 1998

A YAG laser is used to drill turbine blades with the help of a five-axis control system

A YAG laser is used to drill turbine blades with the help of a five-axis control system

'The light fantastic' takes on a new meaning in the latest programme from TWI's television team. The Laser Millennium is the sequel to the popular Day Of The Laser, an introduction to industrial lasers which was released in1993.

Using footage shot in over a dozen UK manufacturing locations the film explores the workings and applications of three types of laser; carbon dioxide, excimer and neodymium YAG.

Funded by the Make It With Lasers TM sponsor group, the film's objective has been to broadcast the message that these industrial tools can weld, cut, mark, drill, surface and machine in a more efficient way than some more traditional technologies.



 

RACH - Reliability Assessment for Containers of Hazardous materials on offshore structures

Connect, March/April 1998

Offshore oil and gas processing plant experience a harsh service environment which can cause degradation by corrosion, erosion and associated cracking problems which, if undetected, may lead to failures.

The RACH project aims to show it is possible to link developments in quantifying inspection (NDT) reliability with damage modelling to give a rational basis for inspection scheduling which provides the required level of safety andis cost effective. Performance trials are currently beginning on about 50 pipe test samples with internal and external corrosion flaws. The work involves trials on both coated and uncoated samples as well as pipes with thermal insulation. The aim is to provide Probability Of Detection (POD) and accuracy of flaw sizing data for a range of NDT methods.

Data obtained will be input into software based on STRUREL TM which enables different NDT methods and inspection intervals to be evaluated against an acceptable level of reliability. Cost-effective inspection schedules may then be determined.

Partners in the RACH project are Technical Software Consultants (TSC), University College London, Bureau Veritas and TWI. It is an EC THERMIE programme with industrial support from Shell, Marathon Oil and the UK Health & Safety Executive.


 

Groundbreaking ceremony for new Science Park

Connect, March/April 1998

Using a CO 2 laser both butt and stake welds are incorporated here in the manufacture of lightweight panels for the shipbuilding sector

Using a CO 2 laser both butt and stake welds are incorporated here in the manufacture of lightweight panels for the shipbuilding sector 

The extraordinarily high accuracy of excimer laser systems make it possible to machine shapes within resolutions of a couple of microns

The extraordinarily high accuracy of excimer laser systems make it possible to machine shapes within resolutions of a couple of microns



 

RACH - Reliability Assessment for Containers of Hazardous materials on offshore structures

Connect, March/April 1998

Offshore oil and gas processing plant experience a harsh service environment which can cause degradation by corrosion, erosion and associated cracking problems which, if undetected, may lead to failures.

The RACH project aims to show it is possible to link developments in quantifying inspection (NDT) reliability with damage modelling to give a rational basis for inspection scheduling which provides the required level of safety andis cost effective. Performance trials are currently beginning on about 50 pipe test samples with internal and external corrosion flaws. The work involves trials on both coated and uncoated samples as well as pipes with thermal insulation. The aim is to provide Probability Of Detection (POD) and accuracy of flaw sizing data for a range of NDT methods.

Data obtained will be input into software based on STRUREL TM which enables different NDT methods and inspection intervals to be evaluated against an acceptable level of reliability. Cost-effective inspection schedules may then be determined.

Partners in the RACH project are Technical Software Consultants (TSC), University College London, Bureau Veritas and TWI. It is an EC THERMIE programme with industrial support from Shell, Marathon Oil and the UK Health & Safety Executive.


 

Groundbreaking ceremony for new Science Park

Connect, March/April 1998

Councillor Shirley Saunders with Bevan Braithwaite, TWI (right) and Gavin Davidson, MEPC

Councillor Shirley Saunders with Bevan Braithwaite, TWI (right) and Gavin Davidson, MEPC

A new Science Park is beginning to take shape at Abington following the groundbreaking ceremony held at the end of 1997. Councillor Shirley Saunders, Chairman of South Cambridgeshire County Council, joined TWI's Chief Executive Bevan Braithwaite and Gavin Davidson of MEPC plc to launch the new project in style.

The infrastructure is due to be completed by summer 1998 with the development programme taking between 5-7 years.

Granta Park, as it has been named, will not only provide new accommodation for TWI's extensive facilities but will also fulfil a need for purpose-built locations for the area's expanding biotech and technology sector.




 

Moving Contact Arc Welding - Group Sponsored Project launch

Connect, March/April 1998

MCAW is a low cost process suitable for either a manual or mechanised operation

MCAW is a low cost process suitable for either a manual or mechanised operation

Moving Contact Arc Welding (MCAW) is an innovative, easy-to-use method for welding, weld overlay and repair. TWI is launching a Group Sponsored Project (GSP) on 8 April 1998 covering this process. The project will develop positional welding and apply wear and corrosion resistant coatings.

MCAW can be used as either a manual or mechanised operation. The process supplies current to a shaped metal consumable electrode through a sliding or rolling contact tool. Moving the tool along the electrode keeps the length being heated constant throughout welding and allows the use of very long consumables. Benefits of using MCAW compared to other process are:

  • greater productivity through reduced interruptions and use of longer and larger consumable
  • requires low rather than high skill
  • low-cost portable equipment and automation potential
  • ideal for restricted access or remote operation
  • weld deposit metallurgical quality compares well with conventional MMA welds
  • electrode geometry can be shaped to suit base material

This project will be of interest to companies in the oil and gas and shipbuilding industries, consumable manufacturers, general fabricators and those engaged in surfacing or remote welding.




 

New projects at TWI

Connect, March/April 1998

The following group sponsored projects (GSPs) have just been, or are about to be, launched. If you are interested in participating, please contact Keith Johnson or the project leader.

Rapid evaluation for high reliability low cost electronics

This project will develop additional methods for a range of applications environments focusing on high reliability and low cost, essential for modern, highly competitive electronics manufacturers.

Launched January 1998
Project leader: Nihal Sinnadurai

Technical and economic feasibility of using titanium for risers

The key research areas are joining process, fatigue and fracture toughness properties, stress corrosion cracking and in-service inspection as well as some of the design aspects.

To be launched March 1998
Project leader: Terry Dickerson

Cathodic protection of ferritic-austenitic stainless steel

This project aims to define more precisely the material, stress and environmental conditions under which duplex and superduplex stainless steels can display hydrogen-induced failure recognising both base metal and welded joints.

Launched January 1998
Project leader: Paul Woollin



 

Job knowledge for welders

Distortion - prevention by design

Strongbacks on girder flange to prevent cross bowing. Courtesy John Allen

Strongbacks on girder flange to prevent cross bowing. Courtesy John Allen


General guidelines are given below as 'best practice' for limiting distortion when considering the design of arc welded structures.


Design principles

At the design stage, welding distortion can often be prevented, or at least restricted, by considering:

  • elimination of welding
  • weld placement
  • reducing the volume of weld metal
  • reducing the number of runs
  • use of balanced welding

Elimination of welding

As distortion and shrinkage are an inevitable result of welding, good design requires that not only the amount of welding is kept to a minimum, but also the smallest amount of weld metal is deposited. Welding can often be eliminated at the design stage by forming the plate or using a standard rolled section, as shown in Fig 1.

Fig. 1 Elimination of welds by: a) forming the plate; b) use of rolled or extruded section

Fig. 1 Elimination of welds by:

a) forming the plate;

b) use of rolled or extruded section 

If possible, the design should use intermittent welds rather than a continuous run, to reduce the amount of welding. For example, in attaching stiffening plates, a substantial reduction in the amount of welding can often be achieved whilst maintaining adequate strength.

Weld placement

Placing and balancing of welds are important in designing for minimum distortion. The closer a weld is positioned to the neutral axis of a fabrication, the lower the leverage effect of the shrinkage forces and the final distortion. Examples of poor and good designs are shown in Fig 2.

Fig. 2 Distortion may be reduced by placing the welds around the neutral axis

Fig. 2 Distortion may be reduced by placing the welds around the neutral axis

As most welds are deposited away from the neutral axis, distortion can be minimised by designing the fabrication so the shrinkage forces of an individual weld are balanced by placing another weld on the opposite side of the neutral axis. Whenever possible, welding should be carried out alternately on opposite sides, instead of completing one side first. In large structures, if distortion is occurring preferentially on one side, it may be possible to take corrective actions, for example, by increasing welding on the other side to control the overall distortion.

Reducing the volume of weld metal

To minimise distortion, as well as for economic reasons, the volume of weld metal should be limited to the design requirements.

For a single-sided joint, the cross-section of the weld should be kept as small as possible to reduce the level of angular distortion, as illustrated in Fig 3.

Fig. 3 Reducing the amount of angular distortion and lateral shrinkage by: a) reducing the volume of weld metal; b) using single pass weld

Fig. 3 Reducing the amount of angular distortion and lateral shrinkage by:

a) reducing the volume of weld metal;

b) using single pass weld 

Joint preparation angle and root gap should be minimised providing the weld can be made satisfactorily. To facilitate access, it may be possible to specify a larger root gap and smaller preparation angle. By cutting down the difference in the amount of weld metal at the root and the face of the weld, the degree of angular distortion will be correspondingly reduced. Butt joints made in a single pass using deep penetration have little angular distortion, especially if a closed butt joint can be welded (Fig 3). For example, thin section material can be welded using plasma and laser welding processes and thick section can be welded, in the vertical position, using electrogas and electro slag processes. Although angular distortion can be eliminated, there will still be longitudinal and transverse shrinkage.

In thick section material, as the cross sectional area of a double-V joint preparation is often only half that of a single-V preparation, the volume of weld metal to be deposited can be substantially reduced. The double-V joint preparation also permits balanced welding about the middle of the joint to eliminate angular distortion.

As weld shrinkage is proportional to the amount of weld metal, both poor joint fit-up and over-welding will increase the amount of distortion. Angular distortion in fillet welds is particularly affected by over-welding. As design strength is based on throat thickness, over-welding to produce a convex weld bead does not increase the allowable design strength but it will increase the shrinkage and distortion.

Reducing the number of runs

There are conflicting opinions on whether it is better to deposit a given volume of weld metal using a small number of large weld passes or a large number of small passes. Experience shows that for a single-sided butt joint, or a single-side fillet weld, a large single weld deposit gives less angular distortion than if the weld is made with a number of small runs. Generally, in an unrestrained joint, the degree of angular distortion is approximately proportional to the number of passes.

Completing the joint with a small number of large weld deposits results in more longitudinal and transverse shrinkage than a weld completed in a larger number of small passes. In a multi-pass weld, previously deposited weld metal provides restraint, so the angular distortion per pass decreases as the weld is built up. Large deposits also increase the risk of elastic buckling particularly in thin section plate.

Use of balanced welding

Balanced welding is an effective means of controlling angular distortion in a multi-pass butt weld by arranging the welding sequence to ensure that angular distortion is continually being corrected and not allowed to accumulate during welding. Comparative amounts of angular distortion from balanced welding and welding one side of the joint first are shown schematically in Fig 4. The balanced welding technique can also be applied to fillet joints.

Fig. 4 Balanced welding to reduce the amount of angular distortion

Fig. 4 Balanced welding to reduce the amount of angular distortion

If welding alternately on either side of the joint is not possible, or if one side has to be completed first, an asymmetrical joint preparation may be used with more weld metal being deposited on the second side. The greater contraction resulting from depositing the weld metal on the second side will help counteract the distortion on the first side.

Best practice

The following design principles can control distortion:

  • eliminate welding by forming the plate and using rolled or extruded sections
  • minimise the amount of weld metal
  • do not over weld
  • use intermittent welding in preference to a continuous weld pass
  • place welds about the neutral axis
  • balance the welding about the middle of the joint by using a double-V joint in preference to a single-V joint

Adopting best practice principles can have surprising cost benefits. For example, for a design fillet leg length of 6mm, depositing an 8mm leg length will result in the deposition of 57% additional weld metal. Besides the extra cost of depositing weld metal and the increase risk of distortion, it is costly to remove this extra weld metal later. However, designing for distortion control may incur additional fabrication costs. For example, the use of a double-Vjoint preparation is an excellent way to reduce weld volume and control distortion, but extra costs may be incurred in production through manipulation of the workpiece for the welder to access the reverse side.

The article was prepared by Bill Lucas in collaboration with Geert Verhaeghe, Rick Leggatt and Gene Mathers.


 

Award of the 1000th European Welding Inspector diploma

Tim Jessop (left) with Allan Saunders and Tony Gillam of Markey Project Support at the diploma presentation at TWI North

Tim Jessop (left) with Allan Saunders and Tony Gillam of Markey Project Support at the diploma presentation at TWI North

Only two years after the launch of the European Welding Inspector qualification, TWI Certification Ltd recently issued the 1000th diploma. The popularity of the qualification demonstrates that inspectors and their employees attach great importance to wider European recognition of qualifications.

The scheme is directly linked to TWI Certification's well-established Certification Scheme for Welding and Inspection Personnel (CSWIP)and people who are successful in the CSWIP Welding Inspector programme gain automatic access to the equivalent EWF diploma. The diploma was presented by Tim Jessop, Chief Executive of TWI Certification Ltd to Allan J Saunders, a project construction engineer from County Durham at a brief ceremony held at TWI North in Middlesbrough. Tony Gillam of Markey Project Support, Mr Saunders's current employer, was also present.

In presenting the diploma, Tim Jessop praised Allan Saunders for his professional dedication and commitment to enhancing his qualifications.


 

TWI training courses add to Phillips's inspection capability

PPCoN's P-scan team getting the most from their training in Norway Photo by Mark Kirby

PPCoN's P-scan team getting the most from their training in Norway Photo by Mark Kirby

As part of an on-going programme of Installations Condition Assessment, Phillips Petroleum Company Norway (PPCoN) need high quality, reliable, repeatable and quantitative inspection information. In response, PPCoN have built up a team of highly skilled and experienced inspectors to perform automated ultrasonic testing (UT) on the offshore Ekofisk complex using Force Institute's 'P-scan' equipment.

Recent courses given by TWI have helped PPCoN's P-scan team greatly enhance their capability. The first course covered the automated UT of welds in duplex stainless steel and another course covered the ultrasonic time-of-flight diffraction (TOFD) and creeping wave techniques.

This know-how, together with previous operational experience, enables the PPCoN team to tackle most of the automated UT applications they are likely to encounter offshore. These courses emphasise 'hands-on' experience of data acquisition and its interpretation. Consequently, P-scan equipment from both PPCoN and TWI and some of TWI's welded test samples were transported to PPCoN's shore base in Tanager, Norway for the training. The courses were very interactive. Lectures were kept informal and delegate contribution was encouraged.

 


 

CSWIP recognises South African welding inspector qualifications

As a result of a recent agreement between TWI Certification Ltd and the South African Institute of Welding (SAIW), holders of SAIW welding inspector qualifications will be allowed exemption from parts of the CSWIP welding inspector examinations.

A session of CSWIP examinations has already been held in Johannesburg for SAIW certificate holders waiting to take advantage of the new arrangement.

 


 

AWS and TWI sign global agreement

The American Welding Society (AWS) and TWI have signed an agreement to provide an international training certification programme for personnel involved with non-destructive testing of welded fabrications and structures.

The programme, which will be managed by AWS as Certifying Body, is based on TWI's long-recognised CSWIP scheme which first developed in the UK in the late 1960s with the onset of North Sea oil and gas production.

For more information, please contact us.

For more information please email:


contactus@twi.co.uk