Issue 161 July/August 2009 TWI has recently finished a collaborative programme to develop thermal spray coatings for high temperature corrosion mitigation in biomass plants. The DTI/TSB funded HiCoat project (High Corrosion Resistant Coatings for Biomass Plant) involved a number of companies (TWI, Energy Power Resources, Talbotts, Monitor Coatings, Metallisation, Ecka Granules and ADAS) and evaluated the performance of thermal spray coatings combined with advanced sealant technologies in specific biomass environments. C hlorine-containing species within many biomass and waste fuels lead to corrosion and hence limit plant operating temperatures and efficiency, and discourage the use of these fuels. The project assessed several coatings based on superalloy derivatives and nanocrystalline/ amorphous materials which are resistant to high temperature corrosion, and can be deposited by affordable coating processes. These sprayed coatings were sealed with advanced ceramic-based materials to mitigate erosion and slagging. The developed coatings have enabled plant operators to double the lifetime of superheater tubes, minimise the number of unplanned shutdowns and lost days due to premature tube failures and extend their scheduled maintenance intervals, thus making significant financial savings whilst securing energy supplies. Further work is on-going within the Core Research Programme to develop coatings for more severe biomass and waste-to- energy applications. Partners are sought for joint industry and collaborative projects to develop and commercialise the coatings. For further information contact [email protected]Diary events September 2009 Joint TWI/University of Cambridge event Tue 29 Great Abington WJS event Orbital welding demonstration Wed 16 Great Abington October 2009 Railsafe2 European Seminar Wed 21 Great Abington Technical Group Meeting Pressure and Process Plant Thu 22 tbc Technical Group Meeting Structural Integrity w/b Mon 26 tbc November 2009 Technical Group Meeting Offshore oil and gas tbc Aberdeen Institute of Rail Welding - 16th Technical seminar Wed 11 London Seminar Don’t go bust: understand the metallurgy! Thu 12 Middlesbrough WJS Yonger Members’ Seminar Renewable energy for the future – can young engineers generate a solution? Sat 14 Birmingham AWFTE Conference Supporting the skills provider Thur 19 – Fri 20 Great Abington Annual Dinner Tue 24 London Workshops and seminars are recognised Continuous Professional Development events The magazine of TWI www.twi.co.uk e-mail:[email protected]Successful project on corrosion resistant coatings for biomass plant Typical corrosion observed on uncoated superheater tubes which can lead to wall thinning and tube failure within a matter of weeks or months in the most aggressive environments. A typical large scale biomass plant
Transcript
Issue 161 July/August 2009
TWI has recently finished a collaborative programme to develop thermal spray coatings for high temperature corrosion mitigation in biomass plants. The DTI/TSB funded HiCoat project (High Corrosion Resistant Coatings for Biomass Plant) involved a number of companies (TWI, Energy Power Resources, Talbotts, Monitor Coatings, Metallisation, Ecka Granules and ADAS) and evaluated the performance of thermal spray coatings combined with advanced sealant technologies in specific biomass environments.
Chlorine-containing species within many biomass and waste fuels
lead to corrosion and hence limit plant
operating temperatures and efficiency, and discourage the use of these fuels. The project assessed several coatings based on superalloy derivatives and nanocrystalline/amorphous materials which are resistant to high temperature corrosion, and can be deposited by affordable coating processes. These sprayed coatings were sealed with advanced ceramic-based materials to mitigate erosion and slagging.
The developed coatings have enabled plant operators to double the lifetime of superheater tubes, minimise the number of unplanned shutdowns and lost days due to premature tube failures and extend their scheduled maintenance intervals, thus making significant financial savings whilst securing energy supplies.
Further work is on-going within the Core Research Programme to develop coatings for more severe biomass and waste-to-energy applications. Partners are sought for joint industry and collaborative projects to develop and commercialise the coatings.
Joint TWI/University of Cambridge event Tue 29 Great Abington
WJS event Orbital welding demonstration Wed 16 Great Abington
October 2009
Railsafe2 European Seminar Wed 21 Great Abington
Technical Group Meeting Pressure and Process Plant Thu 22 tbc
Technical Group Meeting Structural Integrity w/b Mon 26 tbc
November 2009
Technical Group Meeting Offshore oil and gas tbc Aberdeen
Institute of Rail Welding - 16th Technical seminar Wed 11 London
Seminar Don’t go bust: understand the metallurgy! Thu 12 Middlesbrough
WJS Yonger Members’ Seminar Renewable energy for the future – can young engineers generate a solution? Sat 14 Birmingham
AWFTE Conference Supporting the skills provider Thur 19 – Fri 20 Great Abington
Annual Dinner Tue 24 London
Workshops and seminars are recognised Continuous
Professional Development events
T h e m a g a z i n e o f T W I
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Successful project on corrosion resistant coatings for biomass plant
Typical corrosion observed on uncoated superheater tubes which can lead to wall thinning and tube failure within a matter of weeks or months in the most aggressive environments.
A typical large scale biomass plant
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July/August 2009
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A novel underwater digital radiography inspection system
New Members of TWI
TWI is pleased to welcome the following as Industrial Members.
Arc Energy Resources Ltd UK Specialist welding for the oil, gas and petrochemical industry
Azzawiya Oil Refining Company Inc Libya Crude oil refining
Brecknell Willis & Co Ltd UK Electrif ication/traction for all types of transportation systems
C.E.A. Towne (Ship Riggers) Ltd UK Manufacture, supply, testing, inspection and repair of lifting equipment
CWT Limited UK Mechanical engineering
Dickinson Legg Ltd UK Design and manufacture of tobacco processing equipment
JV Eaststroy LLC Russia Engineering, materials procurement, construction and commissioning
National Aerospace Laboratory NLR The Netherlands Consultant and research laboratory for aerospace industries
Neptune Marine Services Ltd Australia Subsea inspection and engineering
Nuvia Limited UK Nuclear specialist
Santos Ltd Australia Oil and gas exploration and production
TNB Research Sdn Bhd Malaysia Research laboratory services and quality assurance
Flexible risers are typically used for transporting oil from the sea bed to
offshore platforms and Floating Production Storage and Offloading (FPSO) units but there is currently no method of examining in situ, the cross section of underwater risers and flow lines to ensure their continued reliable high-integrity operation.
TWI is leading a consortium, known as FlexiRiserTest, which is developing a prototype system for the inspection of flexible risers. The prototype uses an external gamma radiography technique comprising an oppositely positioned gamma source and marinised digital flat panel detector to image the internal walls of flexible risers. Existing radiography-based inspection of underwater structure uses film or phosphor plates which have to be returned to the surface for development and processing.
It is believed to be the first time that a digital flat panel detector has been used for underwater radiography inspection. The application of a marinised digital detector allows for rapid production of radiographic images of the riser, which can be instantly relayed via an umbilical to a host computer at the FPSO.
A robot has been developed for deployment of the radiography based
inspection technique. The robot is capable of crawling along the external surface of the flexible riser, and is also able to rotate 360° about the riser axis so that the riser can be completely imaged. Since this new inspection technique facilitates the acquisition of a significant amount of images, Automated Defect Recognition (ADR) algorithms have been developed to help reduce the number of images presented to the operator. Specifically, ADR has been demonstrated to detect broken tensile wires in flexible risers. The inspection technology has been tested to a depth of 20 metres, but the partners in the project are confident that much greater depths would be possible with further development.
The work is as a result of a two-year European Framework 6 part funded project, shared between nine organisations. The marinised detector technology is deployable now and can be used in underwater inspection solutions for TWI Member companies. Future work includes more underwater trials including sea trials and further development to achieve deployment at greater depth.
Diver verifying the setup of the radiography inspection system prototype
Marinised digital radiography detector and control unit
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July/August 2009
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Development of the friction stir welding process has just literally turned a corner. In a radical adaptation of the process FSW has been used to perform right angled inside corner joints.
The process now opens the door to specialist fabricators using T, L, and V sections, external
and internal angle joints and even round sections. The technique uses much of the recently developed stationary shoulder friction stir welding technology (SSFSW). It has been adapted to penetrate 6mm into an inside corner joint in an aluminium alloy.
In SSFSW, a pin rotates through a non-rotating shoulder which slides along the joint. This technique originally evolved as a means of welding high temperature low conductivity materials and it became clear during development that it could be adapted to corner welding. During corner welding the shoulder is profiled to match the joint being welded.
Recently Nippon Light Metal approached TWI to apply the SSFSW concept to their particular application of internal corner welding. Nippon Light Metal was granted a patent on the technique in January 2009 in Japan. However TWI continues to hold the rights to use it outside the Far East
and can publicise homegrown results with the agreement of NLM.
Since the internal angular join is an abrupt one it is by definition a stress concentrator. So the next task is to devise a way of forming a fillet in the capping bead.
Applications for the process are many and varied. Says TWI’s Jonathan Martin ‘Aluminium is used extensively in the transport industries and this technique offers the potential to extend the areas where FSW could be applied.’
Friction stir variant open new doors for transport applications
Metallography of heat-affectedzones in carbon-manganesesteelsQuality assurance in weldedfabricationVisual inspection of weldedjointsIntroduction to welding processes - 1 & 2Safe working with arc weldingSafe working with gas cuttingand welding
More to become available throughout 2009
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£15 each or £90 for the full set of 7 (exc VAT and P&P)Colour, 840 x 596mm (A1), laminated
TWI Technical Posters now sold on-line
TWI Training & Examination ServicesTel: +44 (0)1223 899500
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Technology Transfer
Stainless steels are ‘stainless’ ie are corrosion resistant, due to the presence of chromium in amounts greater than 12%, where it forms a passive film on the surface of the steel. Note that these stainless steels are not the ‘stainless steels’ that generally first spring to mind; the 18% Cr/8% Ni austenitic stainless steels of the Type 304 or Type 316 grades; but two separate groups of alloys with different mechanical and corrosion resistant properties.
The ferritic stainless steels contain up to some 27% chromium and are used in applications where good corrosion/oxidation resistance is required but in service loads are not excessive, eg flue gas ducting, vehicle exhausts, road and rail vehicles.
The martensitic grades contain up to 18% chromium and have better weldability and higher strengths than the ferritic grades. They are often found in creep service and in the oil and gas industries where they have good erosion and corrosion resistance.
Now for a little metallurgy! Chromium is an alloying element that promotes the formation of ferrite in steel; in the case of the ferritic stainless steels, this ferrite is the high temperature form known as delta-ferrite. Unlike the low alloy steels, therefore, this type of steel undergoes no phase changes as it cools from melting point down to room temperature; they cannot therefore be hardened by heat treatment and this has implications with respect to the properties of welded joints.
Carbon and nitrogen, however, are two elements that promote the formation of austenite so, as the percentage of carbon and/or nitrogen increases, the ferritic steel can be designed to transform, wholly or partially, to austenite before transforming back to ferrite. This series of phase changes are similar to those in a low alloy steel, enabling the steel to be hardened by producing martensite – the martensitic stainless steels. Compositions and typical properties of some of the alloys are given in Table 1.
There are a number of welding problems with the ferritic steels. Although they are not regarded as hardenable, small amounts of
martensite can form, resulting in a loss of ductility. In addition, if the steel is heated to a sufficiently high temperature, very rapid grain growth can occur, also resulting in a loss of ductility and toughness.
Although the ferritic steels contain only small amounts of carbon, on rapid cooling carbide precipitation at the grain boundaries can ‘sensitise’ the steel making it susceptible to inter-crystalline corrosion. When this is associated with a weld it is often known as weld decay. Developments in recent years of extra low carbon, titanium or niobium containing grades have, however, improved this situation.
The ferritic stainless steels are generally welded in thin sections. Most are less than 6mm in thickness where any loss of toughness is less significant. Most of the common arc welding processes are used although it is regarded as good practice to limit heat input with these steels to minimise grain growth (1kj/mm heat input and a maximum interpass temperature of 100 – 120°C is recommended) implying that the high deposition rate processes are inadvisable. Preheat is not required although it may be helpful when
Job Knowledge101 Welding of ferritic/
martensitic stainless steels
Table 1 Typical properties of ferritic and martensitic steels.
AISI Number Steel Type Chemical Composition (max %) Mechanical Properties( annealed cond; typical)
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Technology Transfer
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welding sections over, say, 10mm thick, where grain growth and welding restraint may result in cracking of the joint.
Welding consumables for the ferritic steels are generally of the austenitic type; type 309L (low carbon grade) is the most commonly used. This is to ensure that any dilution that occurs does not result in a low ductility austenitic/ferritic/martensitic weld metal micro-structure. However, provided care is taken to control dilution, types 308 and 316 may be used. Nickel based consumables may also be used and will result in better service performance where the component is thermally cycled. A matching filler metal is available for welding of Grade 409 steel, often used in vehicle exhaust systems.
Post weld heat treatment (PWHT) at around 620°C is rarely carried out although a reduction in residual stress will give an improved fatigue performance: nickel based fillers are a better choice in this context than the Cr/Ni austenitic consumables.
The martensitic grades are used in more challenging environments and, as the name suggests, present rather more problems than the ferritic steels. Both the higher carbon (>0.1%) and low carbon (<0.1%) versions, with a few exceptions, require preheat and PWHT to avoid weldment cracking problems and to provide a sufficiently tough and ductile joint.
Matching welding consumables are available for most grades so that corrosion resistance and mechanical properties can be matched to those of the parent metal. To reduce the risk of hydrogen induced cracking, low hydrogen welding processes are essential and preheat temperatures of 200 to 300°C are recommended. A weld that has been completely transformed to untempered martensite by allowing the joint to cool to room temperature can be
extremely brittle and great care is needed in handling to prevent brittle failure. In addition, such joints are sensitive to stress corrosion cracking even in a normal fabrication shop environment. It is highly advisable therefore to PWHT as soon as possible on completion of welding.
A conventional heat treatment cycle would be to cool the joint to below 100°C to ensure full transformation of the weld and HAZ to martensite, closely controlled heating to minimise stresses from temperature variations, PWHT at around 700°C for one to four hours and controlled cool to ambient.
A hydrogen release treatment from the preheat temperature, say 350°C for four hours, is unlikely to reduce the risk of cold cracking. If the steel is not allowed to cool to a sufficiently low temperature so that full transformation to martensite takes place then there will be austenite present during the hydrogen release treatment.
This austenite will retain hydrogen and may generate cracks when it transforms to martensite as the joint is cooled to ambient. If cold cracking is a real issue, even with good hydrogen control, then it may be necessary to PWHT directly from the preheat temperature, cool to ambient and repeat the PWHT to temper any martensite that was formed following the first cycle of PWHT.
Welding consumables matching the base metal composition are available for most of the martensitic stainless steels, often with small additions of nickel to ensure that no ferrite is formed in the weld. Nickel
lowers the temperature at which martensite transforms to austenite so it is important with such filler metals that the PWHT temperature is not allowed to exceed about 750°C otherwise untempered martensite will form in the weld as the item cools to ambient.
Conventionally, when welding dissimilar metal joints the filler metal is selected to match the composition of the lower alloyed steel. Experience has shown that this can cause cold cracking problems so filler metals matching the martensitic steel should be used. An alternative is to weld with austenitic stainless steel fillers, type 309 for example, but the weld may then not match the tensile strength of the ferritic steel and this must be recognised in the design of the weld. Nickel based alloys may also be used; alloy 625 for instance, has a 0.2% proof strength of around 450MPa; and will give a better match on coefficient of thermal expansion.
The metallurgy of these types of steels is complex and they are frequently used in challenging and safety related environments. An article such as this can only give a partial picture so if there are any doubts surrounding their fabrication it is recommended that advice is sought from suitable specialists.
This article was written by Gene Mathers.
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July/August 2009
In response to positive feedback from students of earlier friction stir
welding courses another opportunity to learn the pros and cons of FSW is planned for the Autumn.
Since its invention in the early nineties, when it was little more than a laboratory curiosity, to its present position as a major manufacturing process, friction stir has played a key role in manufacturing for more than a decade.
World class makers of trains, planes and automobiles have all benefitted by adopting it. It requires no consumables. It’s solid phase. And it possesses great repeatability and mechanical properties.
The course, scheduled for 20-22 October 2009, will feature a mixture of classroom lectures, tutorials and practical demonstrations on TWI’s equipment. There will also be opportunities for individuals to meet TWI engineers.
The course agenda will include the process’s history, licensing, patents and
standards, process fundamentals, process advantages and disadvantages, process control, comparison with other processes, machine technology, tool technology, materials and weld performance issues, quality control, economic benefits, current/planned applications.
By the end of the course, attendees will:
• understand current FSW technology
• be able to assess FSW compared to other joining processes
• have a practical appreciation of different types of FSW equipment and fixturing
• understand FSW weld parameters/programs and their influence on weld quality
• appreciate relevant quality control techniques related to FSW
The course is aimed at technicians, engineers and senior managers
involved in fabricating metal components, and with an interest in friction stir welding.
To maintain quality standards, comprehension and manageable group sizes, numbers will be strictly limited to 12 students. A comprehensive set of notes will be provided to all attendees.
For more information and details of how to register, log on to www.twitraining.com
Returning by popular demand ...friction stir welding training
The WJS hosted a very successful Materials Technical
Group meeting on 2 June 2009 which captured both the historical and present issues on the subject of surfacing and particulate engineering.
Presentations were delivered by key industry speakers and there was also a tour of TWI’s surfacing
facilities and opportunities for attendees to network and take part in discussions. An introduction to the new MI21 metals database by information services staff was also given.
For further information on future meetings of the WJS Materials Technical Group, visit the TWI website www.twiprofessional.com
Surfacing and particulate engineering
July/August 2009
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News in brief
Steel and Aluminium: how can we cut carbon emissions to 50% by 2050?The reduction of carbon emissions is a topic of interest to all TWI Industrial Members. For this reason TWI is working with the University of Cambridge and a number of major industrial partners in a project with this aim. Dr Julian Allwood at the University of Cambridge has been awarded a £1.5m Leadership Fellowship by the UK government to lead a five-year project, Well Met 2050, to explore the full range of options for reducing the climate impact of steel and aluminium production and usage.
On 29 September 2009 TWI will be holding a one-day conference to communicate the range of options
that could be pursued, illustrated with numerous industrial presentations of emerging business opportunities.
If you would like to be kept informed of details for the conference at TWI on 29 September, please contact [email protected].
A survey of ‘Failure Incidents Experienced in P91/92 components’Since Grade P91 and P92 steel was introduced in the 1980s as creep strength enhanced ferritic steel, it has been used extensively in high temperature headers and steam piping systems in power and steam generating plant. However, evidence from premature weld failures in P91 steel suggests that design standards and guidelines may be non-conservative for P91 welded pressure components. Incidences of cracking in P91/92 welds have been reported in components with service life significantly less than 100,000 hours leading to safety and reliability concerns worldwide.
As a result, ASME has now introduced a reduction factor of two to the allowable stress for P91 welded pipe. The application of this reduction factor has the effect of reducing the design life of existing P91 components by more than one order of magnitude.
This survey aims to set up a database capturing failure incidents experienced in P91/92 components and correlating their fabrication methods and service histories where possible to enable
plant operators to assess the risk of failure of their plant and make an appropriate decision to manage that risk of failure.
The survey should take no more than 5 - 10 minutes to complete and can be saved at any time, to be finished at a later date. If you do not wish to answer any questions, simply leave them blank. It is emphasised that this survey is confidential and you will be given access to the results if you provide a response to this survey.
ACFM training ... call in the seasoned professionalsTWI has been conducting Lloyd’s and CSWIP Levels 1 and 2 courses around the world since 1997. Courses are run routinely in the UK and Thailand and are also available at clients’ premises worldwide. In-house courses have been particularly popular in the United States.
ACFM is a modern and innovative inspection process, suitable for ferritic and non-ferritic metals, using non-contact methods. Exhaustive trials have proved that ACFM has as good a track record for detection defects as MPI and eddy current inspections.
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What non-destructive tests does ASME IX specify for welding procedure qualification test pieces?
What are the common properties of oxide dispersion strengthened (ODS) alloys?
Should I apply a sealer on thermal spray coatings?
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Issue 161 July/August 2009
A radical new advance in friction surfacing has been
invented at TWI.
For years friction surfacing has been used to apply a metal coating onto a metal substrate. TWI has now pioneered the deposition of metal coatings onto ceramic substrates, a process previously considered impossible.
An aluminium consumable some 3mm in diameter is rotated at many thousands of revolutions per minute while it is pressed against a wafer-thin ceramic substrate. Once the aluminium is known to be plastic, at approximately 300°C, the rotating consumable is traversed across a ceramic token under sustained pressure.
TWI has laid tracks of aluminium less than 50µm thickness onto an aluminium substrate. It is a solid phase process. No melting takes place, only plasticising of the material.
From examination of highly magnified cross sections of the joints between the aluminium and the ceramic there is some evidence of mechanical keying taking place. But from adhesion tests it is also evident that excellent bond strengths are achievable.
Although total failure eventually ensues, it occurs at the adhesive joint, rather than between the aluminium coating and the substrate.
‘Clearly, there are more questions than answers just now’ says project leader
Abbas Mirlashari. ‘What we can say for sure is that it works, and has enormous potential in several disparate technologies. The key thing to identify first is the range of consumables that you can put onto these substrates. We know for instance that it also works for aluminium nitride and silicon carbide substrates, and the technique can be used to lay copper tracks.’
Friction surfacing is a unique way of metallising ceramics. Potential uses include heat sinking applications on temperature sensitive substrates, and as tracks for providing electrical conductivity.
Future work at TWI will explore a range of deposition materials and substrates now possible. ‘The technology offers huge potential in thermal management for electronics, medical and automotive industries and we are keen to exploit this’ said Mirlashari.
To explore what the technique can achieve for your business, contact [email protected]
