An Innovative Friction Welding Platform for Creep Damage Assessment and Repair of Thermal Power Plant Components
Submitting Institution
Plymouth UniversityUnit of Assessment
General EngineeringSummary Impact Type
TechnologicalResearch Subject Area(s)
Mathematical Sciences: Applied Mathematics
Information and Computing Sciences: Artificial Intelligence and Image Processing
Engineering: Manufacturing Engineering
Summary of the impact
This case study deals with research undertaken at Plymouth University
leading to the development of an innovative friction stir welding process
(friction hydro-taper pillar processing, FHPP) and a bespoke welding
platform that improves the assessment and repair methodology for creep
damaged thermal power station components. This technology, developed in
collaboration with Nelson Mandela Metropolitan University and with
industry investment, enables power station engineers to extend the life of
power generating plant leading to multi-million pound cost savings (over
£66M in direct financial savings are demonstrated in this case) plus
significant safety and societal impacts. It has been patented in South
Africa and a spin-off company has been formed.
Please note that economic impact values were achieved in Rand (R) but are
expressed in £ and therefore worth less in £ today than during the period
when the stated impact was achieved.
Underpinning research
Friction stir welding (FSW) is a solid-state welding process with major
advantages in cost and performance, compared with fusion welding. However,
process parameters are chosen empirically and no direct measurement of
weld process parameters such as force and torque was available on
commercial welding platforms until recently. It has therefore been
difficult to assess optimum welding parameters in terms of residual
stresses, defects and fatigue performance or to make a priori
process-property-performance predictions. In order to introduce a new
repair technique into the power station industry a detailed research
understanding of the process-property-performance relationships is
required, and individual weld repair techniques require full
certification.
Process-property-performance relationships in FSW have been one of the
core research programmes of James (1996-to date Professor of Mechanical
Engineering) at Plymouth University over the last 14 years, involving a
substantial collaboration with Hattingh at Nelson Mandela Metropolitan
University, who led the FSW platform development, and with industrial
collaboration and funding from the Dutch steel and aluminium-making firm
Hoogovens, subsequently continued through Corus R&D, Rotherham when
Hoogovens was acquired by Corus. Latterly, partners from ESKOM, the South
African power utility, have been actively involved in the FSW work and in
associated neutron diffraction residual stress experiments led by James.
This research, in which Plymouth has made a major contribution to the
understanding of defects, process optimisation and residual stress (PhD
projects with Lombard and Bradley), has enabled moving away from an
empirical approach to choice of FS welding parameters, (James et al. 2003)
The research has resulted in a thorough understanding of the influences of
tool speed, feed rate and geometry on residual stresses, microstructure
and defects, and hence on mechanical and fatigue properties, which was
developed in collaboration with Hattingh over the period 2003-2012 [2-5].
One significant outcome from this work, from a jointly supervised PhD
(Blignault), was a unique technique for assessing optimum process
parameters via a graphical FSW interface, the force footprint diagram [2]
and the development of an instrumented FSW platform measuring forces,
torque and temperature. Extensive research into the primary influential
parameters on weld output properties as a function of tool geometry
(2005-2008 — Blignault, Lombard) further demonstrated that maximum force
on a tool during its rotation (the force footprint apogee) and its angular
rotation during welding captured aspects of the plastic deformation in the
stir zone which were fundamental to achieving a high performance,
defect-free weld (Blignault). This research showed that fatigue
performance and defect population in FS welds could be correlated with
frictional power and heat input into the welds (Lombard). This allowed a
priori prediction of optimised regimes of tool feed and rotational speed
in FS welding (Bradley, Lombard).
The complementary range of expertise contributed by the three partners in
this project was fundamental to taking research into platform development
for industry. James has driven the fundamental research insights, Hattingh
the platform design and development, and Newby/Doubell have provided a
direct link into the South African power utility, ESKOM, in the highly
important areas of stress analysis and welding (Doubell — Chief Welding
Engineer, ESKOM, Newby — Stress Consultant). Support was provided by ESKOM
to manufacture the prototype FTPP platform and to make the internal
business case for qualifying the machines for power station use.
References to the research
The following publications have all appeared in high quality journals and
have been through a peer review process. Plymouth staff are indicated in
bold.
1. James M N, Hattingh D G and Bradley G R (2003), Weld
tool travel speed effects on fatigue life of friction stir welds in 5083
aluminium, International Journal of Fatigue, 25
pp.1389-1398. 51 citations in Scopus at 31/7/13; journal impact factor
in 2012 1.976.
2. Hattingh D G, van Niekerk T I, Blignault C, Kruger G and James M N
(2004), Analysis of the FSW force footprint and its relationship with
process parameters to optimise weld performance and tool design, Invited
Paper (INVITED-2004-01), IIW Journal Welding in the World, 48
No. 1-2 pp.50-58. Journal of the International Institute of Welding;
non-members papers by invitation only.
3. Lombard H, Hattingh D G, Steuwer A and James M N
(2008), Optimising FSW process parameters to minimise defects and maximise
fatigue life in 5083-H321 aluminium alloy, Engineering Fracture
Mechanics 75 pp.341-354. 38 citations in Scopus at 31/7/13;
journal impact factor in 2012 1.413.
4. Hattingh D G, Bulbring D L H, Els-Botes A and James M N
(2011), Process Parameter Influence on Performance of Friction Taper Stud
Welds in AISI 4140 Steel, Materials and Design, 32,
pp.3421-3430. 4 citations in Scopus at 31/7/13; journal impact factor in
2012 2.913.
5. Blignault C, Hattingh D G and James M N (2011), Optimising
friction stir welding via statistical design of tool geometry and process
parameters, Journal of Materials Engineering and Performance, 21,
No. 6, pp.927-935. 5 citations at 31/7/13; journal impact factor in 2012
0.915.
6. Lombard H, Hattingh D G, Steuwer A and James M N (2009),
Effect of process parameters on the residual stresses in AA5083-H321
friction stir welds, Materials Science and Engineering A, 501
pp.119-124. 23 citations in Scopus at 31/7/13; journal impact factor in
2012 2.108.
Details of the impact
This case study describes the impact of James' fundamental research into
welding and residual stresses which enabled development of fundamental
insights into FSW, resultantly to development of the automated Friction
Hydro Pillar Processing (FHPP) by James' long-standing collaborator,
Hattingh (in conjunction with James), the technology's development and
patenting as WeldCore and early industrial application in collaboration
with the South African Power Untility, ESKOM. A spin-off company has been
formed to further develop the technology and apply across the globe.
Savings of more than £66M, in addition to significant process and societal
impacts have already been achieved.
This technology has been piloted in providing power station engineers
with evidence that secures confidence in life extension of the current
power generating plant. It has impacted on business performance by
allowing the postponement of major capital expenditure and a multi-million
pound cost saving. The underlying research provides the necessary direct
link between FHPP welding conditions, the service performance and residual
stresses; this enables welding to be performed on safety-critical power
plant components using an automated platform. Automated FHPP has been
termed Weldcore and provides structural information that was previously
unobtainable, which resultantly leads to longer service life of critical
structures due to improved monitoring; deferment of capital expenditure;
lower risk of catastrophic failure; and increased plant uptime, hence an
increased widespread operational profits [Source 5.1].
Weldcore allows cost-effective assessment and repair of creep exhaustion
in steam power plant components that would otherwise be difficult or
impossible to repair and to certify for continued safe operation. The
technology and the impact thereof has only been possible because of a
long-standing collaboration between Hattingh and James, instantiated by
sabbaticals, shorter professional visits, collaborative research projects
and joint publishing, allowing James' fundamental insights to be applied.
WeldCore was developed at NMMU and was awarded first prize in the South
African National Innovation Competition in August 2010. The process was
also awarded the prize for "research leading to innovation by a group" at
the South African National Science and Technology Forum awards in May
2011.
The underpinning research carried out by James on welding and residual
stresses facilitated focussed development of fundamental insights into FSW
and led to a number of collaborative strain scanning experiments with
James as PI. Accurate knowledge of weld-induced residual stress
distributions and their modification by heat treatment is fundamental to
the all-important certification of new welding processes in the power
generation industry. James has taken a leading role applying neutron
and synchrotron diffraction techniques to steam power plant via peer
reviewed experiments [5.2]. Making the weld certification case for
incorporation of the FHPP into power plant repair would not have been
possible without the detailed knowledge of residual stress fields afforded
by neutron and synchrotron diffraction experiments [5.3, 5.4]. Equally,
the process has to be controlled to deliver specific and reliable outcomes
in terms of microstructure, defects and residual stresses, which would not
have been possible without the type of in-depth knowledge and
understanding of process-property-performance linkages provided by the
research.
One example concerns blade attachment holes in the steam turbine rotor
discs of Hendrina Power Station in South Africa where original equipment
manufacturers (OEM) life calculations led to a replacement recommendation.
Turbine component design is complex and historically the industry follows
OEM replacement recommendations without testing true life exhaustion of
components with complicated geometries.
Testing the WeldCore FHPP platform for creep assessment and repair on
Unit 6 at Hendrina Power Station in 2011, showed that the creep life of
the high pressure turbine was less than 50% exhausted. This was in
contrast to the OEM recommendations to urgently replace the turbines on
all ten units after their calculations indicated creep exhaustion levels
of >>100% at the unit life (270-300,000h of operation). Unit 6 was
returned to service without further outage delay or operation with a
reduced output. To meet the OEM's recommendations the alternative would be
to remove two stages of blades and run with reduced output until a
replacement turbine could be manufactured (2 years) and then enter into a
long replacement outage again (an additional 80 days [5.3]). The work on
unit 6 demonstrated that the scheduled replacement of the turbines for all
ten Units at Hendrina Power Station (with a cost of over £6.5M per unit)
could therefore be delayed until the decommissioning date of the Power
Station. This condition monitoring and life extension of the discs saved
the power utility some £65M in direct replacement costs and an extended
outage period [5.5]. Aside from the significant cost savings made,
the avoidance of any outage is particularly pertinent for ESKOM as whilst
"the international norm for spinning reserve is 15% ... Eskom currently
has on average 3% ... Any loss of generating capacity increases the risk
of load shedding (blackouts)" [5.3]. The extended outage period
avoided has been estimated as at least 8 weeks [5.3]. The work by Plymouth
on the performance-processing-weld parameters in FHPP was fundamental to
the certification and to the parametric design of the welding platform.
To maximise the impact of the research, James has also provided ESKOM
with CPD short courses on failure analysis. During the most recent course
in 2011, 33 mechanical and materials engineers from ESKOM's Research,
Testing and Development department attended. James delivered this training
at below market rates (R40000 paid rather than estimated market value of
R132000 [5.3, 5.4]) as part of the technology transfer process. ESKOM
clearly regard the training as important stating: "failure analysis
knowledge is critical for engineers operating in our environment"
[5.3]
Since initial use on the turbine blades WeldCore has also been applied to
two main steam pipework applications at Lethoabo Power Station and a main
steam valve inlet pipe at Kendal Power Station. At Lethoabo, application
of the technique (in 2012) proved that the components had to be replaced
(total cost = R318m (~£19.8m)) in order to prevent a major safety incident
of these safety critical systems. ESKOM views safe operation as extremely
important [5.3] and in early 2013 at Kendal, WeldCore proved that the
serviceable life of the steam valve inlet pipe could be extended, thus
saving ESKOM a further R16m (~£1m) in parts/down time/etc. costs. [5.3]
Now that initial technology transfer and development work has been
completed a spin-off company (MantaCor (Pty) Ltd) was registered on 28
March 2011 and has been assigned the rights to conduct commercial
activities to develop and market the machines on a commercial scale. As a
result of the early commercial work two further commercial projects with a
combined value of £100k have been completed outside the scope of that
taken on for ESKOM and resultantly, a systems engineer, a process engineer
and two technicians are employed in South Africa [5.4. 5.6, 5.7].
Sources to corroborate the impact
[5.1] Eskom internal intelligence brief "INTELLIGENCE BRIEF —
QUANTIFICATION OF CREEP EXHAUSTION IN TURBINE ROTORS" RTD/MAT/13/172.
[5.2] Experiments: 1-01-8, 1-01-58, 1-01-73, 1-02-83, RB720574, RB910338
(2008-2011) https://club.ill.fr/cv/servlet/ReportFind.
[5.3] Stress Consultant, ESKOM, Research Testing and Development, Lower
Germiston Road Private Bag 40175, Cleveland, 2022 SA.
[5.4] Director of MantaCor/Professor of Mechanical Engineering/Director
of eNtsa, Nelson Mandela Metropolitan University, Summerstrand Campus
(North), P.O. Box 77000, Port Elizabeth, 6031, Tel: 041 504 3608, Fax: 041
504 9123
[5.5] Technology Strategy and Research Manager (acting), ESKOM,
Sustanability Division, Research Testing and Development, Lower Germiston
Road Private Bag 40175, Cleveland, 2022 SA.
[5.6] Director: Innovation and Technology Transfer, Summerstrand Campus
South, NMMU.
[5.7] Director: Research Management, Summerstrand Campus South, NMMU.