P91 & P92 Inspection, Quality Checks, Integrity/ Life Assessment

Challenges

Some of the biggest challenges facing P91 users worldwide are: • Minimising the risk of premature failure of P91 welded components operating under creep conditions due to

accumulation of damage in the fine-grained region - or Type IV position - of the heat-affected zone (FGHAZ).

• Dealing with P91 components/welds with abnormal or aberrant (i.e. non-martensitic) microstructures and sub-standard rupture strength.

• Thermal fatigue or creep-fatigue cracking in thick-section components. • Steam-side oxidation leading to overheating and premature rupture of thin-walled tubes.

ETD has unrivalled knowledge and experience of the potential issues. The company has developed and offers an arsenal of techniques for inspection, monitoring and life assessment that can help P91 users to maintain safe and reliable operation of their plant.

Non–Destructive Examination/Inspection and Life/Condition Assessment • ETD’s site services team provides a comprehensive range of inspection techniques for detection and monitoring of

creep damage and other forms of degradation or cracking. As well as conventional NDE techniques, ETD uses ToFD and Phased Array ultrasonic techniques for detection and sizing of defects, together with metallographic replication for monitoring creep damage accumulation and portable hardness testing for monitoring degradation of mechanical strength.

• Type IV creep damage at butt welds in P91 piping tends to be less apparent at the surface of the component until late in life, which may restrict the usefulness of replication. Therefore, ETD uses its unique Scanning Force Microscope (SFM) to detect nanometre-scale creep cavities that appear at the surface earlier in life than the micron size cavities that can be detected by replication.

• Using ETD’s P91-specific damage assessment guidelines, the remaining creep life of components can be estimated using the on-site observations of creep damage.

• ETD can also carry out code-based calculations of creep life using either deterministic or probabilistic (another of ETD speciality) methodologies.

SFM in use for inspection of a high-temperature steam pipe

Miniature Sample Removal for Quality Checks and Life/Condition Assessment • ETD offers a non-destructive sample removal service using Electric Discharge Sampling Equipment (EDSE). Using the

EDSE, thin samples of material (typically 1-5mm thickness) can be removed from the component surface for analysis under laboratory conditions. Mechanical tests can be carried out using miniature specimens, such as small punch or miniature uniaxial creep tests, and thus remaining life assessment can be performed.

EDSE (left), mounted on a pipe (middle) and removed sample (right)

Please turn over

Miniature Sample Removal for Quality Checks and Life/Condition Assessment (Continued)

• At-site inspection using replication and hardness testing can identify P91 material or welds with abnormal microstructures and/or unacceptably low (or high) hardness levels, but only at the surface of the component, so the results of these at-site inspections can be affected by decarburisation and may not be representative of the bulk of the material.

• The EDSE has proven to be a particularly useful tool when verifying the material condition of P91 components in order to resolve material quality issues. Examination of microstructure and hardness changes through the thickness of the EDSE sample can differ- entiate between a surface effect and an abnormal material condition.

Microstructure and hardness results for EDSE sample from an ‘abnormal’ P91 component

Defect/Crack Assessment • If cracks are found by NDE, then ETD can carry out crack

growth and remaining life assessment using ETD’s ‘Crackfit’ defect assessment software. This tool contains a range of crack and component geometries so enables the analysis to be performed rapidly which is crucial when run- repair-replace decisions have to be made.

• ETD’s material property database includes crack growth properties for HAZ and weld metal as well as P91 base metal at different temperatures, so the specific properties for the actual crack location can be used, leading to more reliable life estimations.

Failure Assessment Diagram generated using ‘Crackfit’ for the case of creep crack growth analysis of a circumferential, semi-elliptical defect at a

weld in a P91 pipe

Root Cause Failure Analysis • ETD carries out metallurgical investigation and root cause analysis of failures and cracking in P91 components/welds. In

the recent past, ETD has investigated a range of failures of T91 tube components and welds, and cracking in P91 pipe and drain-line components in conventional boilers and HRSGs. In these investigations, ETD has identified the damage mechanism and the root cause of the damage, and thus remedial actions and solutions to the problem have been identified.

Thermal-mechanical fatigue cracking at superheater tube-to-header weld of a HRSG that was operating in two-shifting mode

Consultancy and Guidelines on Materials and Welding • ETD can provide P91 users with advice and guidance on the metallurgy and welding issues of grade 91 materials,

such as reviewing weld procedure specifications (WPS) for similar and dissimilar metal welds, inspection of NDE records, and guidelines for repair welding of P91.

International P91 Users Group • ETD formed the ‘P91UG’ for international industry in 2006. Members benefit from the industry experience and latest

R&D knowledge provided by ETD in Technical Review Reports (3 per year). The reports cover new information that becomes available from fabricators, and from plants / utilities on component cracking, welding, failure, oxidation, and repair or replacement issues worldwide.

For more information on our P91 Users Group, www.etd-consulting.com/p91-users-group

European Technology Development Ltd Fountain House, Cleeve Road, Leatherhead, Surrey, KT22 7LX, United Kingdom

Tel: +44 1372 363111 Fax: +44 1372 363222 www.etd-consulting.com enquiries@etd-consulting.com