LTS Technology Review

“The Role of Metallurgical Analysis in Gas Turbine Maintenance”

Frame 6 Users Group Conference– Vendor Presentations.

Randy ThompsonLiburdi Turbine Services

June 14, 2010

Presentation Outline•

Maintenance Considerations

•

Metallurgical Analysis•

Component Deterioration•

Base Alloy

•

External Coating / Surface Condition•

Internal Surfaces

•

Use of Metallurgical Assessment Findings•

Summary

Maintenance Considerations•

Component Life Cycle costs, next to fuel are the highest Gas Turbine O&M cost

•

Hot Section Components are the most costly

•

Single sets of components – blades/buckets or

vanes/nozzles can cost from $800K to $2Million per set

•

Risk of component failure increases as unit technology increases.

GER3620 “considerations”. Gas turbine maintenance, based on

independent counts of starts and hours

Maintenance Considerations•

Is the component reparable by conventional means?

•

What repairs should be completed for continued service?

•

Are there modifications which would be more suitable for the engine operating conditions (ie. coating change or addition)?

•

Are there any engine issues which should be known about and/or corrected?

•

Could the service interval have been extended?

•

Is my service interval going to change?•

How can we relate hours and starts to fundamental material changes and measurements of degradation?

7FA Transition post service interval.

7FA Stage 1 Nozzle post service interbal

Metallurgical Life Analysis

• Component history

• Future Conditions.

• NDE inspections

• Dimensional data.

• Destructive examination

• Sectioned in areas of interest

• Microscopic examination

• Chemical analysis

• Mechanical testing

Metallurgical Analysis

Metallurgical analysis can provide information useful to determine:

• Reparability and repair process definition (triage)

• Minimize risk of otherwise undetectable damage.

• Evaluation of coating performance/selection.

• Provide indications of abnormal/detrimental engine operating conditions.

• Assess for potential of service interval extension.

• Validation of repair process or design improvement.

Deterioration During Service•

Ideal Operating Conditions•

Thermal degradation –

base alloy, coatings etc.•

Thermal-mechanical fatigue damage•

Used to establish component life and service interval length.

•

Non-Ideal Condition•

Corrosive environment•

Over-firing or un-even thermal condition•

Foreign object damage•

Coating loss•

Deposits •

Rub•

Dynamics•

Trips

BASE ALLOY DETERIORATION

Nickel-Based Superalloys•

Superalloys

used in

turbine blades are strengthened primarily by the precipitation of γ’-Ni3 Al phase throughout the austenite matrix

•

Maximum strength is developed at an optimum γ’

size

Base Metal –

Alloy Aging

•

During service, γ’

precipitates change.•

γ’

spheriodize, coarsen and agglomerate.

•

Degradation function of time and temperature.

•

Results in reduction in material strength

a) New Condition b) Moderately Degraded c) Severely Degraded

NEW Time & Temperature

Determination of Creep/Rupture Properties of Engine Run Blade

Base Metal –

TCP Formation•

Formation of Topologically Close Packed phases can occur during service.

•

Function of time, temperature and alloy stability.

•

Reduces impact properties and reduction in creep strength.

TCP Phases

•

Aging of Nimonic 263 can result in eta (η

Ni3

Ti) phase formation

as a result of a relatively high titanium to aluminium ratio in this alloy

Nimonic 263 alloy from a relatively cool region of a DLN2.6 liner after 8000 hours of service

Nimonic 263 alloy from a relatively hot region of a DLN2.6 liner

after approximately 8000 hours of service. Note acicular eta phase throughout the microstructure

Base Metal –

Brittle Phase Formation

Base Metal –

Creep Damage

•

Due to elevated temperature and centrifugal loading, turbine blades undergo creep deformation.

•

Diffusion-based mechanism correlated to time, temperature and stress.

•

Eventually forms creep voids and can result in blade failure.

Creep Voids

7FA Stage 2 Bucket –

stress analysis.

Repair of Base Metal Damage•

Both microstructural

and creep damage can be

repaired with the appropriate heat treatments.•

Creep damage requires Hot Isostatic

Pressing (HIP)

•

Microstructural

degradation requires super-solvus

heat treatment.

• Knowledge of the material condition critical to:

•

Ensure the appropriate heat treatments are applied.•

Verify the effectiveness of heat treatments

•

Minimize repair costs.

Thermal-Mechanical Fatigue Damage

•

Repeated, rapid changes in temperature or steady- state differences can result in TMF cracking.

•

Life limiting for high engine start applications.•

If initiating internally, no detection method

Internal Surface

External Surface

TMF Cracks

Coating Deterioration

• Protective nature of coating-

based upon β-aluminide

remaining.

• Function of time at temperature with contribution from starts/stops.

Minimal Depletion Partially Depleted Depleted and Oxidized

Coating Deterioration•

Coating breach occurs by either (a) complete coating depletion or (b) thermal-

mechanical coating cracks or (c) foreign object damage (FOD)

• Deemed as coating failure when irreparable damage occurs to base alloy or damage increases risk of part failure.

Base Alloy Oxidation

Coating Deterioration or Loss

• Abnormal Oxidation damage occurs as a result of coating loss.

Base Alloy Oxidation7FA Row 1 Bucket post service interval

As Deposited Combustion Coating System

Coating Deterioration (TGO) in Combustion Coating System

•

Local Oxidation present on many combustion components.

•

Scaling and effective thickness reduction can be found.

•

Aluminide

coating of the surface arrests this affect potentially improving component life.

Diffused LSR Coating on N263

Diffused layer

Additive layer

Combustion Aluminide Applications

Coating –

Aluminide

Called to Duty

Coating/Blade Deterioration in Corrosive Environments

• Hot corrosion is an accelerated attack to the coating and base metal.

• Corrosive environments:

•

Sulphur

combined with either sodium or potassium

•

Vanadium or lead

• Corrosive elements interfere with formation of protective oxide layer –

accelerate attack.

Coating/Blade Deterioration in Corrosive Environments

• Metallurgical analysis can identify the type of hot corrosion and the corrosive elements present.

• Elimination of the corrosive elements the ideal source to prevent future corrosion.

• Less ideal option is to apply a coating to provide corrosion protection.

• Identify reparability of blade set.

INTERNAL SURFACE CONDITION

Internal Damage•

Most internal damage cannot be repaired.

• Decision to continue using a blade set based upon tolerating damage.

• The majority of internal damage cannot be detected by NDE methods.

• Destructive metallurgical analysis is necessary to determine extent of internal damage and assess if safe to continue using.

Uncoated Internal Surfaces

•

Surface Oxidation and Alloy Depletion•

Consumption of base alloy strengthening elements (Al, Ti)•

Decreases fatigue strength and effective wall thickness.•

Intergranular

Oxidation (IGO)•

Preferential oxidation along grain boundaries.•

Results in brittle phase. Detrimental for thermal-mechanical cracking.

•

Assessing and monitoring internal damage critical to manage risk.

Surface Oxidation and Alloy Depletion

Intergranular Oxidation (IGO)

Internally Coated Surfaces

• Same degradation mechanism as external coatings.

•

Consumption of aluminium content.•

Function of time and temperature.

• Determine if replacement is required.

Hot Corrosion on Internal Surfaces•

Type II (low temperature) hot corrosion

•

Corrosive agents accelerate coating/base metal attack.

• Internal coating provides minimal corrosion resistance.

• Assess reparability based upon extent of damage.

• To prevent corrosion, must eliminate source of corrosive agents.

•

Corrosive agents generally from the cooling air off-take.

•

May indicate filtration inadequacies.

USE OF METALLURGICAL ASSESSMENT FINDINGS

Repair Decisions and Development•

Metallurgical assessment is the most thorough method to characterize deterioration.

–

Measuring the unknown factors.

• Information can be used to:

•

Identify the reparability of the blade set.•

Identify scope of repairs.•

Determination of coating utilized.•

Determine if coating change would be beneficial.•

Determine if “what you see is what you get”.•

Design modifications such as cooling changes. •

Determine life optimization boundaries.

Liburdi Turbine Services Technology

•

Various conditions can exist within a single section depending on the design and operating factors.

•

Knowledgeable and consistent sampling over time generates significant data.

•

Good decisions can be formed from solid analysis.

Repair Decisions and Development

Risk Management•

Metallurgical analysis can minimise the risk of damage undetectable by typical NDE methods.

7FA Combustion Liner at Borescope

Risk Management•

Previous metallurgical data indicated bond coating provides protection and oxidation levels would not cause excessive damage given the predicted time left before removal.

Previous 7FA Liner Sectioned at Damaged Zone.

APS MCrAlY

bond coat

Engine Operating Indicators•

Microstructural

degradation provides a thermal

map of the part in services.

• Once benchmarked, can be used to detect abnormal engine operating conditions: Short term/excessive overheating, long-term overheating, temperature profile shift.

• If limited-to-moderate base alloy damage has occurred, there may be the potential for a service interval extension.

Summary•

Metallurgical assessment is the most thorough method for characterizing component deterioration.

• Information can be used to:•

Identify the reparability of the components.

•

Identify scope of repairs.•

Determine if modifications would be beneficial.

• Provides key information that is normally hidden to assist in financial decisions.

• Metallurgical assessment plays a valuable role in gas turbine maintenance