GAS TURBINE HOT PATH MATERIALS

HOT GAS PATH HOT GAS PATH COMPONENTS COMPONENTS

JOSEPH BASTIANJOSEPH BASTIANRONY JOHNSONRONY JOHNSONM S D KURUPM S D KURUP

TOPICS OF DISCUSSIONTOPICS OF DISCUSSION

INTRODUCTIONINTRODUCTION

HOT PATH MATERIALSHOT PATH MATERIALS

COOLING TECHNIQUESCOOLING TECHNIQUES

COATINGCOATING

Pressure and Temperature

HOT GAS PATH COMPONENTSHOT GAS PATH COMPONENTS

CombustorsCombustors

Transition PiecesTransition Pieces

Stationary VanesStationary Vanes

Rotating BladesRotating Blades

GAS TURBINES & CC PLANTS EFFICIENCY TREND

EFFICIENCY OF CC PLANTS HAS FOLLOWED TIT

Turbine blades in a gas turbine Turbine blades in a gas turbine engine experience:engine experience:

Mechanical forcesMechanical forces• • creepcreep• • fatiguefatigue• • thermo-mechanical fatiguethermo-mechanical fatigue

••High temperature environmentHigh temperature environment• • oxidationoxidation• • hot corrosionhot corrosion

FAILURE MODES OF HIGH TEMP. FAILURE MODES OF HIGH TEMP. MATERIALSMATERIALS

CREEP -CREEP - Time dependent thermally activated Time dependent thermally activated inelastic deformation of material. For constant stress, inelastic deformation of material. For constant stress, the rate of creep increases with increase in the rate of creep increases with increase in temperature.temperature.

HIGH CYCLE FATIGUE (HCF)HIGH CYCLE FATIGUE (HCF) - Microstructural - Microstructural damage that results from small stress amplitude damage that results from small stress amplitude cyclic loading, such as vibration. Failure occurs after cyclic loading, such as vibration. Failure occurs after a relatively large number of cycles.a relatively large number of cycles.

LOW CYCLE FATIGUE (LCF)LOW CYCLE FATIGUE (LCF) - Microstructural - Microstructural damage that results from large stress amplitude damage that results from large stress amplitude cyclic loading, such as thermal stresses induced by cyclic loading, such as thermal stresses induced by start/ stops and tripping of machines. Failure occurs start/ stops and tripping of machines. Failure occurs after a relatively small number of cycles.after a relatively small number of cycles.

FAILURE MODES OF HIGH TEMP. FAILURE MODES OF HIGH TEMP. MATERIALSMATERIALS

HOT CORROSION - HOT CORROSION - Electrochemical reaction Electrochemical reaction between substrate and molten salts, typically sodium between substrate and molten salts, typically sodium and potassium sulfates. and potassium sulfates.

HIGH TEMP. OXIDATION -HIGH TEMP. OXIDATION - Chemical reaction that Chemical reaction that produces oxide(s) of constituents within the solid. It produces oxide(s) of constituents within the solid. It increases exponentially with temp. increases exponentially with temp.

EVOLUTION OF GAS TURBINE BLADE MATERIALS

700 Celsiu

s

950 Celsiu

s

FIRING TEMP. TREND AND MATERIAL CAPABILITY1300 Celsiu

s

950 Celsiu

s

Coating – Protective + TBC

High temperature High temperature environmentenvironment• • oxidationoxidation• • hot corrosionhot corrosion

HOT PATH COMPONENTS

MATERIAL

COOLING

COATINGS

HOT PATH MATERIALSHOT PATH MATERIALS

Making Matter

-Only 100+ number of atoms exists

- Same atoms are used again and again .

-Structure or arrangement of atoms in matter determines not only the appearances but the properties also

- Materials can be broadly classified into

-METALS

- POLYMERS

-CERAMICS

- COMPOSITES

Material Structures

• SHORT RANGE AND LONG RANGE ORDERING

Crystals Glasses

Basic Structures

• BCC

• FCC

• CPH

BCC

FCC

CPH

Alloys

An alloy is a phase comprising of one or more components

• Interstitial : Solute does not occupy the site in the lattice but resides in crystallographic pores

• Substitutional : Solute substitutes the solvent in the crystal lattice without structural changes

• Transformational : A completely new lattice is formed . Usually occurs as a result of intermetallic compound formation.

Home Rothery Rule 1

• Extensive substitutional solid solution occurs only if the relative difference between the atomic diameters (radii) of the two species isless than 15%.

• If the difference > 15%, the solubility is limited.

• More the difference the possibility of interstitial compound is more

Super Alloys

• Superalloys is a name for a group of alloys that retain high strength at elevated temperatures.

• Most of the superalloys are Substitution solutions, where one of the components tends to form covalent bonds (Al in Ni3Al,W in Ta or Nb).

Ni Based Super AlloysMicro Structures

• The turbine blade is made out of a nickel-base superalloy with a microstructure containing about 65%- 70% of gamma-prime precipitates in a single-crystal gamma matrix.

The γ on the left has a random distribution of Ni, Al and Ti atoms, whereas the γ' on the right has its nickel atoms located at the face-centres

Alloy Composition

• Chromium and aluminium are essential for oxidation resistance small quantities of yttrium help the oxide scale to cohere to the substrate.

• Grain boundary strengthening elements such as boron and zirconium segregate to the boundaries. The resulting reduction in grain boundary energy results better creep strength and ductility.

• There are also the carbide formers (C, Cr, Mo, W, C, Nb,

Ta, Ti and Hf). The carbides tend to precipitate at grain boundaries and hence reduce the tendency for grain boundary sliding.

Alloy Compositions

•Elements such as cobalt, iron, chromium, niobium, tantalum, molybdenum, tungsten, vanadium, titanium and aluminium are also solid-solution strengtheners, both in γ and γ'.

•There are, naturally, limits to the concentrations that can be added without inducing precipitation. It is particularly important to avoid certain embrittling phases such as Laves and Sigma.

Composition of alloys

Single Crystal , DS and Equiaxed

Single Crystals

No Grain boundaries .

Hence No diffusion through grain boundaries

Grain boundary forming materials are avoided. Increase in localised melting temperature

Directionally solidified

Grain Boundaries are in parallel to the major stress axis

Equiaxed

Has crystals of same axis length ( same size).

Has the worst creep resistance

Microstructure of Superalloy

Service Induced defects•Two types of service induced microstructural changes.

•Type I:

Breakdown of primary carbides into secondary carbides, i.e., MC + M23C6 + ' (' envelopes the M23C6).

Coarsening of strengthening phase '.

Strength loss due to depletion of solution hardening elements.

Formation of embrittling TCP phases (, , P).

Formation of continuous g. b. M23C6 causing embrittlement.

–Type II:

Cavitations at grain boundaries.

Type I defectsCoarsening of Gamma Prime

Type I defectsCoarsening of Gamma Prime

NDE-Microstructural Damage Assessment of 1st Stage Bucket

• Continuous service exploitation for 48,000 EOH in GE frame-9E machine.

• Material: GTD-111, equiaxed, Tfiring = 1124oC.

• Non-destructive microstructural assessment for its suitability of rejuvenation.

LE-T LE-MBase

TE-M

LE-R

Base-rib

Oxidation spots

GTD-111GTD-111

Microstructure: ’ coarsening

• Coarsening/coalescence of ’.• Unusual degeneration at trailing edge is

caused by coating loss leading to excessive high base metal temperature.

Base LE-mid TE-mid

GTD-111GTD-111

Refurbishment

24000 h

rejuvenated

new

Type 2 defectsDegeneration of grain boundaries

Grain Boundary cavitations

Microstructure: Degeneration at Grain Boundaries (TE-M)

• Complete disappearance of grain boundary (marked by dotted lines) precipitates.

• Grain boundary cavitation with ’ normal to cavity. This can be removed by HIPping

GTD-111GTD-111

Improvement of cast superalloy turbine blade properties by hot isostatic pressing (HIPping)

• Cast alloys often contain pores, these aredetrimental to the mechanical properties

• HIP = simultaneous application of high temperature (up to 2000°C) and pressure (up to 200MPa) via inert (argon) gas

• HIPping can remove sealed porosity from castings (cast +HIP = forged) (NB the casting remains solid - you don’t want it to melt....)

• 90+% of high pressure turbine blades are HIPped

• Blades can be “rejuvenated” by HIPping

Improvement of cast superalloy turbine blade properties by hot isostatic pressing (HIPping)

To illustrate the effectiveness of the HIP process a 25mm diameter hole was machined in two halves of a stainless steel block 75mm square.

The edges of the block were welded together, the air evacuated from the hole and the evacuation pipe sealed to create a subsurface pore.The block was HIPped and subsequently cut in half to reveal fully dense material and complete absence of any pore.

HOT PATH COOLINGHOT PATH COOLING

FIRING TEMP. TREND AND MATERIAL CAPABILITY1300 Celsiu

s

950 Celsiu

s

EFFECT OF COOLING ON TIT

CFD in COOLING DESIGN

SINGLE AND MULTIPASS COOLING

X-ray Radioscopy

LE: Straight TE: Straight

Mid: Corrugated

• 11 cooling holes.

• At LE the straight hole has largest diameter without any discernible defect.

• At TE two straight holes have least diameter, at the edge while next one has larger diameter.

• Rest of the eight cooling holes in the middle of the airfoil have intermediate diameter but with corrugation.

• Corrugationeffective heat transfer. GTD-111GTD-111

Bucket Top

CLOSED LOOP STEAM COOLING

Cooling by air detrimental to cycle efficiency because of irreversible pressure losses, reduction in gas path temp., and internal losses.

Closed loop steam cooling by convective heat transfer avoids losses and yields a 2 % increase in Efficiency.

In a Combined cycle plant with G/ H-class Gas Turbines, steam cooling is used as parallel IP reheater for the bottoming cycle.

9FA and 9 H – A Comparison

Source: General Electric (GER 3935B)

HOT PATHHOT PATH COATING COATING SYSTEMSSYSTEMS

Protective CoatingsProtective CoatingsThermal Barrier CoatingsThermal Barrier Coatings

IN SERVICE DEGRADATION AND COATINGS

DEGRADATION PROCESSES: • Hot Corrosion - Caused by combination of

oxidation & sulfidation of material in contaminated environments by deposition of corrosive salts.

• Oxidation

PURPOSE OF COATINGS: To limit in service degradation of hot gas path components caused by high temperature and severe environmental conditions.

TYPES OF HOT TEMPERATURE ATTACKS

690 860>900

HOT CORROSION TYPE I or High Temperature Hot Corrosion

- Known since 1950s, takes place at 810 0C- 927 0C.

- Chiefly caused by deposition of Na3Fe(SO4)3 (mp= 860 0C).

- Molten sodium sulfate gets deposited on surface of components.

- Appears as: oxide scales, internal sulfidation, and depletion of oxides of Al & Cr from surface.

- Protective Alumina scales offer best protection. TYPE II (Low Temperature Hot Corrosion): - Recognized first in 1970s; Occurs at 595-760 0C.

- Caused by deposition of K2SO4/ Na2SO4 and V2O5 (tm= 690 0C). - Characterized by localized pitting form of corrosion attack. - Coatings forming scales rich in chromium offer best protection. Hot corrosion can be minimized by Stringent Fuel Quality & Air

Filtration, Use of material exhibiting good corrosion resistance and Use of Protective Coating.

HIGH TEMPERATURE OXIDATION

Superalloys exhibit inherent resistance to oxidation up to 9000 C.

Above 9000 C, rapid oxidation of constituents takes place in absence of effective barrier to oxygen on the metal surface.

Oxidation results in a continuous loss of oxide forming elements in the base metal adjacent to surface. It appears as visibly observable changes in microstructure.

If aluminum content in the material is high, a dense protective and adherent layer of Aluminum Oxide (Al2O3) scale, formed in early

stages of service, provides protection against oxidation.

For many superalloys Aluminum content can not be increased due to other considerations like high strength and metallurgical stability.

TYPES OF HOT TEMPERATURE ATTACKS

690 860>900

PROTECTIVE COATINGS - PURPOSE

Hot gas path components of Industrial Gas Turbines operate in a very corrosive gas path environment.

Degradation rate is crucial to the performance and longevity of gas turbine.

Role of hot section Coatings is to protect the substrate from the corrosive gas environment.

Coatings are designed to protect the alloy from high temperature oxidation, and hot gas induced corrosion.

Coatings allow operation at even higher temperature and use of a broader variety of fuels.

PROTECTIVE COATINGS DFFUSION COATINGS (Chromides or Aluminide): Suitable up to

8000C. Early gas turbines (before 1980s) were provided only with Diffusion Coatings.

In some of the current design gas turbines, Chromides/ Aluminide coatings find application in last stage bladings.

OVERLAY COATINGS: Overlay type MCrAlY (M-Co and /or Ni) coatings for higher Temperature applications.

Aluminum forms dense oxide layer on the coating surface that is thermally very stable. Other materials control Al activity, hold the oxide layer in place and adapt the coating to the base.

MCrAlY coatings are applied using the vacuum plasma spray process. Max. thickness = 0.40 mm; Life = 20-25K hours.

DUPLEX COATINGS: High Temp. Duplex coatings (GT-29 IN Plus, GT-33 IN Plus) provide protection against high temperature oxidation. In such coatings a diffusion aluminide top coat is provided over the MCrAlY Coating.

COATINGS

PERFORMANCE OF PROTECTIVE COATINGS

THERMAL BARRIER COATINGS

TBC allows operating temp. increase by 100-170 K.

Outer Ceramic Layer - Zirconia, thickness < 0.25 mm.

Metallic bonding layer - MCrAlY

Methods of application -Thermal Spraying (atmospheric plasma) & Physical Vapor Deposition (PVD).

EB - PVD yields better result but is much more costlier.

Combustor components and Stator blades - Thermal Spraying (APS), 1st Stage Rotor Blades – EB - PVD.

TBC – Working Principle

THANK YOU

Generic Information on Coating Types

Coatings in Industrial Gas Turbines

TBC - DESIRED PROPERTIES

EBPVD YIELDS BETTER QUALITY THAN PLASMA SPRAY

Trends in Coating Evolution Overlay Coatings (MCrAlY) in place of Diffusion Coatings. Use of Chromides and Aluminide is limited to last stage

vanes/ blades, coating of internal holes and for contaminated fuel applications.

Duplex Coatings (a diffusion layer over bond coat) are the latest as these provide benefits of both.

Extensive use of TBC – Earlier use was limited to Combustor parts. New machines require TBC for Vanes and Blade as well.

Earlier generation TBC (suitable for combustor parts) are not suitable for turbine section (particularly moving blades).

Developments in TBC mainly directed to get thermal expansion coefficient nearer to Nickel superalloys.

EB – PVD yields best quality TBC. But being very expensive, its use is limited to Bucket applications.