APPLICATION OF 12% CR STEEL FOR GEOTHERMAL TURBINE ROTOR

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APPLICATION OF 12 CR STEEL FOR GEOTHERMAL
TURBINE ROTOR MITSUBISHI HEAVY
INDUSTRIES, LTD. GRC 2007 OCTOBER 1 YOSHIKAZU
SAKANAKA
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CONTENTS 1. INTRODUCTION
2. 12 CR STEEL ROTOR FORGING
FOR GEOTHERMAL TURBINE 3. WELDING
REPAIR BY 12 CR STEEL FOR GEOTHERMAL
TURBINE ROTOR 4. SUMMARY
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1. INTRODUCTION
12Cr Steel for Geothermal Turbine Rotor
Problem to be solved now
SCC of turbine rotor has occurred in some
geothermal power plants after long term operation.
Solution Plan

• Change the steel type from low alloy steel to 12
  Cr steel, in order to increase SCC resistance for
  new turbine rotor.
• Apply welding repair by 12 Cr steel which has
  good SCC resistance, in order to re-use the
  damaged rotor

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1. INTRODUCTION
Progress of Rotor Steels for Geothermal Turbine
in MHI
2.5NiCrMo Steel
Rotor
Section View of Turbine
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CONTENTS 1. INTRODUCTION
2. 12 CR STEEL ROTOR FORGING
FOR GEOTHERMAL TURBINE 3. WELDING
REPAIR BY 12 CR STEEL FOR GEOTHERMAL
TURBINE ROTOR 4. SUMMARY
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2.1 VERIFICATION STUDIES ON 12 CR STEEL
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2.1 VERIFICATION STUDIES ON 12 CR STEEL
? Verification Studies (Material Test)

• Chemical Composition
• Micro Macro Structure
• Tensile Test
• Charpy Impact Test
• SCC (Stress Corrosion Cracking) Test

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2.1 VERIFICATION STUDIES ON 12 CR STEEL
? Produce a full size turbine rotor forging made
of 12 Cr steel.

• Weight of rotor
• before manufacturing 23.6 ton
• after manufacturing 22.1 ton
• Length of rotor 6.7 m

Full size rotor forging after machining
Full Size Turbine Rotor Forging
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2.1 VERIFICATION STUDIES ON 12 CR STEEL
? Chemical Composition

• Homogeneous Chemical Composition
• Impurity Elements (phosphorous, sulfur, )
  Lowest Level

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2.1 VERIFICATION STUDIES ON 12 CR STEEL
? Micro Structure (Optical Micrograph)

• No major differences in micro structure among
  various locations
• Tempered martensite containing a considerable
  amount of
• austenite
• No d- ferrite

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2.1 VERIFICATION STUDIES ON 12 CR STEEL
? Macro Structure (C-3 center core bar of
rotor forging)
No Network segregation No d- ferrite
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2.1 VERIFICATION STUDIES ON 12 CR STEEL
? Tensile Test Charpy Impact Test
12 Cr steel
Compared with CrMoV steel, 12 Cr steel has
almost the same mechanical strength and better
toughness.
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2.1 VERIFICATION STUDIES ON 12 CR STEEL
? SCC (Stress Corrosion Cracking) Test Specimen
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2.1 VERIFICATION STUDIES ON 12 CR STEEL
? Cross Section of SCC Test Specimen
acceleration SCC test in real geothermal steam
Bending stress 0.2 proof stress
No SCC is observed in 12 Cr steel ? Good SCC
Resistance
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2.2 APPLICATION EXPERIENCE OF 12 CR STEEL ROTOR
? Application Experience
Full size 12 Cr steel rotor forging after
machining
12 Cr steel rotor with blades
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2. 12 CR STEEL ROTOR FORGING FOR GEOTHERMAL
TURBINE
? Summary of Chapter 2

• It is verified that there is no major
  segregation in rotor forging made of 12 Cr
  steel.
• 12Cr steel is verified to have enough
  mechanical strength and toughness.
• 12Cr steel is verified to have good SCC
  resistance.
• 3 turbine rotors made of 12 Cr steel have been
  in commercial operation without SCC damage until
  now.

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CONTENTS 1. INTRODUCTION
2. 12 CR STEEL ROTOR FORGING
FOR GEOTHERMAL TURBINE 3. WELDING
REPAIR BY 12 CR STEEL FOR GEOTHERMAL
TURBINE ROTOR 4. SUMMARY
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3.1 VERIFICATION STUDIES ON WELDING REPAIR
BY 12 CR STEEL

• ? Welding Repair Procedure

Base Rotor Material CrMoV steel (MHI standard
rotor material) Welding Material 12 Cr steel
(good SCC resistance)
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3.1 VERIFICATION STUDIES ON WELDING REPAIR
BY 12 CR STEEL

• ? Tensile Test Result

Welding material (12Cr steel) and HAZ have
higher strength than base material (CrMoV steel).
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3.1 VERIFICATION STUDIES ON WELDING REPAIR
BY 12 CR STEEL

• ? Measuring Result of Hardness

Material with high hardness High susceptibility
to SCC
Hardness of Welding material (12Cr steel) and
HAZ is enough low due to appropriate SR.
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3.2 APPLICATION EXPERIENCE OF WELDING
REPAIR BY 12 CR STEEL
? Application Experience
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3.2 APPLICATION EXPERIENCE OF WELDING
REPAIR BY 12 CR STEEL
TIG Welding
Repaired Rotor
Stress Relief Heat Treatment
After Machining of Blade Groove
After Welding of 12Cr steel
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3. WELDING REPAIR BY 12 CR STEEL FOR
GEOTHERMAL TURBINE ROTOR
? Summary of Chapter 3
It is verified that welding material (12Cr
steel) and HAZ have higher strength than base
material (CrMoV steel). It is verified that
hardness of welding material (12Cr steel) and
HAZ is enough low Weld repaired rotor for
Philippine geothermal turbine has been in
commercial operation without any trouble until
now.
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CONTENTS 1. INTRODUCTION
2. 12 CR STEEL ROTOR FORGING
FOR GEOTHERMAL TURBINE 3. WELDING
REPAIR BY 12 CR STEEL FOR GEOTHERMAL
TURBINE ROTOR 4. SUMMARY
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4. SUMMARY
? It is confirmed that 12 Cr steel rotor forging
and weld repaired rotor by 12 Cr steel are
sufficiently reliable to geothermal turbine.
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END
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1. INTRODUCTION

• Example of SCC (Stress Corrosion Cracking)

High static (centrifugal) stress Accumulated
contaminant
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SCC (Stress Corrosion Cracking)
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Development of 12Cr steel
1. Fundamental study Parameter Alloying
elements, Heat treatments Analysis Metal
microstructure, Mechanical properties,
Corrosion resistance, Stress Corrosion
Cracking(SCC) resistance
2. Production of a full size rotor forging
verification study Check analysis Chemical
composition, Microstructure,
Mechanical properties, SCC resistance
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Summary of Fundamental Study

• The stress corrosion cracking resistance of
  Fe-12 Cr alloy was improved by decreasing the C
  content and increasing the N content.
• The combination of 5Ni addition and double
  tempering treatment was found to be effective in
  controlling a martensite structure that includes
  stable austenite.

Solution Treatment at 9501000?
Double Tempering at 550600?
Temperature
Time
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Reasons for good SCC resistance of 12Cr steel

• Based on the very fine mixed structure of the
  martensite lath and austenite.
• Improved by decreasing the C content and
  increasing the N content.

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Microstructure of Test Sample
Bright Field Image
Dark Field Image
100µm
0.2µm
TEM (Transmission Electron Micrograph)
OM (Optical Micrograph)
Lath-martensite
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DETERIORATION OF GEOTHERMAL TURBINE
Stress Corrosion Cracking
Rotor Groove
Moving Blade
Rotor Groove
Corrosion Fatigue
Moving Blade
Last Stage Blade
Erosion Corrosion
Gland Sealing Parts
First Stage Nozzle
Scaling