Hilfe2 Ma terials Sci ence & Technolog y The effect of high-temperature exposure on the interfacial microstructure of Co-based coatings on a 9% -Cr steel for steam turbine applications Reinhard Hitzek, Andrea Al-Badri, Dr. Christian Leinenbach Power Gen India & Central Asia 2012 19.-21. April 2012
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Introduction Stellba Outline History of materials - From steel to nickel based materials Why special treatments are necessary - High temperature exposure - Failure of coatings occurring at these temperature Materials and Method - Base and coating materials - Coating technique and heat treatment Material Characterisation Results - SEM investigations, element distribution, hardness measurements, TEM investigation Phase diagram and thermodynamic calculations Conclusions Stellba, Power Gen India & Central Asia 2012 2
Introduction of company Stellba Address: Stellba Schweisstechnik AG Wohlerstrasse 51 5605 Dottikon / Switzerland Phone +41 56 201 43 43 Fax +41 56 201 43 41 info@stellba.ch, www.stellba.ch Founded 1957 in Switzerland Certified ISO 9001, 14001, 3834-2 GTS Certification for Thermal Spraying Stellba, Power Gen India & Central Asia 2012 3
History of materials Steam turbine valves and materials Year Temp. C Materials / Coatings HP-control HP-stop IP-control IP-stop 1960 460 C -Low alloy steels X X X X -Nitrided -Some Stellit 538 C -Low alloy creep resistant steels -Stellit X X X X 565 C -High alloy creep resistant steels (12%Cr) -Stellit -Chromium carbide X X X X 585 C -High alloy creep resistant steels (9-12%Cr) -Stellit -Chromium carbide X X X X 600 C + -High alloy creep resistant steels (9-12%Cr) -Nickel based materials -Special coatings X X X X 2020 700 C -Nickel based materials -Special coatings R&D Participate in Comtes 700 project R&D Participate in Comtes 700 project R&D Participate in Comtes 700 project R&D Participate in Comtes 700 project X = produced by Stellba X = produced by Stellba HP = high pressure IP = intermediate pressure / reheat Remarks: All type of valves including actuators and casings can be produced at Stellba Our Customers are all major OEM s world wide Stellba, Power Gen India & Central Asia 2012 4
Why special treatments are necessary - Reduction of CO2 emission into atmosphere - Therefore improvement of efficiency of steam power plants - One important parameter is the working temperature in turbines (efficiency increase 1% per 20 C increase of steam temperature) - valve seat coatings are used for temperatures up to 550 C Problems at temperatures T > 600 C: Delaminating of Stellite coatings on stainless steel on valve seats SIEMENS AG WHY SO? Alstom Stellba, Power Gen India & Central Asia 2012 5
Base and coating material Materials and Method Base: 9% Cr-steel X10CrMoVNb 9-1 Coating: Co-based hardfacing alloys Stellite 6 (Co-29Cr-4W-1C) Stellite 21 (Co-27Cr-5Mo-0.3C) Ultimet (Co-26Cr-9Ni-5Mo-0.04C) Coating technique and heat treatment X10CrMoVNb9-1 - Co-based alloy Plasma Transferred Arc (PTA) welding After coating process heat treatment at 690 C for 4 h to minimize residual stress Heat treatment was performed at 650 C, respectively for 1 000, 3 000 and 10 000 hours in a furnace in standard air atmosphere. Stellba, Power Gen India & Central Asia 2012 6
Material Characterisation Microstructural characterisation - analysed by scanning electron microscopy (SEM) in BSE mode Element distribution - taken perpendicular to the interface by energy dispersive spectrometry (EDS) - three spot measurements at a defined distance from Steel- Co-basedinterface were taken and averaged Hardness measurement - a Fisherscope HM 2000 with WIN- HCU - software was used for micro hardness measurements - Vickers hardness was measured perpendicular to the interface on 5 different spots at a defined distance and averaged Stellba, Power Gen India & Central Asia 2012 7
TEM investigation a transmission electron microscopic (TEM) image was taken in the interlayer region with the assumed interface between steel and Stellite 6 preparation of a thin semicircular disc with 3mm diameter and 80 µm thickness thinned to approx. 30 µm thickness by grinding and polishing a section with a width of 20 µm across the interface was thinned by Focus Ion Beam (FIB) milling, using a gallium ion beam Thermodynamic simulations CALPHAD based software ThermoCalc version S database TCFE3 including thermodynamic data on Co-Cr and Co-Cr-C systems Stellba, Power Gen India & Central Asia 2012 8
Results SEM Investigations X10CrMoVNb 9-1 Stellite 6 Stellite 6 Steel as-coated 1 000h 3 000h 10 000h Stellba, Power Gen India & Central Asia 2012 9
X10CrMoVNb 9-1 Stellite 21 Stellite 21 Steel as-coated 3 000h 10 000h X10CrMoVNb 9-1 Ultimet Ultimet Steel as-coated Stellba, Power Gen India & Central Asia 2012 1 000h 3 000h 10
Interlayer thickness as function of annealing time 35 30 Stellite 6 Stellite 21 Ultimet 25 d interlayer [µm] 20 15 10 5 0 20 30 40 50 60 70 80 90 100 110 t 0.5 [h 0.5] ] Stellba, Power Gen India & Central Asia 2012 11
Stellite 6 Element Distribution X10CrMoVNb 9-1 Stellite 6 Stellite 6 Steel 20 10 0-10 -20 Steel 30 20 10 0-10 -20 c [ma.-%] 100 90 80 70 60 50 40 30 20 10 0 X10CrMoVNb9-1 Stellite 6 as-coated -20-10 0 10 20 d [µm] Fe Cr Co Ni W c [ma.-%] 100 90 80 70 60 50 X10CrMoVNb9-1 Interlayer Stellite 6 10'000h 3'000h Fe Cr Co Ni W 40 30 20 10 0-30-20-20 -10-10 00 10 20 30 30 40 40 50 50 60 d [µm] Stellba, Power Gen India & Central Asia 2012 12
Hardness measurement Microhardness HV 750 700 650 600 550 500 450 400 350 300 250 X10CrMoVNb9-1 Interlayer Stellite 21 Stellite 6- as-coated 10'000h Ultimet- 1'000h 3'000h 3'000h 10'000h -60-40 -20 0 20 40 60 d [µm] Stellba, Power Gen India & Central Asia 2012 13
TEM investigation EDS mapping: Cyan: Cobalt, Co Green: Iron, Fe Purple: Chromium, Cr Yellow: Tungsten, W point Fe Co Cr W C possible phase 1 28.2 38.7 32.0 0.8 <0.3 2 45.8 45.6 7.5 0.8 <0.3 α ordered bcc Stellite 6 annealed at 625 C 3 8.4 7.3 71.7 8.1 5.1 M23C6 4 7.7 25.6 42.0 20.0 4.7 M23C6 spot measurement [wt%] Stellba, Power Gen India & Central Asia 2012 14
Phase diagram point Fe Co Cr W C possible phase 2 45.8 45.6 7.5 0.8 <0.3 α ordered bcc α (B2-type): believed to result in intergranular brittle fracture E.P. George, A.N. Gubbi, I. Baker, L. Robertson, Material Science Engineering A329-331 (2002) 325-333 Massalski Stellba, Power Gen India & Central Asia 2012 15
Thermodynamic calculations Thermodynamic simulations Stellite 6 at 650 C 0 Average composition at 20 µm from 0-point in the interlayer: 44Fe-42Co-9Cr-1Ni-0.45Mo-3W-1C Possible phases: M 23 C 6 µ- phase M 7 C 3 α- phase Stellba, Power Gen India & Central Asia 2012 16
Conclusion Failure of Co-based coatings on 9% Cr steel at temperatures >600 C: development of a thin interlayer with a high hardness interlayer thickness depends on the annealing time Fe diffuses into the Co-based coating TEM investigations assume the development of a brittle B2 interlayer and M 23 C 6 -carbides thermodynamic simulations support the development of M 23 C 6 -carbides and α-phase, as well as the development of µ precipitates but development of µ precipitates could not directly observed Stellba, Power Gen India & Central Asia 2012 17
Thank you for your attention! The project (N 8523.2 E) is financially supported by Stellba, Power Gen India & Central Asia 2012 18