The effect of high-temperature exposure on the interfacial microstructure of Co-based coatings on a 9% -Cr steel for steam turbine applications

Transcription

1 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 April 2012

2 Slide 1 Hilfe2 Diese Folie enthält zwei Mastergruppen (Master und Titelmaster), welche den Corporate-Design-konformen Auftritt definieren. Der jetzt zugewiesene Empa-Master 1 sieht für die Titelfolie das Empa-Logo vor. Den weiteren Folien ist kein Logo zugewiesen. Für längere Vorträge mit Zwischentiteln empfehlen wir, den Folien mit Zwischentiteln den Empa-Master 2 (mit Logo unten rechts) zuzuweisen. Dazu öffnen Sie via Ansicht > Aufgabenbereich > Foliendesign-Entwurfsvorlage rechts die Masterauswahl. Nun markieren Sie im linken Ansichtsfenster die Folien, denen Empa-Master 2 zugewiesen werden soll (mindestens zwei, ansonsten für den ganzen Satz Empa-Master 1 verwendet wird). Weitere Hilfe erhalten Sie bei Monika Ernst, 4995 (Empa, Dübendorf) M. Ernst, 2/4/2005

3 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

4 Introduction of company Stellba Address: Stellba Schweisstechnik AG Wohlerstrasse Dottikon / Switzerland Phone Fax info@stellba.ch, Founded 1957 in Switzerland Certified ISO 9001, 14001, GTS Certification for Thermal Spraying Stellba, Power Gen India & Central Asia

5 History of materials Steam turbine valves and materials Year Temp. C Materials / Coatings HP-control HP-stop IP-control IP-stop 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 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

6 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

7 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, and hours in a furnace in standard air atmosphere. Stellba, Power Gen India & Central Asia

8 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

9 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

10 Results SEM Investigations X10CrMoVNb 9-1 Stellite 6 Stellite 6 Steel as-coated 1 000h 3 000h h Stellba, Power Gen India & Central Asia

11 X10CrMoVNb 9-1 Stellite 21 Stellite 21 Steel as-coated 3 000h h X10CrMoVNb 9-1 Ultimet Ultimet Steel as-coated Stellba, Power Gen India & Central Asia h 3 000h 10

12 Interlayer thickness as function of annealing time Stellite 6 Stellite 21 Ultimet 25 d interlayer [µm] t 0.5 [h 0.5] ] Stellba, Power Gen India & Central Asia

13 Stellite 6 Element Distribution X10CrMoVNb 9-1 Stellite 6 Stellite 6 Steel Steel c [ma.-%] X10CrMoVNb9-1 Stellite 6 as-coated d [µm] Fe Cr Co Ni W c [ma.-%] X10CrMoVNb9-1 Interlayer Stellite 6 10'000h 3'000h Fe Cr Co Ni W d [µm] Stellba, Power Gen India & Central Asia

14 Hardness measurement Microhardness HV X10CrMoVNb9-1 Interlayer Stellite 21 Stellite 6- as-coated 10'000h Ultimet- 1'000h 3'000h 3'000h 10'000h d [µm] Stellba, Power Gen India & Central Asia

15 TEM investigation EDS mapping: Cyan: Cobalt, Co Green: Iron, Fe Purple: Chromium, Cr Yellow: Tungsten, W point Fe Co Cr W C possible phase < <0.3 α ordered bcc Stellite 6 annealed at 625 C M23C M23C6 spot measurement [wt%] Stellba, Power Gen India & Central Asia

16 Phase diagram point Fe Co Cr W C possible phase <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 A (2002) Massalski Stellba, Power Gen India & Central Asia

17 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

18 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

19 Thank you for your attention! The project (N E) is financially supported by Stellba, Power Gen India & Central Asia